Prop Issue Question

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Originally Posted by rick_iles

Skypilot is on the right track. Years ago, I saw a similar issue with a 115 Merc. The cause was narrowed down to the motor being mounted too high on the transom.
Usually this also causes an increase in RPM.

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Thanks, Rick, the sad part is the fact that many dealers are not very good on setup for proper operation. Many look at it as if it floats, runs, and goes into gear, it is setup properly. I have never purchased a "packaged" hull that didn't need rework like completely removing the O/B and remounting.

My last packaged deal was so bad it would only run 51mph(by gps) at full throttle(150HP Evinrude on tournament hull). However that particular setup was way to low on the transom and therefore "slow."

The less than optimum motor mounting is so bad in the States(likely for liability)that even a BassPro friend gets his sponsor's boat with NO holes drilled in the transom nor bow(for trolling motor mount).
He gets the motor in a crate and the boat/trailer delivered and we do the complete installation from drilling transom holes to hydraulic jackplate installation, to hydraulic steering, motor mount position and then field test for optimum prop performance.

Seems odd to me that this happened after adding 2 batteries. How did it run before?

S.

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Originally Posted by Sinker

Seems odd to me that this happened after adding 2 batteries. How did it run before?

S.

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My guess is it's been an ongoing issue. Prop damage by cavitation doesn't happen over night...

Rick I think you are correct that this may have been going on for awhile and that I just noticed it. I am have added a 8hp merc on the back 3 years ago, now the 2, 31 series batteries. Originally I could trim the engine up a fair bit, but now nothing. I am using an aluminium prop. Here are a couple pictures, the level is on the flat of the bottom, not on one of the ridges. I used the level to make sure the boat is level.

http://i659.photobucket.com/albums/u...psvqcyp7w3.jpg

http://i659.photobucket.com/albums/u...ps5tziftzt.jpg

Sinker (post #4) gave good advice.

You have added more weight in the rear of the hull coupled with a poor performing prop. It happens frequently to all of us.

An aluminum prop will not be as stiff and efficient as a SS prop nor can the rake needed for a "bite" every RPM be as efficient in an aluminum prop like it can in a SS prop. The aluminum would have more flex and have easier deformation.

The quickest semi fix, if you want the boat setup like it is(with 2 motors and 2 batts. in the rear) is to simply go with something like a 20-24" SS or try the same pitch in aluminum for an on the cheap attempt.

On the face, it seems a motor adjustment on the transom and a raker prop in SS is needed.

Either the aluminum prop is slipping on the hub, not biting(and cavitating) or ventilating for some reason like exhaust discharging into prop or water displacement from rear hull is allowing ventilation.
A prop MUST run in the cleanest water to perform correctly.
An electric/hydraulic trimmed motor should be able to be trimmed the full range during regular boating operation. Anything less is indication of a motor problem or a rigging problem.

Just some free advice....cavitation is a serious problem, ESPECIALLY when the collapsing of the bubbles are damaging the metal. That situation alone is serious yet the out of phase vibrations that are created from the cavitation are worse on the shafts, bearings, and overall motor, etc.

All else being normal the correct/optimum motor height on transom coupled with the correct prop application will resolve the issue if there are no hull issues(like a hull transducer or speedometer skeg interrupting water flow to prop.)

When I competed, I had over a half dozen props for different tournament locations. For a "heavy boat" and skinny water, with a bottom of marshy mud and oyster shells, I used a 4 blade SS to "jump" the boat up on the pad almost instantaneously to prevent bottom and prop damage.

For long runs across coastal salt water to get to a freshwater river where I had found tournament fish, I used a 24" raker for high top end , get there and back fast results.

I honestly believe if you either work on the problem yourself or ask the dealer to redo the O/B motor mount for proper height and get the appropriate prop, you will be stunned at the difference in performance.

The dealer wants to look at the boat, they are going with pretty much what you have said about or motor burr. The boat has always had a weld at the bottom edge that I have always been concerned with and interferes with my transducer. They asked me to check all leading edges of the motor, which I did and no burrs. So they want to look at the boat bottom. I believe that the problem has been there for quite awhile and I have just now noticed the damage. I put my spare prop on that has only been used a few times, the only marks in the pain are in the exact place at the damage to the other prop.

Sky, I do want to thank you for your help. I will post what the results are. Thanks again all.

Good deal and your very welcome. Probably best to let the dealer have the 1st kick at the can anyway.
Hope it all works out and you are thrilled with the results.

And yes, please do let us know, I'm always interested in fishing boat performance problems and what was found and how it was resolved.

So brought the boat in to dealer today, to have a look at the bottom edge of boat. He agrees that there seems to be to much weld left, but he is thinking the water intake on my etec. It lines up exactly where the scoring is happening, and it is not flush, so he thinks it might be the problem. For temp solution put silicone on the edges to smooth out and put paint on there the paint has been removed from my spare prop. See if this stops it.

I would doubt the intakes as I always run 3 out and 2 in the water out of 5 total on a 150 evin, but something is causing it.

I'd also check the cutting edges of the prop for nicks and dings(those usually cause bland erosion rather than hun though.

It sounds like your are on the right track. Seems it is a problem of too high RPMs for the current load(which is easy to do as one adds weight to a hull with the original prop.(that was OEM selected for the original weight and motor rpm)