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Thread: SOF/Fuselage frame design questions

  1. #1
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    Default SOF/Fuselage frame design questions

    Hi folks. I built a SOF kayak a few years ago and have been reading about fuselage frame boats recently. I am intrigued with the idea of converting a small catamaran design (like the Wharram Hitia 17) to fuselage frame construction and have a couple questions about sizing the frames:

    1) Is there a rule of thumb for what thickness of plywood to use? 9mm or 3/8" seems to be the norm but would it be better to go thicker (maybe 1/2") to stiffen the hulls for sailing? Or is hull stiffness more determined by the stringers and number of frames?

    2) Is there a standard size (thickness) for stringers?

    3) How do you determine the number of frames needed and spacing between them?


    Thanks for any help or thoughts.

  2. #2
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    Default Re: SOF/Fuselage frame design questions

    Quote Originally Posted by evansm22 View Post

    3) How do you determine the number of frames needed and spacing between them?


    Thanks for any help or thoughts.
    Take a degree in structural engineering and hydrodynamics for the wave loadings?
    It really is quite difficult to build an ugly wooden boat.

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  3. #3
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    Default Re: SOF/Fuselage frame design questions

    Your biggest worry will be connecting the hulls to the cross beams at the shear stringers, and how to deal with the wracking forces as the hulls move in different directions in a sea way.
    It really is quite difficult to build an ugly wooden boat.

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    Default Re: SOF/Fuselage frame design questions

    An oldie but a goodie, available at Abe Books:

    Hope for the best, but plan for the worst.

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    Default Re: SOF/Fuselage frame design questions

    I actually don't think connecting the hulls will be too difficult. The design I am using is the Wharram Hitia 17 which connects the hulls and crossbeams via lashings to blocks bolted to the gunwales. I don't think skin on frame construction will interfere with that.

    As for movement of the hulls, I think that's my bigger concern, making sure that they are stiff enough not to cause problems by flexing under sail.

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    Default Re: SOF/Fuselage frame design questions

    mmd-

    Thanks for the book referral, I have ordered my copy already!

    Cheers

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    Default Re: SOF/Fuselage frame design questions

    Quote Originally Posted by evansm22 View Post
    Hi folks. I built a SOF kayak a few years ago and have been reading about fuselage frame boats recently. I am intrigued with the idea of converting a small catamaran design (like the Wharram Hitia 17) to fuselage frame construction and have a couple questions about sizing the frames:

    1) Is there a rule of thumb for what thickness of plywood to use? 9mm or 3/8" seems to be the norm but would it be better to go thicker (maybe 1/2") to stiffen the hulls for sailing? Or is hull stiffness more determined by the stringers and number of frames?

    2) Is there a standard size (thickness) for stringers?

    3) How do you determine the number of frames needed and spacing between them?


    Thanks for any help or thoughts.
    I suggest you search for the archived or mirrored versions of yostwerks.com. His wood frame building manual will answer many of your questions.

    Good luck!
    Dave

  8. #8
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    Default Re: SOF/Fuselage frame design questions

    Quote Originally Posted by evansm22 View Post
    I actually don't think connecting the hulls will be too difficult. The design I am using is the Wharram Hitia 17 which connects the hulls and crossbeams via lashings to blocks bolted to the gunwales. I don't think skin on frame construction will interfere with that.

    As for movement of the hulls, I think that's my bigger concern, making sure that they are stiff enough not to cause problems by flexing under sail.
    But your gunwales are glued to the side plank which forms the web of a box girder. Totally different hull structure than skin on frame.
    It really is quite difficult to build an ugly wooden boat.

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  9. #9
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    Default Re: SOF/Fuselage frame design questions

    Skin on frame has very little shear stiffness.
    Meaning the hull will deflect vertically under load.
    The only thing preventing vertical deflection will be the collection of stringers, and it will be little stronger than just clamping all the stringers together and trying to bend them.
    So make the stringers heavy.
    I had two central plywood frames buckle when I tried to use 1/4" on a kayak.
    This was before it ever hit the water.
    The bent stringers put lots of compression on the frame (which was hollowed out) on a Yost 15 kayak.

    If you put in frames which are solid, this will not be as bad a problem.
    But I would suggest marine 1/2" - not fir.
    BUT, a sailboat will have lots of bigger loads than a kayak.
    I would suggest at least two frames in each hull for each crossbeam.
    Put internal wood doublers (fore and aft where the crossarms go) and glue it all together with epoxy.

    Personally I wouldn't do it, but Dave Gentry has designed some sailboats I wouldn't have thought would work well.
    So it probably can be done.

    Good luck.
    Please let us know what you are doing (with pictures)!

    Marc

    OBTW, kayak stringers for Yost are 3/4 x 1/4 cedar. There are about 5 frames in 15'.
    Dave Gentry makes bigger stringers for the same size kayak.

    I'd put frames at about 1 1/2' but that is a guess.
    I'd also bond the stringers to the frames with a good fillet of epoxy.
    Also the heavier the stringers, the more I suggest using two pieces of 1/2 the thickness and epoxying them together while assembling the frame.

    Of course, if it was me I'd follow the plans or strip plank the whole thing.

  10. #10
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    Default Re: SOF/Fuselage frame design questions

    Thanks for the thoughts Marc. I would love to build a Hitia 17 some day but don't have the time or funds right now so my current project is actually just building a model. Converting the hulls to SOF is a fun design challenge but if/when I get to build the real thing I'm not sure if I would actually try that or just stick to the plans as drawn.

    With that said... I drew a model on Sketchup to see what the SOF version might look like. I am basically following the build process from Christopher Cunningham'sBuilding the Greenland Kayak. Wood would be 3/4" spruce/cedar for the stems, 7/8" for the gunwales, and 3/4" for the stringers (glued up from 2 halves). Frames would be 1/2" marine ply spaced @ 16".

    Hitia 17 SOF.jpg

    After reading High Speed Sailing, I want to make some modifications. I agree with your recommendation of 2 frames where each of the cross-beams connect, I think I would also reinforce those frames with a solid wood cross member. After looking at some of Dave Gentry's boats I think I might also add some plywood "planking" to the top for more rigidity. I am working on figuring out the correct placements for the cross beams, when I have that then I will update this model.

  11. #11
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    Default Re: SOF/Fuselage frame design questions

    Have a look at Dave Gentry's thread on building Chautaqua - and compare the frame with those you are familiar with.
    http://forum.woodenboat.com/showthre...-sailing-canoe

    Last edited by P.I. Stazzer-Newt; 01-20-2019 at 08:54 AM.
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  12. #12
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    Default Re: SOF/Fuselage frame design questions

    Quote Originally Posted by P.I. Stazzer-Newt View Post
    Have a look at Dave Gentry's thread on building Chautaqua - and compare the frame with those you are familiar with.
    http://forum.woodenboat.com/showthre...-sailing-canoe


    There is also a Proa, "Palindrome" that uses a wood/epoxy deck that helps with the loads of the Aka's onto the SOF.

    https://proafile.com/multihull-boats...a-for-the-r2ak

    Take a look. What you want to do is possible, and has been done before.
    Last edited by bjdbowman; 01-20-2019 at 09:46 AM.

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