Torque's Impact On Couch's Speed Equation

[Archive]

With the new big four-stroke outboards coming out, I am hearing a lot of discussion concerning repowering. In particular, several mechanics have said that any given 2-stroke outboard powered boat could be repowered with lower horse power 4-stroke outboards and keep the same performance. The mechanics explain this due to the higher torque of the four-stroke engines allowing for a higher pitch propeller.
Does this make sense?

If so, how does torque impact the speed equation?

i.e. Speed in Kts = [(h.p./weight)^.5]*C

Originally Posted by Archive

With the new big four-stroke outboards coming out, I am hearing a lot of discussion concerning repowering. In particular, several mechanics have said that any given 2-stroke outboard powered boat could be repowered with lower horse power 4-stroke outboards and keep the same performance. The mechanics explain this due to the higher torque of the four-stroke engines allowing for a higher pitch propeller.
Does this make sense?

If so, how does torque impact the speed equation?

i.e. Speed in Kts = [(h.p./weight)^.5]*C

probably in the C constant. which is certainely averaged for typical hull form and propeller efficiency.

If a prop can turn slower (absorbing the same power), and has a bigger diameter, generally, it has a better efficiency. so it will move the boat at the same speed with less shaft power.
Some interesting thoughts about propellers :
http://www.realtrawlers.com/prop.htm

Dont forget that HP = RPM * TORQUE * Constant. (a true mathematical constant that only depends from units). 5252 remins me something with US units. (With usual Eur units, HP (hp) = RPM * TORQUE (m*kg) * 0,001422)

FC.

[Willallison]

Don't (necessarily) believe everything you hear....
most of the comparison tests between 2 & 4-strokes (of equal power) I've seen, show that both acceleration and top speed are higher with the 2-strokes - though I think that the very latest generation of 4's may change that. Of course, the reluctance of o/b manufacturers to release any power / torque / consumption curves on their motors makes it difficult to compare them.
The other thing to consider is that 4's are generally considerably heavier than the equivalent 2 - so any increased torque may be more than offset by the additional weight