Subject: Re: [harryproa] Hull cross section ?s
From: Mike Crawford
Date: 4/19/2009, 11:40 AM
To: harryproa@yahoogroups.com.au
Reply-to:
harryproa@yahoogroups.com.au




  The semicircular bottom is likely to be the fastest build and fastest boat -- provided you don't have to walk through it.

  In a big, heavy boat, the difference in hull shapes might result in a significant difference in drag.  In such a lightweight multihull, the difference might not be noticeable.
 
  Just as important, the semicircular bottom will require taller topsides, as well as a floor, which will add windage and weight in the case of the topsides, and weight and build time in the case of the floor.

  You might be able to get your 20" wide walkway with 5" radius sides with only another day's effort (or less?) during the build, so if you want to walk through the hull, the flat bottom with radiused edges will probably take less time than a floor.  I'd say your proposed design is either less work, or equivalent work, and definitely lighter weight. 

  It's tough to argue against it, especially when Rob plans to use it for the charter proa.  Rob's word doesn't come down from on high, but he has an incredible amount of racing experience compared to most, and he is rarely likely to add a design feature that will slow his boats.

  The flat bottom might not be perfect for heavy weather, but there are two mitigating factors:

  - It's only 20" wide.  You can only slap so hard with something that's narrower than two feet and has rounded edges.  This is going to be a lot less slapping than happens with the bridgedecks of cruising cats, even those with the full-wing undersides (better than nothing, but not a cure-all).

  - Your first three to six feet on each end will be sculpted foam anyway, so you can have any cross section you want.  This will certainly help with any slapping.

---

  The designer at Skimmer Catamarans would argue for your choice.  He claims that his bottom with a flat center (u-shaped section at the bow) will let you walk on the hull without adding a floor, provide "about the same wetted surface area" as a semicircular bottom, and will even plane if the weight is kept low enough.

    http://www.skimmercatamarans.com/about.html
    http://www.skimmercatamarans.com/design.html

  Planing would be impressive.  I doubt that you'd get the leeward hull up on plane, but a flat bottom on the windward hull could product a semi-planing hull that gets more surface out of the water for the same heeling/righting moment than a semicircular bottom.

  Not having been on a Skimmer, nor heard from anyone who has, I can't comment on the designer's claims.

  Regardless, there are good reasons to use your proposed designs.  Especially since I imagine that your first maxiElementarry/miniHarry boat will spend much more time in the Chesapeake than in the middle of the Atlantic.


     - Mike

 

Gardner Pomper wrote:

Hi,

I have questions on the "best" cross section for the ww hull in a
harryproa of around the harry size (25-30' ww hull).

I have been intending to use a flat bottom, which would reduce the
above waterline height while still giving standing headroom. I figured
a 10' flat section, with the hull curving up over a 4-6" radius would
give enough flat space to walk. I think the hull needs to widen out to
at least 2' beam above the waterline to make it comfortable to walk
around in.

The downsides of the flat bottom are possible slapping it short waves,
a wider than minmal waterline beam (it will be close to 24' wide at
the waterline; maybe 22", assuming a draft of 8" or so), and some
complication getting the hull to bend uniformly when making it.

What do the people on the group think is the "best" cross section to
use for both simplicity of construction and performance (min beam and
wetted surface) ?

Would I be better off with just a 12" radius semicircular keel and
vertical sides? It would be the simplest build and the min wetted
surface for a 12" draft, but it probably would not draw that much.
Displacement on a 12" radius for a 26' boat with a 0.75 prismatic
coefficient would be 1960 lbs.

Thanks for your thoughts!

- Gardner

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