Hollow out a solid mast?

[Helle]

Dear forumites,

the mast of my golant gaffer is very well built by the man, who also built the hull in England, but it is far too heavy, about as twice as a comparable mast. To some extent this is a result of the type of wood, I think it's Douglas fir. But the main reason is, that the mast is solid, not hollow, glued from two halfes with just a small notch in the inside for the cable.

My question is: Would it be usefull to saw the mast into two halfs, hollow it out (e.g. with a router) and glue together afterwards?

I need a lighter mast because I've to lower the mast frequently. Now, although it's only 6,70 m, you need three person, and it's still a lump!

Any ideas?

Best wishes
Helmuth

[Steve Lansdowne]

Your solution would make the mast lighter, for sure, but strength (in various directions) is also an issue. Masts are sized to withstand the pressures that the sail plan puts on them, and going hollow would affect other characteristics that might lead to breakage under the right, er wrong, conditions. Folks with more knowledge than I who know what the boat is and the mast composition will likely have more to say about this.

[Helle]

In the original plans of the Golant gaffer there is the choice between a box-like and a solid mast, with the same over all dimension/diameter.
Helmuth

[merlinron]

if the plans show the same dimensions for both solid and hollow masts, i would think hollowing out your present one might work......
i say "might"...for two reasons.....1st, the plans do illustrate a "box-like" hollow built mast as an alternative, suggesting strength needed in the fore-aft direction. it is most possible that the box shaped built up mast may be stronger in fore and aft strength than your round mast after being hollowed out. that box built spar strength matching the strength of the solid spar in the same plane.
2cd..... considering the work involved in hollowing out the solid spar, you could probably build a new bird's mouth stick out of doug fir, with the two side splines widened to match the overall dimensions of the box built spar with about the same amount of work/time. the end result would most likey be lighter than the hollowed out solid spar and compareable, if not stronger, to the box shaped spar.

[Graeme Forrest]

I have the "box" mast on my Golant Gaffer, 30mm square hole down the centre with 110mm round section outside, Douglas Fir. I think it is very heavy for the size of the boat, it is a real struggle to lift if single handed. When Roger Dongray visited here recently he told me he had just had a new mast made for Irena, by Noble Masts I think, which he said was 30mm wall thickness (I understand 8 stave birdsmouth construction), and considerably lighter than his original solid mast. Perhaps a phonecall or letter to him would be useful.

[Helle]

Thanks for your replies!

The box-like mast in the plans is actually a rounded mast, just glued from four identical pieces of wood, not two sides with kind of lids on the fore and aft end, as a real box-mast would be. It results in an almost square space in the the middle of the mast.

I think, hollowing out would be far less work than making a new mast. In addition, it would be very easy to let the mast solid, where mastbands are, to save the strength at these parts of the mast. Sawing into two pieces would also be easy, because I can use a big circular saw.

To what extend could I hollow the mast without weakening it too much? The mast is 6,6 m with a diameter of 100 mm, at the top the last meter above the main mastband is tapered to about 80 mm. Over all sail area is about 25 sqm (main ca. 15 sqm, jib ca. 5 sqm, staysail ca. 5 sqm), two pairs of shrouds, running backstays.

How should I calculate?

Thanks again
Helmuth

[P.I. Stazzer-Newt]

There are the usual jokes about teams of highly trained woodworms....

More seriously, if the mast is already at the correct finished size, and you then saw it in half and re-glue, it would then be undersized by the thickness of the kerf - I cannot imagine wanting a smaller diameter hollowed-out mast.

It's probably less work to start anew.

[orphan314]

Helle, I've built many hollow masts and in general the interior 60% of the diameter is a nuetral axis that provides no strength whatsoever.In many cases this "core" actually weakens the spar because it can force the outer skin to check when it shrinks and swells and the core remains constant. Just make sure that you leave enough core at places where there might be large compressive forces on the skin. You will want to keep the overall diameter the same as the original. Thus , after you cut the mast appart, glue a veneer equivalent to the saw kerf back on to one side of the spar. Remember the rule of thumb is that wall thickness is to be 20% of the diameter of the spar at a particular location in the spar. Ie, your wall thickness can decrease as the spar diameter decreases towards the top of the mast.

[Ian McColgin]

There's a reason hollow masts are built of four or eight pieces. It's the grain. I'd be leary of just hollowing out the middle of a two piece mast for fear of wood failure on either side of the fore and aft glue line. If you must go ahead, get a real expert to look at the grain of the mast pieces first to be sure it will work.

The bird's mouth eight piece mast is easy to build, economical of material, and the tapers for each side and the forward face can be put into the staves.

My experience with Goblin has convinced me that compression blocks inside a hollow mast are about as useful in wood as they might be in aluminum were anyone to ever do that.

G'luck

[orphan314]

Ian, actually, Herreshoff built many of his early spars in two halves that were hollowed out and then glued up as Helle wants to do. I have plans for the Watch hill 15 stick on my office wall and it clearly delineates this 2 halve approach. This spar is 32" long and 3.5 inches in Diam at the partner. I've built seven of them over the last 20 years and all are fine and dandy. Larger spars were built in more pieces to save on material and on labor of material removal. Spars that are traditionally stayed remain in compression and gain no strength by the number of pieces used to make them (As long as the stock is relatively deffect free.) Freestanding and partially stayed more modern rigs do gain by these laminations as they gain stiffness by them helping to maintain compressive forces.

I built the larger rig for Coquina (see cover of WoodenBoat 187) and built the spars hollow . These spars are freestanding and the boat is rigged for hiking out (it will plane) and I've had no trouble with them.

[Tom Robb]

Rant follows:
Why doesn't anyone ever want to consult the original designer or at least some competent N.A.?
I understand the urge to make one's own mark, but if one doesn't have the expertise, don't start hacking away at the stick and hope it works out.
End of rant.

[Steve Lansdowne]

The safest thing might be to build a second hollow mast and transfer the hardware, then if things don't work out, you're not 'up a creek without a mast.'

[Jay Greer]

A solid mast has the advantage of not changing its cross section when under load. This is the reason that bendy rigs such as used for the Finn Class boats used solid wooden masts. A hollow stick under bending load will change from a round cross section to an oval that,
when subjected to over loading, will collapse.
If you do decide to hollow out your stick, consider that what you are thinking about is a lot more work that it appears to be.
Just stabilizing the two halves ,first to be separated, and then to be hollowed will present some interesting challenges for creative clamping and jigging!
Normaly, If I am going to build a two piece hollow mast, I work on the inside prior to shaping the out side. I use a special plane, known as a "Core Box Plane" after first hogging out as much waste is practical. The core box plane will cut a symetrical arc between two given rail points. After this, clean up can be done with curved scrapers and shaped sanding blocks. I always shellac the interiors of my spars.
JG

[P.I. Stazzer-Newt]

WOW.
http://stinsonvoyager.com/BayleyCoreBoxPlane.htm

[Canoeyawl]

I have made several masts by hollowing out the center and reglueing; if you need reinforcement at certain areas, just leave material there with a reasonably smooth transition. I always left the base solid to a foot or so above the boom and solid again a bit above and below the gaff at full height and the reefs. It was all done by hand, splitting with a thin (.025 ) band saw blade, roughing the depths with a skill saw and removal with a big gouge then reglueing with West System. There was appreciable weight savings, they worked well, and they are still working well 20+ years later.

[George Ray]

I agree w/ CanoeYawl.

[Helle]

Thank you very much for all your replies!

Now I've time to think about all your arguments until the boat goes out of the water, and this will be in october or november!

Best wishes
Helmuth

[Boatmik]

Hi Helle,

You are working blind unless you do some basic calculations to compare the 2nd moments of intertia for the designed mast and the modified mast.

Much of the advice above is good advice in general terms - but I would suggest contacting someone who can work out the numbers for you - so you can know just how far you can go with the hollowing.

If the current mast is just strong enough for the stability of the boat (which limits the load on the mast - more stability, the stronger the mast has to be) and you hollow it out you will be making it weaker and it might not stand up.

Get someone to run the numbers for you or do some research and work out how to do it yourself.

At the moment you don't know enough to go ahead with security.

Best Regards
Michael Storer