Johnson Short Shaft NOT so short.

[SeaHorse]

My old Seahorse 25 Short Shaft (RD14 not RD14L) throws up a big spray from the lower unit so I looked up some advice regarding the cavitation plate height and found that it should generally be level with the boat bottom for flat rear end boats. I always thought that "Short shaft" motors were for 15 inch transoms and "longs" were for 20 inch. I measured my motor and it is 18 inches from the top of the transom clamp id to the cavitation plate. My QD20 10 hp is not much shorter at 17.5 inches. If I jack the motor up three inches the clamps will be almost off the top of the transom. How do you get the cavitation plate level with the boat bottom when the motor is too long?

 

[jegervais]

Re: Johnson Short Shaft NOT so short.

The measurement is taken from the underside of the transom clamp bracket (part that sits on top of transom) to the anti-cavition plate, not from the top of the bracket (I'm not sure what you mean by "top of the transom clamp id"). You may want to measure your transom height too. I've seen short transoms as short as 13 1/2" and as long as 16 1/2". You are correct about the motor shafts being a little long. This seems to be especially true on older motors and I suspect this was due to a lack of an "industry standard" in those days. You may be able to purchase or have fabricated some type of bracket to attache to the boat which will lift your motor a bit higher.<br /><br />-John

 

[SeaHorse]

Re: Johnson Short Shaft NOT so short.

Thanks John. The transom is 15". The motor measures 18 from the underside of the transom clamp bracket to the cavitation plate.

 

[oldboat1]

Re: Johnson Short Shaft NOT so short.

I wonder if the spray is a result of the trim angle rather than the shaft length. If the motor is trimmed out, it would tend to kick up spray from the prop angle. If a shaft is a bit too long, but the trim adjusted at the same angle as the transom, I wouldn't think you would be getting that effect. (just guessing)<br /><br />A thought on changes: If this is an old (older?) hull that may be due for a transom rebuild, you might consider customizing the motorwell to fit your motor. You can find a lot of information here on rebuilding transoms in the boating section.