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Thread: Do immersed transoms create less drag?

  1. #21

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    I always said there wasn't much point in having the transoms 200mm above the water didn't I?

    I was wondering how deeply immersed the transoms were in your tests? Did you try a range of different depths?

  2. #22
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    We ran at 50 mms for the hulls, just both equally immersed, on most points of sail one hull will be deeper the other higher, if we add ton we get around 4 cms more...

    I know that a deeper will lower that hulls resistance at normal cruising speeds, but as in all these issues, compromise is needed.....

    Alan

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    Quote Originally Posted by Sandy Daugherty View Post
    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. .
    I sail the Catalac 8M. She certainly is no speedster. Catalac hulls are by design deep Veed with lots or rocker. This has considerably more wetted area than newer hull designs, and is therefor slower due to water resistance. However the wide hulls allow for a decent cruising payload.

    Sandy, no offense was taken.

    I too am curious about immersed transoms. I was taught they should always be above the waterline. With my Catalac, I even went to the trouble of removing and relocating heavy items in order to make this happen.

    Was I wrong?

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    Hi Rick,

    I was of the same belief as you, until I read a thesis that proved otherwise! That provoked me to get these numbers run, and they of course are only valid for this specific set of hulls, with the hull shape and spacing they have.

    If you look at the curves I've posted, you see that the resistance is split into its 2 major components.

    1.) Friction drag. This is primarily a function of wetted area and with minor influences from hull shape and surface roughness for a given speed.

    2.) Wavemaking drag: This is due to all the waves generated, primarily the bow wave and the transverse waves. Key factors here are hull shape and hull spacing. This drag really starts growing when you get close to the so called "hull speed". The "growth" in wavemaking drag is primarily defined by the Hull beam/Lwl. The beamier the hull is compared to the length, the greater the increase in drag.

    For beamy boats (Hb/Lwl < 1:5) the drag will grow with approximately the cube of speed when you reach hull speed.

    An example:
    Lwl = 36 feet => hull speed = 8 knots with e.g. 2 x 20 hp
    To get this boat to 16 knots (2 x speed) we need 2^3= 8 times more power or 2 x 160 hp = 320 hp (given constant efficiencies for the power system)

    For slimmer hulls this factor drops.

    At slow speeds, a submerged transom will increases drag slightly due to the larger wetted area

    The submerged transom can be beneficial especially for boats with a square or "cut-off" transom, as at above certain speeds the water will be "cheated" into thinking the boat is longer than it actually is. For canoe sterns there are no benefits.
    Another factor is that an immered transom will often increase the actual waterline length, and thereby increase "hull speed"

    In conclusion, this subject is very complex, and one can not just conclude that a submerged transom is an advantage, it depends on the shape of the hulls, and transom, Prismatic coefficient, the amount of aft rocker etc.
    On the other hand, we can say that a non immersed transom is Not always an advantage as so many believe.

    If I was to come to a "rule of thumb", then I would say that for modern hull shapes with rounded hull bottoms, a submerged transom will typically be an advantage from around 40% of hull speed and up.

    Sorry about not being able to give you a clear answer..

    Alan

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    Thanks Alan....You certainly cleared that up...... yep..clear as mud

  6. #26
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    Thumbs up

    Ric: You done right. With your hull form there's not much else you can do to reduce drag. In fact, you should try to trim your center of gravity to a "most happy" speed where the lea transom is just kissing the water. The key will be your wakes. Wakes and waves are the product of wasted sail power. Your hull is most efficient when it passes thru the water with the least disturbance. If you can't keep up with the jackrabbits in higher breezes, you can get where you want to go by optimizing your light wind performance.

    Don't do this by adding to the weight in the bows, do it by shifting what you can from the aft ends to the middle of the boat. I accomplish this by building a bigger storage area under the dinette with heavy stuff, in my case tools. My biggest load are four ridiculously heavy (123# apiece!) L-16 batteries, which are under the helm instead of in the aft compartments. Take some pictures form the same perspective under a variety of wind conditions, and see if that helps you visualize the differences.

    I don't have a clue about how to figure the speed at which a truncated stern shifts from liability to asset, but it seems likely to be related to the speed at which the inertia of water excedes the suction of laminar flow. That suggests to me that it might be beneficial to leak some air into what may pass as a boundary layer. I wonder if I could take a hose from the exhaust side of the vacuum cleaner to that thru hull for the head intake....

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    Are we saying that if you are steaming along and you have two rooster tails shooting skywards from the two transoms the boat is not well trimmed

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    Quote Originally Posted by ireaney View Post
    Are we saying that if you are steaming along and you have two rooster tails shooting skywards from the two transoms the boat is not well trimmed
    Actually, in my case I do get a 'rooster tail'. Only one though and it's about 10' (3 meters) behind the boat where the hull wakes come together. It appears at speeds around 6 knots and over.

    I've tried hanging over the stern to watch the hull wakes interact under the boat, but my glasses keep falling off

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    Quote Originally Posted by Tropic Cat View Post
    Actually, in my case I do get a 'rooster tail'. Only one though and it's about 10' (3 meters) behind the boat where the hull wakes come together. It appears at speeds around 6 knots and over.

    I've tried hanging over the stern to watch the hull wakes interact under the boat, but my glasses keep falling off
    I suggest that if it has happened before it may be the time to think about tying them on

  10. #30
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    too geekish

  11. #31
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    Default rooster tails?

    Yes Ian. The horsepower required to stir up all that water came from the sails, and is not being used to make the boat go faster. The next time a big Grand Banks trawler goes by, smelling like a city bus, and dragging along a four foot swell, think about how many of his 800 horses are just pumping water into the air! Fireworks are fine when you've got more wind than you need, but on those days when a screaming reach is not in the plan of the day, lengthen your water line, and minimize your rooster tail. Send out an extra bow bunny* or two, however many you can spare!

    *Bow Bunny: "lightly clad unskilled eye-candy persons sent forward to trim the bow down; more useful for ornamentation than vessel management."
    Last edited by Sandy Daugherty; 24th October 2008 at 04:46 PM. Reason: added seasoning to taste.

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    Quote Originally Posted by Sandy Daugherty View Post
    *Bow Bunny: "lightly clad unskilled eye-candy persons sent forward to trim the bow down; more useful for ornamentation than vessel management."
    OK......now I'm officially envious as I'm without a supply of bow bunnies

  13. #33
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    Its simple; for an unlimited supply of bow bunnies, date their grandmothers.

  14. #34
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    Quote Originally Posted by Sandy Daugherty View Post
    Yes Ian. The horsepower required to stir up all that water came from the sails, and is not being used to make the boat go faster. The next time a big Grand Banks trawler goes by, smelling like a city bus, and dragging along a four foot swell, think about how many of his 800 horses are just pumping water into the air! Fireworks are fine when you've got more wind than you need, but on those days when a screaming reach is not in the plan of the day, lengthen your water line, and minimize your rooster tail. Send out an extra bow bunny* or two, however many you can spare!

    *Bow Bunny: "lightly clad unskilled eye-candy persons sent forward to trim the bow down; more useful for ornamentation than vessel management."
    Hi Sandy, Thanks, mind you regarding the bow bunnies, do you use the standard eared bunny or lop eared, the problem being at the speed they multiply within a couple of days the bows would be under water - silly me you mean the other type of bunny.

    But seriously, I suppose as earlier mentioned trimming is very important and to have some sort of moveable ballast, especially for the long cruising runs (so long as it was not extra ballast) ie being able to move the boats water supply to different tanks would be a very good idea.

  15. #35
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    In the mid sixties, when punch cards were the latest and greatest way to deal with data, your ballot, or paycheck, or enrollment document bore the legend "Do not fold, spindle or mutilate."
    I was in flight training in Pensacola. The back of my helmet, clearly visible to the flight instructor behind me read "Do not scold, befuddle or Confutilate."
    It seemed to work. It has become something of a motto for me. Life is less befuddling when the urge to improve things by making them more complicated is firmly restrained. But the urge is frequently overpowering. I hope you can find a simpler way to achieve fore and aft trim!
    Airplanes have pitch trim, and its pretty easy to over-control, but you can eventually find a happy place. But air is much smoother than open water, and a few dozen feet high or low is perfectly acceptable. We don't want that much variance around sea level.
    There are expensive anti-pitch and roll stabilizers on gold-platers, which work more or less well. And there are water ballast systems on many megabuck sailing machines, but the keywords are gold-plate and megabuck. I suspect the simplest and most effective solution to the problem is to just limit weight on the vessel, and concentrate what you just can't live without in the middle.
    Last edited by Sandy Daugherty; 24th October 2008 at 06:20 PM. Reason: speeling chock

  16. #36
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    Sound advice Sandy.

    Regarding water flow at the transoms, what is important at all speeds is to get a good separation of the flow from the hulls, without creating alot of turbulence.

    If you get water flowing back towards the stern, and then down, this causes a suction that increases drag, and is wated energy, as is the creation of excessive stern and bow waves. With the current levels of fuel prices, even some of the motor boaters are starting to learn about hull speed

    The difficult thing with optimising a sail boats lines, is that you have to compromise, as they have to work well over a large speed range, and at different angles of heel, or for multis, more different levels of immersion.

    A ship or motor boat is alot easier to optimise, if you select a well defined speed window that isn't to large, this is the reason why bulbs on bows are often used. They can optimise the vessel for a speed and displacement.

    This is also the reason why bulbs aren't any use on sailing vessels.

    Alan

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    Quote Originally Posted by Tropic Cat View Post
    OK......now I'm officially envious as I'm without a supply of bow bunnies
    Known as "deck fluff" in UK

    Rick, our conversation about the at-rest position of the transom, I recommended that it was just above the waterline (about 1cm) That is because the stern naturally digs down a bit when sailing, and you will get an immersed transom anyway.

    I suspect the 12m is exactly the same as the 8 and 9m (same underwater shape, just bigger) These boats are very sensitive to weight. the sail area is exceedingly conservative even when the boat is light, fill it up with stuff and it stops being a sailing boat, and turns into a motor sailor because you are fed up with how slow she is!

    Spend this winter taking everything out and seeing just how much extra weight you have added.
    Insanity is continuing to do the same thing and expecting different results

  18. #38
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    Quote Originally Posted by Talbot View Post
    Known as "deck fluff" in UK

    Rick, our conversation about the at-rest position of the transom, I recommended that it was just above the waterline (about 1cm) That is because the stern naturally digs down a bit when sailing, and you will get an immersed transom anyway..
    Mark... There is no winter here!! We sail 360 days a year. If there is any time we take a break, it's during hurricane season. One season of sailing here in Florida is equivalent to 3 or 4 seasons of saiing in The British Isles.

    As to the stern. My boat is extremely light in that I have emptied the boat out years ago in my quest to find the perfect balance. At rest my transom is 4" (10 cm) out of the water. This is with 2 gallons of fuel in each fuel tank, and no onboard water in the water tanks.

    A note... under sail my transom is clear of the water. However, the boat has considerable stern squat while motoring. I've been giving this some thought lately and believe the Lacks either didn't properly calculate the boats center of gravity or puposely designed the boat to motor with the stern squatting. In my way of thinking, if power is transmitted to the hull of a boat aft of it's balance point, the bow will dip when power is applied, if mounted forward of the balance point, the stern will squat.

  19. #39
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    In my way of thinking, if power is transmitted to the hull of a boat aft of it's balance point, the bow will dip when power is applied, if mounted forward of the balance point, the stern will squat.
    And you are right to a point, it is more to do with higher or lower, to have the transom out of the water under power you need a prop out of the water, you could pinch one out of those everglades boats....

    My 12.5m cat used to squat under power and leave behind a wake not dissimilar a power boat, while when sailing you would hardly know I had gone past.

    As I mentioned on another forum, I had several power and sailing cats of all sizes. Immersed transoms create a lot of drag until the water detaches from them; to do that you need high speed, flat water and uninclined ride, hardly possible in open waters.

    For those who want immersed transoms, it helps to have a very sharp trailing edge. I will stick with out of water ones.
    Keep Smiling
    Stefano

  20. #40

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    Quote Originally Posted by Nordic View Post
    Hi Rick,

    On the other hand, we can say that a non immersed transom is Not always an advantage as so many believe.

    If I was to come to a "rule of thumb", then I would say that for modern hull shapes with rounded hull bottoms, a submerged transom will typically be an advantage from around 40% of hull speed and up.

    Sorry about not being able to give you a clear answer..

    Alan
    So have we taken into account waves or were the figures run for cyber world conditions. Gee it appears Morrellie and Melvin have so much to learn.

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