Shaft too short?

[Bprice8983]

I have a 14 ft. Starcraft V-hull with a 15 HP older model Johnson on the back. The motor runs fine, it has recently been re-built, but the boat wont go any faster than around 5 miles an hour, and it definitely won't plane. There are a lot of add-on's to the boat which makes it heavy, but it might be that the shaft on the motor is too short. I know it is a vague question, but any help would be great. If it is a shaft problem, I have another motor with a longer shaft that won't run, is it possible to switch the shafts? <br /><br />I can get serial numbers for motor ID's if it would make any difference.

 

[R.Johnson]

Re: Shaft too short?

The model no. are what we need. If you are running a to short of a shaft length the prop would cavitate, does it do that?

 

[Chinewalker]

Re: Shaft too short?

The anti-ventilation plate, sometimes referred to as the cavitation plate, is the flat horizontal plate just above the propeller. It should be just about even or a hair below the bottom of your boat. If it's significantly above the bottom of the boat, then you've got a short shaft motor on a long-shaft transom. <br /><br />The nominal length for a short shaft is 15-inches, long shaft is 20-inches. Measured on the boat: plane of the transom where motor mounts to the plane of the bottom of the boat, measured perpendiularly, not at the angle of the transom. On the motor it is measured from the horizontal flat of the clamps to the plane of the A-V plate.<br /><br />- Scott

 

[ED21]

Re: Shaft too short?

Is the shear pin or prop hub OK? If it ran alright before rebuild, I would check this.

 

[Bprice0809]

Re: Shaft too short?

Motor ID's:<br /><br />Johnson Sea Horse 18 <br />SN: J2429627<br />Model # FD-18E<br />Also, if it is possible, or if anyone knows were to find a rebuild kit for this engine, let me know.<br /><br />Johnson 15<br />SN: R08548488<br />Model # J15RESR

 

[phatmanmike]

Re: Shaft too short?

measure the transom on your boat.... from top to bottom... then measure the shaft on yur motor... its just that simple... measure from the cav plate up to the part that mounts on the boat

 

[BrianTindall]

Re: Shaft too short?

Unless Starcraft change this boat, it is a short shaft boat (I had a 1970 model). So I would think you have another problem. Is it possible it's running on one cylinder? Have you checked for spark and a compression test? If it's thru-hub exhaust, how's the hub? I know it's been rebuilt, but stranger things have happened.

 

[steelespike]

Re: Shaft too short?

My guess is if the motor was rebuilt and reassembled correctly that you have one cylinder with no spark.Possibly a failed coil.

 

[Bprice8983]

Re: Shaft too short?

I'm pretty sure its a long shaft boat. The cavitation plate is about 4 inches above the bottom of the hull, so I'm pretty sure that is the problem.

 

[Spidybot]

Re: Shaft too short?

If your antiventilation plate is 4" above ideal then a 5" longer shaft is what you need.<br /><br />It should be like this:<br /> http://www.spidybot.com/prop/prop_047.htm

 

[Bprice8983]

Re: Shaft too short?

I hear they make some sort of conversion kit for this problem, is this true or is this some vicious rumor?

 

[Spidybot]

Re: Shaft too short?

Experience shows that trading the wrong lenght to a proper is by far easier and better value.<br /><br />If you have or can get a motor to take the parts from it may be different.<br /><br />Trading will even give you a chance for changing hp/brand/type.