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View Full Version : Outboard shaft length vs transom height



Andrew
12-24-2011, 10:33 AM
I just bought a little home built skiff for my wife, a 13.5 ft Dark Water by Gator Boats. The distance for the keel to the top of the transom is 17.5 in. The boat came with a long shaft (20 in) 9.9 Evinrude (The gator web site says HP up to 20.) It wouldn't plane. I put dolphin fins on the cavitation plate and increased the angle of the motor with shims to point the thrust downward an extra ~5-10 degrees. This helped but not satisfactorily. I removed the shims and put an extension on the transom to raise it to 20 in. It planes better and doesn't feel as under powered but it hesitates at full throttle. I believe its cavitating intermittently. I'm guessing I need to lower the transom but would like a little tutorial on OB motors before faffing round anymore.

Thanks
Andrew

PS I'll post a pic if someone can point me to how to do that (it's been some time since I was on WBF)

wizbang 13
12-24-2011, 12:39 PM
It IS a matter of futzing with it to get the best perfotmance, 'specially when using a small engine.
I would encourage you to fair/sharpen the trailing edge of the boat , first.
Then , ditch the dolfin thing. Its pur[pose is to help dogs plane. It steals top end speed.
The height, AKA, "jack" of the engine is to be played with, get a buncha paint stir sticks and have fun adding /subtracting them every time you go out. Trick is , of course, to enjoy monkeying with it.
It is cheaper and cleaner than turning wrenches.
The trim of the engine , ( as opposed to trim of the boat), is the pin thingy which likes being played with too.
Some boats benefit from "tucking" the engine, which is adding a wedge to trim the OB down beyond what is stock.
But, like I said, ENJOY the process.
Down the road, you may borrow a portable tach, and fine tune the prop, but do not waste money on props untill the back edge has been made SHARP(square)
"ideally" in other words , theoretically, the anti ventilation plate should end up very close to the bottom depth.
do you have a hand held gps/ what speeds are you getting?

SchoonerRat
12-24-2011, 01:46 PM
Instead of lowering your transom, it might be easier to raise your prop.

Take a look at your motor's shaft down by the prop. You will likely see an extension piece bolted in there to make the shaft longer. If it's there, it can be removed to shorten it down to a standard shaft.

SchoonerRat
12-24-2011, 01:47 PM
Never mind the last post, add another if you can find one to lower the prop!!!

Andrew
12-26-2011, 08:49 PM
Thanks for the responses. I measured the 9.9 hp and it's 17.5 inches. I was relying on what the former owner told me i.e. 20 in. So while the motor may have needed raising, probably not to the extent I did. I'll try lower shims and see what happens. I also have a 20 hp Yamah 4-stroke that has a 20 in. shaft. I think I'll try that with the 20 in shim also though I would like to stay with the 9.9 if that can get the job done. I do have small hand held gps that can give me mph. I guess the procedure is to make changes and see which configuration gives the max speed at full throttle. Again thanks. I post the results though weather, tide and job may slow the process.