Saturday, April 23, 2011

More About Fine Lines and Lapstrake Planking

Dennis has written a comment about my last posting which is quite interesting: -

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

Well, this is a very good selection of questions, and the answer to all is, "Yes and No".

Yes, flare can contribute to a fine entry angle at the waterline and below, but it is only one of the design elements involved. To get a combination of flare, adequate breadth (or beam, as it is commonly called), and fine entry angles, you usually need to have a construction method which allows for compound curves in the structure. I will describe an exception shortly, but compound curves normally eliminate sheet material (for us that means plywood) as a building material.

The forward sections of Phil Bolger's Harbinger design
In the above photo, you can see a hull which is very wide (7' 1" on a 15' LOA) and yet has extremely fine entry lines and substantial flare.This boat was designed to be built using bent-frame carvel planking - a system which allows one to build a hull which contains compound curvature. Other options would be strip planking, cold molded using muliple layers of diagonal veneer, or, as I did with this boat, strip/diagonal, which is a combination of stip planking and double diagonal.

There are other options. Take as an example this lobster boat - also a Phil Bolger design, which I built using glued-lapstrake planking.

Planty of flare, but lots of planks

Phil Bolger Hope showing her fine lines

Once again, there is plenty of flare and a fine entry angle - all in a boat which is wide for her length. In this case I was only able to achieve the required shape by using a lot of narrow planks. In therory, if I had used an infinite number of planks, I could have achieved any shape I wanted, but the building process would by infinitely time-consuming and infinitely heavy - so a compromise had to be reached.

When I was designing Phoeinx III and Periwinkle, I was determined to use the minimum number of planks which was consistent with an nice hull shape. This allowed me to capitalise on the wide planks which could be cut from standard sheets of plywood, while at the same time reducing the marking, cutting, and gluing labour time for the builder. Once again a compromise. The individual planks were wide enough to need to be 'developable' shapes, but I still had more latitude with hull shape than would be the case with a sheet plywood boat.

The first Periwinkle awaiting her launching
 One of the design elements which allowed me to get a nice shape with so few planks was that the boats were relatively slender for their length. There are other good examples in the work of Joel White and Iain Oughtred and others.

Periwinkle, showing that she is relatively long and narrow.
There is another option, and that is to take sheet plywood construction to the limits of plywood's ability to twist and bend at the same time. This approach requires very careful design to ensure that all planks (or hull panels in this case) are what is called "developable". What this means is that the hull shape is designed so that the hull panels, which are cut from flat sheets of plywood, steel, or aluminium, are never required to bend in more than one plane at a time.

To visualise this, consider taking a sheet of card, bending it into a section of a cylinder, and then trying to bend it in another plane - it just won't work. If you bend a section of sheet material, it must take up a shape which is a segment of a cone or a cylinder. Therefore, you will always find a place on the curved surface where you will be able to lay a straight edge, and have it contact the surface along its length.

Now, it is possible to draw, using geometry, a surface which is composed of a number of adjacent cones and cylinders, but it is very time-consuming, and the resulting hull shapes are limited. Many thousands of stringer/frame plywood boats have been built this way over the decades, but they are generally fairly blunt and full in their forward sections. The resulting design options are limited.

Here is a good example of a plywood boat with developable panels taking up the normal sort of shape common to this design method. A nice boat, but fairly blunt up forward.
These days, the combination of computer modelling and stitch-and-glue construction has allowed developable shapes to enter new realms of shaping. The computer's number-crunching ability has allowed thousands of calculations to be carried out per second, and the stitch-and-glue method of construction has facilitated the un-folding of bottom panels which have been cut from pre-computed shapes and sewn along their edges.

Bottom panels of Flint cut to shape.....
...and made into a boat....
...which goes nicely with a beautiful, flared and fine entry - all from sheet plywood.
The final answer to Dennis' question is that I think it is possible to design a wide, flared, plywood boat which has fine entry lines - but the main problem is the "wide" bit. Perhaps glued-lapstrake or multi-chine plywood is the best approach. One day soon I'll have a go at a dingy hull of 15.5' x 6' in stitch-and-glue to see what I can achieve - but it will be a while before I get around to it.


  1. Thanks for your insightful comments, Ross. I appreciate them very much. Happy Easter, Dennis

  2. Hi Ross,

    Have really enjoyed the last couple of posts. I like thinking about the laps as spray rails, it added another dimension to what I thought of as simply a nod to tradition.

    On the matter of fine entry + flare I would like to make an observation - is this discussion of fine entry and flare dependent on hull type? I am thinking of flat bottomed sharpie hulls in particular. These hulls often have nearly plum or vertical sides, but are designed to be sailed with a fair amount of heel, thus introducing a degree of flare. Also, the most successful sharpies have a fair amount of rocker in their flat bottoms, this means that more often than not they carry their stems at or above the waterline. I think that Bolger's Black Skimmer is an excellent example.

    Are the needs for flare and fine entry different between say, displacement hulls versus planing hulls, or between boats that are designed to be sailed fairly flat versus those with a little heel?