Bulding a mast

[Adamant]

Hi,
I need to build a 43' mast.

Is there a good source for information, designs etc. I'm basicly looking for a book title or author to get an education. any leads would be appreciated.

Thanks

[paladin]

what are the basic specz

[Adamant]

4 1/2 x7 box tapered to 2 1/2 square, 39'9" from the deck. About 3'6' deck to mast step. Spreaders 18'#' from deck. Shrouds at spreaders, 4 1/2' above, and 2 !/2' below top. Sitka Spruce exterior, unknown core.

[Jagermeister]

For those dimensions you could use a solid spar - if you can find the wood. My Kettenburg has a 40 foot solid mast, 3-1/2 x 4-1/4 rectangular at the base, tapering to 2 x 2-5/8 oval at the mast head. For a mast that narrow I wouldn't think you would save much weight making it hollow.

[Adamant]

jagermeister, is you're mast one piece or spliced? Can Sitka spruce be found in these dimensions or would I need to splice it? Is you're mast SS or another kind of wood? It would be a lot easier to make it solid.

[Bob Smalser]

http://woodenspars.com/k.jpg

http://woodenspars.com/

Box section masts are no more complicated than their rabbet joints, tapers and blocking....and are made of readily available 4/4 or 5/4 stock scarfed to length. I have only one mill locally (Wagner) that can cut 40'+, and they aren't cheap.

Mast grade Sitka is spendy to the extreme. Also consider using Doug Fir instead and save your money for something else.

DF is around 20% heavier than SS, but it is 40% harder, 24% stiffer and anywhere from 5 to 28% higher in every performance category except shear, where it is comparable.

You can reduce SS scantlings by 10-15% and have a stronger mast of only slightly more weight for less than half the money. Rot resistant, too.

[ 10-08-2005, 02:51 AM: Message edited by: Bob Smalser ]

[John B]

spruce 25Lds a foot ?through to DF perhaps up to what? 35 Lds depending on quality?

a 30 ft hollow approx in 43 ft of mast. Even if its just a 6 square inch cavity it's starting to add up to what I'd call a pretty significant weight saving aloft.

[Bob Smalser]

Relative weights for comparison...varies a bit by moisture content, ring count and how computed:

Sitka Spruce 28 lbs/cu ft

Doug fir 33lbs /cubic foot

Western Hemlock 30 lbs/cu ft

Port Orford and Alaska Yellow Cedars 29 lbs/cu ft

Western Red Cedar 24 lbs/cu ft

Northern White Cedar 21 lbs cu ft

White Oak 43 lbs/cu ft

[ 10-08-2005, 02:48 AM: Message edited by: Bob Smalser ]

[Jagermeister]

My mast is a single piece. I think it is spruce, but to be honest I'm not sure. I can lift and carry it by myself, and I'm not exceptionally strong (48 years old, work at a desk job :( ). The boat was built in 1947, and I'm sure the mast dates from the original construction. Spreaders at 18 feet from the step, and another 12 feet above that. Jumpers on the front (fractional rig) from the lower speaders to the mast head. Sail area is available in the wooden boat registry (Kettenburg PC). The mast starts as a 4-1/4 x 3-3/8 rectangle, stays rectangular for the first 4 feet (through the cabin top), and then is carved to a teardrop for the remainder, with the edge at the sail track. It's about a 13 inch circumfrence at the bottom of the sail track, and 7-1/2 inch circumfrence at the mast head, with a pretty constant taper for the entire length.

In my case I'm sure the rigging carries the load. It's a pretty flexible spar, but has lasted 58 years, and carried a spinaker in the past.

I would think any mast with a larger cross section would have to be hollow, but at least some were build from a solid piece.

Hope this information is of some help.

[ edited to add ]

Oh, yeah, Bob's picture points out one big drawback of a solid mast. Either daylight sailing only, or you are going to have ugly wires on the shrouds. A hollow mast gives you someplace to put wiring - a BIG plus!

[ 10-08-2005, 02:39 AM: Message edited by: Jagermeister ]

[Bob Smalser]

I'd think a 1950-vintage Kettenburg would be Sitka.

Countless tons were shipped south to the aircraft industry in California during the war, and it was readily available.

[Boatmik]

Originally posted by Adamant:
Hi,
I need to build a 43' mast.

Is there a good source for information, designs etc. I'm basicly looking for a book title or author to get an education. any leads would be appreciated.

ThanksThe structure should be part of the plans for the boat. A good rundown on the method of calculating is in "Skenes Elements of Yacht Design" as revised by Francis S. Kinney.

Note that he gives a couple of methods in the front of the book and then in the back part gives a worked example.

If you are starting from scratch and have no information about what was on or should be on the boat the biggest stumbling block is having some reasonable estimate of the boat's stability.

The more stable the boat is, the greater the load on the rig. So the mast needs to be stiffer to deal with the buckling loads (if stayed) or the bending loads (if unstayed).

If you can find information about what should be on the boat or has been used successfully on a sister boat - even if it is Aluminium or another material it is possible to design a wood mast to match the stiffness and strength of the alloy (or whatever section) - it will weigh more than an alloy section, but can match the structural requirements.

The calculation set out it Skenes are not too difficult though it can take a little going to and fro between the various sections in Skenes.

If you can't come to grips with the calculations yourself most good boat designers would be able to sort it for you - maybe only a couple of hundred dollars.

Michael Storer
web page (http://www.ozemail.com.au/~storerm)

[Adamant]

Bob Smalser, If I were to use Douglas Fir would I need to use !/4 sawn wood and is it avalible?

[Bob Smalser]

I suspect somewhere your boat has plans drawn for it with the mast construction details you need. Otherwise copying/consulting sparmaker Bruce Tipton at my link above will work.

No, there isn't much advantage to riftsawn in this application, although many will use it because it has a more attractive appearance. Generally used when seasonal movement can be troublesome, wood movement across the 7" width of your mast base isn't an issue.

And yes, DF is available nation-wide. So much, you should be fussy. You want 100% heartwood. 8 rings/inch or tighter is ideal, but so long as your stock is a deep pink heartwood, you can use less successfully. Tight pin knots are OK, too.

The published strength figures are for 4 rings/inch plantation lumber. Tighter is stronger, so if you find 8 ring stock, you may want to be more bold reducing scantling size.

[Adamant]

Thanks

[Bob Smalser]

Originally posted by Bob Smalser:
Sitka Spruce 28 lbs/cu ft

Western Hemlock 30 lbs/cu ft

Also be aware that these woods are dead ringers for each other and can be easily substituted by the unscrupulous.

In the yard, it looks, feels and weights identical, but isn't near as strong in compression, crush or shear as SS. Clear, VG stock worth maybe $1.20/bf against $8.00+/bf for SS, and rots just as quick.

Buy SS only from reputable folks...and another reason to use DF. I'm sure there are a lot of WH spars out there in the hands of folks who paid for SS.

[ 10-09-2005, 10:56 AM: Message edited by: Bob Smalser ]

[yorgie]

Richard Jaegels wrote an article in WB 118 (june 1994) comparing the various spruces.He found one of the eastern spruces to be nearly as good as SS and you'd have better quality control if you could find a local miller.

Also Jaegels suspects that many of the remaining SS stands are at higher elevations and are subject to heavy snow loads and suffer compression stress fractures across the grain.

If I were to build a hollow spar I'd go with D-fir for the strength and better rot resistance but properly selected and milled eastern spruces might be a good choice.

Chris

[ 10-09-2005, 05:41 PM: Message edited by: yorgie ]