Gaff mast...hollow or solid ?

[PeterSibley]

It is going to be a while before I need a mast but if it has to be solid I need to start early.I have a several good 33' poles drying in my shed but upon examination,I think I am going to need something thicker than I already have.The ones I have will become spars.

At the timber yard yesterday I saw some very nice 35mm thick material,(1 3/8"),high ring count pine that would be good mast building material.I have always planned a solid mast but this material made me consider the alternatives.With it I could produce a 7 1/2" diameter mast using the birdsmouth method.That is the diameter I would make the solid mast.

Any thoughts on hollow gaff masts v solid for offshore use.

[Smacksman]

Smacks used to have solid masts and I got a 44 foot pole x 10" finished dia. russian pine for my Alberta CK318. Weighed about 0.33 tons undressed.

Now most owners are going for built up masts (and spars) as it is getting difficult to find good poles.

Here is Sallie CK224's solid mast-

http://www.alberta-ck318.freeserve.co.uk/image66.jpg

[TonyH]

Hi Peter

This is just my opinion, but as Smacksman says (and we've discussed previously I think) finding a really good stick of wood that size may be quite a challenge these days. For a gaff rig I think you could still use a hollow birdsmouth spar, but I would be inclined to make it rather thicker-walled than usual and with a correspondingly smaller hole down the middle. While this may involve a bit of weight penalty up high (shouldn't be too much of a problem for your boat anyway) without any significant addition to bending strength, I expect that beefing up the timber will make it less susceptible to failure caused by damage from the gaff jaws chewing on the mast, which I understand is the main objection to hollow masts on gaffers.

Another advantage of a hollow mast (even one that's only a little bit hollow) is that it makes it easier to get leads to the masthead for things like nav lights and VHF aerials that may be required in this modern world. I'm also told that you can pack the void with scrunched-up aluminium foil and it then becomes an effective and very unobtrusive radar reflector, a good thing for any blue water boat to have.

Cheers

Tony

PS Smacksman - nice to hear from you mate!

[fair&fair]

go hollow....birdsmouth. Very easy to make while using hose clamps for clamping pressure. These are also quite a bit of fun to make, and quite a bit lighter too.

[DerekW]

Thoughts....

As remarked by others, it almost always seems easier to find smaller stock of better quality.

I believe there are lots of good offshore boats rigged with hollow spars.

Hollow at the same O.D. is slightly weaker, but the necessary increase in O.D. to meet or exceed the specs for the solid stick would be very small.

At these sizes, the hollow spar might be significantly lighter aloft, leading to a change in roll.

Birdsmouth is popular, with good reason I think. Hose clips for that big a spar would be expensive, but spanish windlass style clamps from rope grommets would do a fine job.

Judicious use of internally tapered reinforcement should alleviate any concerns with jaw pressure &c.

Rig design is a lot more involved than stick selection, and I'm no expert. If my life depended on getting the right answers I'd probably ask someone like Brion Toss to cast a professional eye over my plans.

Just my 2c
Derek

[PeterSibley]

Hello Roger,hello Tony smile.gif good to here from you.

Please excuse the rerun of the perviously dicussed query.I have a suitable stick of P.Ellioti in my wood lot and its actually straight !The reason for the post was that pine in the yard was of such nice quality that I started to think of ways to use it.
smile.gif
Its rather rare to see pine that you would like to use here in Australia (except hoop)For your interest Tony,it is LaserFrame brand....from Mt Gambier in SA.Amazingly fine grain,say 15 to 30 rings per inch,just like the Russian stuff and very white .A$2.15 + gst for 35x90 in 6m lengths.About A$240 for sufficient for a mast 12m long x 180mm diameter.Weight about 160 kg.A solid mast would weigh around 250 kg.

I agree that the wall thickness would be a bit thin...it would be possible to do it the old fashioned way with tapered segments, that would give a 45 mm wall.Lots and lots of work :(

Thanks for your input Derek.

[George.]

Go for solid. You don't need to save weight aloft on a gaff rig as much as on a Bermuda rig. In fact, a little extra weight aloft will help dampen the roll of a heavier traditional hull.

If it is hollow it needs to be reinforced, i.e. made solid, not only where the gaff jaws normally go but also where it goes when reefed. That's because the jaws exert quite a bit of pressure against the mast (not an issue with Bermuda rig). If you have three reefs on your main, plus need for reinforcements at the foot, deck, crosstrees, and masthead, you might as well make the whole thing solid.

[Ian McColgin]

Hold that thought a sec.

Weight aloft is not always a bad thing.

Many many many otherwise fine boats had their seakindly motion destroyed and replaced by a dangerous whipping roll when they got hollow spars.

If you can't do it yourself, hire a proper NA or designer to look at your stability moments with hollow versus solid spars.

On hollow spars I've come to believe less and less in the need for compression blocks or stiffeners inside the structure. Done wrongly, you can create a hard spot in the mast's structure.

Hollow spars require drainage at the bottom as temperature changes will cause moisture in the wood to load in the hollow space and condense on the inner wall. It'll then roll down to sit atop a compression block and cause rot. I've seen too many nice hollow spars with well hidden rot at the spreaders or partners where there's often a compression block and at the heel.

If you go with compression blocks, have them properly engineered to not upset the dynamics of the mast and build with drainage in mind. If you go without, be sure the matter was examined closely with a qualified rigging engineer. In either even, put a drain down at the solid pard of the heel.

G'luck

[PeterSibley]

Thank you for yor excellent advice gentlemen,much appreciated.

[Charles Burgess]

Not only can you use the mast's hollow channel for the general electrical wiring of lights or gadgets attached to the mast...you can use the channel for the grounding wire of the lightning protection system. The ground wire is easily attached to the through-keel grounding plate, all the while keeping the whole system tucked away safely from passengers and crew.

Usually a built-up hollow mast results in the wood edges being the only bonds holding the whole thing together(Building hollow masts is a lot like putting together a bamboo strip fishing rod...but on a Paul Bunyan scale).

In my hollow masts I like to line the channel's interior with fiberglass reinforement during the final construction...now using kevlar cloth to provide more overall strength. The result is that each wooden section has a more stable bond in relation to the entire mast, not just with the ones on either side of it.

The FRP lining also cuts down on the chance of internal rot getting started.

[paladin]

...you can't get a large enough "grounding" wire inside that mast....it struck by lightning you will probably have the spar explode in your face as the wire evaporates......standing rigging OUTSIDE the spar with proper grounding is a heckuva lot better......and provide a good counterpoise just in case you wanna put a radio antenna on top....and you need a lot of aluminum foil up high to make a really good radar reflector....

and HI Tony...and Peter...