24 ft Proa Design

Discussion in 'Multihulls' started by Jacobra, Nov 21, 2020.

  1. Jacobra
    Joined: May 2017
    Posts: 3
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    Location: Seattle

    Jacobra New Member

    Hi all, I have been working on a pacific proa camp cruising design to recycle a hobie 16 rig. I don't have it completely modelled but it’s worked out far enough to get the idea.
    The hull shape is based around seating ergonomics which give the main hull it’s flair, and a self bailing floor ~4 inches above the waterline. Below the waterline are semicircular hull sections with 18.5” waterline beam. Steering will be done through Newick style dagger rudders. The ama rotates up and over for trailering sorta like Richard Woods’ Strike trimarans. Auxiliary power will be an outboard mounted to one of the main beams.
    Designed displacement is 1,000 lbs and the self bailing floor goes awash at 1,850 lbs assuming the ama sinks evenly.

    Construction is strip built and fiberglassed with plywood bulkheads and decks. Preliminary weight estimates show that if I’m lucky I can keep it around 500lbs.

    Any thoughts on the design so far?
  2. BlueBell
    Joined: May 2017
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    Location: Victoria BC Canada

    BlueBell . . . _ _ _ . . . _ _ _

    I think it looks great!

    Why would you calculate it including the sealed ama?
    Remove the ama from self-bailing vs floor-awash calculation.
    Igor likes this.
  3. garydierking
    Joined: Sep 2004
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    Location: New Zealand

    garydierking Senior Member

    Are you using the dagger rudders for all of your lateral resistance? No daggerboard through the ama?
    Manfred.pech likes this.
  4. redreuben
    Joined: Jan 2009
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    Location: South Lake Western Australia

    redreuben redreuben

    I love the shapes, but to build them will require a high skill level and many more hours of labour, if it's a labour of love thats fine. I assume the design is a one off for own satisfaction ? If you intend to sell plans then be prepared to be asked for a sheet ply version.
  5. Jacobra
    Joined: May 2017
    Posts: 3
    Likes: 1, Points: 3
    Location: Seattle

    Jacobra New Member

    That's a fair point, I was thinking of it as max weight with reasonable weight distribution when in reality additional load would likely be in the main hull. Unfortunately I forgot to add the ama to that figure so they are actually as follows:
    Main hull loaded to floor level with ama at designed displacement - 1,850 lbs
    Main hull loaded to floor level with ama remaining level - 2,176 lbs

    My current plan is to only use the dagger rudders for lateral resistance, but it's probably worth building a case into the ama anyways. I just don't like the idea of going all the way out to the ama to raise or lower the board. Seems like a pain when beaching and makes it harder to adjust center of lateral resistance while underway.

    Thank you, this is planned for a self build only. I expect it will take more time both in hull construction and fit out, but I think the curves will pay off with more motivation to finish the project. :)

  6. Jacobra
    Joined: May 2017
    Posts: 3
    Likes: 1, Points: 3
    Location: Seattle

    Jacobra New Member

    Well, it's been a while but I'm back. Construction has started but unfortunately I didn't finish stripping the hull before the outside temperatures dropped below the chalk point of my glue... So it's back to designing!

    I'll start with the foils and work through my process here. Below is a diagram of the boat with some rough dimensions added.


    By summing the crew weight and boat weight moments we get the max righting moment.


    Using a max crew weight of 600 lbs and a boat weight of 650 lbs including motor

    RM=600*6+650*2.25=5,062.5 lbft

    This is about the same righting moment as two 175lb crew trapped out on a hobie 16, so with a slightly improved staying angle this seems like a reasonable maximum for the rig.

    By summing all moments about the longitudinal axis we should be able to determine the maximum sail force and force on the daggerboard before capsize.


    And since Fsail must equal Fdag we get
    Fy=RM/15=5062.5/15=337.5 lbs
    So the dagger rudders need to provide 338 lbs of lift to windward in order to balance the side force of the sails.

    Now I’ll take the sum of moments about the Z axis. Taking the moments starting at the COE of the forward foil we get
    ΣM=0=FdagF*0-338*8 ft+FdagR*11.5
    FdagR=2704/11.5=235 lbs


    Which means the lift on the fwd foil needs to be about
    338-235=103 lbs, hmm that’s more on the forward foil than I expected.

    Now if I assume a daggerboard area I can find the necessary coefficient of lift. So assuming 3 ft deep with 16 in chord accounting for planform shaping I get about 3 ft^2. Now we have to assume a velocity. Let’s assume the max righting moment is needed upwind and optimistically use 10 knots (17 ft/s) boat speed.
    CLRear = 2 × F / (A × ρ × V²)=2*235/(3*1.94*172)=0.279

    If the forward foil has ½ the area down
    CLForward = 2 × F / (A × ρ × V²)=2*103/(1.5*1.94*172)=0.245

    Those lift coefficients are nearly equal, so if I only want to use ½ the front foil depth maybe bidirectional foils aren’t such a bad idea. The lift coefficients are pretty low but lower speed will require higher coefficients, so let's see how they do at 5 knots (8.44 ft/s) instead.

    CLRear = 2 × F / (A × ρ × V²)=2*235/(3*1.94*8.442)=1.134

    CLForward = 2 × F / (A × ρ × V²)=2*103/(1.5*1.94*8.442)=0.994

    Still seems pretty reasonable but lift coefficient can be adjusted with foil area, so this may take some fine tuning once I start evaluating the foils in xfoil. That’s all for today, next up I’ll attempt to predict capsizing wind speeds.
    Flotation likes this.
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