Adventure 40 Hull Design

Chapter 8 of 16 in the Online Book Adventure 40 (Free)

Elecvation

Well, it’s been a long haul, over two years since I first came up with the concept that became, with the help of many of you, the Adventure 40. A strong, fast, and reliable offshore voyaging boat that you will be able to buy, ready to circumnavigate the world, for US$200,000.

And now, at last, thanks to Erik de Jong, we have a preliminary design. I say preliminary because before Erik does the structural engineering, final hydrostatic analysis, and optimization, we are going to share the design with you, our readers, so that the boat can benefit from the same great process of collaborative thought that yielded such a great specification.

Once we have, over this summer, shared first the hull form–in this post–and then the rig and deck and finally the interior arrangement–in two more posts–Erik will return to his computer and complete the design. The final step in the design phase will be a full check over and evaluation by another naval architect, and then construction of the prototype starts.

As you read these design posts, it’s important to keep in mind that, as I explained in this post, the fundamentals of hull design are well understood, so the key to success is not the application of some high tech black art, but rather formulating a clear understanding of what the boat is intended to do, and then keeping a laser-like focus on that as the boat is designed. Erik has succeeded in this admirably.

If you have not read the original hull form specification post, or even if you have but it was some time ago, please read it now so that you are clear on the goals for the design. Please pay particular attention to the sections on why speed and windward ability are so important.

A Sailor’s Sailboat

Back from reading that post? Great, let’s summarize. The Adventure 40 is not a floating condo, she is not the boat with the biggest interior for a given length, she is not a motor-sailor (although she will motor faster and more economically than most motor-sailors of her length), and she is not the boat with every gadget and contraption known to sellers of marine gear.

She is none of those. She is a sailboat that will be a gas to sail for a day or across an ocean. She will be fast, easily driven, sea kindly, and fun to steer. She is a boat for people that love to sail. If that’s not you, that’s fine, but she is not your boat and we are not going to turn her into your boat, so please don’t ask.

Hull Optimization

OK, now we have the goals firmly in view, let’s dig into the design. The first and most important decision a designer makes is deciding what the hull form should be optimized to do. You see, you can only design a boat to be at its very best in a limited set of conditions. For example, if you design a fast planing hull that will excel downwind in big breeze, the boat will do poorly in light winds and when going to windward. And worse still, if you then load such a boat up with cruising gear, you will end up with a dog that won’t even go downwind well because she can no longer plane.

Given that the Adventure 40 is a voyaging cruising boat, Erik optimized her to be most efficient for speeds that are 90-95% of hull speed, or a little over 7 knots.

This is in keeping with our thinking, proven by tens of thousands of miles in our own boats, that it’s not how fast you go that determines the speed of the passages you make, but rather how often you go fast, even when heavily loaded. In other words, successful offshore boats are optimized for high average speeds over a wide range of wind angles and speeds.

There is one other aspect that determines passage times and that is how comfortable the boat is to be aboard when going fast. Many modern racing designs are theoretically faster than the sea-kindly hull that Erik has drawn, but with a cruising crew aboard both boats, the Adventure 40 will arrive first because the crew on the lighter boat will be forced to slow down due to the motion and slamming.

This brings up an important issue: If a boat can only be truly optimized for one set of conditions and we have aimed the Adventure 40 at around 7 knots of speed with the wind forward of the beam, what about running off, the conditions us cruising sailors love?

Well, by slightly sacrificing performance at low speeds in light air, Erik has pulled off the neat trick of producing a hull form that will be great off the wind too.

To explain why this makes sense, let me digress for a moment. A few weeks ago I was talking yacht design with Steve Dashew and he said something that had me scratching my head:

One of your most important design decisions will be what speed the boat will start motoring at.

And, if you think about it, that makes perfect sense because, if you are, like most cruisers, going to start motoring when things get slow, well, you don’t need to worry about hull optimization for speeds under that threshold.

And he went on to point out that even if you are a purist (or short on fuel) the light air efficiency hit you will take in a hull optimized for higher speeds is in the order of 5%. And for a 40′ boat doing three knots over a 24 hour day, that’s just 3.6 miles–hardly significant. But, on the other hand, if you are in the trades reeling off the miles—where we all want to be—optimizing for higher speeds (in comfort) will get you into port at least a day or three earlier on a typical trans-Atlantic.

Not only that, the hull optimized for higher speeds will steer better when the breeze is up, particularly off the wind in big seas, and said hull will be more efficient at cruise speed under power. One small trade off, many benefits.

By the way, if you are wondering why you can’t optimize for all conditions, the reason is that at low speed it’s skin friction that slows a boat down and at higher speeds it is wave resistance, and you can’t draw a hull that minimizes both perfectly, so we went for the lowest wave resistance.

A Hull That Can Heel

If Erik were designing a motor boat, he would be all done after the above. But sailboats have a major complication that motor boats don’t: they heel. So, not only did Erik come up with a hull form that will work well when flat, but also at 5, 10, 15 and 20 degrees of heel.

I won’t burden you with all the technical details, but this boils down to making sure that the Longitudinal Center of Flotation (LCF) and Longitudinal Center of Buoyancy (LCB) don’t move around as the boat heels because if they do, really bad things happen, like poor steering and scary broaches.

Erik will be putting even more work into this area during the final design process using sophisticated hydrostatic analysis software.

And Carry a Load

There is another important attribute that Erik designed into the hull, and that is the ability to carry high loads, relative to her displacement, without significant performance problems, unlike lighter fat-sterned boats that may outperform the Adventure 40 off the wind when lightly loaded, but will become difficult to steer dogs when fully loaded with cruising gear.

The Right Shape at The Ends

Iso

Erik has drawn a lean, mean, sailing machine. But also a boat with enough beam and form stability to be comfortable,

All of this results in a boat that is relatively narrow and fine at the ends with the following trade-offs against a wider, blunter boat:

Benefits

  • Less strain on the boat, the crew and the rig.
  • Requires a smaller, less expensive and more easily handled rig.
  • Higher speeds in more comfort, particularly with the wind forward of the beam.
  • Less slowing down and smaller leeway angles after being hit by a wave while sailing upwind.
  • A drier cockpit.
  • Much less tendency to pound when going up wind.
  • Easy steering at all reasonable angles of heel.
  • No tendency to bury the bow as the boat heels.
  • The same hull volume for a given displacement.

Disadvantages:

  • A little more water over the bow area of the boat.
  • Less hull volume in the extremities of the vessel. There goes those two aft cabins.
  • Slightly slower speeds when running off in big breeze.

Keel

Sections

This is where the tradeoffs get hard. There is simply no getting away from one basic fact: the deeper a boat’s draft, the better she sails when the wind goes forward—faster and lower leeway angles. But, on the other hand, every cruiser wants the shallowest draft they can get. After much agonizing Erik and I settled on six feet (1.82 m).

Based on Phyllis’ and my experience over some two decades and conversations with other experienced cruisers, six feet is a sweet spot. Go shallower on a boat this size, say to 5 feet, and the performance hit is huge but the cruising grounds really don’t open up that much. Go deeper, to say 7 feet, and a lot of doors start to close.

And, if you really want to open up shallow water cruising, you need a draft of about 3 feet and the ability to dry out. In other words, you need an Ovni or a Boréal–great boats but with a very different mission than the Adventure 40.

Erik has gone for a modern keel profile, with much of the weight concentrated at the bottom, while still designing a foil that will forgive steering errors without stalling, and that can be joined to the hull with a large enough area to absorb hard groundings without structural damage.

He has kept the lateral area as small as possible, without losing the ability to resist against leeway at the speeds that are expected to be achieved under sail. This also helps reduce wetted surface, which helps in light air and decreases the chances of a knock-down when hit by a wave in severe weather. I know the last is counter intuitive but, believe me, it’s true. AAC engineering correspondent Matt Marsh has a great explanation of why over at his site.

Rudder

Erik has gone with a partially balanced outboard hung rudder, turning in a slot in the sugar-scoop stern, a decision that yields many advantages, including:

  • No rudder shaft. At one point we had decided on using carbon fibre for the shaft to get rid of the maintenance problems caused by metal rudder shafts in composite blades, but now the need for a shaft has gone away completely, which will save money we can use in other places.
  • Easier to inspect and repair.

Erik has not yet done the engineering on the rudder, but he is looking at “fusing” it so that it will kick up, rather than break, if hit and/or making the blade comparatively easy to replace at sea, using a cassette type head.

The rudder may look small to you, but that is simply because a boat designed to steer easily like the Adventure 40 just does not need the huge rudders required by boats with shapes driven by interior volume rather than sailing ability.

The Numbers

 Metric  Imperial
Displacement (light ship)  8.00 t  17,600.00 lbs
Payload  2.00 t  4,400.00 lbs
WL Length  11.57 m  37.96 ft
Length Hull  12.72 m  41.73 ft
Beam  3.67 m  12.04 ft
Draft  1.82 m  5.97 ft
Immersed depth  0.59 m  1.95 ft
Prismatic coeff. (Cp)  0.54  0.54
Waterpl. area coeff. (Cwp)  0.66  0.66
Length:Beam ratio  3.79  3.79
Beam:Draft ratio  5.15  5.15

 

As you can see, although we are just about right on our original target displacement, in this current iteration of the design, the hull length has grown to nearly 42 feet. This change, while surprisingly inexpensive, has yielded all kinds of benefits that will become obvious when we go on to look at the rig, deck and interior arrangement.

Please keep in mind that these numbers are preliminary and subject to change, as we explain here.

What All This Buys You

Let’s just daydream for a moment and put all of this in context:

It’s the fall and we are three and a half days out of Newport bound for Bermuda. We left in company with some friends in a light wide boat with a very wide stern. It has been a broad reach most of the way, so our large tooshed friend has actually pulled away from us a bit and is now just visible on the horizon ahead of us.

But now clouds are building and the wind has been veering all day and finally settles into the southeast, blowing 20 knots true, gusting higher, and quickly building a nasty sea, as the low pressure area to our north drags a warm front over us.

Bermuda is now dead to windward and still 100 miles away. We strap the boat down, trim the vane gear, and shelter under the dodger, as Erik’s hull form starts to show that this is exactly the conditions he was thinking about when he drew her.

A few hours after the shift we pass our friends. Their boat is staggering and rounding up in every gust and being stopped dead and knocked off the wind by every wave. They are barely making 2 knots good toward Bermuda as we romp by, pointing high with a velocity made good of better than double that toward the St. Georges Dinghy Club bar.

We chat on the VHF and they tell us that their lightly built boat is flexing badly and so every hatch and port is leaking, turning the interior into a sodden mess. And, to further add to their misery, the erratic behaviour of the boat is making it impossible to use their vane gear or autopilot, so they are hand steering and getting soaked by every wave while doing it.

There is nothing we can do for them and soon their sails disappear in a rain squall behind us.

114_1450

A little less than a day later, aided by a bit of tactical cunning playing the shifts, we beat through Town Cut just for the fun of it (don’t try this if you don’t have a Bermudian aboard who knows the waters) and tie up to the Bermuda Customs dock, four and a half days out of Newport.

Four days later our friends are towed in looking battered and bedraggled. Apparently, the slamming got so bad that they were forced to heave-to for a day, which let a secondary low that formed on the front catch them with its associated gale force winds. That resulted in two more days heaved-to before the wind finally veered into the NW and let them make Bermuda. At least the wind didn’t die completely, as it often does after frontal passage, because the water that was getting into their boat sloshed out of the shallow bilges and flooded all the electrics, taking out the engine.

Just a day dream? Hang out at the customs dock in Bermuda in the fall and you will see many realities that look just like the experience of our imaginary friends.

Next up, in a few weeks, we will take a look at the deck layout and rig that Erik has designed.

Sign Up

The current plan is to build the Adventure 40 in batches of 10 boats. At the time of writing, we have 150 people signed up as interested in the boat and 86 who say they are ready to buy a boat. If you are interested in an Adventure 40, get your name on the list now.

Comments

If you have questions suggestions, please leave a comment. Erik is at sea, on his way to Greenland, but I am in communication with him via satellite email, so if your question is beyond my ability to answer, I can pass it on to him, although it may take a day or so to get his answer and post it.

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{ 136 comments… add one }

  • Bill Attwood July 25, 2014, 3:17 pm

    Hi John.
    I find it difficult to understand the insistence on a below deck autopilot, and wonder if the people who are such strong advocates for them have tried the alternative. I sail a Rustler 36, a long-keeled 1980´s design from Holman and Pye. I have converted from wheel to tiller steering – Rustler Yachts still offer both options. My steering systems consist in: Windpilot Pacific; small Tillerpilot rigged in place of vane when necessary for coastal sailing or under motor; large Tillerpilot connected direct to tiller for rough conditions when for any reason the Windpilot cannot be used – I see this mainly as a back-up for the Windpilot if it becomes non-operational. The large tillerpilot has only the ram mounted in the cockpit, with all electronics and fluxgate compass mounted below decks.
    The price of the small Tillerpilots is such that one could easily buy a spare as back-up. I have used all these systems extensively and can report that they function really well. If they can work for a Rustler, then they should find the A40 no problem. I would welcome a summary of the pros and cons of a below-deck autopilot from the advocates, particularly the con (as I see it) of a system which is hard to get at and therefore won´t be properly checked and maintained.
    Yours aye,
    Bill

    Reply
    • John July 26, 2014, 9:09 am

      Hi Bill,

      That all makes sense to me.

      To answer your question about the pro of an under deck autopilot, on a boat under about 12 metric tons it is purely a convenience item when measured against a vane gear. I think they have become as popular as they are because they, to a large extent, obviate the need to really understand how to balance the boat properly with the sails, a requirement when using a vane gear–no fiddling around, just push the on button.

      On boats over about 20 metric tons I think said pilots become more a need. We have one on our boat and would not be without it, but she is 22 metric tons light ship and really on the upper end of the loads that are practical for a vane gear. Having said that we have a vane gear and it has sailed us for many thousands of miles, including a tran-Atlantic.

      On the cons side of underdeck autopilots, I think what people miss is real expense of one. A good one, well installed, is going to run >US$5000, and as much as $10,000 if all the work is done by a yard. But that’s just the start. These things are power hogs and there is no practical way to feed them using solar and/or wind power. So that means a diesel and/or hydro generator, unless you are willing to run the main engine for several hours a day to charge. Doing a full underdeck autopilot and the support systems properly can easily run you US$15,000-20,0000. That 5%-10% of the cost of the A40…for a convenience item!

      Reply
  • RDE July 26, 2014, 9:57 am

    Hey guys,
    Let’s think just a little bit outside of the box.

    1- Below deck autopilot installations need cost $15-20,000? Certainly if they are the usual suspects we are familiar with—power hungry machines necessary to control a large boat like MC.

    On the other had, the Cape Horn wind vane was designed for exactly that purpose. Below decks addition of a small tiller pilot for use while motoring. Because it drives the servo rudder which in turn provides the steering force it uses almost no power. Here is a photo (third one down) of just such an installation. http://www.cruisersforum.com/forums/f13/tiller-pilot-on-windvane-98164.html

    Combine the Cape Horn with a Raymarine Evolution (http://www.raymarine.com/view/?id=7008) with tiller pilot ram for little more than $1,000 and you have a low power draw electric autopilot with relatively sophisticated electronics, remote control, and all components mounted in weather protected locations.

    2- “Transom mounted rudders are incomparable with wind vanes or below deck autopilots. ” We need to stop visualizing a Westsail 32 rudder and envision a design suitable for a boat like the A40-42. Vertical transom inside of a sugar scoop swim platform. Large diameter tube rudder shaft down to the water line held in place by two saddle style split bearings on the outside of the transom. Rapid removal with only four bolts. Pie shaped cut-out in the swim step, again removable with only four bolts. Full end plate effect. Only a couple of inches of offset for the Cape Horn windvane horizontal shaft. Rudder removable in minutes while in the water using main boom and halyard for lifting.

    Reply
    • John July 26, 2014, 11:48 am

      Hi Richard,

      I absolutely agree. With a boat like the A40 we can come up with an innovative, elegant and inexpensive self-steering system without resorting to the kind of under deck power gobbler that many cruising boats have defaulted to. I expect that experimenting and perfecting self steering will be one of the larger parts of the prototyping process (fun and interesting too) and that we will end up with something very like what you suggest.

      Having said that, the point I’m trying to clearly explain here is that an electronic autopilot that steers at all times when offshore is a big and expensive beast that will cost the kind of numbers I’m talking about. As soon as one changes ones thinking to say that a vane gear will steer when the wind and waves are up, most all those costs go away.

      Reply
  • Dick Stevenson July 26, 2014, 5:13 pm

    Dear John,
    You have made a number of comments about below decks autopilots being amp hogs and the how the subsequent cost of producing those amps must be added to the overall cost of the autopilot. I have no experience with the autopilots you are usually referring to and suspect they are the amp hogs you report. I do have 13+ years of living full time and wandering widely with a below decks autopilot. It is an Alpha 3000 which is known for being miserly with amps as well as its other attributes. It is hard to be accurate with how many amps are actually used: conditions vary dramatically and we are always running with much other equipment going on & off at intervals. That said, at no time have I experienced the Alpha as being a significant drain on the batteries. Having a big freezer more or less demands a genset, so we can’t say that the Alpha could be run on an ocean crossing with solar and/or wind generators, but I would not be at all surprised if it could (given a well balanced boat).
    The above in no way is meant to suggest a below decks autopilot for the A40. I think your reasoning is very solid in that regard and the anticipated alternatives more than adequate.
    Speaking of boat balance, what you say about below deck autopilots and boat balance bears repeating. Modern day autopilots are so powerful that a boat whose sails are out of balance may not exhibit the kind of symptoms that would lead a skipper to adjust things so his helm was eased. This will lead to a geometric increase in amps consumed and, it has been suggested, to rudder damage (especially if the rudder was not well designed, old or suffered a previous knock). A veteran observer of the ARC and other rallies once commented to me that he has observed a rise in rudder damage with an increase in autopilot use. He went on to say that inexperienced groups go out, sail their boat unbalanced with the autopilot sawing away. A week or 2 later there is rudder damage and the blame goes to a random whale they must have hit and they remain unaware of the abuse they have been delivering to their rudder.
    Every vessel with a wheel should have a Turks head indicating rudder amidships and practice keeping that knot in an area indicating just a bit of weather helm. Then, when underway, have it be part of the watches regular rotation of observations to check on where that knot is spending its time.
    My best, Dick Stevenson, s/v Alchemy

    Reply
    • John July 26, 2014, 5:49 pm

      Hi Dick,

      All good points as always. When I say “amp hog” one does need to put that in the perspective of the boat: Amp hog on your Valiant 42 with big batteries, big alternator, and generator? Maybe not, although I think that if you actually accounted for all the amps consumed by your autopilot over 24 hours steering down wind in trade wind swells you might be surprised. I have conducted this experiment on a trans-Atlantic and the results were eye popping, and this on our boat that tracks as if she were on rails. (We turned everything else off for a full watch and tracked amps and if memory serves the power use of the autopilot alone was in the order of 100 amp hours/day.)

      Now even if you half that for the Adventure 40 to 50 amp hours a day you are going to exceed the available solar and probably wind power after taking into account other loads like electronics, nav lights etc. Yes, this problem is solvable in many ways, but to me the way that fits best with the A40 ethos of simple and elegant is just don’t have the large autopilot and use a vane gear.

      I 100% agree that autopilots promote poor sail trim. Woe betide the watch stander on Morgan’s Cloud if the king spoke is a long way from 12 o’clock when the skipper comes on deck!

      Reply
    • John July 26, 2014, 5:55 pm

      Hi Dick,

      I just checked the specs on the Alpha 3000 autopilot. They say “input Current 15amps max. 2-3amps typical cruising on a well balanced, well sailed vessel.” That means 48 amp/hours a day at the very best. I would stick with my estimate that down wind in the trade winds the daily load would be close to 100 amp hours a day and certainly no less than 75.

      Reply
  • patricksynge July 26, 2014, 7:41 pm

    I agree that reliable and powerful autopilots can be relatively expensive to purchase, install and use.
    But despite this I would no longer enjoy shorthanded, long distance, sailing without one – and the costs can be minimised.

    Having a reliable pilot to take over the steering in any situation is like having another crew member on board. There is no way any wind vane is going to be of much help when you are caught in a sudden squall with too much sail up and something jammed. And this is exactly the kind of thing that happens – usually in the middle of the night. Having ocean sailed extensively with 2 young children aboard my wife and I know how fatigue can sap you. And fatigue is one of the greatest dangers.

    I have previously mentioned the simple hydro generator I built from easily available components. It provides at least twice the current required for a powerful autopilot and only cost a few hundred dollars in components. Keep the sails trimmed and reduce canvas reasonably early and a correctly adjusted autopilot doesn’t use that much power anyway. Many people seem to forget that pilots have sensitivity settings and don’t adjust these to suit the conditions.

    Autopilots need not be that expensive either if you keep your eyes on ebay and the classifieds. Over the years I have purchased a “failsafe” collection of Coursemaster units comprising 3 course computers, 3 rudders sensors, 2 compasses and 2 drive units, 3 control heads. All in good second hand condition and easily interchangeable. Total cost $4000. Yes, I like autopilots!

    Reply
    • John July 27, 2014, 10:52 am

      Hi Patrick,

      I agree that powerful underdeck autopilots have many advantages. After all, I have one.

      However, what we are talking about here is the Adventure 40 where we must stay simple to achieve the goals for the boat. Otherwise we are just going to end up with yet another $400,000 boat, or alternatively a poorly built boat because we will be robbing money from construction to fund convenience items.

      There are plenty of other ways to deal with a squall at sea or resting, including heaving-to, to name just one. The bottom line is that people cruised successfully short handed for decades before the advent of any kind of autopilot. Also, a good vane gear will be perfectly capable of steering an A40 in extreme conditions.

      There is even a pretty solid argument (see Dick’s comment) that clicking in a powerful underdeck autopilot “when you are caught in a sudden squall with too much sail up and something jammed” is not a good idea because it can exert huge force on the steering gear and maybe kick out or break something at the worst possible time.

      As to current use. The numbers I quoted in answer to Dick’s comment assume good trim and proper set up. There is simply no way to get away from the fact that steering with electricity at sea uses a lot of electricity. Yes, there are ways to make that electricity, but all of them add complication and expense.

      Reply

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