Friday, February 16, 2018

Aluminium Expedition Cruiser Cockpit Takes Shape

When I last wrote about the aluminium expedition cruiser build in British Columbia the cabin was being built and the side decks had been fitted.

There is a lot of detailed construction in the cockpit that takes time to build. It includes the casing of the swing keel, the drainage trough that runs full-length of the long cockpit, the seating for rowers and helming, and the outboard engine well. All of the seating houses storage lockers for equipment and personal gear, with watertight lids that require careful construction. That cockpit work is now nearing the end.
View along the cockpit, nearing completion. The channel down the centreline is a drainage trench to dump water fast through the keel slot and engine well if she were to find herself in rough seas and take a big one aboard. It will be covered by a flush perforated plate. There are 8 rowing positions, with watertight crew stowage under them. Between the seats are food and ice lockers with watertight hatches through the cockpit sole. The horseshoe seat aft is for helming and has outboard engine stowage under it. In the U of this seating is the well for a small outboard engine for in-harbour use. The swing keel is ballasted and raised with a heavy duty trailer brake winch.
Overhead view of the cockpit.
Owner Tom McPherson has an Instagram site that has more photos, construction videos at various stages, as well as videos of the beautiful wilderness areas where his boat will be working, offering adventure cruises for teenagers. View them at Seaforth Expeditions.

I am still detailing the last drawings of this design, so it is not yet on our website. To see our other designs, go to our main website or our mobile website.

Thursday, February 15, 2018

Sportfisherman 26 Sheer Clamps

Since my last post about Kevin Agee's project he has been working on the second 9mm layer of  the bottom skin. With this layer there is no chance of using clamps along the keel to assist by holding that edge tight against the structure while you work elsewhere on the panel. For that reason this second layer is most easily done in narrower pieces than what worked for the first layer.

Kevin is dry-fitting this layer from the bow, toward the stern. In the bow the pieces are approx. 300mm (12") wide, which has worked well. Further aft the twist reduces rapidly so wider pieces can be fitted without problem.
Second layer of 9mm bottom skin being dry-fitted.
The sheer clamp is the next major piece of longitudinal framing that must be added to the skeleton. This is a fairly complicated component and must be laminated in place. The sheer clamp defines and strengthens the top edge of the hull and joins the hull to the deck. The complication comes from the fact that it curves on plan and in profile, at the same time twisting to conform to the changing angles of the upper edge of the hull side. At the transom the hull has tumblehome, so the side leans inward at the top. At the bow it has a lot of flare, so the side leans outward at a large angle.

To further complicate it, this boat has a broken sheer, meaning that there is a hard break in the side profile instead of having smooth curves that flow from one end of the boat to the other. That means that the sheer must be done in two sections. The aft flat part of the sheer clamp is done first, then the forward curved part is built off the aft section.

Kevin laminated the aft section of the sheer clamp during the week, ahead of me arriving to help with the more complicated forward section.
This photo shows the twist that is formed in the aft part of the sheer clamp as it changes from tumblehome at the transom to the beginnings of flare at the break in the sheer.
The aft part of the sheer clamp is run through to the next permanent frame forward of the break and will be trimmed off flush with the front face. The first layer of the forward part of the sheer clamp is also in position, dry-fitted so that we can check fairness of the curve and also mark the bevels on the frames and the stem.
Preparing the framing to receive the sheer clamp needs to be done carefully, so should not be rushed. The frames are not difficult, the angles can be established with a shallow cut with a back saw to show angle and depth, followed up with a rasp to take the frame edge down to the saw cut.

The Stem is more complicated. The angle of the bevel for the skin must be marked and cut first in the area where the sheer clamp will fit. This angle can be established by running a batten across the forward 3 or 4 frames to check that it contacts all of them correctly and lies flat against the bevel on the stem. Once that stem bevel angle is correct the rebate to receive the sheer clamp can be formed.
In this photo the first layer of the sheer clamp has been glued in place. The narrow piece of stem bevel that shows above the sheer clamp is the bevel for the inside face of the hull side and is flush with the edge of the sheer clamp.  The broad piece of bevel that shows below the sheer clamp extends from centreline on the front face to the aft edge of the stem to form the bonding surface for the sheer clamp.

A slightly different perspective of the intersection.
The sheer clamp must also be prepared with its ends trimmed to properly meet the aft sheer clamp and the stem. The aft end is relatively simple, with an angle in only one direction that is easy to mark, rough-cut with a saw, then fine-tune with a sharp plane.
The junction of the two sheer clamps at the sheer break. The cut angle for a good fit is quite easy to work out.
The forward end is more complicated so it is best to first work out the cut angles (which run in two directions so that the bevel is not parallel to any of the surfaces of the timber) on a length of off-cut of identical size to the sheer clamp. Cut this off-cut and fine-tune to a perfect fit, then transfer the angles to the sheer clamp on the workbench, where they can be accurately drawn, cut and planed. Test fit on the boat.

Now comes the part that demands ingenuity, to deal with the enormous amount of twist in the bow sections. We were working with very nice clear Douglas fir, which is stiff and not keen to twist as we needed. It didn't work to rely on clamps alone to do this work, the timber came in most of the way but still needed about 20 degrees of twist. We set up a Spanish windlass to do all of the work of pulling the timber in against the stem so that the clamps would only have to work on the twist. 
This was our setup for the dry-fitting phase of the first layer of the sheer clamp.
We were not confident of being able to pull it in totally with slippery glue complicating the work, so we decided to leave it clamped for two days to set in some of the bend. When we returned to it the pre-set of the timber made it easier to pull it back in. Again, we used a Spanish windlass to apply the bending forces. We used a different clamp setup to do the twisting, with the handles of the clamps lashed together to apply the twisting force.
Our setup for the gluing phase. The Spanish windlass is visible just above the sheer clamp, pulling the two sides against the stem. The two black clamps with their handles tied together are twisting the wood to align the outer surface with the front face of the stem.
We were working with timber thickness to laminate the sheer clamps in three pieces. Doing it in four thinner strips will make it easier to pull to shape.

The next step will be to laminate the other two layers onto the sheer clamp, which should be completed in the next few days.

Saturday, February 10, 2018

Sportfisherman 26 Bottom Skin Begins

This new offshore fishing boat being built by Kevin Agee in Hampton Roads, Virginia, is progressing nicely. It is entirely his build project and I get to do the drawings for it and to help out occasionally at weekends, sometimes offering guidance, other times supplying an additional pair of capable hands. I always return home after a day working on the boat feeling tired but happy with the progress.

Last weekend we did the last preparation work on the framing to receive the bottom skin, dry-fitting the plywood panels from stern to bow, fine-tuning bevel angles on frames, girders, keel and stem in the process. The bottom skin is two layers of 9mm plywood and this post is about fitting the first layer.

Kevin had already done most of the bevel work and dry-fitted two full sheet-lengths both sides, where the sheets fall nicely into place under gravity alone. The further toward the bow that we get, the greater the twist that the plywood must adopt. It doesn't do this readily and must be coaxed by various methods. There is only 3 degrees of change to the bottom angle (dihedral) in the aft half of the hull but there is 51 degrees of change in the forward half. This is to form an efficient planing surface aft, combined with bow sections that will give a soft ride into short seas, so the bottom has to twist to give the characteristics that we want.

The photo below shows some of the preparations that have been made to the structure ahead of fitting the bottom. The large member across the top of the photo is the keel, which has been planed at an angle to match the edge of the frame. There is a shallow saw-cut visible on the top of the keel, which is used, one on each side of the hull, to gauge the correct bevel angle. The intersection of the two cuts also shows exactly where the centreline of the keel is, needed when fitting the bottom panels. The white is a structural epoxy fillet to bond the frame to the keel, easier to do now than in the bottom of the bilge when the boat is right-way-up. The green semi-circle is a fibreglass pipe that has been cut in half and bonded into the edge of the frame. This is to form a scupper for bilge water that may get into the bottom of the hull. Each frame will have one of these scuppers fitted each side of the keel, to lead any water to the transom for draining or pumping.
The next photo shows the hull with the aft skin panels dry-fitted and the forward panels fitted on the starboard side and held by clamps. The port side, facing the camera, has the contact surfaces of all structure planed to the angles needed for the plywood to lay flat against them, to maximise the bond strength. Here the rapidly increasing dihedral angle toward the bow shows nicely.
This next one shows the other side of the hull, with the skin panels dry-fitted all the way to the bow. Whereas in the aft half of the bottom full 8ft sheets were used, further forward the sheets must be progressively reduced in length. This is because the longer the sheets the greater the internal stresses built up inside the sheets due to the twist, so short sheets accept the twist more easily. It is not a good idea to scarph all of the bottom panels together then trying to fit them in one piece. First, that single panel becomes very difficult to handle because of the size and weight, so it will need an army of helpers to manhandle it. Second, glue must be applied to the mating surfaces before the panel is finally fitted, so the glue will be an almighty mess by the time that the panel is properly in place.
Forward of the two 8ft sheets we fitted the next one as a half-sheet, so a 4ft sheet, which worked well. For the following sheet the 4ft length proved difficult, requiring a lot of clamping pressure to pull it in, so we cut it in half again, down to 2ft lengths. Those worked fine and also showed us how much stress can be built up in the plywood from the twist. Although the two plywood edges are straight when lying flat, the second sheet had to be spiled with a plane to fit against the first sheet, to compensate for approximately 3mm of curvature between the two edges. The 3mm curve is caused by stretching/shrinking of the material in the two panels from internal stresses. Due to the nature of wood, the internal stresses will relieve as the fibres take on a permanent deformation and assume the twist as their permanent form. Lamination of the next layer of plywood will also lock in the new shape.

In the process of dry-fitting the panels they must be trimmed to their final shape. Kevin cut the sheets to shapes that I provided in the form of full-size patterns but they must be fine-tuned along the centreline and chine for a perfect fit. Once that perfect fit has been achieved and the panel is securely clamped, it should have some locating screws fitted, which will assist greatly with getting it back into that exact position when gluing it into place, with slippery glue trying its best to put the panel anywhere other than where you want it to be.

In this next photo the forward sheets of the first side have been glued in place. The temporary screws with fender washers are holding the plywood securely against keel, stem and girders but the clamps have been left along the chine. The strips of plywood on waxed paper across the joints in the middle of the panels are there to align the joints in case some glue has found its way into the joints; we don't want it to glue the edges misaligned.
This one shows the forward sheets of the second side glued on. This is when the previously pre-drilled screw holes to locate the sheets become very important. With the opposite side already skinned, clamps can't be used to position the plywood before any screws are fitted. This is a two-person job, with one person holding the sheet close enough to the framing for the first two locating screws to be fitted, then the other screws follow in a logical order to pull the sheet back into its proper position.
The aft sheets were glued on by Kevin and a friend during the week. The photo below shows the completed first layer of the bottom skin.
And the last job is cleaning up the glue that has squeezed out of the joints and runs down the structure. It is worthwhile to do this removal of excess epoxy as a matter of course throughout the build. If it is left to be cleaned later then the epoxy will be cured, difficult to remove and likely to break out pieces of plywood in the process. Trimming off when soft will save some hassle later.
This design is not yet complete, so is not on our pricelist or website. See our full range of designs on our main website or our mobile website.