A better proa hull?
Discovered an interesting thing about hull shapes. This is apparently the original patent for Jim Brown’s trimaran which became the WindRider 16 (not the 17, which is completely different): http://goo.gl/65pLeP We can discuss the advantages and shortcomings of this boat as a production model, but it’s the fundamental hull shape concept I’d like to draw your attention to. Excerpted from the patent abstract:
“The resultant boat hull… has an upper hull portion which is identical to the lower hull portion. The forebody… is identical to the afterbody except that one is rotated 90 degrees with respect to the other about the longitudinal axis of the hull…The vessel has a… bow which creates an asymmetric water plane even though the forebody and afterbody are conceptually identical. The asymmetric water plane reduces pitching of the hull in rough water.”
So what? Multihulls hobby horse. It’s a matter of long thin hulls in waves. Hobby horsing is not only uncomfortable, it detrimentally effects the flow of wind across the sails. Recently (much more recently than this patent, which explains it in detail), designers have “discovered” that designing with a marked asymmetric waterplane—for instance narrow and deep forward, wide and shallow aft—can significantly reduce hobby horsing, and also reduce “squatting” at the stern at high speed; both very good things.
Unfortunately, asymmetrical waterlines aren’t available to proa designers, and our boats suffer for it. Watch vids of proas under sail and you’ll see the problem (cf Jzerro, others See for example: https://www.youtube.com/watch?v=HevgDyupZeY from 3:30 – 3:50). Building the hulls longer helps and adding overhanging ends helps even more, but though it yields healthy high L/B ratios, long thin boats aren’t preferred by actual builders and owners of, especially, small proas.
Enter Jim Brown (again! The man just won’t stop innovating) offering us a hull design that not only features an asymmetric waterplane, it does so in hulls of any L/B ratio we’d care to name. It does this by (virtually) cutting a symmetrical hull in half and rotating the after half-hull 90 degrees. The hull is quadrilaterally identical; it can be built of 4 identical quarter-hulls, as can many proa hulls.
It’s a generally very healthy shape and offers, within its severe design constraint, a huge family of shape options. The center section can be round as shown. It can also be square, or square with rounded corners, even triangular if we rotate it 120 degrees rather than 90. More esoteric mid sections are possible—cruciform, star-shaped, on and on. We have complete control of block and prismatic coefficients and the bow profile can be semi-circular as shown, or straight plum, or even hollow.
The challenge, of course, is how to get that 90 degree rotation at the shunt. I can think of lots of bad ways to do it, but the concept deserves an elegant and beautiful solution. There are some great minds who post here. What would you do?
Difficult to do without moving parts, but perhaps this is a workaround.
Difficult to do without moving parts, but perhaps this is a workaround.
Dang. So simple! Now all you need to do is to fly that windward ama—all the time. 😉 Seriously, I’d never have thought of this. Cool idea for a “proa”!
Dave
I call it the M. C. Escher proa, because it only appears to work at first glance.
A hull like that can be folded up out of flat sheet using four quarters with the join for the forward half hull being horizontal and the aft half having the join vertical.
Flipshunter?
A hull like that can be folded up out of flat sheet using four quarters with the join for the forward half hull being horizontal and the aft half having the join vertical.
Yes, I believe it can, Jim. Though you’d have to give up a fair amount of control over the shape—I think a single sheeter will, for instance, force you into a too-low prismatic (not enough buoyancy near the ends of the hull)
You might actually do better to make it dead square in cross section. Now each hull half is four flat pieces of ply—and the fatness/fineness of every inch of the boat is under your control.
Naw, I don’t like square sections either, but does imagining them help to visualize the control of the hull shape the designer can/cannot achieve? Frankly, I don’t think the circular/elliptical sections are attractive—it should be a round-cornered square, with pretty big radiuses in the rounds. This would not only look more “wholesome” to my eye, it would get rid of those hideous circular leading and trailing edges. 😉
Hey, I just noticed that the Editor’s hulls don’t actually switch ends at all—each hull is utilized for one direction of travel only—meaning it ought to be possible to build both hulls optimized for running in one direction only—like any other multihull except for proas—and just mount them one facing forward, one facing aft. Not sure if that is the coolest thing ever—or that it sucks! 😉
Dave
I played around with these sort of shapes a lot a few years ago and found it surprising how full the ends could be made. You need a pretty floppy material though so the best way may be to fold up a thinish sheet of fibreglass to make a mould. The bow profile can be made any shape you want from semi circular to square. There is a tendency for the keel line to hog, though. But that can be got around.
A hull like this would suit a podcat very well. Position the cabin aft so the forward half of the hull is inclined a degree or two and you have rocker effectively. 😊
Surely it works, but one direction is Pacific style, the other Atlantic?
Or the hull could be rotated 90 degrees around its axis on the shunt.
While you’re at it, add two simple spade rudders. The forward one is horizontal and can be used as a lifting foil.
Sorry our revered editor, but I do not see how that shape can be used most efficiently…
From the opening post, my understanding is that the vertical end has to be the bow, and the horizontal end has to be the stern to be hydrodynamically most effective.
In the case of your boat concept, that means that the yellow end has to be the bow. Since going in one direction one hull has the “right” bow and the other has the “wrong” bow, I assume that it should be the most “loaded” hull that has the right bow; i.e. the lee hull.
Right?
In that case, this boat ends up being a port tack optimized boat only… in both directions!!!
If you visualize the boat on a starboard tack, in either direction, it is the horizontal end which will be the bow of the leehull, digging into the waves… I would not want to have that hull nose dive; with its shape, it would have a very hard time to “pop up” before pitchpoling…
Don’t you think so?
What did I miss?
Cheers,
Laurent
What did I miss?
Cheers,
Laurent
You may have missed post #3, by the Editor:
I call it the M. C. Escher proa, because it only appears to work at first glance.
fooled me, too! 😉
In that case, this boat ends up being a port tack optimized boat only… in both directions!!!
If you visualize the boat on a starboard tack, in either direction, it is the horizontal end which will be the bow of the leehull, digging into the waves… I would not want to have that hull nose dive; with its shape, it would have a very hard time to “pop up” before pitchpoling…Don’t you think so?
What did I miss?
I might be a moron but I don’t see the problem. You simply need to move the rig to the lee hull for each tack, and always fly the windward hull.
So first of all, I think that “simply” moving the rig to the lee hull might be over… simplifying, don’t you think?
Secondly, it does not work.
It is not on which hull the rig is which determines the leehull; it is the direction of the wind, in relation to the craft. If you have the rig on the leehull, it is a Pacific proa, if you have it on the windward hull, it is an Atlantic proa (let’s put aside for now the “And what is a Harryproa?” debate, please…)
Let’s look at the picture on post #1.
Le’t imagine that as an observer, the wind is coming from behind us and “blowing into the picture”, if I may say.
In that case, the leehull is the one that is further away from us…
If I am on staboard tack, that means that the boat is moving towards the right of the picture; it is the blue end of the leehull that is the bow…
Well, maybe that is because we took the wind from the wrong side of the boat… So let’s do the opposite. Let’s imagine the wind blowing along the red thin line on the paper, towards us, this time, the wind is blowing out of the picture, into our face. So the leehull in that case is the one closest to us. If we are sailing on Staboard tack again, that means that the boat is moving towards the left of the picture… And once again, it is the blue end of the lee hull that is the bow…
I think that on a geometry point of view, this is because this boat does not have any vertical plan of symetry. It does not have a longitudinal plan of symetry like most boats (monohulls, catamaran, trimaran), but it does not have a transverse plan of symetry either! like a (shunting) proa.
It only has a vertical axis of symetry.
It is like the N letter or the Z letter, you can turn then 180 degrees, they still look the same. Not like a B or a K ....
The only way I can see make this work, without rotating hulls along the longitudinal axis that you “dial” by a quarter turn on each shunt (with both hulls with the same bow configuration…)... is to… (ready for this?...)
flip the boat upside down on each shunt: what was the underwater bottom part becomes the deck top part and vice versa…
This can simply be done by an intentional pitchpoling at each shunt and a rig that can be swapped from turtled underwater to “on the other side right side up”...
Not a shallow water boat, though…
😉
Unfortunately, asymmetrical waterlines aren’t available to proa designers, and our boats suffer for it. Watch vids of proas under sail and you’ll see the problem (cf Jzerro, others See for example: https://www.youtube.com/watch?v=HevgDyupZeY from 3:30 – 3:50). Building the hulls longer helps and adding overhanging ends helps even more, but though it yields healthy high L/B ratios, long thin boats aren’t preferred by actual builders and owners of, especially, small proas.
The “overhanging ends” bit is what I have trouble with. They help in theory, but in practice they are the worst way to utilize the length of a boat. Overhangs are only useful for two reasons:
1) When setting and retrieving anchors, the overhang prevents banging the anchor against the hull.
2) When well-intentioned folk decide to base a racing rule on waterline length instead of length over-all, overhangs can be used to stretch the waterline once the boat is heeled over a substantial amount. This led to dangerous “rule cheating” boats and then rule changes that made such boats obsolete. That “heeled over a substantial amount” bit should make any multihuller recognize the uselessness in boats intended to sail relatively flat.
People will argue that “as the end is depressed, more volume is presented to reduce further depression” and that argument is technically correct. However, the volume only increases as the end is depressed, which is what we wanted to prevent from happening in the first place. What is all that space *under* the overhang doing? Nothing! So what you really have is unsupported weight hanging off the front of the boat *contributing to the problem it is intended to alleviate*. On a cruising boat this problem is compounded as that unsupported area is filled with chain and rope and topped with a big steel anchor.
Let’s take that same bow and now make it vertical, at least down to the waterline, if not right into the water. You now have a longer (faster) waterline even when sailing flat, and the buoyancy at the ends of the boat is vastly greater, and more importantly, lower- where it will start resisting depression immediately and with more leverage than the overhanging bow. The cruisers will want to put a short bowsprit to get that anchor away from the hull, but at least the chain and rope will have some buoyancy directly under them now.
Taking this to a bit of an extreme are “inverted” bows. These are also called “wave piercing” with good reason: they will stick into a wave and redirect the rest of the hull upwards. They are wider below than above and present a sharp edge to the water even as they dive under, allowing them to cut back to the surface. There is no deck on the very front of these boats- no place to mount an anchor there. They’re great for multihulls that anchor from the crossbeams, except they throw a *lot* of spray!
Divide the hull into thirds. Let the middle section be circular parallel sided. Arrange the ends to rotate together concentrically with the center section. Attach rig and ama to center section.
This fits well with all the people that like take a part hulls.
Skip
p.s. I think that high prismatic dory style hulls with low moments of inertia are a better solution though it’s hard to say the last part with a straight face with BB33 on the screen.
p.s.s. Doing something like this out of large diameter PVC pipe should be dead bone simple, if you have a big enough heat source.