Visit our Popular Forums

Dave_S · 06-12-2021, 05:15

I'm not sure it's the right name for it, but the large plate above thr prop with the fine adjustment tab on it.

Can I cut it off ?
What is it's purpose ?

Its on a catamaran and protrudes through the hull so it will have the hull where the plate is.

I want to do it as it will make the hole through the hull smaller.

**tkeithlu** · 06-12-2021, 05:32

It unbolts. It's a lateral trim plate -it counteracts the torque of having your propeller turn in one direction, trying to roll the boat in the other direction. In your application it should not make much difference if you remove it.

Oh. Wait a moment. You want to remove the entire casting. It's there to prevent your prop sucking air. Removing that would be more problematic. Is the prop going to be safely on the underside away from air?

valhalla360 · 06-12-2021, 06:56

The small vertical tab is to counter the rotational force as indicated but is also a zinc to prevent corrosion. I believe they make flush versions without the tab (or you can cut the tab off. On a larger boat it should be less critical.

I don't see where you will be able to cut off enough to really make it worth the trouble as you still need a hole big enough for the prop to fit thru.

One of the big advantages of outboard power is the ability to pull the entire drive leg clear of the water eliminating corrosion and aquatic growth. If you trim it way back and then mount the prop after it's in place, I suspect any tiny improvement in performance will be lost due to the prop and leg quickly becoming encrusted.

boatpoker · 06-12-2021, 07:04

I think OP is referring to the anti-cavitation plate that is part of the lower leg drive casting (on which the anode is mounted).

JC Reefer · 06-12-2021, 07:12

IMO you’d be better off making something that mounts into place to close the hole with the engine intact.

denverd0n · 06-12-2021, 07:47

As mentioned, it is an anti-cavitation plate. Cutting it off would be a big mistake, in my opinion.

Dave_S · 06-12-2021, 13:16

Thanks all, when you say anti-cavitation, does it help resist cavitation or is it meant to stop the prop drawing down air from the surface.

I am thinking of cutting it off, not just the trim tab but as far back as I can, I can't imagine it being able to draw air from under the hull.

valhalla360 · 06-12-2021, 21:37

Quote:

Originally Posted by Dave_S

Thanks all, when you say anti-cavitation, does it help resist cavitation or is it meant to stop the prop drawing down air from the surface.

I am thinking of cutting it off, not just the trim tab but as far back as I can, I can't imagine it being able to draw air from under the hull.

What are you hoping to gain? As mentioned, the hole still has to be big enough to let the prop thru and you still need the area to mount the zinc which extends to within an inch of the back edge of the plate.

Dave_S · 06-12-2021, 22:13

Quote:

Originally Posted by valhalla360

What are you hoping to gain? As mentioned, the hole still has to be big enough to let the prop thru and you still need the area to mount the zinc which extends to within an inch of the back edge of the plate.

If I cut it back I can tilt the motor (bottom of the leg needs to go back 3°) to be parallel to the waterline. As it is now anti-syphoning plate contacts the back of the well. Also I can move the whole motor back to improve the clearance for the cables coming in for the gears and throttle. Extra points, I can fit a high thrust prop. And I can use a shorter door to cover the hole when the motor is up.

I don't like doing this sort of modification but it seems to be all positives and not many/any negatives. I just wasn't sure what the ramifications to cutting it back would be.

PaulvR · 07-12-2021, 16:57

I agree, leave it, it is a useful trim tab and often the anode.
I notice in your pic though that you leave your outboard in the water! This is the easiest way to ruin an outboard, as growth in the water intake (as you see) will eventually cut off sufficient cooling water, and it will seize.

Leave the tab, and tilt the motor up, always except at a busy dingy dock.

ggray · 07-12-2021, 17:25

Well, saildrives don't have an anti-cavitation plate, and what he wants to do is more akin to a saildrive, so it may not be a problem. I say "may not" because the water in the well does have a free surface, but the water may be deep enough to preclude cavitation.

thinwater · 07-12-2021, 17:37

Not cavitation, ventilation. Different problems.

Nope, can't cut it off unless you want to lose a lot of power.

Boatyarddog · 07-12-2021, 18:43

Quote:

Originally Posted by Dave_S

I'm not sure it's the right name for it, but the large plate above thr prop with the fine adjustment tab on it.

Can I cut it off ?
What is it's purpose ?

Its on a catamaran and protrudes through the hull so it will have the hull where the plate is.

I want to do it as it will make the hole through the hull smaller.

Yes you can, but only the minimum to get the clearance you want.
In a motor well, your still not very deep.

How is the hole smaller if you cut?

Are you looking to tilt it for draft issues or to make the well opening smaller?
Or, just to be able to get the lower end thru the well hole?
There may be a better way.

There maybe cavitation, in waves.
Sail drives are so deep there's not really wave issues that deep.

I would limit the cut to the middle of the trim tab zinc, as the some of the uptake is in that plastic screen.
And, I'd consult a breakdown of the lower end first.
Boatyarddog

Kastaway · 07-12-2021, 22:11

My thinking is that if you just cut off enough of the ventilation plate to give you the 3 degrees that you are looking for it would finish up being very close to the back of the well. i.e. where the shortened plate finished the bottom of the hull would take over. It would also leave enough material to re-fix a flat plate if everything went south or if you wanted to re-purpose the engine for a different application in the future. Nice shiny new bit to be cutting up, I can see it now.....big breath, no turning back now - hoping that you don't find out that it's filled with oil and springs and an important looking flux capacitor. What could possibly go wrong!!