Calculating expected heel for trimaran

Discussion in 'Multihulls' started by Derek_9103, Apr 29, 2020.

  1. Derek_9103
    Joined: Oct 2019
    Posts: 15
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    Location: Waterbury, CT

    Derek_9103 Junior Member

    "Pop top"... that's where my planning started when I first started this journey last August. As I've been putting together my ideas, I have moved away from that - maybe partly because I did 4 1/2 years on a submarine in the Engine Room, and although I managed at the time, I'm tired of hitting my head.

    "More beam adds speed and stability, but crank up the engineering."
    Yes, I realize this is a trade-off. I'll go with wire stays for the beam/aka support to partially counter that.
    I'm willing to take a year or two, and my own research to learn what I can, and outsource the rest to a professional.

    "Long... amas"... right, I don't see any reason to skimp on ama length. I think same length as the hull makes sense.
    For profile, it seems modern Corsairs, Rapidos, Shuttle 39, TF-10, and Grainger tris all have very similar amas, so that seems like a good ballpark to stay in.

    "sugar scoop stern"... yes, my drawing in progress has a stern pretty close to the Chris White tris with a sugar scoop. In addition to function, it's just pretty.

    Balance out pros and cons of designing and building against a second-hand Farrier - I'd consider a used Farrier - I'd weigh that option if it arose as I'm making my final choice of path.

    What do you mean by "chook egg shape profile cabin"? Is this something like Kurt Hughes does for some of his boats? If so, I started with that idea, but moved more toward a Corsair/Rapido profile a while ago. I get there are downsides to my approach in the 32' length, mostly that to reduce weight and windage I'll need to keep hull beam to, say, a little over 6' and round the corners quite a bit.
    Last edited: May 6, 2020
  2. Derek_9103
    Joined: Oct 2019
    Posts: 15
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    Location: Waterbury, CT

    Derek_9103 Junior Member

    Overall, I think "dihedral" was the idea I need to take away, I'll do 80 hours worth of research and calculation, and maybe figure it out, or maybe come back with some informed questions.
  3. trip the light fandango
    Joined: Apr 2018
    Posts: 382
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    Location: Rhyll Phillip Island Victoria Australia

    trip the light fandango Senior Member

    By chook egg I meant an aero dynamic shape as in upright profile, not much room either side for your head and gaining beam toward the waterline..

    Certain wave actions can be quite annoying when laying at anchor on a Tri with high dihedrals, and as you mentioned the mast/ engineering gets loaded up when the sail can't spill wind for the opposite, a compromise and SOR meet here.
  4. guzzis3
    Joined: Nov 2009
    Posts: 484
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    Location: Brisbane

    guzzis3 Senior Member

    Displacement is the volume of the float. The amount of water it displaces as it sinks into the water. The weight of that water is the upward force the float exerts to stop the tri falling over. Sinkage is the draft, the depth, that the float sinks to. Imagine your float is a rectangular prism to make the calculations easy. Say it's 6 meters long and .3 wide. Lets say it sinks .3 into the water. The immersed volume is 6x.3x.3=0.53 cubic meters. Water is about 1000 kg per cubic meter so that float displaces 540kg at that draft. sinkage 0.3 or about 1', beam of float 1' length 20' if you prefer imperial units. 540kg is about 1200 lbs or something...

    The downward force on the float is the sum of all the turning moments on the boat, wind on sails, lifting of main hull out of the water etc. The calculation isn't impossible but it isn't trivial either. You normally don't need to do it. The longer the beams the smaller the force. If the float has the displacement to carry the loaded main hull it will never sink completely, the boat will rotate up and over the float to capsize. However that almost never happens. Usually tris capsize diagonally. It's called a pitchpole, look it up. It is why tri floats now carry most of the buoyancy forward

    I don't understand why you are considering a poptop on a 32' trimaran ? You should be able to get well over 6' headroom on a boat that length. 6'6" at a pinch.
  5. Derek_9103
    Joined: Oct 2019
    Posts: 15
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    Location: Waterbury, CT

    Derek_9103 Junior Member

    "considering poptop"...
    At this point I'm not.
    That's the first idea I ran with (for a few weeks at the end of last summer).
    So, *I was long ago* is what I meant to say.

    Thanks for confirmation on sinkage, that was my conceptual understanding also,
    but it's nice to have a new phrase like "turning moment" that I can do an internet search with a high wheat to chaff ratio to learn more.


  6. gonzo
    Joined: Aug 2002
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    Location: Milwaukee, WI

    gonzo Senior Member

    Displacement is the weight of the water it displaces at rest, which is equal to the weight of the boat.
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