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split backstay

below deck fine tune

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#1 quasi-expert

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Posted 28 November 2012 - 01:34 PM

Hi,
we are thinking about getting a square top mainsail and therefore splitting the backstay would become necessary. No problem until here.

The difficulty is that we have a below deck setup which we would want to keep, because it keeps the cockpit clean. Right now we have a block attached to the backstay end ca. 2m above the transom and from there two ends going through turning blocks in the transom below deck where additional 3:1 purchases are located. The backstay has a flicker at the top so the mainsail can pass through easily when eased.

Now for the split version it has to be setup in a way that the leeward side can be eased all the way to the mast which requires a longer adjusting range (transom to mast=4,5m, mast height =12m so ca. 3,7m). IMO we need a coarse and fine tune (winch is not an option) which have to be located below deck, because we cannot put additional fttings on the transom (reaching the transom from inside is impossible) or change the turning blocks.
The distance from transom to the cleats on the cockpit floor (approx. 2m) limits the adjusting range if you put purchases in there. The idea I came up with was to put in a 1:2 coarse tune doubling available range and a 6:1 fine tune on each side (see drawing).

What do you think about the system. Any mistakes in my ideas? Will the cleat in the cockpit be able to hold it?

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#2 SoAPieceOfStringWalksIntoABar...

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Posted 28 November 2012 - 04:51 PM

Yikes. The course cleat sees 2x the backstay load. This sounds like a 30'er, so you have something like 2k pounds on the backstay when loaded? Or, from your description you have the above deck 2:1 cascaded to a 3:1 under, so 6:1 total, call it 300 pounds if tensioned by hand. A cleat to take 1.2-4k is a big ask. How about going 1:1 on the backstays, and adding a pair of small winches?

#3 karua

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Posted 28 November 2012 - 10:32 PM

My experience in going from single to dual backstays with a square-top is that the amount of line that needs to run when tacking or gybing becomes a big issue, even with shock cord pulling the lazy side forward. On my 10 metre yacht I had 35 metres of 5mm line on each side with a 6:1/12:1 system. Every tack or gybe was a hassle, even more so shorthanded or solo. I then tried using an endless 35 metres of line, but again it would not run easily enough, and it is a lot of line that can and will get tangled! If it is below deck you will need to be sure you can keep it running smoothly.

I now have a 2:1 system with winches, much easier. So each side goes from deck to backstay tail to block on deck to winch. I then have jammers on each side to free the winch up for the assy downwind.

Or at least I will have this system again when I get the new mast to replace the one that broke when the backstay block (BL 1000 kgs) broke, arguably because of the increased loads with a bigger main.

#4 bammiller

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Posted 29 November 2012 - 11:11 AM

Hi,
we are thinking about getting a square top mainsail and therefore splitting the backstay would become necessary. No problem until here.

The difficulty is that we have a below deck setup which we would want to keep, because it keeps the cockpit clean. Right now we have a block attached to the backstay end ca. 2m above the transom and from there two ends going through turning blocks in the transom below deck where additional 3:1 purchases are located. The backstay has a flicker at the top so the mainsail can pass through easily when eased.

Now for the split version it has to be setup in a way that the leeward side can be eased all the way to the mast which requires a longer adjusting range (transom to mast=4,5m, mast height =12m so ca. 3,7m). IMO we need a coarse and fine tune (winch is not an option) which have to be located below deck, because we cannot put additional fttings on the transom (reaching the transom from inside is impossible) or change the turning blocks.
The distance from transom to the cleats on the cockpit floor (approx. 2m) limits the adjusting range if you put purchases in there. The idea I came up with was to put in a 1:2 coarse tune doubling available range and a 6:1 fine tune on each side (see drawing).

What do you think about the system. Any mistakes in my ideas? Will the cleat in the cockpit be able to hold it?


Lets start off with size and type of boat and the diameter of the upper leg of the backstay wire. This might work on a fractional with afterswept spreads to bend the tip of the mast, but if the backstay is holding the mast up - probably not going to work...

Bam Miller

#5 quasi-expert

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Posted 29 November 2012 - 01:41 PM

8m, 11,80m Mast height above sheerline, 1350kg, slightly swept spreaders, fractional rig, masthead spinnakers. I think the current backstay is well oversized (10/12mm Dyneema, probably Dux), 5 or 6mm would also do the job. IMO the backstay is more of a trimming device upwind, but probably important downwind with the MH kite up.

#6 WHL

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Posted 29 November 2012 - 05:10 PM

8m, 11,80m Mast height above sheerline, 1350kg, slightly swept spreaders, fractional rig, masthead spinnakers. I think the current backstay is well oversized (10/12mm Dyneema, probably Dux), 5 or 6mm would also do the job. IMO the backstay is more of a trimming device upwind, but probably important downwind with the MH kite up.

Wouldn't a couple of pics of your boat, its limitations, and sail plan make it much easier for everyone else to think about your problem?

#7 markvannote

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Posted 29 November 2012 - 05:41 PM

One thing to keep in mind is your current backstay load is being shared by the two blocks in the transom. If you are talking about having the backstays go through the existing blocks then start your design a single block is most likely not man enough for full backstay load.

You are probably into a small clutch if you are going to have a gross tune.

Is there any chance of sending the backstays down the rig to the pit? Gross tune being a single tail in the pit to small clutches and fine tune in the transom? (still, the transom block loads are a potential roadblock) The Tripp 41's had this setup for the checkstays. It had it's drawbacks for sure but it was a functional system.

Easiest solution may be an inspection port in your aft deck.

Thanks.
Mark

#8 quasi-expert

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Posted 30 November 2012 - 01:34 PM


8m, 11,80m Mast height above sheerline, 1350kg, slightly swept spreaders, fractional rig, masthead spinnakers. I think the current backstay is well oversized (10/12mm Dyneema, probably Dux), 5 or 6mm would also do the job. IMO the backstay is more of a trimming device upwind, but probably important downwind with the MH kite up.

Wouldn't a couple of pics of your boat, its limitations, and sail plan make it much easier for everyone else to think about your problem?

probably yes, but I'm a bit far away right now to take some showing the important areas.

#9 Speng

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Posted 30 November 2012 - 06:52 PM

Is there any chance of sending the backstays down the rig to the pit? Gross tune being a single tail in the pit to small clutches and fine tune in the transom? (still, the transom block loads are a potential roadblock) The Tripp 41's had this setup for the checkstays. It had it's drawbacks for sure but it was a functional system.


Thanks.
Mark


You could be adding a fairly decent amount of extra mast compression

#10 markvannote

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Posted 30 November 2012 - 07:28 PM


Is there any chance of sending the backstays down the rig to the pit? Gross tune being a single tail in the pit to small clutches and fine tune in the transom? (still, the transom block loads are a potential roadblock) The Tripp 41's had this setup for the checkstays. It had it's drawbacks for sure but it was a functional system.


Thanks.
Mark


You could be adding a fairly decent amount of extra mast compression


You WOULD be. Absolutely. Just a potential option for what he is trying to accomplish.

#11 quasi-expert

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Posted 05 December 2012 - 03:36 PM

found some pictures on my portable HD, if it helps:

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#12 markvannote

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Posted 21 December 2012 - 01:30 PM

From the picture it appears those chainplates may have a shot at being strong enough. I was picturing some harken thru-deck block. Your issue is the angle, the fact that they are angled in to accound for the bridle that exists now. That lead will not work for split topmasts unfortunately. I am thinking one of the easier solutions would be a winch on the side deck and hang the deck loops off those pins with loops.

Mark

#13 quasi-expert

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Posted 22 December 2012 - 10:57 AM

From the picture it appears those chainplates may have a shot at being strong enough. I was picturing some harken thru-deck block. Your issue is the angle, the fact that they are angled in to accound for the bridle that exists now. That lead will not work for split topmasts unfortunately. I am thinking one of the easier solutions would be a winch on the side deck and hang the deck loops off those pins with loops.

Mark


I'm not worried about the chainplates, they're reinforced with carbon UD going through the deck.
The best solution IMO would be to have the coarse tune above deck and use the existing underdeck systems for the fine tune. Only problem is I can either use the chainplates with the turning blocks (as on the picture) leading the two ends below deck or I can use them to lash blocks to them, but not both at the same time.

#14 Mr. Fixit's brother,, Mr. Fixit

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Posted 22 December 2012 - 03:25 PM


From the picture it appears those chainplates may have a shot at being strong enough. I was picturing some harken thru-deck block. Your issue is the angle, the fact that they are angled in to accound for the bridle that exists now. That lead will not work for split topmasts unfortunately. I am thinking one of the easier solutions would be a winch on the side deck and hang the deck loops off those pins with loops.

Mark


I'm not worried about the chainplates, they're reinforced with carbon UD going through the deck.
The best solution IMO would be to have the coarse tune above deck and use the existing underdeck systems for the fine tune. Only problem is I can either use the chainplates with the turning blocks (as on the picture) leading the two ends below deck or I can use them to lash blocks to them, but not both at the same time.

It looks like there is a pin passing through the chainplate which secures the turning block correct? If so maybe you can replace the pin with a modified version that carries the coarse tune kit. Some type of straddle plate that crosses over the turning block. It could even be a soft connection if you weren't too worried about it falling away and getting fouled.

#15 Grind4Beer

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Posted 22 December 2012 - 03:35 PM

Going from String's comments: Looking at that diagram, each side will need two blocks and a cleat carrying 2x the backstay load, with the boat structure between the mountings carrying 3x the load. That's up in the range where you might chat with your builder about the layup in that area.

Also, the description of your current rig reads for total effective purchase of 6:1, but your diagram shows only 3:1 effective.

Since you've got limited throw on the fine-tune, and there's got to be enough transom access to at least replace the current backstay, I'd suggest installing two more entry blocks, exactly like the two already on the transom. Keep your fine-tunes as is, but run a line from each thru their sides' backstay block, then back thru the new entries, and forward below deck. Turn those out to cleats at some place convenient for crew-work, preferably with workable lead to a winch, just in case you run out of fine-tune.

Yeah, you'll still need to pull 2x string on the coarse-tune, but that should keep the working loads reasonable.

G4B

#16 markvannote

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Posted 22 December 2012 - 03:54 PM


From the picture it appears those chainplates may have a shot at being strong enough. I was picturing some harken thru-deck block. Your issue is the angle, the fact that they are angled in to accound for the bridle that exists now. That lead will not work for split topmasts unfortunately. I am thinking one of the easier solutions would be a winch on the side deck and hang the deck loops off those pins with loops.

Mark


I'm not worried about the chainplates, they're reinforced with carbon UD going through the deck.
The best solution IMO would be to have the coarse tune above deck and use the existing underdeck systems for the fine tune. Only problem is I can either use the chainplates with the turning blocks (as on the picture) leading the two ends below deck or I can use them to lash blocks to them, but not both at the same time.


I figured those chainplates were uni "straps" tying into the boat when I saw the pic. That is kind of the problem. It would be hard to change the angle of those pins and no configuration coming from the top of the rig will work with the lead as it is now. I am also assuming that you are not interested in calling in and paying for a proffesional boatbuilder for major modifications?

One other option you have, and then I will bow out of this, is hanging blocks on the pins like I mentioned before and leading them forward in the cockpit to some blocks which would allow you to kick them out to the primaries. You mentioned your spreaders being "slightly swept" so 2:1 and an XAS clutch on the floor before the deflector block will keep things safe on tacks and gybes. It put a slight spotlight on your trimmer to get onto the windward winch quickly for proper headstay tension upwind but hand-pulling 2:1 should keep you plenty safe. Total cost is for four blocks, which you may have, and two clutches.

In the end, hard to sort it out completely without sitting in the cockpit and running some mock-ups. Hope you post some pics of your final design.

Mark




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