Let's talk transoms...

Andy Garrett · **Posted:** Thu Feb 17, 2011 5:06 am

I just re-read the entire chapter on transoms, studied the designs, and looked at every picture in the archives involving Zip construction.

One issue that is not made entirely clear is the method for bolting on the transom knee and how to hide the holes on the rear (visible part) of the transom.

On some, I see putty spots. That method I can figure out for myself--It even seems to make sense as the motor will hide these spots anyway.

However, I am installing a swim ladder on the starboard side of my transom and intend on having double-thick 3/4" plywood all the way across with the frame members on the interior, 1/4" mahagany plywood on the exterior, plus three full size transom knees at the keel and two of the battens, and smaller bolted-on horizontal knees at transom-sheer junction. This may be a tad 'over-engineered', but I want piece of mind that my 230lbs won't peel the transom off with the motor as I climb in. This means that putty spots will be visible on all but the regular center knee.

I was thinking that I could assemble the transom with bolts and knees attatched, get everything tightened down and puttied, then just glue the exterior mahagany ply on the outside of the whole works. At most, use a few nails or screws around the perimeter.

Does this seem like a sound idea, or am I missing some obvious issues?

I'm very open to suggestions as the literature is a bit hazy on this issue.

upspirate · **Posted:** Thu Feb 17, 2011 6:09 am

That's exactly how I was going to do it on my Squirt....attach a cover lamination/veneer after all the rest is done,but before the plywood side & bottom planking is on

Iggy · **Posted:** Thu Feb 17, 2011 8:20 am

If I was to do extra knee braces I would cover the transom with an extra layer of plywood to hide the bolt heads.

I have my transom ladder on order, a narrow 9" with fold-down steps, and there is not much room left over from what I can spot on my wider Malahini transom. You might want to plot out if you can even fit a transom ladder on a ZIP taking into account the engine clearance you will need at full turns.

Andy Garrett · **Posted:** Thu Feb 17, 2011 8:26 am

Thanks for the replies guys.

My ladder will be stowable. I'm leaning toward a dive style with a single center pole and steps on the outside, but I'm also considering a calapsable type recomended in another thread.

Either way, if I can't buy what works, I'll fabricate it, but this boat will be a swimming platform. :twisted:

As for the plywood covering the bolt heads, I agree. I just wanted to make sure I wasn't overlooking some issue well known to the initiated and unknown by me. :wink:

Thanks again fellas!

vupilot · **Posted:** Thu Feb 17, 2011 8:28 am

Thats the way I did it, glued the 1/4" transom ply last to cover it all up.

Andy Garrett · **Posted:** Thu Feb 17, 2011 8:29 am

Did you nail it too Chris?

pamelalynne1 · **Joined:** Thu Jul 29, 2010 3:55 pm **Posts:** 360

I laminated a 1/4 inch meranti layer over my transom too. Another thought would be to countersink the carriage bolt and insert a wooden dowel over the bolt. With a couple of coats of epoxy it should be nearly invisible.

gdcarpenter · **Posted:** Thu Feb 17, 2011 8:06 pm

Had the same concern with my ZIP - decided to kill two birds with one stone and used the 'transom eyes/U-bolts' as the fasteners for the transom knees, I used two transom knees. Did not want to 'bury' bolts under a veneer, just too much potential for rot in my mind.

pamelalynne1 · **Joined:** Thu Jul 29, 2010 3:55 pm **Posts:** 360

If you don't want to build another one I'll be glad to build one for you. I'll build anything, for the right price.

Iggy · **Posted:** Thu Mar 31, 2011 9:52 am

I am curious.. is there a great number of failed transoms out there that would warrant builders adding 2nd & 3rd knees to transoms?

I could see an issue if the design did not include a motor well, which essentially adds 2 more 'knees' in the for of the motorwell plywood sides already as well as top-bracing not found in other designs.

And I don't ever want to suggest someone 'under' engineer the transom.. however outside of asthetics of covering the knee bolts on the exterior... I have yet to hear of any design flaws in the Malahini/ZIP or any Glen-L design boat that suggests the transom/knee/motorwell shown in the drawings are insufficient.

Now.. I am NOT talking about builders who are strapping on a completely oversized motor that is double the weight and horsepower of the original design. Those who do that do so at their own peril.

What I am thinking is that by adding all this additional bracing & thickness is only just adding weight in the end.. causing our boats to sit with the transom sunk down below the actual intended water line.

I am only concerned about 2 things with my engine and ladder.. and it has nothing to do with the engine or my bear-like weight tearing the transom off the boat. My only concern is with the fasteners pulling through because of insufficient load-bearing area... moreso than the actual material thickness.

I am not going to be able to tear off any ladder from even a mere 3/4" plywood transom using my body weight no matter how often I jump up and down on it... so long as the bolts used include a good sized washer to distribute the shock over a larger bearing area.

The way I see it.. so long as the engine, stern eyes, ladder and whatever else you bolt on the transom has sufficient bolt-bearing surface area... the entire BOAT will move well before the BOLT will tear through. Unless you rope it to a rock and purposely try to snap the sucker off... and will probably be tossed out of boat yourself in the process or eating your steering wheel.

The only other concern I have is vibration issues. Engines and waves can shake and bang a boat up pretty good over time... and I've some thin epoxy joint crack under those conditions. However with a 1" (or 3/4") frame structure thats over 3" thick laminated to all 3 sides of our transoms... there is a lot of surface area to deal with vibration issues already.

I wouldn't mount a big 250# motor to just a 3/4" plywood transom... not what I am saying.. but I am hard pressed to see the need to make that transom 3" thick either with 3 extra knees in addition to the motor well.

I don't want this to be a knock against other builders who have decided to do this, however I do trust that the Glen-L designers did their homework and have the feedback of hundreds, if not thousands, of boat builders over the years in all kinds of operating conditions. Unless you are doing something totally out of the ordinary.. I wouldn't overthink this too much.

Onplane · **Posted:** Thu Mar 31, 2011 11:53 am

Andy / Iggy,

I see both sides to this coin...

You would have to think Glen-L planned for water skiing, or similar behind these boats, but then again, nowadays people take things to the extreme as far as towables. And sadly (including myself) people seem to be a little bigger nowadays too.

With this in mind, I added 2 extra knee braces to my transom as well, using 3" long countersunk silicon bronze screws and epoxy, on the middle batten of the Malahini. I didn't have to drill holes through the transom, and the screws were really only there for clamp pressure as the epoxy cured.

I like having the peace of mind as well.

ttownshaw · **Posted:** Thu Mar 31, 2011 12:35 pm

Just MHO, I believe the motor well sides act like a 2nd and 3rd knee...thus eliminating need for additional bracing not shown on the designer's plans...but again...that's my amaturish opinion.

Roberta Hegy · **Posted:** Thu Mar 31, 2011 12:42 pm

I agree, Bill. The extra plywood and timbers in the motor well have to tie the upper part of the transom nicely into the frames and deck.

Roberta

Iggy · **Posted:** Thu Mar 31, 2011 12:43 pm

To clarify.. 2 extra batten knees are not going to add a lot of weight in the big picture.

I just believe we often overthink the design and start second-guessing and over-compensating for an issue that does not appear to be common at all with our well-designed boats.

I would be just as secure riding in a boat built with 3/4" frames as I would be in a boat with 1.25" frames... same goes for 1/4" bottom hulls vs 3/8"... or 1.5" transoms vs 3" transoms... PROVIDED the boat was constructed to plans, epoxy was done right and the engine is not oversized to the hull.

Honestly guys (and gals).. the wood itself is a natural product with defects that can crop out of nowhere. My transom is 2" thick. IF it ever looks like it needs more for whatever reason, I will add more. But I don't expect it to be ripped off the first time I put the balls-to-the-wall on the throttle and more than I expect my plywood sides to split in half when I hit a big wave at speed.

Transom design deserves some thought and 'considerations' based on intended use and engine specs, but only up to a certain degree proportionate to the change. We are all pretty lucky that there are litterally dozens (if not hundreds) of people who have gone ahead of us and tested this out for us.... including the naval architect who designed the boats in the first place.

ttownshaw · **Posted:** Thu Mar 31, 2011 12:47 pm

Quote:

I just believe we often overthink the design and start second-guessing and over-compensating for an issue that does not appear to be common at all with our well-designed boats.

Well said...and, ummmmmm, I'm guilty of that too! :oops:

Glen-L.com

Let's talk transoms...

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