Power and rev limiters

[JonD]

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. 

[noelm]

The max HP starts to drop off, it doesn't just go to zero, lots of engines will rev beyond their max HP RPM.

[zmk1962]

Further to what Noel said - remember that HP is calculated, and only Torque is measured. 

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). 

Cheers Zoran

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.

 

[zmk1962]

14 hours ago, JonD said:

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. 

Interesting !

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.

Cheers Zoran

[JonD]

Thanks for those fantastic explanations, that really helps.

Early days at the moment as Ive only done one day on the water with the new boat prop testing, yes changed boats again!!!!

Currently fitting out electronics, not to mention horrendous bar conditions lately but should be right to test some more later in the week. 

 

 

[dunc333]

can i ask what you updated to Jon

[JonD]

2 hours ago, dunc333 said:

can i ask what you updated to Jon

Dropped down to a smaller boat now that the kids all have their own things that keep them busy. Picked up a Formula 15 with a 90 Honda, had one of these boats a few years ago which are great for solo trips. Just fitting it out with new bits and pieces.

 

[Blackfish]

Very nice Jon.

[dunc333]

very nice mate and z"s answer to your question was very educational for me to ,greeat question and great answer ,gold