outboard height question w/pics

[Maxum21]

I have a new to me 18' larson with a 115 Johnson 20" shaft. I had a 13.25 x 19 prop on it but did not run right with it. Seemed to possibly "cavitate", ecspecially while making turns. Another member on here told me i was over pitched and recommended trying a 15 or 17 pitch which i'm going to do. But before i start buying props i wanted to make sure my motor height is correct. This motor isnt original with the boat, so wanted to get opinions on whether the shaft length is correct for this hull. The transom from top to bottom measures 20.5", however the transom tapers foward a few inches and then drops down another approx 2.5" which is the hull bottom. Now what i'm unsure about is should the anti-cav plate be level with the bottom of the transom or level with the lowest part of the hull? The motor is currently in its lowest mounting position and the cavitation plate is level with transom bottom, not hull bottom. I attatched some pics to help explain what i'm saying.
thanks in advance

 

[Maxum21]

Re: outboard height question w/pics

side view pic of transom/hull

 

[Sea Rider]

Re: outboard height question w/pics

Once boat is at full plane and well trimed so anticav plate rides paralell to water level, ideal water flow (red line) should be passing slightly under small upper deflector plate, check with a spotter to see where is water flow passing now and adjust/shim engine height as needed. It's more of trial & error than adjusting transom engine height by sight if wanting to achieve top boat/engine performance.

Check this video as a refference : http://www.youtube.com/watch?v=oBeZjaA76VM&feature=youtu.be

Happy Booating

 

[Maxum21]

Re: outboard height question w/pics

thankyou for the helpful info! i will get on the water and check it out. if it turns out the anticav plate or upper deflector plate is too high in relation to the "ideal water flow", would i need to replace it with a 25" shaft motor? my current motor is mounted in its lowest possible position

 

[Sea Rider]

Re: outboard height question w/pics

If water flow hits upper small plate will have tail drag and unwanted water splashes (RED x) Small plate should be riding through edge slightly over and AC plate under water flow. Check water flow and adjust engine height upward if needed, can't tell if will need to cut transom down a bit or get a long shaft engine to match.

Happy Boating

 

[steelespike]

Re: outboard height question w/pics

The boat appears to have a pad that is lower than the transom bottom. this usually allows the motor to be higher.
As the motor is actually lower in the water do to the planing surface actually being farther forward.
A 4 blade Solas Amita prop will resist venting(cavitation).Also the Turning point Hustler is very effective. Both are aluminum.
The Solas has a good reputation.Check out the Turning point site.
Its normal to have to trim down for turns.

 

[metriccrescentwrench]

Re: outboard height question w/pics

Seemed to possibly "cavitate", ecspecially while making turns.

Are you trimming down as you're entering a turn? You'll need to trim down a bit in anticipation of turning, and trim back up to your cruising position as you come out of the turn.

Just from those photos it looks like the engine height is at least in the ballpark. As someone mentioned it looks like your boat has an offset transom, which will make it act similar to a boat with a jackplate. I run a 6" jackplate and the AV plate is WAY above the bottom of the pad - my prop centerline is about 2-1/2 inches below the pad. (But of course no 2 boats are created equal).

 

[Chinewalker]

Re: outboard height question w/pics

My bet is the problem could be solved with a prop change. Maybe something with a cupped edge or different blade design. Something to hold the bite better.

 

[Texasmark]

Re: outboard height question w/pics

Are you trimming down as you're entering a turn? You'll need to trim down a bit in anticipation of turning, and trim back up to your cruising position as you come out of the turn.

Just from those photos it looks like the engine height is at least in the ballpark. As someone mentioned it looks like your boat has an offset transom, which will make it act similar to a boat with a jackplate. I run a 6" jackplate and the AV plate is WAY above the bottom of the pad - my prop centerline is about 2-1/2 inches below the pad. (But of course no 2 boats are created equal).

Mr. Maxum,

Check out this man's Avatar. Look at the pitch of the boat and the speed. Even though your AV plate is above the actual bottom of the boat, it is not too high when you consider where the water level introduced to the engine is under this configuration. Do you run like that at WOT?

This is called setback and Mr D. Hadley and other guys on here with a lot more experience than I have can help direct you to the fact that there is a rule of thumb on setback and it is something like for every 3 inches of setback, your engine needs to come up 1" from where it would be with none. Reason is clear in the Avatar I mentioned.

So get a cupped prop as mentioned or get a SS.

Mark