Bulkhead/Hull Spacer?

[CapnSantiago]

I'm replacing a structural bulkhead and have been reading up on it, searching this forum, and seeking expert advise. I understand the bulkhead needs to be isolated from the hull with a coved or chamfered spacer but don't find much info on what that spacer should be other than "foam". From the expert advice (boat fiberglasser) I was told to use a 1/8" to 1/4" thick piece of high density foam with coves or chamfers.

I have purchased the material from the marina and am surprised at how dense/hard it is as it seems to me it would just transfer any loads from the bulkhead to the hull (which would defeat the purpose of it). It cannot be compressed by hand but only with tools such as plyers/vice/hammer.

I'm wondering if I have somehow missunderstood what I was told or have purchased the wrong product.

I would appreciate any experienced input on this before I proceed any further. (I hope to have the opportunity to rediscuss with the expert input I got but that is not readily available).

[sailingdog]

I would use some medium density closed-cell foam, cut in to a trapezoidal shape, with the wider end against the hull. This allows for a nice fillet and smooth transition for the fiberglass tabbing, making the tabbing stronger and less likely to break. It needs to be a ductile (flexible) foam, rather than a rigid one.

Usually, the foam, at least when I've done this, is at least 3/8-1/2" thick at a minimum. The larger and thicker the bulkhead, the larger and thicker the foam spacer generally, with 3/8" being a minimum IMHO. Fiberglass has problems when asked to bend along too tight a radius. Having a 1/8-1/4" radius is too tight IMHO for the tabbing to make and still retain reasonable amounts of strength. Mind you, if this were an outside radius, it might be okay, but it isn't—so 3/8-1/2" is a much better minimum.

[Faster]

My understanding of how that works is that it spreads the loads over the wider base of the foam section, avoiding the (often visible) 'hard' point at the bulkhead when it is simply tabbed in directly. It also makes for a much easier/smoother glass job without the 'hard' turn on the inside 'corner'.

I was also of the understanding that this foam would be more like 3/4 or an inch thick between the bulkhead edge and the hull, and once angled would give you a 1 1/2 or 2 inch surface contact along the hull.

[mitiempo]

I think 3/4" or so would be too thick. The reason the foam is there is so that the tabbing takes all the load evenly and the bulkhead doesn't touch the hull. Once the tabbing is finished the foam has no reason to exist. Morris yachts uses spacers and after one side of the bulkhead is tabbed the spacers are removed and the other side is then tabbed. And if you are not able to compress the foam without tools it is way too hard. Tim Lackey (Northern Yacht Restoration) uses pink foam that you can buy at Home Depot, I have used soft ethafoam that I had already. But once tabbed it really doesn't matter. Closed cell is important so it doesn't draw resin away from the tabbing. And as suggested cut it trapezoidal to allow for easier filleting.

[CapnSantiago]

Thanks to all for the input so far. There seems to be some varying thoughts on the spacer size/material.

It's interesting that most of what I have read/researched indicates the spacing is required (for hull isolation) with a chamfer (for filleting the tabbing) but none of them seem to define the size or material. Obviously this is something I want to know and get right befoe I proceed.

I'll check out the Home Depot pink stuff. I understand the closed cell issue (also so it doesn't absorb/hold moisture) and as originally mentioned was concerned with how hard/rigid the high density material is.

I'll be monitoring for any further input to sort this out and finalize it.