A better proa hull?
What about trim tabs in both ends. The bow tabs could be lifted flat against the hull. Perhaps it would be enough to have a tab on the in- or outside only. It could be very long and narrow triangular with the long side along the hull and the outer edge about parallel to the keel. The effect should be substantial, if such is needed.
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.
This is not correct. To resist pitching, the only thing you need to consider is the moment of inertia of the waterplane. The waterlpane is the area defined by the intersection of the waterplane (water surface) with the sides of the hull. A hull with a fatter waterline (blunter entry) fore and aft will resist pitching more than a hull with a finer waterline fore and aft, regardless of how much buoyancy is below the waterline. What matters is how much hull volume you add or remove as the boat trims, not how much is already immersed.
Mal.
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.
This is not correct. To resist pitching, the only thing you need to consider is the moment of inertia of the waterplane. The waterlpane is the area defined by the intersection of the waterplane (water surface) with the sides of the hull. A hull with a fatter waterline (blunter entry) fore and aft will resist pitching more than a hull with a finer waterline fore and aft, regardless of how much buoyancy is below the waterline. What matters is how much hull volume you add or remove as the boat trims, not how much is already immersed.
Mal.
The boat without overhangs has greater volume to be added- and sooner -than the boat with overhangs. All that empty space under the overhang is wasted volume that the plumb bow utilizes. I didn’t mention how fine the bow was- use a blunt one if you like.
Alternatively, use an inverted bow which has it’s greatest volume just above the waterline. Volume which comes into play immediately while the boat with overhangs is still diving down to find enough volume to stop… diving down.
The boat without overhangs has greater volume to be added- and sooner -than the boat with overhangs. All that empty space under the overhang is wasted volume that the plumb bow utilizes. I didn’t mention how fine the bow was- use a blunt one if you like.
Alternatively, use an inverted bow which has it’s greatest volume just above the waterline. Volume which comes into play immediately while the boat with overhangs is still diving down to find enough volume to stop… diving down.
Ah….. it depends on your frame of reference. For boats of the same overall length, you are correct, but for boats of the same waterline length, the opposite is true. The fairest comparison would probably be for boats of the same surface area (material weight and cost), in which case the boat with overhangs would be longer overall but would have a shorter waterline.
Whether longitudinal or transverse, stability is maximised by being able to move the centre of buoyancy relative to the centre of gravity as far as possible for the smallest change in trim or heel angle. A boat with overhangs does this better than a boat without overhangs, in the same way that a boat with flared topsides has more reserve stability than one with vertical topsides.
With hobbyhorsing, the main topic of discussion, apart from longitudinal stability there are also dynamic resonance issues, which may or may not be exacerbated in certain conditions by the particular configuration of any hull. As a general rule, having fuller ends helps to dampen oscillations. Transom sterns help in this respect by providing a wider waterplane where it is most needed at the end of the hull and by moving the centroid of the waterplane area aft, further from the bow end, which also helps to increase the moment of inertia of the waterplane. For a shunting proa, the best you can do is to design a hull with blunter waterlines fore and aft and perhaps add overhangs if you can afford them.
The danger with overhangs is that you think you have a longer boat so you load it up more. Best to design the boat as if it doesn’t have overhangs, then add them on as an extra.
Mal.
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…
I do recall seeing a method—on a catamarans, I believe—to swap mast and daggerboard after a capsize, and sail the overturned boat home. Anybody recall where I may have seen this? AYRS, most likely, or the [MHml] Multihulls forum perhaps.
Dave
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.
Chris made a similar suggestion, but rotating the whole hull. Either way is fairly inelegant; it seems to me that the disadvantages will most often outweigh the advantages—though it’d be a cool boat, very cool.
How about inflatables (again??!) If one takes “slices” of this type of hull, each slice’s circumference is identical to it’s twin at the other end of the boat—just oriented differently. Which suggests that the “skin” of the hull, if of a flexible material, will conform perfectly and smoothly; all that’s needed is a second differently-shaped bladder inside. Heck, not even very much air needs to be introduced; the “emptying” one will half-fill the “filling” one during the shunt.
Even simpler, all we need is a pair of high-pressure inflated “struts” right at the bow (inside the hull skin). These struts are already oriented at 90 degrees to each other. On inflating, the strut defines the bow entirely, and the remaining bladder and skin simply serve to “morph” that bow shape back to the midships section. Let’s make the struts of small volume and really high pressure. Now it doesn’t take much pumping to shunt.
This concept—morphing via inflation—is a much larger field. Everything from aircraft wing sections to BMW bodies to fluidic muscles for robots are being done with variable inflation. Proas should get some of the love, too, don’t you think? 😉
But seriously, backing up and taking a 30,000 ft view, what we were after wasn’t a clever looking hull, but rather an asymmetric waterplane, on a proa. Changing shape via inflation could achieve this in a number of ways if you think about it.
Dave
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.
I saw a very clever method for shaping PVC using hot sand rather than a flame or other heat source. You just fill the pipe with ordinary sand, heated in an oven to ~350 degrees and the PVC goes limp—in a very controllable way. (PVC starts to deform at 280f, melts at 360f)
Very clever, very simple, though you’d need to support a big PVC structure or it’d deform under its own weight when it goes limp (boat-shaped hole dug into the ground?) It’ll also take a long time to cool down, so perhaps an ambient air source needs to be inserted into the hot sand, or you’ll wait days for it to solidify (maybe a simple water sprinkler is sufficient?)
This whole concept really intrigues me; building curved akas (for instance) would be dead simple—tapers should be possible, too. In smaller diameters, with heavy gloves and a source of “pressurized” hot sand, it could almost be like glass-blowing.
Dave
To resist pitching, the only thing you need to consider is the moment of inertia of the waterplane… A hull with a fatter waterline (blunter entry) fore and aft will resist pitching more than a hull with a finer waterline fore and aft… What matters is how much hull volume you add or remove as the boat trims, not how much is already immersed.
Mal.
Mal is correct. What RiskEverything may have under-valued is the issue of leverage, also the rate of increase of waterplane area. A pitching moment is the product of a force times a distance. With stacked buoyancy, the distance is nearly constant, so the added resisting moment increases more or less linearly with rotation in pitch. With overhangs, both the volume and the distance increase, thus the moment increases exponentially with pitch. Yes this moment begins more slowly, but increases at a far greater rate.
Whether longitudinal or transverse, stability is maximised by being able to move the centre of buoyancy relative to the centre of gravity as far as possible… A boat with overhangs does this better than a boat without overhangs, in the same way that a boat with flared topsides has more reserve stability than one with vertical topsides.
Precisely.
... apart from longitudinal stability there are also dynamic resonance issues, which may or may not be exacerbated in certain conditions by the particular configuration of any hull. As a general rule, having fuller ends helps to dampen oscillations. Transom sterns help in this respect by providing a wider waterplane where it is most needed at the end of the hull and by moving the centroid of the waterplane area aft, further from the bow end, which also helps to increase the moment of inertia of the waterplane. For a shunting proa, the best you can do is to design a hull with blunter waterlines fore and aft and perhaps add overhangs if you can afford them.
The variable rate of change helps to avoid this resonance issue as well. The resonant frequency is a moving target so unlikely to be brought into play. As to the transom option, unless the boat is designed for planing, such a transom wants to end above the waterline, though I completely agree with Mal regarding the potential. If so, there’s no fundamental reason not to put a bow transom on a proa. Not sure one could actually realize any advantage to it though…
Which brings us full circle, back the MC Escher hull shape. We can have a strongly asymmetric waterplane, we can have very full sections at the bow = high prismatic. We can even draw shapes which will encourage clean breakaway of the wake as the hull approaches planing speeds. Now if we can just solve that pesky reconfigure-at-shunt thing… 😉
Dave
“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.”
I don’t doubt that Jim Brown came up with the idea for that hull shape independently, but he was not the first. I first saw it in a book that I read in about 1980, and I am pretty sure it was “Katamarane und andere Mehrrumpfboote” by Heinz Juergen Sass, first published in 1973. The last chapter, on future boats, had a few sketches with that hull shape. I think there was a proposal for a catamaran, and there definitely was a Bruce foiler. I don’t remember to whom he attributed the idea.
Piet Viegers used fairly short and fat versions of that hull shape for a late 1970s iteration of his Amaran, a triscaph. There are pictures in one of the AYRS publications. According to the index, it should be #95. Mine are at home, so I can’t check now. Online pictures only show later designs with other hull shapes.
Regards
Robert Biegler
I don’t doubt that Jim Brown came up with the idea for that hull shape independently, but he was not the first.
That’s very interesting, Robert. The Sass book is listed available through isbns.net at $18-20. I probably have AYRS #95 at home, will check this evening.
The Brown patent had multiple claims, only one of which regarded the hull geometry. The remainder of the patent (a patent for the Windrider 16, that he may or may not have sold to Windrider Corp) might still stand, but it’s moot—the patent lapsed for non-payment of fees back in 2000. So, everything in it is now in the public domain. 😉
I cited it because it’s an interesting hull shape, not envisioned (perhaps!) for proas, and because Brown (and likely others) recognized the value of an asymmetric waterplane in double-ended, narrow hull pitch stability, long before Farrier made it a household word.
Dave
Here’s Amaran from AYRS 95. There’s a 2-page article on the boat as well, but little about the hulls. They were 800mm diameter at midships by 4.61 meters OA. Their volume was 2000L apiece. They appear to be the bi-longitudinally symmetric, same as Brown’s, but that isn’t certain from this pic or article.
Interesting boat. The editor added a note promoting the rig—pretty darn similar to Gary’s Gibbons/Dierking rig, don’t you think? 😉
Dave
Here’s Amaran from AYRS 95. There’s a 2-page article on the boat as well, but little about the hulls. They were 800mm diameter at midships by 4.61 meters OA. Their volume was 2000L apiece. They appear to be the bi-longitudinally symmetric, same as Brown’s, but that isn’t certain from this pic or article.
As far as I remember, Viegers later cut off the top half of each hull, to save weight, because he didn’t need all that volume.
It is a bit ironic that the only existing example I know of this hull shape, other than the Windrider 16, is the sort of boat that least needs pitch damping for each hull.
This early iteration of the Amaran sailed as a proa of sorts, at least if you counted the superstructure. The hulls wore around underneath the superstructure. A later design, which Viegers tried to commercialise, was configured like an ice yacht, steering with a single, permanent forward hull.
I have wondered why there are so few triscaphs around, when they give you so much longitudinal stability. Someone suggested the problem is the short hulls, that they will have more drag. I have wondered whether there is something more to it than just a short waterline. Parlier’s stepped hull catamaran was built at full size because test tank data indicated it was far superior to the trimarans with the banana foils, but in an actual race, it failed to keep up. And I read that it gave a far rougher ride than the trimarans. I remembered reading that racing motorbikes used not to have suspension, to save weight, until people found that suspension made the bikes faster. Some of that probably was being able to push harder without wiping out when tyres bounced off the ground. But some of it probably also was that the suspended bike didn’t waste a lot of energy on lifting the whole bike every time there was a bump. A rough sea is all bumps, and climbing every single one probably takes some energy. Long narrow hulls can slice through the water, planing hulls follow the surface. Perhaps that was what slowed down Parlier. And most of the triscaphs I have ever seen had planing hulls. An exception is the most recent one, by Hans Schilder (http://hansschilder.blogspot.no/p/amaran-tri-point-semi-proa.html), which uses beach cat hulls. But apart from that one post, I can’t find anything about how it sails, and with that much reserve buoyancy, those hulls probably don’t do a lot of slicing, even if they don’t plane.
So I had an idea. Design a triscaph with submerged buoyancy and a small waterplane area. SWATH doesn’t work terribly well if stability depends on shifting the centre of buoyancy, because the whole point of SWATH is that buoyancy doesn’t change much with immersion. So combine it with a non-heeling rig. Something like the attached image. Weight is to windward, At rest the upper hulls are immersed, the submerged buoyancy carries perhaps half the weight, and foils controlled by wands (not drawn) can lift the top hulls out of the water at modest speed. The weather hulls might have foils only on the inside, to make life easier in harbour. The lee hull could be designed along the same principles, with less volume but the same ground clearance. Or one could simplify and make just a fat, self-aligning foil. Lots of wetted surface in light wind, but simple and with limited response to immersion.
Here’s another idea for a Proa hull that could achieve fore and aft asymmetry:
Start with a “normal” Proa hull Symmetrical and narrow bow’s both ends, then we attach to one or both sides an elastic bladder when deflated the bladder conforms to the hull shape. this bladder would be attached from near the center of the hull to the bow/stern and from the keel to at least half way up the side of the hull.
Now for the “Inflating” of these bladders we use water, by use of 2 venturii’s below the water line one facing each way, the one with the opening facing the now stern sucks water out from the bladder(s) attached to the sides of the now bow and the one facing the now bow pulls water into the bladder(s) on the sides of the now stern.
Now we have a hull that is wider at the stern and narrow at the bow and shunts without intervention from the sailor, leaving him/her to attend the sails. The bladders don’t need to be entirely watertight, just as long as the water comes in faster than leaks out, leaving open a lot more materials for the bladder.
If you wanted to be really tricky you could also rig a switching mechanism to the side of the hull above the water line such that when the now stern swelled the appropriate navigation lights automatically came on for that shunt, and potentially when between shunts your anchor light comes on to highlight that you are stationary.
Will this idea work? or are elastic type cloths too fragile for the skins of a hull? at least if the bladders had some kind of catastrophic failure and couldn’t inflate you are left with a symmetric Proa hull that is still watertight and will get you home.
Dave.
Here’s another idea for a Proa hull that could achieve fore and aft asymmetry:
Start with a “normal” Proa hull Symmetrical and narrow bow’s both ends, then we attach to one or both sides an elastic bladder when deflated the bladder conforms to the hull shape. this bladder would be attached from near the center of the hull to the bow/stern and from the keel to at least half way up the side of the hull.
That would give you both an asymmetrical waterplane and more weight aft, but would the bladders have good shape?
You reminded me of an idea I had a while ago. Build a step all along the side of the hull, pivot it in the middle, see-saw fashion. Let the step descend to the waterline at the stern, let it be parallel to the waterline there. You can even attach rudders to the step, like in the attached picture. If you make them shorter than I have shown, they could be above the hull bottom when they are at the same level, and then you could beach without worrying about your rudders.
You don’t have to make the step full length. That is just mechanically simplest. You could also have two separate steps, pivots closer to the ends, and a connecting rod or line that raises one step as you lower the other.
Regards
Robert Biegler
The problem is to change the buoyancy characteristics of the ‘new stern’ after a shunt. A more buoyant stern will dampen hobby horsing by canceling out the teeter totter motion around the center of buoyancy. If we accept that a flared hull (well, any hull, but especially those with flare and over hang) become more buoyant as they go deeper, what if we shift some weight aft?
If we have the means to add water ballast to the ama, why not pump a couple dozen liters into a tank at the extreme bow? The extra weight would keep us from gaining any actual buoyancy, but the inertia should help prevent the teeter totter.
Of course, this is not a solution for every tack, I see it as something a cruiser would consider while trimming for a long tack.
Also, no matter how much volume you give the stern ballast (trim) tank, you would want to gain volume by stretching the tank vertically if possible to minimize free surface effect. Ballast water sloshing back and forth could have worse results than hobby horsing.