Saturday, April 30, 2011

Discussion about Flat Bottomed Hulls

As I expected, publication of an opinion on the subject of flat bottomed boats has resulted in some discussion. Graham brought up the matter of adequate rocker and running the forefoot above the waterline (see my previous post), and he used Phil Bolger's Black Skimmer as an example. Black Skimmer is one of my all-time favourite designs, by the way - I think she is a superb example of sophisticated simplicity, and even after thirty-one years of studying her shape and construction, I still find her to be awe-inspiring.

Here is a photo of Black Skimmer, copied from Woodenboat Magazine - I hope I haven't infringed any copyright. She is close to being my absolute favourite design.
Dennis has written in with the following: -

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. Best, Dennis

I know what Dennis is talking about, and it is a good demonstration of how everything in boat design requires compromise. The shape which provides the excellent sailing behaviour (adequate rocker, matched curvature of the topsides and the bottom panel, forefoot run above the waterline) is very likely to pound badly when floating level.


There are plenty of flat-bottomed skiff designs around which have the forefoot immersed, and scores of William and John Atkin boats provide excellent examples to study. One of my favourite Atkin designs is Ration and she shows exactly what we are talking about.
Lines of Ration - courtesy of Motor Boating's Ideal Series - Chapman and Horenburger
Ration shows a rowboat which is less likely to pound in a small chop - immersed forefoot and very fine entry at the waterline and bottom - but she will still pound as soon as the waves get large enough to make the forefoot clear the water. The main problem is that the shape of the chine-line is such that there will be turbulence formed as the water running around the sides at the bow inevitably runs down and across the chine, and subsequently runs back across the chine in the aft sections (although this will be less of a problem than at the bow). A mitigating factor in this design is that the boat is relatively slim. It is in wide, flat boats that the problem is at its worst, causing excess drag and wild, unpredictable steering - particularly downwind.


To get an appreciation of what I'm trying to describe, compare the shapes I show below: -

A typical flat-bottomed skiff with the heel of the stem immersed, and the bottom of the transom coming to above the waterline. I'm only showing the body plan here, but the boat I've drawn as the example is fairly long and slim.
The same boat heeled 25 degrees. In reality, the stern would probably be forced a bit higher and the bow lower than I've shown here, which would make matters even worse.
A clearer view of the same boat. See how the chine line will generate turbulence and drag, and will tend to force the boat to round up.
For comparision, here is the underwater shape of the example I drew for the previous posting. While this is not a perfect shape by any means, it is vastly superior to the example shown above - but it will pound more when flat in a ripple.
It is all a matter of degree - the Featherwind mentioned by Dennis shows close to the ultimate in matching the curve of the topsides and the bottom - at least for a sharp-bowed boat - but she is best used as a sailing boat, or a rowing boat on flat water.

4 comments:

  1. Hello,

    There was a flat bottom sharpie built in England in 1898. Very unusual at the time. She was 14ft and had a sprit main and jib. Mr. Lillistone could you look at this picture of her from Dixon Kemp, and comment on what you think? I'd quite like to build an historical replica to see how she went (fast by all accounts at the time as she was banned from racing).

    https://picasaweb.google.com/lh/photo/OYrhbqLKYyB9iQF9G3RN3HY6h4w9_WFXdAPmTale1U0?feat=directlink

    Regards

    Edward

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  2. hi..Im college student, thanks for sharing :)

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  3. Hi! A fellow going by Ivor Bittle writes about modifying the bow wave, how the separates, that you mentioned. He used both bulbous bow and control surfaces to do this. He has shown that he could, with control planes, greatly affect where the lower part of the bow wave emerges.

    My question to you is playing off the idea of modifying the bow wave in relation to hard chine boats: what do you think, in a displacement power boat, might be achieved by developing an aft tunnel of sorts, maybe something like a shallow inverted V or inverted dead rise (something as old as the work of either Andrade with his "new type" or Hickman with his sea sleds so no innovation there) to give the water from the lower part of the bow wave someplace to go besides back over the chine with the resulting turbulence, not that I imagine that could be outright prevented ... just reduced a bit?

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  4. Doh! ... it should say "how the upper and lower part of the bow waves separate" ...

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