Perplexing Problem ?

Riegfrghtl

Petty Officer 3rd Class
Joined
Jul 7, 2008
Messages
77
Hey guys, I am not sure if this is a prop or engine problem. I have a 1972 Cruisers, Inc 21 Foot Bonanza. It has a mercruiser 888 (188 hp) motor with about 1100 hrs. Total weight of boat with fuel, people, gear is around 3900 lbs. Last week I was out with the family and struck a submerged object. I was at idle when it happen but it dinged up the prop around the edges. The prop is a michigan wheel 15 1/2 X 19 3 blade alum. I was able to make it back to the marina where I keep the boat with no problem. Basically just idled back. The next day I took the prop off and had the marina send it out to be fixed. Prior to this mishap I was able to reach 4150 rpms and go about 41 to 42 mph (gps). Mercruiser rpm range is 3800 to 4200 rpm. While my prop was being fixed the marina let me try out a michigan 4 blade vortex 14 1/2 X 18 prop. I put it on and tried it out but the performance was less than spectacular. The boat would hit 4100 rpms (eventually) but would go only 34 mph. It also took forever to plane. Between 2200 to 3000 rpms it felt like there was alot of cavitation. Almost like a muscle car smoking its tires before getting traction. Today I got my 3 blade prop back and its doing the same thing as the 4 blade prop. It came back looking like brand new and they also said the hub was fine. I also now notice a vibration at 2100 to 2200 rpm which goes away as rpms goes up. I don't know if I am dealing with a prop issue, motor issue, or stern drive issue. Any recommendations or ideas would be greatly appreciated. Thanks fellows!! Tim
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
Re: Perplexing Problem ?

There is a slim chance you somehow damaged the throttle adjustment.Or it coincidently came out of adjustment.Perhaps a plug misfiring or a wire off.
Its very unusual for a 4 blade to struggle to plane especially when down an inch in pitch.As far as the vibration remove the prop and and rotate the prop shaft with a pointer close to it to note any runout.Also move it side tot side.Install the prop and rotate to not any obviopus wobble or blade not alligned.There is a coupler(?)that can fail and slip,vibrate.I think in the area of the flywheel.The I/O forum can probably clearify the description.
 

Riegfrghtl

Petty Officer 3rd Class
Joined
Jul 7, 2008
Messages
77
Re: Perplexing Problem ?

Today I pulled the boat out of the water so the mechanic at the marina can fix a problem with my lower shift cable. As we were pulling the lower unit off he noticed the paint on the prop coming off. He looked at the prop and said he could see marks of cavitation. He then looked at the bottom (which was clean) and then suggested I move my transducer away from sterndrive to outer edge of transom. Can this cause cavitation that bad to peel paint? Other than that there were no other obvious obstructions for flow of water to prop. Should I look elsewhere? Maybe trapped water somewhere! Mechanic verified not prop hub, engine coupler, and engine is running strong. Any suggestions would be greatly appreciated. Thanks everyone!
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
Re: Perplexing Problem ?

Cavitation can cause burns peeled paint and if severe associated damage.
Its normsal for props to have some evidence of cavitation where lowered pressure causes the water to boil.Evidence of cavitation can indicate a prop is really efficient. You generally don't don't feel
any effects from cavitation. Disturbence of the water by something like a transducer could create slip making a prop have higher slip numbers.
 
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