Subject: Re: [harryproa] Hull slenderness ratios
From: Dennis Cox
Date: 12/12/2010, 9:11 AM
To: harryproa@yahoogroups.com.au
Reply-to:
harryproa@yahoogroups.com.au

 

John,
 
I'd like to throw in two cents...
 
As you well pointed out, all boat design is a series of compromise.  That being said... the decoupled nature of Proa's in general and HarryProa's (even more so) allow, permit... (actually encourage) the design of the two hulls to be optimized for two totally different conditions.
 
For MLM, I chose to design for all-out speed without foils.  If your interest is more than for discussion and you have a technical bent and ability, I would highly recommend you look into the software program Michlet especially using its Godzilla enhancement.  Where most closed-form analyses are limited to the fully displaced regime, Michlet permits analysis above the hull speed.  It is based on wave theory.  Rick Willoughby is the undisputed master on this forum.  He helped me out a great deal during my learning curve.  I'd be glad to field questions if you need.  Michlet has a fairly steep learning curve, so it takes some commitment before you'll actually get results.  Once you get to the point that you can do trade studies with it, you will quickly see that changing your top speed expectations will result in significantly different hull designs.
 
For the lee hull of MLM, I gave Godzilla its head and let it maximize performance.  MLM's lee hull is the un-adjusted version as recommended by Godzilla.  Its slenderness ratio came out to be 30.09' / 1.14 = 26.4.
 
Dennis
 
 

From: John <jrwells2007@yahoo.com.au>
To: harryproa@yahoogroups.com.au
Sent: Sat, December 11, 2010 8:52:59 AM
Subject: [harryproa] Hull slenderness ratios

 

I am still trying to get my head around the preferred slenderness ratios. This topic has been discussed on the proafile site but I wish to consider only the Harryproa type as the other types of proa probably have other requirements.
The leeward hull - able to support the full mass of the whole boat plus the downforce from the rig plus a margin. I believe that the outriggers of a trimaran have a similar requirement and currently favour buoyancy of about 300% of the total boat displacement (i.e. Farrier)? Then the length and waterline width of the lw hull uses a slenderness ratio of about 20 using a rounded hull that will achieve this buoyancy.
The windward hull needs to support the intended loading only and the slenderness ratio becomes a compromise between minimum surface area (short and fat) and minimum wave making resistance (long and slender). Current preference of Rob's seems to be about 12 to 1. However the very competent Farrier tris seem to use about 8 or 9 to 1 to handle the same compromise. Additionally some people seem to lean towards 20 to 1 presumably considering that wave making resistance is more important for the speeds at which a proa will travel. Friction resistance from surface area might be handled with a larger rig at low wind speeds considering the stable platform of a proa.
Comments? - especially for the ww hull.

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