Proa hull lines, suggestions/guidance

[Thule]

I am hoping to make a chined hull proa of about 18ft. I know there are professional designs such as Gary Dierking T2, P5 etc. It is an experiment. I am looking for some critique and suggestions on the lines I made in Freeship. Essentially I made two different boats, since Freeship doesn't handle assymetric hull and then stitched the lines plans together in an image editor. The windward side gives a displacement of 390KG, and the flat side 9KG - so about 400KG but considering I will only have half a hull each, the displacement is 200KG, just about enough for 2 sailors.

My thoughts on this boat are, to be built with fuselage frame and skin, to be sailed by 2 adults, overall beam to Ama hasn't yet been planned but about 8ft, to fit in the small slot I have at a local club, will be kept on the dry except when being used, plan to use crabclaw sail setup and shunting to turn around, wish to make it as speedy as possible, sailing will be in protected waters.

My thought was with the way the leeward half and the lower part of the windward side connect, if I heel about 15-20 degrees, it should present a long vee to the water. Will this aid in making it more speedy or am I way off my rocker

Any suggestions, guidance, pointers etc all are welcome.
Thanks

 

[Rumars]

My thought was with the way the leeward half and the lower part of the windward side connect, if I heel about 15-20 degrees, it should present a long vee to the water. Will this aid in making it more speedy or am I way off my rocker

Any suggestions, guidance, pointers etc all are welcome.
Thanks

Fastest is with the ama just out of the water, you should design the vaka shape to have minimal wetted surface when the ama is just an inch above the water surface. You don't want more heel then that because you loose righting moment and the sail also spills wind.

 

[gonzo]

the displacement is 200KG, just about enough for 2 sailors.[/QUOTE/]
Displacement is the weight of the boat. There is light and maximum. The load carrying capacity is the difference. Looking at the submerged volume on the lines you posted, 9 Kg seems too low. That would be a volume of about 2.4 gallons.

 

[Skip Johnson]

It looks like the hull will have a fairly low prismatic coefficient which will limit your top speed.

 

[Thule]

Oh dear - goof moment. I translated 0.09T to 9 KG instead of 90KG!
So, I may have misunderstood the displacement then. My understanding is the displacement is the amount of water displaced when the boat sits at the design water level. And the weight of that amount of water is the total weight available for the design - including boat weight/crew and gear for the boat to float at that level.

 

[Thule]

Fastest is with the ama just out of the water, you should design the vaka shape to have minimal wetted surface when the ama is just an inch above the water surface. You don't want more heel then that because you loose righting moment and the sail also spills wind.

I miscalculated the displacement of the flat half of the hull - misplaced the decimal and calculated 9kg instead of 90kg. That will allow me to bring up the windward side up more. I will experiment with that and the DWL to see about reducing the wetted surface. Thanks for pointing about the sail spilling wind at higher heel.

 

[Thule]

It looks like the hull will have a fairly low prismatic coefficient which will limit your top speed.

My miscalculation of displacement led to me making it more and more protruding. I will try to make the windward side thinner and come back with some more lines. Thanks

 

[Robert Biegler]

to be built with fuselage frame and skin, to be sailed by 2 adults,

Will the skin be supported by stringers at the chines and gunwales only, or also in between? Skin on frame will be pushed inward by water pressure, the more so the greater the unsupported span. If you plan on stringers at chines and gunwales only, I expect that especially the lee side will be noticeably concave, and you will lose some displacement to that.

Consider making the bottom not a stringer, but a plank, even if it is just 10cm wide at the widest point, and put closed cell foam between the plank and the skin. I had an Ally folding canoe, and that closed cell foam is essential to preventing the skin ending up literally between a rock and a hard place. It reduces pressure on any one point on the skin, and so reduces abrasion.

I expect you want asymmetry because the lee hull will provide most or all of the lateral resistance. Then consider making the hull a bit deeper and narrower.

My thought was with the way the leeward half and the lower part of the windward side connect, if I heel about 15-20 degrees, it should present a long vee to the water. Will this aid in making it more speedy or am I way off my rocker

You want to have only one chine in the water, so you don't have to worry about how well the chine aligns with water flow, which is not the same across all speeds anyway? Sounds good to me, but I have no idea how much difference it makes.

 

[Thule]

Will the skin be supported by stringers at the chines and gunwales only, or also in between? Skin on frame will be pushed inward by water pressure, the more so the greater the unsupported span. If you plan on stringers at chines and gunwales only, I expect that especially the lee side will be noticeably concave, and you will lose some displacement to that.

Consider making the bottom not a stringer, but a plank, even if it is just 10cm wide at the widest point, and put closed cell foam between the plank and the skin. I had an Ally folding canoe, and that closed cell foam is essential to preventing the skin ending up literally between a rock and a hard place. It reduces pressure on any one point on the skin, and so reduces abrasion.

I expect you want asymmetry because the lee hull will provide most or all of the lateral resistance. Then consider making the hull a bit deeper and narrower.

You want to have only one chine in the water, so you don't have to worry about how well the chine aligns with water flow, which is not the same across all speeds anyway? Sounds good to me, but I have no idea how much difference it makes.

I had the odd idea of running a lace between the chines like a lattice and place a 1/2 inch blue foam between the spans, foam being locked between the lace netting and the skin. That way, I will have a surface for pressure on the skin to dissipate, something to stop skin from rubbing on the ribs/lace and lighter weight.
Sounds reasonable?

re: asymmetry, yes, I am trying to avoid a dagger board or other appendage and use the lee side for resisting slipping sideways. I was trying to minimize the draft but I can see why all the fast proas like the T2 have a fair bit of draft - needing volume in a narrow hull.

 

[HelmutSheina]

Have you found Chris Grill's T2 blog? He may have information in there that will help you avoid some issues. He certainly tested the boat and himself.
A Wordy Report on My Version of Gary Dierking´s T2 Pacific Flying Proa. https://grillabongquixotic.wordpress.com/2011/07/02/a-report-on-my-version-of-gary-dierking%C2%B4s-t2-pacific-flying-proa/

 

[Thule]

Have you found Chris Grill's T2 blog? He may have information in there that will help you avoid some issues. He certainly tested the boat and himself.
A Wordy Report on My Version of Gary Dierking´s T2 Pacific Flying Proa. https://grillabongquixotic.wordpress.com/2011/07/02/a-report-on-my-version-of-gary-dierking%C2%B4s-t2-pacific-flying-proa/

Hi, yes, read through his report, his adventures and travails. Quite an adventure indeed. I have the Gary Dierking book, have the T2 and P5 details as well. Also have been reading books and reports including Miller PhD report, Wangka, Byde, Jeff Horton etc. I am sure I will slowly process this information in due time but in one go that is a lot of wisdom for my little skull

 

[Thule]

Follow up with changes to the hull shape.

I made a few changes to the bulls and came with these.

Total wetted area is coming to about 80 sqft.

I have a couple of questions.

Will keeping the midsection the same beam help with speed and is it better to have some bulge at the amidships and reduce the buoyancy 3 feet from each end?
I kept the buoyancy at the ends to reduce risk of burying the bow and also because the turbulence and friction on the long midsection is much less then if there is a
change in direction of the flow.

I chose the chines for ease of construction but will a bilge hull with rounded surface make a big difference in this type of hull? I Am willing to try a non chined version if it makes a big difference in speed.

Primary objectives are light weight, and speed.
Are there any other noob mistakes, easy pickings to improve performance?

Thanks all

 

[gonzo]

Flat panels have to be of heavier construction to equal the stiffness of a curved panel. For lighter weight a compound curve would be better.

 

[Thule]

Flat panels have to be of heavier construction to equal the stiffness of a curved panel. For lighter weight a compound curve would be better.

Would it apply to a skin on frame too? Since the material stretched taught in between chines - coated ballistic nylon- doesn't have much rigidity of it's own like a curved ply for eg, will that get affected the same way?

 

[gonzo]

I would thing that particularly on skin of frame. Otherwise the water pressure will make the panel concave.