Wander through any marina or row through any mooring field (or popular anchorage) with an experienced eye and you will see a lot of scary things:
- Hulls that will pound horribly going up wind, both motor boats and sailboats.
- Motor boats that burn two or even three times more fuel than they should to move a given tonnage through the water at a given speed.
- Sailboats that need way more sail area than they should to sail well.
- Sailboats that are just plain slow.
- Sailboats that need huge rudders, and often two of them, to be even remotely controllable.
That’s just a few of the things I see. The list of naval architecture sins could fill pages but, enough, you get the idea. A person could be forgiven for thinking that boat hull design must be some kind of poorly understood black art where coming up with a good hull is a matter of pure luck. After all, what other factor can explain the number of truly terrible hull designs that assault the senses at every turn?
But, in fact, the fundamentals of good hull design have been well known for years and the majority of naval architects know these fundamentals. So how do all the bad boats we see come to be?
I thought it would be useful to this book to look at that on the theory that if we can understand how bad hull designs happen, our readers will be better equipped to avoid bad hulls when boat shopping.
So, let’s dive in to the dark and terrifying world where bad boats are spawned.
Two further thoughts on hull form coefficients (prismatic and the many others):
– The optimal range of many of the important coefficients for a given cruising speed is really quite narrow. Differences that you can’t even see when the boats are side-by-side on land can make a 10% difference in efficiency. The hull shape has to be determined by the performance requirements, period.
– Historically, form parameters (prismatic, block, etc) are calculated for the entire boat. This is fine when the boat’s pretty close to fore/aft symmetrical, but with many modern designs you need three sets of these coefficients – forebody, afterbody and whole hull – to accurately understand what they’ll mean in reality. Fore/aft balance – meaning that the weight distribution, the prismatic coefficients, etc. are comparable in the fore and after bodies – is one of the key traits that make Dashew boats perform so well in rough conditions.
I’m afraid I have to differ with the way John has defined the design criteria for the hull form of the A 40 on a number of occasions. If I might paraphrase: ‘No compromise from the optimal hull form will be permitted for the sake of interior design (or for anything else for that matter). ‘
In point of fact cruising sailboats do have interiors and those interiors have to be habitable for human beings. Thus interior design is one of the trade-offs that must enter into hull design.
Let me elaborate from one of the examples presented in this post. If the prismatic relationships and fore & aft balance of the Dashew designs represent the optimum hull form for long distance cruising sailboat, an A-40 designed according to those principles would have a 40 ft. waterline on 40′ overall length, and a beam of only 9’.
If uncompromisingly designed to the Dashew standard of optimum hull form the A40 would have an interior layout unlike any 40′ sailboat built since 1940 and one that would find no buyers.
It follows that:
1- The Dashew hull form is not sail performance-optimal for a 40′ boat because of scale factors. Perhaps so, but I’d have to see this hypothesis proven.
Or:
2- There are other hull forms with enough beam to permit an acceptable interior layout that deliver acceptable performance characteristics. If you accept this premise then you have taken interior design into consideration in designing the hull form. As you should.
I suggest that if you could weigh, quantify and graph all the desired performance characteristics and desirable interior design requirements and overlay them, the curve would be somewhat rounded on top with a fairly broad range before it fell off the cliff at either end. Within that curve of acceptability you might find a beam of between 10.5′ to 12′, but it definitely will not be a Dashew hull form or an Open 40. For example if increasing the beam from 10.5 ‘ to 12’ yields a 100% improvement in interior liveability while sacrificing 2% in desired sailing characteristics and adding 3% to cost that would be an example where interior design should drive hull design rather than the other way around.
Hi Richard,
All good points, but missing one fundamental of hull design (that you allude to) and that is that beam does not scale in a linear way when compared to length. In other words shorter boats have larger beam to length ratios than longer ones to yield the same optimal performance characteristics. This is well explained on page 23 of Principles of Yacht Design. Beam scales against length at a factor of 0.70, not 1.
In fact a 12-foot beam on the adventure 40 is roughly equivalent of Steve’s boat, although, having said that, read on.
Further I never said that a boat had to be the same as “Beowulf”. The two boats have different missions. For example “Beowulf” is optimized for broad reaching and takes some hits in windward ability because of it (you can’t have everything) while the Adventure 40 is more optimized for up wind work. I will discuss the reasons for this in an upcoming post.
Also, the key point is that length is not what drives cost, displacement and stability do that. So, with the Adventure 40 we selected 18,000 pounds as a displacement that would yield the volume we need for the interior we desire. It so happens that the boat came out just under 40-feet when optimized. The point being that if we decided on more volume we would simply make the boat longer.
It should be noted that one cannot simply scale a boat equally in all directions and expect the result to be usable. Stability, displacement and sail area do not follow the same scaling rules. Directly scaling a Sundeer 60 to 40 feet would yield a very tender boat, unable to fly the necessary sail area.
To (very approximately) scale a hull shape while keeping the result in the same general performance family, one should scale the length as L^1, the beam and draught as L^0.7, the displacement as L^2.4, and the sail area as L^1.7. The form coefficients (Cp, Cb, etc.) stay the same. A 40-foot version of the Sundeer 60 would have a 10’3″ beam, 6.3 tonne displacement and 523 square feet of sail.
In a voyaging design, the hull shape and proportions are set according to performance and seakeeping criteria, and the size is set according to the required load-carrying capacity and interior volume.
This is quite distinct from designs intended for marina dwelling and short inshore cruises, in which the length and cost are set according to marketing requirements, and then all the other parameters are adjusted to increase volume and load-carrying capacity until performance is too severely compromised. This is an equally valid design process, but it’s intended for a different kind of boat with a different mission.
Hi Matt & Eric & John,
Both your responses are excellent, both in scope and detail. Exactly the kind of discussion I was hoping to provoke!
Hi Richard,
The same back at you. Your comment inspired a lot of clarification from Erik and Matt that certainly taught me a lot.
Hi Richard,
I like the way you think!
There is however a significant difference between, let’s say, a Prismatic coefficient (Cp) and a length to width ratio.
If a hull shape is designed with a non optimal CP, there is nothing else in the design that can compensate for that. You will end up with a hull that makes more waves and requires more power to push forward compared to a better optimized hull shape.
The width of the vessel has surprisingly little influence on the waves that a boat is generating, assuming that both the length and the weight of the vessel are kept the same as well as the Cp.
In other words, where the Adventure 40 has a very different length to width ratio, she can still be optimized for the same hull shape to decrease wave generating in the same way as the Dashew’s have done with their designs.
Now you are probably thinking that a wider boat does not have the same weight as a narrow boat, and you are correct there, but optimal Cp is only determined by the relative speed of the vessel, and not by the length-width ratio or any other ratio for that matter. In order to push a hull forward, a certain power is required, the closer you get to hull speed, the more power it will require.
Where the Dashew designs are long, they have very high hull speeds compared to the Adventure 40. In both cases, the designer asked himself what an acceptable cruising speed is. For the Adventure 40, I have pushed that to 90-95% of hull speed, while for Steve and Linda Dashew, the acceptable speed might have been around 75 or 80% of hull speed (I just throw a number in here, I do not have the different parameters for a Dashew designed boat at hand).
Relatively speaking, the Adventure 40 has a lot higher power requirement than a Dashew boat since the relative speed is a lot higher. In order to be able to do that under sail, the Adventure 40 needs a lot more sail than what a Dashew boat would carry, again, relative to the size of the boat. The only way to make that possible is by increasing the sail carrying capabilities of the boat. Te simplest way to do that is by increasing the width (but be sure not to change the Cp while you do that!), the wider the boat, the more sail it can carry. The other way is by lowering the center of gravity, but this usually increases the weight of the boat since ballast needs to be added, or draft needs to be increased. Widening the boat does add some weight, because more material is required to build one, but there is no way around that.
Now back to your first statement, the Adventure 40 will be optimized exactly as Steve and Linda Dashew optimized their boats. Optimization is mainly in creating a hull that goes easy through the water (ie, generates no significant waves), and the width to length ratio decreases when increasing the relative speed of the vessel because of the increased demand for sail carrying capability.
The Adventure 40 interior is actually significantly compromised by the hull shape, where a “modern” 40 footer has three full size cabins and two washrooms, the A40 has only a cabin and a half without getting much more living space back for that compromise. When comparing the empty volume of the hull, I would say that the A40 is about 30% smaller than most other 40 footers that one can buy these days.
Now compare it t an open 40. Well, that is just impossible. This because both the A40 and the designs of Steve and Linda Dashew are displacement hulls, while the Open 40’s are planing vessels. A vessel that goes up on a plane generates much less waves as a displacement vessel, so the Cp of an open 40 is much less a dominant factor in the design. What is dominant is the requirement for speed. In orde to increase the speed, the sail carrying capability of such a racer needs to be as big as you can get it without increasing the weight. There are two ways to do that: increase the beam of the vessel as much as you can, and bring the weight to windward by adding water ballast, canting keels, move sails that are not in use to windward etc.
Best Regards,
Erik
s/y “Bagheera”
I now see that Matt and John type a lot faster than I do, but we are all three basically sailing the same.
On the basis that “skinny and pointy” are highly desirable efficiency attributes, I have often wondered why the MacGregor 65 has had so much derisory comment over the years. Perhaps it’s their execution rather than their concept which is flawed.?
Hi Neils,
I think people run the MacGregor 65 down more because she is so different, rather than any actual problem. Anyway, you won’t find any such derision here at AAC, in fact quite the opposite as you can see in this post.
I suspect the MacGregor 65 has been tainted in some eyes by association with the MacGregor 26, an inexpensive trailerable motor-sailer which shares a name but little else with the 65. Although the 26 is darn near ideal for its intended purpose (introducing new boaters to a little bit of everything without breaking the bank), it is pretty cheap in both price and quality and tends to be operated by people who are still learning the ropes. So the name draws a bit of derision in some circles.
Hei Niels
i had the fortune to sail on a MacGregor 65 for two weeks. As it was a modiefied version, I can only comment on my experience on this boat. But from what I read only and what I have seen, I suspect it is a combination of the following points. The MacGregor 65 doesn’t look like mutch in the harbor beside some other boat of here size. The freeboard is low compared to what you mostly see this days. Because of this, she has a wetter bow then many smaller boats. I have been told that, because of her long and fine lines you see the boat flex quiet clearly heading into the waves. An last, the standart interior looks downright cheap compared to what you see at boatshows this days.
Just my two cents
Regards
Simon
I often marvel when the shrimpers in their 60′ boats pass my slip and barely leave any kind of wake. But when the recreational fishermen in their white plastic 25 footers pass at the same speed, I have to grab on to something! I always think that it must take a lot of wasted fuel to move the water that way.
What are the hull qualities at work that creates this enormous difference and what are the recreational fisher designers after?
Having the ability to do 40+ knots severely compromises the boat’s ability to operate under 15 to 20 knots. The appropriate hull shapes and balance properties for these speed ranges are just too different.
In somewhat more technical terms, the ideal hull shapes for moderate to high planing speeds tend to be flat-faced prisms, with all stations aft of midships having nearly the same shape and the beam at the transom being roughly equal to the beam at midships. The centre of gravity of a high-speed boat must also be relatively far aft to ensure correct longitudinal stability, and the planing surfaces should have an angle of attack on the order of 5 degrees when the boat is at rest. This means the afterbody prismatic coefficient is greater than 1, guaranteeing that the wave-making resistance below planing speeds will be enormous.
In addition to what Matt writes above here, the small fishing boat is probably sailing on 100% of hull speed, which is the top of it’s capabilities if it is a displacement hull. While the 60′ boat sails maybe at half of the displacement hull speed it is capable of doing.
Just for the sake of experiment, motor away with your own boat at 100% of it’s capability, and look at the waves you will be generating. If you slow down to half that speed, you will practically have no wake left behind your boat.
Generating waves like that takes a lot of power, be it diesel, be if wind causing the rigging to move forward. So in fact, if you want to sail efficient, a 25′ boat should not sail faster than 4 knots, while a 60 footer could still sail fairly efficient at 8 knots.
When motoring, stay away from hull speed. Practically, one should avoid 70% of hull speed and faster under those circumstances.
While sailing, go for it! the sky is the limit!
As an amateur conceptual cruising/exploration yacht designer I have to wholeheartedly agree with Richard Elder’s comments:
“In point of fact cruising sailboats do have interiors and those interiors have to be habitable for human beings. Thus interior design is one of the trade-offs that must enter into hull design.”
However, I also believe that the “optimal hull form” which John speaks about is indeed one which does cater for people, although perhaps not lavishly so. Consider that theoretically a highly efficient hull needing to carry very little will probably end up looking like a needle powered by a water jet, while the most efficient hull is having no hull at all, able to carry nothing at all. It is about people and I think we’re all on that same page.
For years I’ve been intrigued by John Shuttleworth’s multi-hull designs (http://www.john-shuttleworth.com) where the hull flares above the waterline to achieve both efficiency through the water and increased volume for people – although perhaps not entirely suitable for mono-hulls.
In my own designs, catering for 6 to 8 crew, I have found myself repeatedly increasing overall yacht size in order to afford more ergonomic room for people, systems, gear and supplies while keeping the hull efficient and safe.
Live-aboard offers plenty of personal challenges for the crew. Design compromises must be made to favour human ergonomics and comfort, as long as the resulting vessel remains acceptably safe, and sound, and performing well enough. If not, then I would personally scale my project up to enjoy greater volume, but by implication would have to deal with greater expenses (build, maintenance, mooring, dry dock, refits, etc.)
Using cars as an example: cramming 6 teenagers, 2 adults and gear into a station wagon on a one-off trip to the softball field nearby is one thing, but imagine doing a 1,500 km trip like that! Even with a spacious 8-seater wagon, with luggage in a trailer out back, and as many breaks as can be had along the way, most passengers are still not at leisure. This is a 16 hour trip, tolerated only because it’ll be over – and then the holiday can begin! Now imagine days and weeks inside a vehicle. That vehicle must act as a home, not just an A-to-B. Your holiday and your living is happening in and on that vessel. For me that’s the key challenge of cruising yacht design.
John, thanks for another excellent post! Thanks to everyone else also; I often pore over the comment section, often coming back again to read further comments. Many nuggets of wisdom to be found around here!
I can’t even begin to speak to this level of hull design, you got me buying books again, but I do desire the all wonderful boat.
Compromise!!
It’s just the bottom line. There’s just no way around it. We all want the perfect boat but what is that? It starts with intended use, inshore/offshore, race/cruise………. and then goes right to compromise. They all have different uses. If this discussion is to assist with input for the A40, and finding the “all wonderful boat”, I’ve got to go 1-tough, 2-comfort, 3-upwind ability 4-a 200+ m/day ability 5-low draft. I’d really like more speed and less draft but at what cost? And then why do I want more speed? I know with a safety consideration tucked in, that’s another discussion. Lately, I’ve been day dreaming about a faster boat. This post has me re-evaluating. Why? What’s the rush? If she not a total dog and she can fall off a 20 foot wave land on her side, roll over without dismasting isn’t that worth a few knots and a little more fuel consumption. I think traditional lines are smart,fast and sexy. Roomy production boats are unsafe and should keep land in sight and dream about an wet uncomfortable Vendee run. Clearly, you have to define the use before you even begin the inevitable Compromise. Sorry if this is off point John. I’d love to hear your readers weigh in on their “all wonderful boat” choice and why. Might be good stimulus for the A40. Who’s doing it well? Offshore cruising class Boreal, HR and little harbor. .
Great discussion. For the nautically obsessed this site can’t be topped. Thanks to all.
Hi Jeff,
Thanks for the kind words.
About boat width & displacement, Damen shipyard started something of a fashion about 10 years ago, by taking one of their military patrol boat plan about 20 meters in length, and building it twice as long with all the other parameters unchanged (width, depth, prismatic, accommodations, engines etc…). Resulting boat had twice the displacement, was only about 10% more expensive and had lower diesel consumption at same speed, much greater range and improved sea-comfort because of its larger length and displacement.
Diesel military patrol-boats are different from sailing yachts notably because they don’t pay marina fees and because they are optimized for a wider range of speeds. As a consequence, a prismatic coefficient fit for a 20m. boat might be correct or acceptable for a 40m. boat with same engines and twice its length & displacement. I guess that for sailing yachts, it should not be very difficult to start from an existing boat plan and simulate cost and characteristics of the same boat with length increased 20%, 40%, 60% etc with most other characteristics unchanged.
Contrarily to the Damen patrol-boat, I guess prismatic coefficient should be adapted to account for a lower hull-speed when length increases, and sail area might also be increased, perhaps as a single order of length (to account for the higher righting moment of the longer boat). The Damen patrol-boat cost increased less than 10% when displacement increased 100%. This limited cost increase was only possible because added displacement was not used for added equipments or accommodations (only some added tanks…) . Looking at sailing yachts instead of military patrol-boats, it might be hard to prevent yacht-owners from installing equipments or accommodations in those added volumes, but anyway, this kind of paper simulation should demonstrate that starting from any given boat plan and adding boat length and perhaps some sail area, but nothing else, is cheaper, or much cheaper than supposed according to conventional wisdom (until you meet the marina cashier….), so it should not be a big surprise to find that blue-water cruiser could, or should, be longer and narrower than usual short-range cruisers. This kind of reasoning might get seriously flawed if you account for yearly marina-fee in some fashionable US or European harbor….
Laurent
Hi Laurent,
That’s a really interesting idea. By a funny coincidence, at just about the time you were writing your comment I was chatting about design with Steve Dashew who was talking about investigating optimal boat size by taking a design and costing several different lengths but leaving the accommodation plan and stability the same. Same thing said a different way.
Interesting article! I can’t comment from a technical perspective but it does reinforce my sense that after many years of sailing, one develops a gut reaction regarding good and bad boat design. I recently flipped through a local sailing magazine which had a review of a 57 ft. European built production yacht. Big boat hence the beam was 17ft. What I found more fascinating was that the transom was almost 16ft wide! I showed the
interior photos to my partner who remarked that the main salon looked larger than my not so small urban condo. My boat is a 1978 Spencer 35 with a 9.5ft beam and a transom that is probably under 4ft. I curse and swear every time I need to access storage, hoses, electricals etc, and one time almost thought I would require fire-rescue to extricate me from the lazarette (happy ending, crawled out).However, it’s also a vessel whose sisterships have made numerous circumnavigations safely. Yes the hydraulic transom gate with dinghy garage is pretty sweet, but for my not so fond memories of the Washington-Oregon coast, my gut tells me the Spencer will keep me safe, prettier boats may not. Thanks for some great information I didn’t know I already knew.
Hi,
I love this discussion, although I am at a loss about the practical aspects of it for a guy who likes to sail and has a circumnavigation on his bucket list.
I read with interest the info on the Mac 65. Sounds like a great boat,but can you get one made ? (and can I even afford it).
So, here is my question: Where can I find an acceptable “compromise” vessel that will get me around the world (if I watch the weather and don’t do something stupid) in safety and decent comfort and at a decent speed?
There are a number of folks that have been around in 38 Island Packets and lived to tell about it. I agree…speed is safety and I think I read that waterline is speed. Longer, narrower boat then, but who makes them?
Sure could use some advice.
Thanks,
Ray
Cape George Cutter does.
Hi Rich,
Good point. A loverly boat, but correct me if I’m wrong, I don’t thing anyone is still building the boat? Also, I wonder what the price for a new one would be today.
Cape George is still in business. They are semi custom factory builds (or will sell you a kit boat you or the builder of your choice can finish[check out Zartman cruising / Abednego marine for an example of high quality simple, done on a budget]). Not only are they building the Atkins/Monk/Lange designed Cape Georges, they own the molds and rights to the Bristol Channel & Falmouth Cutters (and recently completed a BCC – The pictures of her are on their website).
This comment may belong in a different thread, but as for an offshore voyaging hull design, these would be hard to improve on. As a semi custom yard, I’m sure CGC would build one to your, or anyone else specs (simple or lavish, and the equipment you want, and none you don’t)(I’m sure Pacific Seacraft would work with some one to spec one of their hulls too). The cost would depend on the outfitting. It would not approach a hunter or bennateau, if cheap is needed (like the Adventure 40, a completely different market – by design), but would be less, than a full cutom one off, or, the same yacht, with all the teak, bells and whistles. I believe the values expressed on this website could be applied to one of these hulls, with a capable and affordable result. Materials and production techniques may have improved, but both power and sail designs from 50 -75 years ago have continued merit.
Marketing to what the majority of people think they want, and the economics of “production” building, gives us what we see at the boat shows. Most buyers are not cruisers (the way most of us that belong to this website define it, be it costal or offshore), and so the boat designs are compromised – to sell. Unfortunately, our voices are in the minority, and so we’re not provided a mass market product.
Just a personal obsevation; most of the well regarded “classic” offshore boats have much narrower beams, and would be considered “tender” compared to modern designs (both power and sail), but preform with better speed (average over days, not top per H.P.) and comfort of motion (at sea or anchor, not at the dockominium).
For anyone considering a smaller, affordable, power costal cruiser, I encourage you to look at the Seapiper 35 that you (John) have previously mentioned, and read the blogs at Great Harbor Trawlers discussing their new TT35, and the decision making in their design, and equipping, an affordable, live on, costal power boat. Both options are designed to safely explore the coast of one area, then be economically shipped by truck or R.O.R.O. ship, to any other coast in the world. A great way to see the world, without having to build a boat with the ability (and costs) to make the passage.
Hi Rich,
Thanks for the fill on the Cape George, sounds good, although I fear it would be hard to hit the A40 target price point on a low volume Cape George build, no matter how simple.
And I agree with your observations on the state of the market.
As to the TT35, yes, a great concept, although I find their other offerings, while better than many, still suffer from the “lets see how much accommodation we can cram into a given length” problem. For example their N47 weighs in at 70,000 pounds, a displacement that should really be on a hull at least 60 feet long.
Hi Again, Rich,
I just had a chance to spend a little more time looking at the TT35, and have made a note to really delve into it, when I have some focused time. So thanks again for the pointer.
I built and lived aboard a Cape George 36 for seven years. She was on numerous magazine covers and was indeed a lovely boat. The 34, 36, and 38 molds are still sitting in the yard. Amazing boat in a lot of ways: I used to have great fun ambushing Valiant 40’s and smaller race boats like the Express 34 ULDB. But I had 900 sq ft of sail area, 33′ of waterline, and12,000# of ballast when the tanks were full to support all that sail power.
An alternative to the Adventure 40 concept? — Right, if your budget is twice as large. The labor cost alone will be as much as the target price of the A40. and manhandling the tiller of the 36 in a tide rip with a bit of wind requires weekly sessions at a CrossFit gym.
The biggest compromise in hull design is brought about through one of two factors: either the skill of the designer is limited, or the pressure for sales of their design is too great.
The reality is that many yacht buyers make their decision to buy this way: husband likes one design as its quick and handles well, wife likes another as it has a lot of room, kids like a third one because it has a swim platform. So they end up buying a boat that isn’t quick, has loads of cabin space, and a large swim platform. “A camel is a horse designed by a committee” springs to mind.
I sail a Shearwater 45, designed by Dudley Dix, and consider it to be as close to perfect as possible. After 7 years I still cannot find anything material to improve.
Hi Steven,
Thanks for the good analysis of what goes wrong in boat buying.
I agree the Shearwater 45 is a great voyaging boat. The clipper bow and bow sprit would not be my thing, but that’s more taste than any real criticism. A look at the boat out of the water tells the story of a boat optimized to sail, not sit in the marina.
Je pense qu il est difficile de creer une coque optimale , qui reponde a des criteres de vitesse , de stabilite, a la variabilite des vents ? . Il y a un choix a faire de la part de l utilisateur ….
l on peut par le calcul determiner,avec precision la forme decoulant de la portance sur l eau,le centre de gravite concernant la stabilite, le ratio de surface des voiles et leur postionnement etc…….
Je rejoins les analyses techniques des specialistes sur ce poste mais il me semble particulierement difficile de reunir toutes ces qualites en un seul bateau un compromis de choix me semble plus adequat ULYSSE
Sadly, our boat acquisition process was exactly how you described. I wanted a Mac65, wife hated the interior and seized upon the many, many negative public opinions of that boat. We ended up with a “condomaran” which I hated. Our present boat is a Valiant, which despite being grossly overloaded and being saddled with the inefficient canoe stern, still manages to sail quite nicely and has a comfortable motion.
The compromises necessary in the design process make it impossible to ever produce the universal “perfect” boat. Dick Newick, I believe said..”Fast, Roomy, Affordable? Pick any two.”
Hi Neils,
Thanks for he interesting and very frank comment. I think your honesty about what you went through will really help others.
Sounds like you ended up with a good compromise, and all boats are just that, compromises. As you say, the Valiant stern is less efficient than it could be, but it’s still a great voyaging boat.
Yes, it’s that “C” word again. A good compromise is when everyone is equally unhappy. The Valiant 50 has met that requirement splendidly. C.A. Marchaj’s “Seaworthiness: The Forgotten Factor” is a good read but written in an era before accurate offshore weather forecasting was readily available en route necessitating the requirement to ride it out.
Hi Niels,
Now you are scarring me. I have to disagree that modern forecasting obviates the need to ride it out. Two reasons:
More on weather here and surviving bad weather here.
Also, more on what happens when people go to sea on the assumption that they can escape all bad weather here.
Oops.. Sorry, John, I think it came out wrong. I fully agree, forecasting does not obviate the need to ride it out, but it can significantly minimize the likelihood of having to. It’s just that what Tony Marchaj proposed is a heavy displacement boat with high AVS and this was all the more necessary in an era without instant downloads via HF wefax or Iridium.
A designer can now offer a lighter, faster”sled” – a virtual lake boat on the premise that this boat will be able to remain clear of heavy weather. I wonder if the recent offerings of called fast cruising boats are built on this premise. The question is, are these necessarily bad designs?
BTW, hull and separately, boat design are a great topics, with no single right answer. Thanks for introducing them.
Hi Neils,
No worries, gave me a chance to mount one of my favourite hobby horses!
As to you question about whether a boat that can’t take a full on storm at sea is a bad boat in every case, I would say that if said boat is sold to go offshore then the answer is yes, it’s a bad boat. The point being that yes, with modern forecasting and a bit of luck, the crew may go for years without getting caught out, but it only takes once.
There is also another aspect to this. I think that much of the fear and trepidation you see out there among cruisers about heavy weather is because they know deep down in their heart of hearts that their boat just isn’t up to it. I, myself, am an avowed wimp, but I get great comfort from knowing that my boat is designed and tricked out to survive, with little or no damage, anything short of a full on hurricane at sea, and quite possibly even that. Bottom line, if you know your boat can take it, crossing oceans is a lot more fun than if you have doubts.
Czesław Marchaj wrote “Seaworthiness” in 1986.
For my son it is B.C. (before computer) but did that much changed in this 28 years?
Maybe there was no sat phones, but I still don’t have one.
For sure they had HF radio in this ancient times, and they didn’t use it much – just like most of us now.
We have mobile phones, but they loose connection not only on Atlantic, but also in English Chanel when you still clearly see the land.
Do we know more about weather now? Probably, but not sure if we always get the information in time.
What a great post & discussion!
I’m barely a fledgling student of NA, but one of the impressions I’m getting is that, for displacement vessels, efficient hull geometry & dead rise at different angles of heel & wetted-surface area would strongly benefit from double-ended designs (aside from the safety benefits against being pooped & despite the “difficulties” of cockpit/interior configuration)…?
Or am I missing the mark here??
In any event, many thanks all for such informed reading!
Hi David,
That would seem logical, I agree. But in fact one of the advances in understanding in the last 50 years is that in fact very pointy sterns are counterproductive because the move the stern wave forward which increases wave drag, this is particularly so at higher speeds. The key to stern design is balancing this fact as well as wetted surface which affects light wind performance, and making absolutely sure that the centre of buoyancy does not move much when the boat heals.
Also, the studies by Don Jordan of what happens to displacement boats in large breaking waves has shown that double ended sterns provide little or no protection against getting pooped. In fact the whole concept of damage from a wave breaking over the stern is flawed. Actually what happens is that a boat not properly restrained by a drag device surfs uncontrollably down the wave face and then broaches at the bottom and this is when the damage occurs. In fact an excessively narrow stern actually makes this worse since it makes it more difficult to steer the boat on a wave face.
You can learn more in our Online Book on storm tactics. And also a read of Don Jordan’s research is well worth your time.
I think the key word here is “balance”. Take a look at the Corbin 39— a design that has nearly as successful a history as a long distance voyager as the original Valiant 40. But it also had a reputation for weather helm to the extent that a large bowsprit and radical re-positioning of the mast were instituted on later versions. I’d suggest that this is a design with TOO MUCH buoyancy in the stern even though it is a double-ender. Combine that with a deep forefoot and a rudder perhaps lacking in power and you have a boat that will tend to be lifted and pushed off line by the stern buoyancy as she starts to surf down a wave, and then dig in at the bow accentuating the change of direction.
Comments from the real designers here?
Hi Richard,
Volume in the stern does contribute a lot to forces generated on the rudder, especially when a boat starts to heel and trims forward because there is more buoyancy in the stern than in the bow. However, i do not hink that this is the case with the Corbin 39
The “lead” is a larger contributor to weather helm (or lee helm for that matter)
I just printed a profile plan of the Corbin 39, and drawn the center of efforts of both underwater and the sail plan. Normally, the horizontal distance between these two points is around 3-6% of the waterline length (this number is called the “lead”, and is one of the best kept secrets of designers) . The Corbin without bowsprit comes to as much as 12%. No surprise there is weather helm! When drawing in a bowsprit, and forget about an overlapping headsail, the sail area is of equal surface, but placed in a better place, and the lead goes down to 7% of LWL, still not ideal, but significantly better.
Center of effort of underwater area, is also determined by the profile of the keel. There is more pressure on the forward end of the keel and the rudder than there is at the back of these appendages, therefore, the dynamic center of effort is quite a bit further forward than the geometrical center of effort of the underwater profile. If the keel of the Corbin is not optimal, and often that is done as a compromise to get enough ballast or tanks in the keel, the center might even move more forward than the standard approximate 25% of keel girth.
Another contributing factor is pressure in the sails, adding roller head sail furling to a boat that has already a weather helm, is making it worse. Sails need to be reefed in such a matter that the longitudinal location of the center of effort is kept in roughly the same place, or better, moves slightly aft when reefing. One often missed fact, is that when reefing an overlapping head sail, one is actually removing sail area and therefore pressure BEHIND the mast, and gives the boat therefore a stronger weather helm, even tough you minimized head sail area.
All in all, it is all about balance, and there are a lot of balancing acts to be performed by a sailing vessel at sea. In a good design, there is little to no compromise done to this balance, because messing with this kills performance, potentially as much or even more than a Prismatic Coefficient that is messed up.
Hi Eric,
Just going on the basis of reported sailing characteristics of the Corbin design rather than an analysis of the underwater shape—:
1- Perhaps some of the downwind wave instability is due to the high freeboard and fullness of the hull shape above the static waterline— an area that doesn’t come into play until it is overwhelmed by a steep wave.
2- To my eye the forefoot looks like a perfect example to generate what Steve Dashew refers to as bow steering. http://www.sailboatlistings.com/view/33105
3- Weather helm and sail balance (lead) are a somewhat different issue than downwind stability. Indeed, under those conditions all or most of the total sail area should be forward of the mast.
John, I just want to echo what you say about the comfort attained by a boat “designed and tricked out to survive” when offshore in a blow. My pat (but very heartfelt) answer to those who admire Alchemy and her ruggedness is to say that I wished for a boat that was much smarter and stronger than I was.
Dick Stevenson, s/v Alchemy
Hi Dick,
I love it: “I wished for a boat that was much smarter and stronger than I was.” My thoughts exactly.
Dear John & Niels,
I must say, to begin, that Bob Perry, to my ascetic sensibilities, has designed in the Valiant family the best looking tushies to ever grace a blue water boat.
Now that the important stuff is out of the way, I would want to note that these canoe sterns, although pointy, carry a good bit of volume well aft. They have wide hips (so to say) before curving gracefully in to its not very pointy stern. This, I believe, contributes to her light air capabilities as there is markedly less turbulence as the water slips away from the hull, while still providing good volume for heavier weather.
My best, Dick Stevenson, s/v Alchemy (a Valiant 42)
Hi Dick,
Good point that canoe sterns are very different that true double ended sterns because, as you point out, they can still have more volume.
I’ve compared a Valiant 40 to an Island Packet 40 using Tom Dove’s Sail Calculator and it appears to me that the IP 40 beats the Valiant in almost all categories. And yet the IP is slower based on its PHRF rating. And of course the IP’s also have a reputation for being slow. Wonder what’s going on here? Does the additional wetted area of the IP’s keel make the difference?
Hi John,
I have decided to turn this answer into a short article. It’s a great question and deserves a better answer than I can do in a comment.
Thanks for the explanation. For the noobish, what do you mean by a “harder turn to the bilge”?
Hi John,
I’m thinking I might turn this into a short post, and will answer that there.
Looking forward to it
Let me add that I am considering an IP 40 (among others) even though it has poor reputation for speed. That’s what led me to the comparison in the first place.
I am a couple years late to comment on this article, but like a previous poster, I am struggling to apply this information into selecting a hull that is appropriate out of the universe of boats available on the used market (within my budget). For a newbie, what advice would you provide to help identify a good hull from bad. I am unable to find prismatic coefficient for the boats I am considering. Is there anything else I can use as a proxy? Max hull speed is sometimes available. I want to make the right decision…but I don’t know how.
Hi LukeorMary,
Yes, picking a good hull is a complex project. Let me think on it. You have given me an idea for an article. Thanks.
Regarding misc. hull coefficients I just found a PDF that might be quite interesting to some: http://www.kastenmarine.com/_pdf/coefficients_of_form_equations.pdf
Hi John,
It is incredibly easy to observe things that don’t work well on boats used for cruising (please not my not using the term “cruising boat”) and get high and haughty.
While all “ways” can be observed, “it ain’t necessarily though”. Hence, while I generally agree with much you say, having gone through a long design process with a top firm, very little of this stood up to practice.
#1 (besides, the title doesn’t match the content)
The inside, which gives one set of requirements) and the outside (which gives another) have to be optimized together. And while I very much like the work of the Dashews’ work; their shortest boat is 56 ft.
Secondly, why are people even trying to find a cruising boat in a market optimized for charter. What is being produced works perfectly for the latter and for someone who charters a week per year, the 40-foot boat with the large interior might actually be the ticket.
#2
“a wide stern is simply wrong on a cruising boat ”
Really? I just spent 11 days delivering a wide-sterned, flat-bowed aluminium, ice-reinforced high-latitude yacht of recent design – it worked perfectly fine. Actually, it was the nicest boat that I have been on ever, and it was the first time I didn’t get seasick at the beginning of the season. It was definitely not a monster.
But you need a good naval engineer and designer to pull it off. And it needs in-depth know-how of the incredible advance that have been done in CFD and testing.
Let’s agree to disagree on twin rudders, we had that discussion already. If they are done right, they work well. I doubt they add more drag – often there is only one in the water and usually the surface of the twin rudders is the same as that of the typically larger single rudder. If they are built sturdy enough the floating debris isn’t an issue. Plus, you get redundancy to boot.
There are dry boats and there are wet boats. A good naval engineer with the latest know how will give you a dry one regardless of the stern-width.
#4
A designer with manners and people skills will find ways to direct the customer gently into the right direction. A designer without is in the wrong trade
#5
“But most yacht designers have been tied to their drawing boards and computer monitors by the need to build a business and therefore have very little offshore experience”
How in the world do you support that statement?
Someone who has the brains and talent to be a good designer can make a lot more money elsewhere. Look up the people behind the designs and you will be amazed about the extensive off-shore sailing and racing experience. Some with several hundred thousand miles including skippering round-the-world races. For most, designing is a way to stay professionally connected with what they love – sailing.
Since you mention the Boreal – Triple chines? In 2022? It’s like designing your bathroom so it is easy to install for the plumber. How do you brace yourself in her galley? Isn’t that the one with the well right outside of the companionway where the water can collect. Or the freestanding radiator right in front of the bed, where you won’t get any decent circulation, and have to step over. Inefficient approach to stability with the lead all in the belly instead of where it is most effective – the bottom of the keel. A true lifting keel would work much better.
Look, I am sure she makes her owners happy. If I had gotten one as a present I would have gladly accepted. But to present that as the pinnacle of a 21st century cruiser?
Last, but not least, if it were just experience, why do we need then engineering schools? Why do we need research and testing? When I compare my car today with the one I drove 20 years ago, the answer is clear. Are there downsides? Sure. But none of these matter when I consider what I get for it.
Before computers, optimizing the indicators was the way to optimize the design. Today, the indicators are an interesting outcome of the process, but the top designers are way beyond that.
correction
While all “ways” can be observed, “it ain’t necessarily so”.
Hi George,
We will have to agree to disagree on pretty much all of that.
Talking of which, please read, or reread our comment guide lines and particularly the part about attacking the opinions of others. Not allowing that is a lot of why the comments work as well as they do around here, which you applauded in your last comment.
Also see the prohibition of quoting others and straw man arguments.
I have let two of your comments with that unfortunate tone of attack go by, but the next one gets deleted without further warning.
It’s fine for you to disagree with me, or anyone else in the comments, but putting your own thoughts forward, perhaps by telling us more about your boat, without denigrating mine, is the way to do that and have a good discussion: https://www.morganscloud.com/2013/11/10/aac-comment-guide-lines/