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.