Mizzenmast spreaders

Greetings all. I acquired a nice mizzenmast mast and would like to convert the athwart ship twin spreaders to twin 20 degree swept back spreaders. The original spreaders were thru bars. I’ll cover the openings with sufficient formed aluminum so that I will have enough material to weld ears on and increase the spreader lengths. My question is the length of both sets of the spreaders.
Is the length of the spreaders only determined by the shroud angle?
Is there a fixed ratio of lengths if one uses continuous rigging.
The length of the mizzenmast is 42’ .Boom 12’. Sail area 215. Stepped on deck. The design was for a single spreader swept 20 degrees aft with a length of 48 inches.
Kindest regards. Mark

 

Hi Mark - I presume that this mizzen mast is for the Shenanigan design by Ted Brewer mentioned in your first thread on here?
Reducing air draft calculations https://www.boatdesign.net/threads/reducing-air-draft-calculations.62999/

Here is some info on Shenanigan from Ted's site -
Ted Brewer Yacht Design https://www.tedbrewer.com/sail_aluminum/shenanigan.htm

And a copy of the sail plan is posted below.

Your main mast is 62' above the waterline, so I am estimating approx 56' above the deck level - and your new mizzen mast is 42' above deck level.
I am estimating that your new mizzen mast will be approx 3' higher than the one shown in the sail plan below.
Twin spreaders for the mizzen sounds unnecessarily complicated, but you have the mast now, so you might as well use it.
Re spreader length, you might as well make them as long as possible, such that the cap shroud and the intermediate cap come down vertically (or almost vertically) to the chainplate(s)?

 

Thank you very much. The mizzenmast I have is only one foot longer. My main mast will be 64’ above waterline.
I thought two sets of spreaders would be stronger on the mizzenmast. I’m not sure why the design has a swept back spreader but the chain plates are welded in and I might as well use them.
I like the idea of keeping the cap shrouds vertical.
The new mizzenmast is pretty beefy. I just could not find a lot of information on the designing of the length of spreaders.
Lots on the strength and minimum angles but I felt I could use a professional second opinion.
As always, my sincere appreciation for your advice.
Mark

 

Re your mizzen mast, how are you planning on supporting it at the forward end?
Some ketches have a triatic stay from the masthead of the mizzen to the masthead of the main mast.
The sail plan above shows the mizzen stayed independently of the main - but it is a single spreader rig, and yours is double spreader.
If you don't have a triatic stay, would you have forward lowers on each side from both spreaders?
This would require 4 stays, as opposed to one between the mastheads.

 

Hi. I’m not completely committed to two sets of spreaders. I’m going to draw out both options because the lowers might be more of a hassle to walk by with two sets of spreaders. Right now, all the mizzenmast chainplates are at the rail and so is the backstays for the main mast which I’ll run independent to the mainmast cap.
What doesn’t show on the plans is a very large permanent pipe Bimini which begins just forward of the mizzenmast and continues forward to the main boom traveler. It’s about 10’x 12’ all welded 1.5” pipe . Four solar panels on top and the center 2’ is aluminum dock panel. I can walk on it to furl the mast. I should be able to clear the Bimini going from just below the upper spreader to the forward chainplates. It looks like a triatic might be my best option to add support as well.
I’m almost finished building the mainmast. I’d love to have the wishbone ketch rig Bruce King had on his boat but I’m getting too old and poor. The mizzenmast was the mainmast on a catamaran and I can fly a lot of different sails off it.
I’d love to build carbon fiber wishbones and ditch the main boom. Manatee dreams.
Again, thanks for the help.
Mark