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#31
10-01-2005, 06:29 PM
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| If you want to plane, you want as little drag as possible pulling you back. You also want as much lift as possible pushing you up. On most planing powerboats the bottom actually slopes down right up to the transom, usually about -5 deg., and so you get some lifting force at the stern. You then have a sharp corner so the boat and water can break away while maintaining more-or-less laminar flow right up to the transom. Curving the transom double-ender style works well at low speed, where laminar (smooth) flow can be maintained right up to the transom. But try to plane such a hull, and you get massive turbulence developing under the stern as the water tries to keep in contact with a boat that is moving faster than the water can adjust. Turbulence then prevents the hull from lifting and so it can't plane easily. In general, the trailing edge should be sharper the higher the speed. (Note that this refers to the trailing edge of the hull-water interface; ie. the corner between the transom and bottom as viewed in profile.)
__________________ - Matt Marsh - Marsh Design (small craft blog and designs)  |
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#32
10-01-2005, 07:09 PM
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| But in the scheme of things...Really now...How much of an issue is that mini wave at the stern of a double ender sail boat fully loaded for blue water sailing? It's becoming apparent that this thread has hit a "Y" in the road...Speed and efficency vs safety and good looks....  |
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#33
10-01-2005, 07:22 PM
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| A day in the life... Hello... Got back from a sail in my 'fatass'... Here in Vancouver you 'reach out and run back' - I many times would like try a sail in a double ender over my boat when running wing on wing with a pole out on the jib - going home... With the tide aft and running in with me - I cannot pour a whiskey without fear of a wild and crazy broach - bah... Mind the boom... My next boat will have two 'pointy ends' - and a pilot house against the damned rain... Yup.... SH. |
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#34
10-01-2005, 07:48 PM
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#35
10-01-2005, 07:49 PM
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| who's right who's wrong ? Quote:
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#36
10-01-2005, 07:54 PM
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#37
10-01-2005, 08:50 PM
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| kudu: How much of an issue is that mini wave at the stern of a double ender sail boat fully loaded for blue water sailing? When you're broad reaching in a nice fresh ocean breeze, it should make plenty of difference. But zipping along in perfect conditions is the least of your worries when cruising. I wonder if keeping that max speed down might reduce the chances of broaching? |
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#38
10-01-2005, 10:56 PM
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| You gots to have purpose... Hello... I don't drive a NASCAR to work and I don't run my Geo Metro on the track on weekends... Form follows function - necessity is the mother of invention - glass is good if you need to look through it - all that stuff... I think the one we need here is - 'play the game by averages'... You can build the 'perfect' boat - the 'perfect' race car - but you cant guess at what (G)od is going to throw at you on any given day - or for a month at sea... So that said - I like designs for average conditions with creature comforts for those less than good conditions...  Maybe thats why I stay married - and drink whiskEy... SH. |
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#39
10-02-2005, 02:12 AM
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| A few comments here. Latest research has shown the submerged transom to give lower drag BUT it depends on other factors in the UW hull form too mainly how the UW aft lines run. Transoms that are entirely above the WL are essentially double ended hulls but the topside run aft gives a lot more usable space and counters one problem with pointed sterns, which is pooping in a following sea due to lack of reserve bouyancy. Modern heavy weather tactics have changed with a quarter presented to the following sea and the hull allowed to run free rather than stern-to with warps and drogues to slow the boat down. Under these tactics a double ender has only the disadvatage of a lower bouyancy aft. Another problem can be the hobby-horsing tendancy with similar sections fore n aft. Flatter aft sections damp the pitching oscillation better. I have trouble with the loss of a decent cockpit or aft cabin with boats with a pinched stern, I favour a small transom to a true double ender any day.
__________________ Mike Johns. |
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#40
10-02-2005, 02:14 AM
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#41
10-02-2005, 02:44 PM
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#42
12-03-2011, 03:43 PM
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 . Just remember it takes energy to move water !! a object moving through water is separrating and pushing aside the water to move forward and as it gets past the widest point the water presure +energy is pressing backinto fill the void being left behind as the hull moves forward through the water . this presure is taking place a long way fro the actual hull as well not only up close but up to a couple of meters away look at the wave alongside the hull as it moves !! the wave is water pressure up is pressure down is suction so take notice nexttime you see a boat going through the water . same applys to air with two cars passing at high speed they push eaach other aside and then immediatly draw each other into the other slipstream behind is more notice able in two highspeed trains traveling in opposite dirrections in a tunnel . theres high pressure before and suction after . water is fluid air just a lot more dense !
__________________ Making beautiful boats is a passion never a chore ! |
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#43
12-03-2011, 04:01 PM
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| Making boats is a[l]ways a passion never a chore! I couldn't agree more. Thanks for bringing this old discussion to light!
__________________ Hoyt "Lightning is very selective and will not strike crap." Wynand N "We Redistribute World's Wealth By Climate Policy" UN IPCC Official |
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#44
12-03-2011, 05:21 PM
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| aged sour mash...straight. smoothe with an E |
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#45
12-03-2011, 05:29 PM
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| Quote:
A) a canoe stern is a square transom with the edges cut off, B) a transom with the hull extended out further If it is A, then the hobby horsing is 'more', if it is B, then it is even 'less' I like canoe sterns when a huge wave is chasing in behind you across the harbour bar. I like square sterns when I am lounging around in the sun with friends, and there is more room on the end of the boat, while paying less at the marina per foot. I hate canoe sterns when I have to build extra framing at both ends, and bend the timber at both ends, and paint/epoxy the narrow bits on the inside at both ends. I hate canoe sterns when trying to hang a dinghy or an outboard motors of the end of the boat, or trying to climb in from the water. So, I definitely prefer flat sterns unless canoe sterns are better - I think. |
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