Mainsheet block ratios.

[GILow]

Hi all,

So, I’ve got a lovely old Kelly Peterson 44. It has end boom sheeting and has come to me with a 7:1 block purchase setup. The boom is 4.5 meters long.

That seems kinda excessive purchase. My Swanson 42 has 5:1 purchase with a similar boom length, no winch, and it feels overly geared to me.

Anyway, I’m changing the mainsheet arrangement to run the mainsheet from the blocks at the end of the boom to a winch on the coachhouse roof, and I’m trying to figure out what sort of purchase is needed.

A walk around the marina here revealed that most modern boats in my size range have mid boom sheeting with a 3:1 or 4:1 purchase. The only end boom sheeting I could find with my boom size and proposed mainsheet setup was on a Leopard cat and it had a 3:1 purchase.

Any thoughts? Of course I can go out there and try different arrangements, but I’m going to have to buy new blocks (my 7:1 setup is constructed from all single blocks) and I’d like to get close if I can.

I figure a similar sized boat with end boom sheeting led to a winch would be a good reference.

Matt

[cal40john]

You can get an approximate number for the load -

https://www.harken.com/en/calculator...ng-calculator/
This is the 1:1 load. Divide by the parts of your block and tackle, then find the winch power ratio and divide by that to get load you'll be pushing on your winch handle.

[GILow]

Originally Posted by cal40john

You can get an approximate number for the load -

https://www.harken.com/en/calculator...ng-calculator/
This is the 1:1 load. Divide by the parts of your block and tackle, then find the winch power ratio and divide by that to get load you'll be pushing on your winch handle.

Well, that certainly makes some of the maths easier, thank you.

But I’m trying to get a feel for what people have on their boats that works well day to day. Ideally I’d only use the winch for trimming, pulling most of the mainsheet through by hand. A winch could probably do the job with a 1:1 ratio at the end of the boom if you get down to it.

[Jim Cate]

Matt, if you are ok with using a winch to do the final trim, I'd recommend using the least number of falls that you can get away with. Each set of sheaves add exponentially to the friction losses and that means that in light airs the boom just sits in place when you want to ease it out. (You can likely guess how I know this to be true).

My expectation is that a two or three part tackle would work well for your setup. And do try to get large diameter sheaves with good bearings... you will enjoy their performance long after the pain of price fades!

Jim

[cal40john]

Originally Posted by GILow

Well, that certainly makes some of the maths easier, thank you.

But I’m trying to get a feel for what people have on their boats that works well day to day. Ideally I’d only use the winch for trimming, pulling most of the mainsheet through by hand. A winch could probably do the job with a 1:1 ratio at the end of the boom if you get down to it.

My boat had a 3:1 at the end of the boom where that end of the mainsheet came out through a racheting block with a cam cleat. I only ever went to the front of the cockpit to use the winch to close the leech up wind. Most of the time I just pulled in the mainsheet from the back end.

[thinwater]

It sounds like the main is sheeted 7:1 with a cam cleat on the tackle (no winch). My F-24 is sheeted this way and 18-foot beach cats are sheeted 7 or 8:1. So no, I don't see a problem with 7:1 boom-end tackle.

If the tackle is proper, reeved properly (it likely is not), and has the proper line (often POs up-size the line for grip which increases friction and ruins it, it should run out just fine in light winds. It seems to work in Olympic sailing.

I would fix what is wrong. It is probably an excellent system for that boat. Unless you have a spare winch, this means something may have to be in a jammer that probably should not be (like that main sheet, for example). Moving the sheet to cabin roof means you will no longer be able to trim from the helm. That would not be an upgrade. Your call.

---
My PDQ is sheeted 3:1 and then to a winch. Is that a better system? For that boat, yes, because of the location of the traveller (a boom-end tackle would be outside of the cockpit and would be a mess when jibing). Much depends on how the boat is set up.

[GILow]

Ok, some good ideas here thank you all.

Just for clarity…

I sail from in-front of the wheel. The only time I stand behind the wheel is when I am motoring.

So, the plan is to get all sheets and winches accessible from in-front of the wheel. Turns out that won’t be difficult at all.

Currently the main sheet is all alone at the back of the cockpit, on top of the aft cabin trunk, with a 7:1 purchase leading to a self tailing 2 speed winch. That’s just crazy purchase.

AND, the 7:1 purchase on a 4.5 meter boom means you need a staggering 50 meters of rope laying around when you don’t have the boom right out.

I’m inclined to go with Jim’s thoughts on this one and see how it goes with 3:1. I know he’s made of Titanium or some other metal, but if he can handle it then I’ve got no excuse.

Matt

P.S. the concept of a winch at one end of the sheet and a cam at the other end is something I’d never have thought of. Although I cannot see it working for me now it is something I will keep in mind.

[HeinSdL]

In my case 2:1 with both ends going back to a winch, using what I think is called the German mainsheet system. Purchase on a mainsheet is only really needed when tightened by hand.

[SurlySailor]

We run a 38 footer and had a 4:1 purchase with a 4:1 fine tune which gave us 16:1 for the final tuning. We found that the 4:1 course tune required significant stregnth to operate anytime the wind was above 12 knots true upwind (so say 15-17 apparent.) over a distance race or long cruise it would wear out crew members and ended up being an issue where my wife had a hard time playing the main when it was windy.

We ended up changing it out to a German main system with no purchase but led to 40:1 winches. It’s been a fantastic change, and really a big safety factor because we can now have any crew member run the main regardless of stregnth.

Before we went to the German main I was considering going to 6:1 for the Mainsheet. I honestly think anything less than 4:1 on boats these size is dangerous and you’re not going to be able to fully get your mainsail in. You may say that does t matter but you have to remember that with these older boats the main is a very important trim tab on the sailplan. The majority of your power is coming from the Genoa but you neeed to be able to control the boat with the main. With 3:1 the sheet loads in any breeze are going to be huge.

[GILow]

Originally Posted by SurlySailor

… I honestly think anything less than 4:1 on boats these size is dangerous and you’re not going to be able to fully get your mainsail in. You may say that does t matter but you have to remember that with these older boats the main is a very important trim tab on the sailplan. The majority of your power is coming from the Genoa but you neeed to be able to control the boat with the main. With 3:1 the sheet loads in any breeze are going to be huge.

I agree, but remember I am running the mainsheet to a 2 speed winch on the cabin top. So theres another 5:1 purchase equivalent at the very least.

[GILow]

I’ve had a quick look at the German Mainsheet system.

I may be missing something but it doesn’t really feel right for a short handed cruising boat. All the descriptions I’ve read of the tacking technique make it sound a little too complex.

[RaymondR]

Both my boats have had 4:1 main sheets and it's pretty hopeless trying to sheet in a loaded main with both of them without the assistance of a sheet winch.

Having a long tail rope to manage is a pest and downright hazardous to ones health when it accidentally uncleats itself and one is sharing the cockpit with it. And the longer the tail the worse it is.

The only truly effective solution to the conundrum is to sell the raggy and buy a pig boat.

[thinwater]

Originally Posted by GILow

...I sail from in-front of the wheel. The only time I stand behind the wheel is when I am motoring....

In that case, I see it. Makes no sense if you are using a winch anyway.


Obviously, you need to take a serious look at the courthouse structure. The load can be many tons in a crash jibe, in a direction it was not built for. Going to mid-boom only increases that.


Don't go too low on the purchase; it's nice if you can hand-haul in light winds and there will be a lot of grinding to bring it up before a jibe.

[RaymondR]

Pondering on my boom breaking incident I identified the following factors:

(1) The vang placed a severe bending moment at the point where the vang is attached to the boom.

(2) The mid point attachment of the main sheet coupled with the upward pull at the end of the boom occasioned by a loose footed sail generated the moment.

(3) Corrosion between the aluminium boom and the SS vang fitting had weakened the boom at the vang attachment point.

[GILow]

I never said I was going mid boom sheeting.