1988 Force 85hp, tops at 4800rpm, 34mph. prop change?

[TimberwolfFXDL]

This is the boat/hull design:

Here is the only information on the prop I can find:
http://www.timberwolfperformance.com/images/1988bayliner/IMG00034-20101017-1553.jpg
Best I can gather from google, it's a 13x19 Michigan Wheel. Sound right?

It's got a chip missing from one blade, and a crack in another. Considering the max rpm (water was fairly smooth, 3 passengers totalling 450lbs, no real gear to speak of) was only 4800, and adding in the repair needed to the prop, I'm thinking I might as well replace the prop, so while I'm at it, maybe drop a bit in pitch.

Would a 17" get it where it should be? I've read 5200-5600rpm is the range for these motors, and that each step in pitch is worth 300-400rpm. Or should I go for the 15"? I don't need ultimate top speed, but I'd like to get moving quickly for pulling a skier or tube. That being said, a reduction in pitch, but a 15% increase in RPM would effect my top speed how?

Any recommendations on prop?

PS: the motor seems to run out smooth, plugs look great, etc.

 

[steelespike]

Re: 1988 Force 85hp, tops at 4800rpm, 34mph. prop change?

That appears to me to be a light fast boat and while a reduction in pitch will add rpm, I would resist a prop change until your sure the motor is healthy,Throttle opens all the way at the carb,motor is in excellent tune.Does the bow rise when you trim up.Is the speed by gps?If all seems well a change to a 15 seems in order.

 

[Frank Acampora]

Re: 1988 Force 85hp, tops at 4800rpm, 34mph. prop change?

In 1988, the Force 85 engine was rated to run in the 4500-5500 RPM range at full throttle with 85 HP developed at 5000 RPM. So, 4800-4900 RPM puts you at the low end of the operating range.

Yes, increasing the RPM may result in better performance but you need to be careful about how you gain the RPM.

If the prop is indeed a 19 pitch, dropping to a 17 will indeed yeild about 400 RPM more, but top speed may remain the same or may drop a bit. If you drop 4 pitches to a 15, (again assuming a current 19 pitch) speed will definitely drop. You can't go fast without pitch!

IF you buy a stainless prop of the same size, you will gain about 200 RPM with an increse of about 2 mph top speed.

Before you start changing props though, try raising the engine on the transom about 1 inch. If it is mounted with the bolts in the top holes, just move it to the bottom holes--one inch higher and no need to redrill. That may give a couple of hundred RPM and increased MPH.

 

[TimberwolfFXDL]

Re: 1988 Force 85hp, tops at 4800rpm, 34mph. prop change?

Thanks for the advice so far

The speed was GPS verified as my boat speedomoter shows a low of 20mph and a high of 25mph... Not much use there.

The prop either needs to be replaced or repaired regardless. I found the same info about the operating range (4500-5500), so I don't think I'd want to drop all the way to a 15".

I haven't played with changing the trim at all (it's not power adjusted, so any of that experimentation is a bit more of a chore). I'll look at your suggestion of raising the motor next chance I have to tinker with it, and I'll check the throttle blades too.

Anything else I can do to make sure it's running up to snuff? Compression check, etc?

 

[Frank Acampora]

Re: 1988 Force 85hp, tops at 4800rpm, 34mph. prop change?

It would not run at 4800 RPM at full throttle if there were any mechanical problems. I don't think there is much to check there.

As far as adjusting trim, yes, without power trim it can be a chore, but not too much, and a necessary one. Really, all you need to do is move the trim bar and that can be done on the water.

To start, set the trim bar to the holes which put the cavitation plate parallel or nearly parallel to the bottom. Then try the next set out and see if speed increases or decreases. If it increases, try the next set out. If it decreases or stays the same, then try moving the bar back and adding a piece of 1/2 inch inside diameter aluminum tubing over the bar to move the engine out only a bit more and see what happens.

If you like the engine, intend to keep the boat, and have the 400 or so bucks to spend, look on the auctions and buy a power tilt/trim unit. Once you have one, you will never want an engine without it.

 

[steelespike]

Re: 1988 Force 85hp, tops at 4800rpm, 34mph. prop change?

Frank is the guy to get your Force setup.In the extreme a height change and proper trim could net several mph.

 

[jason32038]

Re: 1988 Force 85hp, tops at 4800rpm, 34mph. prop change?

15 would be a bit much for a 3 blade prop on that engine. Even a drop to 17 could noticably affect acceleration and top speed. Check compression. Timing. Give her a tune up and see what happens.