Thursday, December 4, 2014

Hull Turning Methods

I have shown a few projects in this blog over the past few months with the hulls being turned using different methods. The bigger the boat the more critical this operation is. The risks of damage to property and injury to people increase exponentially as the boat becomes more bulky and increases in weight. Imagine the difference between turning over a plywood hull that is 6.4m (21ft) long, 2.4m (7'10") wide, 1.2mm (3'11") deep and weighing 200kg (440lb) or a 50% scaled up version of the same hull. At 9.6m (31'6") long, 3.6m (11'10") beam and 1.8m (5'11") deep, it will weigh 675kg (1488lb).

It has only increased 50% in all directions but the weight is more than 3x that of the smaller boat. The bulk becomes more difficult to manage and the weight to lift and lower becomes a major factor. If, at the same time as increasing the size, you also change to steel as the construction material, that same size hull could weigh 2500-2750kg (5500-6000lb). Now you are talking about some serious loads that can get out of control, yet the boat is still only 50% bigger in each direction.

Those smaller projects are easily turned over by hand, with friends and neighbours supplying the motive power and the boat having a soft landing on tyres or some other cushioning material. Years ago a client of mine in South Africa turned his 32ft hull by himself. He jacked it up on one side until it reached the balance point, then let gravity take it the rest of the way. It fell against a young tree, then slid down the tree and came to rest flat on the ground. Luckily his hull was relatively undamaged but the tree didn't survive the experience.

There are many ways to turn a hull but that is not one of them. I have a whole chapter on this subject in my book "Shaped by Wind and Wave", to give guidance on how to safely turn your hull. The two Didi 950 hulls that were turned recently in USA and Australia both used the spit-roast method. For each of my own big boats I have used a chain block to do the work, another of the methods explained in the book.
Turning the 38ft "Black Cat" using a chain block on a scaffold tower.
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