Cavitation Plate Lower Than Boat Bottom

[michael601]

The cavitation plate of my 2023 20hp Mercury long shaft is 2.5 inches lower than the bottom of my 2023 Starcraft Alaskan 13 (with 20'' transom). Is there any way to build up a transom to raise it a few inches? If not, are jack plates effective. Any advice would be helpful.
Thanks.

 

[FuzzyGrub]

2 1/2" is rather low. One inch low is where many outboard mfg recommend starting. Make sure your straight edge is running down the center of the boat.

If using clamps, you can usually get 3/4 - 1" by shimming under the motor clamp and re-tightening. If bolted, you can raise to what you have available. A high setting, will probably need some transom re-inforcement. Can do it for testing purposes, but just ease into the throttle.

There are small fixed transom jack plates, for small HP. With the offset from transom, the motor can be raised slightly higher.

 

[Axhammer]

You can build your own JP using some angle iron. Check out Youtube.

I’m going to build one for my 1648 with a 25HP Rude, using aluminum angle.

Yes, Jack plates work.

 

[Crazyboat]

When I was a kid I bought a long shaft for my little tin boat, not realizing how it would wreck havoc on how the boat handled. My dad and I simply bolted a piece of 1/2" plywood to the back of the boat, cut her 5" above the old mark and filled in the gap with scrap wood. Painted the daylights out of it with (I'm sure lead) paint and dropped her in. She ran like a champ and I never thought about it again, until the regular untreated bolts started to rust, so we changed them with galvanized ones and it was all good.

If I was going to do it different today I'd built it taller and shave off as needed to peak performance. In your case add 3.5" of height and use a circular saw to drop her a 1/2" at a time until she doesn't cavitate any more.

 

[Makaw]

Yes it’s doable, and I recently addressed the same concern. Mine ran 2” low for years, but it ran fine. No “issue” from running at that height, but I wanted to see if I could improve performance by raising it.

I built my own jack plate from 2.5” aluminum angle to raise the cavitation plate dead level with the hull. What I found was that raising 2” actually decreased my performance pretty significantly, both in top speed and cornering stability. My jack plate is slotted so it’s fully adjustable. I slowly made incremental adjustments to drop it back down and found that it runs ideally around 1” below the hull. So I essentially raised it 1” from the original position. It didn’t have a massive effect, but it was noticeable for sure.

Every boat is slightly different and I’m typically running my boat fairly heavily loaded. A jack plate is a great way to give you the flexibility to find your optimum outboard settings. See some pics below for reference.

 

[LaqueRatt]

That's some nice work right there. Hope I remember how to do this if the need ever arises.

 

[Zum]

Transom elevator will work, if you dont want a jackplate.

 

[airshot]

In my experiences over many years of boating, the motors of yesterday with the exhaust above the prop like those in your pic do better when the cav plate is below the bottom of the hull by mabey an inch or so. Newer motors with the thru hub exhaust do better with the cav plate at or slightly above the bottom of the hull. You do nice work !!

 

[Makaw]

In my experiences over many years of boating, the motors of yesterday with the exhaust above the prop like those in your pic do better when the cav plate is below the bottom of the hull by mabey an inch or so. Newer motors with the thru hub exhaust do better with the cav plate at or slightly above the bottom of the hull. You do nice work !!

Interesting! I’ve never heard this. Good to know if I ever have to repower.

 

[sonny.barile]

Before you do anything with raising you need to know how it’s operating. There is a recommended rpm range for the motor at wide open throttle. If your skeg Is too deep you are probably under revving and creating too much drag. That is bad for your outboard. You should figure out what your rpm’s are first before tackling this.
My Evinrude 30 was mounted similar to yours. I was able to get a hold of the wire harness and the eTec diagnostic software to check my rpm’s. I also installed a tach. I was hitting 5400 at WOT and the recommended range was 5400 to 5800. Boat speed was 22 mph. Raised the motor and rpm went up to 5700 and speed to 28 mph. Handling is better and much less steering torque. In the end it became faster and more responsive. If you were to go too high you will get blow out and probably hit the rev limiter a bunch. It’s a balance………..You may also need to mess with the prop diameter and pitch to dial everything in just right.

As far as Jack plates, be careful as putting the outboard further back on the hull creates other things that will need to be dialed in…….

 

[Bailey Boat]

TH Marine Mini Jacker.....