[bigshrimpin]

I'll take a shot at this one. *Correct me if I make a mistake . . . it's still a little cloudy in my head. * Where's CB on this? *

1. *" Can anyone explain to me the difference in HP and torque as it applies at the propeller?? "

Torque (not HP) is the force required to turn the shaft or spin the prop. *

HP is the speed at which you apply the force or perform the rountine( I think ???)

HP is higher on gas motors b/c they can spin at a faster RPM. *

Engines with equal HP . . . does not mean they can generate the same twisting force (torque). *That's why you have big *** trucks that tow huges loads with only 290hp diesels.

HP = Torque * RPM / 5,252 *

Motors generate MAX HP at MAX RPM.
So if you compare the follow two motors

120hp diesel with a max RPM of 2800
120hp gasoline motor with a max RPM of 5000

120hp = (x foot pounds ** 5000RPM)/ 5252
120hp = (x foot pounds ** 2800RPM)/ 5252

120hp Gas Motor @ 5000rpm * * *=126 ft of torque.
120hp Diesel Motor @ 2800rpm *= 225 ft of torque.


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2. * Diesels produce a lot more torque, does that mean a 120hp diesel can turn a bigger prop than a 120hp gas engine? *

"maybe" * 225 foot pounds of torque is more than 126 foot Pounds . . . . so the engine should be able to turn a much bigger prop . . . except that people had to screw the simple math up with a gear box.
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3. *"If this is true, would a 120hp diesel be able to push a boat a little faster than a 120hp gas since it can turn more screw at the same RPM?" *

It gets tricky . . . Weight is a big factor in determining the speed of a boat. *Also a gear box with a lower gear ratio allows the powertrain??? to generate more torque. *
*
500hp @ 1:1 @ 2000rpm = 1313 pound feet of torque

485hp @ 3:1 @ 667rpm * = 3819 pound feet of torque
(500 - friction of gears) *

Then you get into some thrust equasion and prop efficency and that's about where I gave up. *:-/