Cavitation

[Striperswiper]

I have a 1999 2520 with a 225OX66 25 inch shaft on it.

I put on 24 x9 bennet trim tabs and get cavitation in hard turns to port and starboard.

I don't know if this was happening before the trim tabs as I installed them as soon as I bought the boat.

I'm in the process of repowering and am considering a 30inch shaft because of the cavitaion.

Anyone else have a 30 or experiencing cavitation with a 25?

I'm assuming it's the trim tabs pushing water out of the way that's causing it and hoping a 30 inch shaft will fix the problem.

Capt Brian

 

[Bryan 2530]

Check where the anti-vent plate is in relation to the bottom of the boat, my guess is a 30" motor with have a negative effect on performance if you have a 25" transom, pics would help.

 

[UnWired]

Hi Brian,
I had the same problem. I also have the same 9x24 Lencos. It appears you have ModV with notched transom. Looking at the photo, it looks like you have 1 more bolt hole adjustment..

Move the motor down to the bottom bolt hole, most of the cavitation will go away. You might have just a little bit swinging around in rough seas, but it's so minor now, it doesn't matter.

 

[Striperswiper]

Thanks Chris

Did your boat cavitate before the tabs were installed?

Just wondering if it's the tabs that are the cause.

Capt Brian

 

[Bryan 2530]

Or try a 4 blade prop.

 

[Striperswiper]

Bryan,
Did you have this problem and was it solved with a 4 blade?

 

[Bryan 2530]

I have twins, and once in a while the port motor grabs air, I think (as most on the board do) that the clamshell for the raw water on the bottom of the boat causes this.

I am planning on 4 blade props soon, not just for the cavitation but al;so for ride quality and slower planning speeds. We'll see. Brad did the same on his 2530 and said it was like night and day with the 4 blades, he was also able to raise the motor but you are already there.

 

[grouperjim]

in your particular setup, (if i remember correctly) the inboard edge of the tab should be at least 15 inches from centerline according to tab manufactures. based on your pic, i don't think the tabs are a problem.

25 inch shaft should be what you need. as mentioned, i would definitely lower the engine to hole 1 before making any conclusions about ventilation, or deciding to chase the issue with prop changes.

the only other thing i can think of is to make sure there are no clam shells (intakes), or other protrusions installed on or near centerline of the hull. this will definitely cause ventilation of the prop.

 

[Tabman]

in your particular setup, (if i remember correctly) the inboard edge of the tab should be at least 15 inches from centerline according to tab manufactures. based on your pic, i don't think the tabs are a problem.

25 inch shaft should be what you need. as mentioned, i would definitely lower the engine to hole 1 before making any conclusions about ventilation, or deciding to chase the issue with prop changes.

the only other thing i can think of is to make sure there are no clam shells (intakes), or other protrusions installed on or near centerline of the hull. this will definitely cause ventilation of the prop.

The inboard edge of a standard Trim Tab installation needs to be a minimum of 8" from the center-line of the nearest lower unit, and I agree with grouperjim that viewing the picture there seems to be more than enough clearance.

Does the ventilation occur when the Trim Tabs are fully retracted? Since properly installed Trim Tabs angle up from the bottom of the hull when retracted they should not affect the water-flow in that position.

Tom McGow
Bennett Marine

 

[Striperswiper]

Tom
Thanks for the post.
The tabs were installed per specs and have a negative trim when fully retracted.

I have the cavitation when in the full up position but wonder if on a hard turn they still might cause a problem.

One thing brought to light by the responses to this post was I have a thru hull for my wash down and that might be causing the problem.

I'm not taking a shot at your company, I bought your tabs because of your reputation and quality of your product. I had them on my previous boat and loved them then and still love them now on my new boat.

They work great and look even better, just trying to figure out a problem here.

Capt Brian

 

[Tabman]

Capt Brian,

I never thought you were taking a shot at Bennett and I am delighted you installed our Trim Tabs.

It is possible that they could cause an issue in a dramatically hard turn, but given what I saw in the photo I don't think so.

However there is a "field modification" I can walk you through to make the Trim Tabs retract about another inch. There is a "stop" inside the actuators (rams) that can be removed so the inch of exposed shaft you have now when fully retracted will retract into the cylinder. It can be accomplished without taking the upper hinges of the actuators of the transom or undoing the hydraulic lines. With the boat out of the water it would take about 20 minutes per side.

If you are interested in trying this let me know.

Tom McGow
Bennett Marine

 

[TimC2520]

Brian,

I have the same boat, same motor, right down to the same year and same hole mount.....except the tabs.
I get the cavitation you are talking about from hard turns at a decent speed. I'm sure it's not your tabs.

Tim

 

[Striperswiper]

Thanks Tim and Tom,
I'm repowering to a 4 stroke and if the problem still exists I'll take Tom up on the modification he's suggesting.
Capt Brian