Engine prop sizing

[IrishAndy]

Guys I need advice, I have a 16ft Driver 500 speed boat, I upgraded the engine for a 60hp Mariner and brought a new (2nd hand) 2001 90hp Mercury for the boat. Great engine and runs sweet but I noticed it going flat at the top end and it felt like it wasn't revving. I installed a tach today and gave it a run and sure enough it's only hitting 3600rpm at max. Now that's well short of ideal (5500 rpm). It's currently running a 4 blade 12.5" X 19p prop. Has anyone got the same boat/engine combo on theirs that could tell me what prop to get for a replacement as I feel its to big a jump to try and figure it out through hit and miss. Any help appreciated

 

[penfold]

I don't know that boat but I'd say you're quite overpitched if there's no actual problem with the engine, a 16 would be more like it, but the only way to find out is to try different props; if there's a dealer or outboard service place nearby it's worth asking if they have any used props available to try out. Does it rev out ok in neutral?

 

[FishMan]

What penfold said !
Plus:
When you are checking props, if you load up the boat to what the max capacity you think you would have (ie. people, fuel, fishing stuff, etc. etc.) Then the best prop would take to to less than 5500rpm. That way, if you go out lighter, you won't accidently over-rev it.
I know it's a pain changing out props to get the right one, but it's the only way. There will probably be no-one with exactly the same set-up as you.
Remember, that pitch, diameter and number of blades all interact with each other, as well as the material, and make. It's a nightmare. it took me 6 months to get mine sorted.
Have a google search, there are loads of sites that explain the interactions and rpm changes you can roughly expect.
Good luck !

 

[ChromeDome]

It is possible to estimate the right prop size when knowing the boat hull design and its weight including whatever fuel, stuff, crew and gear normally is onboard.

The principle is that a given number of hp is able push a given load to a certain speed, depending on hull shape. As you are aware the prop pitch must allow the engine to reach the specified rpm range at wide open throttle (WOT), which for a Merc 90 2001 is 5000-5500 (and one would aim for upper limit). The last factor to use is the Merc gear range (2.3)

If running at 3600 rpm and the gear reduction is 2.3, then the prop shaft will spin 1565 rpm. By a 19" pitch prop that calculates to 24,47 knots (not considering prop slip). The prop itself of course has a major impact - by experience all are not made equally good.

I'm afraid I don't know your boat but if you state the weight as mentioned and hull design (Deep V, modified V or flat), I'll give it a shot.

A pic would be good also

Edit: Just realized that you did some work on the boat (other threads, other forums). It indeed looks like a deep V hull, but the weight after your work probably will be a guess.
Looks similar to Finnsport 500. A friend had one with 90 hp Mariner. Measured 46 knots (GPS) and 54,6 with a 115 hp Tohatsu.

 

[Rocksteadee]

Rule of thumb, every inch reduction in pitch gives another 200 rpm

 

[Jamie Dundee]

Rule of thumb, every inch reduction in pitch gives another 200 rpm

This. Trouble is to make up your 1900rpm deficit you’re going to be down to a 9” pitch which isn’t feasible. Sounds to me like something else is going on. Is this the 60 or 90 engine?

 

[gordmac]

Is the tacho accurate?

 

[Rocksteadee]

This. Trouble is to make up your 1900rpm deficit you’re going to be down to a 9” pitch which isn’t feasible. Sounds to me like something else is going on. Is this the 60 or 90 engine?

Should have read the original post better.
if revs are down that much look for engine fault first

 

[Jamie Dundee]

Should have read the original post better.
if revs are down that much look for engine fault first

I did, hence my comment “not feasible, something else going on”. Perhaps you didn’t read my post properly.

 

[Rocksteadee]

I did, hence my comment “not feasible, something else going on”. Perhaps you didn’t read my post properly.

I did, was agreeing with you

 

[Jamie Dundee]

I did, was agreeing with you

Ah right, wasn’t clear the way I read it.

OP’s gone missing anyway

 

[QBhoy]

Either the taxho isn’t right, or you need to check things out. You should easily run a 19” on that with a 90 with the usual ratio. I’d do a compression test. What speed was it doing at those rpms ? Should be a 40 or just over mph boat gps, with that engine.