why has high buoyancy become so important?

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

  1. peterchech
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    peterchech Senior Member

    I have noticed that high buoyancy hulls are the trend in modern high performance (and low performance too) multihulls. I understand the importance of having buoyancy in the ends, to rise over (or through) swells better, but why an increase in buoyancy all around?

    Doesn't having more buoyancy mean also having more freeboard (unless l/b ratio is toyed with) and a subsequently higher build weight?
     
  2. Doug Lord
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    Doug Lord Flight Ready

    Buoyancy

    I haven't noticed it in main hulls but a bit in amas since most high performance tri's these days are designed to fly the main hull. The ama has to have enough buoyancy to support the weight of the boat in as low drag a fashion as possible. A factor in more and more high performance tri's has been the use of lifting foils in the ama that support up to 60-70% of the total weight-but that could(and does in some cases) allow for a smaller ama.
    Can you reference any particular designs where you have noticed an increase in buoyancy compared to past designs? That would help me with the database of info I'm trying to build.
    Thanks....
     
  3. peterchech
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    peterchech Senior Member

    Many many comments on the internet have had to do with buoyancy and volume. Especially in the context of construction. For example, in the "tortured ply" thread I started, many people commented that it would be difficult to get proper volume out of hulls made with that particular method. Here is another quote from that thread alone:

    "Here ist a link to the detailed building blog of the F16 cat "Blade", a state of the art racing machine with lots of round curves and volume. Note the extra middle panel at the keel line that alows this high volume.

    http://www.thebeachcats.com/pictures/?g2_itemId=11955"

    (Emphasis Mine)

    I have seen this throughout the internet regarding high performance hulls having "volume" and "buoyancy", sometimes at the ends and sometimes just throughout. Maybe I am wrong to assume that what is said on the internet is truth, but it is pretty uniform in the commentary I must say...
     
  4. Doug Lord
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    Doug Lord Flight Ready

    Buoyancy

    --------------------
    In performance small catamaran design there are some interesting considerations: do you design the boat to float on its lines when both hulls are in the water-or when only one is in the water? Only one is right and that could lead to the perception of more volume as could the tendency of so-called wave piercing hulls to have hull volume relatively low.
    The "more volume" comment referring to the Blade makes sense especially when compared to something like a Hobie 16. And, I know from talking to the USA production builder, that it was designed to plane-which I've seen it do.
    I'll keep an eye open for comments about buoyancy and volume....
    ==============
    Here are some comments by Martin Fischer in an interview on the catsailingnews blog about his career and design choices:

    CS- What you would have designed or changed if you were head designer for Alinghi or BMW?
    MF: Well, that is a difficult question. On Oracle I like (obviously) the floats and I also like the concept of a trimaran without appendages on the central hull. Overall it was obviously the better boat, although it was not the most beautiful boat.
    I think the concept of a catamaran was not the right choice for a boat without limits on sail area and mast height.
    In order to retain at least some maneuverability they probably could not go (much) wider than what they did, and thus the righting moment of Alinghi was significantly lower than that of Oracle.
    Another point was the volume distribution in the hulls of Alinghi. Especially during the second race the boat showed some very significant pitching, which is an indication that the hulls had too much volume in the centre. Of course it is always easy to explain afterward why the others were wrong, so I don’t want to go too much into detail here.

    full article here: http://catsailingnews.blogspot.com/2010/05/cs-interview-martin-fischer.html
     
  5. Ilan Voyager
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    Ilan Voyager Senior Member

    On sail multis, the analysis must be dynamic. So you have the static weight, the pressure of the sails and the accelerations to manage. Add the fact that water on the decks is a brake. The flow of the water must be around and under the hull, not over, leaving the deck clear or the flow will be disrupted. (to illustrate this point in 1988 the formula 40 tri Biscuits Cantreau (200% amas) was faster and better around than Adrenalin which had problems with its too small amas -110% of the displacement-)
    Actually the race tris not using foils have at least 220% amas.
    That gives a far more powerful boat with no decelerations while cutting the waves or receiving a gust of wind.
    The modern composites permit a such light construction that more surface is not a true penalty. A few kilos are less harmful than a ama submerged in the water...
     
  6. farjoe
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    farjoe Senior Member

    The Blade has been mentioned but look at the AC45. It seems to float so high up. I know it must be very light but I believe very low rocker and also high buoyancy low down and also distrubuted for/aft are also contributing.

    regards
     
  7. Gary Baigent
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    Gary Baigent Senior Member

    Alinghi also had too much buoyancy in the stern sections, hard turning bilges - and buoyant sterns will throw the after sections up and push the fine reversed bows down in waves, hence the pitching. All large volume hulled cats pitch badly in a seaway ... UNLESS they're powered up and flying on one hull. In a transition wind period with both hulls mostly in water, cats bounce and jump.
    Although out of fashion now, small float, low buoyancy ama trimarans WITH foils are excellent, steady platforms in waves. But without foils, the amas will bury too far, drag, and rig will have to be de-powered.
    IMO there is still a place for low buoyancy, light weight tris ... but they need foils front and rear.
     
  8. catsketcher
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    catsketcher Senior Member

    Everything comes back again

    If you have a look at the very early cat designs like the Shark, Harris' Tigercat and the Prout's Shearwater designs they were quite bouyant and full hulls. My guess is that this was due to the fact that they weighed quite a lot. The lee hull has to support all of the weight of the cat plus the crew so a heavy cat needs lots of volume.

    In the 1970s cats went in a different direction. Hobie reduced the weight of his boats by reducing freeboard and installing a platform and Tornados and Hydras, Darts etc were still pretty voluminous aft but fine forward.

    Hull design is always related to rig design. In these times rigs were hampered by the fact that in high winds they were very draggy. Dacron crosscuts would get fuller and the draft would move back. This meant that the lift angle (the angle between the force from the sail and the forward vector pushing the boat forward was small) As such the forward force was small and hull designs had to cater for this. Even in light winds rigs did not develop as much power as the mains were usually non square top sails with reduced area.

    Then new materials came along. Epoxy laminates, stitched fabrics enabled cats to be lighter and stiffer, carbon masts reduced pitching, spectra and PET sail cloth warp orientated allowed much better sails to be produced. What happened was that the lift angle was increased. This meant that there was much more forward push for the hulls to deal with and more ability to produce power.

    So the hulls have changed back to voluminous shapes to cater for the increase in forward vector and decrease in drag. That means you need a hull with fuller ends as the hull is now operating at a higher speed (so you go for a larger prismatic). Also the wavemaking drag at low speeds from a full hull is less of an issue than a stable platform for the power producing rig.

    I see the same thing in offshore multis - from the fine hulls of the 70s and 80s to the power machines of today. Anything to handle the power the rig can make.

    cheers

    Phil
     
  9. Gary Baigent
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    Gary Baigent Senior Member

    Catsketcher Phil, those older designs were not so inept performers as you suggest, just because the higher technologies did not exist; for example the Beowulf's of Steve Dashew put in incredible performances, even by today's standards, Hugo Myers' 44 foot Seabird was another design using basic materials that performed exceptionally well ... and these cats were low freeboard, low volume (by today's standards) hulls, fine ends forward and aft, especially SeaBird which had canoe ends. The later 55 foot extrapolation Double Bullet also was a top performing boat that was around for decades. I'd like to see D Class Beowulf 5 (the boat still exists) take on say, a SeaCart 30 - the S30 would probably beat it, and then again it might not ... but it would be a close run thing.
     
  10. Corley
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    Corley epoxy coated

    I've been thinking along the same lines lately, a low buoyancy float trimaran augmented with retractable foils as an off the beach boat would make a lot of sense. In light conditions when crew weight is enough to nearly balance the boat you have minimal wetted surface area and light weight and when the wind picks up you have the benefit of the foils to stabilise the boat and increase the righting moment.

    You would end up with the same net result and less parasitic weight from the high volume floats, on large boats it's probably not so good but on small boats it could well be the way to go I can see other advantages too such as easier righting of the boat in the unlikely event you go over as your not fighting the buoyancy of a large float to right the boat.
     
  11. Doug Lord
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    Doug Lord Flight Ready

    Low buoyancy ama's of Frank Bethwaites HSP-published in his first book:

    click on image--
     

    Attached Files:

  12. catsketcher
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    catsketcher Senior Member

    Gary - I didn't suggest that old boats were slow. I am very interested in the evolution of boat shapes. One mind experiment to do is to think "What type of boat would I design with crosscut sails, plywood and resorcinol glue to go as fast as possible?" I think a large ocean crosser would end up looking very like Moxie, and a small cat would look like a Tornado. Beaowlf and Seabird were great designs. Aikane did fab mileage over 50 years ago but without the same design constraints today's ocean speedsters are a different breed.

    To my mind the great unsung heroes of multi development are the material scientists who have developed epoxy, carbon, spectra, stitched glass, foam etc that have so changed our boats. Even the moulding processes that the moths use is an example of CNC with incredibly fine tolerances. Contrast this with the heroic efforts of Dave Keiper who sailed to NZ and had to deal with heavier alloy foils that would crack and were not able to be made in a mould.

    In large cats Crowther talked of a pivotal event in his design career. Pennant and Bagatelle were sailing in a harbour race. Inshore there was little between the two. The new Bags was not faster than the ten year old Spindrift. But then they hit the chop in the Sydney heads and Bags just walked away. She had a higher prismatic and in real world conditions she was faster even though she had a higher drag shape in theory. So designers started using higher drag shapes that had greater pitch resistance and they keep doing so.

    cheers

    Phil
     
  13. Corley
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    Corley epoxy coated

    It's interesting to look at the lines of modern racing catamarans and trimarans they have high diplacement amas on the trimarans but they are still almost unbelievably fine for their length just look at a front view of Banque Populaire or Sodebo. In smaller boats the fineness ratios could never be so high as there would be inadequate displacement to support the weight of rig, equipment and crew.

    Also as Phil says advances do track with materials there is certainly no way that lightweight structures of the strength that we are seeing now could have been generated in the early days of development.

    I remember reading somewhere that the Maxi Trimaran Geronimo was 4 1/2ft wide at her widest point on the waterline of the main hull I think she was 110ft long? They are certainly pushing the prismatic to the limit whilst keeping low drag shapes and wave piercing bows. Not much fun for cruising unless you enjoy living in a narrow corridor though :D
     
  14. tspeer
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    tspeer Senior Member

    The issue with trimaran ama buoyancy being > 100%, is, which 100%? The ama has to provide the necessary moments in two axis, pitch as well as roll. At the bow-down design point for diagonal stability, the stern of the ama is out of the water, and you have to take the 100% out of the bow and mid section but not the stern. At the design point for sideways capsize when the boat is sailing to windward, the bow, midsection and stern are all involved.

    You also have to make allowance for waves. The trough will decrease the amount of hull in the water.

    When you add up the buoyancy of what is in the water in the various design cases, while still having adequate waterplane area for the purpose of stabilizing the boat, you get a total > 100% for the union of all the design cases.
     

  15. Samnz
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    Samnz Senior Member

    In a trimaran, bigger floats = faster unless its foil assisted which adds a level of complication not desired in boats under 40ft.

    My example is the Seacart 30, very successful boat. Still cant find a similar size/accommodation tri that can beat it.

    The extra weight is counteracted by a bigger sail plan
     
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