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.
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
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:
- 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.
- 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.
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.
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.
|Displacement (light ship)||8.00||t||17,600.00||lbs|
|Prismatic coeff. (Cp)||0.54||0.54|
|Waterpl. area coeff. (Cwp)||0.66||0.66|
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.
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.
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.