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Zip transom curisioty question

Posted: Thu Feb 23, 2017 12:10 pm
by James P
Hey everyone,

I have not even ordered my plans for the Zip I would like to build, but some thoughts roll through my tiny brain about what I have to look forward to. I decided to look for an outboard motor just to see how plentiful they might be. I'm not finding a lot by the way. At least not in a short shaft.
So I'm wondering. How does the transom get modified to except a long shaft motor? Do the plans give a cut out pattern for both options?
Will there need to be any other changes made to the boat other than cutting the transom differently?
I got lots more questions about this build,but this is a start.


Re: Zip transom curisioty question

Posted: Thu Feb 23, 2017 12:33 pm
by sproggy
The Zip plans show about a 3" cut-out in the transom for the short shaft motor - i.e. the motor mounts about 3" below the top level of the transom. If you went long-shaft (nominally 20" rather than 15" for the short shaft) you would have to raise the centre of the transom 5" (so 2" above the top of the rest of the transom as per the plans) to keep the cavitation plate level with the bottom of the transom. You'd then need a taller transom knee for support and to consider adding strength to the sides of the motor well too (thicker ply) to keep the whole thing strong and stiff.

You shouldn't have trouble finding outboards up to 40hp (recommended limit for the Zip) with a short shaft, I'd have thought.

Re: Zip transom curisioty question

Posted: Fri Feb 24, 2017 12:39 pm
by Andy Garrett
Bear in mind, the 20" measurement is not precise across all engine offerings. It will serve you well to have your engine selected and measured before you construct your transom.
If that isn't realistic, make your transom a bit taller than need be and you can trim it later for the exact motor you end up with.

ALSO, the measurement is taken along a vertical line from the center of the transoms thickness at the top to the plane even with the bottom edge of the transom as the boat sits level. DO NOT measure diagonally down the angled transom. A plum line works well for this measurement.

Re: Zip transom curisioty question

Posted: Sat Dec 30, 2017 6:47 am
by Shelboss
Hi James, I am just getting to the point where you were 10 months ago. I wanted a long shaft motor because they are easier to find and also in the future it does not preclude using a modern 4 cycle engine if the boat is built correctly to take the added weight (typical 40 hp 2 cycle = 150 lbs, 4 cycle = 250 lbs. I scaled all the templates and determined that the transom radius is 115". I then laid out the transom and frames in CAD. Changing the transom radius to 73" raises the transom height by 1.6", or 1.235" when a flat is cut at the top for motor mounting. That puts the planing plate in line with the bottom of the boat. The Zip has a longitudinal radius of 1443" from the transom to Frame #4, so I raised the height of Frame #2 by .8". That gives you a longitudinal radius of 1211" so the deck stays a consistent convex profile.
The long shaft motor puts the motor torque 5" above where the short shaft motor would be. That torque is trying to pull the top of the transom back. Reinforcement needs to start at the top of the transom and extend forward to the back of the seat. The motor well can be that support. I am opting for SS tubes and heim joints that are attached to brackets and thru-bolted at both ends. I am also building the transom with two pieces of 3/4" ply so that strength is maintained across the width, and adding strongbacks to the battens from the rear seat back to the transom. I will post pictures as I start to build what I have modeled, cheers!

Re: Zip transom curisioty question

Posted: Sat Dec 30, 2017 7:09 am
by Bill Edmundson

First, let's make sure we're all talking the same language. Older engines the short shaft was 15". New engines a standard shaft is 20" long is 25". I'm assuming you talking 20".


Re: Zip transom curisioty question

Posted: Sat Dec 30, 2017 9:33 pm
by Shelboss
Yes my understanding is 15" = short, 20" = long, 25" = extra long. They still make short available in modern engines, but only for the smaller horsepower motors like 25 hp.