97 1751 Polar Kraft Tunnel - Boat shaking

[Guest]

Just put my prop lower unit on last week (took off my jet) and when I ran the boat at WOT (5100 rpm) the boat/side console was shaking violently in fairly calm water.

I did adjust the trim tab on the outboard which did reduce it some but I still have to probably adjust it a little more.

Looking at this pic, does my motor position (tilt) w/ manual trim look about right for a tunnel hull?

I just had the prop re-surfaced/re-balanced by a guy who does good work.

 

[steelespike]

Can't speak to the Tunnel boat but it seems like it would squat a little
initially.
It appears the the motor may be pushing the bow down some.
Have you tried it trimmed out one more notch?
I often find that one notch out(up) may not be enough and one more up is too much.
If I remember right your running with about 30% more power with a prop.
Seems like the prop would have more affect on the boat than a jet.?
Most boats with good speed you want about 1/3 or less of the boat in the water.
Of course that would depend on the speed and the load.

 

[Guest]

Thanks for the reply steelespike. I'm going to try one more notch like you said and report back. I'm trying to avoid dropping $$$ into a jack plate power trim setup.

I need to double-check the prop shaft and make sure its true. I would THINK that the prop would be OK after getting it re-balanced, resurfaced, etc. But I guess you never know. I've read some people get good results with the SS cupped props and Tunnel Hulls. I might go that route once I know exactly the cause of the boat shaking at WOT.

 

[steelespike]

Sometimes you can buy or make wedges to adjust trim a 1/2 notch up or down.

 

[Guest]

I checked the prop shaft, looks pretty straight to me. I do hear a slight binding on the bottom of the prop and the bearing carrier assembly. I made a video here, would this cause any vibration, is this common? thanks

https://www.youtube.com/watch?v=PCoEMRo1BFk

 

[MRS]

Not a expert on this but does look like maybe outdrive setting a little high? Could maybe be causing the trouble.

 

[Guest]

Oh, it's just tilted up for the video, hah. Or do you mean in the original pic? I have the outboard as low as it can go, its a tunnel hull. thanks

 

[steelespike]

For the prop to make a boat shake it would have to be missing part of a blade or be way out of pitch or balance.
I'm thinking maybe it's running too flat being buffeted by the water..

 

[Guest]

Just wanted to report back thank you Steelespike! It was just a simple tilt issue list you said, I had the outboard too close to the transom. Once the boat is on plane, no more shaking, runs perfect.

The only issue I have now is that there is some slipping during take off. I'm wondering if I need to look at getting a cupped stainless prop or if this is a common occurrence with tunnel hulls?

 

[steelespike]

I would say that a tunnel could be fussy during hole shot.
I don't think you have to get crazy with a prop. A 4 blade should solve the venting.(slipping)
If you get us your prop size and wot rpm and gps speed lightly loaded we can tell better what size prop may work for you.
Size may be under the prop nut.

 

[Guest]

Thanks again Steelespike.

Currently, I'm running a 10 5/8 X 12 Pitch 3 blade aluminum prop.

Operating RPM Range of my Mercury 60hp is between 5000 to 5500

I went down from 5100 RPM to 4900 RPM yesterday after I installed a Mac's River Runner

When I ran WOT yesterday, my phone GPS said 26 mph.

 

[Guest]

I'm eyeing a Power-Tech stainless prop that has a lot of cupping to fix my slipping issue, p/n SWC3R10P-M70 (10 pitch).

I have to say, the customer service at Power-Tech is top notch. They're very helpful on the phone.

Should I throw $$ at the prop or should I just live with it..