Thread: GIS spars

I am not in any way talking about stress induced twist, engineering issues. Just a simple falling over sideways issue when a plank is pulled from the top one way and pulled the opposite way from the bottom. It could be the stiffest thing on the planet, it would still fall over. On a yacht such booms are fixed at the mast with a gooseneck. On a lug no fixing to stop the plank falling over.

I am only trying to describe what I saw happen.

Brian

I meant to mention this earlier. you might need a point near the back of the boom for the reefing purchases unless you tie everything off to the same point.

If using pulleys or cleats you might need a pad, um about 300mm from the back end pad and another 400mm in front of that one.

Mark their position of the faces of the cedar in pencil and epoxy over.

If you were using 4mm ply on the faces I would probably think about a couple of spacers, but the 6mm will be fine.

MIK

For fixing the mainsheet blocks and downhaul you could make a few extra spacers and drill holes through them towards the bottum of the boom. Similar you could drill holes for lashing the tack and clew through the upper part of the boom.
Since the main forces are tack and clew ( both upward ) and downhaul ( downward ) you could expect the boom to stay more or less upright, even when the sheet-blocks are pulling a bit side-ways. Perhaps this works?

The side panels glued up nicely.

The timbers are coming along nicely, too. Have to use three sticks to get the length, so there are 2 scarf joins for the top and 2 for the bottom. I used the planes to make those cuts and I am pleased to report I did a marvelous job with them this time! Having the sawhorses as a solid base and paying more attention to what I was doing with the plane made for excellent results.

I realize this is my boom to do with as I please but since I don't know what I don't know I need to keep asking. I am still not certain about some of the details... Spacers are not necessary, but could be useful for securing hardware to the boom. What are the hardware possibilities (downhaul, kicker, mainsheet, outhaul (?), reef points(?), what else?) and where would they be placed on the boom. I need to review the rigging info.

I am having fun with this!

The reinforcing blocks don't need to be very big at all - particularly if you know where they are!!!

Certainly keep the corner lashing holes near the top when they get drilled is a good idea ... just keeping the same spacing between the hole and the top face as in the round boom will work.

You could put some blocks inside the boom so you can drill holes through the sides near the bottom edge for the sheeting positions.

I like nice fat 9mm holes or more for everything. This means the block inside the boom to take the hole only needs to be about 25mm or so square.

MIK

Okay, having read (again) the rigging info on Mik's site and Brian's posts about his scow's rigging, I think I have figured out where to put the spacers in the box boom. Thanks everyone for your patience...

I got lazy this evening and didn't get over to the "shed" to do any boat work. I did manage to cut the scarfs for the other timber for the boom so the evening wasn't a total loss of time.

Lots of boat stuff tomorrow.

Hi Bob

Could you get me the places where you make these spacers? It would be nice to have them measured from one ( front?) end of the boom. It would be certainly helpfull, since I am planning to make this hollow boom too............as soon as it stops freezing in my area

Ooh boy...

Okay, here is my estimate for placement of spacers in the box boom.

Downhaul: 400mm from forward end (taken from the rigging article on Mik's site)
Kicker: 800mm from forward end (purely an estimate based on Brian's pics)
Forward Mainsheet Block:1750mm (estimated distance from forward end of boom to position over BH-3 based on plan dimensions)
Aft Mainsheet Block: 2830mm (based on distance between BH-3 and BH-4 less 120mm to place the block a bit forward of BH-4)

Holes for lashing the sail to the ends of the boom will be 25mm in from the ends and 22mm below the top of the boom.

I hope these make sense. Please understand these dimensions are estimates calculated on paper. I think these are reasonably close to where they need to be but I am certainly open to anyone's ideas and suggestions for placing the spacers.

Time to go do some boat work!

That sounds good. And the reefing points near the top edge.

MIK

Thanks. Mik.

Reefing points?

The holes at the ends of the boom? I described those as being 22mm from the top edge, but they could be closer.

Something different? What? Where? Would these be placed with reference to the reef points on the sail?

By the way, I'm already thinking of revisions for the next version of the box boom to incorporate something inspired by the sailtrack on Brian's boom: Side panels of 4mm ply, 12mm x 19mm timbers sandwiching a 6mm ply layer extending above the top edge. This would allow cutting slots in the 6mm ply above the edge for kicker and downhaul straps ala Brian's set-up. The 6mm ply could have its middle cut out to reduce weight. Just a thought.

Bob

The first bit of epoxy hit the box boom this afternoon. I glued the top timber to one side. The tapers are marked along the bottom edge. I'll install the spacers and glue the bottom timber tomorrow followed by the other side panel on Tuesday..



In the set-up I only used 8 clamps. When the epoxy went on it took 15 clamps to close all the gaps between the timber and the side panel.

Re SPAR MATERIAL. Douglas fir is what is recommended, as per my understanding in the GIS plan book .. for the mast. Is Sitka Spruce an acceptable substitute? Any other acceptable woods for the mast?

What about the boom and yard - Douglas Fir only? Just researching the available sources for wood in sunny south Florida. Thanks. Rick

Originally Posted by Rickd

Re SPAR MATERIAL. Douglas fir is what is recommended, as per my understanding in the GIS plan book .. for the mast. Is Sitka Spruce an acceptable substitute? Any other acceptable woods for the mast?

What about the boom and yard - Douglas Fir only? Just researching the available sources for wood in sunny south Florida. Thanks. Rick

Is Sitka Spruce a substitute? Hoo Yeah!!! If you can get good stuff, it's THE best

Spruce is fine. But you do need to increase the diameters a little bit.

Add a millimetre to the diameter of the yard and boom if using spruce - it is probably easiest just to add it all the way along. It doesn't sound like much, but a small difference has a big effect.

A millimetre is easy to sand off too, which shows how important it is to make the spars accurately.

MIK