Prop Issue Question

Well I'm having an issue with cavitation grind or whatever you want to call it, since I put my new 24V electric on bow. So I went from 1, 27 series battery to 2, 31 series batteries at the back of the boat. Boat has been sitting low in the water since the install. Just notice that the prop is now scored on all 3 blades next to the hub.

I'm not much for trimming the motor very much as no need to, but since the install I really can't do any trim. I haven't notice the sound of cavitation or even felt it. I have a 2072 jon boat, with a 90hp on it. I doesn't draw a lot of water, but more now.

So does anyone have any ideas how to resolve this? The shop guys are scratching their heads and are still looking for an answer.

Any help would be appreciated.

So the 90 hp prop has cavitation damage? Of the 5 top holes on the motor transom mount, which hole is it mounted to the transom with?
Number 4? or higher?

I have some other questions when you answer back.

Sky, if I'm on the same wavelength as you on the holes, the pin (bar) is in the 2nd from the bottom, closest to the boat.

Can you move your batteries forward? Its better for the trolling motor to have a shorter run of wire, as well as shifting the load around. 2, 31 series deep cycles are easily 150lbs, moving them would make a big difference.

S.

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Originally Posted by sabmgb

Sky, if I'm on the same wavelength as you on the holes, the pin (bar) is in the 2nd from the bottom, closest to the boat.

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Just to prevent confusion, are you talking about the 90 hp prop or the trolling motor prop as having an issue?

If the 90 hp prop is the issue, which set of the 5 holes on the 90 hp motor mount, is attached via the holes drilled in the transom? From the top hole counting down, is the 4th motor mount hole the one used to bolt the 90 hp motor to the transom?

Most dealers, in my experience mount O/Bs way too low for any kind of reasonable performance. Thus my question on mounting holes.

I have never seen an issue with trolling motor set up from a dealer other than maybe an adjustment to lower the trolling motor shaft in its bracket in order to prevent blowout caused by unique load distribution.

I think I ve got you now, it is the 90hp. I was talking the tilt holes in the bracket not the mounting holes. I will have to check the boat later.

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Originally Posted by sabmgb

I think I ve got you now, it is the 90hp. I was talking the tilt holes in the bracket not the mounting holes. I will have to check the boat later.

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Ok, good.

As a reference/starting point(when you have time to do so), level the boat/trailer on level ground(if what I suspect is "off", it is actually better to do this on soft or normal grassy ground, like in your yard for example.)

Anyway, level the boat/trailer and observe how the cavitation plate on the motor foot(motor lower end) relates to the precise centerline and the very bottom of hull or "pad" if you have a pad AND how many water pump holes are above and below the aforementioned centerline/very bottom of hull.(use a straight edge to determine an accurate horizontal line from centerline/very bottom of hull to past motor foot/lower end.
That will give us an idea of how deep/shallow the prop/cavitation plate/motor foot is, with regards to the very bottom of the hull @hull centerline.

Also, alum. or stainless prop and any holes in the prop hub and are they plugged or unplugged or any ridge ring on prop?
Also make(mercury, etc)

More to discuss, check out and explain later as you provide answers so we can determine cause and solution.

sky, thanks for the replies. Since I was out and about for lunch, buying new prop, I went and checked holes. I'm in the first hole from the top, so motor is as deep as it goes. I have an Envinrude 90hp. I will check out the other info tomorrow after fishing. I will also try and take pictures if I can get help holding things.

Thanks again

Quote:

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Originally Posted by sabmgb

sky, thanks for the replies. Since I was out and about for lunch, buying new prop, I went and checked holes. I'm in the first hole from the top, so motor is as deep as it goes. I have an Envinrude 90hp. I will check out the other info tomorrow after fishing. I will also try and take pictures if I can get help holding things.

Thanks again

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If you are in the 1st hole from the top you may be too high with the motor on the transom.
Anyway hard to remember all the nuances between motor mounts.

Nothing under the hull protruding from the hull like a transducer?

Yes, take a look at all we discussed, post back and maybe we can resolve it.

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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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)

sky, that is something I hadn't thought about, the pitch. I have my spare on now that is a 17 up from a 15 so maybe that will also help. Just happened to be a 20ft jon in the shop with the exact 90 etec on it. The mechanic checked the intakes on it and there was definitely a ridge on mine compared to the other one. I will test on the weekend.

I think you are correct, a more pitched prop may help(if there are no issues with motor height.)
It sounds like, if all else is correct, the current prop has become undersized for the additional weight that has been added. It may have been marginal at best. You would likely recognize that per the speed and performance vs the OEM Max RPM.

Mine would run 51mph at 5200-5500RPM(if I remember)when I tested it from dealer. They had motor way too low in water and the motor advance backwards.

After fixing the advance and remounting the motor(on a jackplate) and trying three props and adjusting the jackplate/motor height it ran 71 MPH with a 23 or 24 pitch prop.

Not only that but when you hook a fish at 71mph it takes all the fight out them immediately........

Anyway glad to see you are investigating the correct things and it will eventually lead to the proper performance fix.