SWL for Main Sheet system?

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giraffe

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Hi
I want to replace the main sheet system on a Swift 18 which has a main sail area of just under 9 m2. The original system has a 58mm block with fiddle, cam & becket made by Main(?). I guess the default replacement would be a Barton no 4. However i quite fancy fitting a ratchet block but I'm unsure what kind of SWL and breaking strain i should be looking for.
For example, Barton no. 4 plain blocks are rated as 630kg SWL 1260 kg Break load.
However the same size ratchet fiddle cam block manufactured by Holt is rated at only 240kg SWL 1080kg Break Load.
If I go to the Harken site hardware selector, they don't list a swift18 but for a similar main sail area they select a Midrange Hexratchet block with a 816kg SWL and 2267kg Break load.
The Harken main sheet system loading calculator suggests at 30kt wind a "mainsheet load" of 365kg.

So i'm wondering if i'll be ok with the loading for the Holt ratchet block which is much cheaper than the Harken?

Can somebody advise me please?
many thanks,
martin
 
Thanks Pete,
However I think I should make myself clearer. I'm not really asking for information specific to the boat so much as how to interpret SWL and Break Load, what's crucial, and what sort of margins one needs when selecting blocks.
 
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The Harken main sheet system loading calculator suggests at 30kt wind a "mainsheet load" of 365kg.

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This seems very high. If you have a 4:1 system, then you are trying to pull up 90Kg odd, if my 'o'level maths recollection is correct! Since I can pull it comfortably with one hand then maybe 20Kg x 4? I suppose you could get high loads due to crash gybing in strong winds, and you could get to the point where you have to decide what you want to break in these circumstances.

Can't really see That the Holt will be a problem under normal conditions.
 
I would not fit a fitting that might break at 240kg if I expect to have a load of 360kg. out on the briney with the kids and a broken mainsheet system - buy the right gear or pay the price in other ways, you know it makes sense.
 
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Thanks Pete,
However I think I should make myself clearer. I'm not really asking for information specific to the boat so much as how to interpret SWL and Break Load, what's crucial, and what sort of margins one needs when selecting blocks.

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SWL is what I would look at.

Breaking load is of limited interest, but it does give you an idea of what factor has been used to arrive at the SWL (and sounds impressive on the packaging...).

If it looks like a reasonable factor has already been allowed for SWL, then I probably wouldn't add in any more allowance.

Andy

[Footnote for pedants, etc: The terminology "Safe Working Load" - SWL had a specific meaning when applied to equipment used for lifting, and various standards existed for the factors to be allowed for different equipment and applications. As far as I know, there are no such standards for (e.g)blocks, etc. not specifically intended for lifting, such as those available in chandleries. Officially, SWL is now an obsolete term, having been replaced by 'WLL' (Working Load Limit). Thus the term is almost meaningless in this context, and is probably best interpreted as the manufacturer's opinion of the maximum load that the block should be used for.]

/forums/images/graemlins/crazy.gif
 
[ QUOTE ]
I would not fit a fitting that might break at 240kg if I expect to have a load of 360kg. out on the briney with the kids and a broken mainsheet system - buy the right gear or pay the price in other ways, you know it makes sense.

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I agree with your sentiment entirely. I guess what confuses me is if you're right and the thing would break at 240kg (the SWL) then what does the quoted 1080kg break load mean? And why would Holt design a block to optimally take 10mm rope with a break strain of around 2000kg if the block would break at only 240kg?
 
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[I agree with your sentiment entirely. I guess what confuses me is if you're right and the thing would break at 240kg (the SWL) then what does the quoted 1080kg break load mean? And why would Holt design a block to optimally take 10mm rope with a break strain of around 2000kg if the block would break at only 240kg?

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The breaking load of the mainsheet rope is not relevant, 10mm is the smallest I want to hold without gloves!
With ball race blocks, damage often occurs at quite low loads, in the form of flattening the balls etc, it usually becomes inefficient rather than failing in a dangerous manner.
for a simple mainsheet system i would guess a rope tension of say max 75kg, so parts of a 4:1 system will see 300kg.
Your choice of safety factor times that!
A yacht system probably needs to have a much higher breaking load than a dinghy system, because the mass of the yacht will resist the forces of a bad gybe, where a dinghy will just fall over.
I would not want a mainsheet to break in any 'likely' incident, as the boom could well take the rig down and injuries in the cockpit seem likely. Not the place for a weak link imho.
Have a look at the Barton range, it works well for the price and is designed for yachts not dinghies. otoh I have holt autoratchets on my dinghy jib sheets and they work very well.
 
"Officially, SWL is now an obsolete term, having been replaced by 'WLL'"
I thought that was to stop people reading it as "She will lift" /forums/images/graemlins/grin.gif
 
[ QUOTE ]
I guess what confuses me is if you're right and the thing would break at 240kg (the SWL) then what does the quoted 1080kg break load mean?

[/ QUOTE ]

Wrong ... SWL is Safe Working Load - a load that you can safely go to without strain of the component. Breaking strain or load is the manufacturers estimate of the actual load that would damage or break the component based on prototype testing.
On many items SWL is 1/3rd of BS to ensure adequate margin of safety.

Given that comfortable sheet size to handle is way above that required in strength for a boats sail - the blocks that conform to that sheet size will likely be over safe as well. Maybe I'm daft - but I would be happy with blocks and sheets of good size and really would not be looking at SWL / BS because you buy reputable gear.
 
Many thanks for all your replies. I think i get more of an idea now. In the absence of a standard it seems that SWL is more useful for comparing across ranges by the same manufacturer rather than between manufacturers who may adopt different safety margins (for whatever reasons).

Interestingly Ronstan don't provide any SWL figures. Instead they suggest:

"An appropriate factor of safety should be applied to Breaking Load figures to suit each application before choosing or specifying a particular product. For many industrial and safety related applications, and for some yachting applications, a factor of safety greater than two (2) should be used or may be required by law or other regulations. It is the customer’s responsibility to ensure that an appropriate factor of safety is used, and it should allow for safety implications, service life, fatigue (as may be caused by wave action, wind stresses or repetitive cyclical loading), type of load, exposure to ultraviolet light, corrosion and stress corrosion (such as in high humidity or chlorine environments). Note that a ‘safe working load’ is not specified as this is dependent on the factor of safety, which must be determined by the user relative to each application."

http://www.ronstan.com/marine/definitions.asp
 
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