Thread: GIS Build in New Jersey, USA
14th May 2011, 10:09 AM #31
Richard Harvey has a wooden yard that I think bends about 50 mm or more with 10 kg on it (anyway, standard as far as the plans go as far as I know), and very good sail shape from McNamara sails, though they had to re-cut for greater head edge round after seeing how much the yard bent.
Woodeneye went to an alu yard that bent much less (20 mm with 10 kg weight?) and that worked well with his sail.
So, bendier yard - more edge round, stiffer yard - less edge round.
How much? Stewart Dabbler works it out by hanging 'one third of the square foot area of the sail in pounds' off the middle of the yard, and measuring the deflection of the yard at that weight, using that amount of edge round along the head of the sail. 'one third of the square foot area of the sail in pounds' equals 35 pounds or 16 kg for the GIS sail.
My sailmaker agreed with this, I gave him bend data on my yard at 16 kg weight hanging off the middle, he cut the edge round based on this. He said the shape is in the broad seaming. I guess if you were mainly using edge round to establish shape in the upper part of the sail you'd have to add a bit more edge round than this.
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14th May 2011, 10:49 AM #32
Thanks Ian. I happened to print out the Dabbler method earlier today and was just headed into the garage to hang 35 lbs. I have a 5 gallon bucket I can use.
I have some broad seaming in the seam that hits closest to the halyard point and virtually none in the top most seam. The rest comes from edge round. Together, here's the shape as she stands now:
That's about 55mm or so of round (which is more than what I set as my design parameter). So maybe my somewhat bendy stick (32mm) will mate up well once I hang more weight. I'm looking forward to doing the Dabbler measurements.
And if I don't like these blanks, I can put them aside for use on... a future SON OF GOAT!
14th May 2011, 11:13 AM #33
Is that too much? Initial sailng two up in light winds says it's fine. But 6lb 15ft up in the air is a fair bit of moment so who knows. My guess is that solo in heavy air is when the extra weight hurts in keeping the boat flat.
I'll measure the round in my sail and yard bend soon so we get another data point. Won't know if my setup is good until I get into some stronger winds but I suspect it's not too far off as is.
14th May 2011, 11:42 AM #34
Well so far, so good.
Using the Dabbler method, I get in the low to high 40's, depending on which side is under tension. Once I've done some shaping, those figures might rise to the 50mm or so that The Designer said it would flex if made from Oregon and shaped to the tenth of a millimeter per his master plan.
That MIK is a clever one isn't he?
So I'm not ready to hide them in the shed yet. I shall be cautious with the shaping though...
15th May 2011, 04:34 AM #35
On the other hand, tapering the ends according to Mik's specifications won't change the bend value that much - perhaps 10% or less.
15th May 2011, 06:16 AM #36
15th May 2011, 09:02 AM #37
Interesting to see if my estimate of the change in stiffness/deflection following tapering is correct.
If you want to get more reliable data on stiffness, the best is to measure deflection at increments of weight added, say at 5kg, 10kg, 15kg, 20kg (but don't go high enough to break it!) Even better is data points in increments of 3 or 4 kg.
If I see all your data points on a nice straight line on a graph, I'll know I can be confident in them. One measurement and there is always the question of errors due to where the zero point you measured from was, was it affected by the spar being slightly bent to start with or how tight/straight the straight line you measured from was.
For instance, from above there are measurements quoted:
"douglas fir yard that deflects 20mm with a 10kg load and 37mm with 16kg"
There's a fair bit of difference in the stiffness I'll get depending which of these measurements I use, but with three or four points on a line, we should have a value we can be confident in.
17th May 2011, 10:59 AM #38
17th May 2011, 12:56 PM #39
I'll grant that Ian's methodology will provide for good analysis, but in the end I've got a boat to build...
So I picked up my plane and commenced to butchering.
And just as Ian predicted...
I'd expect a spar that bent 40 mm when square to bend about 65 mm when rounded.
I think I just made boomkin or a mizzen sprit. Or maybe a yard for a SOG? In any case, I'm not using this stick as my yard. I have another blank that is stiffer and heavier and I'l begin shaping that one soon. And then it'll be time to start making a new boom. Which I'm seriously considering a box section for...
17th May 2011, 06:39 PM #40
Good feedback there Dave. Ian will be happy!
25th May 2011, 01:22 PM #41
25th May 2011, 01:30 PM #42
Correct Bruce. I'm putting this one away for some future use. I might do the yawl version and this one could serve as the sprit boom or the boomkin.
I'm about to start shaving the other blank which was stiffer. If it bends too much as well, I'll assemble my spar as box sections. My mast staves are coming along nicely so I wouldn't mind making the spars that way too.
26th May 2011, 02:08 PM #43
Square to round drops stiffness by 60%/increases bend by 70%.
I think you could expect spar bend to double or triple as you go from an square section un-tapered blank to a round section tapered spar, so if you're shooting for a 50mm bend with 10 kg weight, the square section un-tapered blank shouldn't bend more than 20mm with 10 kg hung off the middle.
31st May 2011, 11:37 PM #44
I've tapered my second version of the yard and kept track of the bend measurements. Much as I believed, this one is stiffer (and heavier) and held up tot he shaping better. I'll radius the corners rather than rounding it off fully, partially because I don't want it to get too bendy and partially for the aesthetics. The mast will be square with radiused edges and I think matching spars will look good. Finally, I found another nice Doug Fir 2x8 (x10') that I think will yield a quality boom.
My mast is 75% constructed. Both narrow staves are shaped and connected by spacers. One wide stave is glued up.
I'll post pics and more commentary soon...
31st May 2011, 11:47 PM #45
Now that I've made some good progress on Yard 2, I'll go back and take better measurements of Yard 1.2. You may be right; I may have reduced the cross section too much. I've learned that in order to get a 40mm finished part, don't start by cutting the lumber to 40mm. I't much better to cut over size and work your way down to size. Let's see if I can remember that with future parts!
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