The good thing with tabernacles is that they are fairly easy to make stronger than the mast because of the simple fact that they must have bigger sections. Bending strength varies with the cube of the cross section.

During the short period of time that I used hinged battens, I actually made the hinge parts so that they sat on the outside of the battens, for the very same reason. These never failed.

Arne

David wrote:

Hi Scott,

[...] A much better shape is a top hat section that has wings on the forward edges of the side members, of a combined cross sectional area equal to that of the back face. [...]

Hi David,

Thanks for the response and the suggestion to use the top hat section. If I decide to use an alloy mast I will most likely use a beam made in this way.

I have not been able to find any information on how to calculate the Section Modulus for this type of section.

I found the math to calculate the Section Modulus for a C section. My idea was to determine if C section is strong enough. This would ensure that the top hat section is also strong enough. If I was able to get the equations for a top hat section then I would use the result directly. Do you know how I can calculate this value?

Anonymous wrote:

I'm planning a tabernacle for the Brenda B, my Com-pac 19. The sail is in two pieces, roped with webbing and waiting for some heavier material to finish the short batten pockets by the mast. The 4 ½ inch x .125 inch 6061 t6 aluminum mast tube is on order and the snow is melting off the boat cover. My recent sailing has been from a dock on a lake a several miles from home which means I can be sailing in minutes, and with even less hassle with the new junk sail. While this increases my sailing time and allows things like for spur of the moment supper and evening sails, I miss time sailing out among the islands of Penobscot Bay. The tabernacle will make getting to the coast for a cruise or a temporary mooring much simpler than a keel stepped mast.

Pictures of Daze Z's Slacktide tabernacles and Pete Hill's tabernacle write up in JRA mag #61 have given me some new ideas. I'd been thinking that to hold a round aluminum mast in a square tabernacle I would need half round shapes at the tabernacle's top and base to hold and clamp the mast. Slack tide does well with a round solid wood mast clamped square in a square tabernacle. It seems that that this would would work with hollow aluminum. Is this sound thinking? After seeing how the two tabernacles pivot I decided to use a wide heavy duty s.s. strap clamp designed for pipes to clamp the mast to a rugged hinge with the other flap of the hinge lagged to the back of the tabernacle. The intended pipe size is 4 3/8 inches and the space could be shimmed tight with leather providing a little cushioning.

Personally, with only .125 wall thickness, I would want to put an insert inside the lower end of the mast tube to prevent flattening from pressure.   6061 T6 is superb material, probably the best aluminum for both strength and resistance to cracking.    I would avoid drilling it if possible, and if I did drill, I would take great care, using a drill smaller than the final size and a fixed size chucking reamer, and paying great attention to the corners of the holes so they are extremely smooth.... Even the smallest irregularity here can be the point where a crack can start. (this based on aviation experience) Perhaps building the bottom of the mast up with carbon tow and embedding a bushing in it similar what Pete did on Oryx.... see JRA#61.   Also there is a Utube slide show on the KD 860 that has a couple clips of someone building a chainplate this way:   https://www.youtube.com/watch?time_continue=1&v=Bv-ath_xmjY

                                                           Howard

I've had to cancel my offshore sailing plans again, I relaunched about a month ago and everything looked fine, but on my way to Nova Scotia stress cracks have appeared in the mainmast tabernacle and are spreading rapidly. The cause seems to be a tortional stress which has been a problem all along. The trouble is that the lateral stresses are not being taken up evenly fore and aft and are causing a twisting moment which I hadn't expected. I'll be hauling out again for repairs and hopefully get things right this time, I was planning to write an article about these tabernacles thinking they were a success, now I'm glad I didn't. When I do get things right I'll share what I've learned but more work and more testing are needed

I have sailed a pointy sailed boat once with a wooden mast counter balanced with a length of concrete bolted to it , it came down and up no trouble , probably acted as ballast as well   .     alloy tubes don,t do  well in  compression perhaps a hard wooden or steel former of a fair length in side might do .