Working load on gennaker furlers - SailNet Community
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post #1 of 2 Old 02-27-2012 Thread Starter
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Working load on gennaker furlers

I looked at getting a gennaker furler. In some places the sizes are given wrongly but eg the manufacturer gives for Facnor -

Expected loads on Code Zero and Gennaker furlers:

FX 900 FX1500 FX2500 FX4500 FX7000 FX12000 FX20000
Boat length 6
to 10,5 m
20' to 30' 10
to 12 m
30' to 40' 12
to 14 m
40' to 45' 13
to 16 m
42' to 55' 16
to 20 m
55' to 70' + 20 m
+ 70' +25m
+ 82'

Sail area
advised 30 m2 /
322ft2 55m2 /
590 ft2 80 m2 /
861 ft2 140 m2 /
1500 ft2 250 m2 /
2690 ft2 350 m2 /
3770 ft2 +500 m2 /
+5380 ft2

load 900 kg
1984 lbs 1,5 tons
3306 lbs 2,5 tons
5511 lbs 4,5 tons
9920 lbs 7 tons
15432 lbs 12 tons
26455 lbs 20 tons
44100 bs

Now my boat is 32’ say 9.6 m. Theoretically the FX 900 would work or say the FX1500 with 322 sq ft of sail advised or 590.

The safe working loads are 900 kg or 1.5 tons.
When it comes to calculating the force on the sail it seems that is more complex. The starting point is WL = SA x V2 x 0.00431
WL = wind load in pounds
SA = sail area in square feet
V = wind velocity in knots

Downwind the force may be the wind pressure but seemingly at other angles according to Marchaj etc the force is 1.5 x WL

According to this comment which I don’t really follow in Ocean Navigator the force can be analysed in vectors. Ok I get three dimensions. But then it goes –
RL= 0.125 WF x LL / LS
Where LL is the length of the luff and LS is the sag of the luff.
Assuming 6” stretch in a forty foot luff he gets a force 10 x WP!
Ocean Navigator | The magazine for long-distance offshore sailing and power voyaging.
I don’t understand the first term that an eight of the force is directed in some direction and that that is relevant. I would have thought the majority of the force was perpendicular to the sail and the fitting should be adequate for at least that load in any direction. The second part related to stretch may relate to dynamic load an area of which I have little knowledge but twenty times?

Just taking the WL as 1.5 x SA x V2 x .00431 and solving for wind speed on the recommended area one gets 31 knots if my arithemetic is right.
At say 25 knots taking 15 as the basic upper wind spped for a light air sail and allowing for 10 knot gusts one gets a sail area of 495.
Since the local unit I was looking at has a safe WL of 950 kg or 30m2 and my recommended sail area is 640 sq ft, I conclude that that size unit would only work up to gusts of around 22 knots, maybe slightly more as the sail cloth area is greater than the projected area. That may be ok but seems pushing it a bit. The alternative would be a different brand eg the Facnor 1500.
Am I missing something?
Frankly I suspect a smaller gennaker of 500 sq ft would be fine, but using an existing spinnaker and just getting a snuffer would be far cheaper compared to a new sail and furler, although the latter would be easier for singlehanding
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post #2 of 2 Old 02-27-2012
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Too many calculations for me. As important as the size of the boat is its weight.

On my 36 ft with 5500kg I used (recommended by the designer) a 64m2 code 0.

Of course this also depends on the RM of the boat but if you find similar boats to the one you have in what regards size beam, ballast and type of keel with information about the size of the geenaker you can safely assume a similar value.

Pay attention that on a furling gennaker there is more pressure up, so if you use a pole it is better to attach it to the hull by a cable or a rod.


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