shaft length decision help needed

[Philip_G]

Hello,

I need to decide how to set up my outboard for my boat. First a few dimensions:

transom from top to keel: 20"

motor, from where it rests on the transom to the anti cav plate: 18"

motor with the long shaft extension installed from the clamps to the anti cav plate: 23"

So my conundrum is, would it be better to have the cav plate 2 inches above the bottom of the hull, or 3" below it?

Motor is a 15hp and the boat is 14', I'd be happy with 20mph or so, speed isn't a huge concern.
I already have the 5" extension so it's actually cheaper to leave it long.

 

[tashasdaddy]

Re: shaft length decision help needed

it would really help to know what motor you have. older motors are longer, due to the water pick up and water pump location.

it would seem you have a older motor, and 23 inches would be correct for a 20 inch transon.

 

[Philip_G]

Re: shaft length decision help needed

it is older, 55 johnson 15hp

 

[tmcalavy]

Re: shaft length decision help needed

I would start off by mounting the motor as is and see what kind of speed/handling you get out of it. If it seems doggy, or you want more speed, put shims under the motor clamp...in one inch increments and then compare performance. It will do okay with the cav plate 3 inches below the keel, but will be heavy in turns. I just raised both my late 50's Big Twins 2 inches with a homemade wooden transom riser for this very reason...now the cav plates are 1 and 1/4 below the keel. Should get a chance next week to do a splash test. I'll report back after that...hoping for a little more speed and maybe better handling.
The older motors like yours and mine need to run a bit deeper than modern motors, to keep the water intake/pickup well into the water to feed the water pump. Make sure you chain that puppy to the boat after you clamp it down...safety chains are cheap insurance.

 

[LORDY611]

Re: shaft length decision help needed

I have a 14' aluminum rowboat with a 9.9. IT FLIES. 15hp should not want for more speed.

 

[Philip_G]

Re: shaft length decision help needed

I have a 14' aluminum rowboat with a 9.9. IT FLIES. 15hp should not want for more speed.

I have a 9.5 that's too short and it can barely do 5.5mph

 

[LORDY611]

Re: shaft length decision help needed

I have a 9.5 that's too short and it can barely do 5.5mph

You, sir, need either a shorter boat or a longer motor!