Anonymous wrote:

I have spent days searching the internet for a formula to check the thickness of aluminum I need for my mast. Still I can't find an answer. The dimensions of my mast is attached to the link. Boat data:

LOA : 10m

BEAM: 3.574m

DISP: 5.5T

SAIL AREA: 53.7sq.m

BALLAST: 2T

I found a standard flag post product with 200mm dia base, 85mm dia top, 186.68mm dia at deck level, length 14m and wall thickness 4mm made of T6 Aluminium. If the thickness is not enough, I can buy 2 and sleeve them together to make up a wall thickness of 8 because the inside is smooth.

It will be great if the Calculator can help me find and answer. But I still have to show to the authority my calculation process. I wish to know the deflection on the mast top too. Can someone help me with a formula or an xls spreadsheet. Thanks

John Kwong 

John, maybe this chapter 6b will give you a hint on getting the right mast dimension at deck level. However, if you are to use 2 or 3 masts, you will have to decide how you want to spread the load.

Arne

Hi John,

The Junk Rig Calculator should help with this.  Good luck!

Mark

Now I have looked up in the book where I took the strength data of woods from:

Ian Nicolson: BOAT DATA BOOK

He has three numbers about strength,

Tearing force: 700kg/sq cm (69MPa  -  Mega-Pascal)

Crushing force: 457kg/sq cm (45MPa)

Breaking force: 863kg/sq cm (85MPa)

I guess it was (and still is) a bit unclear for me which number to pick (that is, what they mean), so I picked the lowest number.

I am open to be corrected on this.

Cheers, Arne.

Malcom,
I am sure your numbers are right.
It is just that I am a pessimist, and I want the mast to stay up even if a bit water finds its way into the wood. 

Arne 

Oscar Fröberg wrote:

Paul et al, thanks to JRA member Malcolm I just updated the JRCalc with different modulus of rupture for the different wood types. Source:

http://www.conradfp.com/pdf/ch4-Mechanical-Properties-of-Wood.pdf

Before I just used a generic value of 50 MPa for all of them, as I (as far as I remember) couldn't find any data at the time. The following values are now used:

Cedar, western red: 52 MPa
Spruce, Norwegian: 72 MPa (although it's USA spruce in the PDF, so just guessing here)
Douglas fir: 87 MPa
Oak: 96 MPa

http://jrcalc.oscarfroberg.com/

I notice that the strength of spruce is set to 72MPa (72Newton/sq mm). I see these numbers vary quite a bit in data books. I have used a more conservative number;

45 MPa for spruce. (corrected 17.7.)

My guess is that the higher numbers can be obtained on hand-picked, flawless examples. Since my practice has been to just cut down a suitable tree, I think it is better for me to use that lower strength number when calculating the mast. It is still possible to rig a mast which does not add much over 3.0% to the boat's displacement, and I find that to be tolerable.

Arne