Deep V continuous rocker proa. version 2.0
Reading from real experience of sailing proas, very little rocker is required and is not beneficial,
When gathering information about proas, I have found that they seem to have used lots of rocker in the pacific islands. Considering that trees are not normaly bent in a wide arc, that seems counterintuative to our modern thinking about boats.
I believe the pacific people where extremly good sailors and boatbuilders, and would not use a lot of rocker without any good reason to do soo. I want to test something more similar to the micronesian Walap, but built out of plywood and epoxy.
I believe some rocker gives the boat a lot of desirable characteristics, such as a soft motion, a natural lift of the bow - preventing pitchpooling and to much wash over the bow, a stable and wide center of lateral resistance even though it is pitching and working its way through confused waves - which makes it easier to steer and maintain the course. A hull with very little rocker will react much more to changes in weight distribution, which we have seen in the Proud Mary thread. This is a good thing on a 24 foot beach-proa, but not so good on a 64 foot bluewater cruiser for a family. A hull with a deep rocker will never pound, and will always rise softly over the waves, almost regardless of the size and slope. This is much less tiring then the pounding and the jerky behaviour of a rockerless hull.
Enewetak walap sailing, Youtube
Cheers,
Johannes
Today I have been playing some more with my new proa.
I think nothing beats real sailing, but playing with models comes in at a close second when it comes to learning about boats.
I did not sail it, but pulled it back and forth with a cord, to watch the wake and general behaviour.
I get more and more impressed by it.
It is easy to see the wake climb halfway up to the aft end, and then let go of the hull. The hull gets more directional stable and has a much softer motion when at speed then when laying still.
Cheers,
Johannes
Johannes
Love the new testing method and the new model. My next proa will definitely be developed extensively with models.
Would be great to see and hear your views of ballasting the model down
Cheers
Tink
Perhaps the rockered deep V hull on the traditional craft was there to provide lateral resistance?
Mark
provide lateral resistance
That is probably one of many reasons they used rocker. One can not isolate one single aspect of such an important attribute. It is like saying the bicycle has wheels so it can stand upright when parked.
Rocker is not good nor bad. It is a part of a whole concept. One can not say that rocker “is not beneficial” without specifying the exact circumstance where it is not beneficial.
Can you please specify why and how “rocker is not beneficial”. I am very interested in your reasoning behind this.
I am trying to get a thorough understanding of these concepts, and this discussion is forcing me to focus and define everything more clearly for myself.
Cheers,
Johannes
My next proa will definitely be developed extensively with models.
Would be great to see and hear your views of ballasting the model down
http://www.proafile.com/archive/article/testing_with_models_i
http://www.proafile.com/archive/article/testing_with_models_ii
Two articles i like a lot.
I have not made any detailed studies of ballasting the model. I have tried with “some” ballast and without ballast.
The most important aspect is not the weight, but the center of gravity. With some ballast deep down in the keel, the center of gravity is much lower and there is much less pitching.
It seems like this model can take a lot of extra weight without changing character. Since there is no sudden change in the curvature anywhere, it does not matter much how deep the hull is in the water. There is some speed penalty, but not much (within reason) It can sail quite well with only 3 cm freeboard.
I think that is a major advantage with a continious rocker. The hull has an even and predictable behaviour in a wide spectrum of circumstances. Any modern plumb bowed boat will throw a lot of spray when diving into a wave. There seems to be a lot of sudden changes and variations in behaviour in modern hull-shapes.
I will have to test this deep V vaka some more, but I like it enough to abandon my flat bottom vakas.
It is fun to learn and try new things. If one is not prepared to reevaluate and re-rank ones knowledge, there can not be any progress.
Cheers,
Johannes
Reading from real experience of sailing proas, very little rocker is required and is not beneficial,
When gathering information about proas, I have found that they seem to have used lots of rocker in the pacific islands. Considering that trees are not normaly bent in a wide arc, that seems counterintuative to our modern thinking about boats.
I believe the pacific people where extremly good sailors and boatbuilders, and would not use a lot of rocker without any good reason to do soo.
The amount of rocker on traditional proas seemed strange to me as well at first, as when it’s looked at from a modern round hull/fin keel mindset there doesn’t seem to be any point. Rocker on modern sailboats is (I think) primarily to allow the boat to turn through the wind more quickly, but proas don’t need to turn through the wind, so why have any rocker at all much less such a large amount, right?
But the thing is, with those deep V hulls traditional proas are a lot more like an old fashioned full keel monohull than a modern finned boat. They taper to a deep sharp point down the whole length of the hull, rather than being basically flat bottomed. And from this perspective the rounded keels of the old proas look, to me, less like rocker and more like the cut away forefoot of boats like Nat Herreshoff’s Gloriana, and I think it may have been done for some of the same reasons in both cases; namely reduced wetted surface area and better stability downwind.
Extending the bow of a traditional proa down so that it’s at the same depth as the midpoint of the keel would create little additional buoyancy at the cost of a great deal of additional wetted surface area. The result would, IMO, be a noticeably slower vessel. As an experiment, I drew up two quick sharpie hulls in Solidworks, both 20 feet long at the waterline with one foot of beam, one had no rocker and 12 inches of draft, the other had 15 inches of rocker (circular) and 15 inches of draft. They had equal displacement, but the rockered hull had 12% less wetted surface area.
But I think that downwind stability may have been an even bigger advantage. I don’t have any direct experience, but I’ve read (and the physics agree) that crab claw sails generate considerable weather helm when sailing downwind, by virtue of being swung well out to leeward. With that in mind, I believe that a deep forefooted, v-hulled crab claw proa would have been a nightmare to sail downwind.
Because water molecules circulate inside of a wave (with the molecules at the crest moving in the same direction as the wave, and those in the trough moving in the opposite direction) when a boat is being overtaken by a wave, or is surfing down the face of that wave, the aft surfaces have a reduced relative speed through the water while the fore surfaces have an increased or at least less decreased relative speed. Not only does this reduce the effectiveness of the rudder or steering oar, if the hull ever presents any angle of attack to the flow of water any sharp edges (ie. the keel) will shed stronger vortices (more drag) in the front than in the back. This is an unstable system, the more the hull yaws, the harder it tries to yaw even further. Add in the reduced effectiveness of the rudder and that’s how you get a broach. Deep, sharp forefoots are the worst things to have in this respect; they shed bigger, stronger vortices. They “dig in” and try to steer the boat from the front, and when you’ve got a great big sail out to leeward constantly trying to yaw the canoe to windward and your steering oar isn’t working as well as it should that’s the last thing you’d want. So a more highly rockered hull, shedding smaller vortices from the bow, probably allowed the Micronesians to push their canoes harder downwind, perhaps even to the point of surfing.
I believe some rocker gives the boat a lot of desirable characteristics, such as… a stable and wide center of lateral resistance even though it is pitching and working its way through confused waves - which makes it easier to steer and maintain the course. A hull with very little rocker will react much more to changes in weight distribution, which we have seen in the Proud Mary thread.
Has there been any empirical, back to back evidence to support this? I’m not sure I agree. I tested this with my Solidworks sharpie hulls and for the same pitching moment (enough for 5 degrees on the rockerless hull) the center of lateral area or the rockered hull actually moved fractionally further than the rockerless hull but they otherwise behaved nearly the same. And with the rockerless hull pitched 10 degrees (and one end well out of the water), the centroid of the rockered hull (pitched 13.5 degrees by the same moment) still moved 96% as far. I think that a vaka’s immersion rate, and the distribution thereof, has a lot more to do with its butt-steer-ability than the shape of its keel. Of course, I have no real world proof to support that theory, but the numbers seem to agree.
Greg
Thanks for a great explaination of the possible benefits and use of deep V, deep rocker hull shapes.
I think your idea makes a lot of sense.
Since I tested the deep V hull I have abandoned the barge/ASP hulls. There is no comparison. The deep V is soo superior in soo many ways, except maximum displacement from a given total length.
I have been making some progress on my 24 foot proa, but it takes some time.
Has there been any empirical, back to back evidence to support this? I’m not sure I agree. I tested this with my Solidworks sharpie hulls and for the same pitching moment (enough for 5 degrees on the rockerless hull) the center of lateral area or the rockered hull actually moved fractionally further than the rockerless hull but they otherwise behaved nearly the same. And with the rockerless hull pitched 10 degrees (and one end well out of the water), the centroid of the rockered hull (pitched 13.5 degrees by the same moment) still moved 96% as far. I think that a vaka’s immersion rate, and the distribution thereof, has a lot more to do with its butt-steer-ability than the shape of its keel. Of course, I have no real world proof to support that theory, but the numbers seem to agree.
Since the hull has a very high directional stability - it is even hard to turn it with the rudder - and the hull will have the same underwater shape regardless of pitch, any minor change in CLR compared to the CE of the rig, does not seem to make any differens to the heading. It is easy to balance the hull to track straight without a rudder. It will track straight with only the sail set at a correct position and angle. With a schooner-rig i dont think one would need a rudder except when sailing into a harbour or similar place.
This is the reason I said the CLR is stable and wide.
Cheers,
Johannes
This is one side of the 24 foot vaka. 732 X 60 cm. I am drawing the keel-line and figuring out the amount of curvature in the gunwale. If I draw a negative curvature for the gunwale, lowering the ends of the hull, the keel-radius increases substantially. That is a good thing, but the ends of the hull gets much lower and much more prone to scooping water up on the deck in steep chop. The freeboard is low as it is already…
Why is boatdesign so much a constant balance of different and often opposing properties?!? Why can’t anything be simple, easy and obvious?!?
Cheers,
Johannes
You got it Greg.
This is the reason I said the CLR is stable and wide.
I think you might be mis-attributing that stability to the rocker instead of some of the other big differences between the sharpie, the barge and the deep V. Yes, the underwater shape of the deep V doesn’t change when it pitches, but that shape still moves wrt the rest of the boat and the CE of the rig. I think that rather than rocker creating a more stable CLR, what you’re seeing is happening because the big overhangs and flared sides give the deep V more form stability in pitch than your earlier models. The CLR moves less, not because its location is more stable wrt trim, but because the hull’s trim is more stable wrt imbalance. So for a given amount of weight moved a certain distance the deep V pitches less and by extension the CLR doesn’t move as far.
This is one side of the 24 foot vaka. 732 X 60 cm. I am drawing the keel-line and figuring out the amount of curvature in the gunwale. If I draw a negative curvature for the gunwale, lowering the ends of the hull, the keel-radius increases substantially. That is a good thing, but the ends of the hull gets much lower and much more prone to scooping water up on the deck in steep chop. The freeboard is low as it is already.
Have you considered taking the cut-off portions from below the keel line and stitching them back on above the sheer line? That way you can stretch out your LWL and still have plenty of reserve buoyancy in your bows, and you don’t even need to buy extra plywood to do it.
Have you considered taking the cut-off portions from below the keel line and stitching them back on above the sheer line? That way you can stretch out your LWL and still have plenty of reserve buoyancy in your bows, and you don’t even need to buy extra plywood to do it.
I have been thinking about making some kind of “collar” of plywood in order to get some more reserve buoyancy, but your idea is very simple and economical. I like it a lot! Thanks for a great idea!!!
I like your rendering. It looks really nice. A very classic shape, but cleverly adapted to modern materials.
Thanks for great ideas and a very instructive discussion about the theory and workings of these kinds of hulls.
I learn a lot from your posts!!
Cheers,
Johannes
The CLR moves less, not because its location is more stable wrt trim, but because the hull’s trim is more stable wrt imbalance. So for a given amount of weight moved a certain distance the deep V pitches less and by extension the CLR doesn’t move as far.
I think the behaviour of the deep-V proa in my videos confirmes your explanation. It does pitch some, but mostly in order to closely follow the angle of the surface (waves). These does not seem to be much pitching and hobbyhorsing (resonant behaviour) , except for following the waves in a smooth and calm way.
Cheers,
Johannes
Have you considered taking the cut-off portions from below the keel line and stitching them back on above the sheer line? That way you can stretch out your LWL and still have plenty of reserve buoyancy in your bows, and you don’t even need to buy extra plywood to do it.
That’s a cool trick! Its like making a chined hull where the topsides drop away to nothing in the middle.
I have a question. If you look at Marshallese canoes, they not only have lots of rocker, its almost a series of straight lines in profile. The stem is straight, the bottom of the stem to the flat spot in the keel is straight and the flat spot in the middle is straight—with a bit of blending in between. Why do that instead of the more circular rocker? Just a more deeply cut away forefoot?
This is a great thread. Thanks for your contribution Greg. Welcome to the forum.
Best,
Chris
If you look at Marshallese canoes, they not only have lots of rocker, its almost a series of straight lines in profile. The stem is straight, the bottom of the stem to the flat spot in the keel is straight and the flat spot in the middle is straight—with a bit of blending in between. Why do that instead of the more circular rocker? Just a more deeply cut away forefoot?
I always assumed that the keels were rounded. I still think the portions of the keels between the stem and any flat spot on the bottom are rounded, they certainly look rounded to me in every picture I’ve looked at since I read your post last night. Albeit, the radius is certainly large enough that they might as well be straight.
With the stems, it’s probably the result of a constant angle of flare in the topsides. Since the topsides of the Marshallese canoes look to have been made from more or less straight planks with little or no “torturing” involved, if they were all at the same angle they’d naturally intersect in a straight line up the stem. On the other hand, it may have simply been tradition. Western sailors have certainly become stuck in their ways about how a sailboat should look above the waterline, maybe that’s a universal trait.
As for a flat in the bottom, most of the pictures of Marshallese canoes I had seen where the underside of the hull was plainly visible were all of the canoe on the beach. I guess I always just assumed that they had sunk into the sand a little, but I can think of a couple of reasons why they might have had flat sections on the keels of their canoes. First, it may have made the canoe a bit easier to drag up on the beach; by spreading out the load on the bottom it would be a little more like dragging the keel over the sand and less like dragging the keel through the sand. And second, they were limited by the size of the Breadfruit trees that they had access to. Breadfruit trees seem to have fairly small girths for their height, and I remember reading somewhere that they only grew really big on the tall islands like Yap or the islands in the Chuuk lagoon (which was why the Micronesian canoes had bigger dugout sections and fewer planks - they had bigger trees to work with). So if they wanted more surface area amidships they would have had to stretch out the deepest point of the keel because the maximum depth was dictated by the girth of the breadfruit tree that they were working with.