Zip Transom 20" Design Thoughts

Outboard designs up to 14'

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Zip Transom 20" Design Thoughts

Postby bjfrater » Tue Jul 15, 2014 3:06 am

Hi everyone
I would be interested in what your thoughts are about strength and the best design for the Zip transom top (deck) frame for a long shaft motor. Specifically the frame member that is at the top of the transom that the motor will attach too.

There seems to be a few ways of doing it. Either the frame is two separate pieces with a 3/4" motor board running all the way to the top Or a single frame similar to the other frames on the zip.
Here are a couple of pictures to illustrate from other peoples builds.

Option (1)
Frame member is one piece across
Transom with single top frame

Option (2)
Frame is 2 pieces with a larger ply section.
Transom with two top frames and large ply section

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Re: Zip Transom 20" Design Thoughts

Postby vupilot » Tue Jul 15, 2014 7:12 am

I'm sure either would be plenty strong. I checked Glen's Boat building with Plywood book and it didn't give any guidance on that specifically so I think its something you can just select your preference on.

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Re: Zip Transom 20" Design Thoughts

Postby BayouBengal » Tue Jul 15, 2014 10:54 am

If I were to do it, I'd go with the second option shown in your picture.

But honestly, please don't do it at all. I don't think there will be any structural or performance issues, but the boat is simply not going to be as aesthetically pleasing to the eye with the motor sticking up 5" higher than design. The Zip is a big project, after you've put all the time and sweat into building it you'll want it to look the best it can look. Trade your long shaft motor for a short shaft and build your transom as designed with the dip where the motor hooks on. Personally, that dip in the transom seems simple but is one of the details of the boat that I find very stylish.

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Re: Zip Transom 20" Design Thoughts

Postby gdcarpenter » Tue Jul 15, 2014 3:25 pm

Top one looks 3/4" thick, bottom 1-1/2" thick. The taller you transom the greater the 'moment ' or stress on it. Basically thicker is better. In the bottom one I would have made both top frames full width for greater flex resistance. Personally my ZIP transom is 1-3/4" thick slab of white oak, had it so used it.

A lot cam
N be done to strengthen the transom by how you attach the motor/splash well to the transom,plus possibly extra transom knees if desired. are you concerned with strength or looks?
This is my first, last and only boat build.

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Andy Garrett
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Re: Zip Transom 20" Design Thoughts

Postby Andy Garrett » Wed Jul 16, 2014 9:32 am

With respect to BayouBengal's response, I disagree. Short shaft motors can be harder to find in the used market, and who can afford a new outboard? My Zip has a long shaft and I am very pleased with the asthetics (and I'm a professional artist, so this is vital to me).

As for strength and thickness..., I tortured myself over this for a long time. I even bought bronze bolts to build horizontal knees where the transom and sheers met for massive strength. I bought these early in my build, before I had developed the confidence that would come later in the strength of the basic design. Those bolts now sit unused on my shelf as a testament to overthinking the build.

In the end, I opted for a 3/4" marine ply transom with 1" (3/4" nominal) solid frame stock affixed to its perimeter, and a 3/4" marine ply motorboard. My total transom thickness is 1.5". I have it tied to the rear seat back and the vertical stiffeners I installed on my battens and keel. My long shaft Mercury 500 weighs 176lbs. I even towed the boat once for 40+ miles with no transom saver by accident. I have no transom issues.

I noticed after I flipped the boat and started getting in it to work that it never even creaked..., NEVER. I weigh 250lbs, and I was walking on the bottom battens and keel and sometimes the bottom hull. It never complained with the slightest creak. This phenomenon above all other experiences with the build inspired confidence in the strength and rigidity of this design.

When I hung my motor for the first time (before the decking was installed), I lightly imagined the motor just peeling the transom of like a crowbar opening a wooden crate. Again, it never even creaked.

Build a good motorwell and tie it to the structure everywhere it is logical to do so. use good wood, good fasteners, and good epoxy, and have faith. It will be a strong boat!

As for the motorboard, I agree with Bayou Bengal on this point. Go with the second option you pictured.
Andy Garrett

Perhaps the slowest Zip build in Glen-L history...

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Re: Zip Transom 20" Design Thoughts

Postby Cabron » Fri Jul 18, 2014 4:23 pm

My Zip transom is 1.5/8" thickness in total. The transom knee fits on to it, so
the transom should at least be flat down the middle where the knee attaches.
I glued in 4 other smaller knees at the battens. I also put in corner bracing at the Sheer and Chine.

I will be running a 35 HP long shaft. It weighs about 120 lbs. I`m sure it would take a much heavier
motor to cause any significant damage to the transom.

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Re: Zip Transom 20" Design Thoughts

Postby bjfrater » Thu Jul 24, 2014 5:11 pm

Thanks for your opinions, much appreciated.
I have decided to go with option 2. Andy you are correct the reason for my going for a long shaft motor is that there are just more available. I also have seen several zips configured for these that looked pretty damn good to my eye so im not worried about the look.

I will consider the smaller knees, did you bolt or screw these to the battens and transom?
What does the corner bracing do?

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Re: Zip Transom 20" Design Thoughts

Postby mdweber » Thu Jul 24, 2014 6:34 pm

Hey bj. Just weighing in on the corner brace. In the plans (I'm building a squirt, but I'm pretty sure the zip plans are similar in this respect), there is a brace, like a mini knee, that attaches to the chine and transom. I believe the fastener schedule calls for 2x 2" screws. This mini knee helps bind the chine and transom. The plans don't call for it, but putting a similar knee between on sheer and transom seems like a good idea.

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