Surface preparation theory

Discussion in 'Boat Design' started by 81Pantah, Jul 11, 2007.

  1. 81Pantah
    Joined: Jul 2007
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    Location: Cambridge, MA

    81Pantah New Member

    Hello everyone,

    I am a new poster, but have been sailing for most of my life. I race Laser IIs in Newport, and will be sailing in the Newport Regatta this weekend. I have been reading up on race preparation and have run into some confusion as to the general theory of hull prep.

    The Laser II is a small racing dinghy and is by no means an America's Cup contender... but I want to treat it as such. After sitting around the lunch table with my coworkers (engineers) we ended up with more questions than answers. I am hoping this community can help.

    What is the basic theory of hull surface design; not hull design. Strange viscosity reducing chemicals aside, when it comes to wax, sanding, and polishing... what in the world am I actually trying to accomplish. I have heard that the smoother, shinier, and more glossy you can get the hull the better. I have also heard that a matte finish with 100 grit sandpaper will create microeddys and help reduce drag on the hull. What is correct?

    I have an entire day to deal with the hull, so I am hoping you can point me in the right direction. Technical papers, bar stories, or explanations of tried and true methods welcome!

    Thank you for your insight!

  2. messabout
    Joined: Jan 2006
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    Location: Lakeland Fl USA

    messabout Senior Member

    Marchaj covers the topic in one of his books. He is using Reynolds numbers to make his argument. It seems that the front third of the hull and appendages deserve the most attention. Not to say that the rest of it should be ignored.

    Wax is a no no on the wetted part. The idea is to wet the surface so as to minimize the boundry layer. Wax is, perhaps, worthwhile on the deck. The reason being to get rid of the excess weight you are carrying as water on a wetted deck. A miniscule factor maybe, but if it gives you one more boat length on the weather leg, then do it.

    Opinion only: Microeddies be damned. You want to make the boat slower? Then just rough up the bottom. 100 grit.... sheeesh! An eddy of any size is caused by a disturbance of some sort. An eddy is the result of having accelerated a particle. If you believe in F=Ma then I rest my case.

    Chemical applications or graphite coatings are mostly an exercise in selling you something you dont need. When I was a hard nosed racer, I'd finish the bottom with at least 800 grit wet or dry. Then I'd buff it carefully with 3M Finessit. Whether the boat was faster was arguable but I thought it was. That was a confidence builder and I did not miss quite so many wind shifts as a result. Part of this excercise is mental as you must know. Before you get carried away with polishing, make up a longboard with about 400 grit wet or dry. The first order of business is to fair in the undulations. Yeah, I know the Laser should have come out of a perfect mold and had been laid up by a perfect workman. Fat chance. Use the longboard judiciously and with attention to the smallest bit of waviness. You might well have to do some filling to get it right.

    None of this will matter much if you get a crappy starts, get covered badly on the weather legs, or be on the wrong tack when the shifts come.

    The real guy to address this, in a technical way, is Tom Speer. If we can get him to weigh in here then you can take his advice as reliable. Meanwhile I contend that slick is better.

    Best of luck
  3. tspeer
    Joined: Feb 2002
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    Location: Port Gamble, Washington, USA

    tspeer Senior Member

    I defer to Frank Bethwaite - he's actually done the experiments. Frank's results: the smoother the better.

    Bethwaite, Frank, "High Performance Sailing," Waterline Books, Shrewsbury, England, 1993.

  4. Raggi_Thor
    Joined: Jan 2004
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    Location: Trondheim, NORWAY

    Raggi_Thor Nav.arch/Designer/Builder

    Yes, the smother the better, but the forward part is most important because you can extend the area of laminar flow, also on keel/centerboard and rudder.
    I think it make sense to see that water "flow out" on the surface, not building droplets.
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