First off, thanks to all for keeping the discussion collegial on the first chapter on motion comfort, let’s keep that going right to the end of the series so we end up with something really useful for those trying to make the critical decision of how many hulls will be best for their type of cruising.
When writing this chapter I assumed well designed and built boats. There are a lot of dangerous boats out there, regardless of number of hulls, that simply should not be taken offshore, but that’s another article.
Now let’s take a look at seaworthiness and dive right in at the deep end with capsize risk.
Monohull fanboys love to come at this one simplistically to get a win, but the reality is more complicated since we need to think not just about being capsized by wind but also waves, often a bigger danger.
Wind Capsize Risk
Still, let’s start with wind:
Interesting article making placing the data side by side for the hull types. The stability data for a catamaran, which should include wind speeds and reefing plans, should be stated in the owners manual, which in general I have found to be in place for new builds and coded charter cats in the UK. It may be that when buying second hand, this data is missing, hence potential purchasers of second hand cats should insist that the owners manual is included in the sale or that the seller includes such critical information.
Hi Alastair,
Good to hear that you have generally found the reefing information in the owners manual, and a great recommendation that buyers of second hand boats should insist of seeing the manual.
Thanks for continuing this series John, great information here.
Reefing at prescribed wind speeds is the rule of law on our boat. The reality is that nearly all modern cats and tris actually perform better and are more comfortable when you set the reefs at those wind speeds so there is really no excuse. On our Dragonfly 32, we will still see boat speeds of 8-10 knots broad reaching with a 2nd reef in the mainsail and a full jib in 18-25 knots of wind and it’s very calm and comfortable.
Thunderstorms and squalls are our biggest fear on our boat so we typically take the mainsail down if we are getting in close proximity (typically less than 5 miles) to any storm cell. If we are sailing at night with the possibility of any storm activity, we set the 2nd mainsail reef (we being my wife and I).
So far, we have been successful reefing our full battened mainsail at downwind angles in wind speeds ranging from 18-26 knots. Downwind angles for us are 140-160 degrees TWA. The wide platform of our boat will allow me to center the traveller and pull some mainsheet in to stall and depower the mainsail and I have the ability to ease the leeward shroud to minimize interference with the upper portion of the mainsail. You must also have someone on the helm who is very attentive to maintaining the sailing angle and/or a reliable autopilot. It’s interesting that Quorning Boats (the Dragonfly builder) stresses to turn up to an upwind or close reaching angle any time you reef. The Dragonfly trimarans all have what I would call a “mild” square top mainsail and that is what can potentially get hung up when you are trying to set a reef sailing downwind, thus centering the traveller and keeping some mainsheet in (not tight) to minimize this interference. In truthfulness, I’ve not have to do this in anything over 26 knots of breeze so even more important to set the appropriate reefs at prescribed wind speeds.
I agree with John’s recommendations on getting the boat through the “death zone” (I hate that term) safely during a bear away. I’ve raced small catamarans (Prindle 19, Tornado, and currently A-Class catamaran) since the early 90’s. For those boats, when you got to the weather mark in big breeze and had to turn down to downwind, the safest bet was to stop, blow the mainsheet traveller and most of the mainsheet, and then turn down with the jib still sheeted tight. With the advent of curved daggerboards and rudder winglets (at least on the A-Class catamarans), the maneuver is a lot less risky and in up to 17-18 knots, you can get through nearly full powered up. The foiling multihulls have it even easier if the sailor has his technique right. We are seeing some performance cruising multihulls now sporting curved daggerboards to provide some lift to the forward sections of the boat but so far I’ve not seen any rudder winglets and for practical purposes they may not make sense since you have to maintain them at the correct angle of attack and they are magnets for weed and debris in the water.
Hi Bob,
Great real world information that should be invaluable to any multihull owner, or prospective buyer, thanks.
Also, good point about stalling the main by overtiming it. I have to admit I had never really thought of that, or at least not quite in that way, until you mentioned it in connection with the jib in another comment. Always more to learn.
Interesting that Quorning want you to turn up in the wind to reef. Despite my huge respect for them, I still think you are right in your approach. Point being that if reefing down wind is not our standard practice we can get in a situation where we have too much sail up to turn into the wind and so are stuck with the boat going every faster and no way out of the situation.
If I remember right, this happened to the super-cat Play Station on one of her early voyages. They got away with it, but apparently the crew saw God.
Whereas if you reefing down wind is our default we will have it cracked and so can do it in most any situation.
I also think that not having to round up makes us more likely to reef frequently because it’s just so much less scary. The result often is that we will, like you two, have less sail up more of the time.
Also, great tip on just taking the main down with T-storms coming. The ultimate safety move.
As to the weather mark, as a 505 sailor I totally get that. Always a trick one. On the 505 the key move was to get the board mostly up during the rounding. Forget that and a swim was pretty much guaranteed.
Regarding our downwind reef technique, for a lot of folks it may seem counterintuitive to center the traveller and pull in some mainsheet at deep angles but what this does is present less projected sail area reducing the load on the sail. The main thing I am trying to achieve is to keep the top of the sail clear of the rigging. That said we will have the preventer attached and again it requires a steady hand on the helm and control of the boat to keep the boat in the safe zone. It’s good to practice the steps in lighter air so it becomes very familiar.
Bob, also as an owner of Dragonfly 32, mine is the touring version, I confirm that I have found reefing downwind to be effective. Drawing the boom towards the centreline using the mainsheet and preventer is an effective way to keep the sail under control, away from the rig (mostly) and to de-power it. I will take some convincing that easing the leeward shroud is a good idea – I would prefer to use the boom position and the load on the leech to minimise interference.
Hi Simon,
To clarify, I typically ease the leeward shroud about 24”-36” on the control line so with the 4:1 purchase on the control system, it is only a 6”-9” ease on the shroud. We will do it when sailing at 120-160 TWA’s to keep the leeward shroud from chafing the mainsail and for the above to take pressure off the square top when setting a reef downwind. The caveat is to remember to tighten it back up if you turn back upwind but even if you don’t it does not cause any problems.
I just love the preventer and barber hauler systems on the Dragonfly’s. You can really dial in to very close to perfect sail trim in nearly all conditions with these trimming tools and probably the best feature is mainsail preventer can just about eliminate mainsail flopping in lighter air and chop sailing downwind.
Cheers,
Bob
Very, very interesting article.
I wonder if in terms of capsize risk a Pacific Proa (like Russel’s Brown Jzerro for example) could be assimilated to a trimaran or it’s a complete different beast.
You calculate the data for a proa exactly the same way as for a cat or a tri, and then you see what comes out of the math. There’s such a huge variety of them – some inherently stable, some requiring movable ballast or active management to stay upright – that it’s impossible to generalize.
Hi Michele,
My guess , and it is just that, would be that a since the smaller hull is always to windward and the proa not generally as beamy as a tri the stability curve and relationship to wind pressure and heal angle would be more like a cat than a trimaran and therefore less forgiving than the latter, and probably even less forgiving than a cat. Anyway, Proas are interesting but in my view special interest only and not really viable cruising boats.
Hi John,
Capsize risk has long been an interest of mine too but in a very different context. Many large traditional vessels have met their ends due to wind induced capsize. These vessels are generally not as susceptible to wave induced capsized by shear size but they also tend to have very heavy tophampers and not great GZ curves. To make this more complicated, depending on flag state, sometimes good information is available to the master and at other times, it is nothing more than a number like GM. Roger Long who you mention stands out as being someone who really helped the industry (and is a nice guy too). I just wish I could find a copy of the study he worked on when they were coming up with sailing school vessel stability criteria in the US but it seems to be hard to find.
I like to look at a graph of wind loading laid over the GZ curve when looking at it statically although it is very rarely presented this way. The wind loading curve is a gross simplification but it is still helpful. If you look at it, you see it goes to 0 at a heel angle of 90° which of course is not accurate (there is some difference in mathematical convention here), the wind is still pushing on a lot of boat. The good news is that it is more accurate at the lower angles of heel where it actually matters.
Once you get to the point where the wind moment hits only at a single point, any more wind and you will capsize. It is interesting where these hit as it is at an angle significantly less than the AVS. What this means is that in a perfectly steady wind with no seaway, once you reach this heel angle, the boat will capsize despite having significant remaining AVS unless the wind is suddenly removed.
To me, the really interesting/scary thing happens somewhere around 90° heel angle. If the boat has an AVS above this, an infinite amount of wind cannot capsize it (theoretically only as the hull windage is assumed to vanish here which it doesn’t in practice). However, if the AVS is below this, it is always susceptible to a wind induced capsize. How susceptible is due to what the actual GZ curve looks like and how much windage there is.
I quickly put together 3 graphs that show this. The 120° AVS boat has no worries of a wind induced capsize if it is kept watertight. The 90° AVS boat is extremely close so a small wave or something could push it over the edge. The 75° AVS boat is definitely susceptible to wind induced capsize. You can do this same thing with a multihull but you need to more carefully calculate windage as the projected area of a cat or tri up on edge is quite high.
There are a few things that I always try to keep in mind from this:
Agreed to your conclusions. I hope this is not too off topic.
Eric
Next graph
Final graph
Hi Eric,
A very good point that a boats with an AVS of under say 120 can reach what one might call the capsize vulnerability point (CVP) at heel angles that seem safe. I did know that (kinda) from reading Roger’s stuff, but it did not really sink in until I read his full analysis of the Bayesian sinking, and even then I’m not sure I fully “got it” until I read your comment above. Thank you, it’s important, particularly (as you point out) when thinking about internal ballasted centre board boats. Definitely something I need to think about more.
While on the subject, in recent years I have learned, much to my surprise, that we are generally much safer going to sea on a well found 40 foot yacht than many much larger sailing vessels—definitely counter intuitive. This was exposed by “Pride of The Sea” and “Tall Ships Down” in relation to traditional vessels but I suspect (and hope) that the Bayesian sinking is going to bring the same sort of scrutiny to super yachts, and particularly allowing them to be classed with very low AVS numbers, just because they are big.
In my own case reading Roger’s stuff on Bayesian made me think a lot about the super yacht I went to Greenland on, and not in a good way, particularly the location of her engine room venting coupled with the difficulty of easing her heavily loaded sheets quickly in a gust. And worst of all, the unreliability of reefing her huge mainsail using an in-boom system. Add it all together and I’m very happy that on that trip the skipper basically handled her as a motorboat with a little main rolled out for steading.
Hi John,
Yes, I definitely think there is truth in bigger is not necessarily safer. A lot of large vessels have marginal stability, really poor downflood angles and the ingress points are not ones that can be shut off quickly. These vessels also take a long time to turn into the wind or run off and all sail handling is slow. Plus, there is no ability for the crew to manhandle things when they go wrong, everything must be done with a purchase or leverage and there are some things that simply require a dockside crane or other piece of machinery. Just like small boats, extreme weather events are rare and the chances of everything lining up to actually cause a problem that is newsworthy are also relatively rare. There are also plenty of unbelievably sea worthy large boats out there, it is just that size is no guarantee.
My own personal comfort zone is somewhere around 100° AVS on large vessels (say 100’+ and displacement >150 tons) that haven’t had significant modifications since the calculations and maybe 110° for smaller vessels and I definitely like more. In the US this is complicated as many of the vessels fall under regulations that are more based on a comparison to the rig size so boats with really poor stability are allowed as long as they have very little sail area but these vessels are actually quite susceptible in squalls as even that sail area is way too much. In many of the accidents with traditional vessels, it is found that the AVS had significantly decreased from when it was last calculated. This applies to smaller vessels as well and I can definitely see one of the smaller centerboarders that has had 1kW+ of solar bimini added, a dinghy in davits, a furling mainsail, 3 headsails on furlers, etc. being at the point where they could be capsized by a wind level that they would expect to see many times during their life and only by having no sail up and not being broadside do they not have issues. Some people are fine with those margins, I am not as I know I make mistakes and want the boat to be forgiving.
I have talked to a few people who have been part of some of the wind induced capsizes a decent amount and it definitely seems to me like they had no idea how little margin was left in some of their normal sailing. I believe that this is due to not understanding the implications of the AVS angle being when there are no overturning forces and not realizing that the point of no return occurs much earlier if the wind continues. If you look at the Pride incident, they only had the staysail and double reefed main up which is not a large portion of what you might call their normal sail area of 4 lowers and was an extremely small portion of their overall sail area they could fly. On a boat like that, I would expect that to be pretty close to full on storm canvas and yet it was still significantly too much. For anyone who wants to look at this incident more, it is interesting to note the difference in what the implied cause of loss is in Tall Ships Down versus Pride of the Sea. Tall Ships Down gives what I would consider to be Tom Gillmer’s (the boat’s designer) version and Pride of the Sea gives closer to what Roger Long’s version is and they are quite different and somewhat contradictory.
One other random thought. As I learned more about this, I came to see bulwarks quite differently. If you assume that the top of the bulwarks is defined by aesthetics and windage and can’t move up, the boat with short bulwarks and high deck has a much better stability curve than one with tall bulwarks and a low deck. Under 100′ or so, I simply don’t think you can have bulwarks that are high enough that the extra safety they provide is worth it compared to a low bulwark to give your foot a place to go and good railings/lifelines.
Eric
Hi Eric,
Lots of good points as usual. I wonder how many super yachts would pass the Eric Klem test, particularly since so many of them have machinery space venting, which must be open to operate the boat, situated so that their downflood angle is way before their AVS, which itself is way less than 100°. Not many I bet.
Also a very good point about the danger to stability of modifications like solar panels and roller furling sails. We have seen the same problem, or at least suspected problem, as the cause of several fishing boat accidents here in Atlantic Canada.
And finally, the part of your comment that most resinated with me was :”Some people are fine with those margins, I am not as I know I make mistakes and want the boat to be forgiving.” I have been thinking for some time about an article on just that, and you have inspired me. Thanks.
Following up on this, I tried playing around a bit with a curve that approximate’s John’s GZ curve for a catmaran. While I have had great fun playing around on a few Hobie’s when I was a kid (and going swimming more than a few times) and done some relatively tame sailing on other multihulls, I am not very knowledgeable on them. I love the concept of multihulls for certain applications so this is just an interested person taking a superficial glance.
I plotted a curve that represents the boat about to capsize and called this the “normal gust curve”. The interesting thing to me is that the shape of this curve is extremely similar to the shape of the GZ curve after max GZ. What that means is that if you reach max GZ, you have almost no margin left. If you look at the monohull curves, as long as you have a range of stability greater than ~100°, you can go well past max GZ and still avoid a wind induced capsize.
Then I plotted the steady state wind for the normal gust curve using the gust wind speed as 40% higher than the sustained wind which is what a few UK MAIB accident investigations have found to be the maximum difference in open waters. The boat is heeling a barely noticeable amount and at least to me, I would not expect to realize how close to disaster I was.
Finally, I plotted a “squall” curve which I arbitrarily set at twice the windspeed of the sustained wind which seems kind of reasonable if you have a boat that could capsize at 30 knots sustained as that would be a 60 knot gusting squall, something which happens with some regularity. There is no question that this boat is capsizing.
My initial thinking on this is that there are some similarities to monohulls with low AVS but also some differences. The similarity is that squalls represent significant dangers and must be treated very cautiously including not waiting to see what the wind will do. The difference is maybe in knowing how hard you can push outside of squalls. Even for low AVS monos, it is pretty easy to make sure that you never heel more than half the angle that it takes to immerse the deck edge in the sustained breeze and be confident in your stability in all gusts (the GZ curve is close to linear in this region and gusts exert ~2X the wind pressure compared to sustained). With the idealized cat example here, just looking at the immersion of the windward hull or heel angle seem like poor indicators of the margin left for gusts.
Note, I am using the same calculation for wind heeling moment as before and I suspect the idealization is even worse for a cat than a mono. In particular, once the windward hull is well in the air, the underside of the bridge deck presents a lot of area that I am not accounting for which makes this all a bit worse than the graph shows. Additionally, multihulls are usually lighter and accelerate much more quickly in a gust and this is a static look as if the sails were rigid, flat boards sheeted in all the way so that they provide no lift with a beam wind. It is probably instructive of what happens when caught beam-on with full sail up by a squall but won’t get the actual values things happen at right and doesn’t look at the problems of the death zone.
Eric
John, I had an interesting incident years ago with my Ranger 33. I motored down the East River in light winds, and approaching the Battery a breeze developed so we made sail and continued around into the Hudson. I went below to make lunch and suddenly we got hit by a huge gust (clear weather) that knocked the boat WAY over. As I rushed up the companionway my friend, an excellent seaman, was bringing the boat into the wind when we got hit by another strong gust from the other side. Each gust only lasted a few seconds, but I noticed that a cooler that had been on the starboard settee now resided on the port settee, not on the cabin sole, indicating either a 90 degree knockdown or a major knockdown quick enough to catapult the cooler across without landing on the sole.
On a couple of occasions I have seen the effect of gusts heeling the boat (as you say) maybe 50 degrees only.
Hi Leonard,
Interesting, particularly about the cooler. One of the things that I intentionally left out of the above article was dynamic stability including capsize inertia, but in your case with the violence of the experience those factors would have come into play, rather than the more simplistic static stability that I used in the comparisons above. That’s a long way of saying that my guess is that you did not go all the way to 90 degrees, but rather, as you say, the speed of what happened was the factor that made the cooler end up on the opposite settee.
Hi John,
Thanks for this.
I fear waves much much more than wind in terms of capsize risk, in fact a lot more than anything else on our mono. This article sure did not change that.
I fear what an important wave, not even necessarily a rogue one, which by definition I think would need to be 2-2.2 times the other waves we would be in, could do to our boat if other serious conditions are already upon us: winds in the 40s, minimal canvas out, significant heeling, 4-6 meter waves… How bad a would a wave have to be? Of course, it remains a healthy fear, one that is tamed and looked at from a scientific perspective and rooted into deep respect for the ocean and what it can do. This fear that I have won’t ever prevent us from adventuring. Hopefully, I never get the answer to how big the waves needs to be it if you understand what I mean. But I wonder about it when I read dreadful accounts of boats that did capsize and recovered or met their demise, offering mere survival to their crew to tell the tale. It’s a combination of factors, but let’s face it, it’s always a LOT about the waves; how big the wall of waters were.
In the deep of this subject, I expected maybe a mention about the relation between capsizing risk and length. I know you are comparing mono and multihulls, but very few multihulls, if we are talking about offshore sailing, are under 40 feet. Much shorter monos have proven their seaworthiness. But I’m curious, does added length both on multihulls and monohull, provide the same protection? I think they could be compared toe to toe there: is an added feet in lenght a lot more beneficial on a cat than on a mono when it comes down to considering capsize risk?
On capsizing, a capsized cat is pretty much game over out there, although it nearly never happens. A capsized mono is an absolute nightmare where people break bones, stay stuck under heavy objects, die from the capsizing event itself or drown. However, it’s not necessarily final. Depending on the shape of the boat and it’s center of gravity, the lower the better, if a narrow boat, only one tenth of the force of the capsize energy could be necessary to recover from that belly up situation. John Lacey, the naval architect, came up with that post FastNet of 79. That’s where I console myself with our old 70s racing hull shape from William H. Tripp Jr. when I think of the worse the waves can offer us out there. Although it for sure is more than one tenth of the capsize energy for our boat, it’s less than some large sail boat. I also think about this when stocking up, everything heavy as close to the keel as can be and tied up… Cuz, why not always plan for the worse?
I’ll go read your articles on survival after capsize, I feel it’s right up my dark alley of prepping for the worse.
As always, receive my best John,
Marie
Hi Marie,
I cover most all of that in the heavy weather book, so would rather not get into it here. I will also be covering survival after capsize in all types in the next chapter.
Great article, thanks for sharing. Was caught in a sudden increase in wind yesterday with too much sail out on Lake Superior. Luckily there was no damage, but did create some unwanted excitement. Thanks again.
One small correction for the multihull section: “Heeling angle is not an effective warning of impending disaster.”. That is not quite corrrect. Heel angle, indicated by windward hull rising, is absolutely an effective warning on a cruising catamaran. We have by the numbers warning (apparent wind speed), rig load warning (if you have shroud and/or mainsheet load sensors) and windward hull underbody exposure warning. Each of these in isolation or together indicate need to reef and/or alter course.
The death zone on a multihull is that 20-30 degree zone (at about 60-80* apparent wind angle) where turning into the wind increases apparent wind speed and increases the centrifugal force trying to trip the boat and where bearing away accelerates the boat too quickly with too much sideways force on the sails. Multihull cruising sailors need to be aware of this zone and either head up or bear away to avoid it.
Another correction: dumping the sheets, especially the mainsheet, is absolutely the wrong thing to do when more the 60* off the wind. The reason being that easing the mainsheet powers up the sail. When sailing off the wind the proper gust response is 1) bear away hard and 2) bring in the mainsheet. Only when less than 50* apparent wind angle is dumping the main sheet the correct response.
We survived a 60 knot storm cell on our 54′ catamaran with full jib and full main by thankfully being close hauled when the wind increased from 15-60 knots. We didn’t ease any sheets, just steered up to 20* apparent wind angle and feathered the sails. That was 30 minutes of anxiety!
The modern design fashion for very sharp reverse bows is not a forgiving feature as it reduces the amount of flotation with immersion. We are very thankful for our old-fashioned rounded destroyer bows – a little more drag at low speeds but heaps of volume.
Hi Tom,
I agree with nearly all of your points and in my earlier comment about releasing mainsheet tension and/or the traveller when making the turn from upwind (TWA less than 50 degrees) to downwind, I should have clarified that I never turn to dead downwind for the same reason you state above. The turn is typically to around 120-130 TWA which (at least on the boats I’ve sailed) keeps the AWA typically no less than 90 degrees and IME keeps the eased mainsail depowered. After the turn is made safely I immediately re-trim the mainsheet. I would bet John’s experience as a 505 sailor would be similar. If on our Dragonfly 32 we are sailing downwind (TWA greater than 100 degrees) and the breeze builds to where we want to add a reef, we will turn down to at least 150-160 degrees TWA without easing the mainsheet and then center the traveller and bring some mainsheet back in to further depower the sail (and reduce twist) before easing the halyard and pulling on the reef lines.
Like you I’ve also survived some high winds (thankfully typically short lived) summer thunderstorms hove to at an upwind angle on my A-Cat with the traveller fully eased, but keeping the mainsheet tension on. On that boat, if the wind picked up to over 30 knots, it would probably get knocked over no matter what (180 lbs total boat weight,18’ overall length, 29’ tall mast, and 7’6” beam).
I’ll qualify the above in that I’ve never had to do these maneuvers in over 30 knots of sustained wind. Experiencing gale force conditions is hopefully a situation I will continue to avoid for the rest of my sailing life.
I’d like to add some comments about reverse bow designs as the bow design is part of the overall package in the performance and stability of the boat. At least on the higher performance catamarans I have sailed, there is actually more volume in the reverse bow design (look at the lower section profile) than there is in older straight or destroyer looking bows (like the older Nacras, Prindle 19, and Tornado). I still actively race A-Class catamarans and it is now rare that pitchpole induced capsizes occur with the current reverse bow designs on the boats. As a Dragonfly 32 owner (but with the older conventional amas), I’ve noted that Jens Quorning went to the reverse bow design first on the amas of the DF 28 and DF32 and then followed by the DF 40 (on those boats, the center hulls retained a plumb bow design) and now on the new DF 36 (the center hull on this boat is reverse bow). The stated design goal was to get more volume in the forward sections of the amas allowing you to either push the boat harder or giving you a better safety margin against the boat tripping over the leeward ama and when you see the boats sailing in big breeze, it appears to be very effective. The following links showing a Dragonfly 28 Evolution and a Dragonfly 40 sailing upwind and reaching in 25–30 knots of wind and severe sea state (short period waves) seem to bear that out:
https://www.youtube.com/watch?v=IQAFcQkbRn8
https://www.youtube.com/watch?v=bbvsTsaBqoE
You still have to obey proper reefing guidelines as no matter what bow design you have, pushing too much sail area downwind in big breeze is very risky. A lot of naysayers on reverse bows bring up pictures of boats being pushed off wind at their limits while racing. I believe the reality is they are safer if the boat is managed properly in terms of sail area for the breeze the boat is being sailed in. I will agree they are wetter but that can be a moot point dependent on the number of hulls and the beam and length of the boat. I will be very interested in seeing video of the newer Dragonfly 36 sailing in big breeze and waves with the reverse bow on the center hull.
Functionally I think we are seeing a similar thing in the high performance offshore monohull world with the development of scow type bows in the Mini-Transat, Open 40, and IMOCA classes. Whether that trickles down to offshore cruising monohulls remains to be seen. The scow type bows looks to be potentially a better solution than a reverse bow for a monohull application in terms of improving safety and performance. But it sure is ugly!
Cheers,
Bob
Hi Bob,
Great comment full of interesting food for thought, and thanks for linking to those two videos. I have just watched both twice and I can certainly see how the reverse bows are working well in this case with comparatively small waves. I do wonder what would happen in say a 3 meter swell though, and am guessing that too much wave piercing may be something to manage. That’s not a criticism in that every boat has strengths and weaknesses, just an observation. In this case I think Dragonfly are making a good call given the conditions that most of their target market will sail in most often. And once offshore in bigger seas and >30 knots true no boat is going to make a lot of progress up wind anyway, and further I would guess the DF would slow down and fore-reach well.
One other thing, watching the videos reactivated all my lust for one of their boats. What a gas!
I also watched a couple of their sail handling videos and like how strict and emphatic Jens is about when to reef and the red line upper limits for each configuration.
This one might be a better demo of the Dragonfly 40 in last years Round the Island race:
https://www.youtube.com/watch?v=GIw66L01_XU
The wave heights in the video look to be 2m to 3m with relatively short periods. Smart reefing (2nd reef) and minimal heel appear to allow a fast and relatively safe and comfortable ride. I think the reverse bow design for the amas is doing its job based on reducing pitching and shedding the water/spray quickly. It would be even better if in open ocean with longer wave periods.
Hi Bob,
Interesting video, thanks for the link. That said, it looked to me as if there were trade offs at play here. Yes, the wave piercing was working well most of the time, but I think I saw several waves were more buoyancy in the ends might have worked better (more pitch, but softer and less bury) so I’m not sure that things would get even better in larger waves offshore. I could, of course, be wrong, but in all my years of studying boats it has been rare indeed to see a design feature that was universally positive, there are pretty much always trade offs.
Just to be clear, I’m not saying that Dragonfly have made a mistake. For the places their boats sail most of the time they may have easily made the right call.
Hi Tom and Deb,
I get your points, that said I stick by my warning that healing angle is not a reliable indicator on cats. Sure, a skilled and experience sailor of cats will note the heal angle and that may help them react to a not too extreme situation. But in the case of a sudden high gust that’s not going to work reliably since the difference in healing from normal sailing ~5° and 10° after which stability starts to fall off fast, is just too narrow to be used reliably as a capsize warning given that in a gust wind pressure increases by the square of wind speed.
The key concept here is that of a reliable indicator, and healing fails that test on a cat. Or to put it another way relying on heal angle will work…until one day in a strong gust it does not.
Also, while centrifugal force may play a small role I don’t think it’s significant.
And sure, if you are undercanvased in a bear away in the death zone not easing the sheet may work, but given that the apparent wind will come forward as the boat accelerates, again, I don’t believe that’s as a reliable as holding course and dumping (not easing) the main and possibly the jib to unload everything, before changing course, probably to windward.
And yes, as I will cover in the next chapter on storm tactics, fore-reaching while feathering, what you were doing, can be a very good tactic on multihulls.
And I generally agree on the drawbacks of reverse bows, on cats. I’m still thinking about trimarans, but my guess is it’s far from open and shut.
The maximum righting moment increases until the windward hull fully rises out of the water – generally 10-15 degrees of heel. The righting moment after that doesn’t reduce at all that fast and at 45* of heel (I certainly hope we never see that) the righting moment is still pretty good. and there’s a HUGE amount of difference on our cat between 10cm of windward hull lift and the other 70cm to expose the entire hull. Heel is a reliable indicator loading and unlike with beach cats doesn’t change all that quickly in non-racing modes. But of course, the primary reefing indicator is apparent wind speed (AWS) combined with apparent wind angle (AWA). Just, don’t discount heel angle.
At the higher speeds of a performance cruising cat, centrifugal force is absolutely a factor. Bearing away hard in a gust literally slams the windward hull down as the boat accelerates. We’ve felt that in squalls, and have also experienced the reverse of things feeling light and uncomfortable as we head up to feather the sails without easing the sheets.
In the death zone, it is generally safer to bear away and ease both sails until 90*+ AWA. From that point further downwind, only ease sails forward of the mast. The problem with dumping the main in the death zone is that it powers that sail up. This is a feature of high aspect ratio mainsails not present in pin-headed full or partially battened sails. So if you do want to turn upwind, you must slow the boat down by easing the sails (forward of the mast with the sheets, aft of the mast using the traveller), then slowly turn upwind so that centrifugal force lifting the windward hull is minimised.
Hi Tom and Deb,
I hear you on heal angle, however, I still stick by my warning that multihull sailors should not rely on it. The key thing I missed really punching up in the article above is the way that wind pressure scales quadratically in a gust, which with boats that have an angle of vanishing stability of under about 100 degrees means that a capsize happens very quickly in certain circumstances.
Have a look at Eric’s comment further up and the associated graphics that explain it better than I did. The same factor will, I think, generate situations where leaving the main sheeted in a dearth zone situation could also result in a capsize regardless of whether or not it is stalled.
No this does not happen every time but even thought it is rare it should be factored into our thinking. The classic low frequency, but high consequence event.
I need to do a tip, or maybe a whole post on this because it’s not at all intuitive but is important to know not just for cat sailors, but also for those with internaly ballasted monohulls.
On the centrifugal force, perhaps you are right and I can certainly see that it would feel that way but I have a hard time seeing it as a decisive factor when measured against the quadratic effect of increased apparent wind.
Hmm, again! I’ve seen the same general discussion on various forums and while the basic physics is correct, stability graphs etc for mono vs multi hulls are well known, how that translates in actuality is far less clear.
A catamaran will struggle to recover from a 10-15% heel, rough figure here, but how much wind at full sail would it take to get it there? Is this really a reasonable expectation for a cruising cat? I think most/all cruising cat capsizes are due to tripping over a hull due to excessive speed on a turn etc, I’ve never heard of a cruising cat being simply blown over in anything less than a tornado.
Given the exceedingly low percentage of capsize incidents for cruising catamarans it would seem that this is an edge case at worst. I think Prout catamarans still has the record for the highest number of production boats circumnavigating, 600+, with only one capsizing and that at anchor in a micro burst without sails! Of the three boats that were caught abeam during the Queen’s birthday storm the cat stayed upright, without dagger boards, while both monos rolled.
BTW as a mountaineer I’m uncomfortable with your use of the term ‘death zone’ which seems such an emotive term to use here in comparison to it’s use on a mountain like K2 where it referred to the 90% likelihood of death at one point, not really the same level of risk.
Hi Nick,
I think we are both on the same page in that I started the part about wind capsize by stating clearly that cruising cats don’t often capsize, and as to the Queens Birthday storm I clearly stated that monos are more susceptible to wave capsize than cats.
That said, capsize is, while low frequency, a high consequence disaster and therefore I think it’s important to talk about it in a book that’s designed to help people decide what boat to buy, if only to turn down some of the hype, by explaining the physics.
That said I’m not comfortable with the idea that a cat is so stable no amount of wind will capsize it. When thinking about this we must remember that the relationship between wind speed and wind pressure is quadratic, so, for example, doubling wind speed increases wind pressure four times.
Therefore I find it hard to credit that a cat that has enough sail to sail even half way decently in say 20 knots of wind can’t be capsized in a 40 knot gust if not reefed (a common occurrence in a front) where the wind pressure will suddenly increase by a factor of four. And of course we can easily get a rapid increase of wind speed from 20 to 60 knots in a thunder storm, which would increase wind pressure by an eye popping nine times.
On reflection I should have punched up this relationship more than I did in the above.
It’s also interesting to note that responsible multihull designers like, for example, Dragonfly have very strict wind limits for reefing and carrying light air sails. There are, I think, good reasons for that.
As to death zone, once again, we agree (see the foot note). Still, that’s what people have called the phenomena for as long as I can remember, so that’s what I called it. Sometimes we just have to be pragmatic about common usage—don’t get me started on the use of “in terms of”.
The problem is that the physics is too generic. There are a huge variety of multihulls and the points you have highlighted only apply to a few of them so don’t really help when making boat decisions.
For example you state that heavy cruising cats without daggerboards are more likely to be subject to wave capsize than those with retractable daggerboards, yet this was precisely the type of cat that was not rolled in the Queens Birthday storm in precisely the conditions that should have rolled it if your generic statement applied, breaking wave conditions sufficient to roll the monohulls multiple times dismasting both.
Your ‘death zone’ is only a thing in hobby cats and modern high performance cruising cats that will accelerate in the way described. Most cruising cats out there just aren’t that performant partly because of their design and partly due to being overloaded; they’ll make better speed than the same length mono but won’t accelerate dangerously.
I’m sure that there are multihulls that can be blown over, I’m equally sure that there are others that will lose their rig before it happens. There’s nothing in the stability curves that tells me whether for any particular multihull it’s in danger of being blown over in an F4 or needs an F12+ and so although interesting the curves are practically useless to choosing a hull type.
The point is that to make educated decisions on boat choice you need actual data, not general theory, and I haven’t seen any data that indicates that multihulls are being lost to capsize more or less often than monohulls in practice and so I find it hard to see it as a significant criteria in boat choice.
Hi Nick,
Sorry it does not work for you, however I think it has value, and it seems others do too, so let’s leave it there.
Hi Nick,
One more thought. Once again, we are on the same page in that I clearly state that before buying any multihull we should discuss capsize risk with the designer and/or builder and the safe wind speeds for that particular boat. You are saying that this is required. We agree.
For some reason you just don’t like this article, and that’s just fine, but repeatedly suggesting that I have missed something that I have in fact clearly stated is not helpful to anyone.
Hi John,
I’m actually trying to be helpful by pointing out what’s incomplete or inaccurate in your analysis.
You haven’t shown that there is an issue that needs to be addressed that results specifically from hull type. I mean here empirical evidence of statistically significant difference across your three hull types and possible failure modes that would indicate particular mediation should be looked at. Any discussion with a boat designer should include discussion of safe wind speeds and reefing profiles whatever the hull type, that’s a given, but your article tries to highlight differences where there isn’t empirical evidence that they exist in the target of interest; cruising boats.
To be blunt, I don’t give a fig if you can flip a hobby cat or a 52ft racing cat that can clear 20knts, that’s not what I’m buying.
I’ve given some examples where your statements are empirically incorrect or too general to be useful; dagger boards, death zone etc. I apologise if this has come over as overly critical but I’ve seen far too much generic and generalised statements around hull and rig configurations that without empirical specifics are used to justify just about any position to not comment, especially given the outstanding quality of the resource you’ve produced over the years.
I’ll shut up now and leave you to it!
Regards
Hi Nick
Yes, we will have to agree to disagree.
In my view I have clearly shown with basic and well accepted physics why multihulls must be careful of wind induced capsize and why mono hulls are more susceptible to wave induced capsize and well as many other things to be aware of such as that adding fixed keels to multihulls or overloading them reduces their intrinsic resistance to capsize.
These are all useful things to know when buying a boat. Next I will build on that by looking at how this basic physics influences things like storm tactics and down flood risk.
Conversely, you have criticized, by claiming that I have missed things that in fact I clearly wrote about, and without any constructive addition or reasoned correction to this basic physics. Easy to do, but not useful.
The Kelsal Stability Index attempts to quantify gust resistance – generally most cruising and performance cruising catamarans are somewhere in the region of 25-40 knots (full sails, beam reach, knots of wind required to capsize). By the time of 20 knots true wind speed, a performance cruising cat can be reefed down to 30 further knots of resistance. Charter type catamarans typically don’t have enough sail area to be in trouble either. With a 60 knot gust, that’s trouble for anyone and for a multihull, surviving right side up requires either proactive reefing and/or selecting a safer wind angle (either upwind or downwind, but not reaching).
Hi Ton and Deb,
I agree, broadly, particularly your point about reefing. That said, I think 60 knots is too high a number and we should get concerned way below that. For example wind pressure at 40 knots is four times higher than it is a 20 and an eye popping nine times at 60 knots.
Hi John,
In 2019 we joined friends on their Leopard 47 for a passage from Tonga to Fiji with a couple of weeks of exploring added in to the trip.
We left the northern Vava’u Group with a forecast for 20 -> 30 knot easterlies, completely overcast, with a weak front bringing heavy rain. The top battens in the fully battened mainsail had been destroyed and new ones were being flown in to Savusavu to meet us. This meant we would rely on the headsail and engines and plenty of fuel for the two night hop. Not ideal.
My watch from 23:00 to 03:00 with instructions to keep her below 8 ->12 knots. Before the skipper went to his bunk we wound in some genoa to 80% jib size, as the wind was at the upper end of the forecast. On watch, the wind increased with occasional gusts to 35 knots, but the bows were lifting to each surge and with the hulls humming, the speed display showed surfs to around 13 knots boat speed.
So I helmed for about 10 minutes – the steering was light and responsive and the autopilot coping just fine. I considered waking the skipper to take in more jib – there was a trick to their system and in the dark we didn’t want errors. But the boat seemed to be quite comfortable, so I left him until 03:00.
His first action was to examine the boat speed instruments and “maximum boat speed” setting I didn’t know about – much to my chagrin it showed 18 knots. Together, we rolled up to around 50% jib and I retired with a lesson on how a monohull sailor experiences speed differently than a multihull sailor, especially on a dark, dark night, without visual clues.
The seas were 2->3 metres which we were keeping pace with or overtaking in the gusts. The Leopard showed no tendency to bury a bow, with decent flare in the forward sections.
But something “new to multihulls” sailors, may be easily caught out by?
Hi Rob,
A very good point. As you know so well in a mono hull we can get fooled about how things are escalating when off the wind, but your story makes the point well that the same can happen in a multihull and might be even more difficult to discern intuitively so having hard limit numbers is even more important. Thanks.
We’ve seen that on our catamaran when sailing with monohull-experienced sailors – they are surprised at the actual speeds when the wind builds and don’t have the heeling indicator that they’re used to. I’ve awoken when off-watch to the feel of the boat really picking up her skirts and going, while the watchkeeper is thinking all is well. Catamaran sailors do have to pay attention to numbers, particularly when they have less experience!
Glad you enjoyed the passage.
Hi Tom and Deb,
A very good point and exactly why I get so concerned about multihulls being aggressively sold to those new to sailing, or even experienced monohull sailors who buy one without understanding that sailing a multihull safely takes different skills, and a general higher level too. That said, there is the same, or perhaps even worse, problem of the unskilled taking a monohull offshore and getting rolled because they don’t understand how suddenly things get dangerous for that boat type when the waves get higher.
Hi Tom & Deb,
Thanks, good passage although not your typical trade wind sailing but really enjoyed my first offshore passage on a cat.
On our 14.5m Beneteau, off the wind as we were on that passage, when the wind picks up over 25 knots, in 2->3 metres waves we will surf just like the Cat (unless we take in a third reef or even fourth reef). And when this happens we actually stop rolling and come upright like running on rails. This is a very noticeable change of state even to a novice.
But there was no big difference in the cat ride between 8->12 knots surfs and 8->18 knot surfing, except the harmonics which I hadn’t the experience to interpret. Not sure the skipper did either to be fair, as we set a new boat speed record for them..!
Cheers, Rob
I’ll just add that trend towards higher performance cruising multihulls can find its limits quickly. The first generation of Morelli and Melvin Gunboats (the 48, 62, and 66) are and still are very powerful boats that must be sailed with a very keen awareness and respect for safety and proper boat handling. I was tactician and alternate helmsman on a Gunboat 66 for six years and one year racing in St. Maarten we had three days of 18-25 knot tradewinds. I made the call on the reefing and every day I called for 1st reef in the mainsail. Upwind, we were typically doing 10-12 knots of boatspeed, reaching we were doing high teens, and downwind (120-140 TWA) we were surfing at up to 25 knots. On the last day of the regatta, I was on the helm for most of the race until the finish. We had been having an issue with a sticky clutch on the mainsheet system (basically comprised of a large electric winch followed by the clutch) and mainsheet trim was being handled by the boat’s captain who knew her well. We were approaching the finish line that day on a close reach which was in Simpson’s Bay and were hit by a gust line (probably close to 26-28 knots). The boat had been sailing well on her feet all day in similar gusts and on this leg we trimmed to keep the heel angle no more than about 10 degrees (about 1’-2’ of the windward hull clear of the water). On this gust band, the clutch would not release and I could not turn down. The weather hull flew high enough that tip of the weather daggerboard cleared the water (confirmed by a 3rd party photo and probably close to 40 degrees of heel). I really thought we were going to lose her and the only thing I could do was head up to try to depower. Luckily, the captain got the clutch to release and we got the boat back. She accelerated from 12-13 knots to 19-20 knots during this incident. I thought the owner was going to be real pissed with me but he gave me a high five! The postscript to this was in 2023 the same boat capsized sailing upwind in 18-22 knots with full sailplan in the Round the Island race in the UK. I believe they had upsized the sailplan based on the recommendations of some “pro” sailors and sailmakers, some of who I think were on board. They were pushing the boat and pics before the capsize showed them sailing upwind with the weather hull 3’-4’ clear which was too narrow a safety margin for me in my experience on the boat. I had some serious conversations with another naval architect at M&M after our close call and it was a wake up call for them as they were experiencing more owners who wanted to push that performance envelope and still be able to cruise on their boats. It was not a trend he was comfortable with.