Transom Height Question, Again..... Sorry

[therobzilla]

OK,

Here is what I have, and I think I have the correct size for my boat, I could use your expert advice if possible.

Based on this sketch, my "A" dimension on my boat is 15 1/2", now that dimension is not to the lowest point at the center-line of the boat, towards the front of the boat, the "A" dimension if running a straight edged from the lowest point to the top the the transom is more like 17".

Now my dimension of the motor from where is hangs off the transom to the anti ventilation plate is like 17 3/4" on my motor, it's a 1986 Johnson 15 hp.

Based on a bunch of keyword searching on transom heights off this site, it sound like my lengths are correct based on what I have read, and what I have measured, I just was hoping to have someone confirm I'm within range. My ID plate on the new to me motor is missing, and it only identifies what size motor and year it was built, nothing about length of shaft?....

On another note, the motor I'm replacing a Mercury 1979 7.5 hp, the lengths of hanger on the motor to the anti ventilation plates are the same as the Johnson, however I'm not saying it was right when I originally purchased the boat years ago.

Thanks
Rob

 

[Colbyt]

I am probably the least experienced guy who will reply. My OB manual says my plate should be 1-1.5" below the lowest transom point.

The PO had added a well built riser to the transom and even metal clad it. I decided to test it before I cut it down as my SS motor hangs about 3/8" above the bottom. I have not finished my break in period so I have not gone WOT yet but it seems to perform fine in that position. In discussing this at another site with another poster, he swore that getting it even or even just above the bottom was far more efficient.

Bottom line: I think you are fine. You won't know for sure until you test it. Recommendations are always just starting points.

 

[Rich27028]

do you have power tilt and trim-

the anti cav plate should - be even or 1.5 to 2 inches above the keel line - trim in to keep bow down - when at full throttle the anticav plate should just be out of the water- you dont want it draging .

 

[turbotodd]

Looking through a major manufacturer's rigging guide, they specify even as a starting point, then raise the motor up until performance suffers, then back down a hair. Setup takes a while if you do it this way unless you have a cut-and-dried measurement based on another person's exact same rig that performs as you'd expect it to.

This is assuming a flat bottom. Semi-V can get away with a little more lift based on my experience.

I've had to do them at the lake before, with the heavy 25 4 stroke motors (180 lbs +/-). I'll drop them onto the transom, clamp it, then try it like that. IF the owner says he's satisfied with its performance and doesn't care about making it any better, then it's fine at that point (even with the bottom). However there are some guys/gals that want every last tenth of a MPH, better ride, less spray..and for those I'll keep raising the motor until we start to see RPM rise and no further speed increase, then back down a hair and we're done. Many times on flat bottom rigs, it'll blow out during turns being raised that high. Thats where a jack plate or even PT&T comes in real handy. The CMC allows one to raise the motor up a good bit (to get the prop in cleaner water) because of it's inherent setback, while at the same time adding PT&T. Properly installed, they work very well.

 

[marshman]

most motrs are fine or run better with cav plate even or 1 inch higher... i really only have experience with mercurys though...i usually start at 1 inch above and go from there...

 

[therobzilla]

I'm definitely even or low, I am going to the lake next week and will bring some different sized shim stock material and experiment with the higher height and see if any improvement is found.

Thanks for your replies!

I will report back and let you know what I find.

Thanks
Rob