Jet/prop - is one harder on your motor?

Rimshot · 04-30-2006, 01:14 PM

Just wondering if there is a difference in stress to your motor between the two. I just installed my jet for the first time and it's obviously less effecient on gas and power. Does it work the motor harder too? Or maybe the prop does because it requires more torque to turn it? Just curious.

Tacklebuster · 04-30-2006, 02:13 PM

I am no expert by no means but I think RPM's are RPM's. If you have to push your motor to WOT on a pump rather than 1 grand less on your prop, then I would say the pump is harder on your motor. If you are keeping the about the same RPM's for cruising and running around, I would then say the pump is just harder on your wallet

Great question, I have wondered that myself.

crabbait · 05-01-2006, 08:10 AM

I would say whichever allows the highest rpm at WOT is easier on the engine (as long as the max rpm is within engine specs). If the engine will only make 4500 rpm with the prop but runs out to 5500 rpm with the pump, the pump is easier on the motor.

SnowDog · 05-01-2006, 10:24 AM

Great point CB. My experince is that the prop on my F225 requires the boat to work harder to reach a comparable level of RPM, and thus a heavier load. That does not relate to speed as the prop obviously produces higher speeds at lower RPM.

Flatfish · 05-01-2006, 10:40 AM

What rpm is the jet running at when on plane(not necessarily wide open, just a running speed)?

What rpm is the motor running at with the prop at plane?

Our dual purpose outboard runs at 2800 rpm with the prop. 4000 with the jet. Both run within 100 rpm of themselves at WOT, so assuming your motor is not running rich (less rpm=rich) the prop is far easier on the engine.

rpm = wear. High rpm = much accelerated wear.

Think about it, would you question if running a car down the freeway is easier at lower engine speed, or higher engine speed?

Your prop is the equivelant of overdrive in a car.

No brainer.

Mark and the dog.

Rimshot · 05-01-2006, 12:10 PM

Quote:

rpm = wear. High rpm = much accelerated wear.

Hmmm, wouldn't torque factor into that somewhere? I'm pretty sure that pulling the boat with the truck turning 4k is harder on it than just cruisin down the road with no load at 4k.

I get what you are saying about being able to run lower RPM's with the prop, and I'm sure that is less wear on the motor. But, if you factor in the torque required to turn the prop is it really less wear on the motor? I can tell you it sure feels like the motor is working harder with the prop on at any RPM.

So I guess what I'm saying is that it would seem that not all RPM's are equal. It seems like you could be turning less RPM's but still causing more stress/wear to the motor because of the torque.

Just thinkin out loud, I could be way off. Thanks for the replies

Chrome Bumper · 05-01-2006, 12:32 PM

There is a very positive correlation between engine life and total fuel consumption. I.E. when an engine burns a given amount of fuel, it is used up. So since a prop is 30% more fuel efficient it would cause the engine to last 30% longer for a given service life. (BTW you can vary the engine torque caused by a prop by changing its pitch.)

BassinFever · 05-01-2006, 01:22 PM

If the jet and the prop are set up correctly, you will have the same life out of either at Wide Open Throttle..

Your prop should be set in the neighborhood of 5,500 RPM at Wide open throttle, and your jet pump should have an impeller that puts you there also, so the load on your engine will be the same at the same RPM. The only difference is like stated ealier that you will have to push the jet harder to achive a cruising speed of say 35 MPH than you would with the prop.

backlash · 05-02-2006, 10:19 PM

Cavitation is a factor on pumps, especially prone on a flat bottom. That'll suck the life out your motor as well. Even if only briefly running through chop, it will take it's toll.

MattPark · 05-02-2006, 10:57 PM

Chrome Bumper, I don't really understand what you are saying. The engine's output is the engines output, you can't change torque without changing some aspect of the engine. The torque at the propeller is also always the same, it's just whether or not it's sufficient to turn the prop you are using.

Higher rpms do cause an engine to wear faster, but the word I don't see in this thread is load. An engine with no load spinning 5000 rpm is under much less stress and will wear nowhere near as fast as an engine at WOT under load at 5000rpm.

The answer to this question is application specific.

The fuel used is probably the best indication of wear occuring. With a lower pitch prop you will turn more rpm with the same amount of fuel being used because the engine isn't under as much load. The loaded engine will likely wear out just as quickly as one with less load, even though it is at a lower rpm.

Another thing to consider is that the engine makes peak torque at a certain rpm. This is more or less the most efficient rpm to operate the engine at (by efficient I mean the most power is created for the amount of fuel used, not necessarily the rpm at which you will use the least amount of fuel).

If you are propped/jetted so that you are at a too low of an rpm, you may be wasting fuel and lugging your engine. Just like when you are towing your boat up a hill in a high gear at a low rpm with lots of throttle. There are occasions when you can drop a gear and put the engine at an rpm where enough torque is created, and let off the pedal quite a bit. At the same time you will burn less fuel, have a happier transmission, and have a longer lasting engine.