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Thread: Do immersed transoms create less drag?

  1. #41
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    Quote Originally Posted by catty View Post
    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.
    Catty, have a look here: http://www.gunboat.com/gunboat_flash.html

    Most transoms seem immersed to me when they are sailing. Looking at the newer designs like the 66 it seems that the submerged transoms become more evident.

    If we look at the VO70 boats for this season, ALL have submerged transoms on most points of sail.... Coincidence?

    Of course waves have an influence on what is optimal, what kind of waves should one optimise for? Wavelength, size, shape ?????

    There are many other parameters that also have an influence on the drag, I'm not preaching for or against, all I'm doing is presenting the data and based on this I will make my choices and have to live with the consequences.

    10 -15% less drag in the "comfort/speed" window is a sizeable amount IMO. Even if it was only 5% it's still worth a shot.

    The amount of time I expect to be sailing at less than 4 knots is minimal, so IMO immersed is what I will go for.



    Alan

  2. #42

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    Quote Originally Posted by Nordic View Post
    Catty, have a look here: http://www.gunboat.com/gunboat_flash.html

    Most transoms seem immersed to me when they are sailing. Looking at the newer designs like the 66 it seems that the submerged transoms become more evident.

    If we look at the VO70 boats for this season, ALL have submerged transoms on most points of sail.... Coincidence?

    Of course waves have an influence on what is optimal, what kind of waves should one optimise for? Wavelength, size, shape ?????

    There are many other parameters that also have an influence on the drag, I'm not preaching for or against, all I'm doing is presenting the data and based on this I will make my choices and have to live with the consequences.

    10 -15% less drag in the "comfort/speed" window is a sizeable amount IMO. Even if it was only 5% it's still worth a shot.

    The amount of time I expect to be sailing at less than 4 knots is minimal, so IMO immersed is what I will go for.



    Alan
    I'm not sure a catamaran necessarily floats where the designer intended it after the owner has filled it with the necessities of cruising life. Try this drawing of the gunboat 48., the transoms appear well clear of the water.(one should not confuse the boot topping with the lightly drawn waterline)

    I'm not sure a cruising cat hull and a racing mono hull have a lot in common.
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  3. #43
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    Catty, this is why I started my design process "backwards". I wrote down every conceivable piece of kit someone would want on board a cruising cat, included 2 months of provisions for 4 people, more than a ton of fuel and water, aircon, dive compressor + tanks, generator, 4 anchors, 2 anchor winches, 150 m chain rode, parar-anchor, series drogue, spare parts, life raft, fishing gear,washer/dryer,full nav equipment, radios, sat comms, radar,solar panels, wind generator, watermaker, etc.. you name it.

    This came to around 4.5-5 tons of load.This is what I used as the maximum payload for the empty boat.

    In normal cruising trim with my wish list, I expect to be about 1.5 tons less, and have used this figure for the drag analysis. Immersion will be around 240 kgs/cm, so 1.5 tons is around 6 cms more. That is where the Dwl will be.


    Alan

  4. #44
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    Catty, here is another new cat by VPLP that has submerged lee transoms when sailing. As this is the third day they are sailing, they must be pretty light.. I rate VPLP up amongst the top designers of cats at the moment.

    http://www.youtube.com/watch?v=9nkKxMJxZMU


    It seems like the guys who know what they are doing these days have submerged transoms by choice and not due to overloading the boat.

    Alan

  5. #45

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    Does this mean the wife can carry ALL her hair care equipment?

  6. #46
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    Quote Originally Posted by Nordic View Post
    Catty, here is another new cat by VPLP that has submerged lee transoms when sailing. As this is the third day they are sailing, they must be pretty light.. I rate VPLP up amongst the top designers of cats at the moment.
    http://www.youtube.com/watch?v=9nkKxMJxZMU

    It seems like the guys who know what they are doing these days have submerged transoms by choice and not due to overloading the boat.

    Alan
    Now, this is the proof that I am definitely biased... I only see bigger rooster tails in the lee hull

    Of course, being deeper immersed would increase the overall wave induced, but it seems the immersed transom actually lifts water up in the air from time to time.
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    Keep Smiling
    Stefano

  7. #47
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    Quote Originally Posted by 44C View Post
    Does this mean the wife can carry ALL her hair care equipment?
    Not sure on that one I have factored in 130 kgs per person including them and their luggage, so if she isn't too heavy, there is allowance for more maintenance equipment The inverter WILL handle her hair dryer though..

    Alan

  8. #48

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    Quote Originally Posted by Nordic View Post
    Catty, here is another new cat by VPLP that has submerged lee transoms when sailing. As this is the third day they are sailing, they must be pretty light.. I rate VPLP up amongst the top designers of cats at the moment.

    http://www.youtube.com/watch?v=9nkKxMJxZMU


    It seems like the guys who know what they are doing these days have submerged transoms by choice and not due to overloading the boat.

    Alan
    Nice clip Alan. Whats the world coming too if this is the latest cat thinking. Aft beam on transom, hence no thought to centalizing weights. No transom scoops to get in and out of water.No where to get out of the sun. It must be a lake boat. If thats an average of 17-19 knots then there must be a heck of a lot of current judging by the bow wave.

    The windward hull wake looks great.

  9. #49
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    The boom is a bit on the low side as well

  10. #50
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    Default centre of balance

    Hi On the point of balance, when I purchased my 12m, the was a letter from Tom Lack about adding extra water tanks forward under the galley floor. He addvisec not as it would upset the balance of the boat.

    Peter

    Quote Originally Posted by Tropic Cat View Post
    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.

  11. #51
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    Quote Originally Posted by peter fake View Post
    Hi On the point of balance, when I purchased my 12m, the was a letter from Tom Lack about adding extra water tanks forward under the galley floor. He addvisec not as it would upset the balance of the boat.

    Peter
    Peter, it sounds like the Lacks were very aware of the balance of their boat designs. I can only wonder why they designed in stern squat with the inboard diesels?

  12. #52
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    Quote Originally Posted by Tropic Cat View Post
    Peter, it sounds like the Lacks were very aware of the balance of their boat designs. I can only wonder why they designed in stern squat with the inboard diesels?
    Rick
    Stern squat is inevitable with aft mounted engines, move them further forward and you will decrease squat, so it wasn't rocket science to "design stern squat in.."

    Alan

  13. #53
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    Alan,

    I can't physically move my engines forward in the hulls as they're just about touching bulkheads as it is.

    I'm trying to visualize what you wrote. I've been considering adding a hard bimini, which would be mounted forward of the current boat balance point. What I've been considering would weigh at least 300 Kgs and would have the effect of moving the boat balance point slightly forward. If I understand you correctly, this will amplify the stern squat issue. Or am I misunderstanding the issue?

  14. #54
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    Default power squat vs sail dive

    Picture your vessel in a side view. Draw in the center of gravity. tie a line to the center of effort of the sails, and pull on it. Tail up, nose down right? Now tie a line to the props, and pull on it. Nose up, stern squats. As you approach hull speed, the bow wave gets higher, the stern goes lower, and the engines are pushing the boat up hill!

    The power squat can be modified somewhat by pointing the thrust line down. High powered planing boats use trim tabs. The reverse can be accomplished by increasing the area of the genoa, which has some lift as well as thrust, based in part on the angle between the luff and vertical.

    Chopping a transom doesn't add speed, it slightly reduces drag and greatly reduces the cost of ownership. It only works when the boat is going fast enough, and thats not going to happen on most catamarans around today. The reason the V70s are truncated is explicit in the name. Its the "70" part! I THINK the magic happens at traditional hull speed, when the stern wave breaks away from the back of the boat and moves aft, and the angle of the bow wave (top view) becomes more accute.

    Rick, it's likely you could increase the Catalac's load carrying ability somewhat with transom extensions, and enhance your boats sailing qualities too, but not enough to justify the expense of having it done. As for doing it yourself, see www.pdq36.blogspot.com

    If you can't reduce weight, increase bouyancy!

    EMERGENCY NOTE: Avoid grandmothers with too many bow bunny offspring: she may be interested in your willingness to divert boat-bucks to tuition and fees.

  15. #55

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    Quote Originally Posted by Tropic Cat View Post
    Alan,

    I've been considering adding a hard bimini, which would be mounted forward of the current boat balance point. What I've been considering would weigh at least 300 Kgs and would have the effect of moving the boat balance point slightly forward. If I understand you correctly, this will amplify the stern squat issue. Or am I misunderstanding the issue?
    300 kg seems amazingly heavy for a bimini. Are you sure it'll weigh that much? What are you making it from?

  16. #56
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    Default exit brise ?

    hi sandy , i am intrigued by your last comment , regarding exit rise . i have extended my transoms by 3ft to bring the trailing edge of the transoms exactly level with the waterline . this was done because my transoms were always submerged by 6 inches and as you can imagine ,produced a drag inducing rooster at anything over 4 knots . the length of the extensions were created to follow /continue the hull shape, and rise , until it met the waterline . it has produced a clean wake . results were good for speed ,pitching and, IMO ,aesthetics.so , had i continued aft ,unfair ,this would go along with your theory of no exit rise ? and what would the effect be ? , regards , gramos

  17. #57
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    Quote Originally Posted by Tropic Cat View Post
    Alan,

    I can't physically move my engines forward in the hulls as they're just about touching bulkheads as it is.

    I'm trying to visualize what you wrote. I've been considering adding a hard bimini, which would be mounted forward of the current boat balance point. What I've been considering would weigh at least 300 Kgs and would have the effect of moving the boat balance point slightly forward. If I understand you correctly, this will amplify the stern squat issue. Or am I misunderstanding the issue?
    Rick,

    I think it was Sandy, who gave the example of where to "pull the strings"

    I don't think that adding weight just forward of the balance point will do anything to lessen your stern squat.

    The only viable solution would be to add bouyancy, i.e. extend the transoms. If you are considering this, have a look at the Katiecat site where they did a bum job on a Seawind..

    There is a second advantage to a transom extension, and that is you increase your waterline length, and thereby your hull speed.

    Hope this helps...

    300 kgs for a bimini sounds like an awful lot, don't you mean 300 lbs?

    regards

    Alan

  18. #58
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    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??

  19. #59
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    The sterns on the sig look like they will open. Could that be steps?

  20. #60
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    Hello Gramos! Congratulations on extending your transoms. It is no mean feat! If you gained speed under sail and fuel economy under power, you have a successful design. You will find that just the added bouyancy alone will improve the boat's motion in short chop, and help it surf sooner in following seas. By reducing the pitching motion the sails will remain effective for a longer proportion of time, and you will experience higher daily miles. If the shape is right, you will increase the speed at which power tuck becomes a problem, and widen the lines on your polars. The only downsides COULD be structural issues, and dock fees! Just keep them dry. How about some pictures and details?

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