Thread: GIS Build in Queenstown New Zealand

Originally Posted by callsign222

Didn't your Mom tell you to keep your fingers out of doorjambs and other tight places? Jeezo, Bruce! Careful, but better your thumb than your noggin'.

Here's a pic of 4 sudden stops, and corresponding daggerboard damage. Cherry trailing stem up against cherry spacer in the CB case. A strip of dense yet yielding foam on the aft spacer is ideal. I can only imagine if I brought the daggerboard down to 1/16th of an inch.... (impact 1 and 3 are temporarily filled with epoxy, 2 and 4 still too fresh, the wood hasn't had a chance to dry in the past week...)

My rudder, however, still looks brand new!

That is pretty minimal damage for the screeching stops you have managed!

I think the Cherry may have been a good choice. It is sometimes difficult to find a timber that is hard enough without being too brittle. Like Kapur is really strong and heavy but it kinda shatters leaving quite a bit repair to do ... when you get round to it.

And something too soft will tend to get a bit more crushing too.

MIK

It was available for free, in the right size, and it was good quality. I used it for my skids too, on the bottom of the boat.

Cherry is somewhat rot resistant too, isn't it?

I don't know. I haven't seen much in my boatbuilding life.

The skids are a different kettle of fish. They are quite big cross section compared to that relatively fine trailing edge of the centreboard. Also the type of load will be usually be more a sliding one.

I don't think timber durability is a big issue for boats we are building so carefully epoxied. Gaboon ply is not particularly durable, but tied in with this building method it is very reliable indeed.

Best wishes
MIK

Originally Posted by woodeneye

When the breeze is above 15knots, the boom bends horizontally as well, so again, I hope the tow will provide stiffness here too.

Most of the bend was caused by the mainsheet tension, not the downhaul, so my thoughts now are not to taper the boom at the ends at all. Looking at the recent RC44 series, I noticed the carbon booms are much like the old Finn style planks, so are vertically stiff all the way to the clew.
]

Here is a clip of the RC44. Note the boom and how stiff it is. Also note the hull shape of the RC44. Notice anything familiar?

[ame=http://www.youtube.com/watch?v=QC4TxCvjxVY]YouTube - RC44 - RC44 Valencia Cup 2010[/ame]

Originally Posted by woodeneye

The boom was made of stock sized 40X10mm radiata pine. So it finished at 60x40. The sides were clear stock but the top and bottom caps were finger jointed. It's tapered down to 45mm at the ends. Spacers are paulownia with the front of the boom having a continuous spacer of about 800mm (from memory) to stiffen the downhaul area.

With 10kg, the deflection is 15mm. At the time I thought this was relatively stiff, but it's not nearly stiff enough. I'm hoping the tow will bring this down to 5mm or less, but I'm not sure if this is just wishful thinking.

When the breeze is above 15knots, the boom bends horizontally as well, so again, I hope the tow will provide stiffness here too.

Most of the bend was caused by the mainsheet tension, not the downhaul, so my thoughts now are not to taper the boom at the ends at all. Looking at the recent RC44 series, I noticed the carbon booms are much like the old Finn style planks, so are vertically stiff all the way to the clew.

I ran the numbers through on your boom section, calculated deflection with 10kg weight and span of 3.5 metres matched your measured deflection of 15mm (using a ballpark E (elastic modulus) for the pine of 9.0GPa).

Running the numbers through again for the same section on it's side, the sideways stiffness of your boom would be half it's vertical stiffness.

The section I drew above https://www.woodworkforums.com/attach...ss-section.pdf
comes out at about twice the vertical stiffness for the same weight if it has 70mm x 50mm outside dimensions, 4mm ply sides with top and bottom of 8mm thick Paulownia capped with 6mm Vic Ash. Haven't checked what it's sideways stiffness is yet.

How much more sideways stiffness do you think you need?

The main point for you adding carbon tow to increase stiffness is that if you want to increase sideways stiffness as well as vertical stiffness, make sure you have a reasonable proportion of the tow towards the outside edges of the top and bottom of the boom. A thin strip of tow down the middle of the top and bottom of the boom won't increase sideways stiffness much at all.

Ian

Howdy, The excellent question to always ask is Ian's ... just how much stiffness do you need?

It's important to keep the idea of minimising the maximum as much as possible to keep it simple and light. Otherwise you end up like the plywood kayaks with glass sheathing inside and out when we all know that for most use glass tape is enough.

I think tow much narrower than 25mm (1" wide) will not make enough difference. So it will have some effect on the side bend as well. I suggest starting with top and bottom using one inch wide tow and seeing how it goes.

One thing you have to remember with the RC44 boom is the amount of vang/kicker they will be cranking into the rig. This requires the boom to be fabulously stiff in the vertical plane. The sideways forces are also distributed by the mainsheet system along the length of the boom rather than operating at a small distance from the mast like the vang.

I have already set up the temporary WIKI to include the carbon tow data and the box boom data. When it is available.
WIKI for setting up and tuning Lug and Sprit Rigs#|#Michael Storer Wooden Boat Plans

MIK

Hi Ian and MIK

Something has happened to your pdf link Ian, and I could not access it.

The sideways stiffness is not all that bad, but as my sail is loose footed, sideways bend is as bad as vertical bend, so I want to eliminate it as far as possible. Having said that, this is a bit of an experiment as if I can’t get the stiffness into the yard and boom at a cheap cost in terms of weight, I’ll go the aluminium or carbon tube route.

As for the carbon tow, I could not get any wider than 6mm, so I will need to lay down extra lengths, that’s all. I bought 120metres of it so no problem there. I’ll take your suggestion to apply the tow to the boom edges, rather than the middle. I’ll be doing the work on it this w/e. However, I may run out of time as we have an air show this w/e in Newcastle and I’d love to go check out the planes, especially the Red Bull race planes. Matt Hall is a local, and ex-RAAF pilot.

I’ll also keep the measurements for the WIKI in mind.

Ian, it will be interesting to see if your boom with ply sides turns out as stiff as your calculations suggest. Bob (Sacramento) made his boom with ply sides and it was more flexi than mine. It’s logical given that two of the plies run in the vertical plane, but maybe you’ve allowed for this in your calcs already?

Those RC44s are really serious kit. The booms are just amazingly stiff for the small section that they are. Other boats of this size most often have much larger sections and are usually round or oval. Did you notice the hull shape at all MIK?

Originally Posted by woodeneye

Hi Ian and MIK

Something has happened to your pdf link Ian, and I could not access it.

Ian, it will be interesting to see if your boom with ply sides turns out as stiff as your calculations suggest. Bob (Sacramento) made his boom with ply sides and it was more flexi than mine. It’s logical given that two of the plies run in the vertical plane, but maybe you’ve allowed for this in your calcs already?

Hi there, I've fixed the link to the PDF.

Ply sides: Yes, I took account of the plys, I figured it for 4mm 3 ply so worked on the basis that only two of the three plys were doing any work. Most of the stiffness of the section I drew comes from the Vic Ash top and bottom, not from the sides. (Though the sides are needed to hold top and bottom apart) My worry regarding the ply sides is more around whether they reduce the sideways stiffness too much.

I'll run the numbers through again as I did them in a bit of a hurry, and also check sideways stiffness but they don't look implausible to me as my section is about 20% bigger than yours in outside dimensions.

I think Bob in Sacramento used WRC for top and bottom of his ply sided boom, I think the lighter timber had a greater influence on the lower vertical stiffness than the ply sides. I'll try and find exact dimensions of his boom and check if the assumptions I'm making to calculate stiffness hold for his boom.

Ian

Originally Posted by woodeneye

Hi Ian and MIK

Did you notice the hull shape at all MIK?

Did you mean the kinda boxyness and plumb stem?

Yes.

The boxyness is kinda unavoidable - the boats follow the small beam tendency of the America's Cup boats. Because the crew doesn't hike out on the America's Cup boats there is little value in having a lot of beam.

They have found that the AC boats are faster with a minimal beam - upwind and down with minimum drag.

My understanding is that the boats are faster upwind because of a reduction of wave impact drag. At least that is the story of some of the OZ racing dinghies - lots of flare was a lot slower in rough water.

The place where beam might make a difference is reaching where the larger form stability - stability from the hull shape - is not included in many modern race courses.

So making the assumption of keeping the beam down and the light displacement, there ends up being quite a boxy hull.

Which I rather like!

MIK

Hi Ian, would you be able to add some dims to your box boom section drawing? That way once we know what the stiffness is like, it will be more useful info for the WIKI that people can use.

By the way, are you using spacers in this boom?

Hi MIK, I have quite enjoyed watching the RC44 series on TV and on YouTube. For the size of these boats, the performance is quite dinghy-like and the result is some quite exciting racing.

Originally Posted by woodeneye

Hi Ian, would you be able to add some dims to your box boom section drawing? That way once we know what the stiffness is like, it will be more useful info for the WIKI that people can use.

By the way, are you using spacers in this boom?

I'm thinking in terms of overall dimensions of 50 x 70 mm. Sides of 4mm gaboon ply. Top and bottom of a laminate of paulownia and Vic Ash with the Vic Ash on the outside. Vic Ash say 6mm, Paulownia 8mm thick. The Paulownia is there basically just to provide gluing area, and enable the thin higher density timber to be where it needs to be for maximum stiffness.

Calculated stiffness twice yours in the vertical direction, but only 20% more than yours in the horizontal direction, so it should only bend 8mm with 10kg on top of it.

Stiffness calcs based on E of 14.0GPa for Vic Ash and 4.0GPa for the Paulownia and ply sides. I think that low value for the ply GPa should take account of only two of the three plys running along the beam.

I ran the same calcs on the dimensions of your radiata pine boom and came out with a deflection of 16mm for 10kg weight in the middle, so very good agreement with your measurement there.

I did the same for what I think are the dimensions of Bobwes's boom, (6mm ply sides, WRC top and bottom, overall dimensions of 45 x 70mm), it should be exactly the same stiffness as your boom, but his bent a little more than yours I think. I'm not sure how thick the WRC on the top and bottom of his boom ended up, or how enthusiastic he was with the plane after he'd glued it up.

Main source of inaccuracies in these calculations are how accurate the dimensions are, vertical stiffness goes with the cube of that dimension, each extra mm off the top and bottom with the plane makes a difference of about 10% to the stiffness.

Conversely, round timber spars seem to come out stiffer than you would calculate as you round them till they look round, diagonal diameters are probably a millimetre or two bigger than vertical/horizontal diameters in most cases (not a bad thing). I'd be curious to get some circumference measurements at the same time as diameters when people are reporting dimensions of round section spars to see if this is true.

Other factor is of course the timber modulus (E) which is a ballpark average value off the internet, actual values will vary a bit. Especially with something like WRC, I suspect the reported values were measured some time ago on good pieces of old growth timber so actual values now days are likely to be a fair bit less.

I'll report exact dimensions and stiffness (deflection) measurement for my boom compared to these calculations once I've made it. I guess I'll put a couple of light Paulownia spacers in it, possibly some harder blocks once I figure what fittings I need and how/where to attach them.

Carbon mast and yard should turn up sometime later the coming week, I'll report some data on how stiff the yard when I have it.

Ian

Originally Posted by IanHowick

I'm thinking in terms of overall dimensions of 50 x 70 mm. Sides of 4mm gaboon ply. Top and bottom of a laminate of paulownia and Vic Ash with the Vic Ash on the outside. Vic Ash say 6mm, Paulownia 8mm thick. The Paulownia is there basically just to provide gluing area, and enable the thin higher density timber to be where it needs to be for maximum stiffness.

Calculated stiffness twice yours in the vertical direction, but only 20% more than yours in the horizontal direction, so it should only bend 8mm with 10kg on top of it.

Stiffness calcs based on E of 14.0GPa for Vic Ash and 4.0GPa for the Paulownia and ply sides. I think that low value for the ply GPa should take account of only two of the three plys running along the beam.

I ran the same calcs on the dimensions of your radiata pine boom and came out with a deflection of 16mm for 10kg weight in the middle, so very good agreement with your measurement there.

I did the same for what I think are the dimensions of Bobwes's boom, (6mm ply sides, WRC top and bottom, overall dimensions of 45 x 70mm), it should be exactly the same stiffness as your boom, but his bent a little more than yours I think. I'm not sure how thick the WRC on the top and bottom of his boom ended up, or how enthusiastic he was with the plane after he'd glued it up.


Ian

Hi Ian,

Just FYI;
my boom ( with loose-footed sail ) has a 36 x 72 mm square midsection with diam 12mm rounded corners, tapered towards 36 x 55 to the front and 36 x 45 to the back ( kept the max height more to the front part, where the downhaul is attached ).
It is made by a massive laminate of 2 pieces of 18mm European fir ( around 0,6-ies weight pine ) .
It works well and appears to be stiff enough to handle lots of downhaul tension and does not show any sign of excessive bend in any direction. I am not shure if it is still bullet-proof with 3 grown-ups hiking in force 6-7, but it works well in force 5 with and without reef single handed ( 90+kg hiking and waves).

Be aware there is much more up-and-down tension in a GIS rig, but not that much sideways if you compare to a triangular main.

I am considering a box-boom to save weight, but with similar dimensions. 4 or 6 mm ply sides with 12mm pine top and bottum.

Howdy, please feel free to put any sizes you are going to try out in the WIKI too. Just mark as "unbuilt". MIK

My carbon mast and yard have arrived. Sorry I don't have pics yet, my wife is out and she has the camera.

It looks like C-Tech have done a good job, and put quite a bit of extra reinforcement in at all the stress points. The overall wall specs are also up a bit above what I reported above, so it should be completely bombproof.

Mast:
76m ID mandrel, tapering to 45mm ID at the top. Wall thickness also tapering: Bottom meter 2.4mm wall, then 2.0mm wall for the next meter or so, then 1.5mm wall to the tip. Extra sleeves on the inside at the base and partner height. Calculated stiffness at least 30% above the hollow wooden box mast throughout, so it should be bombproof enough to have all my (4) children hiking out on the rail on a screaming reach in a gale when they're teenagers.

Weight - about 3kg from the bathroom scales. Can't say more accurately than that at this moment, but I'll try and get some more accurate scales to check.

Yard:
38mm ID, tapering to 20mm ID at the tip. 1.8mm wall with some reinforcement at either end and the middle where the halyard attaches.

Weight: 1.15kg

Stiffness: 10kg hanging off the middle gives a 20mm deflection. Very close to what I'd calculated/was aiming for, and about twice the stiffness of the wooden yards people have reported.

I need to make some more progress with the wooden parts now, though it's snowing outside as I write!

Ian