Saturday, April 13, 2013

Scram Pram Progress

I have previously written about a Scram Pram which I am finishing off for a customer from Sydney. Scram Pram is a design from the board of Jim Michalak, and makes use of the so-called, "Birdwatcher" form of cabin. You can read some of what I have written previously in the post which you can find by clicking this link:-

The Scram Pram which I am working on was started and taken to about two thirds completion by a builder in Sydney, but work commitments and lack of time prevented him from finishing the job. He asked me whether I would be prepared to finish off the boat, and I told him I would be very glad to do that as it would give me an opportunity to gain experience with the "Birdwatcher"-style of hull.

The boat is now nearing structural completion, and we hope that she will be in the water in a week or two. Internal painting will still need to be completed but we will have a good opportunity to test the boat on the local dam. Below are some photos which may be of interest.

Windows installed, but without bedding compound at this stage.

Windows installed, but without bedding compound at this stage.  The large decks forward and aft, are fitted to the hull using silicon-bronze screws, and set in bedding compound. This allows them to be removed periodically for internal inspection and maintenance. 

The large side windows are simply screwed to the internal faces of the hull-sides, with the screws passing through timber battens so as to cover the unsightly bedding compound, and to spread the point loads so as to help prevent cracking of the Perspex.

This is a view of the leeboard on the starboard side of the boat, looking aft. You can see the 1/2 inch pivot bolt with a very large washer, and at the top the uphaul and downhaul tackle. This is all very simple, although since this photograph was taken I have added a pulley block to the fairlead which you can see on the far right of the photo. This has reduced friction and made the uphaul easier to operate.

 Note the bronze plates which prevent wear on the timber surfaces.
 Note the bronze plates which prevent wear on the timber surfaces.

Scram Pram makes use of water ballast, with three tanks underneath the cabin floor. These are filled and emptied through drains in the bottom of the boat. This photo shows how I modified the brass tube and bung arrangement which was supplied so that the brass tube would not protrude beneath the bottom of the boat, and that water would drain right down to the level of the bottom. The brass tube was cut just long enough to fit the entire length of the bung, and holes drilled laterally level with the bottom of the boat. The block of timber I made to support the tube has been thoroughly sealed with many coats of epoxy, both inside and out.

Here are the same components, but with the bung fitted.

In this view you can see the wooden block, the brass tube, and the bung all fitted permanently to the inside-bottom of the boat. The photograph has been taken looking downwards through a screw-in hatch which is set into the cabin floor. In use, it is an easy matter to launch the boat, reach in through the hatches, and open the bung. When the tank has filled, just replace the bung and screw in the hatch. Emptying is carried out in the reverse manner once the boat has been hauled out onto a trailer.
This is the mast partner which is bolted and glued to the forward bulkhead slightly to port of the centre line. The plan called for either steel or aluminium alloy for the closure plate, but specified half inch thickness. I have taken the liberty of using 8mm aluminium alloy, and set-up the closure plate as a pivoting gate. Time will tell whether this will be strong enough, and also whether it should simply be a plate with two holes and held on by butterfly nuts.

The owner of this boat asked me to modify the design of the rudder so that it did not protrude below the depth of the skeg. In order to do this I designed a shallow rudder with roughly the same area as that shown on the design, but I then added substantial endplates. Testing will prove whether this is adequate.

The rudder is very robust, and the endplate is glued on with epoxy and is supported by a substantial epoxy fillet reinforced by 450GSM woven glass. This should be strong enough to act as a step to assist people re-entering from the water.

I also shaped the upper edge of the rudder so that a foot can be placed on it without sliding off. The combination of the endplate, top of the rudder, and the outboard motor well should make for a serviceable ladder.

This shows the tiller position looking aft towards the rear bulkhead.

Here you can see the leather chaffing guard sewn around the mast where it enters the partner.

This is leathering on the yard at the point where the halyard is attached. The lump in the middle is the leather sewn around a commercial fairlead.

Here you can see leather glued to the started side of the boom, which in this view is upside down. The downhaul is attached through the black cleat and pulls downwards when the boat is rigged.

This is a pulley block with beckett attached to a pad eye which has been very securely screwed to the masthead. The mast is square sectioned and solid Hoop Pine.
Once the boat has been launched and initial test sailing has been conducted, I will publish another post.

Water Rat in 4mm Plywood - Sailing Version?

Water Rat is a 3 foot wide plywood kayak which can be built with a length of either 9' or 10'10". This boat has an unusual, but attractive shape, and has proved to be quite popular. She was initially drawn by my friend Doug Laver back in the days when we were working at my workshop in Brisbane, and we had started experimenting with Gregg Carlson's Hull Designer program.The plans that Doug and I used originally were very difficult to interpret when it came to the shape of the developed panel sections, and I subsequently redrew them in a CAD program, incorporating a few variations and conventional dimensioning. You can read about the boat in a previous post here:-

Doug's original Water Rat with my son, Steven, showing off the boat's stability. Steve was about 12 then, and I haven't seen this activity performed by an adult - but for a canoe she is a very stable boat.

Ross Trinder, from Toowoomba in Queensland, Australia, has built a number of boats recently including a Green Island 15, one of my First Mates, and a Water Rat. My good friend Al Burke also built a Water Rat, but used 4mm plywood instead of the 6mm plywood specified. Al applied some strategic stiffening materials and as you can see in the comments, his boat weighed in at 17 kg.

Ross Trinder is now building a second Water Rat, and this time he also is using 4 mm ply. Another variation on the theme being experimented with by Geoff Leedham in his boatbuilding program at a school in Alice Springs, Outback Australia, is to use 4 mm plywood for the entire boat with the exception of the bottom panel, which is made from 6 mm plywood.

This boat is a very simple project which involves very little time and very little money. However, it represents a great way to gain experience in the use of  epoxy and plywood, and the resulting boat is great fun to use, is very stable, and is surprisingly seaworthy in protected waters.

Ross Trinder has asked me whether it would be practical to fit Water Rat with a sailing rig. Given that she is 3 feet wide and carries her breadth out into the bow and the stern, I think that she would be able to carry sail reasonably well, and the only real challenge is to decide how best to give her some lateral plane. My first reaction is to make use of the leeboard, but I'm beginning to think that an off centre dagger board would suit the boat very well. I will be doing some preliminary drawings shortly, and will post them here.

In the meantime, here are some photographs of the early stages of construction of Ross Trinder's 4 mm Water Rat. You can also see Al Burke's finished 4 mm version via the link at the end of the first paragraph.

In this photograph the panels and bulkheads have been cut out accurately, have had holes drilled along the edges of the panels, and the boat assembled using plastic cable ties. After having ensured that there is no twist in the boat, Ross has placed a series of short tabs of thickened epoxy between the cable ties in the forward and middle sections of the boat.

Here you can see how the "tack-welds"of thickened epoxy remain clear of the cable ties, and are very small in cross-section. After the epoxy has cured, the cable ties will be removed and a larger fillets of thickened epoxy will be applied in a continuous run over the top of the "tack-welds" and have glass tapes laid over them while still wet.

Note how the bow and stern transoms, and the bulkheads, have centrelines marked. These are lined up with the centreline on the bottom panel, and as long as there is no twist allowed, the boat must come out the correct shape - even though there is no strong-back or mold.

Here you can get a good idea of the shape of the boat, and as long as the panels are cut out accurately, the shape of the boat is smooth and fair. This particular boat goes together very easily, and it may be possible to get away with as few as half of the cable ties you see in the photo.