In the last chapter, John wrote about a new motor yacht design that's quite different from just about anything you'll find coming off a production line. Now it's time to put our preconceptions aside for a moment and consider if, from a technical perspective, the boat is likely to work as expected.
I’m aware that the helm station is well positioned in relation to the pitching centre but its low height means that vision directly over the bow will be poor. I like it being low because the roll arc at head height will be minimised but I’d opt for giving it a little more height and slightly more distance from the pitch centre to get better forward vision.
Hi Bob,
I’m not sure that the “vision directly over the bow will be poor”. Dennis gave Phyllis and I a Skype video tour of the boat and the visibility from the inside helm station looked pretty good to us. These things are deceptive. I was on a motorboat the other day where I could have sworn looking at the drawings of said boat that the visibility from the helm would be poor, and it was in real life very good.
Also there is a raised outside helm station just aft of the wheelhouse on the Artnautica. I’m going to guess that the visibility from both her stations would be significantly better than that from the average aft cockpit sailboat helm.
Hello Bob,
The boat has a fairly high bow and looking at the profile is would appear to impede vision a fair bit. I just posted a picture taken from eye height at the helm station on the Facebook page. You will notice that since the bow is fine the visibility is actually better than one would think. I also posted a picture from the exterior steering station.
View from the helm
https://www.facebook.com/300243140002724/photos/pb.300243140002724.-2207520000.1412038773./968876629806035/?type=1&theater
The outside helm is raised.
Here in Europe the cost of berthing this very fine vessel would be prohibitive that is if one could even find a berth.
Great looking design. Put a short, freestanding carbon spar on her and you’d cut the fuel burn in half when motorsailing off the wind.
I think that has merit, the short mast idea, as would a free-flying Parasailor chute off the bow for downwind “sail assist”. A short mast would also dampen roll in a quartering sea, something to consider in a narrow boat like this.
This comment, however, gave me pause: “In other words, fully 25% of the boat is fuel – an unusually large figure that implies a serious long-range cruiser.” It also implies (to me) a serious issue with stability numbers as said fuel, which by definition comprises part of the ballast, is used up. If you are down to your last 100 gallons, isn’t the actual waterline going to change along with handling characteristics? Or am I missing something?
She’s a beauty, regardless. I get the sense I’m looking at the future of distance powerboats instead of the prevalent “trawler/Nordhavn” model (not that I don’t like them as well).
You’re not missing anything, Marc. High-fuel-fraction boats often do get a bit twitchy when the tanks run low.
I’m not sure how much seawater ballast the Artnautica 58 is capable of carrying. She does have freshwater tanks that are good for up to 2.3 tonnes; if a watermaker’s fitted, perhaps they could be topped off en route to partially compensate for lost weight due to fuel burn.
I suppose the alternative might been ballast tanks that could be selectively filled and later blown with seawater to maintain trim…but that affects power requirements (for pumps and automatic valves), weight and fuel burn, and of course, interior volume.
I just recall one of the few times I’ve been in a “light load” powerboat in any sort (3-4 feet) of a running sea…it got pretty brutal and not just on the flybridge.
Gidday Matt,
First, good job on writing this post.
I made a bit of a design change a while ago. My original plan was to have two 500 litre water tanks in the engine room but when the quote came in for fabricating said tanks I decided to purchase a simple water maker in stead. The extra cost was not much and the space saving benefits even greater.
The unit is an Open Ocean made right here in NZ.
So now I have the option to keep the built-in 1300 litre tank topped up at all times while on passage. This tank is as low in the boat as possible under the lower accommodation sole.
Hi Marc,
Actually, I don’t think that using up the fuel would have much, if any effect on stability. The reason is that the fuel is in vertical saddle tanks and so when full I’m going to guess that much of the fuel is above the center of gravity. In fact the boat might even get more stable as fuel is used.
As to the waterline, with the plum bow and stern immersion is going to make very little difference to length, I would think.
Matt, Dennis?
Anyway, I gather from Dennis that the boat has great stability and recovery from capsize numbers—way better than most any trawler.
OK, John…I am familiar with saddle tanks. My buddy in a steel Goderich 40 ketch has them. He uses a cross-transfer pump to improve trim when he’s around half-tank level. I plan to do the same with my water tankage (my fuel tanks are all keel situated).
Thanks for the explanation.
The self righting capacity of the boat is unaffected by the fuel load. If anything, she will right herself more readily at lighter loads.
No doubt she will feel more lively when the fuel load gets lighter. How much that will affect levels of comfort onboard is one of the more interesting questions I’m looking to answer when I get the boat in the water. I have not planned seawater ballast tanks but these could be incorporated in the design if deemed desirable – at a cost to interior volume of course.
Thanks, Dennis, and may I say it’s a privilege to have the fellow who drew and is building Hull No. 1 to go deeper into the design elements.
One thing I *think* I see from the 3D drawing are cambered decks…is this the case?
Hello Marc,
Yes, the fore deck is cambered apart from an 800mm flat strip along the centreline. I did this to make hatch and ventilator installations easier.
The pilothouse roof is in three flat sections and the cockpit sole is flat but with one degree fall aft.
Hi Roger,
Actually, in my experiences, and I have motor sailed a lot offshore, motor sailing does not work off the wind because the apparent wind is so low, particularly in the lulls, that the sail starts crashing about. It also has very little fuel burn benefit when off the wind for the same reason. A sailboat may wish to tack down wind in this case, but that would not make sense for this boat because the increased distance would outweigh any fuel burn savings from filling the sail.
Motor sailing does work well reaching but of course not dead up wind. So we have to ask ourselves whether or not the additional cost and complications of a rig and the design changes required—you can’t just stick a rig in the boat as is, it will have effects on the whole design—would be worth it. I don’t know, but I’m going to guess not. Matt, Dennis?
Hello Roger,
I’m planning an emergency backup sail rig to help me drift controllable towards land in case the engine gives up.
The plan is to use one of the two booms as a mast, hoisting it up and securing it up with a forestry and shrouds to the aft corners of the cockpit.
This is not fully detailed up yet, it’s a bit of a tricky one since I want the whole shebang to be able to fold down for getting access into the European canals.
An alternative could well be a kite by Omega Sails in France. If anyone has first hand experience with one of these I would be very keen to hear. The French Ecotroll uses one.
A minor point but why the forward raked front to the deckhouse? Surely the wind resistance is significant. A sleeker profile would provide more space below while reducing wind resistance. Perhaps not significant in calm conditions but when pushing into 40 knots it would be noticeable.
Hi Pat,
As part of my project of learning a bit more about power boats I have spoken to several experienced owners, including one circumnavigator. They have told me that reverse raked windows are highly desirable and much better than the those where the bottom of the window is forward of the top for three reasons:
I guess those reasons outweigh any slight drag advantage.
This is probably why most commercial vessels that I’m seeing today have reverse slanting windows like this boat.
The air resistance on a 4 square metre (frontal area) deckhouse with vertical or forward-raked windows in 40 knots apparent wind is just a hair over 1.0 kN. That takes about 4.2 hp to overcome, if the boat’s doing six knots. Streamlining the wheelhouse like a car windshield would save up to 2 hp of that, but you’d lose wheelhouse space and get a lot more glare on the windows. I think the improved visibility is a fair tradeoff for burning an extra cup or two of fuel per hour on those rare times when you have a 40 kt headwind.
All the necessary comments have already been made in favour of the forward slanting windows except my favourite one: they look the business!
Hi Dennis,
I secretly really wanted to say that!
Hi Matt,
Thank you for the analysis. I was wondering whether you could comment on how you think that this boat will do going right into bad weather as you have mentioned that this will necessarily be an important storm tactic. I guess that my question is really about how much energy is dissipated by the motion of the hull and the wave action against it at different speeds as opposed to how much power is available. In my own boat which pitches a lot more than I would like in these conditions, we need a lot of power to maintain a very slow speed (and sometimes we don’t have enough and can’t keep the bow into it using the engine). A few years ago, I was sailing up the New England coast and passed Wind Horse as it was headed south into quite choppy ~8′ waves. While the conditions were certainly not very bad, I was impressed by how level the boat stayed and how little horsepower it appeared to take to move along at a nice pace.
I have no experience on this type of hull form but I have been on a number of boats with much higher power to displacement ratios and reasonably quick lines that are unable to stay head to wind in conditions that I would not consider very bad. What I am really getting at is how do you think this boat would do in really rough conditions? If the main storm tactic is to slowly jog into it, is 75 hp enough for the hull design to keep the bow into 30’+ breaking waves and 50 knots of breeze should you get trapped and be unable to outrun the storm?
I am not particularly familiar with this specific design but a vessel that relies on a single form of propulsion does scare me a bit. Maybe I missed a get-home option but it seems like you would need this and a backup option for heavy weather as well. In general, I think that it is a really appealing design and maybe I am just being too conservative.
Eric
Hi Eric,
On the issue of backup power. I discussed that with Dennis. For his own boat he is planning to experiment with the new French kite sails that are designed for that purpose.
He also stated that there is plenty of room on the port side to fit a small get-home engine. Having said that, and talked at length with several motorboat owners, I think that get-home engines are often an illusionary security, particularly in the situation you postulate, since they don’t have enough power to be useful.
I think that if I were to buy an Artnautica I would first need to satisfy myself that she could safely lie to a Jordan Series Drogue off the stern in storm force conditions without power. My guess, and it is just that, is that she would do fine in that configuration since the JSD would prevent her from accelerating down a wave face to a speed where any tendency to bow stuff or steer would become a problem.
Having said all that. I’m just as interested as you in her ability to jog upwind into storm conditions. Here I think that she might surprise us to the positive side because the rig in sailboats, as you would know, has a huge effect on pitching moment—no rig, much less pitching.
This was graphically demonstrated to a friend of mine two falls ago when motoring into a nasty swell trying to get out of Buzzards Bay in a 60-foot ketch equipped with 150 hp engine and a big fixed prop. Another friend in a Nordhavn 46 came past them like they were standing still, despite being a much smaller boat with less horsepower. Of course there are a lot of variables, but as far as the two friends could see, the big difference was that the motor boat was just pitching less.
We have also noted the same thing in our own boat: since we replaced our old cracked aluminum mast with a carbon mast, our speed under motor upwind into waves is much improved even with no sail up and we just don’t get stopped by pitching, even though we only have 87 Hp in a boat that is much heavier than the Artnautica.
On the topic of “getting home”; the majority of the cargo ships that log around 1.5 to 2 million NM during their economical lives, do with just a single engine as well. They just have to fix it to such a level that it can bring them to port. In the old days, major main engine repairs had to be done on a monthly basis or even more frequent, in the modern days this barely ever happens.
I don’t think that there is a big probability that one will encounter an engine failure that can’t be (temporarily) repaired at sea if the proper tools and spare parts are carried. Assuming that the level of maintenance of the engine and attached systems is raised to an art.
Hi Erik,
I would agree with that. In some 45 years of going offshore with diesel engines I have only had two failures and in each case we were able to fix them at sea.
The secret seems to be good maintenance, some parts and tools, and a bit of basic common sense.
My friend Bob T, a professional mariner, fleet fisherman, and sailboat owner tells me that in his some 50 years (he started young, fishing with his uncle) he has never had a diesel engine fail to bring him home. Bob has just bought a single engine powerboat and actively rejected twin engine versions of the same model.
Erik and John,
While I agree with both of you that a single diesel engine is actually remarkably reliable, I fear that this would not work for a large portion of the cruising community due to a lack of comfort with making repairs at sea. Maybe the answer would be that the cruising community needs to become better educated about repairing these engines and their systems as well as caring appropriate tools and spares. Realistically, rebuilding an engine is really not any harder than setting up a jury rig if your mast just went over the side if you have ever done one before (I might be a bit biased here as I used to design industrial air compressors which are basically diesel engines minus the fuel system). On my own boat, the most difficult part is that I would have to take the engine out to pull any major components as I can’t get the head or pan off due to height constraints in the engine compartment.
Something else to think about is that many twin engine setups are not really independent and give a false sense of security.
Eric
Hi John,
Thank you for the reply. I think that all of the points that you make are good.
You make a very good point about the relationship between the polar moment of inertia about the pitch axis and the ability to motor to weather. Yet another reason to keep the weight in the rig and the ends of the boat to a minimum. I couldn’t see in the pictures of the design whether the anchor chain is stored back from the bow but that would probably help a lot.
As you suggest, the combination of a JSD and some form of sail as the get home option seems like the best route to go to me. I wonder what Dennis’ actual plan for heavy weather is and whether it is the JSD?
Eric
I have read a lot of good things about a Jordan Series Drogue and one of those will likely be part of the inventory.
Hi Dennis,
You might want to consider incorporating the strong points required for the JSD now, while you are building. Also, I would think about adding built in storage so that it can remain ready to deploy at all times. We detail all of that, together with chapters on deployment and retrieval in our Heavy Weather Online Book.
Hi John,
I’ve had in mind that the bollards in the cockpit would work well with a JSD. I’ll read the book to confirm what else I can incorporate into the boat to make handling the gear easier.
Re. staying head to wind in bad weather.
Without a model test in a wave tank, it’s hard to say for sure. My suspicion, backed by no calculations at all and based only on what others have reported about boats with similar proportions and similarly fine wave-piercing bows, is that:
Force 7, she’ll be able to make headway dead upwind.
Force 8, VMG to windward will be only a couple of knots but she’ll be able to hold a more-or-less steady course.
Force 9, probably close to the limit for beating into a head sea, might be able to keep head to wind but won’t be making any headway.
Force 10, you’d better have the JSD out by now.
To elaborate a bit more. The added resistance a hull experiences in a head sea is strongly dependent on:
– Its tendency to pitch (a relatively low pitch moment of inertia, i.e. weight concentrated near the centre, is preferred).
– The shape presented by the forebody and bow topsides when cutting through a wave (a narrow, streamlined bow can part a big wave with relatively little resistance; the resistance goes up dramatically when you get a wider, fuller bow that tens to bash into the wave, throwing breakers to both sides). Or, in other terms, how big a change in momentum do you have to impart to the water to move it out of your way.
The Artnautica 58 would appear to fall considerably on the favourable side of average in both of these characteristics.
Re. get-home power.
This is a topic worthy of several articles on its own. My general opinion of low-powered “wing” engines is pretty poor. Many failures that would cripple a main engine (a net wrapped around the prop, running aground, wet cruddy fuel, a lightning strike blowing out the ECU, an engine room fire, etc.) will take out the wing engine at the same time. And, in a storm situation, an off-centre engine sized for limping home at 5 knots may be of little help. I do like the kite idea, and the Dashew approach of jury-rigging a staysail from the dinghy cranes, as “if all else fails” contingency plans.
Matt,
Thanks for ballpark guesses, they seem quite reasonable and confirm my suspicion that the ultimate heavy weather tactic cannot be reliant on the engine alone because it would require more power.
I have always thought that the design of boats is funny in that it often results in an unstable set of forces when pushed past normal limits. The fact that the thrust is back aft and the resistive forces are mostly up forward when pushing into weather mean that the boat wants to spin around. And then at anchor, a lot of boats have a lot of windage forward and a CLR quite far aft (not talking about the Artnautica here) so they are again unstable. I wonder whether someone will eventually be able to come up with a way to overcome these issues without introducing even worse ones or badly compromising everyday efficiency. Overcoming the instability of the location of the thrust could go a long way towards making a boat that can hold its own motoring into heavy weather without a lot of power, just look at how many boats with narrow sterns actually back to weather better than they go forwards because they are not putting a ton of energy into steering.
Eric
Pat and all,
I believe those forward raked windows with such a salty feel are called Portuguese windows/configuration and are designed to allow better visibility. Conventional windshields reflect all the instrument lights right back into the helmsperson’s eyes, unless the “dashboard” is very well thought through (eg kept dark- like an automobiles). and that is all one sees, the reflected instruments. Reverse raking pulls the reflection off the ceiling which should be dark, and then one is able to see forward through the window much better.
Dick Stevenson, s/v Alchemy
While we are all having fun designing Dennis’s boat, I would give a lot of credit to a designer who incorporated a reservoir for good oil waiting to be used, a reservoir for storing one change of old oil plus a tranny oil change and while we are at it, a built in gasoline tank for dinghy engine fuel. Those elements would go a long way to convince me that the designer had cruisers in mind.
Dick Stevenson, s/v Alchemy
Good ideas about the service fluid tanks, Dick. (Although I would personally prefer a dedicated locker with secure slots for several jerry cans of dinghy gas rather than a built-in tank). I have lost count of how many plastic pop bottles full of waste oil are lying around my place, waiting for the day when the hazmat depot is open at a reasonable hour.
Hello Dick,
Very good point. My plan is to finalise the system installations in the engine room and see what sort of space I have left over. So far it looks like I can weld up a couple of tanks to fit between the stringers for clean and dirty oil. These will bolt on so can be removed for filling and emptying.
I have a locker on deck for two propane bottles and a couple of jugs of petrol.
Dennis, There are some things that in home waters one does not deem important. Then when you try to store a few oil changes and worry that the plastic containers will break etc etc. Someone will bless you in the future for your thoughtfulness. Good luck, Dick
Here, here!
I would like to take this opportunity to thank John, Matt and the rest of the team for showcasing my boat and all the readers who made comments. It has been exciting and educational. Thank you!
Dennis, we may never meet in real life, but I’m sure that we share with a lot of the sailors here the feeling of being the only guy in the world trying to transform an idea into reality.
It’s the story of every person who’s bought a boat intending to go out of sight of land. I’m sure a lot of the comments here, of use to you or not, were offered up in that spirit.
Hi Dennis,
You’re welcome. You deserve all the help we can render for going out on a limb and actually building a boat that is not just another too short trawler with too much interior. We can’t wait to see your boat in the water.
Dennis,
Thank you for participating in this thread, it is really amazing to have the designer of a very cool boat take the time to participate and do so in such a constructive and open way.
Eric
John,
I know this discussion ended sometime ago but I wonder if I can ask some questions of yourself, Matt or even Dennis (or anyone else for that matter).
The first thing I am interested in is static stability. Do you have any information on the self-righting behaviour of the AN58?
Secondly do you or others have any comments/information on the pitching and rolling behaviour of the vessel underway and at anchor given the hull shape?
Thirdly, I note your comments regarding pitching – I think you essentially said it is best to keep weight out of the ends. Isn’t the trade off that the period of pitch increases? (Same can be said for roll response.) So that with weight out of the ends and extremes of beam, the vessel will be much more quick in movement and uncomfortable?
Thanks.
Made a mistake in my previous post. When I said the period of pitch increases , I meant to say that the frequency increases.
Hello Henry,
The boat is self righting from any angle of heel.
It is a bit difficult for me to comment about the pitching and rolling behaviour of my boat since I have been on very few other boats that would be worth comparing to. I can say however they I am happy with the pitch motion, the sharp bow reacts slowly to waves. The roll is still a bit quick given that the boat is very underweight, has hardly any fuel onboard and the interior is still very much unfinished. Luckily hull number two will be launched in a couple of months and should be at cruising weight from day one. I will report on the findings.
I would like to think that keeping weight out of the ends of a boat is perhaps more critical in a sailing yacht that has a lot of mass far from the pitch centre already, both in the rig and in the keel.
On a boat like mine you want to have a certain amount of weight in the ends to help keep the bow from rising too fast and causing slamming. Ideally the fore-aft distribution of weight should be adjustable so the boat could be tuned to the conditions! That would be fairly easy to do with integral water ballast tanks at the extremes of the hull, these would add a degree of safely being watertight compartments… Food for thought!
Hi Dennis,
Thanks for the response.
I have to say I like what you have done with the design of the AN58. Not every thing about the boat is my cup of tea but it is a breath of fresh air – very much like Steve Dashew’s breaking the mold (who has done this ongoingly it seems!). I also very much admire the way you went about personally putting it together – quite amazing.
I am sure water ballast could be used judiciously to modify the dynamic behaviour of a vessel such as yours (any vessel I would say). Stern/bow ballast tanks for pitching, ballast tanks in the extreme beam of the vessel to modify rolling behaviour.
Stern ballast could be used to pull the bow out of the water when running before in extreme conditions to reduce broaching.
Personally, being of the sailing persuasion, I like the idea of a lump of lead beneath my feet. That would mitigate the inertial forces generating fast pitching and rolling. A keel of a couple of feet deep and several feet long I imagine would also add lateral resistance to rolling (and yawing).
I am pleased to hear the vessel is self-righting. I would definitely believe it if it had a lump of lead dangling from the keelson!
If you have any other comments, I would like to hear them.
I have some other ideas about and roll and pitch control which I may float a little later.
Cheers.
Hi Henry,
Just to clarify a couple of points. I know it’s counterintuitive but actually adding a lead filled keel to the AN58 would give the boat a far less comfortable motion. The reasons are complex but the proof is simple: there are few things that have a more violent and dangerous motion at sea than a dismasted sailboat.
Further, in all likelihood the addition of a large ballasted keel would make the boat more susceptible to knock down or capsize in heavy weather, not less so. Again the reasons are complex, but there is also a simple proof for this one: lifting keel boats like the Ovni are safer in heavy weather, in fact very safe, with the keel retracted so that they can skid sideways.
If you are interested in pursuing the reasons for all of this I would suggest reading this article over at Matt Marsh’s site: http://marine.marsh-design.com/content/dynamic-stability-monohull-beam-sea
Hi John,
I hope you don’t mind my tossing this around some more because I’m not sure that I can agree with your points. I think there is a contradiction in your first paragraph, for starters. As you say, this is a complex area and I don’t pretend to understand it.
The first thing I would raise is what do you mean by “dangerous motion”?
When a yacht is dismasted it’s polar moments are reduced or expressed elseways its transverse moment of inertia is reduced. This means that the dismasted boat will roll at a much faster rate. This would be more uncomfortable for the occupants of the boat, I presume you would agree with that. If this is what you mean by “dangerous motion” then I would agree with you that this is dangerous as well as uncomfortable.
In the case of an “umasted” and “unkeeled” boated, all other things being equal, this vessel has low polar moments and when excited transversely will roll more quickly than a keeled and masted yacht, all other things being equal.
So are you saying you would be happy to be in a vessel which rolls very quickly? From what you say above you would not. So on this score, any vessel with a ballasted keel (and we are focussing on inertial and gravitational forces only here) will be preferable than one without.
Matt’s article focuses on inertial and gravitational forces – I don’t think he mentions the role of damping specifically. Hull shape and hull appendages contribute to damping roll motions. The harder the bilges (chines in the extreme), the deeper and longer the keel – the greater the damping effect. I’m not telling you anything new I’m sure. (And of course the sails also increase roll damping which is another reason a dismasted yacht rolls violently.) So keels damp roll motions. I would imagine you would think that a good thing.
Now to skid factors.
Firstly, Matt in raising the question of skidding uses the example of a centreboarder. Raising the centreboard has minimal impact on roll inertia. However, adding roll inertia (that is, adding a ballasted keel) to such a boat will slow the rate of roll which will give the wave time to pass under the hull of the vessel before the vessel knocksdown completely, allowing it then to be rolled in the other direction as the wave front passes. Also, a keel will add to the hydrodynamic resistance of the hull as it skids down the face of the wave. Yes, this same resistance is the force that causes the tripping (rotational motion increased) but it also reduces the (translational) motion of the vessel down the wave front. This reduced speed down the wave front will reduce the tripping force generated by the deck /hull interface as the boat slides down the wave face. So there’s a balance of forces to be considered and I’m betting that increased mass in the keel will be critical to having the vessel respond benignly. Sure, if you have a vessel with no ballasted keel it seems reducing Flat, as Matt calls it, would be a sound manoeuver. However, I don’t think I would feel safe going to sea in a centreboarder, all things considered.
Henry.
Hi Henry,
I’m not going to get into a detailed discussion of stability and roll simply because I’m not qualified to do so.
All I was trying to point out in my comment was that simply adding a ballasted keel to the AN 58 would have undesirable outcomes and I stick by that. I also wanted to make sure that others realized that there are are many truths about stability and seaworthiness that are counterintuitive.
I guess the my real point is that design is a holistic process and I’m always uncomfortable with others trying to add stuff to boats without doing all of the modelling and calculations necessary.
Sure, there are ballasted deep keel motorboats like those from Michael Kasten, but they have radically different hull forms than the AR58.
My thinking on boat selection is that it is best to find a boat with a hull form that meets your needs and desires rather than take an existing design and then try to add bits to turn it into something that the designer never intended.
In summary, it sounds to me like you would be happier with a design from Kasten, perhaps this one.
Hi John,
Thanks for the suggestion regarding the Kasten yacht – however it doesn’t suit my aesthetic tastes and for me is too heavy and too beamy for the length.
You say “there are are many truths about stability and seaworthiness that are counterintuitive”. I understand enough about stability theory (and a little knowledge can of course be a dangerous thing) to know that static stability does not necessarily translate into dynamic stability – in fact they can work against each other to the point that in naval architecture this phenomenon has been labelled the paradox of stability – the more statically stable a vessel is, apparently the less stable she is in waves. So it’s not enough to characterise a vessel’s behaviour in a seaway with it’s transverse gyradius and metacentric height. There are resonance effects (if the natural roll period of the vessel is equal the period of the waves it could theoretically capsize in very low amplitude waves) and the other effects mentioned above, viz. damping and skid factors.
Anyway, I seem to have given you the impression I’m for motoring around in lead mines –very far from it.
I have seen the promotional videos of Dennis’ Koti. In one scene she is seen crossing behind the pursuit boat. As soon as she hits the boat’s wake she gives a little quick wobble. I know that she was underloaded, so not the normal situation. I look forward to seeing her perform in good sized waves under normal loading.
Cheers,
Henry.
Hi Henry,
The bottom line is that I think you are right, the AN58 will be a lively boat at sea, no question about it.
Where we differ is on what to do about that, if anything. To me it is simply a trade off that we pay for the benefits (and drawbacks) of an incredibly low displacement to length ratio.
I guess a good simile in sailing would be trying to modify a Santa Crus 50 to give it a high motion comfort index like a Whitby 42…simply can’t be done, or at least not well.
I would stretch the simile further to say that if it were me and I wanted a boat with a high motion comfort I would go to Brewer to design it, rather than try to convince Bill Lee to change the SC 50.
Dennis,
Have you published a curve of the transverse RM characteristic?
Dennis,
I may have missed something here.
In following your activities on your facebook page I’ve never seen a keel in any of the construction photos, however I see above there is a perspective drawing of the vessel showing a keel.
Is this correct?
What is draft as a result and how much lead is there in the keel and what is the ballast ratio?
Thanks.
Hello Henry,
I appreciate the kind words about my boat and the effort I put into her. I have learned a truckload in the process. Mostly not to build any more boats singlehanded 🙂
I have not published a RM curve but could do so, no worries. I’ll pop one onto Facebook one of these days. Might take a while, in sort of snowed under at the moment, doing a big contract job for a former employer.
There is indeed a keel, or perhaps more accurately called a keelson, in front of the propeller. The engine sump fits partly inside to help keep the shaft line near horizontal. The draft is about 800mm to the bottom of the keelson. No ballast is added.
Playing with adjustable weights to tune the motion was first suggested to me by one of the instructors at the landing school. He was musing about lead weights on tracks that would allow adjustment. Great except in way of a watertight bulkhead… I would love to hear your thoughts.
Hi Dennis,
Lead weights on tracks sounds like an interesting idea. I guess the advantage of water ballast is that it can be jettisoned if you wanted to lighten the vessel for any reason (e.g. motoring through the doldrums).
I would like your opinion on some ideas, probably all of them hare-brained!
Firstly, for roll frequency control (and also pitch control for that matter), I am thinking of a shortish vertical mast section (could be the aerial, radar mast), of appropriate diameter, and pump filled with water when fast rolling/pitching was a problem. Being up high, not so much water would be required and of course space robbing ballast tanks would not be needed. Although it may require dimensions (but I suspect it won’t) such that it looks like a ship’s funnel and ruin the aesthetic. I’d be interested to know if anybody’s done this somewhere and how it performed.
Modern roll stabilization is effected by bilge hung foils. Obviously, these are a collision/grounding risk and increase hydrodynamic resistance. They can also have an impact on the vessel’s steering, being off the centre line.
My idea is to hang a single foil behind a keel placed at or near the vessel’s turning centre (so there is minimal impact on steerage). If the keel is already designed in then there would be little additional hydrodynamic resistance and no additional collision/grounding risk. I posted the idea into Steve Dashew’s blog. He seemed to think it might have merit.
Now modern stabilizer fins are driven by hydraulic rams, I believe, and controlled by sophisticated sensors and electronics. My idea here is to take the keel hung foil and from the foil’s shaft suspend a mass which can swing in a horizontal (when the vessel is level) plane. When the vessel rolls the mass will swing in such a way to turn the foil so as to generate lift opposing the roll sense. The mass and the radius of the arm it is suspended from would have to be appropriately dimensioned relative to the foil and the speed of the vessel. Arrangements would have to made to prevent the mass swinging violently if a knockdown or inversion occurs. This kind of arrangement might also work for bilge hung foils, if they were more vertical than horizontal, with some thought. No power to be consumed, no fancy electronics or hydraulics to fail. (Just a potentially out of control rogue mass!)
The other idea for roll stabilization is to use high pressure water jets.
There are water jet systems used for bow/stern thrusters e.g.,
http://www.jetthrusters.com/
The systems as configured are very straightforward. Of course roll stabilization would require sensors and control electronics. The jets could be located at the base of a keel or at the extreme edge of the bilge pointing downwards. I asked Steve D. about this. He said they had used jet bow thrusters in the 80s without success and saw no reason to consider them. I cannot see why they would not function as required if adequately sized. Maybe this is the problem. Too much power might be required. If they could be made to work, then there is minimal collision/grounding risk and minimal additional hydrodynamic resistance.
The same approach could be used for pitch control with stern and bow thrusters although I would imagine even more powerful jets would be necessary.
(By the way, the idea of suspending inertial masses in a vessel stirs the notion of connecting generators to them such that rolling and pitching might be used to harvest the energy of waves impacting a vessel.)
I’ve unburdened myself now, and let loose all the twittering birds in my brain. I don’t know if there is a practical solution to roll/pitch problems in there somewhere. I do look forward to your (or other’s) comments.
Cheers.
Hi John,
I’m glad we can agree on something.
However, you seem to want to characterize the available possibilities by extremes.(I suspect it’s more about your wanting to make a point, so I will give you leeway here. 🙂 ) Why not something in between, or towards one end or the other?
I can’t see what would be the problem with making a vessel like the SC50 a tad more docile with beam disposed water ballast, for instance. OK she might lose a little downwind speed or surfing performance (and maybe a small amount of internal space), but if that’s a worry, discharge the water. As you say. It’s a trade off. As long as you are conscious of the trade offs, no damage done. Design might be beset by compromises, at least we can make choices about the final outcome.
I understand that the quoted heavy displacement of the AN58 is 14 tonnes and rated fuel capacity is 3,800 litres and water capacity is 2,300 litres. Fully loaded with fuel and water, about 5.8 tonnes of fuel and water makes up part of the 14 tonnes. So if in the videos, Koti was unloaded with fuel and water (and accommodation not complete), then she will be a different vessel when fully loaded with different seakeeping behaviour.
If she can tolerate a displacement range from light to heavy of 8.2t to 14t, I can’t see why beam water ballast or even a shallow lead ballasted keel would ruin her capabilities. It will change them to a degree, for sure.
If being fully loaded or somewhere near doesn’t quieten her down, then I’m betting a tonne of water (given her displacement) in each of two beam tanks would make a significant difference (or even a few kgs pumped into a mast up high as suggested by me elsewhere).
I’m guesstimating (without knowing the waterplane shape and precise dimensions) that it would take about 1.2 – 1.5 tonnes to sink the AN58 one inch.
Sinking the boat another inch or two will up hydrodynamic resistance and fuel burn marginally – but that’s the price for not spilling your coffee when you hit somebody’s wake.
A shallow keel may not need to be ballasted but I’m betting it would add a good deal to dampening of roll tendencies without compromising steering too much. OK you lose a couple of feet of draft. You choose.
And it’s not my business to tell Dennis how to design his boat. I think he’s done a terrific job. I’m merely expressing where my preferences would lie.
Henry.
Hi Henry,
It’s an interesting debate, but one I think we will always differ on.
Anyway, our discussion has spurred my thinking and I think a post will result. Thanks
In the mean time a lot of my thinking is detailed in this chapter, and point #3 is particularly relevant.
John,
You make some great points in your article on “bad boats”.
I had to have a chuckle, though, over your recommendation to study Larsson et al’s book. The boat depicted on the front cover looks like one of those “bad boats” you’re talking about.
I have the first edition, entirely different boat on the front cover.
HI Henry,
You might want to reread the article in question. There was no inconsistency in what I said. The boat depicted on the front cover is designed to be light and plane. I think light planing boats with wide sterns are way cool. In fact I’m a huge fan of the Open 40 and 60 classes. What I said was that bad boats result when the same sterns are grafted onto displacement cruising boats.
Yes, I get that John, but the irony was a little too much for me.
But you would have to admit that the last two paragraphs of the section were a little scathing (“the monster they have created”) and you were speaking in the context of the their designed purpose. I would not want to see what you’d say if you disliked something. 🙂
I know I’m flogging a dead horse here, but I note that the AN58 already has a substantial prop skeg. It could easily be extended forward to morph it into a longish keel and still keep the draft as it is. Roll damping would have to be vastly improved with a minor if not negligible hit to fuel burn and maybe steering sensitivity.
Thank you all for the very interesting comments above, but am surprised no one mentioned one of the first, I think, but correct me if I wrong, aluminum FPB boat designed by George Muller, named Idlewild for owner Ben Gray, who, with his two sons, navigated through the North West Passage , to Greenland, South Africa, Australia, Russia and back to Canada. Have seen the boat up close, as I almost bought it, 6 years ago. Powered with a 55HP single diesel, without thrusters and hydr. stabies and didn’t see any lead ballast.
With a length of 53 ft and only an 11 ft beam, Ben did tell me he would have liked to see a little more beam and power, however, it was very seaworthy.
My present 60ft aluminum flushdeck, round bilge has about 6 ton of lead ballast and instead of the 8mm hull, would have preferred a much thicker hull and less ballast.
Since I am not planning any deep water voyages, like to remove some of the ballast.
In between the master cabin and engine room are 2 x 1500 gln fuel tanks and act as a great sound insulator.
Have one question : plan to protect the 2 stabies by welding a skeg, about a foot infront of each, which also should prevent rolling somewhat. They will be approx. in the center length wise , but might these skegs come with un-intended consequences ??
Also have purchased a 2000W gasoline inverter-generator and won’t replace 2 of the 6 large house batteries. The generator sits in an alu chest on the fly-bridge and can be used to run the large domestic fridge, if needed and the 2 large submersible pumps incase of a complete brown-out, i.o.w more redundancy.
Please let me know if I have overlooked something. Thank you .
Rene
Hi Rene,
I agreed “Idlewild” is a very interesting boat and proof of concept. I spent quite a bit of time looking and thinking about the boat and corresponded briefly with the owner some years ago.
As to the skegs, I just don’t know. If it were me, I would have the idea checked and specified by a naval architect before proceeding with it. I know Dennis at Artnautica will do third party work and the other option would be Michael Kasten. I do wonder if turbulence from the skegs would reduce the effectiveness of the stabilizers.
As to the gen set you are asking the wrong guy since I have a visceral, and probably unreasonable, hatred of gasoline engines, the result of years of struggling with recalcitrant outboards, so I would always default to a diesel. That said, we had a little Honda on the German boat we took care of in Greenland and that worked great.
I find little Hondas generally do work. I know they all seem to have the same “trick”: if they don’t start on the first two pulls with the choke fully out, lean the choke back to the halfway position. This has worked for me on one Honda 2000 and three Honda outboards. It seems to be particular to Hondas.
Thank you John and Marc for your comments and for correcting me on the name Bhueler instead of Mueller.
Had not thought of the skeg causing turbulence, but that may well be a possibility. Could place them further forward, but that may make the bow thruster less effective. Less rolling at anchor would be a big plus………..more homework to do.
I understand your views with having gasoline onboard, but outboards require it too.
Many years ago, an instructor at Perkins diesel, threw his cigarette into a pail of diesel to make his point. The good old days, ………today he would be crucified.
Like you, I prefer diesels too, but even our small genset is quite noisy inside, although quiet outside. The gasoline gensets as Marc mentioned are now quite good, and very quiet, but have to be used on a regular basis in moist climates. Ethanol is an evil, especially when it sits thru the winter, as it segregates from the gasoline. That’s why I use the top Shell brand which comes with zero ethanol, more money, but less problems. Unlike most Americans, who appear to be in love with batteries, if Tesla is anything to go by, now with a market cap just below that of Ford!! Yet with very little profits to show for it? Not that I would install any lithium batteries on a boat yet. Marc referred to possible starting problems with small gasoline engines, I usually spray some “Quick Start” in the airfilter, ………. works great and much easier on the old body.
Thanks again,
Rene
Love this design. Likely my next boat when the twin engine version comes out. Hopefully Dennis will continue to forgo the glitz which doesn’t make the boat safer nor easier to liveaboard nor longer range.
Tend to be on the thicker plate less framing school of thought. Particularly for this boat. It’s much easier to find good steel welders than tig and cruising you inevitably bump into things or others bump into you.
Hi Lee,
Yes, the twin boat will be interesting, however, if you are thinking of building this boat in steel, that would not be a good idea since it would be impossible to keep the boat light enough to perform properly and still be strong enough, because aluminium has a much higher strength to weight ratio than steel. Also, while there are things to like about thick aluminium plate construction, generally thiner plate and more framing yields a higher strength to weight ratio.
No. Want unfinished 6000 series aluminum. Steel defeats the the beauty of the design. Light hull means better avs, less fuel consumption, and for this boat paradoxically a better ride in a seaway
Hi Lee,
Good to hear that I misunderstood your comment.