This is *crazy* !

[s/v Jedi]

Yes, that’s what you will say when reading my post below

But I bet many of you are, like me, crazy enough not to just trust what “they” say and find out for yourself

This is about your sails, the forces on the sails and everything that attaches to that. The ratios of your sheeting system and choice of rope for sheets.

Below I worked this out for my mainsail. I now know at what wind force my sheet will break, or the blocks etc. unless I reef. I tried to explain every step but not in the finest detail, i.e. not how to convert knots to mph etc. But with the numbers shown, everyone should be able to replicate this for their boat.

I will post the same for my mizzen in this thread. It is very different because of a different sheeting system and the sheet is sized for not just performance but also handling.

I must add one thing before including the calculation sheet below: I have sails with a very big roach. You can see backstay overlap in the data from the sailmaker, it’s more than a meter. Because of that, I use a more complicated methid to calculate surface areas. If you have a full batten big roach sail (multihulls) then you need to do the same method I do below, but if you have a more regular sail with modest roach, you can use this formula instead:

P x E / 2 (no roach)

or

P x E x 0.585 (with roach)

Anyway, here is the sheet:

Jedi rigging Main

Calculations of sail areas incl. for each reef and all the forces at the clew and sheet for different wind speeds. Shows how to select the line for sheet and wind speeds for reefing with that sheet.

First the numbers from the sailmaker (we used Elvström/Sobstad):

Mast
⁃ P 54.58’
⁃ E 18.5’
⁃ Rake 0’

Perimeter
⁃ Luff length 54.3
⁃ Head width 6”
⁃ Foot length 18.3’
⁃ Foot round 1”
⁃ Luff to max round% 50%
⁃ Leech roach 48”
⁃ Head to max roach% 36%

Clew location
⁃ Leech length 54.127”
⁃ Clew height 42”

Reefs
⁃ 1: luff height 5.9’, leech height 9.4’, offset 0.08’
⁃ 2: 12.2’, 15.7’, 0.08’
⁃ 3: 19’, 22.5’, 0.08’

Battens (full battens, leech angle 19.473 deg, parallel battens)
⁃ 0: leech height% 87.5%, length 63.54”
⁃ 1: 75%, 112.79”
⁃ 2: 62.5%, 145.43”
⁃ 3: 50%, 169.09”
⁃ 4: 37.5%, 187.64”
⁃ 5: 25%, 200.90”
⁃ 6: 12.5%, 210.55”

Next my calculated/estimated numbers:

⁃ 0.5’ MHB – Main Head Board width
⁃ 5.3’ MUW – Main Upper Width at 7/8 height
⁃ 9.4’ MTW – Main Third Width at 3/4 height
⁃ 14.09’ MHW – main Half Width at 1/2 height
⁃ 16.74’ MQW – Main Quarter Width at 1/4 height

We have a very large roach in the mainsail because we have no backstays. This means we need to do a more elaborate calculation for sail area. Mainsail measured area is calculated by the simplified trapezoid formula, dividing the luff in amounts of 1/4, 1/2, 3/4 and 7/8:

Area = P/8 x (E+2·MQW+2·MHW+1.5·MTW+MUW+0.5·MHB)
Area = 54.58/8 x (16 + 2x16.74 + 2x14.09 + 1.5x9.4 + 5.3 + 0.5x0.5)
Area = 6.8225 x (20.5 + 33.48 + 28.18 + 14.1 + 5.3 + 0.25)
Area = 6.8225 x 101.81 = 694.6 ft^2 (64.6 m^2)

full sail: 695 ft^2
1st reef: 590 ft^2 left (used online trapezoid calculator for reefed area)
2nd reef: 480 ft^2 left
3rd reef: 375 ft^2 left

Samson provides a formula to calculate the amount of force on the clew: (windspeed in mph)^2 x 0.004 x (sail area in square feet) = Sheet Load at Clew in pounds

Our sheet attachment is 40% from end of boom so force at attachment is 1.67x higher. Sheet has a 2:1 ratio so sheet load is half. Both factors combined is 0.83x factor for sheet load.

Rule of thumb for selecting sheet: FOS (factor of safety) of 2:1 at maximum wind force should result in a FOS of better than 4:1 for less wind, reefed sail etc. so for line choice: 7,668lbs maximum sheet load at 50kts wind (see below) x 2 = 15,336 pound breaking load. Samson MLX3 12mm $2.10/ft has a tensile strength of 12,000lbs. Samson recommends a maximum loading of 20% of that for longevity so that is 2,400lbs SWL.

With full sail:
= 828 pounds at 15kts wind (690 sheet load)
= 1,472 pounds at 20kts wind (1,227)
= 2,301 pounds at 25kts wind (1,918)
= 3,313 pounds at 30kts wind (2,761) (MLX3 sheet needs 1st reef)
= 4,509 pounds at 35kts wind (3,758)
= 5,889 pounds at 40kts wind (4,908)
= 7,454 pounds at 45kts wind (6,212)
= 9,202 pounds at 50kts wind (7,668)

With 1st reef:
= 2,814 pounds at 30kts wind (2,345)
= 3,830 pounds at 35kts wind (3,192) (MLX3 sheet needs 2nd reef)

With 2nd reef:
= 3,116 pounds at 35kts wind (2,597)
= 4,070 pounds at 40kts wind (3,392) (MLX3 sheet needs 3rd reef)

With 3rd reef:
= 3,180 pounds at 40kts wind (2,650)
= 4.024 pounds at 45kts wind (3,353) (MLX3 sheet needs sail taken down)

When caught by a 50kts squall with full sail up, the MLX3 sheet will see a 65% load so should easily survive with just some loss of lifespan.

[s/v Jedi]

On the picture I attached above you see a main backstay…. but that is a running backstay. I don’t need to set it at the conditions from the picture (reaching under working canvas at 10kts STW) but offshore, in big seas, I do this anyway. Sailors are conservatives

Below the sheet for the mizzen; you’ll see I end up with Samson MLX3 for both main and mizzen sheets, 12mm for main and 10mm for mizzen.
Currently I have something similar to XLS3 in 12mm for both, which is what “they” tell you to do, but what is obviously wrong for offshore cruising

Jedi rigging Mizzen

Calculations of sail areas incl. for each reef and all the forces at the clew and sheet for different wind speeds. Shows how to select the line for sheet and wind speeds for reefing with that sheet.

First the numbers from the sailmaker (we used Elvström/Sobstad):

Mast
⁃ P 48’
⁃ E 16’
⁃ Rake 0’

Perimeter
⁃ Luff length 47.6’
⁃ Head width 6”
⁃ Foot length 15.7’
⁃ Foot round 1”
⁃ Luff to max round% 50%
⁃ Leech roach 42”
⁃ Head to max roach% 36%

Clew location
⁃ Leech length 48.397’
⁃ Clew height 24”

Reefs
⁃ 1: luff height 5.9’, leech height 7.9’, offset 0.08’
⁃ 2: 12.4’, 14.4’, 0.08’
⁃ 3: 19.2’, 21.2’, 0.08’

Battens (full battens, leech angle 18.855 deg, parallel battens)
⁃ 0: leech height% 85.71%, length 62.67”
⁃ 1: 71.43%, 107.8”
⁃ 2: 57.14%, 135.63”
⁃ 3: 42.86%, 156.25”
⁃ 4: 28.57%, 170.58”
⁃ 5: 14.29%, 180.31”

Next my calculated/estimated numbers:

⁃ 0.5’ MHB – Main Head Board width
⁃ 5.5’ MUW – Main Upper Width at 7/8 height
⁃ 8.33’ MTW – Main Third Width at 3/4 height
⁃ 12.5’ MHW – main Half Width at 1/2 height
⁃ 14.17’ MQW – Main Quarter Width at 1/4 height

We have a very large roach in the mizzen because we have no backstays. This means we need to do a more elaborate calculation for sail area. Mizzen measured area is calculated by the simplified trapezoid formula, dividing the luff in amounts of 1/4, 1/2, 3/4 and 7/8:

Area = P/8 x (E+2·MQW+2·MHW+1.5·MTW+MUW+0.5·MHB)
Area = 48/8 x (16 + 2x14.17 + 2x12.5 + 1.5x8.33 + 5.5 + 0.5x0.5)
Area = 6 x (16 + 28.34 + 25 + 12.5 + 5.5 + 0.25)
Area = 6 x 87.59 = 525.54 square feet (48.9 m2)

full sail: 525 ft^2
1st reef: 435 ft^2 left (used online trapezoid calculator for reefed area)
2nd reef: 340 ft^2 left
3rd reef: 250 ft^2 left

Samson provides a formula to calculate the amount of force on the clew: (windspeed in mph)^2 x 0.004 x (sail area in square feet) = Sheet Load at Clew in pounds

Our sheet attachment is at end of boom. Sheet has a 3:1 ratio so sheet load is a third of load on the clew.

Rule of thumb for selecting sheet: FOS of 2:1 at maximum wind force should result in a FOS of better than 4:1 for less wind, reefed sail etc. so for line choice: 2,353lbs maximum sheet load at 50kts wind (see below) x 2 = 4,706 pound breaking load.

Samson MLX3 10mm $1.50/ft has a tensile strength of 7,700lbs. Even 8mm $1.20/ft still has a tensile strength of 4,500lbs but this is just below the calculated 4,706lbs and 8mm is more difficult to handle, so for the price difference I keep it at the 10mm.

Samson XLS3 10mm $0.90/ft has a tensile strength of 6,300lbs so would work but it’s elastic stiffness is 62,600lbs vs MLX3 10mm 344,000 so it stretches 5.5 times as much, negatively affecting sail trim. For these reasons, I select the MLX3 in 10mm for line performance and handling.

Samson recommends a maximum loading of 20% for longevity so that is 1,540lbs SWL.

Samson provides a formula to calculate the amount of force on the clew: (windspeed in mph)^2 x 0.004 x (sail area in square feet) = Sheet Load at Clew in pounds.

With full sail:
= 680 pounds at 15kts wind (227 sheet load)
= 1,100 pounds at 20kts wind (367)
= 1,760 pounds at 25kts wind (587)
= 2,570 pounds at 30kts wind (857)
= 3,360 pounds at 35kts wind (1,120)
= 4,440 pounds at 40kts wind (1,480)
= 5,680 pounds at 45kts wind (1,893) (MLX3 sheet needs first reef)
= 7,060 pounds at 50kts wind (2,353)

With first reef:
= 4,668 pounds at 45kts wind (1,556)
= 5,763 pounds at 50kts wind (1,921) (MLX3 sheet needs second reef)

When caught in a 50kts squall with full sail up, the sheet will see a loading of 38% so no trouble at all.

[Chotu]

And you heel in those 50 knot gusts further adding to your safety margin.

[Snore]

Jedi,

Thanks-- while I cannot add to the conversation, I will subscribe to this thread so I can find it.

Next time some knucklehead says you don't need to reef a main on a catamaran, I want some science to show the difference in stress on the rig.

Full Main loads

3,313 pounds at 30kts wind
4,509 pounds at 35kts wind

First Reef

2,814 pounds at 30kts wind
4,000 pounds estimated for 35kts

Second Reef

3,116 pounds at 35kts wind

[s/v Jedi]

About these sail areas: we are at almost 70% of the area formed by the mast - boom rectangle. Compare to in-mast furling where it is normal to be at 45% of that same rectangle.

This is the mistake people make: they state that their boat performs good enough with in-mast furling but they don’t realize that their mast is much longer because of that.

[Matt Johnson]

It's why we chose the size catamaran we did. The need for very expensive hardware triple in cost the next notch up in size. The loads get insane fast.

[s/v Jedi]

Originally Posted by Snore

Jedi,

Thanks-- while I cannot add to the conversation, I will subscribe to this thread so I can find it.

Next time some knucklehead says you don't need to reef a main on a catamaran, I want some science to show the difference in stress on the rig.

Full Main loads

3,313 pounds at 30kts wind
4,509 pounds at 35kts wind

First Reef

2,814 pounds at 30kts wind
4,000 pounds estimated for 35kts

Second Reef

3,116 pounds at 35kts wind

Yes, I believe that the head of the sail pulls with the same force as the clew. But of course you have all those cars on the track sharing that load which is much tougher to calculate than a sheeting arrangement.

But there is a lot more force, especially on cats which have the same high roach sails as in the math above.

[s/v Jedi]

Originally Posted by Chotu

And you heel in those 50 knot gusts further adding to your safety margin.

This is exactly why I am not in panic replacing my sheets, but of course cats don’t have that safety net and really need to get this right from the start.

I’m sure you can easily plug your numbers in there to get your loading. Your mast is way taller I think….

[s/v Jedi]

Originally Posted by Matt Johnson

It's why we chose the size catamaran we did. The need for very expensive hardware triple in cost the next notch up in size. The loads get insane fast.

Not many ketch cats out there

Really why we love our ketch. Our main is comparable to a 45’ sloop mainsail.

[hlev00]

Very cool. Now I have something other than my honey-do list for today.

I would definitely put those numbers up against the load specs for the blocks, shackles holding them, and the terminating end of the main sheet that looks to be a knot.

Harry

[s/v Jedi]

Jedi rigging Genoa
Fore triangle
⁃ I 58.5’
⁃ J 17.5’
⁃ Jc 17.5’
⁃ Rake 0’

Perimeter
⁃ Luff length 57.5’
⁃ Head width 1”
⁃ Foot round 6”
⁃ Leech hollow 14”
⁃ Head to max hollow% 50%

Clew location
⁃ Leech length 51.75’
⁃ Foot length 20.75’
⁃ Lp% (Jc) 106.743% (18.68’)
⁃ Clew height 43.775”

Area = (I x J / 2) x Lp%
Area = (58.5 x 17.5 / 2) x 1.06743
Area = 546.4 square feet (50.82 m2)

Samson also provides a formula to calculate the amount of force on the clew: (windspeed in mph)^2 x 0.004 x (sail area in square feet) = Sheet Load at Clew in pounds

Rule of thumb for selecting sheet: FOS of 2:1 at maximum wind force should result in a FOS of better than 4:1 for less wind, reefed sail etc. so for line choice: 7,236lbs maximum sheet load at 50kts wind (see below) x 2 = 14,472 pound breaking load. Samson MLX3 12mm $2.10/ft has a tensile strength of 12,000lbs. Samson recommends a maximum loading of 20% of that for longevity so that is 2,400lbs SWL.

That said, for the genoa we must also look at light weather performance. I want to have a very light sheet and MLX3 is strippable, with a coated core so if I go to 10mm (7,700lbs, $1.50/ft) I limit my SWL to 1,540lbs.

= 651 pounds at 15kts wind
= 1,158 pounds at 20kts wind
= 1,809 pounds at 25kts wind (must partially furl with MLX3 10mm)
= 2,604 pounds at 30kts wind (must partially furl with MLX3 12mm)
= 3,546 pounds at 35kts wind
= 4,631 pounds at 40kts wind
= 5,862 pounds at 45kts wind
= 7,236 pounds at 50kts wind

Unfortunately, I do not know the position of the reef marks on the foot of my jib so I can’t calculate forces when furled to those positions. I will need to include that later. Just so that I remember: my genoa is a number 3 (I think) at 106%. So I multiplied the surface area by 1.06 as per formula I used. With those reefing marks, you simply make it a smaller jib, like a 95% so multiply by 0.95 instead of 1.06

But I don’t trust that so I will also measure the luff at those reefing marks and calculate it with that as well. Also, I have no foam in the luff so lousy sail shape when partially furled.

Another big point: I saw YoungBarnacles use a Dyneema leader to the clew of the jib. He explained it’s for adjusting to different sizes head sails so that the different position of the clew is adjusted by that leader so that the sheets are always the same. Brilliant, but I only have this one genoa. However, I can not reach the clew while the sail is furled, so I will make a leader and fix this. I can then also take my genoa sheets off to protect against UV.
For this leader Samson Amsteel Blue in 8mm (13,700lbs, $2.30/ft) is perfect. Eye splice direct to the clew and either an attached soft shackle or just another eye splice and a separate soft shackle.

[s/v Jedi]

Originally Posted by hlev00

Very cool. Now I have something other than my honey-do list for today.

I would definitely put those numbers up against the load specs for the blocks, shackles holding them, and the terminating end of the main sheet that looks to be a knot.

Harry

Definitely a problem. The sheet is tied to the pad eye with a bowline knot. That is 50% loss of tensile strength right there.

The calculations above are for spliced sheets, no knots. MLX3 is a core dependent line that is strippable, which makes splices on the stripped end easy. You can also strip and re-cover for easy splice but more UV protection.

[SaylorMade]

Wow, that is one cool boat.

I think it's safe to say "We are not gonna need a bigger boat"

[s/v Jedi]

Originally Posted by SaylorMade

Wow, that is one cool boat.

I think it's safe to say "We are not gonna need a bigger boat"

So you like the 8mm halyards to work with? I think 10mm is the smallest I want to handle. When I sailed a 30’ boat I also had 10mm halyards

[s/v Jedi]

So for CF members from “metric land” (I’m Dutch myself and use both systems, sometimes one is handier, next time the other )

Some metric numbers. My mainsail is 65 square meters. The mizzen is 49 and the genoa is 51.

All those loads in pounds… divide by 2.2 for converting to kilograms.

And for US members: the 12mm halyards are just under 1/2”. The 10mm halyards are just above 3/8”

I think those are very modest sizes for any boat?

Does anybody know more about halyard loads?