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Thread: Do immersed transoms create less drag?

  1. #141

    Default Re: Do immersed transoms create less drag?

    Quote Originally Posted by tamicatana View Post
    newer Catanas (post-1994?) are very different boats than the older models, weight included
    I'm pretty sure the older Catanas, which were true Crowther designs are lighter than the newer Catanas. When I say old I'm thinking mid to late eighties, or the original Catanas.
    After reading my post, I think this may have been the point you were trying to make?

  2. #142
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    Default Re: Do immersed transoms create less drag?

    Quote Originally Posted by 44C View Post
    Catanas are very heavy boats too IMO. Nearly 10 tonnes for a 42 footer!
    I guess it is relative - the Catana 431 is 8.5T, while the Lagoon 450 is 19T.

    But heavy or not, it seems like all new designs have immersed transoms. Even looking at Bob Oram's designs I see immersed transoms. Chris White also.

    Mark
    Mark Cole
    Manta 40 "Reach"
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  3. #143
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    Default Re: Do immersed transoms create less drag?

    Quote Originally Posted by colemj View Post
    . .
    But heavy or not, it seems like all new designs have immersed transoms. Even looking at Bob Oram's designs I see immersed transoms. Chris White also.

    Mark
    Do you think that could be because they have very heavy engines as near to the stern as possible so as to maximise internal space. This weight distribution then means that they need plenty of buoyancy right aft.

    Mike
    Nothing works on an old boat, except the skipper.

  4. #144

    Default Re: Do immersed transoms create less drag?

    Quote Originally Posted by dmmbruce View Post
    Do you think that could be because they have very heavy engines as near to the stern as possible so as to maximise internal space. This weight distribution then means that they need plenty of buoyancy right aft.

    Mike
    ….and it's probably easier/cheaper to build, and reserve buoyancy/more rocker to elevate the transoms might serve to increase hobby-horsing?

  5. #145

    Default Re: Do immersed transoms create less drag?

    Quote Originally Posted by colemj View Post
    Even looking at Bob Oram's designs I see immersed transoms. Chris White also.

    Mark
    You do? Which of Bob's boats are designed with immersed transoms?

    To me a transom which is maybe slightly immersed is of little relevance anyway. At sea it will be alternately deeply immersed and high and dry. If the same transom were 1" higher, how much difference would it make?
    Last edited by 44C; 31st January 2014 at 08:36 PM.

  6. #146
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    Default Re: Do immersed transoms create less drag?

    Quote Originally Posted by 44C View Post
    You do? Which of Bob's boats are designed with immersed transoms?

    To me a transom which is maybe slightly immersed is of little relevance anyway. At sea it will be alternately deeply immersed and high and dry. If the same transom were 1" higher, how much difference would it make?
    Neither Chris White nor Bob Oram designs are meant to have immersed catamarans. I suspect that both designers are prone to optimism about the loading discipline likely to be exerted by builders and especially, owners.
    Currently concentrating on http://earthnurture.com .

  7. #147
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    Default Re: Do immersed transoms create less drag?

    Well, maybe I'm just not seeing these boats correctly. Here are pictures of Chris White's new A47 and Bob Oram's "School's Out" and another.

    From the pictures, these transoms don't look much different than those for Lagoon's barge.

    As for "slightly immersed", well then I don't understand this thread much. I have been on many boats with their transoms statically out of the water, which spend much of their time immersed when sailing.

    Mark
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    Mark Cole
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  8. #148

    Default Re: Do immersed transoms create less drag?

    Yes, you're not seeing them correctly. Looking from side on doesn't really show you the transoms. They're at the back of the boat.

    And there's a huge difference in the width. School's Out's transoms are maybe 18 inches wide at deck level. At water level, maybe a foot. A Lagoon's would be more like a metre.

    I agree about transom immersion being different under sail form static. That was exactly my point - what difference does an inch of static immersion make, when underway it will alternately be deeply immersed, and well clear of the water?

  9. #149
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    Default Re: Do immersed transoms create less drag?

    Would adding a pair of dihedral foils inside the tunnel towards the aft to shape the exit flow of the water change transom height? Fastcc has told me this won't work with an old light plywood Bobcat but nevertheless it would be better than squatting.

    Talbot at times has suggested adding trim tabs to the engines. That in the case of on outboard is a foil of sorts. I'm suggesting some smallish foils attached where the deadrise occurse somewhere after the bulb in a Catalac or Bobcat.

    Does anyone other than the Hysucat experts have data - test tank - computational - real life or otherwise on the effects of foils on lifting part of a displacement hull out of the water?

    Not looking to go fast but checking to see if all the numbers have been ran and all the possibilities considered. Would a couple of tabs on either hull aft keep the transom above water and the bow down some, slow the boat by induced drag, simply be not effective or end up tearing open the hull due to log strike?

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