The Hinge for the Main Mast

I have been making the parts of the hinge for the main mast. These are three 316 grade stainless steel plates, one aluminium packing piece and a couple of pins. Machining stainless steel can be a problem because it easily work hardens. Any rubbing, rather than cutting, quickly raises the temperature causing hardening. I had the stainless supplier cut the plates to shape for me on a CNC plasma cutting machine. This left me with the job of accurately drilling a number of different sized holes through the plates. Fortunately I have a small CNC milling machine to do this job. Here is a photo showing the aluminium packing piece being cut; the white surface is powder coating on the scrap aluminium being reused.

Here are the two identical plates for the upper part of the hinge having the holes cut. Note the clamps and bolts through the first couple of holes to keep the plates together and stop them moving relative to one another. Note the coolant stream to prevent both cutter and material heating up and also to carry away the chips.

Here are the completed parts, upper hinge plates (4mm thick) on the left and right, lower hinge plate (6mm thick) left of the steel rule and the white packing piece (6.35mm thick).

 

This next photo shows the parts assembled. The notch in the corner of the lower hinge plate hits against the pin which passes through the two upper plates to provide a positive stop when the mast is vertical. The packing plate (white) is slightly thicker than the lower hinge plate to ensure that the hinge doesn't bind.

The next two photos shows the hinge clamped to the top of the lower part of the mast its up and down positions. In the up position there is a locking pin (just below the lower bolt in the upper hinge), this will probably only be used when the mast is pushed up until the forestay is tightened and when the forestay is released to lower the mast.

The lower part of the mast is still square at this stage so that the lower hinge plate can be used as a template to drill the five dowel holes. Having the mast square will also help with cutting the slot in the mast for the hinge plate. Doing either of these jobs on a circular mast would be difficult to get right and the position of the hinge plate in the mast is critical.

The plate will be glued into the mast with epoxy after roughening the surface of the steel. Hardwood dowels will reinforce the glue joint. For some reason that I can't explain I have put 5 dowels through the lower plate and four through the upper plate! Maybe I decided that the hinge pin which passes through the upper part of the mast provided some additional support.

It's now a day later and I have fitted the lower hinge plate into the lower part of the mast. Here it is in place in the boat. The mast still needs to be shaped (square to circle) and I am pleased that I drilled the holes and cut the slot while it was still square. I will do the same with the upper part of the mast.

 

 

The Centreboard and its Lifting Tackle

After returning from my break I turned my attention to some of the steel parts for the boat. The biggest item is the centreboard (CB). I drew the CB using a CAD package on my PC and did some phone research to find out how and where I could get it cut. A few calls led me to Midway Metals at Yatala in Brisbane. I emailed a DXF file (standard drawing interchange file) to them on Tuesday evening, received a quote on Wednesday morning, I accepted the quote and picked up the CB on Thursday - pretty good service. The CB is cut from 316 grade stainless steel, 12mm thick and it weighs 48 Kg. The CB cost $380. The cutting was done on a "plasma" cutter and unfortunately plasma cutters can't do small holes. I had to take it to a machine shop to get the 2 holes drilled and this cost $80, expensive holes!

While I was drawing the CB I made a sketch of it in 3 positions with the lifting line. Here it is:

 

It is hard to see the detail so click on the image to get it enlarged. There are 2 dimensions for each position of the CB. One gives the moment arm of the dead weight of the CB, the other gives the moment arm of the lifting force. The moment is the force or dead weight multiplied by the moment arm and the moments about the pivot point must balance. The lifting force needed in each position id shown in the table below.

 

CB position	Lift moment arm	Dead weight arm	Lift force Kg
Down	450	75	8.0
Middle	597	460	37.0
Up	509	575	54.2

 

The CB in my Welsford Navigator is 2/3rds the weight of the Able CB and it is quite difficult to lift. This made me wonder whether lifting the Able one with block and tackle was a practical proposition - maybe the winch called for by the plans is a better way to go. I decided to make a test rig and try the tackle approach.

 

Here is the CB mounted in the test rig. The pivot point and lifting "eye" are located as they would be in the boat.

 

 

I tried the 5:1 tackle that lifts the CB in my Navigator(the boat on the trailer) but it didn't work very well. It did lift the CB but I had to pull very hard to provide the 54 Kg lifting force as the CB approaches the up position. Theoretically the 5:1 tackle should only need about 11 Kg on the tail of the rope to pull 54 Kg. I guess that I had to apply 25 to 30 Kg to lift the CB. Most of the force applied to the tackle is used to overcome friction in the small (20mm) sheaves.

 

I searched through my boxes of old boat bits and found some blocks with larger sheaves and cobbled together another 5:1 tackle and tried this. The result was much better but still getting close to 20 Kg. Here is a photo of the CB in the up position.

 

Last photo shows a close up of the 5:1 tackle, 3 sheaves at the fixed end and 2 at the moving end. A tidy version of this would probably do the job but I will look for 2 triple blocks with ball bearing sheaves to get a 6:1 purchase. Hopefully this will dramatically reduce the friction forces which increase as more sheaves are added and also increase as the gets bigger.

 

I will also look further into the suggested worm drive winch. This is what the small Dutton Lainson winch, suggested on the plans, looks like.

 

 

 

The body of the winch is 140 mm high and the end of the handle is 300 mm above the base. This winch has a 30:1 ratio and a drum 40mm in diameter. To pull in the lifting line 900 mm (CB fully down to fully up) would need over 200 turns of the handle! It would undoubtedly work but it is ugly and, to me, 200 turns of the handle is 10x too many.

 

Thursday, 9 May 2013

Problem Solved

I bought a pair of triple blocks with 30mm diameter ball bearing sheaves. Using these gives me a 6:1 purchase so in theory the force needed on the tail of the tackle is only about 9 Kg (ignoring friction effects). Here is a photo of the new blocks in place in my test rig.

 

 

 I pulled on the tail of the tackle with my spring balance  and raised the CB completely with less than 12 Kg (that's the limit of my balance). Using one hand (gloved) I could raise the CB completely in 6 pulls. So, the ball bearing blocks make a huge difference by reducing the friction in the tackle.

 

I didn't like the winch and now have a solution to the problem of raising the CB.

 

Back to putting epoxy fillets in all the corners inside the hull.