Replacing the Centerboard Cable Underwater Sheave on a 1986 Bristol 35.5c

By Frank Cingel
winter 2013-14


As it became more and more difficult cranking up the centerboard I started looking for a cause. Thought at first it might be the winch. Removal and inspection didn't show anything, lubrication didn't help. Thinking the gear teeth were a bit galled, I turned it around to put the smooth unloaded side of the gear teeth to work. That didn't help either.

An internet search hinted at a sheave failure. Did find one person that showed a picture of a failed sheave. The only clue, besides a difficult raise of the board, was the creation of a new exit hole offset from the normal exit hole of the cable, seen when one looks up into the centerboard well.

I had no idea how to go about attacking sheave replacement. Hence this documentary in the event it might be useful to other Bristol 35.5c owners.


It soon became apparant better tools were needed to cut into the hard fiberglass. Not enough room to swing a hacksaw, leaving a chisel - brutal, difficult, and it causes separation in adjacent fiberglass. Enter the Sears multitool. Uses vibration to move the cutting blades. Worked great, can't imagine doing this job without it.


After breaking through, one can get an idea of how to proceed.


The sheave is contained between imbedded walls of fabricated fiberglass sheets about 3/16 inch thick, 5/8 inch apart. This eliminates any chance of the cable jumping the sheave.


Held in place with an imbedded 5/16 bolt and nut. No additional hardware support.


The opening has to be long enough to get the sheave and cable out. Despite all the dragging the cable over the bolt when raising the board, only saw one whisker, here or when the board was down with the boat in the lift. Decided cable replacement can wait.


No doubt what this is. The wall of the sheave just corroded away, the cable wearing a grove in the bolt as the cable was dragged over it.


There also was significant crevice corrosion at the bolt head end, happens when stainless occupies stagnant water.


I thought I was fortunate to find the Ronstan RF580 as the exact match for this sheave. Although I don't think brass under the plating is a good choice; once the integrity of the plating is penetrated brass will deteriorate rapidly. But there are few, if any, alternatives out there in a sheave this size and function.

sheave14 There are two underwater sheaves. The other takes the cable upward, is buried under cabinetry, and much harder to access. But I still needed to know its condition so I bought this usb endoscope to take a peek through the opening I had created.
sheave15 Sheave intact! Yea!

Time to put it together. Here smoothing the surfaces with epoxy in preparation for covering.

sheave16 New sheave in place. The original bolt was 5/16 whereas the sheave hole was 3/8. Didn't make sense to me so I used a 3/8 ss bolt.
sheave17 Took a while figuring how to close the opening. A piece of fiberglass cloth, laid on a piece of saren wrap and filled in with epoxy resin. Once set up it was still somewhat flexible. Shape it with scissors and place it over the opening, supported by a small shelf I created.
sheave18 Tacked it in place with resin and filler, thick enough to prevent epoxy from seeping past where it might mess up movement of the sheave.

Continued fiberglassing the opening and the bolt end holes.