Long or Short Shaft?

[jumpjg]

How can I tell if my 1978 Merc 402 is a long or short shaft motor? Are the dimensions standardized across all outboards, or is a Merc long shaft different than other mfg's?

Thanks for the help,

Joe in St Louis

 

[wired247]

Re: Long or Short Shaft?

The mercury long shaft is 22 inches shaft length from the base where it enters the lower to the end of the shaft. Mercury calls that a 20 inch transom height but its really much higher than other manufacturers and later Mercury offerings. Combined with the higher mercury lower and the higher mid section a 20 inch Mercury 4 or 6 cyl inline sits MUCH higher. A 15 inch "short shaft" Mercury motor is almost equivalent to a 20 inch Johnson.

 

[carholme]

Re: Long or Short Shaft?

http://www.boatmotors.com/ref/shaft_length.html

Gerry

 

[jumpjg]

Re: Long or Short Shaft?

Thanks Guys. By studying the diagram, is it safe to say that the general rule of thumb is the bottom of the cavitation plate should be equal with the bottom of the transom? If so, are boat transom height standardized?

Appreciate the help,

Joe in St Louis

 

[wired247]

Re: Long or Short Shaft?

That chart really doesnt work with inline Mercs. The lower sticks down much farther than anything modern. A "long" shaft inline merc is 21 inches from the anti cav plate to the clamp on the transom but the lower still sticks down 3-5 inches lower than comparable modern dimension motors.

You only want the cav plate at the same level of the pad or keel if you want your boat hard to steer and slow. You'll get best performance with the centerline of the prop at 3-5 inches under the bottom of the transom.

 

[Chinewalker]

Re: Long or Short Shaft?

You only want the cav plate at the same level of the pad or keel if you want your boat hard to steer and slow. You'll get best performance with the centerline of the prop at 3-5 inches under the bottom of the transom.

Not really a good idea with the OP's motor, as that would put his cooling water intakes near the waterline, with the possibility of sucking air instead of water. The Merc 402 isn't a high performance motor and running the AV plate flush with the bottom of the boat is just fine.

 

[jumpjg]

Re: Long or Short Shaft?

Thanks so much for the replies, appreciate the insight. My situation is I bought a boat with a bad motor. It must have overheated pretty hard because the water pump was burned up, pistons are scored (115 & 100 psi comp), water jacket gaskets were leaking, and the "honey" is dripping out of the stator harness sleeving. It looks like fixing this motor will cost much more than it is worth, so I'm on the lookout for a replacement or a donor power head with good parts. If I go the replacement route I want to make sure I get something that fits.

Thanks again,

Joe in St Louis

PS: Leads appreciated