Cruisers Forum

  This discussion is proudly sponsored by:
Please support our sponsors and let them know you heard about their products on Cruisers Forums. Advertise Here
Thread Tools Search this Thread Rate Thread Display Modes
Old 24-02-2019, 11:51   #16
Registered User

Join Date: Oct 2007
Location: San Francisco
Boat: N/M 45
Posts: 290
Re: Rigging boom vang

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
beetle is offline   Reply With Quote
Old 24-02-2019, 21:43   #17
Registered User

Join Date: May 2018
Location: Washington DC
Boat: Bristol 30
Posts: 104
Re: Rigging boom vang

Thank you Rob.
Bristol 30 S/V Sonora del Viento
Dancing with Sonora
svsonora is offline   Reply With Quote

boom vang, rigging

Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off

Similar Threads
Thread Thread Starter Forum Replies Last Post
Rigging boom vang on main sail help theglo Deck hardware: Rigging, Sails & Hoisting 29 29-01-2019 21:45
Navtec Boom Vang Hydraulic - Leaking avazquez Construction, Maintenance & Refit 35 04-05-2018 15:49
Boomkicker vs Rigid Vang vs 'Soft' Vang finleydc Deck hardware: Rigging, Sails & Hoisting 24 20-06-2013 15:54
Boom Vang SkiprJohn Deck hardware: Rigging, Sails & Hoisting 20 09-05-2008 14:33
To Vang or not to Vang ihhartley Multihull Sailboats 10 07-12-2006 07:46

Advertise Here

All times are GMT -7. The time now is 06:12.

Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2022, vBulletin Solutions, Inc.
Social Knowledge Networks
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2022, vBulletin Solutions, Inc.

ShowCase vBulletin Plugins by Drive Thru Online, Inc.