short shaft long shaft

[Bob1944]

I know how to measure shaft and transom length. But how do you determine if you have a long or short transom. For example, I have a 15 inch transom and an engine with an 18 inch shaft.

Then I have a 75 hp with a 23 inch shaft and a glasstron with a 20 inch transom.

What do you folks think?

 

[tashasdaddy]

Re: short shaft long shaft

you have older motors, where the water pump is higher in the leg, your 18 inch is a short shaft, the 23 is a long shaft.

 

[Bob1944]

Re: short shaft long shaft

I wrote an incomplete question. so what is the rule in putting engines on transoms. Can I put an 18 inch shaft on a 15 inch transom (15 foot aluminum starcraft) and:

a 23 inch shaft on a 20 inch transom (15 or 16 foot glasstron).

Is there there a rule of thumb like, the shaft may not be longer than 2.5 inches than the transom except where said transom is thicker than the square root of the sum of the horse power less half the sum of the shaft and transom divided by the age of the boat or the boat owner whichever is the less multiplied by the last two digits of the hull number.

bb

 

[tashasdaddy]

Re: short shaft long shaft

older motors had longer shafts than the newer motors. the 18 inch belongs on the 15 inch transom, the 23 inch belongs on the 20 inch transom. the new motor measure more closely to 15 and 20 inches.

 

[Bob1944]

Re: short shaft long shaft

what i hear you saying, or read you writing, is that transoms and shafts are uniform either 15 or 20 inches?

 

[Chinewalker]

Re: short shaft long shaft

Hi Bob,
Boat manufacturers have some variation between brands and models as to what "short" or "long" shaft measures out to. Hull configuration, keels, etc. can make a difference in how a motor's lower unit will cut the water. With that in mind, motor manufacturers leave in a little bit of "buffer" by making the shaft length a little on the long side so that their motors can be used on different brands of boats without modification needed. The standard shaft lengths are more of a minimum than a set-measurement. .ie, your 18-inch is considered a short shaft, as it falls between 15 and 20. Your 23-inch motor is a long shaft, as it falls between 20 and 25.
You can "alter" your shaft length by setting the motor up higher on the transom when rigging. This may involve some testing on your part to see where the motor is happiest and performs the best, keeping in mind that you want to maintain cooling and propeller bite.
- Scott

 

[tashasdaddy]

Re: short shaft long shaft

on the off shore fishing boats they go to 25 and 30 inches also.

 

[PaulyV]

Re: short shaft long shaft

My 14' starcraft has a 1965 40HP Big Twin. Transom measures 15" motor shaft 18.5". ( I just purchased this boat)They built up the transom 2.5 inches with a piece of wood. Not sure why.The motor locks are 1" away from the top of the transom. There is also a metal bracket that the motor locks are tightened on to. What purpose is that? The 2 bolts you see below the motor mounts are not from this motor. The motor is not attached to the transom by anything accept the 2 screw clamps. This to me does not seem too secure but I was told it runs fine. Is this motor (shaft) to long for the transom? I am planning a complete restoration of the boat and want to replace the transom and make it its original height. By doing this the motor will sit lower in the water. Again I haven't been able to test her out so I am speculating. With all the experience here I am sure I will be led in the right direction.

 

[Chinewalker]

Re: short shaft long shaft

Hi Pauly,
As noted in my explanation above, your motor is a short shaft. However, as you have found out, the motor and boat transom heights are often off by a few inches and some provisions have been made to bring the motor's height closer to that of the transom height. As long as the motor is securely fastened, pumps good cooling water when underway, and doesn't blow out the prop in turns, you are good to go. Less lower unit in the water equals better efficiency/speed/mileage/etc.
- Scott