On my first two builds with rudders, I just ran the rudder line tube through the hull/deck and glued it in place.  Not the prettiest but it is easy and works.  Here is my Wahoo:

On my later builds, I bought these through deck fittings from Stellar Kayaks.  They are are designed for thin decks on surfskis so not sure how well they would work on a Duck.

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Are these through-deck line ports designed for thinnish lines while also limiting water ingress somehow?

I've 'cast' line guides a few times in laminated sub-structures using epoxy packed into a channel in all the necessary parts, with a piece of waxed-on-the-outside clear vinyl tubing chosen for appropriate outside diameter as a mold 'plug'.

Once epoxy's cured fully it's never been a problem pulling the tubing free (maybe a few quick twists with pliers) with some gentle tugging. Needs a bit of clean up at entry & exit points but that's pretty easy with sandpaper around a dowel.

   Thanks Mark N! The first pic you posted is, I think, what's intended.  It just seems so makeshift, and I was wondering if it would be likely to pull out if the lines got snagged in the cargo area, for example. 

The second pic you posted looks a lot cleaner.  Maybe I'll track down those fittings and see if they can be adapted.

There is also this video from Nick Schade for through hull fittings for deck lines and I was thinking something like this might be adapted to this purpose and would be pretty slick.

https://www.youtube.com/watch?v=mCv7oDhiJ7w

Thanks too spclark  You asked "Are these through-deck line ports designed for thinnish lines while also limiting water ingress somehow?"

Yes, exactly.  There is a braided stainless cable that runs through a plastic housing.  Think about brake or derailer cables on a bike going through a cable housing.  The plastic housing has to exit the hull at some point and I was just thinking about whether there is something more elegant than drilling an angled hole and embedding in epoxy and/or silicone.

The through-hull fittings in the pic I posted at the top of this thread seem like a good solution too and they must be out there somewhere, but I haven't been able to find something similar.

Thanks for the clarification Bob... I might have gotten something out of that pic you posted when you started this thread, but even now I can't see it.

Nothing I can find Googling my keyboard comes anywhere close to what Mark N's shown us: a tube through which a control line passes through a solid barrier.

Lots of electrical wiring 'ports' that seal but nothing for control lines other than this product from Racelite in stainless.

If you want to pass that image file from your initial post to me via e-mail, feel free:

spclark   icloud   com

- you need only deleted the spaces then add appropriate @ and . in their place.

   FWIW I used these from https://topkayaker.com/  

https://www.imagevenue.com/ME13GHZV

There are others available. Should be somewhat water tight . Find out tomorrow when I finally launch this boat 🎉🙂🍾

   CLC verified that just putting the plastic housing through the hull with as gentle a bend as is possible is what they recommend.

Thanks for those links Pappy!  Glad to see something like that exists. They are out of stock but easy to add later I suppose. They may or may not be any more water tight but I think they look more finished.