Prop is not slipping...

[jzumi]

Kind of a long story but I am perplexed and could use some help.

First of all, this is my 2005 2520XL with modified vee and 2005 Yamaha F250. Everything was fine and dandy for 5 years (I'm the second owner) until August. I replaced the lower unit then when the gears ground themselves up. I installed a brand new genuine Yamaha lower unit. When I installed the new anode on the cavitation plate I did not have the old position to reference so I took a guess and just placed it at a slight tilt to starboard if you look at it from the back.

After break in of the new lower unit, it felt like the prop was starting to slip. On flat water with no wind, I would be at say 4200 rpm and the rpm would rapidly increase without the throttle being moved. Rpm would increase to 5000 quite rapidly and the boat speed would slow slightly. This had never occurred during the previous 5 years. So, I thought the prop hub must be starting to slip. The prop shop replaced the hub but said the old one was very difficult to press out and the rubber did not show signs of slip.

Re-installed the prop, did the break in. Same rpm increase occurs. Only happens at 4000 rpm and above and does not happen during hole shot.

Could this possibly be that the angle of that little anode is causing cavitation? Or...when we replaced the lower unit, we took the opportunity to upgrade to a CHIRP fishfinder and new chart plotter. We installed a transom mount transducer just starboard of where the Armstrong bracket meets the transom. Could that be causing high speed cavitation?

What's the best way to sort this all out? I'd appreciate your ideas on how to approach this. Thanks! John

 

[grouperjim]

We installed a transom mount transducer just starboard of where the Armstrong bracket meets the transom. Could that be causing high speed cavitation?

Would need to see a pic.

 

[Brent]

How big is the transducer and location. Ask Jim stated, pics would help

B2

 

[Megabyte]

When you had the prop off, was there any indication that the hub was damaged or spun?

 

[jzumi]

The prop shop guy said it did not look spun and he said it was difficult to press out the hub so he rather doubted it was spun... He said if it wasn't such a nice stainless steel prop he'd recommend getting a new style prop just so rehubbing could be done without the big press.

I'll take some pictures next time it goes on the trailer. Trailer first needs to get new brake lines this Thursday. It's always something.

 

[shawnee83]

Based on what you have described and the fact that you mounted the new transducer when you did the lower unit, my money is on the transducer is causing the problem. If you can, take a pic from behind the boat with the motor down and straight so we can see the position. Did you read and follow the instructions as to how far from the prop that it needed to be mounted? Without pics, I can tell you that you should have that transducer at least 6” beyond the swing radius of the prop. More is better for both your transducer readings and the cavitation issue. Also, don’t mount it over a strake. Good luck with the fix.

 

[WAJSKI]

I had a 2320 and a Garmin gt50m about 6" to starboard of drain plug with no problem> Armstrong bracket with F225

 

[coolbreeze]

I had the same problem in following seas . I put in a shoot thru hull transducer and no more problem. My boat is a bracket boat and I also had a new hub installed and prop guy said it was not spun.

 

[Brent]

I agree with Shawnee. How big is the transducers and location? M2cw

B2

 

[jzumi]

Finally got the boat onto the trailer and took some pictures.

Transducer looks to be too close to the centerline of the hull and may be throwing bubbles at the prop at higher speeds. So I guess we'd call this an aeration problem.

At the suggestion of the marine electronics shop, I'm going to move it starboard about a foot. They actually want the transducer on the port side of the hull since I have a clockwise rotating outboard, but IO will try this to see if I can solve the issue without having to drill another hole in the aluminum swim step...

I hope the pictures come through.

Thanks for all the replies. John

 

[FishWhisperer]

That would definitely cause cavitation mounted there. Do you have a thru Hull transducer also? If so, aren’t they supposed to be mounted next to each other so they read more accurately? I have the structure scan HD (Lowrance) from my old boat and could not find a good place to mount it on my 2520XL so it’s sitting in my shop. I used it a few times on my old boat but was not too impressed with it. You have to go so slow for them to work anyways that I wonder if you could mount it up higher so when you are running, it would not be changing the flow of water going towards your prop, and when you slow down to fish, it would still be under the surface.


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[Brent]

It looks too deep to me and in wrong location.

B2

 

[jzumi]

Old thru-hull transducer failed so has been left in place but abandoned. This new transom mount transducer is one of those all-in-one kind that does some imaging out to the sides and towards in front of the boat, meaning it has to be mounted low enough on the transom to get those views. So the trick is to find the sweet spot where it doesn't cause problems with the prop, still can "see" what it needs to, and keeps sending useful info at reasonable forward speeds.

Today, I'm remounting it one foot over to the right. Splash the boat and test it all in a week or two.

 

[shawnee83]

It’s definitely in the wrong place and may be a little to low as well? I feel sure moving it will fix the problem you are having with cavitation. Make sure that you read the instructions as to how much below the bottom of the hull the transducer is supposed to be mounted. Good luck.

 

[FishWhisperer]

Yes it looks like it should be mounted so the middle of the unit is level with the hull. So a couple inches higher and to the right should be good. Probably don’t want to be in line with the strake though either as that would probably not read at higher speeds. Good luck!


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[Brent]

Did someone drill a hole in the bracket for the cable?

B2

 

[jzumi]

I moved the transducer about 14 inches to the right. We did a sea trial and no more aeration/cavitation. And, the fish finder keeps workng all the way through 30 miles an hour! In the previous location it would lose the bottom at any speed over 10 mph.

During the install we drilled through the swim step and then through the transom to route the cable. For the relocation, I was able to simply swing the transducer over to the right, remount the bracket, and add a new cable clip to keep the cable snug to the hull. Easy peazy!

Time to go throw the crab pots back out and go fishing before the New Year!

 

[shawnee83]

Good news. Glad it worked out. Happy New Year!