For calm water, a given load carrying capacity and design speed, a variable flare 3 panel hull will inevitably have lower drag than a semi-circular section. The 3 panel hull will be shorter than the round section because the block coefficient is higher. However when you start considering performance in waves, hull length becomes a factor as longer means less pitching in given waves and that means less added drag.
One of the features of the HP is that you only need one long hull and its shape is not compromised to suit accomodation.
In my view you would start the design spiral with the longest lee hull you can fit in wherever you build and store the boat. You then do a preliminary estimate of displacement for what you plan to do with the boat. With those two factors you can come up with a design speed for the lee hull carrying say 80% of the displacement. You then optimise the hull shape for that design speed, maximum length and estimated displacement. I can guarantee it will be a slender hull. You need enough freeboard in the ends to avoid burying the nose, making allowing for your ability to shift weight; for example the crew on a smaller boat.
The design speed is a function of the length. There is no point having a design speed of10kts for an 8m boat displacing 1 tonne. That vessel would need to be semi-planing so wide and flat. The HP is constrained to canoe ends so as soon as the minimum drag shape has a transom, your design speed is too high for the length and displacement.
The design speed does not imply a maximum. It is the speed where the hull achieves its best efficiency. When sailing you know you are in the groove. On the 18m proa here in Melbourne, which displaces just under 5t, the groove is around 15kts. The shape is very close to an 18m hull with semi-circular section displacing 4t and optimised for 16kts. On that length and displacement, a 3-panel section hull would have a slightly higher design speed - say 18kts.
Rick
On 12 Oct 2018, at 4:50 pm, ryanonthebeach@gmail.com [harryproa] <harryproa@yahoogroups.com.au> 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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Posted by: Rick Willoughby <rickwill@bigpond.net.au>
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