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Old 24-02-2019, 11:51   #16
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Join Date: Oct 2007
Location: San Francisco
Boat: N/M 45
Posts: 290
Re: Rigging boom vang

Quote:
Originally Posted by svsonora View Post
I imagine if I go smaller than 30 degrees, the 'downward' force on the boom will be deminished and that I'd be wasting too much force into compressing the boom instead.

Is there a way for me to optimize this angle?
Sure, it's straight forward in a simplified way; it's rather more complicated to account for loads when they aren't on boom centerline such as offset from mast due to gooseneck, offset from boom centerline for vang attachment, clew attachment above boom centerline, boom vertical dimension, angle the boom makes with the mast, etc.

The simplest way to see the effects of vang location is to ignore the effects of gravity (acceleration), keep the boom horizontal, and put the loads into equilibrium - which doesn't exactly reflect reality as a boat is a moving dynamic system. That's where fudge/safety factors come in (often 3x gravity/acceleration is a safety factor for a cruising boat, you might go as low as 1.5-1.2x for a race boat if you don't mind breaking things).

Forces on the boom:
Force vertical (up) at gooseneck (FG)
X1 distance from gooseneck to vang attachment point
Force vertical (up) at clew (FC)
X2 distance from clew to vang attachment point
Force vertical (down) from vang at vang attachment point (FV)

(FG)(X1) = (FC)(X2) -> a balanced lever arm or teeter-totter

FG + FC = FV -> system at equilibrium (no translation up or down)

for a Bristol 30, the numbers I found are:
boom length 156"
P = 29.17', E = 12.91'
I don't know the height of the gooseneck above a possible vang attachment point on the mast, I'll guess 36" - you can measure it and play around with the numbers.

Harken's mainsheet load calculator comes up with 334 pounds mainsheet load at full hoist sailing upwind in 15 knots of wind - this factors in righting moment by letting you set how much breeze you can carry a full mainsail upwind in. We'll go with it, it's a number for purposes of demonstrating the effect of moving the vangsheet attachment around on the boom.

vang attachment point on the boom aft of the gooseneck for a vang angle of 30 degrees at the boom with the boom perpendicular to the mast):

X1 = (36")(sin(60))/sin(30) = 62.4"
X2 = 156" - 62.4" = 93.6"
FC = 334 pounds
(FG)(62.4) = (334)(93.6), FG = 501 pounds
FV = 501 + 334 = 835 pounds vertically down

vang loads using a 45 degree angle attachment to the boom:

X1 = (36")(sin(45))/sin(45) = 36"
X2 = 156" - 36" = 120"
FC = 334 pounds
(FG)(36)=(334)(120), FG = 1113 pounds
FV = 1113 + 334 = 1447 pounds vertically down

What you want to do is try out (calculate) different vang angles to minimize FV and FG while maintaining FC. FV will reach a point where the boom cannot handle the forces (boom will bend), you can also exceed the shear strength of the gooseneck clevis pin - usually the boom breaks first. You'd like to position the vang attachment point to minimize bending while still holding onto the mainsheet loads (this all assumes the mast is strong enough to take these loads at the gooseneck).

To size the vang blocks, an 835 pound force down in a 30-60-90 arrangement requires 1670 pounds along the hypotenuse (vang), multiply by 3 (fudge factor) and you're looking at a block & tackle setup that can handle a 5010 pound load before breaking.

I believe I did the math right, if there's an error up there please let me know.

- rob
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Old 24-02-2019, 21:43   #17
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Re: Rigging boom vang

Thank you Rob.
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