Saturday, September 10, 2011

Thoughts on V-Bottomed Hulls

There is a lot of prejudice about flat-bottomed hulls, even though ones of good design can make excellent boats. But in western culture there is a far greater acceptance of the V-bottomed hull-form. I guess it is just a case of what we are used to seeing - westerners are familiar with the V-bottomed form and see it as being quite normal, just as I imagine that Chinese and South-East Asian people would have little argument with the virtues of a flat-bottomed boat.

Flint showing her V-bottomed hull-form
More than anything, I believe that concerns about pounding prevent the wider acceptance of flat-bottomed hulls.

A good example of a flat-bottomed sailing hull - this one is a Green Island 15
Any light-weight boat will pound, but if well heeled over, a flat-bottomed hull will run softly because the sharp angle between the bottom and the topsides acts as a "V". But if the boat is upright pounding can be a real problem.

I've designed a number of V-bottomed hulls, but my main reason for using this form has been to gain stiffness in the bottom panels. The V-bottom picks up stiffness because the bottom is broken into two halves, and the keel line is very stiff indeed due to the convexity of the bottom in two directions - somewhat similar to the strength in an eggshell, which would break easily if a flat panel, but is extremely strong for its weight when in a compound, convex shape.

That is me sanding the keel line of Flint. You can easily imagine how stiff the bottom is, even though it is made from 6mm (1/4") plywood. If it was a flat panel it would be very flexible if not supported by internal framing.
My primary concern with hard-chined hulls, regardless of whether they are flat-bottomed, V-bottomed, or multi-chined, is the matter of turbulence where the flow of water crosses the chine line. Without access to tank-testing or sophisticated Computational Fluid Dynamics (CFD) programs, my assessment of correct hull design for hard-chined forms has been based on intuition and common sense backed-up by insights I've gained from the writings of the late Philip C. Bolger.

One method of reducing turbulence around the chine is to keep the chine out of the water altogether - both Jim Michalak and Phil Bolger have used this technique a number of times - but the problem with that is it generally works well only if the boat is a light displacement design intended to run on an even keel e.g. rowing boats and power boats.

When more displacement is required, I use a different approach. A good example for discussion is my Alby design, which is a short pram dinghy designed to be short, but able to carry a heavy load. She was designed for my friend, Allan Burke. In order to maximise the displacement (therefore increase the carrying capacity), I used a lot of keel rocker, and reduced the angle of the "V" in the bottom amidships to only about 3.5 degrees. However, in order to reduce pounding, I twisted the bottom panels to give a relatively sharp "V" at the waterline foreward and aft - 18 to 20 degrees of deadrise.

In the drawing above, you can see how the chine-line (drawn in red) sweeps down strongly from the bow and stern to the lowest point amidships. The resulting hull is almost flat-bottomed amidships but quite well V'd at the bow and stern. I could have made her totally flat in the middle, but I left enough "V" to give the "eggshell" stiffness of a convex shape.

Some people will tell you that running a chine deeply into the water will cause excess turbulence and drag, but that is only the case if the hull is incorrectly shaped in 3D. You must consider a boat shape in profile, plan and body plan (i.e. end elevation) to get some idea of how the water will flow.

Above you can see the same hull viewed in body-plan . See how the designed curve in the topside panels and the bottom panels has resulted in a clean and well-streamlined chine-line (drawn in red)?

Now look at the same hull heeled at 20 degrees. The chine-line is forming a beautiful "V" with very little to cause turbulence, or to hold the boat back. This is one of the primary reasons why a well-designed pram or scow can be so fast - particularly in flat water.

This may give a clearer idea of how well the shape will slip along with minimal turbulence.
The only way to achieve the same effect on a sharp-bowed boat is to run the chine very high at the bow. An excellent example of this approach is Jack Holt's Mirror 16 design.

Mirror 16 showing chine-line. (courtesy Terence,
In contrast, have a look at a hull with a hard chine and the chine-line left low at the bow - you can imagine how much turbulence and resistance would be generated by the chine (drawn in red) being forced forward through the water.

These are just my own thoughts, but I'm sticking with them at this stage.

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