New boat owner prop, whats your thoughts?

[Trihull&error]

I got a hell of a deal on a 1972 Forester for a 1st boat. and want to know if it is set up properly, I put a couple small dings in the prop nothing that needs replacement but a back up or better prop would not hurt.

Boat is a 72 forester tri hill fiberglass 750lbs dry
Engine is a 1979 85HP Johnson V4 79txl85
running a OMC stainless steal 13 1/4 17 pitch
plate is set even with the keel
Iboats specs this boat as follows
This outboard runabout is 15.08 feet long and weighs 750 (pounds dry). This weight does not account for passengers, fuel, or gear. The beam of this craft is 72 inches
Motor is 350LBS
acceleration is quick tops out at 35mph on the GPS @ 5400RPM WOT
idles around 900

So stick with 17? Move up to 19 or 21 or 23?

 

[Sea Rider]

Re: New boat owner prop, whats your thoughts?

What are wot parameters factory stated for that engine ? When you say 35 mph at 5400 at wot, is that with you alone ? For instance if you were to go boating with family, you'll need to record max achieved rpm at wot to determine if over or under propped, then match the correct pitch to optimize engine rpm.

Happy Boating

 

[Trihull&error]

Re: New boat owner prop, whats your thoughts?

That was me out testing the new tach solo. I have yet to do a family run with the tach, but I know that the speed has been the same top end with a buddy or with all the kids in the boat. still hits 35, 36 with a tail wind 34 loaded down with a lot of chop. I can do a loaded run with the tach soon here, but the extra weight does not seem to affect top end. What it does for RPM is yet to be seen.
Motor is rated for a 4500- 5500 WOT 2.1 - 1 gear ratio, so Slip of about 16%?

 

[jestor68]

Re: New boat owner prop, whats your thoughts?

I suspect with a family run, you'll find that the 17 pitch prop is the right size.

A 19 will bring your rpm down to about 5000 rpm running light (based on what the 17 does light), and would slow your "hole shot" a bit.

If you plan on doing any water sports, stick with a 17 pitch.

 

[steelespike]

Re: New boat owner prop, whats your thoughts?

If your speed doesn't change with added weight I suspect the speedometer is not accurate.
A gps speed is likely to be more accurate.
It appears that a 17 is right on.If its an older prop you might benefit from a newer design like the
Solas Amita or Turning point Hustler might improve the slip numbers and add a touch of speed.

 

[Trihull&error]

Re: New boat owner prop, whats your thoughts?

No spedo on the boat using GPS, but thanks guys looks like the person who owned be before me had it set up the right way to begin with I will keep it the same.. Just was a little concerned about the slip # and the fact that it is 100RPM short of max.

 

[jestor68]

Re: New boat owner prop, whats your thoughts?

Your motor is rated 85 hp @ 5000 rpm.

The reason to prop it 300-400 rpm over that with a light load is so that it will still be at or above 5000 rpm when you add some people/gear and pull something.

As long as you're turning 5000 rpm, you're developing full power.