Adventure 40 Keel—Draft and Shape

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.


The Adventure 40 will draw about 6.5 feet (2 meters).

We chose this draft based on Phyllis and my long-term cruising experience, because it provides the best tradeoff between being able to access most cruising grounds and performance.

For example, with that draft we were able to cruise the Bahamas (with limitations and care) and get into most river mouths on the East Coast of the UK and inlets on the Eastern Seaboard of the United States.

And the A40 will be more close-winded than our McCurdy and Rhodes 56 since, on the smaller boat, the vertical measurement of the keel (the span¹) will be greater for the same draft and it is that parameter that, all other things being equal, governs performance.

¹We sometimes see “span” misused to describe the horizontal measurement of a keel, but that measurement is the “chord”. I will use those terms for the rest of the article.

Not A Shoal-Draft Fin

This span depth issue is also why shoal-draft fin-keel boats don’t make a lot of sense for offshore cruising. For example, if we reduce the draft to say 5 feet (1.5 m), one might logically assume that’s only a 23% reduction and so maybe worth it for more harbour accessibility.

But that’s not the way it works. Let’s assume that the boat hull draws say 1.5 feet (.45 m), then the keel span on a 6.5 foot draft boat is 5 feet (1.5 m), and the span on the same boat drawing 5 feet becomes just 3.5 feet (1 m), a difference of 30%.

But wait, it gets worse.

The optimal relationship between chord and span is deep and narrow—look at racing boats—so the further we get from that ideal the less lift and more drag we get, but to end up with anywhere close to the same resistance to leeway, and to carry the required ballast, the shallower boat must have a longer keel chord.

And worse still: making the keel shallower with a longer chord raises the ballast, which, in turn, means we need more of it to balance the same sail area, but that added ballast drives the boat down in the water, which means we need more sail and have less payload…OK, I will stop, you get the picture.

The key takeaway is that shoal-draft fin keels carry a lot of downside for a limited upside, in this case a big decrease in performance for just 18″ (.45 m) of draft reduction.

Relentless Offshore Focus

Bottom line on draft, as always with the Adventure 40, is we are putting sailing performance offshore ahead of all else.

Not For Everyone

Of course, the downside of this focus is that there will be a fair proportion of potential buyers for whom the boat won’t work, either because they want to keep the boat in a shallow area or because their desired cruising grounds are shallow.

That’s unfortunate, but it’s also the right thing to do, since we only have to look around to see that much of the reason there are so many poor offshore boats on the market today is that the designer attempted to make everyone happy to maximize sales.

It’s a great truth that a design that tries to be good for everything, in fact ends up being really good for nothing.

The Right Answer To Shallow Draft

The other angle on this is, if we truly want a shoal draft boat, the right answer is to buy a boat designed from scratch with that goal, like a Boréal with a really shallow draft of a little over three feet (1 m).

Also, at least to me, it makes no sense to buy a shoal-draft boat that can’t dry out—either go the whole hog or not at all. So if we truly want shoal draft, a centreboard is the way to go. (Some multihulls are a good option, too.)

A shoal-draft fin-keel boat drawing say five feet (1.45 m) is neither fish nor fowl—once again, not really good at anything.

No Options

Of course, some might suggest that we should just offer a shoal fin keel as an option, even though we know that’s a poor compromise, just to get more sales, but that violates one of the most important core Adventure 40 principles: no options.

And this would be one of the worst ways to break that rule, since offering a shoal keel would be particularly onerous and expensive because doing it right not only requires managing the complexities of some boats on the production line having a different keel, but also different keel-attachment engineering and build, since the chord would be longer on a shoal keel.

As always with the Adventure 40, if the builder is going to be able to deliver the boat for our target price without compromising quality, and make a fair profit, complications like keel options must be eliminated.

Lifting Keel?

So what about just building the standard boat with a centreboard? Yes, there are a lot of things to like about moving appendages, but they also add a lot of expense and complication, not only to the keel but also the rudder, so not a viable option for this project where we are focusing on delivering a high-quality boat at the most reasonable price possible.

Also, all other things being equal, a fin keel offshore boat will always be a better sailor than one with a centreboard.

That said, who knows, maybe one day there will be an Adventure 40CV (centreboard version), purpose designed from scratch—the hull and interior arrangement will be very different and probably the rig, too—to do the shallow draft thing right and sold as an alternative to the Adventure 40PV (performance version)—each boat optimized for its mission.

Keel Shape

Enough on draft, on to shape.

One of the things that has advanced a lot in recent years is optimizing keel shape to make the boat close-winded, while keeping drag as low as possible for speed, and without having to make the draft crazy-deep.

As an example of how things have advanced, the smaller and more race-oriented version of our McCurdy and Rhodes 56, Selkie, designed in the eighties, draws the same at 6.62 feet (2 m), even though she is 18 (5.5 m) feet shorter, at only 38 feet (11.5 m)!

But today, using keel shape advances, we can beat her legendary performance at the same draft, particularly since the A40 will not be constrained by the IOR rule as Selkie was.

And we have a designer on the team who has the skills, experience and design tools to take every advantage of those advances.

We should not miss that compared to even boats designed ten to twenty years ago, the Adventure 40 will have noticeably better performance, particularly to windward—this is not going to be your father’s keel…or even some 70-year-old, old-fart’s keel.

Why Performance Matters

Some will argue that windward performance does not matter in a cruising boat, but speaking as someone who cruised a boat with less than sparkling close-windedness² for over a quarter of a century, I would have given a lot to reduce her leeway, even by a couple of degrees, while picking up even a couple of degrees of pointing.

²Due to a short-span keel, coupled with a keel designed before the importance of optimizing the shape of the bottom part to reduce cross flow from the windward to leeward side over the bottom edge was fully appreciated.

That might not sound like a lot, but five degrees made good toward our desired course when the wind is forward, can make days of difference on a long rugged passage, or hours of difference over the course of a long day.

And, anyway, pointing higher and going fast is more fun…particularly if there are other boats around…oops, my closet racer is out and about again.

Seriously, beating all day, or worse still for days on end, on a boat that barely makes good 100° to 105° tacking angle as most cruising boats are actually capable of, at best, at least once the water gets lumpy, gets disheartening and tends to lead to that well-known cruiser’s affliction: itchy engine-ignition finger—good sailing performance is green, too, since we will motor less.

Bottom line, fast is fun, as long as it’s not taken to extremes that make the boat uncomfortable, and the A40 team has already committed to preventing that.

Bulb Keel

The boat will have a bulb-type keel to both move the weight down, which results in a lighter keel for the same stability and more payload, and increases the end-plate effect as I discussed above, which contributes to efficiency—bulbs are a big and multiple win.

That said, the bulb will not extend forward of the keel leading edge, since that’s a gold-plated invitation for something in the water to get fouled on the keel—fishing gear, for example—and would also screw up the French team’s plans to make the keel grounding resistant.


In conclusion…hmm…no conclusion until:

Part 2

In Part 2, all written and ready to publish in a few days, I will delve into some very exciting work that the French team has already done on keel strength and grounding resistance, as well as write about materials and attachment…and then I will come up with a pithy, insightful, and, of course, brilliantly-worded conclusion.

Sign Up

If the Adventure 40 is for you, please sign up as interested.

Adventure 40 Swag

And don’t forget that there is some very cool Adventure 40 swag available, thanks to the efforts of member Scott. Phyllis loves her A40 hoodie and both of us are very impressed by the quality of our tee-shirts. I particularly like my long-sleeved one.

Wearing A40 gear is a great way to help promote the boat.


If you have added thoughts or questions, please leave a comment.

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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.

Matt Marsh

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

Matt Marsh

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.


Matt Marsh

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 a tighter angle and stronger apparent wind, which means more power from the same wind. The actual tacking angle and sailed distance increases, but still VMG increases significantly. (VMG = Velocity Made Good. The advancement directly into the wind.)

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.

IMG_3025 (1) Large.jpeg
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?

Richard Tomlinson

Have you considered a performance twin keel option? Shallower draught, ability to dry out, easy to park the boat on the hard when necessary.

With a well-designed twin keel, there is no appreciable loss of performance on a cruising boat. Look at the Sirius and RM yards who sell identical boats with a choice of deep fin keel or performance twin keel. The yards themselves have match raced their own boats, and found no difference in performance, and at the end of the days the crews did not even know what keel they were racing on. The French sailing mags have also done their own controlled tests – with the same results – no appreciable difference.

At both yards, the twin keel option greatly outsells the fin keel – so there is a clear commercial advantage of offering a twin keel.

Finally, you already have a designer who has quite some experience at successful twin keel designs.

I really would like to see some serious consideration of such an option .

Maxime Gérardin

Hi Richard,

you know of the no-option rule:

A twin-keel option would require a substantial dose of additionnal engineering and keel tooling, so something that could be considered only in the very very long term, when everything runs smoothly.

More generally, I’m a fan of twin-keel designs and have beaten to the wind or dried-out numerous times with them. That said:

1) their drawbacks are on matters that are important to the Adenture 40 mission: propeller and rudder protection, risk of fishing pot fouling (I’ve done that too with twin keels…), ability to disengage from a sandbank (the draft doesn’t decrease when you heel…), and arguably higher capsize risk, as it takes a higher heel angle to suppress the hydrodynamic effect of the keel – this one I have no experience of, but I do have the experience that when you play the game of heeling hard, the twin-keel boat keeps going straight on instead of sliding to the side, which in a sense is not very reassuring.

2) the trade-off is less favorable to twin keels on relatively narrow boats such as the Adventure 40 – the twin-keel Surprise might be an exemple of this.

P D Squire

“I would have given a lot to reduce her leeway …while picking up even a couple of degrees of pointing.”

Would you have given another foot of draft?