Saturday, January 16, 2016

A Yawl Rig Proposal


I've worked up yawl rig here that provides a total sail area of 553 s.f, with a 506 s.f. main and a mizzen of 47 s.f.  I have yet to design a rudder and lee boards but can adapt their design and placement to match this rig.

My plan is to build a mast from 6061-T6 aluminum extrusions.  It can be in two pieces telescoped together, with a lower section of 7 inch diameter by .125 wall thickness 15 feet long, and an upper section of 6 inch diameter by .125 wall thickness 16 feet long.  The mast would weigh about 92 lbs.  The large diameters provide a lot of strength but allow lighter construction - important for ease of lowering for the bridges.  I will need to make a spacer to adapt them together since the inner diameter of the 7 inch and outer diameter of the six inch are off.

The mast extends approximately 25'4" above the tabernacle.

The yard and boom are 3 inch extrusion, with battens of 1-1/2 inch extrusion.  Boom and battens are 22 feet long and the yard is 18-1/2 feet long.

The mizzen is mounted on a tabernacle over the engine well, and pivots forward.  I've eliminated the need for a boomkin by carrying the boom forward with a tiller arrangement. I can haul the boom into desired position with the tiller and drop pin it to secure.  The tiller can fold out of the way.  I'll just have to move it when I tack.  The main sail sheet can be secured on the mizzen mast.  At the dock it can all be swung 90 degrees out of the way.


I am going to need some help to determine if the mast is strong enough with the 1/8 inch thick material.  If I have to (the boat has a HUGE righting moment) I can always go to the .250 thickness on the lower section.  I shouldn't need to beef up the upper one.





5 comments:

  1. Looking good, but a few comments:

    RE (Fore)Deck Slope... This looks a bit steep, to me. I'd be a little concerned that it would be less sure underfoot (while handling anchor gear in bad conditions, say) and be uncomfortable for sitting without compensating cushions (butt creep). Also things tend to roll, easily with more slope.

    We use 1/2in per foot as our standard slope for and aft. Sheds water well, but without side-effects.

    RE Mizzen Boom (and boomkin)... One advantage of the sprit boom (angling down and aft from well above the tack to the clew) is that it's self-vanging. This keeps the sail spread and flat without undue sheet tension, which only has to control lateral swing (not boom lift).

    Given the sheet lead and foot boom, you may get some lift/twist to the sail.

    A boomkin (even a very short one) can improve this lead under either scenario.

    RE Mizzen Boom 'Tiller'... The extension, as shown, could be simplified? For instance, a solid boom could be set and extend fwd on one side of the mast. Alternatively, the tiller could be mounted offset on one side of the boom to avoid in-line complications.

    In LUNA, we used a sprit-boom which naturally extends fwd of the mast, at an angle which cleared our (sitting) heads. Thus no swing-up mizzen tiller, self-vanging, easy furling, and the clew is 'hauled out' (tensioned) with its (inboard) snotter. It's possible this may be a simpler, overall system?

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    1. I can and will adjust the fore deck no problem. My head is spinning though as far as rig arrangement is concerned. Based on our talk, and feedback from the JRA website I'm thinking I need to defer to the expertise and go with a double mast arrangement. If I do so to keep the complexity in check I am considering fixed masts. The downside is getting bridge swings when required - some inconvenience but when weighed against the extra designing and building work maybe worth it? I don't know. I'm going to take a little break from thinking so hard on it though. Perhaps something will gel soon...

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  2. Of three different junk sails we have used on our boats the 420 square footer was a handful. Quite a bit easier to manage once we dropped it down to 330 on the same mast. Lunas 225ers were a pleasure. Something to consider for us older sailors. I love the junk rig but on a boat that is not going to challenge you too much regarding a slippery, much tilted deck in seas that will never get super large I'd be open to other rigs. With a reference like one of Bolgers boat rigs books something else might appeal to you. But..... sure is nice to reef from the cockpit!

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  3. Possibility of the traditional three master used on junks? This would keep the aspect lower and thus heeling force. The biggest sail would be centered up weight wise a bit more. Maybe the rear sail a sprit sail for heaving to and raising sail? Lastly Bolger designed a three master with 421 ft. sqr. dipping lug as the main on this boat, the Weston Martyr, with two sprits on each end.
    http://hallman.org/bolger/isometrics#WestonMartyr It's all fun. Best abuilding to you.

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    1. I'm working out a three masted rig right now...will post soon. Thanks Robert.

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