These hulls or not these hulls

Discussion in 'Boat Design' started by GMR, Sep 30, 2013.

  1. Tiny Turnip
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    Tiny Turnip Senior Member

    I reckon the tri is in the ball park of 180- 200 lbs. too heavy to carry very far!

    Yeah, when I did the rough sums on my cat idea - 2x hulls at 90lbs, plus hull decks plus bridge deck, beams, rigs, moveable recumbent seats and pedal drives, and that's before there's any gear in it,... its not a boat you can drag up a beach out of the surf zone, its gonna need an anchor too.
     
  2. GMR
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    GMR Junior Member

    Turns out my nice light little cat actually weighs in a 220 pounds when set up for one rider....I was shocked!! And that is without any gear on board..... Good news is that I should be able to shave that down.

    Since my car is a front drive I am not keen on backing the trailer down a ramp only to end up with the video on the 6 o'clock news. So instead I put two wheels on the side, biased towards the rear, and walk it to the water. On boat ramps where there are no sandy beaches or docks near by, I don't need to haul it on the rocks while I park the car, I just put the wheels on the deck and off I go. Another few pounds. I have made up a fixture so that can mount the trailer to the front of the car which should allow me a bit more flexibility.

    Not an ideal weight of course but I am able to pick it up on the leading edge of the frame and drag it up the beach without the wheels, or up a boat ramp with the wheels.

    Can't seem to upload any images at the moment.....

    I see the Cross 18 described as an older model and that newer models could be as much as twice as fast. Is this talking apples and apples? Is the hull shape and size going to make that much difference compared to other plywood boats? Is the weight of 400 pounds realistic?

    Some boats like the Cross and Sea Clipper have a large square seating area where the seats are are in effect outside the hull perimeter. If one were to extend the front deck starting say a foot or two behind the bow and sweeping out to or towards the seating area, this might make for a drier ride. Suppose this would increase the likelihood of the deck peeling off when the bow gets buried.......or does the buoyancy of floats and bow make this unlikely? I have yet to see a video showing one of these small trimarans with the bow under water, am I not looking in the right place?
     
  3. upchurchmr
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    upchurchmr Senior Member

    GMR,

    I think with front wheel drive you have an ideal setup to back the trailer down the ramp. My 2wd pickup will spin the tires endlessly when they get wet due to no weight on them.
    You already have weight mostly on the driving tires. safer and easier to use.

    I have tried to get more information on the Cross 18 for lots of years. Never heard of the bow burying, I believe the older style has a much higher freeboard than newer boats. Nor have I heard of them burying the outriggers. No reports of flipping over the side by being overpowered although any Tri can be done this way (with enough wind and no sense). I believe the 400# could be achievable if you don't "improve" the strength. The sides of the hulls are 1/8" plywood which would help. Mast height is lower as I remember, so less overturning moment.

    I believe these boats are not driven as hard as a beach cat in competition, so the 2X speed is probably exaggerated. These hulls are fairly slippery, although they have hard chines.

    Your description of the bow/ seating area is accurate. I have seen trampoline material stretched from bow to the crossarm like you describe doing to the deck. Might be lighter with a trampoline and less succeptable to damage.

    I don't think you will ever see a Cross 18 with the bows in the water, it would take lots of weather, when its rare others will have a camera. Those amas are very large displacement also.
     
  4. GMR
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    GMR Junior Member

    Very good point, extending the foredeck outward would mean making it strong enough to stand on since someone is sure to be up there. Tramp is a much better idea. I do feel a bit ripped off on many of these boats because there is often little or awkward access to the area ahead of the mast and/or cross arms. In reality this is probably a good thing since I am a bit of a pack-rat.....

    Pretty sure I can pick a design from what is available in the 17 to 20 foot range and be happy with the results. As far as adding Human Power, that is another story. Obviously the biggest drawback to HP is that there just isn't that much to start and it decreases with age no matter how determined and cranky we get. And every gear, chain, sprocket, belt, pulley, bearing, bushing, and change in direction takes a slice from that pie. That will be a fun challenge!
     
  5. upchurchmr
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    upchurchmr Senior Member

    Good luck, let us know what you decide.
    Obviously lots of interest.
     
  6. GMR
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    GMR Junior Member

    I sure will, but I still have lots of question! For example, some boats use the rig from a beach cat but I see lots of warnings about mast failure and super high compressive loads. I have no experience with masts so I am wondering if there is a common failure point in this application and where this might be? Other than punching though the bottom.
     
  7. upchurchmr
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    upchurchmr Senior Member

    Any multihull has more ability to hold the sail up than a monohull, so there is always high compression loads. That means the mounting point on the beam has to take lots of load.
    If you calculate the righting moment of the donor boat you can compare it to your boat.
    If the shrouds are wider spread on your boat the compression will be lower.
    If you show us your geometry, you can get some help, but there is no one size fits all rule.

    Where did you hear all the gloom and doom? Anything specific?
     
  8. GMR
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    GMR Junior Member

    Not sure of the places where I read that. I did see a comment about the SeaClipper and the mounting point of the sea stays. The author thought they would lower compression loads by moving them out to the amas but clearly this would impact the folding process on this boat. I think the weight pushes this boat off my list so far. Discovery 20 is quite a bit lighter but uses a carbon mast etc so I would only use that if I can put a different rig on it. Cross 18 still looks interesting, W17, Strike 18.......
     
  9. GMR
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    GMR Junior Member

    I think the maximum length boat I can manage is 18 feet and that is more of a weight issue than anything else. I do have the space to build to 22 feet but cost and weight would be outside of the box for me......unless I went with a VERY narrow hull......
     
  10. upchurchmr
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    upchurchmr Senior Member

    If you are talking about the akas (crossbeams) anything without waterstays will put the beam in bending. That means the top of the beam is in compression and the bottom in tension. The sides of the beam are in shear.
    Once you have a sea stay the aka becomes like a mast. The length from the side of the boat out to the sea stay attachment (on the beam) will "primarily" be in compression (there will always be some bending if the beam is solidly mounted to the side of the main hull). If the sea stay attachment is not all the way out to the ama, the portion of the aka between the sea stay and the ama will be in bending.

    The angle the sea stay makes to the aka will also govern the amount of compression. A small angle (example 10 degrees) will produce higher compression and bending than an angle of 20 degrees.

    FYI, Discovery could be rigged with an aluminum mast, the original design certainly was as I remember.

    In my opinion the Sea Clipper is very much the same as the Cross with the obvious changes of how to fold the boat. As I remember it is built much heavier - not always good in my mind.

    If you think about a "very narrow" main hull, you basically need to think about how you want to use the boat. Most of the boats you talk about you sit in (sort of) but a narrow boat you might only be able to sit on the trampoline.
    Did you ever look at the Trika 540 - a much lighter boat?
     
  11. GMR
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    GMR Junior Member

    All good points for sure. The loads are somewhat arbitrary for the windward ama since one must decide how many people might be out on the ama or on the net, and the magnitude of the moment applied. On a 20 foot boat there could easily be two, maybe three people out on a net and they would probably be biased towards the stern. I could assume a round or rectangular section, get an "I" value and work backwards from there. Then decide how much load I want to carry on the aka and how much on the sea stays.
    It's hard to find much in the way of independant user reviews on the Seaclipper 20 and the Discovery 20. I am curious as to how they compare performance wise. The Discovery looks like a more complicated build process...
     
  12. GMR
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    GMR Junior Member

    Sorry, I did look at the Trika 540 when you suggested it and again last night. A beautiful boat and well suited for some kind of pedal drive. I am looking for something bigger that will handle more weather. I seem to have a few boats now for more protected water and want to venture a bit further. Although I don't mean to Sable Island or Ireland!!
     

  13. upchurchmr
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    upchurchmr Senior Member

    I agree about the Trika.
    Looks like a nice boat for sheltered waters with a modest expectation of speed.
    That is why I passed after being tempted.
     
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