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  1. #31
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    Quote Originally Posted by davlafont View Post
    I guess what I'm looking for is how to relate this 10kg measurement to the amount of head round I've got in semi finished form. I have 50mm of round to play with. I also expect that under brutal downhaul conditons, I'll be apply more force than the 10kg metric. I have left my sail unfinished specifically for this reason. I'm also at a decision point with the spars. If needed, I can laminate more lumber, or I can proceed with tapering and rounding which will certaily increase the amount of flex over the current raw form.

    All thoughts are welcome. I'll be spending the weekend cleeaning epoxy drips and taking care of the house's yard.
    People seem to have got good sail shape with a range of yard stiffnesses.

    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.

    Ian

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  3. #32
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    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!
    Dave
    StorerBoat Builder, Sailor, Enthusiast
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  4. #33
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    Quote Originally Posted by davlafont View Post
    So my take aways are: 1) Simon thinks 6+ lbs is heavy for a yard? or heavier than expected for a hollow spar? 2) Simon's sailmaker wanted more flex than 20mm. Boy I've got THAT.
    If you look in the Wiki Woodeneye's aluminum yard is the only one heavier than mine.
    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.
    Simon
    My building and messing about blog:
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  5. #34
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    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...
    Dave
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  6. #35
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    Quote Originally Posted by davlafont View Post
    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...
    Is that bend value (low to high 40's) for the still square and untapered spar? Rounding a spar from square reduces it's stiffness to about 60% of previous. I'd expect a spar that bent 40 mm when square to bend about 65 mm when rounded.

    On the other hand, tapering the ends according to Mik's specifications won't change the bend value that much - perhaps 10% or less.

    Ian

  7. #36
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    Quote Originally Posted by IanHowick View Post
    Is that bend value (low to high 40's) for the still square and untapered spar? Rounding a spar from square reduces it's stiffness to about 60% of previous. I'd expect a spar that bent 40 mm when square to bend about 65 mm when rounded.

    On the other hand, tapering the ends according to Mik's specifications won't change the bend value that much - perhaps 10% or less.

    Ian
    Correct, that's for the raw lumber. My plan is to keep measuring as I progress. Next I'll do the taper and if you're correct and I see very little change, then I'll round the edges and measure. If I reach 50mm, I'll stop. If not, I'll go eight-sided, 16, etc.
    Dave
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  8. #37
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    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.

    Ian

  9. #38
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  10. #39
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    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 ended up with 64mm, 58mm, 64mm, 62mm at the midpoint of the four sides respectively. And that's only tapered, not rounded at all.

    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...
    Dave
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  11. #40
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  12. #41
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    Quote Originally Posted by davlafont View Post
    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 ended up with 64mm, 58mm, 64mm, 62mm at the midpoint of the four sides respectively. And that's only tapered, not rounded at all.
    Mmm, well that's a fairly big variation given the spar isn't rounded yet? Is that right Dave, you haven't yet rounded it? If so, I'd leave well alone and not round it. That way you have a spare.

  13. #42
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    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.
    Dave
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  14. #43
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    Quote Originally Posted by davlafont View Post

    And just as Ian predicted...

    I ended up with 64mm, 58mm, 64mm, 62mm at the midpoint of the four sides respectively. And that's only tapered, not rounded at all.
    Hmm... I predicted (more of a guess) that tapering wouldn't effect spar bend more than 10%, your tapering has increased the bent much more. Did you take any shavings off the middle part of the spar at all? A millimetre off each side takes a full 20% off the stiffness at these diameters.

    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.

    Ian

  15. #44
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    Default Quickie update

    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...
    Dave
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  16. #45
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    Quote Originally Posted by IanHowick View Post
    Hmm... I predicted (more of a guess) that tapering wouldn't effect spar bend more than 10%, your tapering has increased the bent much more. Did you take any shavings off the middle part of the spar at all? A millimetre off each side takes a full 20% off the stiffness at these diameters.

    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.

    Ian
    Keep in mind that the latest figures I provided were with more weight: 15.875 kg vs. 10 kg.

    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!
    Dave
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