Main sheet and halyard sizes - how to work the loads involved out.

xyachtdave

xyachtdave

Well-Known Member
Joined
9 May 2009
Messages
3,227
Location
MYC
Visit site
I'm looking to replace a lot of the running rigging on our boat as I think a lot of it is over sized. It's a 36ft masthead rig with a 4:1/8:1 twin speed mainsheet on the end of the boom with a 32m2 mainsail.

The light airs performance of the mainsheet drives me bonkers, I want to replace it with the thinnest cordage that will safely handle the load (whatever that is!) so I've still got a bit of confidence in it when it's blowing hard.

Any ideas on working out the potential loads on the main sheet and halyards?

At the moment it's a case of wondering around the chandlers touching up the rope without any real idea if it's strong enough for purpose.

Also when going through say a 4:1 does the load on the rope decrease meaning smaller diameter ropes are fine or does the load stay the same but the effort needed to control it less?

Hope that makes sense and any info gratefully received!
 
The rope manufacturers websites give the breaking strains and the Boat Data Handbook is brilliant to all this kind of stuff. IMO a thin mainsheet that is capable of taking the loading is all very well in light airs but will be hard on the hands and difficult to handle in a blow.
 
Presumably what is driving you bonkers is friction, and while it's true that thinner lines will cause less friction, they will also be much less comfortable to handle. It's highly unlikely that you will ever approach the breaking load on a mainsheet.
With a 4:1 block arrangement, the load on the line, ignoring friction, is indeed divided by four. It could be that the blocks for your mainsheet are not big enough. If you use big enough blocks, a lot of the friction will disappear.
 
I haven't checked but from memory I think both the Harken and Selden websites include calculators for sheet loads, not sure about halyards.
Marlow Ropes website (and no doubt others) include tables of suggested rope sizes.
Or Google ...
 
Breaking strain is rarely the deciding factor when sizing sheets and halyards.

As a rule of thumb I reckon a human can easily apply 50kg of force, so in a conventional mainsheet system with pulleys (rather than a winch) the mainsheet itself won't be under much more than 100kg of load. So pretty much the thinnest dyneema you can get would cope with the load. However handling something that thin is very difficult - I would not go below 10mm for anything that needed pulling by hand, and most winches won't handle thinner ropes well either.
 
To be honest, I'd ignore the Jimmy Green website - they want to sell rope.

You can get a formula for the mainsheet load from the Selden website. I've it in a spreadsheet, so I can post that when I'm back on Tuesday if you haven't found it by then.

There will be less strain on the line with higher radius blocks. It matters more for Dyneema (but you'll be overspeced if you go for that anyway) and less for braid on braid, which can stretch to cope with the curvature.

I'm absolutely in favour of thinner line and less friction. I know it's a minority view on here, but it is really frustrating to hop on a cruising boat and find the lines harder to pull than on a bigger racer. The extra force is going into overcoming friction. Only the OP will know how comfortable he is heaving on lines less than 12mm thick. Personally, I have an 8mm mainsheet on a 43 footer and a 10mm main halyard spliced to 5mm wire. 56 square meter mainsail.
 
Thanks for the replies chaps.

I'm thinking of dropping to 10 mm dyneema on all the halyards and the same on the mainsheet which is currently 12 mm braid on braid. I have 8 mm Marlow D2 check stays and vang at the moment which run nicely through the blocks. Was thinking if this would work for the mainsheet but was worried a) it would break and b) it would be too thin to sheet in under load.

I've looked at the Jimmy green website before and our boat falls between their size chart which I think is on the cautious side.

Shall check out the Seldon website later too lpdsn.
 
I have 8mm dyneema for my light spinnaker sheets and that is not particularly nice to handle. It is a bit stiff and shiny (even though it has a polyester braid of course) and so is not easy to grip. My preference would be for 10mm (or even 12mm) braid on braid for the mainsheet. On my boat that is just about the only rope I have to pull by hand under load. As mentioned above the load on the mainsheet is very low, and friction can be taken care of by buying decent blocks of the right size (that is not that cheap).
 
8mm cruising dyneema is the way to go.
Easy to pull and runs through blocks easily plus you can taper it down to 4mm for sections you don't handle.

Mainsheet - worth noting also the further aft your mainsheet feed to the boom is, the longer it needs to be but the load is vastly reduced.
In my experience the purchase systems on mainsheets are ott.
You can easily apply 30 to 40kg to 8mm rope - factor that into a 6:1 or 8:1 system.
We recently fitted a 10mm dyneema 2:1 system onto a 52ft down from a 8:1 16mm braid on braid system to reduce the amount of line cluttering the cockpit.
Below 10 knots you could easily adjust the main and above you had to use a mainsheet winch which you had to use anyway when it was 8:1.

If you have a traveler and a efficient kicker you really don't need lots of purchase on the mainsheet.
The traveler up to windward will centerline the boom closehauled and the kicker controls the twist.

I see a lot of folk killing the power in their mainsail by having way to much mainsheet tension up wind, closing the leech and stalling the sail.
 
Last edited:
Just to check I find a lot of trouble with salt in the mainsheet and stiffness. A good soak in fresh water and detergent can help a lot.
As for main halyard it is stretch that is the biggest problem.Your main halyard is counteracting all the down load of the main sheet. So definitely dyneema or spectra needed there. Then it is a question of diameter needed for winches clutches and handling. Breakingnstrain will be much higher than needed.
Likewise with main sheet it is a matter of size to suit handling. good luck olewill
 
I replaced the 12mm mainsheet with 8mm dyneema on my 30m2 main.
More than strong enough but fairly difficult to handle - but what a difference on getting it in and letting it out.
Sheets all 10mm conventional braid on braid for genoa, spinnaker and genniker - but only 85m2.
 
Javelin said:
8mm cruising dyneema is the way to go.
Easy to pull and runs through blocks easily plus you can taper it down to 4mm for sections you don't handle.

Mainsheet - worth noting also the further aft your mainsheet feed to the boom is, the longer it needs to be but the load is vastly reduced.
In my experience the purchase systems on mainsheets are ott.
You can easily apply 30 to 40kg to 8mm rope - factor that into a 6:1 or 8:1 system.
We recently fitted a 10mm dyneema 2:1 system onto a 52ft down from a 8:1 16mm braid on braid system to reduce the amount of line cluttering the cockpit.
Below 10 knots you could easily adjust the main and above you had to use a mainsheet winch which you had to use anyway when it was 8:1.

If you have a traveler and a efficient kicker you really don't need lots of purchase on the mainsheet.
The traveler up to windward will centerline the boom closehauled and the kicker controls the twist.

I see a lot of folk killing the power in their mainsail by having way to much mainsheet tension up wind, closing the leech and stalling the sail.
Click to expand...


Thanks Javelin, I've mulled over the words of wisdom received in this thread and had the mainsheet made up in 8mm Marlow D2 with an endless splice.

Fitted it today and swung the boom around to check the action - massive positive difference!
 
Last edited:
You must log in or register to reply here.

Share this page

Top