Calculating expected heel for trimaran

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

  1. Derek_9103
    Joined: Oct 2019
    Posts: 21
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    Location: Waterbury, CT

    Derek_9103 Derek

    "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: 21
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    Location: Waterbury, CT

    Derek_9103 Derek

    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: 412
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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: 21
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    Location: Waterbury, CT

    Derek_9103 Derek

    "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.

    Thanks!
     
  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.
     
  7. Sulzerman
    Joined: Jul 2020
    Posts: 3
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    Location: Tasmania, Australia

    Sulzerman Multihull Sailer

    These plans appear to be excellent value for money! I am looking for Farrier F32X plans, unfortunately unavailable to purchase from the designers company any more.
     
  8. Derek_9103
    Joined: Oct 2019
    Posts: 21
    Likes: 0, Points: 1
    Location: Waterbury, CT

    Derek_9103 Derek

    Sulzerman,
    I bought a set of Farrier F-85SR plans (28-ish feet) -- I confirmed via f-boat (the CEO answered the email) before I purchased them that the specific plan # is okay to build.
    I like a lot of what Farrier does, but they were historically very strict about exceptions, so after much consideration, I'm not looking to use the plans I bought. (The plans also come with a bunch of extra bushings for the pivot arms, which by my understanding are no longer available, and the key missing piece if you tried to build a Farrier from scratch yourself.)

    My wife has been bothering me to find a new home for these plans.
    (but she says she doesn't like the word "bothering") :)
    I'd sell for the same price I purchased.
    Would you be interested?
     
  9. Sulzerman
    Joined: Jul 2020
    Posts: 3
    Likes: 0, Points: 1
    Location: Tasmania, Australia

    Sulzerman Multihull Sailer

    Hi mate, could be intertested, what would the cost be?
     
  10. Sulzerman
    Joined: Jul 2020
    Posts: 3
    Likes: 0, Points: 1
    Location: Tasmania, Australia

    Sulzerman Multihull Sailer

    I also understand the boat is quite a performer!
     
  11. buzzman
    Joined: May 2011
    Posts: 509
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    Location: Australia

    buzzman Senior Member

    The original Farrier Command 10 might be a useful source of ideas as well.

    I have a full set of the plans in PDF format on my Mediafire storage, here:

    MediaFire http://www.mediafire.com/folder/56wr7egnhypqc
    Files are huge, as they are scans of actual paper plan set from Farrier that I purchased some years ago.

    Feel free to download and peruse.

    The hull shape is broader than most in period 'performance hull shapes', and the amount of rocker is greater than in more modern designs, and you may not want a folding aka design, but nonetheless interesting, as it was more desinged as a cruiser than as a racer, and was deliberately under-rigged, as all Farrier's early boats were, to prevent the potential 'user-induced-errors' that could have spelled disaster for a fledgling designer.

    You'll note that the Command 10 like the similar vintage TT680 and 720 designs has box beam akas that meat in the middle of the main hull, a practice he went away from on the later F-boat designs in foam/glass, as he found the additional strength was not necessary, and the constraints it placed on the main hull usefulness was not 'market friendly'.

    Over on the TT forum on GROUPS.IO, several TT owners have documented ways they have modified Farrier's original aka designs to make for a larger forepeak access way, and higher roofline in the forepeak.

    The advantage of the earlier Farrier TT desings is they were mostly built in ply with glass over, and so are simple to modify. If you can use a skill saw and swing a paint brush, you can modify a TT.

    One of the principle reasons for so modifying these boats is that they were originally designed with a pop-top which provided for standing room at the companion and galley once at anchor, but not on the move, necessarily.

    My own TT720 is currently undergoing a major refit to 'fix' this issue. I have lower back spinal issues that mean stooping is not an option for me.

    Another of the posters on IO has cut the sides our of the cabin and widened them to enable a better, wider bunk design.

    Ply boats are FUN because you CAN play with them..!!
     
    Last edited: Sep 10, 2020

  12. waynemarlow
    Joined: Nov 2006
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    Location: UK

    waynemarlow Senior Member

    I have a F85SR plan set I may well sell, age is getting to the point racing may not be my priority so much.
     
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