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Thread: Goat Island Skiff
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6th October 2008, 05:51 AM #136Senior Member
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Clint,
Are you planning on a round birdsmouth mast, or a hollow rectangular mast?
We built the solid round mast for our GIS. I wish we'd gone hollow.
For the OZ PDR balanced lug setup I completed recently, we did a combination of types.
Boom = solid, tapered, rounded over square
Yard = solid, tapered, round
Mast = tapered, hollow, square
Everything but a birdsmouth spar. I've built a number of birdsmouth sticks in my career. From a professional woodworkers standpoint - they're fun. Nifty and slick. Aesthetically the final product is very pleasing.
However, I have to say that the hollow rectangular spar - even with a taper - is far simpler to build. Both from a joinery and an assembly standpoint. easier to sand, too. I have a new mast on my "someday" list. It will be hollow. Given that I already have the round mast step & partner, maybe it'll be birdsmouth, but it might still be simpler to modify the step & partner and build a square hollow mast.
"Honor isn't about making all the right choices. It's about dealing with the consequences"
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6th October 2008, 06:46 AM #137Intermediate Member
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Yes, done some sailing, and racing too, Pirat and Hobie 16. Sometimes quite sucessful.
But my concern about topweight is from some years of teaching children to sail with gaff boats. Hoisting a spar or not is clearly a matter of the weight for the kids. Also is righting after a capsize more easy with less topweight.
Fiddly: thats just ok, sometimes I do some violinmaking.
Measurements could come from here - http://www.duckworksmagazine.com/08/...outh/index.htm - so for a 8 sided yard stave thickness is 6 mm, stave width 17.4 mm, tapered to 12.4 mm and 16.1 mm. Manageable.
For the mast stave width would be 36 mm. tapered to 20.7 mm, thickness 12 mm.
With 4730 mm length maybe bit fiddly as well.
And hadn't had Herreshoff preferred the rectangular masts for raceboats for some reason?
Jörn
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6th October 2008, 10:15 AM #138
Howdy Jörn,
With the Birdsmouth spars the diameter needs to go up slightly to retain the stiffness of the spar.
With the mast there is probably no need to increase diameter at all because it is quite stiff.
However the yard and boom are almost too flexible now. So their diameter will have to increase a little to prevent excessive bending.
If you were planning to race the Goat and use some of Keyhavenpotter's rigging to get a more effective control of twist then the yard and boom might have to be slightly bigger again Or maybe as the increased force wil be vertical only two staves would need to be increased in width with the others unchanged to make an oval yard and boom (or boom could be rectangular with rounded corners)
Best wishes
Michael.
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6th October 2008, 12:31 PM #139SENIOR MEMBER
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Spars
Ardordg I do intend to do a B'mouth mast. For the yard I'd like to go as light as possible so will look into doing a B'mouth but ovalize it, which is what I did on my current sailing dinghy and it seemed to work. I have a formula I'vebeen following to do B'mouth. I will post it at some point and maybe discuss...there is a great article on B'mouth technique in Duckworks that Jorn refers to above and when the time comes will look at this again. For the boom I like the idea of a tapered, squarish section...solid. I will be doing a mizzen for a yawl version and intend to use windsurfer spars, so I might use one of them for the lug's yard. It'll need to be affordable though, and as our economy crashes here I am more and more glad that I have chosen an affordable option for the next boat.
Cheers,
Clint
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6th October 2008, 04:16 PM #140Senior Member
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Clint,
I think hollow spars sounds like a great idea. Just keep in mind that Mik has the spars engineered to achieve a particular flex - which translates into proper sail shape when the downhaul is tensioned, and some spillage of wind in response to gusts. If you make it very far from his specs, I'd imagine that you'd needlessly defeat all his neat calculations/engineering. As I said, I really appreciate the weight aloft issue after sticking a solid fir mast into the GIS hull. I'd certainly recommend a hollow mast. A hollow yard will probably not gain you much in that department, but every bit helps (esp. up high like that) - as long as it doesn't end up to stiff, or too limp. Mik can comment better on any change in scantlings that might be required.
"An elephant: a mouse built to government specifications" -- Robert Heinlein
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6th October 2008, 11:13 PM #141Intermediate Member
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spars
Hi,
Calculated measurements give 43 mm yard and 88 mm mast, so I increased diameter of the yard by 7%.
But real stiffness depends much on the wood used, for example is the tensile strength of european grown oregon given ( http://www.holzwurm-page.de/holzarte...uglasie-us.htm ) with 82 N/mm², american (other suorce, forgot where) with 92 N/mm², and hemlock, which I use (actualy no affordable source for oregon/douglasie) with 68 N/mm², with bending resistence for american douglasie 88 N/mm², european douglasie/oregon 70-100 N/mm² and hemlock 80 N/mm².
So actual bending resistence of the actual spar is not really predictible.
Mik, could you give a figure of the bending resistence needed? Like AMO arrow spine measurements? Kind of - spar is supported at the ends, 10 Kg weight attached at position of the halyard / at middle of the boom - how much bending is fine?
By the way - why the spars are so much longer than upper/lower leeches of the sail? Just to be prepeared for bigger main ?
Just two comprehensive problems - page 26 - assembly rear or front faces of bulkheads with lines - and page 30 - seat stringer or surface tanktop at 365 mm down from the ply edge?
Greetings - Jörn
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7th October 2008, 11:29 PM #142
Howdy Joern,
I would have to remember calculus and then it would still be wrong without a very complex method taking all factors into account to get the deflection under a weight .. but you are looking at the right type of physical test.
The completely professional engineering way to allow for species of timber is to use its "Young's Modulus" (E). This is a measure of stiffness.
Most boat structures depend on stiffness rather than strength because most things can bend a long way before they break and we don't want them to bend that far!!!!
There are two parts to stiffness .. the material and the geometry.
Stiffness of Material
However the nice thing about timber is (more or less) the stiffness is proportional to density!!!
Actually if you take most common engineering materials and divide the stiffness by the density you end up with a constant value.
So density is a reasonable guide for stiffness from the material.
If the timber is twice as heavy it will be approximately twice as stiff too. The German site had a very high value for the density of Fir - maybe it was for wood that was still "green"
My reference gives ... just to semi prove the density stiffness you can work out the proportions .. roughly the fir is 20% stiffer. It will vary from piece to piece of course. But this value is close enough.
Material Density / = Kg/m³ / ‘E’ in N/mm2
Douglas Fir Canada / 545/ 12700
Hemlock Canada / 465 / 10400
Stiffness from the diameter.
I am not going to go into calculating stiffnesses from scratch, but for comparing squares to squares and circles to circles. Here we are comparing the original circular yard to a new circular yard of the same stiffness.
The stiffness of the circular cross section of the yard is proportional to diameter raised to the fourth power. Diameter ^4
We know the hemlock material is 20% less stiff so we know the finished yard geometry has to be a size that is 20% MORE stiff so its diameter needs to be increased
So (new diameter/old diameter)^4 = 1.2
1.2^0.25 = 1.046 times the original diameter.
Original diameter is 40mm
New diameter is 40 x 1.046 = 41.8mm
The diameter at all points along the yard (and boom) would have to be multiplied by 1.046
I would actually be betting the original mast diameter will be OK both with birdsmouth and with the lighter timber. I did design it for three men leaning out hard. So if you don't think you will do this often I think the mast could be the original diameter but now birdsmouthed AND out of the slightly lower density timber.
Best wishes
Michael
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8th October 2008, 05:11 AM #143Intermediate Member
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Thanks,
that's helpful. Thaught over increasing diameter, by guess I got 43 mm. 41.8 (=> 42 roughly) sounds better (thinner is easy).
Mast I do rectangularand with 83 mm, better would be 85.7 mm - to late. If it bends to much, I will add a layer of glass (unidirectional) on the outside. or build a new mast.
Tree men leaning out hard - have you rigged ankle straps to prevent falling them out?
Greetings - Jörn
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8th October 2008, 05:23 AM #144Intermediate Member
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Hi Clint
oval spars tend to bend more easy in the direction of the lesser diameter. With straight spars this sometimes causes unwanted turning of the spar, so arched spars like hollandse gaff's are useful.
Building oval and arched with birdsmouth should be no problem, but flats out the upper part of the sail.
So I will just increase the diameter a bit, this 2 mm should do no harm.
Greetings - Jörn
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8th October 2008, 01:38 PM #145SENIOR MEMBER
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Jorn, You are right about the oval spar being a trick to orient properly...it took a couple seasons to finally get my yard on my dinghy such that the oval section stayed oriented along the mast...it works well now.
Do you guys know where my Eastern White Spruce would fall in the stiffness range and therefore the diameter difference for yard and boom? The only stiffness rating I know is the one when it is glued up like a B'mouth spar" "Stiff as hell".
Cheers,
Clint
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8th October 2008, 05:08 PM #146
Sorry to bring up a more basic question, but I'm still working on my hull. After wrapping the sides around the bulkheads and planing down the chine logs, I've found that bulkheads 3 and 4 are riding a little high. They will end up 2 to 3 mm off the floor if I don't adjust something. A bit too much to just fill in with epoxy. What's the preferred fix here, to shave down the sides a bit or to fill in with some scrap wood to close the gap? TIA.
Our motto here at the Ten Thumbs Boatyard: "A good coat of paint and you'll never know."
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8th October 2008, 06:30 PM #147
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8th October 2008, 06:50 PM #148
[quote=paulie;820276]Sorry to bring up a more basic question, but I'm still working on my hull. After wrapping the sides around the bulkheads and planing down the chine logs, I've found that bulkheads 3 and 4 are riding a little high. They will end up 2 to 3 mm off the floor if I don't adjust something. A bit too much to just fill in with epoxy. What's the preferred fix here, to shave down the sides a bit or to fill in with some scrap wood to close the gap? TIA.
Howdy ... I Have had similar problems in building classes where one of hte bulkheads. Generally I am not too worried about 2 or 3mm ... the epoxy will fill that EASILY.
If you do a neat job no-one will ever notice it.
I imagine you have already planed down the chine logs and bulkheads to be flat ready to take the bottom, but the bulkheads are going to have that little gap. You can plane the sides down only so they are flush with the outside corners of the side panel ply (the inside edge is higher than this)
The problem with planing down the side more is that the boat is set up for the sides to be perfectly fair shapes and if you plane below the marked line then it will not be a fair (smooth) curve any more. This is quite a serious problem for the appearance of the boat.
Now obviously if the bottom goes in place with the bulkhead a bit high you will end up with a hollow in the bottom.
To prevent this I put some copper or bronze nails, or a few stainless screws across the bulkhead and adjust them so they match a straight edge that is held from chine to chine. They will act as spacers to prevent the hollow from forming.
Then when the bottom is ready to go on put some extra glue on those bulkhead cleats ... do all the other temporary screwing to hold the bottom on .... then gently push down on the bottom along those bulkhead lines until you can feel it touch down and put a few screws through the ply to maintain that height. If you overtighten the screws you will feel the bottom move in past that point ... so easy on the trigger finger!
Then when all done .. turn the boat upright and put it on two pieces of wood running transversely and clean up the inside using a stirring stick sharpened to a chisel point.
Looks good!
Best wishes
Michael
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9th October 2008, 05:14 AM #149SENIOR MEMBER
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Do you all remember putting on new sneakers as a kid and feeling like you could run much faster because of them? I think that is how I feel when I glue up my favorite wood, Spruce!
But it is a good reminder to beef things up when the time comes and maybe or maybe not ovalize the yard.
Paulie, My solution if the gap is bigger than 1/8" is to simply glue on a strip of wood, a little wider and a little thicker than necessary -- use some tape to hold the wood down until the glue cures. Then plane it down to be flush with everything you need it to be flush with and get a nice tight glue joint.
This reminds me of something I feel -- maybe like running faster in new shoes -- but I swear boats can be made much lighter b/c the glue joints are tight (not too tight!) and the fiberglass is not soaked! I'd love to say you can save a couple pounds in extra glue -- imagine if all the joints were gapped by 1/8"!
Cheers,
Clint
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9th October 2008, 06:55 AM #150
Clint,
Told you I was a lazy boatbuilder! Now you have evidence.
So... two scenarios for fixing the gaps.
Excellent!
MIK
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