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Sail & Rigging - What size blocks should I buy
27 January 2014 - 01:50
#1
Join Date: 02 February 2007
Posts: 126

What size blocks should I buy
A question for the Professor perhaps. I am looking to buy new sheet and guy blocks for the spinnaker on our Swan 76, Tigris to attach to the rail and also a new 2:1 tackle to go at the bow inside the pulpit for using a chute without the pole or prodder. My question is what safe working load should I choose? Gavin

27 January 2014 - 13:01
#2
Join Date: 02 January 2008
Posts: 1547

Dear Gavin
Both sheets and aft guys could use blocks with SWL 10000 lbs, the foredeck tackle half of this. It is suggested that you also consider the influence of winch and line sizes.
Kind regards
Lars

27 January 2014 - 14:19
#3
Join Date: 30 January 2007
Posts: 461

Dear Lars,
a similar question from me. I need to replace the main sheet system of my 411 which presently is composed of:
1) two single blocks on the boom, one with becket,
2) a violin block on the car,
3) a single block on the deck and
4) a winch.
Overall the force ratio is 1:5 plus the winch. Somehow it looks like an overkill but I believe it comes this way from Nautor`s and I like to keep it so; my question is what SWL should I require for new similar blocks?
Daniel, 411/004

27 January 2014 - 19:25
#4
Join Date: 02 January 2008
Posts: 1547

Dear Daniel
Suggest you use 2 kN load in the mainsheet for determining block SWL.
One boom block needs 4 kN SWL, the other 6 kN, the violin block on the car 8 kN, the deck block 3 kN. But maybe you prefer to use one or two sizes of blocks only?

You are right that five parts is a safe choice. If the mainsheet winch is a Lewmar 40 two parts would be sufficient.
Kind regards
Lars

27 January 2014 - 23:09
#5
Join Date: 02 February 2007
Posts: 126

Lars, many thanks I am now searching through the catalogues. Best wishes, Gavin

28 January 2014 - 10:42
#6
Join Date: 30 January 2007
Posts: 461

Dear Lars,
as always your reply is extremely clear; thank you. The winch size is something like 30 and single speed, so I rely on your confirming that a 5:1 ratio is the best choice.
I like the numbers you give because somehow I was expecting higher values and more expensive blocks. I think that the present original blocks do not have roller/ball bearings and I plan to improve this. Very good and I am ready to proceed!
Thank you again.
Daniel, 411/004

02 February 2014 - 17:10
#7
Join Date: 30 January 2007
Posts: 461

Dear Lars,
I hope I am not too impudent in taking advantage of your kindness but it would be extremely helpful to have the SWL of the principal lines in our boats as in the original designs: sheets and halyards. From them plus a tackle ratio whenever needed, one could find the right size of the lines and of the blocks in case of substitution.
May I suggest a few tables to be published in the Maintenance section of this site?
Daniel, 411/004

02 February 2014 - 19:37
#8
Join Date: 02 January 2008
Posts: 1547

Dear Daniel
Good idea, would be glad to assist with this. Assume Rod Stephens running rigging recommendations in the Maintenance section could be used as a basis. There the headstay size is used as reference for the halyards, and sheets and guys could be included.
Would metric dimensions be preferable, and can wire halyards be omitted? Can also do a addition to the original version for the owners used to imperial sizes if required.
Kind regards
Lars

02 February 2014 - 23:08
#9
Join Date: 30 January 2007
Posts: 461

Dear Lars,
whatever you decide is fine, I greatly appreciate your cooperation which will be of invaluable help for all of us. Regarding myself, metric is fine and I do not have wire halyards anymore but it is up to your patience how may cells of the tables you are willing to fill!
Thank you again.
Daniel, 411/004

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