Jami Jokinen wrote:

Phil,

thank you for this!

Does this mean that there is not necessarily need for a keel/bottom -stepped mast support in the cabin at all? That is something that I didn't expect!

I will probably have to go with aluminium mast. I was thinking of getting the tabernacle welded. This probably means that I would have to build a section on top of the curved ply, that makes a straight base for the tabernacle? 

Hey Jami,

The tabernacle itself should be a single structure that runs from the a few feet above the deck, through the deck, right into the step in the bottom of the boat.  My tabernacles are usually like a U shaped tube (viewed from above), made from three pieces of good solid wood, with the inside corners filleted and glassed, the outside corners rounded off and glassed.  The deck is cut with a shaped hole just large enough to let the tabernacle in.  The tabernacle - deck intersection is then filled with thickened epoxy, filleted, and also glassed.  The balsa core will only be exposed for those few days after you've cut the deck and before you've glassed the tabernacle in.  There is no worry there.

For a round aluminum mast, you can build filler pieces that really snug the mast into the socket of the tabernacle. It doesn't have to be perfect, but you want a fair bit of contact area. 

For locking the mast to the tabernacle when in the mast is up, you can fabricate a filler piece for the inside of the mast to support a through bolt.  I've seen many masts for boats your size that don't bother with the filler though, and they haven't broken, so that may be a belt-and-suspenders approach.

It looks like you've actually got a great boat to do this with. 

Regarding the mast and the support needed, my situation is different from your projects, because I want to save the easy trailerability of the boat. This means that I have to build a tabernacle. How do I do this with the curved foredeck with balsa core and without compromising the strenth of the rig is still very foggy to me.

Jamie

I have have done two conversions, an O'day 23 in the 90's and a Com-pac 19 completed last year. The 23 had a tabernacle the 19 does not. The balsa cored deck shouldn't be a worry.  You are gong to re-enforce your deck where the tabernacle comes through. That reinforcement, your deck, including to outer deck, core and inner deck and tabernacle will act as one unit and provide plenty of strength.  Your partners will be simpler than with a stepped mast and the tabernacle will be sealed to the deck with no need to build a collar or boot or deal with wedges. No deck core will be exposed.

Your deck is quite curved and as David pointed out giving very good strength. After deciding on the location I would cut that first piece of ply longer than wide and bend it to your deck along the the grain of the outside layer of the ply. It should follow the curve nicely with a some pressure. If it doesn't, use a longer piece of the same width. Follow mfg instructions preparing the deck, coating the ply and applying thickened epoxy. Temporarily screw the first piece of ply down though the deck. If needed a helper could hold a block of wood under the deck to catch the screws for a good bite. With the first layer glued and cured remove the screws and match the next layer up and do the same and then again. You may be able to do the top layers together. I through bolted the ply on my 23 and 19. On the com-pac I used small squares of 1/2 inch ply like washers under the deck to avoid compression. I don't know if you need bolts If you have enough gluing surface. Cut the hole for the tabernacle to fit tight and epoxy in place or if you think you may have to remove it for any reason just seal it well. The tabernacle base will be secured in its step.

How difficult the tabernacle is can depend on the mast. My previous tabernacle was as simple as possible. It consisted of two 3 x 4.5 inch built up spruce uprights above the deck with separating blocks in between them at deck / partner level and at the step. The 4.75 inch wood mast was square and without any rounding for the first 3 feet and was through bolted at the reinforced pivot point. A tabernacle for an aluminum mast is has to be more involved than that. I played around with a number of tabernacle ideas for the 19 but then moved close to the town dock last year, got a mooring and went with a stepped mast. May be someone here that will have a good design solution if you are wanting to go with aluminum.

Jami,

I have fitted junkrigs on a few boats now, including an Albin Viggen (Malena), and my present Marieholm IF (Ingeborg)  -  boats that you may know well. Here is an album, which shows some details from building the mast step and partners for the IF: Remember that this boat is probably more than three times as heavy as your Joe 17, so you can get away with much lighter dimensions.

In addition, here and here are two little write-ups which show the sailmaking and rigging process of my 6.5m/750kg “Frøken Sørensen”. This one is closer in size and weight with your boat.

Good luck!

Arne

I have many details about the coming structure to ask about, but maybe I'll start a thread about my boat as soon as I get the first drawings ready.

OK, that's good, the hull and deck together form a near-circular ring frame, which is just what we need. So the square reinforcement around the tabernacle can be three layers of 5mm plywood, with temporary screws holding them to the curvature while the epoxy cures.