250 HP to make Amphi plane

R

rogtwo@aol.com

Guest
Well, maybe ....

Based on the Celina video, we can now make an educated GUESS about how much
power it would take to make an Amphicar plane.

In the latter part of the video, Billy (I think) enters the water at high
speed in his yellow car. I would say the car is actually planing for a little
over one-second before it settles into the water. If I make the following wild
guesses, then I can estimate the horsepower that would be necessary to make
the car plane.

Here are my wild guesses:

Speed of car entering the water - 35 mph
Speed when car stops planing - 20 mph
Time on plane - 1 1/4 seconds
Weight of vehicle (including 5 passengers) - 3100 pounds

From high school physics, you might remember that kinetic energy equals
one-half mass times velocity squared:

KE = 1/2 M v*2

Using this equation and the numbers above, we find that the car looses about
85,000 pound-feet of energy in the 1 1/4 seconds it is on plane.* This is
68,000 pound-feet per second. Now, one horsepower is 550 pound-feet per second.
So, 68,000 pound-feet per second is equal to about 124 horsepower.

Finally, water propellers are often around 50% efficient (probably less if
you were to try to get an Amphi to plane :) :) ). Thus, the engine would need
at least twice the 124 horsepower calculated above to get the required thrust
from the propeller(s).

Thus, my 250 horse power prediction. Keep in mind this is a really rough
number. I give it here just for its amusement value. I don't really think an
Amphi can be made to realistically plane without serious hull modifications.
For example, look at the video again -- notice that while "planing" the bow is
so high that the front wheels are out of the water. Even if you added rudders,
I think you would likely have a vehicle that wasn't very controllable. Not
to mention you couldn't see over the hood!

Roger
White '63
Seattle

* If you are doing the math using English units, remember to divide the
pounds (3100 lb) by 32.2 to get the mass in slugs (96.3 slugs).


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