How much safety margin do you really need when choosing blocks?

I have been accumulating blocks as I found them on eBay, Harken closeouts
and other sources and thought I had a full complement but now I am
wondering. .

For example Rutu is designed to take it first reef at about 20 knots so I
figured sheet loads at 25 knots. At 25 knots the main sheet load will be
about 2,750 lb. That is 156% of the 20 knot load. The sheet is a 2:1
"admiral's cup" layout with a single block on the traveler, a double at the
boom end lead to the mast and back to a pair of 40's at the cockpit.

The single block on the traveler and the pair at the mast are 75mm Airblocks
rated for 5,000 lb SWL. More than I thought I needed but I got them cheap.
My problem is the double on the boom. The way I figure it a 90 degree turn
has a load factor of 141% so 3,877 lb SWL should be enough. I have a Lewmar
80mm double with a 4,000 lb SWL but the Harken calculator says I need a
block with a 7,000 lb SWL which costs $550!

Where do you say enough?

--
Glenn Ashmore

I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at: http://www.rutuonline.com
Shameless Commercial Division: http://www.spade-anchor-us.com

On Sun, 2 Jul 2006 17:52:08 -0400, "Glenn Ashmore"
wrote:

I have a Lewmar
80mm double with a 4,000 lb SWL but the Harken calculator says I need a
block with a 7,000 lb SWL which costs $550!

Where do you say enough?

Having a mainsheet block blow up on you at some inopportune time is
very unpleasant. Sooner or later you will be hit with a surprise gust
that takes you over 30 to 40 kts with a full main. That is what you
need to size for in my opinion. Trying to corral a flailing boom with
a jerry rig AND get a reef in at the same time is a very dangerous
job. Another 99% certainty is an accidental jibe or two at some
point. They create huge shock loads on all of the hardware as you
well know.

Bottom line is that I would not cut things too close.

I have some concerns about leading the mainsheet forward to the mast
also, having tried that arrangement once and giving it up. There was
too much friction in the system because of the multiple blocks, and it
was difficult to ease the main out in light air. All of the big 45+
boats that I have sailed usually had a simple up and over lead from
the traveler to a 180 degree turning block on the boom, with a winch
on each side of the cockpit.

Glenn Ashmore wrote:
How much safety margin do you really need when choosing blocks?


For example Rutu is designed to take it first reef at about 20 knots so I
figured sheet loads at 25 knots. At 25 knots the main sheet load will be
about 2,750 lb.

Glenn-
As I understand it, the 'force' required to restrain the end of your
boom at 25kt is 2750#. You have a 3-part tackle between your boom and
the traveller, and each part will experience a force of 900# or
thereabouts.
The total force on each block (not sheave) shouldn't be much more than
3000# or so.
If you're worried about 'airblocks' blowing up, you can always put a
swaged cable through the 'hole' in the block as a backup (as the
Maxiboats seem to do).

Also, the sheet loads generated by the various formulas for a given
sail area tend to assume that the boat is upright. Unless you have a
huge ballast ratio or are sailing in concrete, the 40kt gust mentioned
will cause your boat to heel and spill air from the sails..The force on
the sheet will not necessarily increase as much as one might suppose.

Your sailmaker could be a useful resource for questions like this as
well. Of course, finding a similar-sized boat that's been offshore and
having a chat with the owner about gear choices can be educational.

Having said all this, bigger blocks are certainly preferred. Simce you
seem to have quite a few 'creature comforts' on your boat, you might
find that heavier,'non-racing' style blocks (eg Garhauer) might provide
the safety margins you'd like at less cost.

Cheers
John

sv Manali- (29-ft Joel White Double Ender)

Glenn Ashmore wrote:
How much safety margin do you really need when choosing blocks?

I have been accumulating blocks as I found them on eBay, Harken closeouts
and other sources and thought I had a full complement but now I am
wondering. .

For example Rutu is designed to take it first reef at about 20 knots so I
figured sheet loads at 25 knots. At 25 knots the main sheet load will be
about 2,750 lb. That is 156% of the 20 knot load. The sheet is a 2:1
"admiral's cup" layout with a single block on the traveler, a double at the
boom end lead to the mast and back to a pair of 40's at the cockpit.

The single block on the traveler and the pair at the mast are 75mm Airblocks
rated for 5,000 lb SWL. More than I thought I needed but I got them cheap.
My problem is the double on the boom. The way I figure it a 90 degree turn
has a load factor of 141% so 3,877 lb SWL should be enough. I have a Lewmar
80mm double with a 4,000 lb SWL but the Harken calculator says I need a
block with a 7,000 lb SWL which costs $550!

Where do you say enough?

Glenn,

Your blocks sound the right size to me. Don't forget that:

- the boat will heel to reduce the wind force in a gust
- SWL is not Breaking load. Don't be adding Safety factors onto
safety factors.
- in a gust of say 35 or 40 knots, I would presume you blow the sheet
or traveller to reduce heel and thus sheet loading.

Evan Gatehouse

Glenn Ashmore wrote:
How much safety margin do you really need when choosing blocks?

I have been accumulating blocks as I found them on eBay, Harken
closeouts and other sources and thought I had a full complement but
now I am wondering. .

For example Rutu is designed to take it first reef at about 20 knots
so I figured sheet loads at 25 knots. At 25 knots the main sheet
load will be about 2,750 lb. That is 156% of the 20 knot load. The
sheet is a 2:1 "admiral's cup" layout with a single block on the
traveler, a double at the boom end lead to the mast and back to a
pair of 40's at the cockpit.

The single block on the traveler and the pair at the mast are 75mm
Airblocks rated for 5,000 lb SWL. More than I thought I needed but I
got them cheap. My problem is the double on the boom. The way I
figure it a 90 degree turn has a load factor of 141% so 3,877 lb SWL
should be enough. I have a Lewmar 80mm double with a 4,000 lb SWL
but the Harken calculator says I need a block with a 7,000 lb SWL
which costs $550!

Where do you say enough?

From personal experience it's usually the shackle that goes before the
block. Besides, shackles are cheaper! Always design a weak link into a
mainsheet set-up!