The Case For the Pacific Proa by John DalzielAn essay posted to the Multihull Mail List
Subject: [MHml] Re: really weird proas Date: Fri, 15 May 1998 14:08:32 +0000 From: John Dalziel <firstname.lastname@example.org> To: email@example.com
John Dalziel <firstname.lastname@example.org> wrote:
> > If we are *not* dealing with a speed week flyer, then the adjustability of the windward (small) foil seems to be the main feature. On a proa, there are a great number of advantages to concentrating the weights in the hull.
> Just what are those advantages, John? I understand the pluses of keeping the rig on the leeward hull, but keeping the weight there is not so obvious. In fact, Joseph Norwood promotes placing all the weight and accommodations, etc. in the windward hull, with the rig in the leeward one.
I think Norwood is wrong about that. In the first place, it is comparatively rare for a boat to be sailed at its maximum stability point, and this is the *only* time that the "all weight to windward" is to your advantage. So the rest of the time- which means most upwind travel, but also effectively includes all reaching and running- you are simply adding to your drag by keeping the weight to windward- since you have to keep it from sinking even when it is not needed for stability, you've added skin friction and wave drag. You have also added structural weight, in spades. Even with the rig on the lee hull, the weight of the windward hull will have the same effect on the structure as it does on an Atlantic proa or catamaran. Though the lee hull is fairly light, it will still have considerable skin friction, even when not pressed, for it must be designed for full immersion and have underbody lines to match. It will also be considerably heavier than a comparable log on an equal-sized Pacific canoe, and will need to be structurally able to resist the strains of the backstay and mast(s).
Motion will be much worse on a boat with a strong windward weight bias, since the heavy windward hull, intercepting every wave and reacting ponderously to it, will always be attempting to pitch the lee bow up or down. Roll in a beam sea will be as bad or worse than a catamaran's. Sail efficiency will be a lot lower as well, as the boat is rolling the sail back and forth through the air with every wave, making trim a sometime thing. It is also impossible as a routine matter to fly the heavy hull for cleaner air flow to the sail and less hydro drag.
You've also added considerable torque against which the rudders must fight. With the sail over the lee hull, and the center of hydrodynamic drag quite a ways to windward near the heavy hull, the sails are acting at a large distance from that drag location. So the rudders must be much larger in order to counteract that- adding drag, complication, and expense. In this respect the Atlantic proa is superior, since the force lines from the sail match up pretty well with the centerline of the boat.
Another difficulty with the windward-weighted arrangement is that during lulls, the boat will spin sharply to windward due to the drag caused by the heavy hull. This can lead to speed-wasting (and irritating) backwinding. And if you are caught out in heavy weather, the boat can yaw, and the wind forces on the large hull can now drive the boat into a backwind. And once you are backwinded it is much easier to capsize, since your weight bias now works against you.
Let's take a look at Pacific canoes for a moment- and the first thing we see is that _every_ damn one of them violates the "principle" of keeping the maximum amount of weight to windward! NO Pacific proa has the heavy hull to windward. In fact, the weight of the logs on most of the canoes is almost negligible. WAAN AELON KEIN (in multihulls mag proa issue) is a good example, and typical. In Haddon & Hornell, the largest log I could find, and that on a 45'+ boat, was about 350#. So the "all weight to windward" theory is simply not practiced ANYWHERE in the Pacific.
Now for some of the specific advantages of the traditional Pacific arrangement: In the first place, for a given weight of structure you have the longest waterline possible, and you have that advantage always, beating, reaching or running. You have the least stress on the structure, and consequently the lightest structure; hence, least cost, smaller sails, lighter scantlings, lighter rigging, lower crew demands, etc.
You have the boat hydrodynamically arranged for the least drag in nearly all conditions. The boat has a smooth motion with little pitching, and roll is dampened sharply by the weight of the mast. Beam seas don't rock the boat nearly as fast, since the log can rise above the water and then back into and under it in a long, slow cycle. And under way the boat can be trimmed easily with the log just skimming the surface, allowing the sails to stay still and provide maximum power.
Marshal Isles Proas (270K)
OK, how's it done? The answer in the Pacific has been: a light log with very nearly neutral buoyancy, movable ballast (crew) sent out onto the platform as necessary, and the lifting rig.
If you examine the lateral overturinng forces from a sailboat rig, you see that they begin to fall off as the boat does; in other words, when the boat is hard on the wind the overturning force is greatest, and as you point off it diminishes as a function of how far you point off. So the only time that a sailboat is really in danger of a wind-driven capsize over the beam (ignoring pitchpoling for the moment) is when beating. At some point, around a close reach, ballast to windward becomes little needed.
If we study a crab claw closely (the Taylor rig you are using will show the same behaviour), we see immediately that it is a lifting rig, and also that its lift _increases_ as the boat falls off from a tight beat. So in other words, overturning force from this arrangement drops _much_ earlier, in terms of pointing angle, than it does from a traditional Western rig. So the pointing angles at which ballast, kept to windward, is actually needed for sailing to windward are reduced to those only found on a very tight beat, and even then the amount of ballast needed has dropped substantially.
In normal conditions the Pacific Islanders would provide that ballast on tight beats by moving out onto the platform, and as the boat fell off they would move back to the hull or onto the counterpoise. That's all it took. Now: suppose that a big wind comes up and you must beat to windward. This would be the only time that the "all weight to windward" design would have worked in your favor - or would it? Think: what do you do when beating in high winds; you fall off a bit, don't you? That way the waves don't stop you so much and overall you will make better progress than if you had tried to punch it out on a tight beat. But when the Pacific canoe, equipped with the crab claw, does this, overturning force nearly disappears! So all that weight to windward isn't even helpful then, either.
Moving to modern canoes, there is a large financial price to pay in having two full hulls, but only having storage and accommodation in one of them. But this is what you have under Norwood's prescription. However, if you design along the lines of the Pacific peoples, you can get all the advantages of their boats at relatively little cost. The only arrangement you must make would be to add some sort of movable ballast for those tight beats in moderate weather (in light weather, the necessary weight of structure is plenty of ballast, and in heavy weather you will be bearing off a bit...), and if the boat is too big for crew ballast, or is sailed shorthanded, then water ballast could be used, or some sort of sliding ballast like a chain locker on tracks could be used. Water seems simpler to me.
The modern Western sailor also imposes some requirements on the boat that the Oceanic people do not, so I don't think a slavish copy of the originals will serve particularly well. But the general pattern of these boats will. It is one thing to specify a certain design for a boat that is only intended to go one speed under a specific condition, like during speed week at 55 knots. Since nothing else matters you can design specifically for that one, top speed. But if the boat must be used in all sorts of conditions, it will pay big benefits to come up with a boat that will handle the vast majority of those conditions in a competent way, even at the expense of that last dollop of speed during speed week. This is what the traditional Pacific proa does well, and what a proa with "all weight to windward" does not.
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