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Re: a hypitheticaly stupid question

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In Reply to: a hypitheticaly stupid question posted by Ken-Ohki on March 02, 2003 at 16:16:00:

here is a wild guess I am making without checking anything, just trying to remember from school days..
f = m * a
w = f * d
p = w / t
v = d/t
then
d= vt
and
w = fvt
then
p = fvt /t
and
p = fv
1 newton = .224 pounds
1 mile per hour = 0.447 meters per second
as you are in the water and the boat begins to pull you, lets say you feel like it is 100 lbs on the ski handle..if so then from above 446 newtons
you start a 0 mph and lets say you plane at 10 mph so your ave speed is 5 mph (or should we use the maximum speed? these are all gueses) or 11.2 meters per second
so
p = 446 newtons * 11.2 m/s
p = 5000 nm/s a newton meter is a joule, 1000 joules is a kj, a joule per second is a watt
p = 5 kj/s = 5000 watts
there are .746 hp in 1000 watts
5000 watts (.746 hp / 1000 watts) = 3.73 hp doesnt include hp loses from motor to water to skier
If all the stuff above was correct then maybe your answer is between 2 and 5 horsepower for power required to lift person on plane only, if the boats weight and motor are zero.

Follow Ups:

Re: a hypitheticaly stupid question - machnumber2 10:34:36 03/03/03 (4)
oops I think i spotted at least one error above, 5mph is not 11.2 m/s it is 2.2 m/s.
so then
p = 446 newtons * 2.2 m/s
p = approx 1000 nm/s a newton meter is a joule, 1000 joules is a kj, a joule per second is a watt
p = 1 kj/s = 1000 watts
there are .746 hp in 1000 watts
1000 watts (.746 hp / 1000 watts) = 3/4 hp hmmmmm that doesnt seem right either nevermind!! lol
- Re: a hypitheticaly stupid question - KenOhki 23:13:50 03/03/03 (0)
  I knew i should have been looking at the girls paper next to me in algebra rather then down her blouse ;P
  I actualy understand some of that formula, but man its been a long time hehehe. But I have the anseres I need I think
- Re: a hypitheticaly stupid question - Tailgunner 12:55:52 03/03/03 (2)
  Ken, the easy answer is you need 1 horsepower for every 27.5 pounds of weight. 200 pounds would need 7.27 horsepower to launch and plane. add the weight of the boat motor and another person and this is where the extra hp. comes in, example boat=400lb. skier =200lb. motor =155lb. driver=200lb. You now have to move 955lb. from dead stop to plane on calm water. Theoretically you need 34.72hp minumum.Top speed is not a revelant issue here. There are simply too many variables to be discussed here to even determine what top speed would be. A skier will stay on plane at as little as 2.5 mph. this is the same as a brisk walking pace. Inertia and other factors help maintain the conditions needed for planing.,TG.
  - Re: a hypitheticaly stupid question - KenOhki 23:14:38 03/03/03 (1)
    thanks TG. A bit more Hp then I was figuring, but not that much more
    - Re: a hypitheticaly stupid question - walleyeslayer 07:24:19 03/12/03 (0)
      Too much thinking involved for me!
      All I can say is that I learned to ski behind a 35hp on a 15' Lund. It pulled me up on one ski when I weighed about 130lbs. I could still ski behind it at about 165lbs. I'd get up on 2 & drop 1 right away.
      Last time I tried on that setup was about 185lbs. It took a lot of arm strength to pull me up & it took a while to do! When I did get up, I needed to cut to get any speed, but by doing this, it slowed the boat waaaay down!
      Now I'm 220 & I don't think I'll be trying anytime soon!

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