Thursday, April 28, 2011

Sharpie Hulls and Fine Sections

Graham has written a very interesting comment following-up on the last two postings. As it happens, it touches on a subject which is of great interest to me, as I'm in the process of designing a boat which incorporates the very characteristics he mentions. This boat has come about because of a request from a customer, but she has been in my head for many years and some of the reasons that I am so interested in this hull type are illustrated by Graham's comment.

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? cheers, Graham

Yes, the combination of a fine entry (i.e. sharp waterlines in the forward sections, with a half-angle of less than 19 degrees when viewed from above) a flat bottom, adequate rocker, and a boat which sails at an appreciable angle of heel will produce a soft and dry ride in a chop. I lack suitable photos to demonstrate this in decent wind conditions but here are a couple which may help.

Green Island 15 in light conditions. You can see the chine rising above the waterline, and gently slicing through the water. If the boat was heeling more, the effect would be greater, as the angle between the bottom and the topsides would make a pronounced "V"

Martin Kortlucke's Folding Schooner designed by Phil Bolger. Once again the wind conditions are light, but you can see the way the chine would work if the boat was heeling more - it would cut like a knife.
There are two primary reasons why so many flat-bottomed sailing boats have a bad name: -
  • bottom too wide; and
  • not enough bottom rocker
The idea is to have the curve of the topsides (in plan view) match the curve of the bottom (in profile view). This over-simplification really only applies if the boat has no flare i.e. vertical sides. However, with normal flare of between 10 and 12 degrees, it works pretty well. In theory, such a chine line would slice through the water without the formation of eddies associated with the normal flow of water across the chine.

Here are two drawings to give some idea of what I'm talking about. These show the forward hull sections of the design I'm working on, drawn at an angle-of-heel of 25 degrees. For reasons of practicality, the curve of the bottom and the curve of the topsides are not perfectly matched, but you will get some idea of the process involved.

Forward sections of sharpie heeled at 25 degrees
Same drawing, but with only the underwater sections shown. This gives a better visualisation of why properly designed sharpies can work so well, and be so fast.
What should be obvious from the above illustrations is that (within reason) a narrow hull with the chine line running high will perform better than a wide hull with the chine line running low.

Maybe I'll be able to come up with some better way of describing what I mean, but this will have to do for the moment.

Just to get you thinking, I believe that the fastest sharpie would actually be a scow hull, in which case the limitation on breadth of hull is removed. Look at the work of Phil Bolger and Jim Michalak for some clues...


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  3. Thanks Ross, you explained yourself perfectly :)

  4. Ross, I love it that you are discussing flat bottomed skiffs. I love 'em. My first sailboat was a Bolger Featherwind which exhibited the design principles you mention in your post.

    I am not so sure I agree about the curvature in bottom and sides. W/L length gets shortened, more of the bottom is exposed to waves and the pounding is obnoxious (this is not to say that the boat was not a blast to sail). How would John Atkin's Lark (14'3" sailing skiff) stack up to your discussion? It has a fine entry, but the stem is immersed. It has good flare and I would bet that it does not pound the fillings out of your teeth while sailing in a chop.

    While I have not built and sailed another flat bottomed skiff since owning the Featherwind, I will probably do so one day because the nostalgia of that first boat has a powerful draw.