# 20 or 25 inch shaft outboard?



## dabutcher (Apr 19, 2008)

I am seriously considering repowering my boat and I can't decide if I should stay with a 25 inch shaft outboard or change to a 20 inch shaft. The measurent from the top of the transom to the bottom of the keel on my boat is about 25 inches. The measurement from the top of the jack plate to the bottom of the keel is about 23 inches. When the motor is level the prop is set back about 30 inches from the transom.

I've been doing quite a bit of research on line and there are a ton of different opinions regarding the height adjustment for the setback. As of right now I'm thinking that the 25 inch shaft may be the safer option. 

Any information or advice you could give would be much appreciated.


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## Sailing_Faith (Mar 11, 2009)

25" would be my choice. Better bite, less likely to get cavitation at high rpms...


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## X-Shark (Oct 3, 2007)

Your a little confusing...A pix would help.

You talk about measurments like the motor is mounted on the transom.

Then you talk about a Jackplate.

But Then you talk about the motor being set back 30in from the transom, Like you have a Bracket.

So what do you have?


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## dabutcher (Apr 19, 2008)

Sorry for the confusion. The motor is mounted on a jackplate. The height from the top of the jackplate to the bottom of the keel is about 23 inches. The motor itself is not set back 30 inches, but the prop is about 30 inches from the bottom of the transom. I took the measurement last night and remeasured this morning and the prop is more like 27 inhes from the bottom of the transom.


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## GoneCoastal (May 1, 2008)

From what Ive found out you can supposedly go shorter in shaft length for every 12 inches of setback from the transom.
Ive got a boat with a bracket with approx 25 inches from mounting point to bottom of boat. The bracket, spacer and jackplate brings the mount 30 inches from the transom. Motor mount to center of prop is about another 20 inches. So if the mounted prop position is 50 inches in length from the transom I can theoretically use a 4 inch shorter than 25 in shaft motor.
With my jackplate I could use a 25 in shaft and jack it up 4 in or use a 20 in shaft motor(which I already have) and mount it more or less level...I think.


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## dabutcher (Apr 19, 2008)

GoneCoastal said:


> From what Ive found out you can supposedly go shorter in shaft length for every 12 inches of setback from the transom.
> Ive got a boat with a bracket with approx 25 inches from mounting point to bottom of boat. The bracket, spacer and jackplate brings the mount 30 inches from the transom. Motor mount to center of prop is about another 20 inches. So if the mounted prop position is 50 inches in length from the transom I can theoretically use a 4 inch shorter than 25 in shaft motor.
> With my jackplate I could use a 25 in shaft and jack it up 4 in or use a 20 in shaft motor(which I already have) and mount it more or less level...I think.


 
That is my understanding as well. I would like to put a 20 inch shaft outboard on my boat but I've got a feeling that the 25 inch shaft gives me a bigger margin of error.


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