new ss prop rpm too high

[z rated]

Hey guys could use another opinion. Just got my 21 foot maxum back from the dealership, had the first 20hr servicing done so engine is broken in. The dealership gave me a new ss revolution 4 prop with a 17" pitch. My boat has a 260hp Mercruiser 5 litre mpi v8. I took it out today with 2 other people so not much weight in the boat. The top speed i could get was around 40 knots. Was expecting more like 50ish knots according to what i was told by some people at the dealership who have ran this exact same boat/prop. WOT rpm with trim all the way down was not bad, about what the engine is rated for. But after playing with the trim getting it up a bit to speed up the rpm was around 5300, in fact an alarm kept going off with the trim up a bit. to drop the rpm down to ~5000 Should i move to a 19" pitch prop, or more? With the right pitch prop will i get a better top end? Thanks

 

[Silvertip]

Re: new ss prop rpm too high

I believe WOT rpm for that engine is around 4800 so you definitely need to increase pitch. You didn't say what you had on the boat before and what the performance was with that setup. You need to pull about 500 RPM off the top so a 19P would theoretically pull off about 400 PRM. But keep in mind you said you had a light load. If your normal load is heavier, you should make your test runs with that load. It's easier to throttle back to avoid over reving when lightly loaded than it is to lug the engine when loaded heavy because you have too much prop for the load you are carrying. Also be aware that a 19P prop from manufacturer "A" will generally perform differntly than a 19P prop from manufacturer "B".

 

[z rated]

Re: new ss prop rpm too high

Good advice, sorry the previous prop was a 3 blade aluminum one that came with the boat, im guessing it was a mercruiser as well. WOT rpm with that was 5000 rpm. keeping in mind i had a light load, do you then still think i should go with a 19p prop? I could keep it on and see how it is with 5 or 6 people but i really dont like how high the rpm's were with 3 people.

 

[Tail_Gunner]

Re: new ss prop rpm too high

Is that Knot's or MPH......

 

[etransfer]

Re: new ss prop rpm too high

Hey guys could use another opinion. Just got my 21 foot maxum back from the dealership, had the first 20hr servicing done so engine is broken in. The dealership gave me a new ss revolution 4 prop with a 17" pitch. My boat has a 260hp Mercruiser 5 litre mpi v8. I took it out today with 2 other people so not much weight in the boat. The top speed i could get was around 40 knots. Was expecting more like 50ish knots according to what i was told by some people at the dealership who have ran this exact same boat/prop. WOT rpm with trim all the way down was not bad, about what the engine is rated for. But after playing with the trim getting it up a bit to speed up the rpm was around 5300, in fact an alarm kept going off with the trim up a bit. to drop the rpm down to ~5000 Should i move to a 19" pitch prop, or more? With the right pitch prop will i get a better top end? Thanks

For all around performance I would say a 20" Apollo 4 blade would put you where you need to be and you'll be very happy !