Please select your home edition
Edition
Selden

Handling the load: Selden's new jib halyard turning block

by Mark Jardine on 24 Mar 24 March 2017
Selden's new jib halyard turning block © Andy McCormack

A number of classes, including the Fireball and Flying Fifteen, have moved from 2:1 jib halyards to 1:1 systems. The reasoning behind this has been weight and windage, but it has doubled the load on the turning block in the mast. This also came at a time when the classes started using higher rig tensions, further increasing the stress on the block, leaving Selden Mast's Andy McCormack with a problem to solve and a solution to find... something he excels at.

The previous sheaves had ball-bearings and we asked Andy what problems this was causing in the jib halyard turning block, "The problem with ball-bearing sheaves is that they are designed to take a low load at high speed; they are low friction, with a rolling contact rather than sliding contact. The problem we had with this application (on certain dinghy classes) is that it's a high static load, with very little movement once the jib is up. The ball bearing race wasn't doing anything for you apart from reducing the bearing area, so we decided to move to a plain bearing."

A plain bearing typically has no moving parts and can be made of a polymer, loaded with PTFE which is pressed into a housing and runs around a plain stainless steel shaft.

"In a static load situation the load handling is all about the area. The load is distributed over half the area of the shaft and all we need to do is stop the shaft and the sheave from deforming. The sheave still rotates very slightly as it is transmitting the full rig tension; the only time it has to rotate quickly is when the jib goes up and down which is under very light load where the sheave spins very well, without the need for ball bearings." Andy explains.

So instead of a ball-bearing sheave, where the small contact area of the upper ball bearings take the full rig tension, the load is spread throughout the top of the plain bearing: a far greater surface area.

"We already had a lot of experience with very high specification PTFE plain bearings which we use in our deck hardware, so we first of all tried using a standard one of those in this application, but because the loads on this tiny sheave are huge even that wasn't enough area. So we introduced an additional stainless steel bush inside the sheave to increase the load bearing area, increasing the load bearing diameter to 20mm. This meant - from a production point of view - we could keep the standard sheave box and pin so that it can be retrofitted into an existing mast."

The new sheave is already proving popular and is getting the results. Most recently Steve Goacher and Tim Harper used the plain bearing sheave box on their Seldén Epsilon mast while winning the Flying Fifteen Worlds in New Zealand. The sheave has been thoroughly tested throughout 2016 by Team Seldén riders on the Fireballs and Flying Fifteens.

www.seldenmast.co.uk

Related Articles

America's Cup AC75 boat concept revealed
Bold new high performance fully foiling monohull Here it is! The ground-breaking new America's Cup class race boat concept. The AC75 is the bold new high performance fully foiling monohull. Posted on 20 Nov
DAME Award again
We speak to Drue Kerr, Senior Designer at Zhik We caught up with Drue Kerr, Senior Designer at Zhik, who have just won the DAME Award in the clothing category for their Isotak X Ocean range. Posted on 15 Nov
I am not a number
David Henshall looks at dinghy handicapping You probably have to be at the very least middle aged to fully appreciate the tag line reference to 'The Prisoner' TV series, but as that means you're in the demographic majority it is okay to continue. Posted on 14 Nov
Mixing Work and Play
We speak to Alpine Elements MD James Hardiman We spoke to James Hardiman, the Managing Director and founder of Alpine Elements and Ocean Elements, about his life in business, the overlaps between his work and play, his own sailing and the Solo Wolf Rock Race he took part in this year. Posted on 10 Nov
Interview with Mike Sugden
About the latest B&G software version 4.5 features We spoke to Mike Sugden, Instrument Product Expert at Navico, about the latest B&G software version 4.5 which now includes full integration with PredictWind, and how advanced start line features are now standard on Zeus and Vulcan chart plotters. Posted on 8 Nov
Graphics, Coatings, Creative
A year of diversification for Grapefruit Big changes are happening to Grapefruit this year as we speak to Andy Yeomans, Founder and CEO to find out more. Posted on 2 Nov
Growing classes and exports at Ovington Boats
We speak to Nathan Batchelor about the 2017 season Ovington Boats has had a great season, supporting the classes they build, while adding a couple of new ones to their roster. We spoke to Ovington's Nathan Batchelor to find out more... Posted on 19 Oct
60 years of Crewsaver, 30 years of Crewfit
And the 'beautiful art' of simplification Crewsaver has come a long way since the original hand-welded, orally inflated lifejackets and buoyancy bags sixty years ago. Along the way they've introduced automatic inflation, dual chambers and then in 1984 Crewfit came along. Posted on 18 Oct
Endeavour Trophy 2017 overall
Hat trick for Saxton and Lewis Ben Saxton and Toby Lewis today added another three wins to their already impressive haul of first places from yesterday to win the 2017 Endeavour Trophy series with a race to spare writes Sue Pelling. Posted on 15 Oct
Endeavour Trophy 2017 day 1
Red hot racing leaves reigning champs just one point ahead With three wins in the bag after today's Endeavour Trophy opening five races, Ben Saxton and Toby Lewis (reigning Endeavour champions) lead the Endeavour Trophy series by just one point from Nick Craig and Holly Scott (D-One) writes Sue Pelling. Posted on 14 Oct