Prop Question

[Coolbreeze135]

I have a 1988 Stratos 19 ft. 189 F/S this boat has a 1991 force 150hp motor Model 1508X92C. The prop numbers on the motor are 48 817718 A40 17P

The boat only runs 41 mph. and about 4800 RPM seems pretty slow for a light boat and 150 hp motor. I cant decode these numbers on the prop.. Does anyone know what these numbers are I would like to find a prop that performs better at top speed. Does this motor need a special prop ?

I cant even find the WOT RPM for this motor.

Regards
Mark

 

[Frank Acampora]

Re: Prop Question

The important number is the 17p this indicates 17 inch pitch. At 4800 RPM your engine is running near the bottom of the specified RPM range of 4500-5500, but still within it.

With a 17 pitch prop on a light boat, I would also expect higher top RPM. So before you go playing with props, be certain the carbs are all opening fully and the timing is correct at 30 degrees BTDC at WOT.

At that size boat, I really hesitate to recommend dropping down to 15 pitch. Even though the RPM will rise around 400, the top speed MAY actually drop. If you can try before buying, or if money is not a major concern, you can try a 17 pitch stainless prop. they generally run faster than the same aluminum and give a tad more top speed.

I do have a friend who had subjectively good results over a stock 19 pitch prop with a Vortex 19 pitch aluminum on an 18 foot tri-hull bow rider with a 150. Unfortunately, he still does not have a tach so all we have to go on is the increased top speed from 32 to 38 MPH.

You may experience a small gain in speed, but even with the engine running well, I suspect that 50 MPH is out of the question. Mid 40s would be more like it.

 

[Coolbreeze135]

Re: Prop Question

Thanks will check on those carbs and timing I just cant find a manual on this thing, Im an I/O guys so I know those motors real well dont know my way around an outboard yet..
Regards
Mark

 

[Coolbreeze135]

Re: Prop Question

Had a chance to check out those carbs (since my starter fried) waiting for a new one.

I discovered that at WOT the carbs were not opening all the way in fact they were less than half way open. So hopefully that will explain the low RPMS at WOT.

I adjusted the linkage so that they open all the way at WOT.