Box Mast Fears

Hello,

I am a dreamer with time, and possibly the right amount of money to get into a world cruiser. My hang up, she needs masts. Originally sitka spruce, 11"x7"x50" and 9"x6"x37', boxed. I am completely confident in my handy skills required to construct these. I have found many sites detailing the "How To's", but none of them state any "Beware Of's". For instance, how do you ensure they are straight and true during final assembley? Or, doesn't matter too much because of adjustment once stepped? Just be sure to be relatively straight... Any and all comments regarding wood mast construction is appreciated. Also, anyone with a contact in the Great Lakes area, who could build, advise, or help would be great.

Excited about learning,

C.

 

Box masts are easy.

Get your work surface true and straight and draw on them marks to follow when gluing. If you use work horses get a half dozen of them and use wedges and taut string to get it all straight mark the horses (As in the mast needs to go exactly between this mark and that mark on each one).

Draw chalk around each leg so if you nudge one you'll know about it and you'll also know where it should go. Set up on a hard surface.

Don't use fat bulkheads inside the mast as they create 'hard spots' that actually make it weaker and add weight. Minimal bulkheads made of thin stuff is fine.

It is very much worthwhile to rabbet each side of front and back planks so the side planks have somewhere to press against when you clamp it all together . Once everything is lubed up with epoxy and you're working against the cure time clock you'll be glad you remembered the 6 P rule.

(6P rule ; proper preparation prevents piss poor performance)

Make a scarph box and scarph up the four planks first into their full lengths. Then work in all the tapers paying close attention to precision. Rabbet crisply where needed. Setup everything. Mark everything. Have twice as many clamps as you think you'll need , even if they're jury rigged clamps. (rope, sticks, wedges) Don't forget lots of plastic shopping bags to place between clamps and glue (plastic bags release well from hardened epoxy).

Do a mock run with your helpers to make sure there are no glitches.

Then glue it all up in one go with slow epoxy. Do a final check by sighting along it that it is indeed true and straight, as you still have a ton more clamps to place and a few more minutes of working time. It should be perfect since you set up the guiding marks with care.

If it ends up with a half inch of (it should not if you did everything right) curve, don't get depressed, it will not make the slightest bit of difference.

 

PRTNDR37:

I run a wood boat shop in Vermilion, OH about 40 miles west of Cleveland. We have built and repaired several masts over the years in the sizes you named. I'd be happy to talk to you. Email me directly at the address below if I can help.

 

Thx Tcube

I should clarify, while I'm confident in my general skills in many different areas, proper terminology is not my strong suite. Your sentence regarding "fat bulkheads" in a mast, what does that mean? Or are you refering to, literally, bulkheads, as in a possible stepping?

Thank you for your time,
C.


Riddle, I'll call you tomorrow.

 

I mean partitions like in a bamboo stalk which you sometimes see in hollow wooden masts as well. They are necessary where the spreaders go for example. They are also helpful for keeping the planks spaced and angled correctly during the gluing process.

However , i've seen many masts with totally excessive bulkheads in there apparently done with the idea that it will make the mast stronger but that is erroneous.

 

There has to be enough solid wood to distribute stresses to all four walls.
Also, while a solid section doesn't add a lot of strength, it does add a significant amount. More important may be a bulkhead design that is very much like bamboo---- ideally curving on its inside edges and transitioning gradually rather than abruptly at a hard 90 degree corner.
The bulkheads add so much to the overall strength of the mast (again, look at bamboo) that they demand careful consideration of design and construction.
L Francis Herreshoff (I think) developed the box section mast in the twenties. It would pay to find some of his writings on the subject. Many Herreshoffs were (and probably still are) built with box section masts.

 

Update

Apparently the seller has all of the original blueprints. So construction by the book should be a snap. It was the logistics of building something so long, while keeping it true and straight. I was also looking for common mistakes, the one thing all websites leave out. Continued feedback will be appreciated.
By the way, this is boat is a ferro.....


C.

 

A taunt string can sag a fair amount under it own weight across not that large a distance, a laser is a better tool and much easier to work with.

Working on saw horses is difficult, unless they are fastened to the floor or each other to keep them from moving around. "L" brackets on a wall can work just as well. If a laser is used when positioning the L brackets, you'll defeat two problems with the same stone.

Rather then inserting spacers inside the mast to accept compression loads from attachments (rigging tangs, spreader sockets, etc.) build the mast completely free of internal bracing (temporary braces are fine and often necessary). Instead, insert a hardwood dowel at each hard point. Ideally these would be under every fastener, but sometimes this isn't practical. Drill a hole that is 50% larger in diameter then the dowel, through the mast. Chamfer (countersink) the outside of the hole, then fill it (each side) with thickened epoxy (50/50 mix of silica and milled fibers by volume will do). When cured drill a slightly larger hole then the dowel (maybe a 1/16" bigger) through the epoxy and bond the dowel into the holes, with the same type of epoxy mixture. This is much lighter, places the grain in the proper orientation and internal wires, lines etc. can still be used. It's also work that can be done after the mast is assembled, without worry the internal stiffeners have moved during assembly (a frequent occurrence).

Herreshoff's larger masts have an internal triangular corner piece, which can be a real ***** to fit properly. With epoxy and well lapped corners, the additional glue surface they provide isn't necessary.

Do a dry assembly, several times if necessary to insure you have all the tools, clamps and things you need during the glue up. You're time is limited with epoxy, so you have to be coordinated and organized or things will get forgotten or you'll run out of time (glue kicks off).

 

If you must build the masts, then why not build them in a shape other than rectangular? A birds mouth type (octagonal) is likely to be lighter and will probably be more efficient in terms of sail power. Not hard to build either. The easiest way to get a mast is to find an appropriate tree like the old timers did.

 

Only a couple of things. Make sure the outfeed table on your jointer is set exactly at the height of the knives. And failing a laser, you can get straight enough with a tightwire (piano wire and turnbuckle) set up between two strong anchors. It will still sag, but use it only to keep things straight. Measuring and allowing for the sag is not for the first timer. Even with a pro teaching me it was easy to make a mistake. Steam turbines do not forgive mistakes. A long builders level will help set things level. Assembling the mast on edge will also help.

 

The solid blocks within distribute the loads from stay attachments and spreader loads and stop those loads distorting and working the hollow tube. A fully stayed mast doesn't curve much at all so I don't agree with the hard spot argument unless you had very thin walls.

If you look at the stayed column buckling model you'd only be concerned with long blocks at the max curvature points anyway. This will be different for deck or keel stepped and the staying arrangement.

An arrangement to drain the cavity through each block is recommended it also stops pressure build up in cavities.

Rectangular sections are good since the maximum fibre is present in the highest stressed portions which will make it stiffer. Oval sections are going to feed the wind into the luff better.

Every hole you drill removes fibres and weakens the mast.

Whole trees are actually pre-stressed with the core in tension at rest which is why they work so well and should only be de-barked and smoothed. An easy cheap option for gaffers.

 

Actually a round, hollow mast, regardless of build method will be weaker then a square section of the same diameter and wall thickness. Ditto oval and rectangular sections. The corners of the square and rectangle section masts are farther from the center of gyration, making them more effective at resisting loading with the same wall thickness. Also rectangular sections aren't as bad a leading edge in regard to penetration and disturbance as it would seem. This is well proven.

If you use solid blocks within a mast you will get hard spots at the beginning and ending of these blocks. This is where the stave walls will fail when overly stressed. To prevent this, good spar builders use a "swallow tail" termination in these internal blocking schemes. The gradual taper, slowly increases wall stiffness, dramatically reducing point loading and stress risers. This isn't speculation, but also simply well proven fact. Internal blocks also increase weight aloft.

A hole filled with properly applied epoxy is much stronger in every regard then it was when just solid wood. A bonded dowel (as described above) is also stronger then a solid (un-drilled) stave wall.

If you've ever been on a hard slosh to windward and taken a tension gauge to your leeward shrouds, you see there a whole bunch of compression on the spreader, huge tension loads on the tangs and all the windward fasteners in the spar. Coming down hard off a wave, closed hauled in 20 knot winds can easily exert twice the boat's own displacement in loads on the windward shrouds, so yes, these hard points, even on a stayed mast, can impose impressive fastener pulling, part breaking, mast crushing forces.

Follow the plans and use resorcinol as the adhesive, which is likely what the original builder did. I've seen this type of mast last a 50 years. If you elect to use modern methods and adhesives, you can make some slight alterations to layout, assembly and method.

 

As usual Par comes with the straight talk, he is right . In my first post i just wanted to give you the seat of the pants, get it done without a workshop , operation.

Yes a taut string sags, but if it's good twine real tight it's pretty close and can be corrected using other standard methods. It all depends just how meticulous you want to be.

I've had to build boats for people in their backyard so i've learnt how to get things done quick and basic sans workshop and fancy tools.

Par also detailed the facts of bulkheads or blocks nicely.

And a telegraph pole is a timeless mast too. But don't underestimate just how much work it is to shape it. I've done this too and at first sight it seems way simpler than all that gluing up but it is really a lot of heavy work. (and a very large pile of wood chips & shavings!)

Do keep in mind that resorcinol is not forgiving like epoxy. It will not fill in any gaps you might have and you need even more clamps as resorcinol should be clamped with much higher pressure than epoxy, so i would recommend epoxy. The mast plans for a glued mast will allow for plenty of gluing surface anyway so i can't think of any reason right now why switching to epoxy would require any mods to the plans.

 

Dude,

You guys are great. I love the interaction between the regulars on this site. Many posts have a lame question such as mine, then develope nicely once to educated people start to compare differences, thats where it's at(knowledge). So, any comments on the ferro boat issue? Besides the obvious. Any thoughts regarding this boat to a Samson C-Breeze, with all documention, pics of construction in '72 to current. Two owner boat, on her "third", who inherited last fall to avoid the marina aquiring it. She's solid, but stripped of recyclables, currently floating, full set of sails in decent condition still residing with second owner, engine in place apparently runs, pretty rough all around, but solid. Whats she worth, real world money? I think I've tapped out the internet for information on these particular boats, I would like to here what the grapevine has to say.

Should I post this last question elsewhere for more feedback?
C.

 

Just a thought: I didn't see any reference to internal services such as halliards, electrical wiring etc. Is that an issue here?