I agree with all Mikes points. There are any number of arrangements possible, but the one that fits the criteria of least hassle to build and sail, least weight and cost, most safety, most comfort and usable space is the harry layout. Happy to discuss any possibilities that would do any of these better.
My main concern with biplanes is that they have the sail area lower down, which makes them less good in light air. I want to sail well in these conditions, and am happy to reef if the wind gets up and the natural flex of the unstayed mast is not enough. They are also more expensive.
A rarely appreciated problem with a rig in the ww hull is a stiff neck from sitting directly under it checking the trim. Sitting a boat width away from the mast is much more comfortable. I have always felt distinctly uneasy sitting to leeward of a rig, but I guess you get used to this.
Todd,
Why would the beams not be heavier? The rig weight has to be added to the weight of the ww hull that the beams have to lift. The torque from the rig trying to rotate the ww hull means a lot more off axis material than otherwise required. The beams have to be heavier.
I don't use an ustayed rig just for performance. The benefits are ease of use, safety, lower cost, lower boat weight, less pitching, less windage and no maintenance. For cruisers and short handed racers, these attributes will result in higher performance.
8" x 8" box beams, 17' long x 0.5" walls vac bagged from glass will weigh about 90 kgs/200 lbs each, assuming sg of 1.65 for the laminate and no internal bulkheads or local reinforcing. I suspect they would not be stiff enough (I used a 17' long all glass beam on a 35' x 3,500 lbs cat 30 years ago, it had to be 12" square to be stiff enough), and that as you only calculate the lengthwise strength of the materials, they would not include any off axis material for intra laminar strength or torsion. This will add a minimum of 20% (30% on most cruisers) to the weight.
The 27' long beams on my boat weigh 18 kgs/40 lbs each.
I don't see anything of relevance to the discussion (mast location, number of masts, beam strength, torsion, rig loads) in the video. Nice models, though.
regards,
Rob
On Thu, Nov 11, 2010 at 9:27 AM, tsstproa
<bitme1234@yahoo.com> wrote:
What I find hard to believe is that the beams on Robs boats would be any lighter than those of an atlantic prao design.
Or that the stayed rig vs the unstayed rig is in any way a weight reduction worthy of performance.
According to Arthur Edmunds.
Where as for a craft with a beam of 17' on center with a weight of 12,000lb heavy will create a bending moment of 80,500ft-lb = 966,000 in-lb or 11,139 kg-m.
Sesction moduluse (SM)= BM/S where S=29,000 psi for glass fiber (199,810kPa)
Use a FS= 2. required SM =966,000(2)/29,000=66.6 in*3
Basically using two , 8in x 8in x 0.5 walled box beams.
Whats the weight of each beam compared to robs proa?
I figure if carbon was used drop the weight by 1/3 per beam or more possibly depending on process used for laying up each beam.
Todd
> John,
>
>
> I'm just a fan of designing forces out of the equation, and have been
> since my engineering days in college. Thus, I like the harryproa
> downspiral design: Eliminating torsional (forces in line) and
> compression forces (unstayed mast) means less weight, which means less
> sail, which means lower forces, which means a lighter structure, which
> means less sail, and so forth.
>
> At the same time, less carbon and epoxy mean a lower cost, as does
> skipping additional structures such as crew pods.
>