Hull speed improvement

Discussion in 'Sailboats' started by CASS, Nov 2, 2005.

  1. CASS
    Joined: Nov 2005
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    CASS New Member

    Hi, I have a first 40.7. I have just spent 5 days sanding off all coats of paint right down to the gelcoat.(Hull)
    After all this work I am wondering how to make sure the hull will be as fast as possible when finished.
    I intend to put on 3 coats of epoxy primer and then Antifouling.

    So you specialists: Do I smooth down the Primer? if so with what grain? at the moment I have sanded down using 60 grade discs.

    What do I do with the keel and rudder? Fairing? how is this done?
    How do I make sure the keel and rudder are symetrical? is this really very important? what gains can I expect with a smoother hull, how smooth are we talking?
    Advice please, suggestions please.
    I have come this far, might as well go the whole hog, right?
    :?: :?: :?:
  2. cyclops
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    cyclops Senior Member

    To maintain the highest speed requires a VERY, VERY clean and glass smooth bottom. PLUS a coating of the Slick Bottom coatings used by the Off Shore racers. And pulling it out of the water whenever you can.
  3. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    During the refairing process it would also be a very good idea to insure your appendages are symmetrical and follow the designer's sectional shapes. Most production boats have asymmetric appendage shapes, though not intentional, just not perfect, some, off quite a bit. This usually involves full size templates made to the correct shapes and lots of fairing.

    Considering your lack of experience in this adventure, it would be best if this was a task hired out to professionals. Fairing to templates is exacting work, not easily executed by the beginner.

    Coatings are a way to gain some speed, but truly, smooth, fair and correctly shaped below the water surfaces will provide the best you can ask of her, without resorting to changes to sectional and plan form shapes. I know racers that use 1200 grit on their bottoms before each race, you've got a way to go yet, from the 60 you've started with.

    All this effort is not usually noticed by the average skipper and crew, who aren't skilled enough to twist the last tenth of a knot out of the boat. If your class is very competitive and you're looking for an edge, I'd suggest lots of practice and crew drills. A sharp, knowledgeable, well trained skipper and crew will beat a competitive boat most times. I've taken inferior boats and beaten vessels that should have eaten me for lunch, but I've out smarted them with a good crew, better tactics and a better handled yacht. Many times the fastest boat doesn't win, but most times the better sailed boat does.
  4. lewisboats
    Joined: Oct 2002
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    lewisboats Obsessed Member

    Ever heard of blueprinting an engine? Same Same. Of course... the better driver is usually worth more than the gains in horsepower that are achieved by blueprinting.
    'Course...this is just a Gearhead (or Redneck) way of saying exactly the same thing as the quote above but something stated another way is often better understood.

  5. CASS
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    CASS New Member

    Of course the 60 grade paper was needed to get off the old paint.
    We were thinking of finishing off with 600 (wet rubbing down) but 1200 grade is a bit over the top. I know of a 40.7 in Britain that boasts a hull that has been sanded down with 800 grade paper and is supposedly the fastest 40.7 on the south coast! Obviously we are only club racers and not professionally involved but we have succeeded in beating all the other 40.7 clubbers in our group. It was after being confronted with the work of taking off all antifouling, that we decided to do what we could to make the hull faster. My posting was to find out what (real) difference in speed we could expect by fairing and rubbing down. Obviously letting go the Genoasheet at the wrong moment or hoisting the spinnaker upside down (yes it happens) will kill any advantage through the above application of elbow grease.
    I have been told that a faired and well prepared hull could gain up to half a knot against a non faired badly painted hull (with antifouling flaking off)
    I guess this was rather too optimistic.

    Thank you very much anyway everybody.

    Here is another thought for you though, in slight winds we are faster with our dacron sails than with the MAXX carbon racing sails.
    Now that I did not expect!
  6. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    Tests show that 320 grit is as fine as it is needed. A glass smooth bottom is slower. The reason is that the laminar flow is thicker- there is more adhesion- than a small turbulent layer.
  7. gggGuest
    Joined: Feb 2005
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    gggGuest ...

    Could you name the tests please. Everything I've heard suggests that is a popular myth.
  8. yokebutt
    Joined: Aug 2004
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    yokebutt Boatbuilder

    Cass, wetsand it to 1000-1200 grit or so, then polish with rubbing compound. Once you get into the finer grits it doesn't take that much time or effort, use the sanding blocks on a stick made for sanding ceilings. The simple reason for making the surface very smooth is that it's much easier to keep clean.


    P.S. Never mind the rubbing compound with antifouling.
  9. Guy G
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    Guy G Junior Member

    I have been in the industry for a spell and I have never seen or heard of anyone only going to 320 unless they just don't care! I think Bob Koloti would love to hear me tell him, he has been wrong all these years in training me!
    LMAO! To think I've been sanding my hiny off for know reason. HAHA!
    I don't work on sail boats but I have spent as few hours on off shore race boats and we sand them down to at least 1200 grit!
  10. gonzo
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    gonzo Senior Member

    Check the experimental data in "Principles of Yacht Design by Larsson & Eliasson". The use of rough surfaces in leading edges to promote turbulence and create smaller eddies produces less drag. The myth that a mirror finish is faster has been proven wrong in tank tests. However, the finish of boats is often done for the showroom to increase sales, not to make them faster.
  11. sabre66
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    sabre66 New Member

    No one has mentioned the most basic form of fairing using a long board. I faired our hull last year using petitt fairing compound and west systems.
    I simply went after the obvious defects "free hand" then used an eight foot long board, stretched along the hull, to show any high/low spots, filling them or sanding as needed.
    I also made templates of the keel to transfer profiles from side to side to achieve an even shape. It’s important to remember that you will need to apply a barrier coat if you sand through the gel coat or if your using micro balloons to fair. De-wax the entire area first then apply six or more coats of epoxy, finishing with west systems barrier coat additive. I don’t sand beyond 150 grit, in my opinion, anything below that is left for one design racers with Teflon bottoms.
    I noticed a big difference this year on the race course but that being said I agree with those who mention good sailing tactics and crew as being the biggest advantage. We sail a 1966 Columbia Sabre yet we beat a Tarten 3700 and several C&C 32’s all summer long not just on time but boat for boat.
    Learn to drive your boat well and practice with your crew and leave the sanding to the one designs where every last detail counts.
  12. mattotoole
    Joined: Nov 2004
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    mattotoole Senior Member

    Yeah, this garbage was around too when I was a kid sailing Lasers. Some of them even believed a shiny, brand new hull could be improved by sanding with 400 grit.

    I proved them wrong by fairing, refinishing, and polishing my boards, and then sanding to 1200 grit and polishing my hull with rubbing compound and auto detailers' glaze. I won two national level regattas with that boat, which everyone said was too old and flexy. Others who borrowed it commented on how fast it was and how it outpointed everything else. But then they went back to their stupid 400 grit sandpaper ritual, as the prevailing "wisdom" dictated.

    After I let the maintenance slide, I discovered the finish made the most difference on the boards.

    So you might want to concentrate on your keel and rudder, both shape and finish.
  13. bazza
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    bazza Junior Member

    whatever happened to that ,I think 3m stick on stuff that was supposed to imitate shark skin. I believe it was supposed to improve laminar flow by dragging water along with it therefor creating a water to water interface?
  14. Van Nostrum
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    Van Nostrum Junior Member

    You may not want that turbulence when it comes to actually using the foil.

  15. Leo Lazauskas
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    Leo Lazauskas Senior Member

    Sorry, but you didn't "prove" them wrong. Your winning experience is not scientific evidence that a smooth or rough surface made any difference whatsoever.
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