...prop help/experts needed...any takers?

[Bigprairie1]

Ok,
I've got a 1990 Force 90hp outboard...and I want to get a spare prop for it.
What do I need?
What do I look for?
How do I know what size and pitch I already have?
How do I know what size and pitch I am buying?
Are there different spline sizes for the same motor?
Should I actually measure the hell out of my existing arrangement and use that to launch this search? (seems like a lot of work)
Would any props fit off off....say a 50/70hp Force or say a 125hp Force?
Is there a way to find out if I've got a 'ski' or 'cruise' prop?
What is a reasonable price to pay for one (new vs. used?)
The boat this motor is mounted on is an 18' Bayliner.
Much thanks in advance.

 

[Silvertip]

Re: ...prop help/experts needed...any takers?

You've given us lots of information but little of it useful at this point. The purpose of the prop is obviously to push the boat but it must also be a diameter and pitch that allows the engine to run at or close to the manufacturers recommended wide open throttle RPM (probably 5200-5500 rpm) for your engine with an average load. So we first need to know what the diameter and pitch is of the prop you now have. The numbers should be stamped on the prop or cast into the hub of the prop by the nut. We then need to know what the wide open throttle speed and rpm you now have. We then would like to know what performance characteristics you are trying to improve. Yes, we know -- you want killer hole shot with awesome top end. Got news friend, the two can, but rarely go together. With one prop you can get "acceptable" hole shot and "acceptable" top end. By playing with pitch and prop style you can tweak top end or hole shot. Lastly, check your engine mounting height. The antiventilation plate should be flush or about an inch above the bottom of the hull. The engine must also be tuned properly and making full power when you make your test runs. If the boat is full of water soaked foam you are lugging around a couple hundred pounds of water so consider all this before spending money on props. Right here on iBoats is a prop section that allows you to see what props are available for your engine. Keep in mind that a 13 inch diameter/19 pitch prop from company A may perform differently that a prop with the same specs from company B. That's the way it is. Prop selection is sort of a crap-shoot.

 

[Frank Acampora]

Re: ...prop help/experts needed...any takers?

Prop from a 50-70 horsepower will not fit. It is a different series gearcase.
Props from model year 1984 (Chrysler one piece lower unit) 75-140, through later model 1990 (and possibly later) Force 85 through 125 will fit. Gearcases are all the same splines and diameter with only minor unimportant changes eg. skeg shape and drain plug size.
That 90 is a fairly strong engine with a 2 to 1 reduction ratio gear case and on an 18 foot Bayliner you would want to start with about a 13 X 17 prop. That's what I used on my 18 footer. Should put you somewhere in the 4500-5500 recommended WOT operating range. You may or may not get 1 or 2 MPH more with a 19 pitch prop but the 17 will be better for towing and tubing. The Michigan stainless props I have seen are cupped and a 19 pitch may be a little much for your set-up.
You don't have very much selection in props for these engines. --13 diameter X 15, 17, 19, and 21 pitch and virtually no performance props. You might be able to find a Vortex prop, but again, the selection is limited. If you look closely, some of the Mercury small gearcase (4 1/4 inch diameter) props will fit. The splines, hub, and thrust washer are almost identical.
Look at my avatar in the corner. That is a 1990 90 HP. force with a 21 pitch stock stainless prop on a 14 foot hull. Goes 56 MPH at 5600 RPM. And I have not played with it for max speed yet.
Stainless will give 2-400 RPM and 1-2 MPH more than aluminum but is significantly more expensive (around 300 new). Look on ebay, they often have props for sale there, but they tend to be a little pricey for used. iboats sells props and there are also a few discount sellers on line
Size and pitch are stamped on the outside of the hub on stock props. It is stamped in the center of the hub on Michigan props. Some props just have a number that must be referenced through a catalog to determine size and pitch.