I’m installing a bulkhead into an ABS Zoom. As the picture shows I’ve slotted the thwarts into the top of the saddle. I’m also adding a dowel dropping from the rear thwart into the saddle. Does the bottom of the saddle need to be glued to the hull?
For PE boats the glue keeps grit from accumulating under the saddle. Is that less of an issue for ABS?
In order to glue the bottom, I’m going to have to cut off one side of the thwart slots since I can’t spread the gunnels far enough to drop in the saddle with thwarts on top.
Nolan Whitesell liked the rotomolded Perception saddle and use it to outfit his boats unless otherwise requested. The Perception saddle was held in place by two thwarts, with a bracket screwed into the rear thwart, and 2 machine screws going through the front thwart into the saddle to fix it in place. Nolan maintained that the saddle, which generally had a layer of Ethafoam on the bottom of variable thickness, should not be glued to the hull bottom as he claimed that would increase the likelihood of the hull cracking.
I'm not sure about the reasoning, but I have mounted both minicell pedestals and Perception saddles secured by a pair of thwarts and not glued them down and it has worked fine. Yeah, sand can get under the saddle but at least for me it hasn't been a major problem.
In your case, a dowel extending down into the pedestal from the rear thwart should pretty well prevent side to side motion but I think you need something to secure the front of the pedestal. I would make a pair of minicell knee wedges and that butt right up against each side of the pedestal in the front, and glue those to the hull bottom They should bracket the pedestal and prevent side to side wobble in the front.
The front of the bulkhead will be wedged between sidewall foam and knee pads/blocks like you are suggesting. If the saddle stays put under use without gluing, I'll likely not be gluing it to the hull.
You brought up more stress on the hull with glue; I wonder if the minicell might fail if the saddle isn't glued in place.
I doubt the minicell would fail. The Ethafoam on the bottom of Perception saddles, and on the bottom of a Bob Foote Grand Canyon saddle that I have had did not show any signs of distress despite not being glued down. I doubt Ethafoam is stronger than minicell.
Northwater makes a removable minicell pedestal that secures to the hull using daisy chain loops and straps. I haven't used it, but I haven't heard of any failing.
Not being glued down might actually reduce stress on the foam in certain ways. If glued down, then under certain conditions, there will be shear forces next to the glue line that can break the foam. That's happened with a triple saddle I have in one of my ww open boats.
Now I try to control pedestals and saddles with thwarts above, and by gluing in narrow pieces of foam, right next to the saddle, at the bottom. Careful gluing of narrow pieces seems to reduce shear force damage.
Just had a screwy idea that I will share for the amusement of all. If there is a reason to seal the edge of a bulhead, perhaps one could cut a groove and use an inflated bicycle tube as the "gasket."
About 5 years ago, I was paddling an XL-13, which had a metal thwart with a dowel sticking down into the foam at the rear to the saddle. On the first rapid of this particular day, the contact adhesive failed between the saddle and the hull. The saddle stayed *mostly* where it was supposed to, but for the rest of the day, it felt like I was sitting on a swing sideways as I was paddling. It was very disconcerting.
I'll go with Shep on this one. The glue on my L'edge bulkhead let go earlier this season. At first I only noticed it when the boat was being picked up and there was less pressure on the bulkhead. I thought the thwarts would do a good enough job on their own and paddled it a couple times. However, I noticed an increasinly annoying side to side at the rear while paddling. The movement was amplified upon impacts and I soon pulled it out and glued it down. Much happier now. I'd also worry that the foam might give a bit over time. What's snug today might not last.
You can take the side to side slosh out by glueing 1/2 inch (ish) minicell to the floor on either side of the saddle and then securing things well to the thwarts. It makes the cockpit more comfy and warmer too.
I'm cheap so I buy minicell flooring at Harbor Freight (the puzzle edge kind) when it goes on sale for $10/4 pack. Cut the puzzle edges and you get nearly a 23 inch square just under 1/2 inch thick. You can use a belt sander to remove the texturing without losing much of the thickness and wala. It may sound like a lot of work but it really isn't bad and it is just a little denser than normal 2lb minicell, works great for floor foam. I've also glued it up thicker to make other parts and pieces when I have not had thick stock in hand.
Good idea with the bottom foam. I would not be concerned with sand under the saddle either, but glueing down would definitely make things more solid. If you can't spread the gunnels far enough to put in the saddle with them mounted, put in the saddle first and then add the gunnels - you definitely can push them over the compressed minicell. While you're at it, you could add wooden gunnels too, would make the boat lighter, stiffer and easier to glue in the saddle when they're off, haha ;D
