Trimaran From a Tornado Cat

Discussion in 'Multihulls' started by upchurchmr, Oct 31, 2011.

  1. Doug Lord
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    Doug Lord Flight Ready

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    Moth about 11/1; Tornado about 20/1 Why?
     
  2. idkfa
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    idkfa Senior Member

    Please give pg30 a look, I'm seeing 446mm = 17.5inches for template 3.3???
     

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  3. upchurchmr
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    upchurchmr Senior Member

    Idkfa,

    The tornado overall is 20' x 10', each hull is about 12" wide max on the deck, I don't know the sailing waterline width. If you accept 20' x 1' per hull that = 20/1.

    Ausiebushman,

    Sorry if I let some attitude show thru, I really do appreciate the fact there are any responses at all. Possibly some frustration with work, although this is one of the ways I try to ignore work.

    Possibly I misinterpreted some "attitude" in some of the responses, asking why I didn't want something else. Why is certainly fair game, but it felt like I was being asked why I didn't want an Americas cup boat (I am exaggerating of course).

    I am not the most tolerant of people, but I keep trying here. My experience is less than I wish it were. I really wouldn't mind a direct comment, if I am out of line.

    Corley,

    Thanks for bring up the Warren 23, I had seen his site for some reason I didn't remember this boat. It probably meets virtually everything I intend.

    Marc
     
  4. Doug Lord
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    Doug Lord Flight Ready


    Well that is amazing to me. I learned that the Tornado was 20/1 years ago from an expert I had no reason to doubt. Never double checked it myself.
    You can't take the L/B ratio at deck level so I checked it at the approx. position of max immersion flying a hull and it is around 16/1 depending on the exact waterline length.
    Thanks for pointing it out!

    UPDATE: I just measured the approximate waterline beam with the boat sitting level and this is where the "myth" of 20/1 came from:
    I got an approximate waterline beam of 11.46"(.955') divided by an approximate waterline length of 19.25' for an L/B of 20.16/1.......

    I would assume that anybody doing a design utilizing this hull(or any hull) and making decisions based on the performance of the hull would check this stuff thoroughly when doing the design. I corrected post 25 to reflect this information.
     
  5. upchurchmr
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    upchurchmr Senior Member

    Gentlemen,

    Idkta certainly is right about the template, my apologies for trying to remember the width. However, the boat measures right at 16" on the deck between the beams (I just ran out with a flashlight). I don't know the details about how the templates are used but the actual boat is narrower. W/L width (at rest) is somewhat less but not a great deal (I would guess no more than 1" but my memory has already shown its quality). Of interest is the fact that the stern is very close to the max width, but it is virtually never fully immersed, if at all. The more the boat is nose down, the less the actual sailing width. So the L/B will increase on a tack or on a close reach with the bow burried deeply.

    Doug, how did you get the approximate waterline width?

    Marc
     
  6. Doug Lord
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    Doug Lord Flight Ready

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    I found the template(and hoped that the proportions were correct) that Idkfa referred to and used a ruler to measure the deck beam(that he referred to- 17.5") which was 3.625" off the screen direct, then I estimated where the loaded, heeled(hull flying) waterline would be and measured that in inches which was 3.065". I divided the small measurement(direct off the screen) by the large measurement and multiplied the result by 17.5). The result was 14.8" converted to feet=1.23 and then divided that number into 20=16.26. The actual waterline would probably be shorter around 19.8 making the ratio of the one hull in the water when the boat is flying the weather hull 16.1/1. Did the same for the level case because I was trying to find out where my good friend and very compentent sailor and designer had come up with 20/1, a number I have used for over 20 years in reference to the Tornado. I found 20/1 in the L/B ratio of the hull loaded with the boat sitting level.
    In considering a tri with the main hull a Tornado, your weight calculations will determine what the L/B ratio is but If you keep the all up weight less than a Tornado with two crew the L/B ratio might go up a bit(reflecting the narrower waterline beam) or if it is heavier then that the ratio will go down (reflecting the wider waterline beam). If it goes down to under 15.5 or so I'd consider not using the Tornado hull because that ratio would reflect an overloaded hull.
     
  7. upchurchmr
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    upchurchmr Senior Member

    Doug,

    I understand your process but realize that the template measured 17.5 but the boat was actually 16.
    When I think about the main hull I don't think the L/B will matter as much, since a day sailing boat would only be on the main at low speed. While I understand L/B to be critically important at high speed, I don't think that applies at a lower speed. Actually at lower speed (minimal wave making) surface area should be more important. Possibly slow speed would be optimized at a lower L/B with lower surface area. Is the Weta the proper model for slow speed? I suspect it could be narrower for slow with a penalty at high speed since it obviously planes (not flying the main hull).
    Overloading is bad no matter what, a good reason to make a carbon boat if you have the money. It would be nice, but this is (might be) a hobby, not my reputation, ego or living.
    Actually the Tri in its transition from slow speed (on the main) to higher on less main and more ama, to flying should give the technically oriented designer lots of opportunities to optimize performance continuously instead of in 2 states - slow or drifting and all out on the edge of overpowered.

    Marc
     
  8. Doug Lord
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    Doug Lord Flight Ready

    The Weta is a good model for a tri that can't fly the main hull in that it has a planing main hull. It is an excellent design for the weight it has to carry.
     

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  9. idkfa
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    idkfa Senior Member

    upchurchmr narrower is always faster, but because it is always lighter, you can't change l/b without changing disp. There will be less total drag, the mix between WSA and wave will change, but there will be less in total.

    The moth problem was one of same disp, by changing hull form, they went from being a (fast mono) at 1/6 to a (slow multi) at 1/11. Very big difference in wave drag = less total drag.

    IMHO, it would be a bad idea to go from 1/16 to 1/20 at same disp by using a square hull form instead of building out of carbon and nomex and keeping semi-circular the hull form. More total drag more of the time, right up till the very top speed, but you may not have the speed potential/ability to get there in the first place.

    Sailing multiples of speed of wind is more about efficiency than power. Take a land yacht versus and ice yacht, the land yacht has more sail but more importantly more drag.

    The little AC has stuck with semi-circular hulls and rocker, they've spent millions writing the bible, we don't need the print, just pictures of the boats.
     
  10. Doug Lord
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    Doug Lord Flight Ready

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    Idkfa, a minor point: you mention ratios above of 1/6 , 1/11 and 1/20 when I think you meant 6/1, 11/1 and 20/1. L/B= length divided by beam so to avoid confusion the larger number is on the left: 20/1 means the length is 20 times the beam; 6/1 means the length is 6 times the beam and so on.
     
  11. Corley
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    Corley epoxy coated

    The transistion to carrying the full load on the float would come later with the boat that is being proposed than it would with a C Class cat with 14' beam and extremely lightweight construction. It would be interesting to do a real world comparison and see how the hullshapes would compare on the water. I would not consider a square section main hull but for floats it could be worth considering.
     
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  12. Doug Lord
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    Doug Lord Flight Ready

    ============
    The most critical thing in assessing the ama shape on a modern high performance tri is answering the question: "Is the ama expected to support the entire boat weight with buoyancy or not?"
    The answer determines whether the outside hull itself will be designed to support from 0% to 100% of the total weight when flying the main hull. That is quite a range that will affect cost, weight, build time and involve serious design work so that what happens as the ama is loaded is planned and not
    random chance.

    Picture: 1--20' planing main hull tri with ama shaped like a symmetrical Hobie 16 hull designed and built by me in 1971. 2&3-- ama shape for an 18' oversquare tri designed to be sailed with intermittant water contact-designed by me 2011-test model under construction, main hull of model AND full size boat mostly complete.
    Just to illustrate the changes in thinking over 40 years!
    click on image:
     

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  13. aussiebushman
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    aussiebushman Innovator

    OK Marc, no offence taken. Please remember I am not a naval architect and my comments were based on a combination of experience and guesswork. Doug has provided the rationale for what I was trying to express, namely:

    In considering a tri with the main hull a Tornado, your weight calculations will determine what the L/B ratio is but If you keep the all up weight less than a Tornado with two crew the L/B ratio might go up a bit(reflecting the narrower waterline beam) or if it is heavier than that the ratio will go down (reflecting the wider waterline beam). If it goes down to under 15.5 or so I'd consider not using the Tornado hull because that ratio would reflect an overloaded hull.​

    Regards

    Alan
     
  14. upchurchmr
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    upchurchmr Senior Member

    Alan,

    No offense taken, I understood the comment because I understand my tendencies.
    A real design has not been undertaken, this was collecting these kind of comments to avoid senseless blind alleys, or a wasted build.

    Marc
     

  15. sawmaster
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    sawmaster Senior Member

    hey guys: I apologize for a post that is only tangentially related to this subject but does anyone remember a post a few days ago relating to tortured ply construction as a possible method to build a proa designed to compete w/beach cats --I want to re-read it but it seems to have disappeared.I guess I dont know how to search this forum efficiently.
     
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