<<The logic just doesn't make sense.>>
Okay, I'll give the explanation one more try, and then I'll drop
it. There's no sense in beating a dead horse if the engineering
approach doesn't work for you.
Consider:
- You have a proa where the beams are designed to handle the
maximum bending moment due to sailing/heeling. Let's assume this is
the same for the rig to windward and the rig to leeward, even if
that's not quite accurate. Beams are set for flying the hull, plus
a margin of safety.
- Since we're going for the lightest weight, let's assume that the
beams are as small as possible to resist these bending loads. If
they encounter the max bending load for which they are designed,
they're fine. Anything over that and there's a risk of failure.
- Now add 23,000 lb-ft of torque to the beams for the racking
load, on top of the bending stresses. For the sake of discussion,
let's assume that's 12,000 lb-ft per beam.
Adding that 12,000 ft-lb of torque to each beam will then create
stresses on top of (in addition to, in excess of) the bending
stresses for which the beams are designed, causing them to fail.
Thus requiring a heavier beam.
If you can demonstrate how to add 12,000 ft-lb of torque to a beam
that is already maxed-out due to bending loads, without a) making
the beam fail, or b) making the beam heavier, you'll prove your
point about the logic not making sense.
- Mike
On 11/22/2010 11:23 AM, tsstproa wrote:
So you still think having the mast and sail in leeward hull would be lighter and beams requiring less strength. Compared to just a float to lee. The logic just doesn't make sense.
Todd
--- In harryproa@yahoogroups.com.au, Mike Crawford <mcrawf@...> wrote:
>
> <<Its marginal if all is rigid and designed to function taking that into
> consideration>>
>
> That simply is not true.
>
> If you can quantify the forces to demonstrate that they are marginal,
> you might be able to make a convincing point. If you can't quantify the
> forces, then you don't have enough information to say whether or not
> they are marginal.
>
> For the sake of discussion, let's do a sample calculation with rough
> numbers.
>
> Say you're on a Visionarry with 774 square feet of sail area, in a
> 20-knot wind. The Sail Wind Load calculator at sailingusa.info (
> http://www.sailingusa.info/cal_wind_load.htm ) calculates the load at
> 1300 lbs. Assuming the center of effort on the rig is 1/3 of the way up
> from the attachment to the beam, say 18 feet, that's 23,400 lb-ft of
> moment that the boat needs to resist pitchpoling. The actual force is
> several times that, because you have to design for: a) gusts, b)
> plowing the bows into a wave, and/or c) plowing the bows into a wave
> during a gust. But let's just assume 23,400 lb-ft of moment.
>
> If the rig is in the lee hull, the force is transferred directly to
> the bows through the hull, leaving the beams largely out of the
> equation. The hull takes the load. The beams do have to deal with the
> righting moment, but that's a separate calculation, one which has
> already been diagrammed, discussed, and largely agreed-upon.
>
> If the rig is in the windward hull, that 23,400 lb-ft of moment needs
> to be translated through the beams into the leeward hull so that it can
> resist that pitchpoling moment. That means torque going through each
> beam. Personally, I'd say that 23,000+ lb-ft of torsion is not a
> marginal load.
>
> If you've ever done a torsion calculation in a square box beam, you'll
> know how difficult the forces are to deal with. If you haven't, you
> might not have the background to decide what's marginal or not.
>
> That said, my training was in civil engineering, and I don't know
> enough to design boats.
>
> I'm happy for anyone to correct me with a better assessment of the
> forces at play. Not a flip "that's not significant" statement, but an
> assessment involving actual forces and numbers.
>
> - Mike
>
>
> On 11/21/2010 8:38 PM, tsstproa wrote:
> >
> >
> > Built in safety factor beam still sees less force than weight dominant
> > windward hull.
> >
> > Referring to a different force. Aw I see that too, Its marginal if all
> > is rigid and designed to function taking that into consideration, Lee
> > float skips across waters surface very little torsional load its a
> > float not a hull.
> >
> > Todd
> >
>