Prop stops turning above 1500 RPM??!!

[brad614]

OK - here is what is going on. I have a 1986 Sea Ray 25' with a 5.7 / AlphaOne Mercruiser I/O. Today we ran into a lot of low ground, chewing up some sand and grass (at very low speeds). It was so bad we passed at least 8 boats that were grounded! We finally got out in the channel and got up to speed (around 4000 RPM). About 2 miles later all of the sudden the engine sounded like it was thrown into nuetral. After that we could get along at about 1500 RPM, but anything over that it seemed like the prop would stop spinning.

Got home and took the prop off and the bushing "looks" ok, but not really sure how to test it. If you put the boat in gear (with the engine off) and turn the shaft you can hear / feel a clicking sound. What could be wrong??

 

[DBreskin]

Re: Prop stops turning above 1500 RPM??!!

Sounds like a broken prop hub or broken engine coupler.
With the engine off and in gear, try to turn the prop by hand. If it turns without the prop shaft turning with it, you've got a broken prop hub. If the prop shaft also turns, you've got a broken engine coupler.

The prop can be repaired or replaced easily. Replacement of the coupler requires removal of the engine.

 

[brad614]

Re: Prop stops turning above 1500 RPM??!!

With the engine off and in gear I can turn the shaft by hand and you can feel / hear a clicking sound. Not good, is it...?

 

[NYBo]

Re: Prop stops turning above 1500 RPM??!!

Hopefully, it's just a spun hub in the prop. The hub can be spun without any obvious signs when you look at it.

Make a reference line from the hub to the prop itself with a dab of white paint. Let it dry, then run the boat past the engine speed where you are having the problem. Take a look at the reference line. If it's not intact, the hub is spun.

 

[paultjohnson]

Re: Prop stops turning above 1500 RPM??!!

With the engine off and in gear I can turn the shaft by hand and you can feel / hear a clicking sound. Not good, is it...?

Thats not necessarily a prob If its in FWD The prop should lock if u spin the prop by hand CCW and ratchet if u spin the prop CW in REV the opposite.

 

[DBreskin]

Re: Prop stops turning above 1500 RPM??!!

With the engine off and in gear I can turn the shaft by hand and you can feel / hear a clicking sound. Not good, is it...?

If you're turning the prop shaft by hand while the engine is in gear you've got serious/expensive problems.

 

[ufm82]

Re: Prop stops turning above 1500 RPM??!!

If you turn the prop in the opposite direction of the gear it's in, it will ratchet. The lower unit gear are cut so that they "pull" themselves into each other. The carrier is spring-loaded and the spring pressure holds it in gear- if you are blazing down the lake and suddenly cut the throttle you may even hear the ratcheting noise as the water pressure overcomes the gearing.
Since you ran into the bottom and placed a high load on the prop, you may well have spun the hub. This actually sounds like the issue in this case unlike many other cases here on iboats. As stated above- mark the prop and hub and run- that's the best way to check a hub.
Good luck
UFM82

 

[brad614]

Re: Prop stops turning above 1500 RPM??!!

Thanks for all of the great advice! You guys are awesome! It turns out it is the hub, so I obviously need to replace it. The prop is pretty chewed up too, so I am just going to get the whole ting. I have a few questions: 1) it currently has a 15 14" X 15 P 3-blade prop. I am happy with it getting up on plane, but itr seems a little slow (31 mph on GPS at 4200 RPM). Would changing pitch help the top end without sacrificing the holeshot? 2) What benefits does a 4-balde prop have over a 3-blade?

 

[Robbabob]

Re: Prop stops turning above 1500 RPM??!!

Thanks for all of the great advice! You guys are awesome! It turns out it is the hub, so I obviously need to replace it. The prop is pretty chewed up too, so I am just going to get the whole ting. I have a few questions: 1) it currently has a 15 14" X 15 P 3-blade prop. I am happy with it getting up on plane, but itr seems a little slow (31 mph on GPS at 4200 RPM). Would changing pitch help the top end without sacrificing the holeshot? 2) What benefits does a 4-balde prop have over a 3-blade?

Glad you found the issue to being only a few hundred bucks repair!!!!

Now to changing the prop..... Seems like a whole lot of engine for only having a 15" pitch and WOT of 4200. Either your 25' boat is extremely heavy or you're lacking a bit of HP. I didn't catch what type of boat you have, but I assume you are trimming since you say it gets on plane how you like.

General rule of thumb: lower the pitch will gain hole-shot and increase WOT RPM.

From your stats, I am just confused. I can't see going to a lower pitch with all that engine. Need someone to help with whether I missed something.

Good luck!

 

[brad614]

Re: Prop stops turning above 1500 RPM??!!

The boat is a 1986 Sea Ray 25FC. It is a 25' cuddy cabin. Empty weight is 4200, and I usually have about 60 gallons of gas, 30 gallons of fresh water, 3-4 people, and assorted other gear. It has a Mercruiser 5.7 260 hp with an Alpha One outdrive. 4200 RPM is not necessarily WOT, it's where the engine sounds "comforatble". I would like to be able to pick a few MPH at the same RPM, without affecting my holeshot too much.

 

[Adjuster]

Re: Prop stops turning above 1500 RPM??!!

You need to determine what your current wot rpm's are and go from there. I had the almost exact same set up just different maker(slickCraft). I was running the 14x15 four blade prop. If I remember correctly wot rpm's for your motor should be 4400-4800. My four blade dramatically improved steering especially in reverse backing away from the dock. Keep in mind if you go four blade you need to adjust your pitch for approximately 200 rpm's. It will all be explained on any websites for purchasing props. Another improvement I can personnely recommend is trim tabs for these heavier boats that are a little low on power.

 

[brad614]

Re: Prop stops turning above 1500 RPM??!!

Thanks! I actually have trim tabs and it is AMAZING what a difference they make!