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Thread: Do immersed transoms create less drag?

  1. #81
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    Default pics /extensions and ..... more extensions

    Sandy, pics here of the 3 ft stern extensions , sadly no previous detail shots , they are on another pc in the uk . also pics of another extension i did - on the bows !these were 2ft 8 " long , taking overall length to just about 38 ft .i am with the longer waterline = more speed and have just lifted her out and am looking at further stern extensions of 1 ft . but this would bring my transoms (static) to 3 " above waterline ,but when pressed sailing should be level with it . I dont know if these pics of the bow chop should have a mental health warning ! it took a large helping of courage to put the jigsaw to it !
    Attached Images Attached Images

  2. #82

    Default

    Loks like neat work Gramos.

    On the original "Mango" Bob Oram decided to extend the bows. He caused a fair amount of surprise at a Maryborough slipway when he took to his boat with a chainsaw!

    The old bows are currently being used to mark the end of his driveway.

  3. #83
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    Quote Originally Posted by Tropic Cat View Post
    The bow waves are nearly non existant. What there is comes off the bow and meets the other hull under the middle of the bridge deck. What I'm referring to are actually generated near the stern where the hulls rise up from the waterline.

    I'm going to look and see if I have a photo.
    Rick,
    I don't remember what your hulls look like below the waterline, if there are some chines, then you could well have an intermediate wave being generated. Do you have a canoe stern or is it more a traditional cat set-up?

    Alan

  4. #84
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    Default

    Gramos: the transom extensions look very well done, but the water is a bit too unsettled to really read how well it's separating at the stern. That said, I can't get over the middle picture. You have the proportions SOOO right! It almost looks obscene! What a beautiful job! Do you have outboards? What was the original design?

  5. #85
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    Rick: I'm getting the picture. I'm afraid that the problem arises from the transition from the Vee shape of your hull at its deepest to the hard chines at the transoms, There is nothing I can think of that could clean this up except exhorbitantly expensive work. And it would at best get you a half knot. That's presuming your transoms are above water at rest. You would be better off just fairing the hulls, and perhaps sealing the gaps above your rudders. Do you have folding props?

    I was talking to my secret resource at Ospry yesterday and he showed me a section of hull from a VERY strong boat. It was 1" thick very rigid foam with a single layer of biaxial underneath, and two layers of lighter biaxial under a thin coat of 2-part polyethylene paint on top. It would be strong enough for a square dance floor at the home for the morbidly obese. It would weigh about 50 pounds, but would require vacuum molding. It IS something an amateur could do, though, possibly for less than a thick, wet layup would cost.

    Did you know that the curved biminis on Maine Cats have very little windage? Its frontal area that matters, and with the window out, they are pretty slick.

    p.s. Ospry just soda-blasted 120# of copper off my PDQ.

  6. #86
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    Talking well thankyou !

    Quote Originally Posted by Sandy Daugherty View Post
    Gramos: the transom extensions look very well done, but the water is a bit too unsettled to really read how well it's separating at the stern. That said, I can't get over the middle picture. You have the proportions SOOO right! It almost looks obscene! What a beautiful job! Do you have outboards? What was the original design?
    Sandy , many thanks for your comments , at last someone other than the skipper sees the benefit of the extension work .a potted history of the design ... original design was by Erik Lerouge as a Jeffcat 32 then this design became a Passion 32 , then it went to England and became the new cat from Solaris Marine . they ceased and a company in Dorset bought the moulds and reproduced the Sunstar . I ordered from them a 32 ft sailaway and fitted her out . over the years i have made a few changes ,the biggest being 3 ft stern additions ,then 2 ft 8 " bow additions .she has twin volvo penta 2010 ,s with 120 s saildrives giving 7 knots max . I love the boat , and enjoy developing her ,principally for extra speed . she took a full Biscay F9 storm on the nose in 2006 . skipper and crew felt comfortable and safe .

  7. #87

    Default Test on Stiletto

    The nice thing about a Stiletto is it is so easy to experiment with.

    I have a Stiletto (and now a PDQ too - I'm hooked?) and some years ago when considering hull extension I performed some experiments, both under sail and under power. My crew and I would walk to the bow and stern (enough to put the transoms 4" under water, or just level):

    Speed by GPS in a tide protected bay.

    Under power
    at ~7 kn, 0.3kn+ when sterns up.
    at ~ 9 kn, 0.5kn + when sterns up.
    at ~ 11kn, 0.2kn + when sterns up. (wake starting to exit cleanly at this point)

    Under sail
    at ~ <7 kn beam reach or close hauled, same result as power
    at ~>11 kn beam reach or close hauled, same result as power
    at >11 kn, no difference any course.
    at >9 kn with the wind free, no difference, since the transoms are up because of sail pressure.

    At the time I recall Peter Wormwood (Stiletto designer) saying the advantage of extensions was ride and light-moderate air. As speed the exit was clean and, in a sense, the water did not see where the boat ended.

    On long passages, if the water was smooth, I would move everything heavy (spare water, fuel, sails) onto the tramp. Easily done on a 1400 pound boat. If it was rough down wind I was going fast and there was no worry... other than the silly grin on my face. Up wind I was too tense to notice any difference.

    Thought you might be interested in what I learned under fairly controled conditions.

  8. #88
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    Default Re: Do immersed transoms create less drag?

    Quote Originally Posted by ireaney View Post
    Peter (catabroker) has posted some interesting pictures of an extended 45' Leopard http://www.multihulls4us.com/forums/...=1221#post1221 , the scoops are quite high out of the water, what sort of effect will this have to performance??
    The leeward scoop won't be much out of the water when moving at speed under motor or on the wind, and its waterline will be longer, which will make it faster.
    Currently concentrating on http://earthnurture.com .

  9. #89
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    Default Re: Do immersed transoms create less drag?

    Quote Originally Posted by Tropic Cat View Post
    That's interesting. I had pulled my numbers from this thread on the boat building forum. It's the only chart of it's kind I came across. I calculated weight based on two 1/4 inch glass skins surrounding 1/2 balsa core. The link is here:

    http://www.boatdesign.net/forums/fib...ness-8686.html
    Planning on parking your car on top of the bimini? That would be about the right scantling for that-- seriously, one layer of light biaxial material on each side would be adequate.
    Currently concentrating on http://earthnurture.com .

  10. #90
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    Default Re: Do immersed transoms create less drag?

    Quote Originally Posted by Tropic Cat View Post
    That's interesting. I had pulled my numbers from this thread on the boat building forum. It's the only chart of it's kind I came across. I calculated weight based on two 1/4 inch glass skins surrounding 1/2 balsa core. The link is here:

    http://www.boatdesign.net/forums/fib...ness-8686.html
    Planning on parking your car on top of the bimini? That would be about the right scantling for that-- seriously, one layer of light biaxial material on each side would be adequate.
    Currently concentrating on http://earthnurture.com .

  11. #91
    Evan_Gatehouse Guest

    Default Re: Do immersed transoms create less drag?

    You don't see immersed transoms on 95% racing sailboats because they produce more drag at moderate speeds where they spend most of their time sailing.

    The Volvo 60s (and 70s) spend 80% of their time at high speeds (say > 15 knots) where the immersed transoms are lower in drag.

    So unless you are spending most of your time reaching / running at high speeds, I really doubt the original posters conclusions. I doubt Michlet is capable of figuring out the big eddies you get behind immersed transoms; I think you are seeing the effects of a longer waterline. Talk to Leo who created the program. I don't think Michlet uses RANS code at the transom. The thesis you cited on the boatdesign forum used Shipflow, which does use RANS properly, but they chose to optimize a heavy cat for a 14.5 knot speed under sail, which is clearly uhh, optimistic.

  12. #92
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    Default Re: Do immersed transoms create less drag?

    Quote Originally Posted by Evan_Gatehouse View Post
    You don't see immersed transoms on 95% racing sailboats because they produce more drag at moderate speeds where they spend most of their time sailing.

    The Volvo 60s (and 70s) spend 80% of their time at high speeds (say > 15 knots) where the immersed transoms are lower in drag.

    So unless you are spending most of your time reaching / running at high speeds, I really doubt the original posters conclusions. I doubt Michlet is capable of figuring out the big eddies you get behind immersed transoms; I think you are seeing the effects of a longer waterline. Talk to Leo who created the program. I don't think Michlet uses RANS code at the transom. The thesis you cited on the boatdesign forum used Shipflow, which does use RANS properly, but they chose to optimize a heavy cat for a 14.5 knot speed under sail, which is clearly uhh, optimistic.
    Even without knowledge of the programs, I can tell you that Evan is quite right, because you have only to look at the history of yacht design to see that you don't see immersed transoms in displacement craft. You don't design immersed transoms unless you are designing semi-displacement or full-on planing boats, which operate at above 1.5 times the square root of the waterline in feet, expressed as miles. (ie. for a 49' waterline, 10.5 knots.) No cruising catamaran is going to average over that speed in the long run, even if it is a fast boat, for a cruising boat.
    Currently concentrating on http://earthnurture.com .

  13. #93
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    Default Re: Do immersed transoms create less drag?

    There are a whole bunch of factors that influence the total resistance, what we have done was take an initial design, with non immersed transoms, played around with various Prismatic Coefficients, and with transom immersion, the curves speak for themselves for this specific set of parameters where we have optimised for speeds over around 4 to 5 knots.

    One could of course also play around with hull beam and draft, but we left those fixed.

    I value the input you guys are giving here, and I am not an expert on hydrodynamics, but Michlet is a widely respected tool, so please explain where you think it is not good enough, so I can get the picture a bit more clearly.

    cheers

    Alan

  14. #94
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    Default Re: Do immersed transoms create less drag?

    Quote Originally Posted by Nordic View Post
    There are a whole bunch of factors that influence the total resistance, what we have done was take an initial design, with non immersed transoms, played around with various Prismatic Coefficients, and with transom immersion, the curves speak for themselves for this specific set of parameters where we have optimised for speeds over around 4 to 5 knots.

    One could of course also play around with hull beam and draft, but we left those fixed.

    I value the input you guys are giving here, and I am not an expert on hydrodynamics, but Michlet is a widely respected tool, so please explain where you think it is not good enough, so I can get the picture a bit more clearly.

    cheers

    Alan
    I take it that you are saying that an immersed transom was calculated to have lower resistance than a non-immersed transom at 4 or 5 knots. I'd refer this one to BoatDesign.net. Every now and then someone tries to reinvent the wheel based on calculations, and it usually flops because of some limitation in the calculations, testing procedure, or scaling issue.
    Currently concentrating on http://earthnurture.com .

  15. #95
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    Default Re: Do immersed transoms create less drag?

    Good idea, thanks

    Alan

  16. #96
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    Default Re: Do immersed transoms create less drag?

    Quote Originally Posted by Nordic View Post
    Good idea, thanks

    Alan
    After the failure of Shamrock II, when asked why his yacht was beaten, he (Sir Thomas Lipton-TD) replies: "Because Herreshoff did not carry out tank tests".
    Currently concentrating on http://earthnurture.com .

  17. #97
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    Default Re: Do immersed transoms create less drag?

    Hi everybody,

    New here, I was watching some threads in CF and by accident during a search found this new forum.

    With regards to the original post I found a pdf which goes a bit beyond only calculations but they have done actual tank tests. As a thesis I found it an interesting read and it might give some additional insights. It was a while back that I read it, but as I recall the conclusions are similar to what Nordic states in the first post.
    So far I'm only learning with regards to catamarans and you people are quite informative, here and on CF prior to. . .

    Oops, sorry, the original pdf is 3,2MB. I tried to optimise and it went to 2,7MB but still it doesn't upload. I can PM, but I will search for the site where I got it from.

    Greetings,
    Bardo

  18. #98
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    Default Re: Do immersed transoms create less drag?

    The file is called "Design Optimisation of a 50ft SailingCat Clementine.pdf" it was made by a couple of students who did research for Yapluka.

    Again uploading failed, I'll post it in two pieces. I know it is not as easy to read, but if interesting enough I can e-mail it as a single file if you can't put them together.

    Sorry, I can't get it to work, a small pdf would upload but these files don't. Even when I down size the file name and delete the spaces it won't upload.

    Found the link, it might be reffered to before;
    http://www.boatdesign.net/forums/att...lementine-.pdf

    Bardo
    Last edited by maribar; 2nd June 2009 at 07:50 PM.

  19. #99
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    Default Re: Do immersed transoms create less drag?

    Thanks Bardo,

    a very interesting article/thesis that looked at a number of important design criteria, albeit on a very heavy catamaran. More a motorsailer IMO.

    I would have liked to see some more variations in the transom designs he selected to use, as +16, -16 and -30 cms are far apart. Anyway, it was interesting to see his conclusions on wavemaking resistance at relatively high speeeds. (9 and 14 knots).The transoms with -16 cms came out best, regardless of whether they were concave or not.

    The CFD we did, looked at both the Prismatic coefficient as well as smaller variations in transom heights, and we chose to optimise for the more natural cruisng speeds expected of a 50 ft cat, that is over around 4-5 knots and up to around 15 knots.

    My thinking in this is that it is in lighter winds that you need to minimise rsistance, as when the wind picks up, so does the sea state, but most boats will sail well (relatively speaking)above 15 knots TWS.

    The VPP (velocity prediction) for the Nordic 490 is between 95 and 110% wind speed on a reach depending on displacement. 95% wind speed is for a boat in light cruising mode with about 80% load (don't have the exact figures in my head at the moment). Total load is around 4-5 tons, depending on how many optional pieces of kit are selected initially.

    The latest french designs, as well as the new Morelli and Melvin designed 50 footer all have slightly submerged transoms. My initial thinking when playing around on my exsisting boat, by moving 6 people fore and aft, and looking at the log and wave patterns, was that with the right shape of the aft bottom, I was getting better speed.(The bottom of the hulls on my FP slope up too much IMO). This was evident by moving weight forward when going downwind.

    On a reach, the submerged transom gave me a higher speed.


    So it has been nice to see that my observations, and the latest CFD software produce pretty much the same results.

    All this is interesting for all the theorists, but at the end of the day, displacement (or lack thereof) and sea state will have the greatest effect on speed and resistance.

    I just couldn't resist optimising the hull shape for that 10% gain in speed for a given displacement, and to go against conventional "wisdom"...

    Alan

  20. #100
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    Default Re: Do immersed transoms create less drag?

    I seem to remember a design discussion for some of the very large, very fast new transport ships, that they had needed to design the transom such that the immersed stern was actually lower than the hull somewhat further forward, and this helped to lift the stern and avoid the dig in under power
    Insanity is continuing to do the same thing and expecting different results

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