ski bottom

Discussion in 'Powerboats' started by 650xs, Oct 28, 2005.

  1. 650xs
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    650xs Junior Member

    what are the advantages of having a ski bottom on race /ski boat, compared to solid vee to full length of hull also is it worth having aerodynamics on top side of hull on craft with top speed of 75 mph, with a mono hull single person of about 16 foot , will it make a difference????????. just a few qsssss for you to ponder .....:) taaa john
     
  2. marshmat
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    marshmat Senior Member

    Are you talking of, essentially, building a 1-tonne, 16-foot outboard-powered water ski that runs 75mph?
    A big reason for the V is so that the hull can cut cleanly into waves instead of just slamming onto them. A flat or concave-flat bottom will POUND. Big time. So the waterski hull shape on your boat, I'd say, is probably a no-go if you want to run in anything more than 3" ripples.
    As for aero: Over about 40mph this does become a significant factor. At 60, topside aero will make a fairly noticeable difference in handling and speed. Over 100 and it becomes really critical, to the point of being almost as important as the hull for stability. Look up the BAT- (can't remember the builder now)- a speedboat of about 25-32 feet, that actually has 'wings' built into the gunwales to help with dynamic lift, getting the stern higher out of the water.
     
  3. Jango
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    Jango Senior Enthusiast

    A small vee bottom, 3 - 7 degree deadrise at the Transom with a gradually increasing vee forward to a CONCAVE VEE not a Convex will give the best tradeoff in a small 75MPH boat. Convex bows will pound far more than concave bows since the concave will tend to throw the water away from the boat instead of absorbing it. Larger deadrise will however ride smoother in larger waves at the expense of speed. Unless you are operating in dead smooth water all the time, I would not consider a flat bottom. Everything is a tradeoff.
     
  4. nevd
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    nevd Junior Member

    Flat vs Vee bottoms

    More deadrise Vee not only helps ride, it also improves resistance to porpoising, improves turning and tracking in swells.
     
  5. cyclops
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    cyclops Senior Member

    A word about running 16' above 45 mph. If you accidently jump a 2' wave above that speed 2 things can happen. You go airborne for about 2-3 seconds and look at the windshield up close. Or you stuff the top of the boat into the crest and get thrown into the wheel and windshield. DO NOT try those speeds where a suprise wake from a boat to your blind sides can get in front of you. Only thing to do is say sh**, and wait it out. NEVER open her up on the edge of where shallow and deep water meet. A big cruiser +1/2 mile away will send it's wake downward in deep water. It disappears. When the deep wake hits the bottom where it come up to the shallow area, it instantly is 3' high again. That 3' wave will roll for 500' with ease. I have been hit 1 time like that. 67 years old and airborne at 65 mph for 3 seconds can really pucker up your butt in a hurry. I was lucky in the Chaparral.
     
  6. KCook
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    KCook Senior Member

    A 16' V bottom at 75mph? Look up "chine walking".

    Kelly Cook
     
  7. Jango
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    Jango Senior Enthusiast

    Not much chine walking with a 3 deg. Transom deadrise.
     
  8. KCook
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    KCook Senior Member

    Jango has a point. Even so, 75 is cooking for a 16' boat.

    Kelly
     
  9. marshmat
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    marshmat Senior Member

    3 deg. deadrise on a lightweight 16 footer probably won't chinewalk. But if you expect to run 75 in that thing, do yourself a favour and invest in a full pneumatic suspension chair for the captain- 'cause this boat will pound like hell in 4" pond ripples.
     
  10. Jango
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    Jango Senior Enthusiast

    I'm talking pure inboards which are probably a little smoother running.

    Used to race a "Cracker Box" class in the 1950's with the specs we're talking about (16', almost flat bottom at transom and 65mph+) Never ran in a 2' chop, but ran smooth even in 6" ripples.

    Seriously, my present Boat, a 19' Monoco (w. 3 1/2 deg deadrise) runs reasonably smooth in a 6" to 12" chop at 75mph+.

    Granted more deadrise will make for a smoother ride, and balance and trim will make a big difference too.
     
  11. cyclops
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    cyclops Senior Member

    I don't see how. My 2002 Chaparral about 24 degree V, 186 SSI ,5.oL 220 hp at 56 gps and wet about 3600# is a NO NO over 12" to 18" . I don't like being tossed in chop from all directions on weekends.-----------edit. Glen L Monaco woody? Yea. It is a helluva lot smoother with the engine in the middle.--------------- My boat can jump 3X higher than yours.
     
  12. Jango
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    Jango Senior Enthusiast

    Hey Cyclops, We're about the same age and I also don't like getting tossed around anymore. You're right on about the smoother ride with the engine in the middle, although I moved it back some in order to run the speeds it,s doing. They like to HOP when the CG is too far forward with a lot of power. She's putting out OVER 300HP at the prop with only 2350 lbs total.
     
  13. Jango
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    Jango Senior Enthusiast

    650XS, the advantage of having a Ski/Race bottom is it throws a much smaller Wake with virtually NO roaster tale which is ideal for skeers. Secondly, small deadrise bottoms take LESS power and are therefore favored by racers.

    As shown previously, All this comes at a price - small deadrise means a rougher ride in a chop.
     
  14. 650xs
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    650xs Junior Member

    and so

    thanks guys ,thanks for all info , i m hoping to run 70hp motor turnig out about 90 hp after tinkering ,boat is about 14 feet circuit racer for uk t850 class boats in this class race in all conditions with high winds etc and with a whole fleet of mixed boats like 850 cats and f 2 cats ,we are all some times all out together which makes it vvvvvvvvvvvvery goood fun ,when its on a 1/4 mile circuit.so it does get a bit choppy at times so boats have to be able to ride rough and tur f.....kkkkkk tight, so set up with hull types and engines is crucial to get it wright.answer to rooster tails ,there is lots of them and when you hit one in yer face u know,so i ve found in my small experience:D
     

  15. cyclops
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    cyclops Senior Member

    Put a small windshield 3' in front of you face. The air flow will carry the water over you.-------About 2'-3' wide.
     
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