Take into account that the sole will be wider in a flat bottom than a semi-circular section whether it is directly on the bottom or raised a little. There may be a crossover point depending on flare angle.

Raising the sole is optional. A lot depends on the chine joint configuration and if there is any internal stiffenening.

Resistance to sideslip may be more on a shallower flat bottom because of the relatively sharp change of flow direction over the chine compared to the large radius of the round bottom. Depends on flare angle and chine radius. Not trivial to quantify.

Cheers,

Peter

 

 

From: harryproa@yahoogroups.com.au [mailto:harryproa@yahoogroups.com.au]
Sent: 12 October 2018 16:13
To: harryproa@yahoogroups.com.au
Subject: Re: [harryproa] Re:: Flat bottom hulls?

 

 

    I think there are downsides that haven't been brought up.   For example the cabin sole being the hull bottom is normally going to be narrower than it would be if raised a bit.... Case in point the boat that I was originally looking at, Kohler's KD 860 is flat bottom, but the sole is raised per the image below:

    The other concern I have is the lack of stiffness of a flat surface versus one that is curved in at least one axis.  That means that means more reinforcing or heavier material, or both.  With foam sandwich, thicker core of course accomplishes this with almost no additional weight... In ply construction it's a different story, and an overall weight savings over the rounded bottom may be a realistic expectation.

 

    The shallower draft offered by a flat bottom may offer an a dditional asset on a multihull with daggers or Rob's fore and aft rudders in that it should skate sideways more easily with the boards up offering less resistance in storm conditions.   Without heeling as a "safety valve", the ability to move in response to a gust is said to be important.   A monohull with it's huge keel has to stand and take a beating like a big heavy boxer, while a multihull can dance away and give to blows, either accelerating or skating sideways.  Lightness and a shallow bite into the surface of the sea should be an asset.

 

    Regarding performance differences, I think we are looking at what in real life are pretty trivial differences.  There is far more to be gained I think from light weight construction and from keeping payload down than from the difference between flat and round bottoms.    There is a lso the possibility of surfing with a flat bottom, more so than a rounded one.

 

 

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On 10/11/18 11:50 PM, ryanonthebeach@gmail.com [harryproa] wrote:

 

Thanks Rick, good info.
So from a performance only perspective.
Upwind, lower overall height reduces wind resistance
Light air, weight savings reduces wetted surface overall as opposed to somewhat higher wetter surface of the section...

You mention that a three panel optimized will be shorter than a half round... wouldn't this knock some points off the top end?
I can see some weight saving from floors and lower structure but a shorter hull runs counter to overall performance. Maybe I'm missing somethin.

 

 

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