View Full Version : Boom jaw dimension

01-17-2015, 11:22 AM
Ive been gluing up the solid mizzen mast and boom for my Caledonia Yawl. I'd like to make some jaws for the boom and use parrel beads as opposed to the sprit rig..

Just looking for some advice on what size and shape the boom jaws should be. Is it best to epoxy them on and bolt them through. Does anyone have a template I could see/ use? I have some 1 inch think oak I plan to use.

01-17-2015, 12:38 PM
Just had a look at various threads and about 8" gluing on the boom with two bolts through seems to be what others have done.

I'll get cracking tomorrow, unless someone points out I'm miles off.

01-18-2015, 02:03 AM
From what I can tell, the standard mizzen on a CY is a Sprit boomed Leg 'O Mutton sail. The reason for this is that is is a self vanging style of sail with the sprit boom providing both outhaul and vang via the downward angle of the boom. If you switch to a standard horizontal (ish) jawed boom then you will have to provide some kind of vang to keep the boom from lifting. Problem is is that the boomkin for a CY projects at an angle rather than directly aft of the stem. How are you going to provide a balanced point to attach the vang to the boat?

01-18-2015, 04:30 AM
Good point!!- well made. Hadnt thought of that. Can anybody share photos/ drawings/ experiences of what you've got and how you find it?

Jim Ledger
01-18-2015, 05:53 AM
There's an article in issue #59 called "The Right Jaws for Your Gaff and Boom", written by Bill Garden which I have always used as a reference. All the dimensions are given as a function od mast diameter, so it's scalable to different sized masts.

01-18-2015, 08:38 AM
Here are the results for a google search for sailboat boomkin. You might find something in there. Most tend to be of the Vee or wishbone arrangement.


01-18-2015, 09:06 AM
I followed one of the links and found this:


looks like an interesting site but this is the boomkin building page. He made two (kind of a wishbone) out of laminated Hickory. Looks nice but I don't know if having a sail with a jawed boom is worth the trouble. Like Geoff says over on the Off Center Harbor build of a CY... He hasn't needed to adjust his sprit boom more than a couple of times in 20 years. You might be able to catch the video for free if you visit the site. It is the most recent video posted in the series and the second to last of 42 vids on the build.

Peerie Maa
01-18-2015, 02:49 PM
If you want jaws, these are the ones that I made for Perrie Maa's gaff. Something resilient like oak or ash is best

They are 1 1/8 x 7/8 at the thickest, the arms are 3/8" thick, and the overlap riveted to the spar are 6 3/4" long.

01-18-2015, 04:43 PM
Thanks for these links, most useful. Lots to think about!

I thought it might be worth trying without a boomkin, and leading the mizzen sheet to a block on top of the rudder stock. I appreciate I'll miss out on the leverage, but will be simpler. I suppose fiddling with the rigs is part of the fun on these boats!

01-18-2015, 10:41 PM
Be sure to make the jaws wide enough to take being leathered and slid way down the mast. Mine are a bit tight but haven't broken yet, made from white oak.





01-24-2015, 11:59 AM
Been thinking some more about this....
If I take the mizzen sheet from the boom above the top of the rudder stock ( which I think will be about half way along the boom, will this provide enough of a vang effect, though I do appreciate I'll miss out on some of the leverage effect provided by a boomkin?

I have never seen a sprit boom, and there arent any near me I can have a look at. If the sprit boom is better, then I'm up for it, just not sure how to do it (and cant quite work it out from the plans).

I've got the mizzen mast 16 sided so am making some progress.