sail area for a new design

[DCockey]

Originally Posted by Perm Stress

It has to do that without extremes in design like 49-er, hiking or trapezing do not cahnge righting moment so much as to render WL beam factor out of the dominant position.
that is about selection of sail area, which is closely related to stability of boat.

Take for example a simple plywood skiff, 14 ft LOA, 250 lbs rigged but without occupants. Put two 200 lb people sitting on the rail. They are the dominant contributor to maximum righting moment. Beam does matter in that case because if affects how far out they can fit.

As to pressure, we either:
a) know the exact righting moment to be developed, and appply normal engineering routine with safety factors, or
b) go with old rules of thumb, developed for boats with "normal" proportions; they already (indirectly) contain stability, sail loads, as result of stability and sail type, safety factor for dynamics, for imperfection and deterioration of materials.

Mixing of those two approaches is not so productive, because nobody ever knows where to draw a line and say "strong enough".

Actually, I find mixing the two approaches to generally be very productive. If they agree then it improves confidence. If they don't then further investigation may be worthwhile to determine if the engineering anaylysis is not sufficient, or if the "rule of thumb" is not applicable, or both.

Any suggestions on a "rule of thumb" to use in determining mast scantlings for small boats? (SA/(LWL*BWL)) isn't directly relevant.

The righting moment approach could be used with the simple approximations of the center of buoyancy located a half-beam from center, the CG of the boat on center, and without a trapeze or sliding seat the occupant weight a half-beam on the other side of center.
Max righting moment = (weight of boat / 2 + weight of occupants) x max beam
This would generally be a conservative over-estimate without a trapeze, etc nor a ballasted keel. Then for an unstayed rig the mast can be sized based on that moment applied at the deck/thwart/partners. If the rig is stayed the standard textbook type analysis can be done. In either case the sail area itself isn't needed, though with rigging the height of the "Center of Effort" may be needed to distribute the load.

[Petros]

TimothyM,

Ignore the "experts" and just go build it, what you have in terms of size of sail and mast will work out just fine.

Boat building is as much guess work (based on the practical experience of similar designs) as it is an analytica design process. No one can perdict the maxium loads your particular boat will ever see, all we can is is guestimate on any of it.

But as one of my engineering professors used to say: "one simple test is worth a 1000 expert opinions".

Go build it and have fun with it. IF something breaks, build a stronger one to replace it.

[TimothyM]

Petros
Thank you. that's my plan. I'm not reinventing anything here. I've based everything off of other boats. I just wanted to make sure I was in the ball park. I didn't want to be way under canvased. As far as the spars go, I'll build them on the safe side. I'll post pictures of the build later this winter.

[Perm Stress]

Originally Posted by DCockey

Take for example a simple plywood skiff, 14 ft LOA, 250 lbs rigged but without occupants. Put two 200 lb people sitting on the rail. They are the dominant contributor to maximum righting moment. Beam does matter in that case because if affects how far out they can fit.



Actually, I find mixing the two approaches to generally be very productive. If they agree then it improves confidence. If they don't then further investigation may be worthwhile to determine if the engineering anaylysis is not sufficient, or if the "rule of thumb" is not applicable, or both.

Any suggestions on a "rule of thumb" to use in determining mast scantlings for small boats? (SA/(LWL*BWL)) isn't directly relevant.

The righting moment approach could be used with the simple approximations of the center of buoyancy located a half-beam from center, the CG of the boat on center, and without a trapeze or sliding seat the occupant weight a half-beam on the other side of center.
Max righting moment = (weight of boat / 2 + weight of occupants) x max beam
This would generally be a conservative over-estimate without a trapeze, etc nor a ballasted keel. Then for an unstayed rig the mast can be sized based on that moment applied at the deck/thwart/partners. If the rig is stayed the standard textbook type analysis can be done. In either case the sail area itself isn't needed, though with rigging the height of the "Center of Effort" may be needed to distribute the load.

Sorry, did mix a two points a bit (sail area selection and sizing of rig).

Take for example a simple plywood skiff,.....
Yes it perfectly good approach to get good figures for further analysis or to add a healthy safety factor and build for them.

Actually, I find mixing the two approaches to generally be very productive......

I should have written "not very accurate". By writhing "not very productive" I had in mind "the result of such mixing is not accurate enough to engineer with stress 95 % of permissible , so further analysis is required"

[Perm Stress]

Originally Posted by Petros

TimothyM,

Ignore the "experts" and just go build it, what you have in terms of size of sail and mast will work out just fine.

Boat building is as much guess work (based on the practical experience of similar designs) as it is an analytica design process. No one can perdict the maxium loads your particular boat will ever see, all we can is is guestimate on any of it.

But as one of my engineering professors used to say: "one simple test is worth a 1000 expert opinions".

Go build it and have fun with it. IF something breaks, build a stronger one to replace it.

You are correct on "go on and build".
Many the "experts" started this way, before becoming too lazy -calculations do not ask for so much sweat as actual building .