RATED H.P. ?

[Boilermaker]

My 1978 Johnson is rated at 115hp@5000 RPM.
Its W.O.T. RPM range is 5000 to 5500 RPM.
Is 5000RPM, its PEAK H.P. ? OR will it develope a little more at 5500RPM (or maybe a little less)?
WIll it develope MORE HP at higher RPM? or does the curve start going down after 5000RPM?

THANKS

 

[DJ]

Re: RATED H.P. ?

Ffirst off, your engine is rated at the powerhead. Prop ratings did not start until 1983.

That engine needs to be able to turn at least 5500 at WOT with a moderate to light load. Anything less will "lug" it-not good.

Every inch in prop pitch is good for about 200 RPM, up or down.

If it were me, I'd prop it to run about 5600 to 5800 at WOT.

 

[mfgsuzuki]

Re: RATED H.P. ?

My 1978 Johnson is rated at 115hp@5000 RPM.
Its W.O.T. RPM range is 5000 to 5500 RPM.
Is 5000RPM, its PEAK H.P. ? OR will it develope a little more at 5500RPM (or maybe a little less)?
WIll it develope MORE HP at higher RPM? or does the curve start going down after 5000RPM?

THANKS

 

[mfgsuzuki]

Re: RATED H.P. ?

your horsepower will be a little less at 5500 rpm.
you should never prop an engine so that it runs above its rpm range nor below.
if it runs below 5000 at wide open throttle then there is too much load on the engine it will run hot and burn out
if it runs near the 5500 mark there is less strain on the engine and it will carry loads of people and fuel without problems but you sacrafice a little speed.
If you want the best preformance a stainless prop will make a big difference and try to prop it at 5000 or just above like 5200
for the most speed do this when the boat is loaded with fuel and persons.

 

[DJ]

Re: RATED H.P. ?

I agree and disagree.

5500 is your goal. Lugging that engine (my experience) will kill it-certainly.

Very few run at WOT anyway, just like your car/truck. But, it needs to be "geared" right for optimum performance/life.

 

[Dhadley]

Re: RATED H.P. ?

Where the motor is rated in the rpm band has to do with how it's certified by NMMA. Each manufacturer picks an rpm on a given engine and then it's certified to be xx hp at that rpm. Nobody is saying it's not the same or even more at a higher rpm. All they're saying is that at x rpm it's certified by the NMMA to be x hp.

And nobody talks about torque. That's what does the work. Don't get caught up in a "numbers" game.

Look at the Evin /John carb'd 175's. There is a 90* crossflow 175 thats 2.6 liters. And a 60* looper that's 2.6 liters. They're both correctly rated at 175 propshaft horsepower. But the looper will run circles around the crossflow - and on a lot less fuel too.

 

[Boilermaker]

Re: RATED H.P. ?

Where the motor is rated in the rpm band has to do with how it's certified by NMMA. Each manufacturer picks an rpm on a given engine and then it's certified to be xx hp at that rpm. Nobody is saying it's not the same or even more at a higher rpm. All they're saying is that at x rpm it's certified by the NMMA to be x hp.

And nobody talks about torque. That's what does the work. Don't get caught up in a "numbers" game.

Look at the Evin /John carb'd 175's. There is a 90* crossflow 175 thats 2.6 liters. And a 60* looper that's 2.6 liters. They're both correctly rated at 175 propshaft horsepower. But the looper will run circles around the crossflow - and on a lot less fuel too.

OK Dhadley, thats what I thought. >>> I seem to remember reading that sometimes the Manufacturers spec was set at a lowwwer RPM so the given engine actually makes more HP then the rating. >>>This way it would seem a little stronger then the competetors same HP rated engine. (except it was done by OMC & their competitors).

I was only curious >>> I wanted to know if the engines
HP curve was starting to get stronger/weaker as we prop for higher RPM?

I would LOVE to see a H.P. vs Torque vs RPM chart on these engines. That would be so usefull for propping & finding good cruse speeds!

THANKS to all that took the time to answer!

 

[Dhadley]

Re: RATED H.P. ?

From here it gets kinda complicated. Yes, at some point every motor will start to lose hp as it gains rpm. But (dont ya love those?) as the boat gets on plane and goes faster, theroriticaly, it loses drag or wetted surface.

So.....on a given hull it may take 140 hp to get it up and planed out but at that point it may only take 100 hp to maintain that speed. Even if you lose hp as rpm's increase you're still spinning the prop more and more rpm. Basically you've "fooled" the prop into thinking the boat doesn't weigh as much (as you lose drag).

This is where hull design comes in. Let's assume we're talking about V bottom hulls, not tunnels. The pad bottom hulls are faster than straight V's simply because they have less drag once up on the pad. So it took 140 hp to get on the pad but once there it only takes 100 to maintain.

Here's an example -- we know every motor falls off on hp at some point. Just for the sake of argument lets say that on a V4 looper it's 5800. The higher the rpm after 5800, the less hp there is. We ran a V4 looper on an old Allison. It would run, once on the pad, 1 mph for every 100 rpm. At 6000 it ran 60. At 7000 it ran 70. At 8200 it ran 82.

Now, at 8200 I'd bet it wasnt much over 75 hp (if that) but there wasn't any boat in the water and therefore no resistance so the prop pushed it faster and faster even if the hp dropped off. IE -- as drag deminished it took less and less hp to turn that prop.

Not all hulls are like that. Every set up is a marriage between the hull design, propeller and intended use. Along those lines -- I'd bet that our V4 on that 15' boat with that set up couldn't pull a skiier up at all. That's where intended use comes in. Who'd think that a V4 looper on a 15' boat wouldn't pull a skiier but still ran over 80?