Hi Everybody!

I just wonder what the best combination of rockerline and underwater bulkhead forms are, suitable for a small flat panel ply proa, lets say up to 7.5metres (24’8’‘).

The dory hull with a flat bottom and flared sides, having a relative strong rocker to bring the forefoot’s near the surface of the water.
Will it work with low rocker and deep forefoots es well? Examples for the “dory style hull” are Mbuli, P52, Wa’apa

On the other side is the “cutting” hull with a deepV, low rocker with very deep forefoot’s just as the 31’ “Madness” has it or the T2. Will it work proper in a small proa too?

The intermediate way would be a trapezoid bulkhead with a narrow flat and strongly flairing sides, meaning that there must be a chine somewhere above the waterline, otherwise the hull would be to fat at the sheer.

Opinions please!

Russel Brown, it would be very nice if you write a few words about this. I’am strongly interested in your opinion.

Best regards, Michel

Michel,

My guess would be that a deeper V shape is better for lateral resistance without using a dagger board, but will tend to have more wetted surface for the same displacement.  As to rocker, a flatter rocker is better for higher speed in a straioght line, but more rocker will let you turn easier, and in a monhull at least give you less wetted surface until the boat heels, so it’s better for light air performance.

With a shunting proa, the “tacking” ability of lots of rocker isn’t necessary, so i think it has less value.  Wa’apa for example has very little and works well.  Also, unless you’re seriously flying a hull, I don’t know if you’ll lean enough to extend the waterline at higher speeds with a proa.

Tom

Any more opinions?

I think the deep V hull will have too much wetted surface, Its hard to get enough volume to carry the needed weight into a proa hull so small. Wetted surface penalty is more than 20% compared with the semi circle. This is a lot!

And you have to fit a separate floor to have a flat base for your feet, this means more weight.

Michel

The only hull that I’ve had experience with is P52’s and will have to say that I’ve not had any great desire to change the concept. Basic idea is to have a simple parallel sided strip fitted to a fairly carefully shaped bottom piece, the rocker that results from flaring the top out turns out to be 2-1/2” to 3”. Plus side of the ledger, it’s dead easy to do, makes for a nice flat area for foot well in the center of the boat, bottom can be a nice thick piece suitable for bouncing off the bottom, flare is good to keep bow from burying in steep water. Minus side of the ledger, more wetted surface than some other shapes.

If there’s a successor to P52, it’ll be 3 full sheets long of real plywood, bottom panel will be just a little fuller and there will be a ledger piece glued to the inside at the berth flat elevation before assembly.

cheers,
Skip

Skip - 07 December 2011 09:57 AM

The only hull that I’ve had experience with is P52’s and will have to say that I’ve not had any great desire to change the concept. Basic idea is to have a simple parallel sided strip fitted to a fairly carefully shaped bottom piece, the rocker that results from flaring the top out turns out to be 2-1/2” to 3”. Plus side of the ledger, it’s dead easy to do, makes for a nice flat area for foot well in the center of the boat, bottom can be a nice thick piece suitable for bouncing off the bottom, flare is good to keep bow from burying in steep water. Minus side of the ledger, more wetted surface than some other shapes.

If there’s a successor to P52, it’ll be 3 full sheets long of real plywood, bottom panel will be just a little fuller and there will be a ledger piece glued to the inside at the berth flat elevation before assembly.

Hi Skip,

can you use another word for ledger (ledger piece) or explain it in another way?. I cannot understand what you mean. The translator programm into german is not helping. (Its seldom usefull when I chat about boats)

Otherwise I agree with your opinion. The trapezoid shape of the sections is one of the good ones for a sheet ply proa. The wetted surface penalty is not so bad, I think you have around 4-8% more compared with the optimum semicircular (if the bottom panel is not too fat). But with the flat bottom you can benefit from dynamik lift which results in reduced wetted surface when going fast.

I read somewhere at the forum that you plan to make your botton 3/4’’ thick. I think this is a structural overkill. 1/2’’ and 6oz glass is more than enough for a 24’.

I would not let the rocker spring off from a straight panel side and the flare. I would design it carefully and rely on freeship for panel profiles or develop them during construction. This gives you better numbers and you can shape the bottom ends a little fuller, not continiusly curved as with your method. I would go with the old sharpie rule to bring the forefoots up near the waterline.

Yesterday I asked a experienced pro(a) designer about this and he had the same opinion. So, by now I’am confident that my small singlehander will have that flat bottom with flaring sides. Piece by piece Iam completing my data and detail collection, being ready for the design stage. What a wonderful forum.

Greetings from the North Sea Coast, Michel

Hi Michel,

The ledger piece is a continuous shelf strip or stringer glued to the inside of the hull panels to provide a solid spot to glue the berth flats to and reinforce the hull near the waterline.
Hope that helps.
3/4” is overkill as was the 17 oz biaxial glass on the bottom up to just above the waterline. But..the 3/4” subfloor plywood was the best material for the cost plus the T&G edges facilitated assembly, extra weight was pretty nominal. 17 oz cloth was what was at hand. Whole boat was built down dirty cheap and quick for the Everglades Challenge.
You are probably right about hull shape but I’ll probably stay with the straight panels for now on the basis of keeping it simple.
Actually I’ve wandered off in a slightly different direction in my quest for a simple, light quick coastal cruiser but need to cogitate some more before going forward.

cheers,
Skip