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Chapter 7 (Part)

THE STARBOARD INNER HULL HALVE

TIMBER STRINGERS:

When it was time to start the stringers I arranged for Dale, my son in law, to help me as I reasoned it would be hard to hold the timber in the right alignment whilst fixing it.

It turned out easier than expected and we proceeded great guns until we got to the intersection between the hull and the bridge deck, when I couldn’t see how to proceed.

The pictures on the web sites didn’t offer any solutions and I decided to proceed by fixing a half width flexible batten along what I thought was the intersection.

This helped by illustrating how the stringers needed to be/could be run and completion from there was straight forward.

The picture on the left is the aft section and shows how I used short pieces of timber near the curve junction to get fixing points for the foam.

For the forward section the same shorter pieces were utilised. Also some thinner battens were used to provide guidance for where the foam needed to be positioned.

In Hindsight (Stringers):

• The stringers in the tight curves need to be positioned to hold the foam in the right places (just before the start of, at the tangent, ¼ , ½ , ¾ , tangent and just after). Seems a lot but unless they are provided it is a lot of trouble to control the foam and get a fair result. With the second hull, SB outer I adopted this practice and the result was noticeably better.
• Joining stringers, by using 150 mm long off-cuts to join them, with 30mm screws, works well and keeps them reasonably fair, should however position them away from the frame as the need arises to slide the stringers along from time to time.

FOAM

As mentioned before this will concentrate on the areas that were the greatest challenge to me, a novice boat builder. On the whole I was careful to try and follow the Farrier recommended practice in the ‘Construction Manual’

The things I did differently came about when I had trouble making the prescribed solutions work and may well have been due to my lack of experience and/or material selection

Following my experience with the dingy, I decided to commence in the middle with 200 wide strips. Because of the perceived restriction posed by the 75 mm HD (marine ply) strip for the mini keel, I decided to have a joint on the flat of the hull, before the bridge deck junction.

These part strips were also suited as the curvature of the hull required the use of the hotbox and my hot box was of limited depth.

Initially I tried to join the strips on the centre of a stringer but found that this was not holding them fair and that a kink developed.

I then joined the strip by screwing 4 timber off-cuts together to form clamps and found this quite successful in holding the strip in true alignment.

This method was slow and required the joints to be staggered, which aggravates the waste problem.

As an experiment I tried to fix a full length strip (2450 mm) and it worked as I found I could trim the excess length (which would prevent it from fitting against the HD) after the curvature had set and before gluing.

I found that with the use of the ‘Kwik Grip’ clamps and securing the strips by screwing them to the stringers with plasterboard screws I could, heat , bend/fit , hold while cooling (with clamps), release, apply ‘Purbond’ urethane glue, reposition and fix the strip in the one continuous sequence.

By the time a strip had been fixed, the next strip put into the hot box, at the start of the previous sequence was nicely malleable. The multiple bends keel/bridgedeck join were provided for by sliding the strip up in the hotbox during the heating process.

The little stepladder also made life easy.

As I got toward the aft end of the bridge deck junction with the decrease in radius and taper in the hull lengthwise, I did not manage to heat the strips sufficiently and quite a few snapped on me.

After replacing some, and the replacement snapping again, I tried to repair one, by filling the cracks with ‘Purbond’, letting it set overnight and sanding it the next day. The result was a fair surface, I did find out about the other side until later, but the slight depressions were easily faired.

The bow area presented my first serious access problem, as I weigh 105 kg and I suspected the foam on stringers would not support my weight

 The solution was one Jack Dearden had offered and that was to use a 6 mm sheet of structural ply over the foam after I had positioned it. I did need to move a few stringers, as I had not considered this before.

The hot box was then set up on a platform (the front is visible centre left with strips leaning against the box). By arranging things in this way I had no need to get on and off the platform and proceeded to do the curved deck and keel sections of the bow. I started with 200 mm widths and reduced progressively to 75 mm toward the end. (There is more detail on this area to follow later).

The High Density insert (Marine Ply) I made by cutting the board into slightly tapering (10 mm) strips and arranging them to suit the shape. I relied on ‘Purbond’ to fill the voids, again.

The next section, the front of the bridge deck, was one I had been dreading since I’d asked Ian about the stringers, and his advise had been, that "all will reveal itself".

I decided to have a break in the strips, where my little battens had indicated the junction is and to go for 100 mm wide strips. I found after the first two that I had to use tapering strips to fill the voids. Here the use of small bullet head nails (40 mm) to hold the strip in alignment was invaluable. (the nails are removed with fine nosed pliers after the glue has set).

As I progressed on the bridge deck section, I completed the matching hull portion and decided to revert back to full strip at the point where the curvature had eased sufficiently to do so without ‘snapping’ episodes.

Also a this point I trimmed the excess foam with a handsaw and after a bit of sanding thejunction was remarkably fair.

In Hindsight

• For the next inner hull (Port) I will cut the full strips at the junction towards the aft end of the bridge deck as well.
• The foam bends much better by being pushed into a bend. To do this it needs to be restrained at the front end of the strip (clamp both sides) and then pushed in, after which it is clamped at the end, until cooled and the bend has set.
• The foam in the bend will often snap when inserting screw to fix (especially if not heated enough and still bent to achieve the curve), so in this situation avoid screws in the bend.
• Ensure strips are sitting nice and tight against the stringers before gluing. Too late to align afterwards.
• Cannot rely on self tapping screws to hold down foam, reason covered later.

FAIRING FOAM

Following completion of the foam, the fixing of it has to be reversed from front to back and I fixed the foam from the back with 30 mm self tapping screws and followed up a section (4 bays), with removal of the plaster board screws from the front and sanding the area (mainly to vary the work as I went).

Sanding the second section the foam started to come away from the stringers and the bottom, the self tappers were not holding it.

The initial fix was, to redo some front screws and to clamp the bottom edge followed by fixing a batten along the bottom. That stabilised the foam.

I then decided to abandon the self tappers and substitute them with 35 mm pine screws. In the areas where the foam was actively pulling away (not enough heatforming) I used 45mm pine screws angled away and toward one another, this was able to hold the foam. After this screw substitution was completed and the foam sanded.

Before fairing the foam I ‘prepped’ the fibreglass, the sequence is, - marking the width, red for trailing edge, blue for leading edge, making 70 mm allowance for overlap. - measuring and cutting the glass, trial fitting it, - rolling it up, numbering it, marking leading edge and stacking it on a dedicated table (a half hull of prepped fibreglass takes quite a bit of space)

With all my fibreglass preparations I am always conscious of the fact that I’d probably be laminating on my own, so I try to minimise the lengths I have to handle and have a workable installation sequence. This is where trial fitting helps, as it is also a dry run for the lamination.

Advice from FGI was to seal/hot coat prior to fairing, but I found that makes it very hard to achieve a smooth surface, so after this hull I opted to fair first and then hot coat just prior to lamination. I tested the bond fairing to foam in several spots and always found a good adherence of the fairing to the foam.

In Hindsight

• Originally my fairing compound consisted of Q-Cell and I mixed some talcum powder in to firm it up a bit as I had trouble with the thicker patches slumping. If you mix more Q-Cell in and make a drier mix, it does want to stick. This was solved when I got phenolic micro balloons. I adopted a 4 - 1 mix and found this made, a workable mix, that adhered well, almost stopped the slumping and sanded well.
• The foam strips even when they readily bend into the shape, need to be heatformed to shape at intervals, to eliminate the springback problems.
• Hot coat after fairing, the reverse causes the surface to become very scratchy and a lot more fairing is used.