James Stevens answers your Questions of Seamanship. In this article, how confident would you be about fixing a jammed sheave?

Without professional help, would you and your crew be able to fix a jammed sheave?

James Stevens looks at the problem and provides the solution.

Question: Can you fix a jammed sheave without a rigger?

Pete and three crew are on board Lapita, a fractional rigged Sigma 41 on a world cruise.

They are currently in the Cape Verde Islands off Western Africa, with the next stop planned to be Antigua.

They have a problem at the top of the mast.

A week ago the main halyard sheave jammed, making it impossible to hoist the main.

A sheave at the top of a mast

A jammed main halyard sheave can be a difficult problem to solve away from major harbours. Credit: Theo Stocker

Pete used the topping lift as a main halyard but now that sheave has jammed also.

Pete has spare sheaves but there is no rigger on the islands and in order to get to the top of the mast he would need to send someone up on a bosun’s chair, which would, of course, require the main halyard or topping lift.

Robbie, the youngest member of the crew, is a strong and fit 25-year-old.

He reckons he can climb the mast without a bosun’s chair but there’s a big difference between getting to the top and stopping there to replace the sheaves.

Pete is also unsure whether the truck, the top fitting on the mast, will need to be removed to access the sheaves, in which case the backstay will have to be released.

The other alternatives are to sail the Atlantic with just a headsail or aim for the Canary Islands, 760 miles away to the north where hopefully they can find a rigger.

What would you do?

James Stevens answers

The sheaves will take the weight of a man but the friction will prevent him from being pulled up.

It should be possible to pull the slack halyard or topping lift over the sheaves.

Robbie can be hauled up to the top of the forestay in the bosun’s chair with the jib halyard.

The next part requires some agility and strength.

Continues below…

Robbie should attach the bosun’s chair to the main halyard and topping lift and shin up the last few feet of the mast.

The deck crew should take up the slack on the safety lines.

Once Robbie reaches the top, the halyard and topping lift are secured and should hold his weight.


James Stevens, author of the Yachtmaster Handbook, spent 10 of his 23 years at the RYA as Training Manager and Yachtmaster Chief Examiner

He now has to transfer the weight of the bosun’s chair off the halyards so he can access the sheaves.

If he’s lucky he can remove the old sheaves without removing the truck.

He can secure to the top of the backstay and take a safety line around the top of the mast several times, secured with a rolling hitch.

If the truck has to come off, the next step requires some nerve and a pop rivet gun.

Robbie will have to drill holes and rivet a D-ring to the top of the mast to hold his weight while he removes the truck.

It is not a job for the faint-hearted and it is, of course, crucially important not to drop anything, especially down the mast.

But it can be done and it will save them days on the transatlantic voyage.