why has high buoyancy become so important?

Discussion in 'Multihulls' started by peterchech, May 6, 2011.

  1. Doug Lord
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    Doug Lord Flight Ready

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    I'm not so sure about that. The new Seacart 26 is designed to be able to use curved lifting foils. The Farrier F32SR can be equippped with curved lifting foils as was his F27 years ago....
     

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  2. Samnz
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    Samnz Senior Member

    yes Doug but the Seacart 26 also has very large high buoyancy floats.

    look at the float rocker and large transom. As I said in reply to the original post- the reason is... overall its faster... quite simple
     
  3. gypsy28
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    gypsy28 Senior Member

    Didnt Farrier drop the F27 foils due to added complexity and weight?
     
  4. jamez
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    jamez Senior Member

    Yeh, that was 25 years ago on a boat that was marketed as a sport cruiser. Some of Ians newer designs have them as options, but unless you are going nuts-out for absolute max speed at tip-over time whats the point? Its more crap to build and fart round with.
     
  5. Fanie
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    Fanie Fanie

    In SA there were a few incedents where people drowned due to too little booyancy in the hulls wrt the weight of the vessel. It is normally a case of a guy that doesn't do his homework, doesn't know how to calculate booyancy, and has no experience of how a vessel reacts on the water, but he has a bit of money so it's ok.

    On a catamaran setup, a hull should be capable of carrying the complete vessel and have some booyancy to spare. In windy conditions when there are swells, if the vessel gets swung around then the leeward hull may be forced to have extra booyancy over and above the full weight that could be dumped on it.

    If this happens and the leeward hull gets submerged, it's almost a given the vessel will capsize.

    Sorry probably not what you guys expected, but it works like that. A small sailing trimaran my friend has has the same problem. I sail the leeward hull under the water. It doesn't capsize, but if the ama's had more booyancy it would have sailed faster and leaned less, probably making it a more pleasant sailing experience as well as have more storage space.
     
  6. Samnz
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    Samnz Senior Member

    This might be a case with a cat but not with a tri. My previous tri had smallish floats that we (just) could push under.

    One day I was sailing out to the racecourse with my (non sailing) Girlfriend helming while I hooked the kite up. Gusty conditions with all sheets cleated. A gust hit the boat and the leward float went way underwater, the boat just heeled right over to a massive angle and rounded up. She just looked at me and said... is that supposed to happen? and I said... um... no...

    With high boyancy floats I think we would have much higher chance of capsizing in that gust.

    So in my experience bigger floats are still faster, but not safer.
     
  7. catsketcher
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    catsketcher Senior Member

    Tris and low bouyancy

    Fanie - it may seem strange but in the 70s low buoyancy floats were the thing. Lots of designers liked them as they gave a softer ride and let you know how hard you were pushing the boat. Both Lock Crowther and Farrier liked low buoyancy floats for a while.

    A few other problems that Tom didn't mention are that as you lean over the rig develops a downforce. If you heel 45 degrees then the downforce will be 70 percent of the total rig force. Most multis fly a hull at about 10-20 degrees so the downforce is much less. On a tri I started to design the downforce from the rig was 17% when the hull started flying. So on top of Tom's factors feature in another 17%.

    Then you also have to have to factor in that multis go up and down waves. As you accelerate up a wave you get heavier and so does the boat. The worst time for accelerating is when you fall off a wave with no back and hit the trough on the other side. If it is a 2 metre wave then you hit the water with a velocity of about 6.3 metres per second. (Velocity ^2 = 2 x 2 x 9.8). If the float is say 1 metre in freeboard then it has to slow the boat (an acceleration) in 1/6th of a second (unless the float deck goes under). So the acceleration is change in velocity over time = 6.3/0.16.

    Answer 37.4 metres per second squared. This is over 3 times the boats normal acceleration due to gravity (9.8) and shows how any float will be low buoyancy if you fall fast enough.

    Of course that is the worst case scenario and assumes the lee hull will take all of the load. What will happen of course is that the windward hull will come crashing down as well and assist the leeward hull in floating the boat.

    If someone can find some accelerometer data we could calculate the real world data for wave sailing.

    cheers

    Phil
     
  8. Doug Lord
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    Doug Lord Flight Ready

    ============
    This is what Farrier says: " Such curved lifting foils were originally used on the prototype F-27 back in 1985, but they proved to be very difficult to make, and far too expensive for a production boat. However, CNC machining, along with newer production techniques recently developed at Farrier Marine, have now made such foils more viable, and although still expensive, this can be justified for a true racer."
    -
    Just noticed the price for his new "tight curve" foils: $5990
    " These all carbon curved lifting foils can also be used on the F-82, F-9, F-31, F-32, and F-33.
    Foils are made in one piece with epoxy-vinylester infusion resin system and come with either a white gelcoat or a clear gelcoat finish" 25kg.
     

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  9. Fanie
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    Fanie Fanie

    Sorry I just realised my post was a bit misleading.
    I was referring to power only boats, specifically those on two hulls like pontoon boats. A few were mentioned on the forum that capsized as well.
     
  10. Gary Baigent
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    Gary Baigent Senior Member

    If you want a light trimaran, as light as a catamaran, because a conventional tri has more material and surface area than a similar sized, conventional cat (read Derek Kelsall's comments on the two types) ... then the only way to achieve this is to have a similar surface area trimaran, and to gain this, small floats with folls is the only way to go. Then because shorter floats require less beams compared to long, high volume canter-levered floats, you can go to one main beam with a semi circular mainsheet track connecting to it: straight away, another reduction in weight. Now with the tri same weight or close to the cat, you have a better boat, better to windward, because tris have proven this to be so, (stiffer platform, better mast support, no dolphin striker or equivalent reinforcement) plus the same or better speed when hard reaching (formerly an area where cats have been superior) and absolutely no comparison in light airs because a tri has less wetted surface and cats stick two large volume hulls to the water like epoxy glue. Am I right ... or am I right? Facts gentlemen, facts.
     
  11. Samnz
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    Samnz Senior Member

    uh... if we are sticking to the facts Gary can you please provide an example as I did? i.e is there a tri in the 30ft range (with accommodation) that is faster than a Seacart 30 that follows your ideas?

    or are you denying the fact that higher volume hulls are getting more and more common?
     
  12. Gary Baigent
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    Gary Baigent Senior Member

    No denial, large volume float tris are the conventional norm today - but pointing out that if you want a light-as-a-catamaran trimaran, then you have to reduce surface areas ... and to do that, small floats with foils, less beams is the solution. Similar thing applies with a proa which is lighter than other similar sized multihulls simply because there is less boat involved.
    To make a comparison between foiler tri and high buoyancy Seacart 30 tri, is difficult because I can't think of any examples, maybe Exploder 25 and Seacart 26, although they both carry foils in long, medium buoyancy floats. However, if we go back to when low buoyancy float foilers were around, then the VPLP 50 foot foiler Ker Marine is a good example because that boat beat a large fleet of other multihulls in speed trial course round buoys, all much larger than Ker Marine, in fresh winds and equivalent seas, the outstanding point being that the foiler was the fastest beating, 17 knots, and close to the maxis offwind, 22 knots, third fastest if I remember, but still fast enough beating to take the win.
    You'll have to wait for low volume Seditious Sid to be launched, 7.6 x 8m, accommodation for two, and boat weighing less 250 kgs.
     
  13. Gary Baigent
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    Gary Baigent Senior Member

    Some more details, average speed over each leg in knots:

    name size designer type beat traverse reach broad reach

    Ker Marine (50 foot VPLP foiler) 15.34 19.60 22.89 21.43
    L.St, Marc (50 ft Langevin foiler) 13.63 16.60 22.15 19.48
    Umupro Jardin (60 ft Morrison tri) 15.25 17.96 22.66 24.64
    Credit Agricole 11 (75 ft Ollier cat) 14.93 9.10 24.44 28.30
    Royale 11 (85 ft. GRAAL cat) 14.22 13.22 25.97 24.90
    Elf Aquitaine 11 (75 ft Briand cat) 11.17 11.94 25.34 25.79
    Roger et Galet (45 foot Greene tri) 11.56 11.98 20.08 18.67

    Although these are old figures; there were many more competitors but these are the top ones, the performances of today's designs, undoubtedly faster, would still relate similarly to these in terms of boat size and relative performance. The outstanding point about the two smaller foilers compared to the larger designs, is that Ker Marine was the top boat to windward and on the close traverse, beating all the larger multihulls plus well up on the other points of sail.
     
  14. Doug Lord
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    Doug Lord Flight Ready

    Here's an America's Cup experiment where the ama buoyancy was ,shall we say, too low:


    Blue Arrow,click on image:
     

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  15. Gary Baigent
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    Gary Baigent Senior Member

    I heard that Blue Arrow's demise was a large sea running and a pitchpole - anyone have the real gen on what occurred?
    Got to hand it to them for their intestinal fortitude to go with nothing out to leeward except a foil.
    However here's another near no flotation foil design, Bradfield's NF2 .. and this boat was exceptionally fast, ... but perhaps not in a seaway - achieved over 30 knots according to a poster/sailor here who steered the radical boat.
     

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