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- Posted by Ken-Ohki on March 02, 2003 at 16:16:00

Someone once told me that it takes upward of 35Hp on a skiboat type hull too pull up a waterskier. ( average ) Now, Being resonably inteligent ( even though I cant type worth a crap ) I know there are many variables, weight, boat, water conditions, ski's used and on and on and on. But just for the sake of argument. lets use a 200 pound person. Lets say we are using a motor on a magical boat, one that has no weight or displacement, Basicaly just a motor sitting at the end of a tow rope (just like cartoons)Question 1: How much HP would it take ( apox ) to pull this 200 pound man out of the water and up onto the skies.

Question 2: How slow could this 200 pound man move, and still stay up on his skis? ( average skis, I know that is a variable too ) I know there has to be a certain amount of headway speed too keep him up, and that really has little to do with HP once he is "on plane" I know I could naturaly build skies wider or more boyant, but im talking your average run of the mill waterskis )

These are pretty simple Hypithical questions I have came up with that will answere some other questions I have that are way to complicated to word out properly.

Once Again I will also state I relize it all depends on a giant list of variable,, But I am looking for rough answeres here somewhat grounded to fact. I always welcome opinions, But MOre value a Opinion backed up with some form of facts.

From what I understand,, taking the weight and dividing by the HP will give you a number SOmehting about between 7 and 14 is good, over 14 is worse, under 7 is too much or something like that, but those equations also include boat size,something that is not relivant here.

For some reason I tend to think about 3 too 4 Hp maybe 5, with a min forward velocitey of about 10 mph, But I am just making a guess at it all. I supose that some form of hydrofoil equation would answere my question,, maybe hehe.. Pulling that much weight up out of the water might require much more HP then I am giving it credit for. I have also seen someone waterski down a canale wile being towed by a horse, So the power of 1 real horse could do it, But I also know motors HP is another thing all together.

- Re: a hypitheticaly stupid question - machnumber2
*22:38:59 03/02/03*(5)- 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 * aw = f * d

p = w / t

v = d/t

then

d= vt

and

w = fvt

then

p = fvt /t

and

p = fv1 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/sp = 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.

- Re: a hypitheticaly stupid question - machnumber2
*10:34:36 03/03/03*(4)- In Reply to: Re: a hypitheticaly stupid question posted by machnumber2 on March 02, 2003 at 22:38:59:

oops I think i spotted at least one error above, 5mph is not 11.2 m/s it is 2.2 m/s.

so thenp = 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)- In Reply to: Re: a hypitheticaly stupid question posted by machnumber2 on March 03, 2003 at 10:34:36:

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)- In Reply to: Re: a hypitheticaly stupid question posted by machnumber2 on March 03, 2003 at 10:34:36:

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)- In Reply to: Re: a hypitheticaly stupid question posted by Tailgunner on March 03, 2003 at 12:55:52:

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)- In Reply to: Re: a hypitheticaly stupid question posted by KenOhki on March 03, 2003 at 23:14:38:

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!

- Re: a hypitheticaly stupid question - walleyeslayer

- Re: a hypitheticaly stupid question -

- Re: a hypitheticaly stupid question -

- Re: a hypitheticaly stupid question - machnumber2

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