Torque rating?

[bronxlatetour]

My question is whether boat engines ever get rated for torque? In the automobile world it's an important figure. Does the torque of an engine matter in boating or is horsepower the only important number. For example, I have a 1990 sea ray 250 DA with the 7.4 Bravo rated for 330 HP. What is it's torque rating? Would the boat perform the same with a smaller engine with the same horse power rating? Thanks in advance.

 

[QC]

Re: Torque rating?

The answer is that horsepower is almost the only important number. Propeller loads are fixed, no gears, and no real hills . . . The load rises linearly with RPM. Here is a cut and paste of QC's simple explanation:

"The reason that marine manufacturers do not publish torque numbers is that torque is almost meaningless in a marine application. I said almost, so don't throw anything. If you are satisfied with how quickly your boat gets on plane and dissatisfied with top speed then increased torque will not be important to you. If you are satisfied with your top speed, but not the time it takes to plane, then you want more torque and don't need more horsepower, and you may be able to solve that issue with trim tabs if you don't have them already. It is that simple. Marine manufacturers do publish prop load curves which are actually pretty close to linear with RPM (not curved) and illustrate the point I am making. Everything I said above assumes that it is propped correctly for either example.

Torque and horsepower are two different things, related, but different, and adding them together only makes the situation mushier. It totally misses the RPM part of the equation: bhp=torquexRPM/5252."

Tests I have seen show that a smaller engine with the same rating will be faster due to the weight savings. I have a lot more stuff on this topic if you want more . . .

 

[bronxlatetour]

Re: Torque rating?

Thanks, that does shed a little light on the subject. My boat weighs aprox 6500 lbs loaded and jumps up quickly on plane with out the use of trim tabs. I'm satisfied with that and the 38-40 MPH top speed. Thanks again, Interesting contrast to the automotive world.

 

[mkast]

Re: Torque rating?

From Mercury Marine website:

How do I obtain horsepower curves, torque curves, or fuel consumption data?


Horsepower and torque curves are not published because they are considered confidential company information. Fuel consumption data is not published because it is affected by factors beyond our control, such as boat hull type, temperature, humidity, altitude, gear ratios, propeller, etc. In some cases, fuel consumption or other performance data is available from your boat manufacturer for a specific boat/engine combination.