Hp and rev limiters

[JDP]

Ok Im not mechanical, just do basic servicing and leave the rest to the experts. My question is why does an outboard continue to accelerate well after its reached its full hp. Example being my engine developes full hp at 5500 but pulls away with plenty of power well over that, with the rev limiter kicking in at 6600, 1100 rpm above full power. One of my prop revs out to 6100rpm and the other to 6500rpm just under the rev limiter. Both props are the same size and pitch but from different manufactures, one gets used for heavier loads and the other for light but Im leading towards staying with the one that revs higher as it gives a higher top speed. 

[Saintly Fish]

@JonC can answer this, he’s an absolute whizz with prop pitch and sizes. 

[suzook12]

Just because it's beyond peak power doesn't mean it makes no power. If you find a power curve for your engine compared to say a motorcycle it will probably be quite flat

Not your engine I know but you get the idea. A high RPM engine usually falls off the power much quicker

[JonC]

1 hour ago, Saintly Fish said:

@JonC can answer this, he’s an absolute whizz with prop pitch and sizes. 

I think the best way to answer this question is by asking another question about something else that I almost understand.

Have you tried altering the height of the engine? 

[GPSguru]

11 hours ago, JDP said:

Ok Im not mechanical, just do basic servicing and leave the rest to the experts. My question is why does an outboard continue to accelerate well after its reached its full hp. Example being my engine developes full hp at 5500 but pulls away with plenty of power well over that, with the rev limiter kicking in at 6600, 1100 rpm above full power. One of my prop revs out to 6100rpm and the other to 6500rpm just under the rev limiter. Both props are the same size and pitch but from different manufactures, one gets used for heavier loads and the other for light but Im leading towards staying with the one that revs higher as it gives a higher top speed. 

 

Generally, just like a car, it is the torque curve that is vastly more important than where peak HP occurs.............. this is why some outboards of similar HP seem totally different when used in anger.

In the words of Carroll Shelby “Horsepower sells cars, torque wins races.”

[Gordmac]

The power required to push the boat, car or whatever is basically the force required X speed. When the power delivered by the engine exceeds this it will accelerate until they balance. 

[JDP]

A friend just sent this which explains it in a way I find easier to get my head around.

 

HP = Torque(ft/lb) x RPM / 5252 

Up to Max HP RPM the motor is generating ever increasing amount of Torque, measured on a dynometer.

At Max HP RPM the motor is running most efficiently, converting the maximum amount of fuel energy to Torque. 

Beyond Max HP RPM the motor still generates Torque but instead of increasing, the amount of Torque is decreasing, as the engine becomes less and less efficient ---  higher RPM means more friction, less time to inject, combust, expel ... everything starts to run inefficiently. So Torque production drops off (but some torque is still there) so your still experience boat acceleration but the rate of acceleration is slowing down, until you eventually hit terminal speed.

Look at the chart below as an example and you will see the different HP rating (blue) calculated at different RPM based on the Torque (orange) measured at that RPM (the curves always cross at 5250). 

 

PS - HP is just a term invented by James Watt so that he could equate the Torque produced by his steam engine (the worlds first motor) to the work output capability of a draught horse.  Power  = Force(lb) x Distance(ft) in a linear scenario.  With Rotational movement, distance traveled is a function of RPM hence the HP formula that takes that into account.

 

Generally engine manufacturers would advise you to stick with the one that lets the motor reach WOT.  The difference in performance may be due to the efficiency of the props - thinner blades have less friction giving higher revs, stiffer blades have less distortion giving higher thrust etc.

But having said that, you would need to ensure that you are running the optimum size and pitch for your motor+boat combination. If I was observing that, I'd check the following:

1: both are not spinning to 6600 (WOT) which may suggest you are a tad over-propped, but 6500 is very close

2. The 6500 prop: does it provide good/acceptable holeshot?

3: The 6500: does it show any signs of cavitation marks on the low pressure prop faces?

...and if 2: and 3: are good, then I'd run with it.

Edited July 11, 2021 by JDP

[suzook12]

Shame he got it wrong tho........

Up to Max HP RPM the motor is generating ever increasing amount of Torque, measured on a dynometer.

At Max HP RPM the motor is running most efficiently, converting the maximum amount of fuel energy to Torque. 

As can be clearly seen by the graph provided max HP occurs well beyong maximum torque, as is the norm as can only be the case as HP is a direct result of engine speed. As a direct result of this, max HP does not occur at the most efficient point or the point of maximum torque

higher RPM means more friction, less time to inject, combust, expel ... everything starts to run inefficiently. So Torque production drops off (but some torque is still there) so your still experience boat acceleration but the rate of acceleration is slowing down, until you eventually hit terminal speed.

Obviously increased engine speed reduces the time for each of the 2 or 4 cycles to be accomplished BUT despite the torque dropping off the engine speed is increasing so there are more bangs per minute so more work (HP) is done. Maximum acceleration stops when the power starts tailing off, not the torque. The friction should remain relatively the same

PS - HP is just a term invented by James Watt so that he could equate the Torque produced by his steam engine (the worlds first motor) to the work output capability of a draught horse.  Power  = Force(lb) x Distance(ft) in a linear scenario.  With Rotational movement, distance traveled is a function of RPM hence the HP formula that takes that into account.

That statement doesn't really make sense. Take horsepower as work completed and torque as the force required to achieve it. For example, pushing a car- Initially it's the torque that moves it but once it's rolling and gathering speed the amout of work (distance pushed) is much greater and the torque requirement is much less

 

[Gordmac]

Power is not equal to force times distance! That is work done, for power you need to divide that by time.

As Duckworth said refering to engines, power is the size of the bang X the number of bangs per minute. Basically torque X rpm. When the torque falls off faster than the revs increase power reduces.