Stavanger, Mon

My problem with just calculating a mast’s strength from the boats righting moment is that the boat is also to be sailed downwind.

On my first sailing trip in Johanna this spring there was blowing a F3, rising to F4-F5. The first leg was upwind with full sail up. On the downwind leg the wind picked up, but since Johanna steers so well with the sail fully squared out, I hardly noticed – that is until I noticed that we were touching 7.5knots! Normally 7.0kts is reckoned to be Johanna’s "sound barrier" so the forces involved were very high indeed. I couldn’t see any bend in the mast because you can’t detect it from the helm when going downwind. I still dropped a panel and as we headed up on a beam reach we at once needed another one.

My (vague) conclusion is that for a serious cruiser, I would let the mast strength more or less follow the displacement of the boat (and height of the mast), almost regardless of the boat's beam or ballast position. The different boat types would anyway have to take sudden squalls from behind from time to time and none of them would tip over – forwards.

Arne

Stavanger, Sunday

    Scaling up or down a mast section using the cube rule

Peter

Finding the right scantlings for a JR mast, sloop or schooner will always involve some guesswork. One method is to start with a known boat with a mast from the same material as you want. The thing to remember is that the breaking strength of a mast varies with the cube of the masts diameter. If you want a mast that is twice as strong as another one, just multiply it with the cube root of 2 (=1.260). Likewise, if you want a 3 times as strong mast, the cube root scale factor gets 1.442.

Now if we start with David’s "Tystie" at 8tons, your 20tons boat is likely to need a main mast that is 2-3 times stronger than Tystie’s main mast, at 8 5/8" (=21.9cm)

Then your mast should be between...

21.9cm x 1.260=27.6cm (=10.86") and...

21.9cm x 1.442=31.6cm (=12.43")

Note: I don’t know the wall thickness of Tystie’s mast. This must be scaled up too if the cube rule shall be correct. The 12" pole you mention sounds therefore reasonable if the wall thickness is not too thin.

Arne

Annie Hill wrote:Come on, David.  That's hardly encouraging!  Is there such a huge difference between 'Leto' and, say, 'Pacific Spray'?  They are both big, heavy, beamy boats, but if the rig is working on the Spray - and latest reports imply that your input has made it into a great success - then it would surely work on a Venus.

And Arne - what about 'Samson'?  She's a 49ft heavy displacement boat.  Surely comparable with 'Leto'.  Peter, why don't you go to 'Junk Rig Association - Public Pages/Pages of information generously placed ....'/Arne Kvernland's Page/Samson, A 49ft Schooner'?  That will not only give you information, but inspiration, too.  If my memory serves, 'Samson' is ferro cement, so whatever else she is, she won't be a lightweight flier!

Peter Hosford wrote:A few years ago I designed A JR conversion (based on Derek Van Loan's book ) for my 21 ton gaff ketch but shelved the idea because of the difficulty and expense of finding or making masts. I am considering the idea again as the gaff rig seems to be getting heavier each year and I wonder whether any members know where to acquire masts of 50 to 55 ft.? I saw tapered aluminium lamp poles in Florida USA whilst cruising there about 12 years ago . Presumably hurricane proof, the poles appeared about 12ins. dia. at the base tapering to about half of that  at around 50 ft. An open wiring hatch revealed a wall thickness of about 1/4''.  I do remember thinking at the time that they may be good for a large JR but  I had several years of cruising and  oceans to cross before thinking of converting Leto to JR . Does anyone have experience using large tapered alloy poles  for masts on an ocean going JR ?