G Man Posted January 25, 2012 Report Share Posted January 25, 2012 Well, REALLY pushed Last Mango (Behaven #40) the other day and the mizzen was dismasted. It broke right around the pivot hole, just above the tabernackle. The mast is solid meranti and I found the issue. When originally sighting the hole, I was slighty out and had to oval out a tad to make it fit. Bingo! One of those things that always stuck in the back of my mind. Now granted the winds were gusting 30+kts with 5-8' seas and no reef points, soooo, I sort of got what I had coming. The problem is that the lumber yard ony has 18' pieces and not sure when the 20' will come in. The mast needs to be 18' 9 3/4". The bottom 3' of the mast sits in the tabernackle, so I was wondering if it's better to scarf out the mast with a piece of the old one to achieve the full length, or mount the 18' mast 9 3/4" higher in the tabernackle. I'm leaning toward the scarf method, keeping in mind that with an 8:1, the length of the scarf will be completely contained in the tabernacke, reducing the stress in that area of the mast significantly. Just wanted to get some thoughts from the community on this one. Quote Link to comment Share on other sites More sharing options...
John Burritt Posted January 25, 2012 Report Share Posted January 25, 2012 I think full length scarfed would be better. Think about running the scarf side to side rather than fore & aft. Other folks may have ideas about which way is less apt to separate under load. Quote Link to comment Share on other sites More sharing options...
Tom Lathrop Posted January 25, 2012 Report Share Posted January 25, 2012 The largest stress on a tabernacle mast is at the pivot point and the heel. It sounds like your scarf will be between these two points. The closer it is to either point, the greater the stress on the scarf. While it may do so, don't depend on the tabernacle to provide extra strength to the joint. Scarfing on a new mast top and lowering the bottom may have less chance of failure. Tightly wrapping the scarf with unidirectional glass AROUND the mast would add hoop stength and greatly reduce chance of failure again.. Quote Link to comment Share on other sites More sharing options...
Hirilonde Posted January 25, 2012 Report Share Posted January 25, 2012 8:1 is the accepted ratio scarf for plywood, toe rail sections and a lot of other applications on a boat. For spars I have always used 12:1. Whether I am scarfing an entire spar, like a solid boom or spinnaker pole or whether I am scarfing a component like a side of a box spar or the stave of a round spar I use 12:1 And I agree with all Tom has said, especially important is that the greatest stress is at the partner, which is the pivot on a tabernacle. Quote Link to comment Share on other sites More sharing options...
G Man Posted January 26, 2012 Author Report Share Posted January 26, 2012 Thanks for the advice. I was worried about a 12:1 because at 36", it would protrude past the tabernackle and become invoved with the pivot area. I that if I use a 12:1 plus glassing as Tom mentioned, it would probably be stronger than the original one piece I used before. I've noticed that with wood working projects in the past that a good scraf joint is often times stronger than the original material. In other words, if a scarf joint is involved, it is the last place a failure will occur. Quote Link to comment Share on other sites More sharing options...
G Man Posted January 26, 2012 Author Report Share Posted January 26, 2012 Since the Behaven has freestanding masts, and I notice in stronger winds the mast flex helps provide some stability by spilling air, I'm concerned that scarfing near the top and adding glass would reduce some of that flexibility to absorb gusts and control heeling. Any thoughts? Quote Link to comment Share on other sites More sharing options...
PeterP Posted January 26, 2012 Report Share Posted January 26, 2012 If you scarf a piece on top do a 12:1 with out glass. Glass would definitely put a hard spot in that section. I think Tom was thinking to back up the scarf in the tabernacle. That is the place where the extra strenght is needed. Personally, I don't consider a good scarf a weakness. We all tend to think that once you cut a piece of wood it can never be as "good" again. But the fact is that a good glue/epoxy is a whole lot stronger than any wood. (So long as it sticks together well ) PeterP Quote Link to comment Share on other sites More sharing options...
Tom Lathrop Posted January 26, 2012 Report Share Posted January 26, 2012 I was assuming that the glass reinforcement would only be used near the base of the mast and would not be needed near the top. Very little flexing takes place near the top of a free standing mast, at least that is the way it should be. My suggestion of wrapping the scarf with uni glass is based on the fact that the mast failed there and might need some more help in that area. When a spar bends, the main point of failure is usually on the outside of the bending arc and less often on the opposite compression side. Stop the fibers in tension from starting to break on that surface and failure will be much delayed. That is what the glass wrap would do, not add much to the final breaking strength. For that, the glass strands would need to be along the long axis of the mast. Quote Link to comment Share on other sites More sharing options...
Howard Posted January 26, 2012 Report Share Posted January 26, 2012 Is solid meranti (I am assuming wood, not plywood) a good material to be building a mast with? I assume this was partially, if not wholly, dictated by what was available? I only ask as I've not heard of others using it and I have no clue about the bending characteristics. Based on what most use, douglas fir or a spruce would be a better material? But when in Rome...........(or Dubai)........... Did you have plans for the mast or did you design it on your own? Most solid wooden mast plans I've seen are built with a slight bulge above the tabernacle (parabolic shape), such that the widest, stiffest, beefiest section of mast is at the top of the tabernacle and a few feet of area slightly above. Other options include birdsmouth and square box masts, with solid cores in this section to resist the stresses. The upper sections remain hollow and still maintain the desirable bending characteristics. Long term, you may also want to consider the aluminum masts. Had not seen any of this size until I was on Tony's Princess 24. They worked awfully well. Lastly, if you are going to be in those types of winds on a regular basis, you really should do what you can to get one if not two sets of reef points in your sails. I'd be less concerned about damaging a mast than loosing the boat...........or worse.......the skipper! Quote Link to comment Share on other sites More sharing options...
Travis Votaw Posted January 26, 2012 Report Share Posted January 26, 2012 Also, keep your pivot (hinge points) bolts made up snug so there is NOT ANY play between the washers and the outsides of your tabernacles. Otherwise, you run the risk of splitting out the sides of your tabernacle in strong winds. Been there, done that, but don't want the tee shirt:) Quote Link to comment Share on other sites More sharing options...
Charlie Jones Posted January 27, 2012 Report Share Posted January 27, 2012 Hey bud- you doing the Tex 200 this year? Come a bit early and visit if you are. Been too long since I've seen you. I'm gonna do it in Traveler. Hoping mto find compatible crew. Quote Link to comment Share on other sites More sharing options...
Travis Votaw Posted January 28, 2012 Report Share Posted January 28, 2012 Don't know yet. If John and his boy(s) can make it I will. If John is working that month, we might put something together on the coast in May. For pure sailing, though, the Tx200 would be very hard to beat. I have reinforced Pilgrim's main tabernacle with 12 layers of epoxy and glass and am anxious to see how it holds up. I also snugged up both main and mizzen hinge bolts :-) I gave up on varnish this summer and took all the brightwork back to bare wood and put on Cetol Natural Teak. It really looks good. I hope it holds up, since it is so easy to work with. Quote Link to comment Share on other sites More sharing options...
G Man Posted January 31, 2012 Author Report Share Posted January 31, 2012 Thanks for all the great info. The meranti seems to work very well in terms of weight, strength, and cost. The big problem with the first mast was the mis-drilled hole. This is where it failed. With the scarf, I plan to wrap glass around the pivot and the heel, just to be sure it is very strong. Quote Link to comment Share on other sites More sharing options...
Hirilonde Posted January 31, 2012 Report Share Posted January 31, 2012 The meranti seems to work very well in terms of weight, .............. But I don't think Meranti is a particularly good choice for it's weight, though I have no knowledge of what you can get in your area. It will work, but there are better choices if you can get them. Dark Red Meranti specific gravity - .55 Douglas Fir specific gravity - .45 Sitka Spruce specific gravity - .36 Quote Link to comment Share on other sites More sharing options...
Howard Posted January 31, 2012 Report Share Posted January 31, 2012 Question for those of you who live in regions with marine industries that support a boat building habit.........is sitka spruce typically available locally and if so, at normal lumber yards or are their shops that specialize in marine plywood and related lumber? For those of us located away from water, doug fir is typically available at lumber yards, but is all construction grade destined for some type of building construction. Sitka spruce? Unheard of around these parts. A quick Internet search ID a place about 7 hours away in Wisconsin....but that is a sideline for them and I doubt the selection is much. Quote Link to comment Share on other sites More sharing options...
Hirilonde Posted January 31, 2012 Report Share Posted January 31, 2012 I have recently found this site and found it does find suppliers. I can't say it finds all of them, or even the best, but worth a try. I am very fortunate in that I can find almost anything locally. I am not sure why I can and many others can't, actually, I don't have a clue why. On a related note I found this site very useful for quick comparisons on specs for different woods. Sitka Spruce is usually not available retail as an in stock item in RI and vicinity. Whole sale it is often available and you will find many whole salers are willing to sell retail if you show up in person with cash in your pocket. Good retail suppliers will often order anything for you, but you may not like the price. If your lumber yard (Home Depot, Lowes, etc., are not lumber yards) carries a #1 Douglas Fir as a building material you may be surprised how good it is. By culling through the pile, and by being willing to buy a piece a bit larger than you need you can often get what you want/need. Quote Link to comment Share on other sites More sharing options...
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