Outboard Shaft Length Question

[Jon Frey]

I recently acquired a 1980 Kristiansen glass over cedar sneakbox. It didn’t come with a motor, so when I got it home I measured the transom to start looking for an outboard with the right shaft length. It measures about 20” straight down from the top of the motor board to the bottom of the hull (see below) so I figured I needed to start looking for a long shaft outboard. This afternoon I had a conversation with Marty Kristiansen about the boat and he told me that unless someone had replaced the motor board, this boat should be run with a short shaft outboard. From what I can tell the motor board is original, or at the very least not excessively taller than the original from looking at pictures of other Kristiansen boats that I can find online. So my question is what length shaft outboard do you think I should run with this boat? This is the first true displacement hull duck boat I’ve ever owned, so I’m not sure if that makes a difference on what length motor I should use or not, but I figured I should ask before looking any further. View attachment 20170825_190149.jpg

 

[Tom Scholberg]

Rule of thumb is the cavitation plate on the motor should be flush with the bottom of the boat. Don't know about the hull you have so there may be an exception, I will be watching. Good luck.

 

[gcs]

If it's a displacement hull, and if your going to go at displacement speed, probably 5 mph, then you should be good with the short shaft. BUT, if you try to make that boat go, like a planing hull, then you'll need the long shaft to keep the water intake below water as the cavitation plate will want to be at the water level at the transom.
Those traditional hulls were designed for oars, sail, or poling, ie, slow, and while you can overpower the design to a point, the entire trim will suffer and make the otherwise seaworthy boat unseaworthy.

If you want fast.. you have the wrong hull, if your going to go short distances at pokey speed then your good.

 

[Craig F]

15 hp short shaft and you should be good to go. That's what 99% of guys run on that boat.

 

[Gordon Bartlett]

Isn't a sneakbox a really small boat? Like twelve feet? If so, and it really is a true displacement hull, I'd put a 3 horse outboard on it, preferably a light weight two-stroke.. Beyond that, you'll be trying to plane it. As you add power, you start making bigger and bigger waves as the stern begins to squat and the bow lifts skyward. At some point, the boat will start handling weirdly. Also, as you add power, you will also add weight, which has a big effect on such a small boat. As an example, an old OMC 3hp, the one without a gearshift, weighs about 35 lbs., depending on the exact model. A 9.9hp of similar age weighs almost twice that. Again, I'm talking about a true displacement hull.

 

[Vince Pagliaroli]

A 1955 Johnson 5 1/2 hp, short shaft, outboard worked very well on my wooden sneakbox, in all weather conditions for many years. Always had extra shear pins, and the proper tools for replacement if needed.

The sneakbox is long gone, but the motor still does what it was intended to do.

 

[gcs]

What Gordon said....

 

[Fritz Meck]

15 hp short shaft and you should be good to go. That's 99% of guys run on that boat.

You are right, a 15 will or 9.9 is what you need. If you gun big water a small eng. will get you in trouble.