Long Shaft? Short Shaft?

[Uncle Krusty]

I'm giving up on my cranky old Nissan and looking at motors. The boat is a 16', 68" beam, Valco. Transom height from the cutout is 15.5". This sounds like I need a short shaft? Current motor is a long shaft and the cavitation plate sits a couple of inches below the keel.

I'm looking at a 2009 15 horse 4-stroke Mercury for $1200. Most of what I see for sale are long shaft.

 

[Riverbean]

You need a short shaft and you want your "anti-ventilation" plate to be level with your hull or keel, wichever you want to call it. Lots of ways to raise or lower it an inch or two, however much you need to make it level.. Ive used a 10 inch peice of 1 by 2 before under the clamps to raise it...

 

[maintenanceguy]

Different manufacturers will vary a little and they may call the shaft lengths something different.

This seems to mostly be accurate for smaller motors under 40hp:

Short shaft: 15"
regular shaft: 17"
long shaft: 20-22"
extra long shaft - could be anything over 20"

 

[Weldorthemagnificent]

Post a pic of the transom. Might be possible to make it taller. Also the 15 Merc/tohatsu is the same size as the 20 which may be a better choice for a 16' boat

Sent from my CLT-L04 using Tapatalk

 

[Uncle Krusty]

The transom is welded aluminum. Possible to fill it in, maybe, for someone with more skills than I have. That would give me more motors to choose from. The current motor sits about three inches below the hull. Is it possible to modify the 15hp to 20? Hope the pic comes through...

.

 

[Uncle Krusty]

Trying the pic again.

Transom.jpg

 

[Weldorthemagnificent]

I use the Tapatalk app for forums. All your forums in one app and picture uploading is simplified.

Sent from my CLT-L04 using Tapatalk

 

[Weldorthemagnificent]

I have a boat of similar dimensions. A Naden 16 with a 68" beam. I run a 35 Johnson on it. It's a short transom as well. Not sure what size the old Nissan is and the type of water you fish. If necessary a jack plate can be used instead of modifying the transom. Neither is ideal. Running the correct motor for the hull is. By modifying the transom at least you gain a little height over the water and aren't so endangered by waves coming from the stern. As for modifying motors, I'll leave that to others.

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[Pappy]

First off....how did you measure the transom??

If you just laid a tape from the transom cut-out down to the bottom of the boat you did not get a correct measurement.

Typical transoms are somewhere around 15 degrees or so for outboard use and you measured the angle instead of true transom height.The way to measure is to put a straight edge along the bottom of the boat with a few inches sticking past the bottom.
Now, measure from the transom cut-out straight down and perpendicular to the straight edge. That is true transom height.

You are looking at a short shaft transom one way or the other...just thought you may like to know the correct way to measure it.

 

[ppine]

The cavitation plate needs to reach the water when the motor is mounted and the boat is at rest.

Most motors are 15 inch regular shaft, 20 inch long shaft and 25-26 inch extra long shaft, mostly used on sailboats or larger power boats.

 

[Uncle Krusty]

Correctly measured the transom is 15" even. The cavitation/anti-ventilation plate sits 2.5" below the bottom of the hull. What is the effect of too long a shaft?

 

[eeshaw]

From what I've seen it's just more drag in the water. It will bottom out in shallows quicker also.

 

[RaisedByWolves]

Correctly measured the transom is 15" even. The cavitation/anti-ventilation plate sits 2.5" below the bottom of the hull. What is the effect of too long a shaft?

Slight reduction in speed, porposing and handling issues depending on a variety of things and your intended use.

In the past Ive countered these issues with a whale tail which some consider non correct.

Im in the works of correcting the motor height but will probably still run the whale tail for better plaining with just me in the boat.

Im not quite a porker, but I do smell like bacon if Im out in the sun too long. :wink:

 

[Uncle Krusty]

Bacon, lol. My intended use is to chalk this up to experience and look for a Duroboat or some such. Other than that, it would just be on a small local lake. I'd like something I could trust on a river or bay. This rig, not so much.