What’s a Prow to do?
It occurs to me that, in a serious knockdown, a high-prowed hull—like a traditional proa—brings significant reserve righting moment into the game, as heel nears 90 degrees. I hadn’t considered this before, but wonder whether the ancients may have known a thing or two?
This may be superior to a windward pod in that it’s never immersed until it’s actually needed. When it hits the water, overturning moment will have abated, since the rig is nearly flat in the water; thus the actual buoyancy contained can be relatively small. Because it only comes into play at extreme angles of heel, the crew are free to drive the boat hard and to fly the ama very high, knowing 1) the reserve isn’t going to trip them and 2) it *will* be there, if things get out of hand. Both attributes might lead to driving the boat harder yet safer.
Such a prow would be more useful on a bow than a stern, to stop diagonal pitchpole. Again, the ancients may have known this—tacking outriggers often having the prow only at the bow.
Can it be improved? I’m leery of playing with a feature I may not fully understand, but what if the prow were rotated around the longitudinal axis of the vaka, ~30 degrees to leeward? This would put its reserve into the water at 60 degrees heel and would be at max righting moment around 80-90 as it is completely immersed.
There are other things to like about prows; they bring huge anti-pitchpole buoyancy into play as they’re driven hard—but without wetted surface penalty when not needed—and without the high aerodynamic penalty modern “stacked” buoyant bows suffer.
Dave Culp
Very interesting thought Dave.
This is a lot like what Pizzey was thinking about with his knock down recovery system. You go *almost* over but are at some very high angle there aren’t a lot of things pushing you anymore…except the ama, alas and whatever platform you have on the ww side.
But I’ve noticed something sailing my tacking outrigger—a lot of the stability of the boat is dynamic, not static. Its not JUST a free body diagram of a light thing on a long stick. When that light thing on the long stick starts moving up (rotating around the CB) it becomes an energy storage device. A stone age flywheel. That energy that made the boat roll has to be redistributed. And the sooner you do it, the better, I think.
BTW, what do you think of Pizzey’s other secret sauce—leaning the rig to leeward for a more rapid reduction in sail area as the boat rolls?
Also, maybe the prows are just to cut through waves so that they don’t push the bow down as much—as on Hawiian canoes?
Chris
It occurs to me that, in a serious knockdown, a high-prowed hull—like a traditional proa—brings significant reserve righting moment into the game, as heel nears 90 degrees. I hadn’t considered this before, but wonder whether the ancients may have known a thing or two?’‘’‘’‘'This may be superior to a windward pod in that it’s never immersed until it’s actually needed. When it hits the water, overturning moment will have abated, since the rig is nearly flat in the water; thus the actual buoyancy contained can be relatively small. Because it only comes into play at extreme angles of heel, the crew are free to drive the boat hard and to fly the ama very high, knowing 1) the reserve isn’t going to trip them and 2) it *will* be there, if things get out of hand. Both attributes might lead to driving the boat harder yet safer….....There are other things to like about prows; they bring huge anti-pitchpole buoyancy into play as they’re driven hard—but without wetted surface penalty when not needed—and without the high aerodynamic penalty modern “stacked” buoyant bows suffer.
Dave,
When any boat gets to 80 -90 degrees heel, unless it has shipped a lot of water the prow won’t be anywhere the waterplane, so I can’t see how you idea is going to work. By the time any boat get much past 45 degrees heel, you wont be driving hard, you will be hanging on hoping not to get thrown out of the boat and if you are still on board and going fast at 80 -90 degrees heel, the last thing you want it an small amount of reserve buoyancy right at the end of the boat being driven in and tripping you over.
It is also weight, windage and waveage (to coin a word) in the worst place most of the time as well.
Instead of a localised lee pod approach, which has the (remote) potential to trip up a proa, why not stretch out the lee pod full length and get some high level reserve bow buoyancy???? Like half a aircraft carrier? doing it this way gives you loads of extra buoyancy and accommodation to leeward with minimal windage and wavage.
I agree with Chris also, dynamic lift is the thing, so if you angle the underside of the pod down to the waterline in the middle, as a proa heels, you get a lot of stabilising lift off it, increasing with heel. A lot of performance sailing dinghies have narrow unstable underwater bodies for speed but heavy flares amidships which give reserve buoyancy and lift
.
So I reckon a modified ” Herbie” approach is the way to go….Below is a crude indicative example.
Rob
This is a lot like what Pizzey was thinking about with his knock down recovery system. You go *almost* over but are at some very high angle there aren’t a lot of things pushing you anymore…except the ama, alas and whatever platform you have on the ww side….....BTW, what do you think of Pizzey’s other secret sauce—leaning the rig to leeward for a more rapid reduction in sail area as the boat rolls?
If you use a biplane rig, the windward staysail part does what the Pizzey rig wants to do without the complication of canting the whole rig. (You can indulge in complication somewhere else) There are many other advantages to a bi plane rig anyhow, I have touched on them in previous posts.
So if you combined a biplane rig with a sleeker version of half aircraft carrier hull forms you end up with something like this….
Rob
Very interesting thought Dave.
This is a lot like what Pizzey was thinking about with his knock down recovery system. You go *almost* over but are at some very high angle there aren’t a lot of things pushing you anymore…except the ama, alas and whatever platform you have on the ww side.
I wasn’t active on the list when Pizzey was writing. Can you give me an “inside” URL please?
But I’ve noticed something sailing my tacking outrigger—a lot of the stability of the boat is dynamic, not static. Its not JUST a free body diagram of a light thing on a long stick. When that light thing on the long stick starts moving up (rotating around the CB) it becomes an energy storage device. A stone age flywheel. That energy that made the boat roll has to be redistributed. And the sooner you do it, the better, I think.
I know this well. My kite proas all carry their ama to leeward, yet fly them at speed because the kite is attached at the leeward rail. Many times we capsized to windward—yes, to windward—simply due to angular momentum. Kites come on and off-throttle *hard*The trick is to hang on—even as the boat heels well past 90 degrees, and keep the kite in power mode, pull you back on your wheels with a shower of spray as it accelerates out of the mess. I have drunk huge quantities of saltwater to avoid breathing the stuff aboard high speed, high acceleration kiteboats. http://www.dcss.org/speedsl/Whigmaleerie.html
Re: prows, I’m not suggesting they are optimal for modern boats—though I disagree with Rob; more later—only that they (perhaps) did have a hydrodynamic purpose, not just decoration.
BTW, what do you think of Pizzey’s other secret sauce—leaning the rig to leeward for a more rapid reduction in sail area as the boat rolls?
Cool idea, though I first heard it from Rob Denney c. 2004, and believe he lifted it from Newick’s work in the mid-80’s. “Good artists copy. Great artists steal.” —attributed to Pablo Picasso
Also, maybe the prows are just to cut through waves so that they don’t push the bow down as much—as on Hawiian canoes?
That’s included in my post. Last paragraph.
Dave
here are the Pizzey articles.
http://proafile.com/magazine/article/the-proas-of-john-pizzey
http://proafile.com/magazine/article/the-proas-of-john-pizzey-part-2
http://proafile.com/magazine/article/the-proas-of-john-pizzey-part-3
and here’s a recent design
http://proafile.com/forums/viewthread/236/
LOTS to think about here. The man has made some interesting boats.
And, as you well know, I’m all for stealing!
When any boat gets to 80 -90 degrees heel, unless it has shipped a lot of water the prow won’t be anywhere the waterplane, so I can’t see how you idea is going to work.
I have huge respect for your experience and intelligence, but I have to disagree with you, Rob. If the hull is very wide, like a monohull, sure, but these are 16:1 narrow multihulls. Let alone that they’re often completely flat on the leeward side. I’m sorry I don’t have the appropriate software, especially including hydrostatics, but I’m confident 3-4 people here are capable of simply modeling a basic proa hull with prow like the one I show in the illustration, perhaps 16:1 L/B on the waterline, then generating righting moment versus heel curves.
Similarly, I’m confident that someone here can take that same model and develop pitching moment curves demonstrating what happens as such prows are driven deeper through either wave action or sail thrust? I did mention, didn’t I, that revolving the prow to leeward, to perhaps 30 degrees, might offer an improvement, shifting that righting moment curve towards 60, even 50 degree heel, rather than 80-90?
By the time any boat get much past 45 degrees heel, you wont be driving hard, you will be hanging on hoping not to get thrown out of the boat and if you are still on board and going fast at 80 -90 degrees heel, the last thing you want it an small amount of reserve buoyancy right at the end of the boat being driven in and tripping you over.
I’m not talking about driving hard and efficiently at extreme heel angles, I’m talking about survival. Also not of modern boats nor “optimums” but of what actually was built, and sailed over hundreds of years and for 100’s of 1000’s of miles.
It is also weight, windage and waveage (to coin a word) in the worst place most of the time as well.
I don’t disagree, but 1) please compare/contrast with “modern” stacked buoyancy in multihull bows. (see, for instance, http://www.gunboat.com ) Also 2) is the fact that the ancients actually did this, they did not simply contemplate it from their keyboards.
I agree with Chris also, dynamic lift is the thing, so if you angle the underside of the pod down to the waterline in the middle, as a proa heels, you get a lot of stabilising lift off it, increasing with heel. A lot of performance sailing dinghies have narrow unstable underwater bodies for speed but heavy flares amidships which give reserve buoyancy and lift.
I recognize that this is a favorite of many, but as regards lee pods, you are reciting dogma, however well-regarded. I’m a fan of lee pods, principally for what they do to interior space, but trust me, they are not hydrodynamically advantageous. The pod is far too short, too curved, and too wide to offer any sort of dynamic lift. It is the antithesis of a planing hull.The easiest way to understand this is to consider the pod as a separate hull, traveling in close consort to the main hull. If taken alone, it is easier to see that planing is impossible and that the pod will generate a huge wave system at any loading and velocity a fast multihull might impart to it.
Instead of a localised lee pod approach, which has the (remote) potential to trip up a proa, why not stretch out the lee pod full length and get some high level reserve bow buoyancy???? Like half a aircraft carrier? doing it this way gives you loads of extra buoyancy and accommodation to leeward with minimal windage and wavage.
The only such I have seen which were successful were Dick Newick’s “half moon” amas. It is unfortunate that so few of these were ever built, but the consensus seems to be that they worked very well. All others were disappointments due to their adverse curvature, sharp “anti-chine” and high wetted surface.
Dave,
Equally, I have great respect for you and your achievements.
Every dinghy I have ever capsized has never had its bow ( Prow) any where near the water plane. Similarly, when I have done 90 degree buoyancy calculations on various proa designs, the bows again are well in the air. Slim hulls or no. So I fail to see what help,having got to 90 degrees how a prow is going to help significantly on any boat.
I do not believe in short pods, mainly because of trip potential, which is why I continued in the post proposing full length ones. Never the less, Sven Stevens on Pacific Bee has mentioned elsewhere on this forum the lift other than buoyancy he gets when his (short) pod hits the water.
I started off with half moon amas and vakas on my designs, but changed them to full chines for exactly the same reasons you suggest some time ago, and if you had bothered to follow down and seen my subsequent posts on this thread before replying, your response might have been more constructive.
Rob
Every dinghy I have ever capsized has never had its bow ( Prow) any where near the water plane. Similarly, when I have done 90 degree buoyancy calculations on various proa designs, the bows again are well in the air. Slim hulls or no. So I fail to see what help,having got to 90 degrees how a prow is going to help significantly on any boat.
I understand on a dinghy, or any wide hull, but am considering hulls like the attached. Are you suggesting the ends of this boat won’t fall below the water plane when heeled? I would honestly like to see what the waterplane looks like on a hull like this as it heels from zero to 90 degrees, where the CB migrates to, etc.
I started off with half moon amas and vakas on my designs, but changed them to full chines for exactly the same reasons you suggest some time ago, and if you had bothered to follow down and seen my subsequent posts on this thread before replying, your response might have been more constructive.
I am a fan of yours, Rob (really, honestly), but I have not read your every word. If you’ll provide me with the relevant URLs I will be excited to read them. I try hard to make my comments constructive. My apologies for any offense.
Dave
‘Short’ lee pod:
Russel has a video Jzerro with green water seen outside the lee pod porthole, doing 17kts and not slowing significantly.
There is an other (at a wooden boat show?) of Jzerro throwing up an impressive lee bow wave, which lands just before hitting the pod - which is (unfortunately, they look better) a good argument against a full length / long lee pod.
Hard to beat Russel I guess.
Mark
Mark,
I would argue that those sheets of spray flying up both sides of Jzerro’s bows are wasted energy and lift. Nearly every planing motor boat has spray rails and deflectors to harness that energy (and it keeps the boat drier as well) so I reckon long pods should be better in that respect. Proas, don’t (shouldn’t) heel much, so I don’t know why they shouldn’t take advantage of spray rails in one form or another as well.
Sven Steven’s copious posts in the technical section also serve as a reminder that there is serious dynamic lift to be had from pods if you want it and not withstanding a recent assertion that short fat pod surfaces are the antithesis of planing hulls, you only have to think skim boards wakeboards or even barefoot skiing to begin to wonder, or you can just read the High Speed Hydrodyamics chapter in Principles of Yacht Design (Lars Larsson and Rolf Elliasson) page 187 : “..A wide and short planing surface is….more efficient in generating dynamic lift than a long and narrow one”.
I still prefer long pods.
Rob