Friday, April 22, 2011

Fine Lines and Lapstrake (Clinker) Hulls

I was having a discussion recently with one of my sons about which hobby activity was the most fun. We have far too many hobbies, but flying and sailing are close to the top. Well, we both decided that there was no contest - dinghy sailing came out as number one.

Where I live, the biggest problem with sailing is exposure to the savage sun, but running a close second is being continually doused by water as you drive to windward. Yes, I know that we have a nice climate on the east coast of Australia (at least as far north as we are), but for a large part of the year the water is frequently very cold when it hits a hot body!

Most of the boats that I have sailed have had relatively blunt forward sections, and they tend to bash water into the air when butting into a wave, and the wind blows it back directly over the crew.

Hard to windward with a deep reef tied in - relatively smooth water as we exit the harbour, but we are still getting wet!
There are plenty of advantages in having full sections forward, such as adequate buoyancy when running down waves, and the ability to move forward in the boat while still retaining reasonable trim. In some boats where load-carrying is critical, a very broad bow is essential - for example a praam dinghy

My Alby design - lots of carrying capacity in a short hull
The problem is that the disturbed water pushed up by a blunt boat - at least in a dinghy - consumes power, and as previously mentioned, bashes water up into the wind so that it can wet the crew efficiently.

This is a nice design I built a few years back, but you can see that she pushes up a lot of water, even in light conditions

Here is another good example - a really nice design, but pushing up lots of water.
In both of the above photos, the boats were drawn by exceptionally good designers, and the shape ended up the way it did for important reasons. But it does demonstrate the point I'm making about the water.

When I was asked to design Phoenix III, I really wanted to keep the bow fine. In fact, it has become a bit of an obsession with me, although I do change my attitude when necessary (e.g. Alby and Whimbrel).

Phoenix III showing her fine forward sections
My reasons for going down the path of fine forward sections include: -
  • Dryness when pushing to windward;
  • Reduced pounding - something which is very important in a small and light-weight boat;
  • Easily driven hull - particularly when under oars
Paul Hernes in his Phoenix III under oars - he is too busy smiling, and should be rowing faster!
One of my favourite methods of construction is glued-lapstrake (or glued-clinker). This system is only possible if the planks are made of plywood, because natural timber planks lack cross-grain strength and will crack where the thickness of the planks changes from double to single. That is why traditional clinker hulls have lots of closely-spaced bent ribs, and have their plank laps riveted or clench-nailed - the ribs provide cross-grain strength, and the mechanical fastenings allow the planks to move relative to each other as the boat shrinks and swells in changing conditions. Plywood has almost equal strength in all directions, and can be glued at the lap - this produces a strong, stress-skin hull which requires few, if any, internal frames. The glued laps form an integral "stringer" which further strengthens the hull.


A clean and open interior

A wonderful side-effect of the lapstrake construction method is that the laps produce a series of "spray rails" on the external surface of the hull.

Fine forward sections and prominent laps on my Periwinkle design
The combination of the fine lines and the overlapping planks makes for a dry ride in most conditions.

Here is a good example of the effectiveness of the sharp forward lines and the "spray rail" action of the laps. This is another photo of Periwinkle.
As with all things, there are drawbacks. Any boat with fine lines forward (or aft for that matter) needs to be properly trimmed, as they are sensitive to weight distribution.

Here, the crew of Periwinkle is a little too far forward for the conditions. In gentle winds their position would be ok, but I think the big mainsail was driving her down somewhat at the bow. However, she is still going nicely!
Every boat is a compromise, and there is a place for all different sorts of hull shapes. The more more you understand about hull-forms and construction methods, the easier it will be to make a decision about what is right for your circumstances.

3 comments:

  1. Ross, this is precisely the problem I face in my own region sailing in the Great Lakes USA. The water never gets warm, really, and getting a dousing is not a lot of fun, especially as one ages and gets "thin blood." Your first picture illustrates perfectly the trouble my 15 ft sailboat gives me. And while she sails very well, I increasingly feel the need to replace her with a drier boat.

    I figured the problem was that there was not enough flare in the top sides of my boat's fwd section. Does flare contribute to a fine entry? The relationship between the two is not clear to me and the picture of Phoenix III seems to challenge that supposition. Can one have a beamier boat than the Phoenix III, say 6 ft, on a 15-15.5 ft length and still attain a fine entry to the bow you mention in your blog?

    Best, Dennis

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  2. Periwinkle is such a handsome boat, and practical too...

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  3. Dennis and Graham,

    Thanks very much for the comments. See the next blog entry for more on the subject.

    Ross

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