Engine height question for V21

B

bgreene

Banned
The cavitation plate is below water except when fully trimmed up running a WOT test.
Then it's just about even or above.

So question is, would therefore be worthwhile to move it up the remaining hole ?

Marina thought I shouldn't have gone up the 1 hole higher, but it turned out fine.

Boat runs just about 50.0 mph / 5,600rpm run light, scaly 6" chop, bimini down.
That's trimmed out all the way...........just before losing bite.

Most advanced riggers will suggest cav plate should be even to slightly above water without having to be trimmed up to maximum, while " old school" says that's too high.
 
My opinion, it is not going to make much difference because it is so close to perfect. I have a jack plate on several boats whereby the engine height is infinitely adjustable from stock up to 6" higher and it makes no appreciable difference in speed, or economy where it is set. If you were to change props and motor height it may be helpful but 50 is hauling the mail..You have it close to perfect anyhow.:sly:
 
Thanks for your reply and comments Ridge, appreciated.

Seems dialed in fairly well.

All I'd add is if the cav plate is under water, and engine trimmed, the plate is actually
" digging in".........providing a bit of drag, and likely pulling the motor back creating a little more stress to the transom.

That's the only reason I'd consider possibly going higher but not essential.
 
I'm confused why you think the plate in the water would cause the motor to pull on the transom? The motor pushes the boat, more drag at the motor would just mean a little less push, but no more pressure on the transom.

Or am I missing something?
 
garbubba said:
I'm confused why you think the plate in the water would cause the motor to pull on the transom? The motor pushes the boat, more drag at the motor would just mean a little less push, but no more pressure on the transom.

Or am I missing something?
Click to expand...

Since there is a pivot point at the engine mount there are different torque forces being applied to the transom. While you are correct that the main propeller force is being applied in a direction that pushes the boat forward through the water, there is a lesser force that is actually either pulling or pushing on the transom. While this force does not diminish the driving force through the water, it does do other things depending on how the engine is trimmed. Thus, if the engine is trimmed high, this force has a tendency to actually pull the bow of the boat up out of the water, and if the engine is trimmed too low, it will cause the bow to dig in too much. The plate in the water at the wrong angle will increase this force since it equals additional friction of the engine moving through the water.
 
Last edited:
garbubba said:
I'm confused why you think the plate in the water would cause the motor to pull on the transom? The motor pushes the boat, more drag at the motor would just mean a little less push, but no more pressure on the transom.

Or am I missing something?
Click to expand...


Destroyer explains it with his last sentence and also picture it this way.

With the cav plate under water, when engine is trimmed up for efficient cruising, the cav plate is then running through the water at an angle - downward facing.
The water therefore is pushing against the cav plate.......same as if you're in a swimming pool and running your hand " flat" under water - no pressure right ?
Turn your hand at an angle........but keep it going straight and you'll feel the water pressure and resistance against your hand.
Now imagine the pressure on your hand if you could push it through the water at 30, 40, 50 mph. The outboard is therefore generating " pull" on the transom when the motors trimmed up. I can't tell you how many lbs of force, but it is drag, no question about it.

This is why performance riggers like the outboard higher, so the cav plate is close to even with the surface rather than below.
Got it ??
 
Thats why the cav plate is to be even with the bottom of the boat, the boat blocks alot of the drag you are speaking about. There is going to be drag no matter what you do!!!! So suck it up and enjoy!:laugh:
 
Sounds pretty perfect to me...raising it any more MAY cause you to have more cavitation problems when turning at slower speeds.

I think the drag of the motor would put rearward pressure on the motor opposite of the foreward pressure of the thrust from the prop.
 
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