Thread: Another UK GIS (Gizzle in Hants)

My apologies, missed that when my computer was falling to bits.

Seems slightly odd though to make it more flexible? Also 3/4 of the tow was wasted on the sides and bottom of the spar. That's 60 metres of the 80m.

Carbon tubes do seem to need to bend a bit before they stiffen up, so perhaps "not loading up" until there is already too much bend was the issue.

Hi

The boom flex data is also available on the WIKI (link provided in my last post). If you have made the boom to the same dimensions as the plans, but but left it square, it will likely still be too flexible for the loose footed sail. It needs to be stiff for loose footed.

Based on my experience with encasing the entire yard with carbon tow laid longitudinally (Made no difference to the flex at all), you will need several layers of UD carbon tape to stiffen your yard. As Ian mentions, "the percentage elongation on the surface when it is bending only 40mm is too small to load up the carbon fibres". https://www.woodworkforums.com/f169/b...ml#post1213826

I think the best bet is to make one from some stiffer timber (close grained Douglas Fir if you can get it) and depending on what quality you can get, maybe increase the dims by 5mm. You can always plane it down some more. The other option is to go with a 38-40mm aluminium tube. Mine has a 3mm wall which is heavier than I'd like, but it was all I could get. I'm hankering for a carbon yard when the finances permit. You may as well check out the carbon tube suppliers that Brian has mentioned as they seem to be a lot cheaper than we can get.

Aluminium tube is what all the Scows use for mast, boom and used to be for the yard as well until John ran out of unwanted alloy windsurfer masts.

Ally tubes can be bought very cheaply, and delivered anywhere in the UK by

Aluminium Round Tube | Aluminium Tubing | Aluminium Warehouse

I am sure wood will look nicest but simple alloy spars can work very well for lug rigs. You can always paint them cream. Scows use 50mm for mast and 50mm for boom.

Brian

My feeling, based on some quick calculations (which may or may not take things into account correctly, but are likely to err on the side of underestimating the amount of carbon needed to produce a given gain in stiffness) is that you would need at least 1 mm thick of carbon on both sides of a 40 mm or so dia wooden spar to double its stiffness. 1mm thick is I think 3 - 4 layers of carbon tape on each side, so quite a lot.

I agree with woodeneye, I think you're better trying to get some stiffer timber and go up a bit in diameter, all that carbon will be more expensive and more work to put on than finding another bit of timber. (Though I'm keen to hear from anyone who can report measured success beefing up a spar this way)

Increasing diameter from 40 to 45 mm will increase stiffness by about 60%.
Leaving a spar square also keeps it a lot stiffer, a square section is 70% stiffer that the same section after it is rounded.

Based on your measurements, either your Doug Fir is very flexible or you were a bit too enthusiastic with the plane when shaping it. A couple of things to measure to determine which is the situation:

An accurate circumference measurement at the largest point of the yard you have shaped. Use a dressmakers tape measure or wrap a piece of paper around it, make a mark then measure with a ruler.

Can you give us some data on your unshaped (square section?) spar blank:

Exact dimensions: length , width, height, weight

The same deflection measurement with 10kg hung off the middle. Measure the exact span between the supports at either end when the bucket is hanging off the middle.

Report back with that data and we should be able to figure out what is going on.

All the best,

Ian

Thanks for your help with this Ian!

Nice to have someone else around who can do some ballpark calcs!

MIK

Thanks all, I can check the dimension on the second piece of DF and post.

The comments made about how much CF would need to be bonded around the yard to stiffen it up are more than I would like and I don’t think it is worth the effort, cost and additional weight.

I think I will have another go at it with the DF that was going to be the boom, remove less material and keep an eye of flex as I go, it is possible that I removed a little to much material on the yard but I will have a measure up to confirm.

If all fails I may start looking at the CF tube option as this would look pretty cool.

Thanks again all.

I have often wondered how it would work only part rounding the yard.

For example the face that would be the front is rounded and the back left square.

Bethwaite's research into the wing masts on NS14s and Tasers would indicate that a rounded front and a squared off back is pretty efficient if it can be made to point in the right direction when sailing.

The fear would be that it would flip round the wrong way or the sail could end up migrating around to the side of the spar - but maybe with careful placement of the lashing holes at each end and careful restriction of the block for the halyard it might work.

If it worked as a square blank concept it might be worth looking at making it deeper and narrower - this could lead to some over-rotation being possible. This is where the tip of the yard or mast flips up to windward under leach pressure, effectively reducing the sail twist.

The idea is that the yard ends up being too deep to bend fore and aft much, but when you rotate the yard in line with the wind direction more it suddenly starts bending up to windward.

I don't think we need to go quite this far



Might be as crappy as my inverted L shape lug boom concept!

MIK

Originally Posted by IanHowick

Can you give us some data on your unshaped (square section?) spar blank:

....

Report back with that data and we should be able to figure out what is going on.

Hi Ian,
The spar blank is 40mm square

Supported at each end there is a deflection of approx 4-5mm under its own weight, with a 10KG bucket a further 21mm of deflection can be measured so a total of around 25-26mm.

Note that my measurements on the "bendy yard" did not include any deflection from its own weight, just from adding the 10KG.

To my untrained eye it seems that this DF may be a little more flexible than we would like.

I'm not planning to do anything further on the rig this weekend while I consider the options. The dagger board is sitting on the bench ready to be planned to shape and some of the bulkheads are marked out and ready to cut from the ply sheets so I have plenty to be getting on with.

Originally Posted by Gizzle

Hi Ian,
The spar blank is 40mm square

Supported at each end there is a deflection of approx 4-5mm under its own weight, with a 10KG bucket a further 21mm of deflection can be measured so a total of around 25-26mm.

Note that my measurements on the "bendy yard" did not include any deflection from its own weight, just from adding the 10KG.

To my untrained eye it seems that this DF may be a little more flexible than we would like.

I'm not planning to do anything further on the rig this weekend while I consider the options. The dagger board is sitting on the bench ready to be planned to shape and some of the bulkheads are marked out and ready to cut from the ply sheets so I have plenty to be getting on with.

I'll have a look at the numbers when I get the chance, can you tell me the distance between the two end supports when you were doing the deflection measurement?

Ian

Originally Posted by Gizzle

Hi Ian,
The spar blank is 40mm square

Supported at each end there is a deflection of approx 4-5mm under its own weight, with a 10KG bucket a further 21mm of deflection can be measured so a total of around 25-26mm.

Mmm, it does appear very bendy and you haven't even started to shape it yet. It's looking like this will end up bending around 45-50mm when shaped, but Ian will confirm. This is too much in my experience. Are you able to laminate some 5mm strips to the blank to make it 50mm as a starting point?

Good suggestion.

Originally Posted by IanHowick

I'll have a look at the numbers when I get the chance, can you tell me the distance between the two end supports when you were doing the deflection measurement?

Ian

Hi Ian,
The distance is the standard yard length (3.6m)

George

Originally Posted by woodeneye

Mmm, it does appear very bendy and you haven't even started to shape it yet. It's looking like this will end up bending around 45-50mm when shaped, but Ian will confirm. This is too much in my experience. Are you able to laminate some 5mm strips to the blank to make it 50mm as a starting point?

Hi Woodeneye,
I still need to check the measuements of the bendy spar but I think it may bend more than you suggest. Good idea to make it 50mm as a starting point.

I'm going to make a few calls first to get an idea of cost, size and flexibility of CF spars.

cheers.

Ian in NZ has a specification worked out for the carbon spars - have a look at his thread.

Quick details are here
http://www.storerboatplans.com/wp/de...r-google-docs/

Though I think I have left out the mast wall thickness.

MIK