Engine height on Offshore repower.

[Keyboardman]

Hi guys, I'm repowering a 21' walk around cuddy with an offshore bracket on the transom. It originally had a Johnson 175. I'm putting a 200 Ocean Pro on it now. I've heard that setting the engine cav plate height just a tad lower than the bottom of the hull is better as it will bite more in offshore conditions. What's your opinions? Thanks

 

[Scott Danforth]

the further back you move the motor, the further up you move the prop/vent plate

 

[Keyboardman]

So you're saying I should actually have the cav plate higher than the bottom of the hull because it's further back from the transom?

 

[jbcurt00]

So you're saying I should actually have the cav plate higher than the bottom of the hull because it's further back from the transom?

Yep

 

[dingbat]

Generally speaking, raise 1” for every 12”of setback.

 

[Scott Danforth]

with outboards, its more like 1" up for every 4" of setback (at least from reading on scream and fly)

however you do need to test, and this is where a vertical jack plate pays dividends.

 

[dingbat]

So.. your saying for a 36” bracket I need to raise the motor 9” on a 25” transom?

 

[Keyboardman]

Allrighty then, seems like I have some math and measuring to do. In my research, I ran across that if the transom or bracket is angled then I need to compensate for that too? I'm going to take some measurements and pictures of my bracket and hopefully with you guys knowledge and expertise I can mount this engine correctly. Thanks

 

[Keyboardman]

So if my bracket goes back 27" then 1" per ft would put the cav plate height at 2 1/4" above the keel line. Is the bracket and transom 12 degree angle worth figuring in? I can see where full down trim would be at a greater than 90 degree angle to the keel line and when trimmed up a little to put it at 90 to the keel line it would be slightly lower? Thanks.

 

[Keyboardman]

Looks like if I use the bottom hole on my engine, I can get 2 1/4" above the keel point.

 

[JimS123]

There is no "standard". Every boat needs to be optimized by collecting data at different heights with trial runs.

Just for reference, my 2 boats were set up by the manufacturer with the following specs:
16" setback - AV plate 4" above keel
No setback - AV plate 2" above keel

I am assuming that the manufacturers did the testing before they sold the boat. Right or wrong, since I consider both's performance awesome, I never did an trials of my own.

 

[Sea Rider]

Placing the AV plate above, middle or under the boat's rear bottom in a Dry Installation doesn't say much. t's important to know at which lower leg the water flow is passing by, different heights dials different water performance. Each motor/boat installation it's unique and must be tested on all water conditions from flat calm to windy, wavy, choppy ones.

Happy Boating​

 

[JimS123]

Placing the AV plate above, middle or under the boat's rear bottom in a Dry Installation doesn't say much. ​

When you're on the hard, you have to start somewhere. While above or middle may be viable options, below is not a good idea, especially with a setback.

 

[Keyboardman]

Thanks for all the replies and advice guys. Seeing as I don't have the money for a jack plate and not gonna drill additional holes in the bracket, I'll go with the lowest hole on the engine which will be 2 1/4" above and call it good. Thanks again. Tim

 

[Sea Rider]

When you're on the hard, you have to start somewhere...

Right on, need to start somewhere. Many prefers the flow to skim righr under the small upper plate, some right over the AV plate. For Offshore boating set the motor to a given height for the prop to achieve top thrust propulsion at choppy, windy wavy water conditions with O prop aeration. If worked well on such water scenerios will work top when on flat calm water condition.

Happy Boating​

 

[Faztbullet]

It is never mentioned..is the OP200 a 25" or 30" length?? When mounted the AV should measure 2"-3" above boat bottom. When you add set back you change the fulcrum point from the engine. It also gives you less drag and cleaner water for prop to operate. You should look like pic below when mounted. If a 30" mount it 2 holes higher on set back...