Bowsprit vs. longer hull

Discussion in 'Boat Design' started by Skeezix, Mar 9, 2022.

  1. Skeezix
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    Skeezix Junior Member

    Might we summarize the considerations in choosing between adding/lengthening a bowsprit versus building a longer hull? This is for original design more than adaptation of an existing one.

    As a more particular question, for me this is for a fast nonfoiling beach trimaran that often will sail with no daggerboard due to weeds. Might a sprit make tacking easier, helping to push the bow across by bringing the headsail CoE forward? I do recognize that sprits are more typically for sailing off the wind.
     
  2. BlueBell
    Joined: May 2017
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    BlueBell . . . . .

    How about both, a longer hull and a bowsprit?
     
  3. Skeezix
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    Skeezix Junior Member

    For my plan, I was wondering about that. But why a bowsprit at all if the hull is long enough? And I am considering a furling tube for a spinnaker, so I wonder if that might be better with no bowsprit?
     
  4. bajansailor
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    bajansailor Marine Surveyor

    How big will this trimaran be?
    Do you have any initial concept sketches drawn that you can post on here?
     
  5. Skeezix
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    Skeezix Junior Member

    No sketches yet. All three hulls about 21' (to scarf from three sheets of plywood) and a beam about 17'. Borrowing a lot of design concepts from Mike Waters W17. Narrowest possible main hull that permits forward-facing seating with foot steering, but sailing from trampolines an option. Daggerboard, but hope for it to do OK upwind without the board because I often deal with weeds. So a rudder that easily dumps weeds, and a longer and shallower stem and forefoot than usual to ride over weed tops. Lots of buoyacy in the amas and a flat dihedral. Wharram wingsail, self-tending jib, and a moderate asymmetric chute. Cockpit seating for two, able to sail solo, but high buoyancy amas should permit occassional extras on the tramps.
     
  6. BlueBell
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    BlueBell . . . . .

    Skeezix,

    Sounds like a plan.

    What scarfing method are you considering for the ply?
    What's your length/beam ratio at the water line on your hulls?
    What length of bow sprit were you considering?

    I can't tell you how valuable visual aids are on a forum.

    Cheers
     
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  7. Skeezix
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    Skeezix Junior Member

  8. BlueBell
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    BlueBell . . . . .

    ???
     
  9. Rumars
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    Rumars Senior Member

    Having a bowsprit for the working jib does bring COE forward and that affects mast and daggerboard position. They both move forward, wich can be advantageous in case you need the space. Lead of COE over CLR stays the same, if you want to affect that dynamically there are other methods (weight shift, moving daggerboard, etc.).
    Rocker affects tacking, the more you have the more willing the hull turns.

    For weeds and beachsailing I would use a high aspect ratio centreboard, offset from the keel. This keeps the slot from clogging with mud, sand and pebbles.
    If you want to use a daggerboard, angle it back so that weeds naturally slide off. There were multis where the daggerboard exited in front of the mast because of this.
     
  10. Skeezix
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    Skeezix Junior Member

  11. Skeezix
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    Skeezix Junior Member

    Not having planned it to specific dimensions, using the above-stated figures of 21' LOA and 17' beam gives L/B somewhere in the 0.80 to 0.85 range, depending on the length of the stem above waterline.
     
  12. Skeezix
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    Skeezix Junior Member

    Oops, I bet you meant individual hulls. Kayak seats are typically 20". Give a couple more inches on each side and we get L/B = 20'/2' = 10. The amas beam across the deck is about 15". Waterline beam when half submerged about 10"? So L/B = 240"/10" = 24?
     
    Last edited: Mar 10, 2022
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  13. tlouth7
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    tlouth7 Senior Member

    Think about the optimum length of the rig and the optimum length of the hull separately. If the optimum length of the rig is longer than the optimum length of the hull then a bowsprit is the natural solution.

    For a fast multihull you are unlikely to reach a waterline length where drag is minimised, so you would always benefit in pure drag terms from a longer hull. Thus hull length must be limited by some other function, perhaps towing or storage.

    As for length of the rig, start from desired sail area and righting moment, and see what aspect ratio that gives you. That in turn determines sail foot lengths. Position CoE as appropriate relative to CLR and you will determine where the forestay attachment ends up with respect to the bow.
     
  14. Skeezix
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    Skeezix Junior Member

    Thanks tlouth7! At this point, my rig planning is based on spars I already own. The Wharram wingsail can be pretty efficient. A beamier plan helps to set the shrouds out at a greater angle, since there are no spreaders. Because of the wingsail, the forestay goes to the masthead. So probably a furling self tacker.

    The hull length is based on maximum of three 8 foot sheets scarfed.

    Mike Waters' flat-bottomed hard chines main hull form does seem to create planing effects, and helps resist windage. His amas are quite innovative.
     

  15. seasquirt
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    seasquirt In the beginning there were waters.

    Hi Skeezix. Build the boat and see if your 3 sheets long works with your sail plan in practice, and if not, then add a bowsprit later, just make the bow so one can easily be retro fitted, when you build it. Save time and work, which you can do later if needed. I added one to a dinghy easily, and it does make a difference, but:
    problems up front are inaccessible or difficult on a good day, instead of strolling up to the forestay; not safe or easy to nose up to a pole or behind another boat to tie up to / hang off of; can be a worry if getting towed; if you like getting in close to trees eg. mangroves there are more interactions with getting hooked up (maybe not so much in a tri); easily damaged when coming into a wharf or pontoon; on a trailer it could get close to a van's back window; head bangers while on the trailer; and so on. But they can look cool, and can help turn a sloop into a cutter.

    Now, I'd rather have a long raky bow you can walk up, and get access to, with a small pulpit, or rails if big enough. You can look at it like it's a bow and bowsprit that has just had the sides covered in and decked over, and no exposed struts or chain plates etc, just a clean surface. Having dabbled in a small way, I'd only put one on 40 foot and over, and not ruin the conveniences of a small boat, with a big proboscis. Unless it really needs it. Plan to put one on, but don't yet.
    I haven't read the other bowsprit thread, so probably just repeated some stuff. Good luck with your build. Do whatever YOU want, you are the creator, and you have to look at it; don't listen to me, if you just want one, do it.
     
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