Sunday, March 11, 2007

Compound curve lamination, mast step and rudder

In my previous post, I mentioned a couple of small cosmetic customizations to the design of the Jimmy Skiff ™ I am building. The most visible result of my 'straying' from the published plans is shown in this entry. I thought that with all of the nicely curved lines on this boat, the splash rail/deck support beam would look better if it curved in a way that would compliment the curve of the aft seat. Since the deck has camber which is determined by the shape of the top of the forward bulkhead, the deck rail curves to follow the profile of the deck. The CLC plans show the deck rail with this curve, but as straight across from sheer to sheer. I decided that I wanted the aft edge of the deck to have a compound curve as shown in the photos on this page.

I knew that laminating the deck rail in a compound curve would be challenging at best, and I don't mind saying that I went through more than one scrap wood 'prototype' before getting it right. In the end, the solution really turned out to be rather simple, yet elegant, much like the Jimmy Skiff ™ itself. Since wood of any thickness and width beyond a few millimeters is not favorable to bending in more than one direction at a time, I realized that the way to do this was to laminate thin strips of wood that were wide enough to allow cutting the shape of the second curvature from the laminated stock. I started by taking a length of 4"x4" Douglas Fir and marking it with the curve which matched the fore/aft curve in the deck, then I cut it in half on my band saw. This would be the laminating form. Then I sliced another section of the same straight-grained, knot free beam into six thicknesses of about 3mm each. These were glued together with a generous amount of silica thickened epoxy and wrapped in plastic wrap before getting clamped in the form.

After the lamination cured, I took the blank out of the form, sanded off the excess epoxy, and marked the curve of the deck camber on it. I did this by holding the blank up against the aft edge of the deck and running a pencil along the length of the rail with a 1¼" thick block riding along the top of the deck, under the pencil. Then I just traced the bottom of the deck from underneath to mark the bottom of the rail. Taking the lamination back to the band saw, I cut along the lines for the top and bottom of the deck rail. A little more shaping with the sander, and I had a perfect fit! The next challenge was to figure out a way to clamp the rail in place on the aft edge of the deck. I found that I could place half of the laminating form on top of the deck, and clamp it down to the sawhorse. Then I could clamp between the rail and this block. I also clamped the center of the rail to the deck where the mast hole was. Then I clamped both ends of the rail down to the inwales.

The plans specify the deck material as 6mm marine plywood, and when I was cutting all of the parts for the hull, I cut the deck out of the 6mm okoume with the shape shown in the plans. Later, when I decided that I wanted to extend the sides of the deck back a little to get the curve I wanted, I didn't have any 6mm ply left. I did have ½ sheet of 9mm okoume left, so that's what I used for the deck. Since 9mm plywood does not bend as easily as a 6mm panel over the bulkhead, I set my circular saw to cut several kerfs lengthwise on the bottom of the deck to make it easier to bend. Then to reinforce the deck, I filled the kerfs with wood flour thickened epoxy just prior to putting the deck in place and temporarily screwing it down. I made the mistake of removing the screws the next morning after I glued the deck on, because I thought the epoxy was sufficiently hardened. I went away for the weekend, and when I returned, the edges of the deck had begun to lift. By now, the epoxy had hardened, and there was no way to just screw it back down. I had to remove the hardened epoxy from between the deck and the inwales, using a handsaw and RotoZip ™ tool. Then I was able to re-pack the gap with a fresh mix of silica thickened epoxy and drive the screws back in. The screws will be removed when everything is fully cured, and the holes will be filled prior to sanding and painting the deck.

Here is a view of the deck that also shows the placement of the mast step. I decided not to install the mast step until I have the mast ready to test a trial fit. This will allow me to make sure that I have it placed correctly in relation to the hole that runs through the deck.

Here is a better look at the mast step. This will be sealed, sanded, and painted the same color as the floor.

These rails support the aft seat. They are glued in, and held with screws.

The aft seat is finally glued to the rails and then a fillet is formed around the perimeter, including under the front lip of the seat. This will make the space under the seat a sealed floatation chamber when the hatch plate is added. The toolbox and bench vise are there as weight to assist the clamp in holding the seat top down while the epoxy cures.

This is the rudder attachment. I made the rudder based on the templates and directions in the plans. The lower portion will flip up when beaching or in shallow water. The attachment is accomplished using a 12" bronze rod, fitted through stainless steel eye-bolts. I had read somewhere that the one thing that a particular Jimmy Skiff owner did not like about this arrangement was that it was noisy due to the 'play' between the rod and the eyes. I decided that I would attempt to solve this by buying oversize eye-bolts and filling the eyes with epoxy. Then I drilled a hole through the cured epoxy which the rod fit through without any play. This resulted in a very smooth and quiet steering mechanism. I still need to make the tiller and attach it to the top of the rudder.

Here is a better view of the rudder attachment hardware. If you click on the photo and look at the eye-bolts on the bottom, you can see the epoxy fill that I was talking about. You will also get a better look at the ring clip at the top of the rod, and the pin clip retainer at the bottom. With this set-up, I can be confident that the rod will not slip out, while it is still easy to remove the lower pin and pull the rod upwards to remove the rudder.

I welcome your comments or questions. If you click on the Comments link at the bottom of this page, you will be able to leave a message.


Robert Duncan said...

Hello again,

Rails completed, mast step in, and second sanding of the interior completed after runs were detected from the inwale down. I am mounting the rudder next but I do not know exactly where to place the gudgeons. Will both anchor to the block on the interior of the transom? I hope I do not have to Fasten a backing in the now closed chamber below the aft seat.

Robert Duncan said...

I am putting gudgeons and pintals the top one is a no brained but the lower one is troubling me. The block is not long enough to fasten the pintal high where the wood is stout on the rudder too low and I guess we uses the support under the seat to screw into for the gudgeon but the pintail which requires some routing to fit may enter the cavity for the rudder blade. Suggestion?

Ron Paro said...

Hi Robert, when I used the eye-bolts and rod for the rudder attachment, the lower eye-bolt goes into the support for the aft seat. Later I changed it to using gudgeons, and the lower gudgeon does go into the same backing block as the upper one.