Adventure 40 Keel—Draft and Shape

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Keel Draft & Shape

Before I get going on the keel, a huge thank you to all of you who contributed to funding the initial design. While Phyllis and I, and AAC, won't see any of that money, I'm still hugely excited that finally, after nearly 10 years of effort, the Adventure 40 has passed the ultimate test of any new business idea: are people willing to part with money.

Speaking of which, as at the time of writing, the Adventure 40 gofundme campaign is just €660 away from the goal of €15,000, so if you can help fill that gap, it would be great.

Back to the keel design, which is one of the many things that will make the Adventure 40 better for offshore cruising than most any other boat out there.

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William Pence

Did you mean keel chord? Excellent site, by the way. I have learned a lot.

Michael Feiertag

Perhaps WP referred to chord vs cord.

William Pence

That is correct.


In keels (and rudders, wings, daggerboards, etc.), span is the distance from the root to the tip, or from tip to tip in a bilaterally-symmetric wing. Chord is the distance from leading edge to trailing edge. I think the confusion is just a spelling issue (‘cord’ without an ‘h’ has nothing to do with foils).

Stein Varjord

Hi John,
In my opinion, those are all good choices by the team!
I notice attention to limiting the total weight of the boat, on the concept level. Awesome! That attention is essential, of course, even on a cruising boat. It’s often dismissed as in “the boat will be heavy anyway, so adding this little extra weight won’t matter”. That attitude has destroyed many otherwise good boats.

I also love that fencing of the keel bottom end is in focus. It’s way more significant than most sailors appreciate. In the old “bible” on this topic, ‘Aero- hydrodynamics of sailing’ by Professor C A Marchaj, he states that fully fencing one end of a foil will be about equivalent to doubling its aspect ratio. That’s approaching doubling the lift!

We got great results from experimenting with this on sails in the Formula28 class. It will also significantly reduce the energy in the vortex spinning off the end of the keel, thus reducing drag. A bulb isn’t 100% fencing of the tip, but it’s significant. Details in the shape of the bulb will be important. I’m sure the team will explore the options well.

P D Squire

“Not caring about weight and windage…”

I’m excited in anticipation of the A40’s hank-on vs roller- furling headsail discussion and decision. Quite a lot of windage in a furling headsail that increases as sail is reduced to match apparent wind increases.

Huw Morgan

I agree, the fin design on the Winner 9.50 by van de Stadt is so effective and practical with the bulb behind the leading edge and the flat bottom makes emergency dryouts against harbour walls much more stable as well as the improvement in leeway. Compromise can be good sometimes if the goal is clear


This is all good news, in my opinion.

To really get the benefits of shallow draught, you do need to go shallow enough to take the ground at will, or at least to not have to pay much attention to soundings. In sailboats, that means centreboards or retracting ballast keels, both of which are relatively expensive to make robust and reliable.

The moment you have a fixed fin sticking more than three or four feet below the waterline, you get into the awkward trade-off of performance versus draught, and John’s analysis is bang-on.

I can’t own a sailboat that won’t point high in light to moderate air. It’d be downright unusable as a sailboat in our area, where at least half of any given voyage, regardless of direction, will be sailed close-hauled. Around here, a sailboat that can’t short-tack through less than 100° ends up just being used as a motorboat, or being replaced by one.

The importance of tip and bulb shape to minimize the tip vortex (and the associated loss of lift and increase in drag) went unnoticed for far too long. Learjets had tip winglets in 1978, but as recently as the late ’90s some big commercial planes were still being built without them. The sailing community didn’t really get the hint until CFD simulations became commonplace, and even then, a lot of weird grafted-on keel tip winglets with no real math behind them were (and still are) seen. This is one area where it really is worth burning a few engineer-days on modelling and simulation to be sure the chosen shape really does work well.

Eric Klem

Hi Matt,

I never cease to be amazed how many things are “designed” by eye where small details are very important and the designers seem to be completely oblivious to that fact. In my line of work almost all parts get some form of optimization and it never ceases to amaze me how a few small changes can double the stiffness or strength of the part and most people couldn’t even eyeball that the designs are different, they would need to actually overlay the CAD models and then it becomes obvious. Prior to FEA and other forms of simulation, optimizing complex designs was just not practical so people were forced to take their best shot based on simplistic hand calcs and then do rigorous testing programs with constant iteration to slowly optimize. These days if you don’t do a lot of optimization through simulation first and then check/correlate your results with real testing, you will either never reach any meaningful level of optimization or it will take you a ridiculous amount of time to get there. Yet the world is full of examples of designs without even basic analysis run not to mention things like sensitivity.

End plates are an interesting trade-off. A decent way to deal with the tip vortex is just to lengthen the foil and move it outwards but that means extra draft. In a place like the PNW, this would probably be the right way to go but for those people who are still somewhat draft sensitive, a moderate bulb makes a lot of sense as it gives multiple benefits and represents a good all around compromise.



Lengthening the foil always helps; all else being equal, a greater span and higher aspect ratio for the same area will always yield a more efficient lift-to-drag ratio.
That said, you won’t find very many modern aircraft without some kind of tip vortex control device. Even a very high aspect ratio foil can still benefit from a well-shaped winglet or endplate to minimize the loss of energy in that vortex. And, in a sailboat keel, such a shape is almost always going to result in a lower centre of gravity than you’d get from a plain-tipped keel shape.
Performing and interpreting CFD simulations is now significantly easier than many engineers think at first glance. The hard part is in learning how to set up and configure the software toolchain in the first place; once you’ve done that, rapid iteration of the 3D model -> simulation mesh -> simulation result -> updated model cycle is surprisingly time-efficient.

Stephen Cox

I am really enjoying the development of the Adventure 40 design and specifications, gladly contributing to its funding and looking forward for further investment/purchase opportunities.

I hope you will cover the keel attachment to the hull in Part 2 or a subsequent article. As a structural engineer, I have always marveled at how little there is holding keels on to many modern production boats. Dick Beaumont, founder of Kraken Yachts, is adamantly opposed to bolted-on keels and Kraken is building their boats with integral ballast “Zero Keels” which appeal to my structural design instincts. That is likely too costly for the Adventure 40, but it seems like the keel attachment should be a critical design consideration for a boat designed primarily for offshore use with a relatively high-aspect keel.

Mark Devlin

Is ‘boat that barely makes good 100 to 105 degrees to true wind’ actually what is meant, or is it a typo, intended to be ‘100 to 105 degrees true tacking angle’?

Stephen Cox

I also questioned this, but I currently sail a cruising catamaran and sometimes it feels like I’m not making any VMG windward when tacking ,135+ degree TW tacking angles make me want to weep sometimes, so John’s wording made me feel better for a moment. The moment has now passed…

Stein Varjord

Hi Stephen, This is off topic to this article, but I hope it’s relevant to your frustration with your cruising cat. I also sail a cruising catamaran that has a performance in a whole other world than the extreme multihulls I used to race in the nineties etc. The vast majority of cruising catamarans, especially the latter 20 years, have their sights set squarely on charter companies, their biggest customer. Performance is not at all a priority, and charter customers use diesel to go to windward. 🙂 They’re basically made to be motor sailors, but many of these are actually quite good designs, made by the same people who design the fastest sailboats on the planet. Most Lagoons (the biggest part of the Beneteau-Janneau group) are from VPLP Design (Van Petegheim – Lauriot Prevost). Google what they have done through the years… (!) They really know what makes a boat perform. They have just adapted the parameters to fit the goal of the customers. The Lagoon 38 is a cruising cat we meet all over the world. Most of them are overloaded, have fixed propellers, dirty bottoms and worn out sails, plus are not trimmed at all. Nothing can make that perform, no matter how good the boat is. If all the unnecessary junk is tossed ashore, even without being an extremist like me, one gets reasonably good sails and the other items are fixed, these cruisers actually sail faster than pretty much any similar monohull cruising type boat, also VMG upwind, but not in very light winds, due to the undersize rig. I know this, because I’ve done it a lot of times. Also when letting less race skilled people handle the boat alone. It’s a good boat. (Never owned one myself.) I’m pretty sure that you can also get massive improvements on your boat by looking at the many details. Most cruising cats should have an optimum tacking angle in calm seas and say 10 knots of wind no worse than 115 degrees. Some 5 degrees off the wind compared to a similar mono. You should be totally able to point significantly higher too, but that’ will reduce VMG. The short fixed keels on most cruising cats are decent when the wind is up, but a big penalty in lighter winds. Same goes for the small and very conservative rigs. The reason multihulls point lower isn’t that they can’t point high. Some racing multis can point better than anything afloat. The main reason is that they don’t have a “hull speed” and no proper “planing threshold”. While a mono will often not be able to increase speed more than say 10% by lowering 5 degrees, due to already being at its speed “limit”, a cruising cat might increase the speed by 30% by the same degrees lower course. A thoroughbred racing multi will gladly increase the speed by 50 to 100%. Pointing a bit lower will increase the speed more than the distance. The higher speed will give… Read more »

James Chase

This post has made me more excited about this project than all the other ones put together. And the other ones all made me pretty danged excited.

Whitall Stokes

Great design decisions! Thumbs up to the deeper draft and bulb staying aft of the leading edge! Will be most useful to getting off lee shores and getting upwind around the next Point when cruising. I’ll be curious to hear if/how the fin gets slotted into the hull structure with cross bolts.

Hurry up and take my money!

Matthew Morrison

I hear that! Looking forward to more news on a probable time line for the boat to get into production. Understand that this will be an exercise in patience, but eager nonetheless!

Rob Gill

Hi John,

Nice…a two metre draft and a well secured steel / cast iron keel is a great mix for cruising, we find. On our Euro built Beneteau 473 we have the 2.1 m draft standard cast-iron keel (see picture), with moderate bulb extending aft. A friend has the same 20 year old 473 model, but built as the shoal draft version at 1.8 metres in the USA, for the Eastern Seaboard. It has a slightly larger bulb to compensate.

The spade rudders however are identical between the boats. We both carry the same standard rig and boom-furling Doyle mainsail, although we run a Doyle jib and they run a 120% Doyle genoa.

The difference in performance is very noticeable upwind. We point higher and go faster. Also there is a marked difference in leeway – maybe 3-5 degrees, even when reaching.

In light airs (sub-10 knots TWS) the performance difference is more marked, and our friends will often be motor-sailing.

Perhaps surprisingly, the other difference we notice is when anchoring. We are always anchored closer in to the shore than our friend’s boat, as we are confident with our deeper iron keel, that should we touch the bottom (say I get the tide wrong which I haven’t yet), then we will lose a little anti-foul. If our friends touch down, they may lose their rudder because they don’t have the same delta (and so protection) between the bottom of their keel and bottom of their rudder.

Very often we are moored in a bay close-in with a catamaran. Even the launches are very often further out than us, as they worry about their expensive projecting props and drive shafts!

I should add, we are more conservative anchoring in bays we don’t know, or in water where we can’t see the bottom. And also add that our friends absolutely love their boat too, one which they won’t ever take offshore.

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Rob Gill

Hi John,

After your comment above, I was wondering why go to all that bother to achieve just 0.3 metre reduction in draft unless you transit restricted inland waterways?

So I checked and I was wrong sorry, their shallow draft is actually 1.7 metres. So the designed difference is 0.4 metres. Not sure why I had 1.8 m in my head.

I also checked their cast iron keel weighs in at 3577 Kg vs 3209 Kg for the standard draft. That’s a lot of extra weight on a 14.5 metre yacht with design displacement (unladen) of just 11,000 Kg. And as you say, this extra ballast must lower their hull in the water, thus shortening their fin span further.

I must remember to measure both spans next time we are on the hard-stand. It would be interesting to know.


Will the keel and fin shape allow for easy Heave To and maintaining proper positioning during a gale?

  1. what do you guess the proper sail plan is? Backing the main only or main and innerstay?

i know you are a big fan of the Jordan Drogue. Will there be chain plates on the back of the boat to support the drogue?