Submerged transoms give higher speeds?

Discussion in 'Multihulls' started by Nordic Cat, May 18, 2008.

  1. Nordic Cat
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    Nordic Cat Senior Member

    I have been doing some reading on what is best for a catamaran, transoms submerged or not when sailing?

    I think Morelli and Melvin reccomend at least 200 mms free at the transom when at rest, but not many boats out there are anywhere close to this.

    As I understand, the primary and largest resistance comes from wavemaking at the transom. maximising waterline length should also be a priority,even though this will give a small increase in wetted area.

    To keep this discussion on track, let us specify that the hulls have a 1:12 fineness ratio, and that the Bcl/Lwl ratio is 0.5. Lwl is 46 ft/14 m. (LOA is 50)Displacement is 7 tons, draught is 22" /56 cm. Cp is 0.6

    Should the transoms be free at rest? What about when sailing? At 9 knots (hull speed) and say 12 knots?


    Any experience or ideas?

    Regards

    Alan
     
  2. waikikin
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    waikikin Senior Member

    Nordic cat, I'd go along with the transoms clear at rest, when sailing & you get some lift on the windward hull & corresponding "sink" of the leeward hull it should still just be clear or the corner kissing the wake goodbye, along with this some aft trim in regards to sheer line looks good & when the boats pushed still looks right & "safe" as multies adopt some bow down trim whilst being sailed, the level sheer may still be safe but starts to "look" wrong to me when the boats being pushed .All the best from Jeff.
     
  3. catsketcher
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    catsketcher Senior Member

    Pitching and tacking

    Hello Nordic

    I remember when Lock Crowther launched the 40ft offshore cat John West he got some quiet criticism for the immersed transoms. He argued for them saying that a racing boat should have immersed transoms to reduce the "release angle" and help reduce pitching. Speedboats are always straight aft - a speedboat with rocker in the aft sections is slow.

    But if you have a variable trim cruising boat you will be often sailing at slow speeds - John West was designed to go fast so very rare slow sailing was compromised for common high end speeds. So you want clear transoms at rest for light wind sailing and motoring. I don't think you should worry too much about high end perfomance - most cats sail around slowed down in moderate winds for comfort.

    Tacking is also much easier with clear transoms. My transoms are just kissing the water with full victuals, crew and water. Now they are out almost 100mm. I think 200mm would be way too high. When the leeward hull gets very depressed you are going fast so immersed transoms on this hull aren't slowing you down - probably increasing speed. Shoot for kissing with load - the boat will almost always be heavier than you think so add 20-30mm or so to be safe.

    PS - don't flatten the transoms underwater area if you want to sleep well on a quiet choppy night. Keep them rounded or you may get an annoying slap. Grainger immerses his flat transoms to stop the slapping on a new design but that is slower in the light and worse for tacking.

    cheers

    Phil Thompson
     
  4. Nordic Cat
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    Nordic Cat Senior Member

    Thanks guys, appreciate the thoughtful answers.

    So, the suggestion is around Zero clearance at full load, and anything less in load will create clearance at the transoms?

    What do you suggest the shape of the underside should be from say the drives and aft? (about 3.5 meters on my design) I am thinking in the fore and aft direction. Should it be:
    A) Flat
    B) Convex
    C) Concave

    I have seen some boats that have the run slightly convex, this is said to work like an extension of the waterline, moving the stern wave further away from the transoms.

    Jeff Schionning claims that his hard chine boats are slightly faster in light winds due to the chines, despite the long flat run. So are hard edges better than rounded in this area?

    Lots of questions, hopefully some good answers...

    regards

    Alan
     
  5. Fanie
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    Fanie Fanie

    Hi Alan,

    I design my cat with a flat hull bottom. It is unpleasant to walk or stand on a rounded surface, and I want to save the work and cost and aditional weight a floor would add,
    so -

    I have my hulls parallel aft, so if flat is what you refer to then mine is flat.

    Convex may add a bit of drag, concave might increase rocking a bit. Best is to draw your hull in software and see what the hydro's calculations spits out.

    I drawed my hull in delftship, changing some of the features gives you an idea of what is gained or lost, for what it's worth in software like that. How good it is someone else may be better to compare and give an opinion.
     
  6. catsketcher
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    catsketcher Senior Member

    Aft sections

    Hello Nordic

    I know that Shionnings boats may be fast but there could be many reasons for this other than chines. Chines are not seen on any other boats that move in a sub planing mode so I can't see them actually increasing speed in a displacement sailing cat.

    I would be cautious about going away from the norm with the shape of the keel line. I would go down the normal road of a gentle upsweep from max depth. Don't make it hard to build or sell.

    I don't think there are any speed panaceas you can design in. Weight, length and sail area are the main inputs for speed. On top of that remember that most cruisers sail throttled back - we found an 8 knot average (over the chart) to be comfy most of the time. We could go faster if conditions were good but if I could design a boat that would quickly get to 8 knots and compromised top speed I would.

    cheers

    Phil Thompson
     
  7. Spiv
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    Spiv Ancient Mariner

    Hi Alan,
    I tend to agree with what Phil said here (underlined), except for
    Schionning
    boats being fast, mine was not, nor I read of them winning races..:?:

    Planing power boats benefit from a sharp slightly concave transom to get a little lift and allow the water to leave the boat without turbulence that would increase drag; sailing cats are different beasts.

    I race small cats and I can feel the acceleration as I shift my weight forward and allow the transom to just kiss or clear the water.
    My 12.5m Cosmos also behaved much better with passengers distributed evenly or forward (hence lifting the sterns) than with all people in the cockpit.

    What we need here is to allow the water to fill the holes left by the hulls with as little turbulence as possible and optimize it for the most desired average speed. 8 kn seems a good average, perhaps your design can achieve a better average, but as we know, the more wind the higher the waves, so, when cruising, we slow down for comfort.
     
    Last edited: Jun 6, 2008
  8. Nordic Cat
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    Nordic Cat Senior Member

    Hi Guys,

    I got hold of a copy of a thesis on the subject, quite interesting reading I must say. Optimisation of the transoms for a large heavy sailing cat. Even more interesting conclusion.

    Here is a copy.

    Let's discuss after you have read it....

    Regards

    Alan
     

    Attached Files:

  9. Spiv
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    Spiv Ancient Mariner

    I have skimmed most of it,
    my observation:
    1. this boat has a waterline beam of 2m and a displacement of 23t. Fat and heavy roomaran. I did like Yaplukas....
    2. all he says about concave and immerse transom is correct, but only at the speed at which the waterflow detaches cleanly and effectively lengthening the waterline length. At slower speed it will just increase drag disproportionately.
    3. he did not consider that the sails want to go ahead and the hulls want to stay still, so there is a momentum trimming the boat forward. This lifts the transoms and pushes the bows down. So, at speed convex transoms do not sink as he describes.
    The third point is the most important. Many would have noticed that their cat trims very differently when under power than when under sail.
    Mine at 10kn under power would be squatting with the transom well underwater and leaving behind a wake not dissimilar to a power boat.
    On the other hand, under sail the transoms were much higher and nearly no wake.
     
  10. Nordic Cat
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    Nordic Cat Senior Member

    Stefano, good comments.

    1.) I had the same feeling as I was under the impression that Yaplukas were better performing. One illusion less:D

    2.) I'm not so sure about the increase in drag at slower speeds, but it might be worth the trade-off, if the SA/D is high enough, then this might nor be such an issue if there is enough power.

    3.) I fully agree, in fact I will install my saildrives so that they will give a lifting component to their thrust instead of purely horisontal.


    There is still the wave slap at anchor that can be very annoying if the transoms are too high of the water. Anyone have a solution for this, a hinged bottom step???

    Regards

    Alan
     
  11. Spiv
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    Spiv Ancient Mariner

    I had 3 power cats, a 7m Shark cat with 2x 225Hp, a 9.9m Mantacat with 2x 300HP and a 12m Alluminium cat with 2x 340HP diesel and Jet drive.
    All of them had trouble getting the water to detach cleanly from the transom (I deliberately sharpened all trailing edges to assist in this process). Once that happened, then there was a significant increase in speed and, like monos when they get on the plane, they would trim up. At that point I could reduce throttle considerably and maintain speed.

    What we need here is someone to calculate the "theoretical" speed at which the waterflow would detach from your hulls cleanly; also determine if and how much drag is at lower speeds.


    Have you thought of using nacelles (see pics). For the pros and cons, we could start a new thread.


    True to a point, we are only talking about a very small portion of the hull. I thought a little lapping noise was quite nice:D
     

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  12. BigCat
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    BigCat Junior Member

    Hi, Alan - A 'v' shaped section aft will prevent slap. Immersed transoms only work at planing speeds. At lower speeds, they have very high resistance. Chines increase wetted surface, and catamarans tend to have too much wetted surface for their own good anyway-wetted surface is the enemy in light winds. I think the reason we see transoms under the water in sailing catamarans is that the designers were too optimistic about the displacement either as built or as loaded-Oram seems particularly optimistic in this regard. In other words, I don't think it was intended that the transoms be under water.
     
  13. Nordic Cat
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    Nordic Cat Senior Member

    Hi Big Cat,

    Good to see you are still around!

    I agree that many cats have immersed transoms, due to overloading.

    Stefano, I think there is a big difference between the relatively speaking wider transoms on cruising cats, compared to the thin ones on most beach cats. (relative to max beam WL). I also agree that lightwind conditions is what we need to optimise for, good reasons posted above.

    I have asked Anton du Toit, the NA working on the final design to do some CFD on this subject, to see iff the computer model comes up with any clear answers.

    This is quite time consuming, so we have chosen 3 scenarios. Also playing around with how far aft the max hull beam is.

    We have worked in plenty of extra bouyancy aft to carry all the crap that gets loaded on there.


    The latest crop of VO 70's all run with immersed transoms now, and some have quite a hard chine, as does the new BMW/Oracle tri. There might be something in this, but the big boys aren't telling!

    When we have some results I'll let you know.

    Regards

    Alan
     
  14. BigCat
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    BigCat Junior Member

    Hi, Alan - Hey, it's good to still be around. I don't think cruising catamarans can maintain anything like the speed of these ultra-light, highly specialized racers with big, highly motivated expert crews, even if the cruising catamarans are very optimized for speed, so the designs are bound to have points of difference. If a big catamaran has a 121' waterline, it's square root is 11, and they sail at well over 2.5 times that (27.5 knots,) so they can optimize for planing. If you are that fast, then chines and immersed transoms begin to make sense. So, if the square root of your waterline in feet were 7, you'd have to average 17.5 knots for that kind of thinking to make sense. Surely this is unlikely for a cruising catamaran 49' or so overall?

    As far as buoyancy aft goes, yachts typically have the LCB 55 or 56% aft of 0, so the question is what is the hull shape if the hull squats below it's designed waterline. I designed BigCat to pick up beam at the stern quite quickly if it squats, or looked at another way, if the stern wave starts climbing up aft. This way, you get the benefit of a narrow stern at slow to moderate speeds, and the benefit of a wider stern if you squat aft due to the stern wave as you pick up speed. You also have deadrise in the aftermost sections to minimize that annoying slap. At least, this is my theory!
     

  15. catsketcher
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    catsketcher Senior Member

    You don't want lots of volume aft

    I just had a quick look at the last comment and would like to draw attention to the problem with it. As most cats go stern over bow in a capsize situation it is important to make sure the sterns do not over power the bows. Indeed as the lee hull gets immersed, until it takes the total displacement of the boat, there is a good case for the CB to move forward. This is done by shaping the flare so that the bow sections get fuller quicker than the stern ones.

    To do otherwise is to give waves a really good method to grab the sterns and overpower the bows in following waves.

    cheers

    Phil
     
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