Converting keel stepped mast to deck stepped

If the rig will hold the mast up alone and a suitable deck step is fabricated and the compression post is adequate and continuous from keel compression point to deck step, sure this will work. But all it gains is a small amount of room below, gets rid of a possible source of deck leaks, and will cost at least $20k even using the mast you have.
Several production boats with deck stepped masts I have examined have had inadequate steps which were failing from poor design for the strong compression loads, and corrosion, so be careful what you do.
By going to deck stepped, all rigging loads are transferred to rig tension and mast compression, with little bending strain involved. Going downwind with the big spinnaker up and busting into waves adds more load, so check carefully all chain plates and rigging bits for tiny cracks or imperfections.

 

Northman,

Figure a new rig and rigging will run at least 20k, add in another 5-10 for for new sails, and running rigging. A couple of thousand. For the yard work to replace the rigging. Perhaps 2-3 to rebuild the deck where the current partners are... By the time it's all added up you are in the 30-40k range perhaps more.

This is assuming you don't need to rebuild the chainplates, hull supports, ect. Add in that and you are easily into the 40-50,000 range.

 

I dont understand why this is such a problem and the cost talked about are ridiculous.

I had 62 steel hull Roberts Mauritious with a keel step mast , it leaked and I could not stop it. I took off the boot on the mast, cut off the lip surrounding the mast that held the boot without cutting the mast and drew a line round it at floor level.

I considered not even removing the mast but loosing stays etc to raise it using the goose neck to get the new foot under it. I decided to remove the mast.

I bought some st steel approx 8mm sheet, shaped a nice oval shape and made a 3 inch high ring in similar metal that fit the mast shape, it was quite deep taking into consideration the PVC chopping board that would be fitted in the bottom of the mast seat just for something to sit on. The ring was welded to the plate with little distortion with small knees made from the ring to plate with 1/2 inch holes that became very handy for snapping shackles to.

The knees were just flat 3 inch plate cut into 3 inch squares and rounded on one corner.


The hole in the roof was welded up with matching steel and a 4x4 inch compression post welded in from roof to the massive keel mast support bracket that some one had made.
The new foot was bolted over the welded plate as near as possible to the mast original position with large counter sunk stainless bolts. I think I sat it on a sheet of rubber --again just for something to sit on.

The 70foot mast was laid out on the marina yard and I cut it 8mm plus the pvc chopping board thickness higher than the mark on the mast.

The crane came and scooped it up and put it back in.

6 inch of stainless pipe was welded just for of the foot but still on the foot plate and 2 x90degree bends to make a water proof access to the wiring --radar etc.

Cost errr,--- maybe a hacksaw blade some st steel and a crane lift.

Still think you could jack under the goose neck, keep it tight and slip a new foot under it, even with 70 foot 1/2 ton mast.

I don't think it was much more than a few hours work apart from a visit to the machine shop.

 

Machine shop !!! Crane !!! STAINLESS STEEL !!!!!!!!!!!

Holy Mackeral Frosty, thios sounds expensive to me. Peter Blake on Ceramco converted from keel stepped to deck stepped ....in mid ocean...for a fraction of that cost.

http://volvooceanrace.com/en/Legends-Wings-clipped-but-still-racing/2987/news.html

 

Well you could cut back a bit if you already had a hack saw blade. The pvc chopping block was 2 dollars.

Yup good ole Peter there is those that can and those that talk about it.


Not really a keel step though is it. He strapped the upper broken bit to the lower bit still left sticking out.

 

Why would you do any of that? It's a steel hull not a feeble f/g one.

Walter, check out Euler's Formula, a keel stepped mast is a lot stronger than a deck stepped one so the tube needs to be up to the loads. A deck stepped mast in a properly designed tabernacle is as rigid as a keel stepped one, FWIW, but you need someone competent to design the tabernacle.

I agree with Frosty, if the mast section is adequate then the cost of the swapover is going to be small provided you can do the work yourself.

I'm planning on putting my masts in tabernacles to avoid deck penetrations.

PDW

 

I may be wrong here, but it seems unlikely that you would be able to reuse the same mast. Wall thickness and diameter would be different on the deck step mast. When you replace the mast you are almost defiantly going to need to replace the standing rigging, and may also need to reinforce the chainplates, or move them.

The new sails are dictated by the new rig and shroud design. You might get away with reusing them, but as long as you are going to put in a new mast, you likely will want to either go taller or shorter depending on the prevailing conditions. If not, then I guess you could reuse the same sails and accept the penalty for them being cut for a different flexibility of the mast.

I did miss the fact we were dealing with a steel hull, so reinforcing the deck and closing the old hull may not be such a big deal.

 

I think your thinking about this wrong. What is the difference the mast forces are distributed to the same place.

You have not altered anything but the mast has now a continuation of it under the deck roof. Forces from the sail or shrouds or what ever you want still go down to the base of the keel as before.

 

Not quite. You *have* altered something. You've gone from a mast that was in a fixed/pinned state to one which is now in a pinned/pinned state. Look at Euler's Formula, you will see that pinned/pinned is not as stiff therefore a stiffer/stronger section mast *might* be required.

The vertical load is OK, sure, with a compression post, not necessarily the mast itself.

PDW

 

If the intent is to use a tabernacle and rig it so you can raise and lower the mast without outside assistance then it could be well worthwhile if you plan on keeping the boat a while, i love being self sufficient, i have a friend who has transited the erie canal several times and would have appreciated this feature. On the other hand i dont see much point if this is not the goal. If you go ahead it does not need to be an expensive proposition at all, you can sleeve the bottom of the mast if you need to internally with alluminum or externally with epoxy/glass, i see no reason why you would need new sails or even standing rigging if you are careful although if it is due for replacement anyway this would be the time to do it.
Steve.

 

Frosty, thanks for the thumps up! I really like that BOM!

Peter, you are so right

Eulers formula is way over my head. As I said, the whole rig, mast, standing, chainplates and all, are rock solid. I will take the previous advice given and ask a mast supplier for dimensions for a deck stepped mast of the same height, including dimensions for the standing rigging and compare it to what I have now. Actually I haven't yet; I am still negotiating.

Peter: how is yours coming?

Walter

 

What advantage are you seeking with this modification, Walter?

 

Walter

The real issue chopping a keel stepped mast off at deck level is that the mast will not be stiff enough under compression and buckle if it doesn’t have what we term “end fixity”. ( Unless the section you have already suits which it may since masts are always the next size up from the requirement).

If you add a reasonable tabernacle you’ll achieve the same level of end fixity and nothing else with the existing rig will need changing or calculating to shorten it.

If you choose a fixed tabernacle base arrangement then there’s a bending moment that is transferred from the tabernacle and its best to let this carry on into the compression post via a through bolted bracketed flange connection rather than transferring it into the deck plate.
The steel deck is usually either doubled in way of the mast or has a thicker insert to spread the load into the thinner deck plate. Otherwise there’s no other structural deck requirement.

The only issue is weight of the tabernacle wrt stability. Which I doubt is an issue in your case.

 

The hull is essentially finished. I've ground back all the welds on the deck & hull exterior and am making the 'fiddly bits' now - chain plates, bitts, steering gear, filler bases etc etc. Lots of work but after spending a lot of time with angle grinders, what a relief. I just dug out my TIG welding gear too.

I knocked off for nearly 3 months & went travelling so haven't been back at it for long.

BTW I don't like the location of the shroud bases on yours OK having them inboard like that means you can sheet in harder but it's weaker and an obstruction going forward. That galley flue looks interesting though, I assume a diesel stove? I've been keeping my eyes open for a diesel heater, nothing I can afford so far.

Buy the thing if it passes survey & go sailing. If you head this way then I've got the shop space & tools for you to do whatever mods travelling halfway round the planet indicates you need. I don't think it's much of a drama changing that mast from keel stepped to deck stepped but if it isn't leaking I don't think I'd bother, myself. All you're going to gain is a little extra 'visual' space.

PDW

 

BINGO.

Since we don't know anything about the rig that is in the boat no one can say if the section will allow this change.

Given that, I wonder what the OP will tell the spar builder when they ask him what the RM of his boat is?