Worn Prop causing over-revving?

[chillin014]

I took my boat out for the first time for a test run to make sure it would perform decently before sinking any money into it, (hence the reason for running a worn out old prop). Anyway, its a 50 horse Force and I dont have much experience with boats, but I wasn't impressed. I read that the motor should be in the manufacturers preferred RPM range at wide open throttle. Well, I could rev much higher than the recommended 5500...I'd say I could squeeze it out past 6k easily and maybe TOUCH 30 mph. But I didnt want to hurt the motor. Holding a motor at WOT makes me feel uneasy, I realize this is a boat motor though, and not a car motor.

So my first attempt to cure this was to buy a new prop. I dont have much of a baseline to go off of since regardless of the current props specs it is very worn out and beat. Is my train of thought correct? -That the prop is so beat that its simply not moving enough water and just spinning rather than propelling?

If this is the case I may be very pleased with the performance increase a new prop will get me. My second question is...where do people get props for these motors? The options seem VERY limited...I see some on this website, and here and there on other websites..

 

[bowman316]

Re: Worn Prop causing over-revving?

look on ebay for a prop with the right hub already in it.

But it may not be the prop, what pitch is it?
You may have spun a hub.

 

[chillin014]

Re: Worn Prop causing over-revving?

by hub do you mean the grooved housing inside the prop that slides over the "output" shaft? Its not like the boat doesnt move...it just seems to rev too high and not move very fast.

I have no clue what the pitch is. I need to find out how to get that measurement, i've just been a little lazy. I know I need a new one though for sure.

 

[Frank Acampora]

Re: Worn Prop causing over-revving?

Even thoughthe engine is a Force engine, the question really belongs at the prop forum.

BUT: You really have not provided enough information. For example, we need to know the boat size. If the engine is on a 17 foot fiberglass runabout, then 6000 RPM is too high a reading and something is suspect. If, however, the hull is a 13 or 14 foot aluminum boat, then 6000 is a reasonable reading.

Note that I said reading, not speed. The engine will turn 6000 without harm, but if improperly propped, yopu are wasting fuel and operaing the engine outside of its most efficient range.

We also need to know the year of the engine. Most Chrysler and early Force engines used a 10 3/8 diameter prop in a range of 11-13 inch pitch. I am not sure about later Force engines manufactured by Mercury.

On the older props, these numbers were stamped on the hub, between the blades. Newer ones had a model number stamped in the hub under the tailcone. This code number (for example: AJC 1234) identified the prop as to diameter and pitch. Some of the guys over at the prop forum know these numbers and can tell you what you prop is.

NOW: If, for example, you do have the engine on a light, fast hull, AND IF you are propped with a say--10 3/8 X 11 prop, then the engine will over rev and top speed may suffer too. In this case it would make sense to try a 12 or 13 pitch prop. Do not expect much more than 30 MPH though; you canonly do so much with 50 ponies.

 

[Mark_VTfisherman]

Re: Worn Prop causing over-revving?

Worn prop or not beat up at all, 30 mph is pretty fast in a small boat. I would do what one of the other guys suggested and post over in the prop forum. You simply might not be pushing enough water.

 

[chillin014]

Re: Worn Prop causing over-revving?

yeah part of it could be that I'm expecting too much. the 30 mph was a stretch...for the most part it just seemed like it was working too hard to get over 20

Even thoughthe engine is a Force engine, the question really belongs at the prop forum.

BUT: You really have not provided enough information. For example, we need to know the boat size. If the engine is on a 17 foot fiberglass runabout, then 6000 RPM is too high a reading and something is suspect. If, however, the hull is a 13 or 14 foot aluminum boat, then 6000 is a reasonable reading.

Note that I said reading, not speed. The engine will turn 6000 without harm, but if improperly propped, yopu are wasting fuel and operaing the engine outside of its most efficient range.

We also need to know the year of the engine. Most Chrysler and early Force engines used a 10 3/8 diameter prop in a range of 11-13 inch pitch. I am not sure about later Force engines manufactured by Mercury.

On the older props, these numbers were stamped on the hub, between the blades. Newer ones had a model number stamped in the hub under the tailcone. This code number (for example: AJC 1234) identified the prop as to diameter and pitch. Some of the guys over at the prop forum know these numbers and can tell you what you prop is.

NOW: If, for example, you do have the engine on a light, fast hull, AND IF you are propped with a say--10 3/8 X 11 prop, then the engine will over rev and top speed may suffer too. In this case it would make sense to try a 12 or 13 pitch prop. Do not expect much more than 30 MPH though; you canonly do so much with 50 ponies.

the motor is a 1990 ...and the boat is a 17 foot fibgerglass bayliner, correct.

As you said, I just want it to operate efficiently. I'm not expecting a top speed gain or anything, but I just felt like the boat wasn't "plane"-ing or something..just kind of lugging along. But maybe it was planing and I'm just not familiar with what it feels like.

I'll post in the prop section if there are no moderators that can move it.

 

[Frank Acampora]

Re: Worn Prop causing over-revving?

On a 17 foot hull, that engine should get the boat on plane, but you are significantly underpowered and it will take a bit of time. A 17 footer is usually rated for somewhere around 120. With the 50 HP engine and that hull, assuming average load and one person, I would expect speeds in the low to mid 20s. A bit higher if it is a "Bass" boat. That is usually fast enough to plane--seems like many hulls plane at about 18 MPH-- but she will be riding low in the water and may feel a bit "sluggish." If you are indeed getting speeds close to 30 consider yourself doing well.

Check the accuracy of your tachometer. With that engine and even a poor prop, I would not expect the RPM you are quoting. If the prop hub was worn to the point of slipping, I would expect almost no speed at high RPM. Typically, the prop drives the boat well until it slips and then speed drops to almost nothing as the engine revs up.

 

[chillin014]

Re: Worn Prop causing over-revving?

I need a bigger motor . its a small boat though, technically I think its like 16.7 foot.

if that gives you any idea.
The tach was doing some funky things every once in awhile, it would wander but it seemed intermittent. I will check the wires to make sure everything is getting a good connection.

let me ask this. is it possible to NOT plane and be going 20+ mph?

 

[Mark_VTfisherman]

Re: Worn Prop causing over-revving?

........ is it possible to NOT plane and be going 20+ mph?

I don't think you can run 20mph with 50hp and not be on plane with a boat like that.

You need a bigger engine and not a force. HP numbers are optimistic

 

[Jack Daniels]

Re: Worn Prop causing over-revving?

I have the same size boat but mine has a max HP rating of 90. I have a 85 FORCE on mine and can run 40 on a good day with the wind behind me. Think you just might be to under powered.

 

[bowman316]

Re: Worn Prop causing over-revving?

if you want to keep that same motor, my guess would be that you need to go up in pitch about 2 inches. So you don't rev as high. That will me more efficent, and may get you more speed, if you are over reving.

 

[chillin014]

Re: Worn Prop causing over-revving?

thanks guys. a motor upgrade is highly unlikely since the price of these things are a little ridiculous. I thought 50 hp was the limitation for the boat, but maybe I read wrong. Either way, I would be skeptical about loading down the transom anymore.

I am going to go try to find out what pitch my current prop is and go with a higher one as suggested, to bring down the RPMs.

I read that the prop is like a gear ratio to a car. and it definitely feels like its winding out in 1st gear and not really MOVING to describe it more accurately (in case i hadnt before).

 

[bowman316]

Re: Worn Prop causing over-revving?

are you sure your boat is 17 ft? Maybe it just has a narrow beam, but if you think 50 is the limit, then don't go higher. I guess there are big and small 17 ft boats.

If it can push it up to 25- 30 mph then you have enough power.

 

[chillin014]

Re: Worn Prop causing over-revving?

yes, according to the title/registration its like 16.7 which was unfortunately just enough to push it into the next price bracket when I registered it .

So its okay to run the thing at WOT as long as its not overheating?

i'm going to go take off my prop right now and look for some numbers. i'll come back with pics.

 

[NYBo]

Re: Worn Prop causing over-revving?

Isn't there a Coast Guard capacity placard? That will tell you the HP rating. If it's a Capri 1600, 50 HP may well be the maximum rating.

 

[chillin014]

Re: Worn Prop causing over-revving?

Isn't there a Coast Guard capacity placard? That will tell you the HP rating. If it's a Capri 1600, 50 HP may well be the maximum rating.

yes, and that is what it says if I recall correctly haha.

here are pictures of my prop. Its a "michigan" brand. The only numbers I could find on it said "PR 130" I'm posting in the prop forum as well :redface: so maybe they can interpret that.

oil coming out around the propeller shaft? i just rebuild the damn lower end.