20' and under trimaran beam-less than square, square or oversquare-and why.

Discussion in 'Multihulls' started by Doug Lord, May 15, 2019 at 8:19 PM.

  1. Doug Lord
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    Doug Lord Flight Ready

    Interested in hearing technical or just commonsense or experienced based thoughts on this subject for
    daysailing and/or performance small trimarans.
    Thoughts appreciated......
  2. Ad Hoc
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    Ad Hoc Naval Architect


    Your question, if that is what it is, is too vague.
    Your title suggests a question but is not clear.
  3. Doug Lord
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    Doug Lord Flight Ready

    Sorry about that. I was referring to overall beam and the rationale behind choosing one of the three possibilities for a 20' and under performance and/ or daysailing trimaran from the vantage point of a designer(with experience in this type of trimaran) and/or someone who has owned or wants to own a trimaran in this size range.
    Last edited: May 16, 2019 at 8:06 AM
  4. fastsailing
    Joined: Sep 2017
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    fastsailing Junior Member

    How many crew? How many of them is using trapeze?
    When using those, a tranverse righting moment increases while longitudinal one does not, due to crew weight location, suggesting narrower beam than without those.
    When selecting beam/length ratio, the target should be to balance lateral and longitudinal righting moments with associated heeling moments. For a high performance daysailor, lateral maximum heeling moment is much greater than longitudinal one in same true wind speed with the same sails. But if some assymmetric is used, it shifts the balance point for boats with less performance by increasing max longitudinal pitch down moment. With most high performance boats, such sail is not even used. And in your case foils need to be considered as well. If there is no downforce on windward side, foils do not increase lateral RM, they reduce it, by moving center of support closer to boat centerline. With down force, lateral RM increases. Any increase of lateral RM allows narrower beam with less weight, decrease requires wider boat for the balance. Also consider effect of foils for longitudinal RM. Is there rudder downforce available? If so, the boat can be wider for maximum performance, but with associated risks, if rudderfoil loses grip for what ever reason. (ventilation, fooling by seaweed, structural failure, etc)
    For some combinations over square is needed, while for others beam = 0.5*length might still be the appropriate selection. Too many unknown details involved preventing any more precise answer.
  5. Doug Lord
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    Doug Lord Flight Ready

    Thanks for your reply....

  6. SolGato
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    SolGato Junior Member

    I can only offer my experience as a Farrier Tramp Trimaran owner which is just under 20’ @ 19’6”L with a 14’9” beam unfolded.

    While not as fast or light as slightly larger newer Corsair examples, the Tramp for its size and weight is a wonderful boat. Some of the nice things about its size and proportions is that the boat sits flat when at rest, can be pushed hard without fear of capsizing, and is able to sail well in open water considering it’s size and that it was not really designed to be an ocean going vessel.

    I have pushed my boat pretty hard at times, often completely burying a hull at 12-15kn and the boat just slows a bit and tips to spill the sail until the hull comes up and the boat recovers. The only time I’ve been scared is downwind in swell with full main and screecher flying. If you overtake a wave or get caught by one that’s faster, it can be a bit hair raising but not as hair raising as being sucked up a wave and riding the face. In those conditions, the rudder starts to become ineffective due to the bottom shape of the boat as you are riding on the belly with little rudder in the water praying you don’t get sideways! But with the stock rig, the boat is very forgiving. In mild conditions trimmed correctly it will almost sail itself.

    One of the other things I like which isn’t unique to its design but something you might not think a multi would do well, and Farrier points this out in the Corsair sailing guide I believe, is that the boat can be put into a holding pattern where it will self tack and do circles without loosing much ground, which as he points out is great for staging at the start of a race, but as a single hander something I appreciate as I work through my final checklist just outside the mooring area before heading out.

    Now I don’t know how much of this is directly related the the boats beam to length ratio, I just know that the Tramps configuration seems to work quite well especially for a boat of its size and weight. And on the topic of weight, I really appreciate the heavy build quality of the Australian Haines Hunter Tramp in our waters. If the boat was lighter, I would probably get tossed around a lot more and lose my ability to point in swell. Understandably those who sail in protected waters in mild chop would prefer the boat be lighter.
    Doug Lord likes this.
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