Bury of unstayed mast in tabernacle?

Hi, I have a Bolger designed Sparkler (5.8m LOA x 1.5m beam). it is a flat bottom centerboarder, shallow draft, 150mm board up. I want to add a tabernacle, in a well, for easier stepping of the mast and to make way for a removable cabin and need some help.

The mast is 5.3m from heel to halyard block (standing lug rig), unstayed, and carries 10 sqm of sail.

My questions are;

Is 390 mm bury from heel to deck adequate to support an unstayed mast given the overall height of the tabernacle (1020mm)?

Will extending the tabernacle below the well floor to the bottom of the boat (see dotted lines in diagram) add to overall strength? (i would prefer to avoid the construction complications of this)

I hope the diagram below is clear enough to explain.

 

Absolute minimum "bury for the mast is 10%, in your case 530mm, that would be the minimum of supported mast in the tabernacle. If you can extend the tabernacle down to the sole that will be beneficial, unless it can be thru-bolted in a well bonded and supported bulkhead, as this will be taking all the mast strain.
Edit to add: If you can tie in the rear of the tabernacle with the cabin front with blocking, you will gain in reducing loads.

 

You will note that the build is similar to what you have drawn, but reinforcement timber is on the opposite side of the bulkhead and taken down almost to the sole

 

Thanks for your reply Skaraborgcraft.
But to clarify; does the support for the mast extend to the top of the tabernacle, that is does the 10% include the entire tabernacle? Or only to the mast partners, i.e. the deck in this case?
In the first case I have ample support, in the second, not enough.
The tabernacle will be through bolted to a bulkhead and i can, and will extend the tabernacle to the sole if, as you say, it will be beneficial. The cabin is to be removable (probably my next post question), so i cant use it to further secure the tabernacle.

 

hi again Skaraborgcraft
Maybe your diagram annswers my last question. Here it looks as though the mast steps on the deck and has only the tabernacle for support above the deck. So i should have more than adequate support?
I notice also that the pins in this diagam are positioned aft and forward, above and below repectively. That, i imagine, is to take the strain of the mast pulling backward from the top; is that correct?
thanks again

 

I usually calculate the length of the cheeks that are either side of the mast as the "bury", on the understanding that the tabernacle itself is 100% securely fastened. Your diagram shows you have enough mast support, more than 10% increases the safety margin.

I believe that lower "aft pin", is just used as a stop, to make sure the mast takes the right amount of rake. It does look as though the mast is level with the deck, but is actually an open bow well. Hope that helps. Your drawing looks fine, just be sure to back up the tabernacle on the opposite side of the bulkhead. You could always double up either side of the tabernacle at deck level another 300mm.

 

much appreciated. That gives me confidence to go ahead. I will beef up the bulkhead and through bolt to it, as your drawing indicates.
Your drawing also seems to indicate the tabernacle starting at the well floor, not extending to the sole. do you think it unnecessary to extend it to the sole? it would complicate the build a little but I guess it would mean the load is transferred to the sole and over a larger area of the bulkhead, which is better than having it on the well floor, so probably worth doing, beneficial as you have already said. But if it is a complete overkill and unnecessary, i could do without the complication.

 

I would always prefer to take the tabernacle down to the sole and a cross member, even if it meant cutting for access. The loading being compression, can be done with the bulkhead and some bracing though, if its done right.

The free standing mast on my Michalak family skiff, landed on a built up layer of plywood and a mast gate, with a bury of around 600mm. Simple and solid if the mast is light enough to manhandle.

 

That's a very cute looking little boat, clean too. I'd like to think i could get the well and tabernacle done by summer, but its approaching fast and being an alteration to an older boat there is a lot of scraping to do to get a good gluing surface. but I'll be sure post the results here when it is done.
Cheers, and happy sailing to you.