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Thread: Do immersed transoms create less drag?

  1. #1
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    Default Do immersed transoms create less drag?

    This is a question that I asked on a couple of forums earlier this year, without getting any really clear cut answers.

    As I am involved with the design of a 49 foot cat, I requested that some calculations be done to clarify the situation. Another issue was how high to run the Prismatic coefficient.

    I have just received the preliminary results. At 2.5 knots the immersed transom adds 2.5 kgs of drag from around 4 knots the immersed transom is superior.
    At 10 knots boats speed (this is a 49 footer) there is 20% more drag on the non-immersed transom!!!

    This transforms to around 2 knots higher boat speeds (hulls only) so 12 knots instead of 10 for the same drag.
    Optimum point for these hulls is at around 11 knots where the difference is largest.

    We have also been playing with the Prismatic coefficient, and preliminary results show that increasing this from around 0.58 to 0.61/0.62 decreases the drag from around 4 knots. At 10 knots there is a decrease in drag of around 10 kgs or 8% roughly.

    We are going to fair the lines to have the transoms immersed 50 mm at full load, and end up with a Cp between 0.61 and 0.62, then we can calculate the drag reduction with a higher certainty.

    All this work is being done by Anton du Toit Yacht Design. www.dtyd.co.za who are doing a terrific job using Michlet for the predictions.

    I hope we can publish a proper report when we are finished.

    These predictions have suprised me a bit by the magnitude of difference in the drag, I did not expect to gain up to over 20% drop in drag at typical cruising speeds!!

    regards

    Alan

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    Hi Alan,

    Is this a sailing or motorised cat ?

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    Sailing cats usually have the transom above waterline because they want the max speed at low wind speeds. At higher speeds the bowed shape of the hull aft begins to act similar to a transom, it begins to suck water in after it. Since the hull is shaped gradually up the effect is less than a vertical transom.

    At low speeds the above waterline transom hull has to displace the water in front of it and it glides back into place when the hull passed through. At high speeds however, the water still gets displased from the bow, but now the curved shape of the aft part of the hull begins to drag water behind it.

    A motorised cat will want the transom below waterline, since the motors can provide enough power to overcome the suction cup effect of the water behind the transom, the lower transom means a streighter hull and less water volume that has to be filled up at speed behind the hull, just sheering over it.

    At low speeds the below waterline transom hull has to displace the water in front of it and has to overcome the water dragged after it. At high speeds however, the water still gets displased from the bow, but now the hull speed increased enough to leave the transom water behind, hence less drag and more speed.

    Many compromizes

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    I'd like to see the outcome of the experiment.

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    Hi Fanie,

    Sorry, I forgot to mention it is a sailing cat.

    Good to see you here!


    You are right about the need to get the water to "cut cleanly" from the transoms. I heard about a designer who got it really badly wrong, apparently if you threw an empty can into the wake it would stay with the boat for long distances

    Must have been in the pre-Greenpeace era


    I hope to have the results of the combined immersed transom and increased Prismatic coefficient.

    I'll post when they are available.

    Cheers

    Alan

  6. #6
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    Hi Alan
    This was an interesting read, do all major manufacturers take all of this into account or only to a small degree.
    If they don't are we saying that by some testing you could even have some sort of moveable ballast to utilise the depth of water the transoms sit in and therefore increase the performannce?

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    Smile

    Yes, in beach cats, movable ballast are called "the crew".

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    How stupid of me, I wasn't thinking properly was I.
    So what ideas to do this on a bigger scale without loading on too much weight as this of course would have a detrimental effect on the performance?
    Any ideas?

  9. #9
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    The option on my boat will be to use the 4 water tanks as movable ballast, either fore and aft or port and starboard. My idea was not for lowering the transom per se, more to keep the nose up when going fast downwind.

    The results we have so far seen, are now being used to make a selection as to transom depth and prismatic. These 2 features will be drawn up, and a new set of drag versus speed lines computed.

    If this looks okay we will stick with it. If one had the time and money, you could go on endlessly tweaking all the features, shapes and sizes on a boat, as the interaction of these various parameters is not always fully known, you could keep at for years.

    We need to remember this is only software, so the rsults will never be 100% correct but rather very good indicators of where we are at.

    The main increase in drag at around8-10 knots is due to wave making, so changing of the shape of the hulls would of course influence this, but as we do not have unlimited funds and time, we are only looking at optimising the stern areas.

    Alan

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    I am really intrigued by this discussion, it goes against everything that I intuitively thought to be correct. It is really challenging my perceptions and I really look forward to understanding more of the physics

  11. #11
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    I was never that good at physics or chemistry at school, biology and basic maths ie percentages yes, so if you lot don't mind can you post your concrete findings with suggested solutions to this thread in plain English, I would very much appreciate it, I am not saying that this has been the case so far but with any future posts I hope there won't be any weird formulas.
    Please don't take this post post as a moan as it is certainly the reverse, I am finding this very interesting but wont to make sure I fully understand it and not have to ask silly questions and sound like the thick kid at school.

  12. #12
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    Nordic,

    I have difficulties to buy the water ballast thing in cats. If you increase weigth, you are not going fast anymore.

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    Quote Originally Posted by terhohalme View Post
    Nordic,

    I have difficulties to buy the water ballast thing in cats. If you increase weigth, you are not going fast anymore.
    I fully agree, but when you do have some water on board anyway, you might as well place it where it can do the most good. I'm talking 2-300 kgs maximum here.

    Alan

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    Quote Originally Posted by ireaney View Post
    I was never that good at physics or chemistry at school, biology and basic maths ie percentages yes, so if you lot don't mind can you post your concrete findings with suggested solutions to this thread in plain English, I would very much appreciate it, I am not saying that this has been the case so far but with any future posts I hope there won't be any weird formulas.
    Please don't take this post post as a moan as it is certainly the reverse, I am finding this very interesting but wont to make sure I fully understand it and not have to ask silly questions and sound like the thick kid at school.
    Hi Ian,

    I will try to keep it as simple as possible, for my own sake as well

    I have opened a new thread here on the multihull design forum with a few links to places where anyone interested can learn a bit more about some of the terms used here like Prismatic coefficient etc.

    Hope it is helpful

    Regards

    Alan

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    Hi Alan,
    Thanks, very much appreciated.

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    Default Results of drag analysis

    I have recieved the results of the analysis on drag, and I'm posting a couple of graphs here.

    This will not be the final result, as we have decided to drop the Prismatic back down a bit as it did increase the wavemaking drag between 6 and 8 knots. This is primarily due to transverse waves. So the final solution will be a Cp of around 0.59 and a submerged transom (50 mm) at full load.

    Notice how the wavemaking resistance has dropped from the original design with transoms out of the water and the improved design.

    The conclusion:

    This little excercise has reduced the total drag of the boat by around 15%

    To put this into perspective, let's look at a typical Atlantic crossing.

    Canaries to Barbados = 2800 Nautical miles.

    At 8 knots average = 14.6 days

    At 15% faster = 9.2 knots or 12.6 days

    2 days less

    Alan
    Attached Images Attached Images
    Last edited by Nordic; 18th October 2008 at 02:51 PM.

  17. #17
    peter fake Guest

    Default 12m catalac

    hi
    I have a 12m catalac, the problem is I just cant reach double figures.

    It's like sailing with the Hand(park) brake on, light winds are fine, as the speed builds so does the problem, 8kn fine, 9kn comes and goes, then it's 9.2/9.3/9.4/9.5 if lucky, never 10.

    Tried sailing at best sailing speed on flat water, starting both 35hp engins at full revs still cant reach 10 kn.

    I am told the boat should sail 12 to 14 kn.

    any suggestions

    many thanks.
    peter

  18. #18
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    The problems with Catalalac 12 are based more on heavy weight and small sail area than immersed transoms.

    You know how to gather apparent wind speed?

  19. #19
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    Quote Originally Posted by peter fake View Post
    hi
    I have a 12m catalac, the problem is I just cant reach double figures.

    It's like sailing with the Hand(park) brake on, light winds are fine, as the speed builds so does the problem, 8kn fine, 9kn comes and goes, then it's 9.2/9.3/9.4/9.5 if lucky, never 10.

    Tried sailing at best sailing speed on flat water, starting both 35hp engins at full revs still cant reach 10 kn.

    I am told the boat should sail 12 to 14 kn.

    any suggestions

    many thanks.
    peter
    Hi Peter,

    As Terho says, SailArea/Displacement is a key issue. The other issue I can imagine is the ration between the hull beam and length water line.

    As the vessel gets up to its "hull speed" which is roughly = 1.34 x SQRT Lwl the wavemaking resistance starts to grow. The size of this resistance increase is bigger for lower Hb/Lwl ratios.

    For a 12m or 39ft boat, the hullspeed is roughly 8.4 knots. I do not know what the Hullbeam or Lwl is exactly for the 12M Catalac, but probably 1:8 to 1:10 so the increase in resistance will be alot more than you see for the graphs above where it is 1:12.

    So due to the conservative SA/D and the beamy hulls, you will find it difficult to get past the resistance "hump".

    This is the reason that monohulls with their 1:3 or 1:4 ratios can not exceed their hullspeed unless thay can plane. Cats never plane, but can surf a wave as all other boats also can....

    This is a simpified explanation but I hope it helps in the understanding..

    regards

    Alan
    Last edited by Nordic; 21st October 2008 at 07:13 PM.

  20. #20
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    Default Wetted surface and induced drag

    I do not wish to criticize the Catalac design in any way. It's a durable and proved boat, and a great value for safe cruising. However, it represents a trade off in performance. By simplifying the hull design, The wetted surface area and frontal area were increased. As you can see from the above charts, the penalty is drag that increases geometrically with speed; you've hit the wall at less than 10 knots. It would take double the thrust to increase you speed through the water a knot and a half.

    Another vessel with a semicircular cross section instead of a deep vee provides enough buoyancy with the absolute minimum wetted surface area. This boat will be very sensitive to weight because its most efficient at one specific waterline.

    Its remotely possible that efforts to improve the fairness of the surface will reap minor improvements. If you have occasion to haul the boat and strip the bottom, some tedious long-boarding and fairing could profitably fill a few idle moments, however: A sow's ear is perfectly adequate for its intended purpose, and you would look damn silly with a silk purse!

    Water that is displaced by the bow has inertia. That must be overcome to fill back in behind. Water tends to stick to a surface it moves past. Thats why we see a vee shaped section of lifted water behind a long overhanging transom nearing hull speed. If the afterbody is long and very gently curved, this effect is minimized. If there is more pitch up at the stern, this is increased. Its a function of boat speed. For power boats faster than hull speed, its better to cut off the wetted surface drag with a sharp angle, letting the water do what it wants. Faster catamarans can do the same.

    But at slower speeds, a cut off transom developes as much as half the drag of a flat plate being pulled through the water. The energy to part the water, and the energy to bring it back to rest all come from the same place, the engine or the sails. So the designer must compromise; minimise surface drag, facilitate water flow around the boat and back to rest, without letting the suction of an abrupt change in profile pumping up a big stern wave. If you are going to add scoops to an older catamaran, DO NOT INCREASE the exit rise. It will be adding speed brakes, rarely useful on a cruiser.
    Last edited by Sandy Daugherty; 21st October 2008 at 05:12 PM.

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