Rookie Prop questions

[Reming870]

OK, I'm new to boating and have a concern here.

WOT, 3,000 RPM max.
Boat and gear "estimated" at 2500#
Starcraft Islander 191, 1984
140 HP, 3.0 L Mercruiser and Alpha 1 outdrive. 1.89:1 ratio
Motor list max RPM 4600.
Prop. 16D x 17P

The RPM is way off. I tried a friends prop (16D x 13P) and got the RPM to 4000 and the motor sounded much better. I am thinking about ordering the 11P but these pitches seem off from everything else I am reading. It appears that similar boats are running the 17P but I cannot.

Any input is appreciated here.

 

[steelespike]

Re: Rookie Prop questions

You may have a motor problem.A couple of things to check.Bottom clean and smooth?Prop in excellent shape?Throttle plates opening all the way?Running on all cylinders? Your about 1600 rpm short you can't make that up" right" with a prop.
Typically a 17" prop is suggested for skiing with the warning to watch over reving with a light load.For speed typically a 18 or 19 ft 3.0 would run a 21-23" prop turning about 4,600 at 40+ mph.

 

[45Auto]

Re: Rookie Prop questions

You shouldn't need to be anywhere near an 11" pitch.

What kind of speed (GPS, not speedometer) are you seeing at 3000 RPM?

It could easily be your tach that's not accurate. Giving us speed and RPM will allow a little more insight into what's going on.

When's the last time you had a complete tune up and a compression check? A 1984 motor might be getting a little tired. Could be a motor problem instead of a prop problem.

 

[Reming870]

Re: Rookie Prop questions

The boat is definetly sluggish and slow with the 17P. When I put the 13P on the motor actually sounded great.

I am not sure of the speeds. I will try again over the weekend with more accurate data.

Whats the best way to measure the RPM?

 

[45Auto]

Re: Rookie Prop questions

When I put the 13P on the motor actually sounded great.

Probably sounds great in neutral too!

Doesn't mean anything, though!

Whats the best way to measure the RPM?

Lots of timing lights have built-in tachs. There are also "tune-up" tachs like the "Tiny Tach" for around $50.

 

[Reming870]

Re: Rookie Prop questions

I've been around engines long enough to know that with the 17P the motor is not responding. There was no way I was anywhere even near 25 MPH let alone 40. The boat struggled to plane (15-seconds WOT to plane) and the motor just isn't working hard with the 17P. When I put the 13 P on I noticed right away that the boat jumped onto plane (5-seconds WOT to plane) quickly and I was moving a lot faster.

I am confident that the RPM's are close enough to trouble shoot the problem just by the way the motor responds. I will accurate when I can determine the problem.

Motor was redone 4-yrs ago.

Is it possible its in the throttle?

No idea when the compression or tuning was last done. I purchased the boat site unseen and took a risk. The price was right.

 

[45Auto]

Re: Rookie Prop questions

I am confident that the RPM's are close enough to trouble shoot the problem just by the way the motor responds. I will accurate when I can determine the problem.

Then you should be concentrating on getting it to run correctly with the 17" pitch prop you have. Spending money on a ridicuously small prop isn't going to do you any good. The 17" will get you within a few hundred RPM of where you should be if everything else is good.

Is it possible its in the throttle?

It's possible that it could be LOTS of things:

Throttle cable
Timing
Anti-siphon valve
Fuel pump
Carburetor
Coil
Burnt valve (low compression)
Points (if it has them)
Distributor cap
etc, etc, etc

Look at the top of the I/O forum, under the "stickies". There's whole lists of stuff and how to check it. Propping it is the LAST step once everything else is working correctly (unless you just like giving money to prop companies)!

This link will take you to the "sticky". Check out #13 there:

http://forums.iboats.com/showthread.php?t=342393

 

[steelespike]

Re: Rookie Prop questions

My ink must have been invisable.

 

[thompy]

Re: Rookie Prop questions

thats kind of weird all the 3.0's ive seen run the 1.98 ratio lowers

 

[whit71]

Re: Rookie Prop questions

Motor was redone 4-yrs ago.



Who redid it? sounds like it may have an auto camshaft. which does not produce enough power at low rpm.

 

[Reming870]

Re: Rookie Prop questions

Then you should be concentrating on getting it to run correctly with the 17" pitch prop you have. Spending money on a ridicuously small prop isn't going to do you any good. The 17" will get you within a few hundred RPM of where you should be if everything else is good.



It's possible that it could be LOTS of things:

Throttle cable
Timing
Anti-siphon valve
Fuel pump
Carburetor
Coil
Burnt valve (low compression)
Points (if it has them)
Distributor cap
etc, etc, etc

Look at the top of the I/O forum, under the "stickies". There's whole lists of stuff and how to check it. Propping it is the LAST step once everything else is working correctly (unless you just like giving money to prop companies)!

This link will take you to the "sticky". Check out #13 there:

http://forums.iboats.com/showthread.php?t=342393

I guess the problem I have with some of the other options for low RPM at WOT outside of the prop is the fact that when I went to the 13P the engine reponded without a fuel, carb, etc... issue. It ran great.

Is there an easy way for me to tell if the gear ratio is wrong? BTW it was a typo it is 1.98.