J9 Mast
I was taking a close look at the mast on J9 and it is looking its age (Proctor mast maybe not original). As a trapeze boat I am concerned about stresses etc. How do I know what the probability is of mast failiure?
Uffa Fox Jolly Boat - J9
Mirror - 34359
Hornet - K140
Please see http://www.justgiving.com/160miles for my charity ride to Paris in May
Mirror - 34359
Hornet - K140
Please see http://www.justgiving.com/160miles for my charity ride to Paris in May
<blockquote id="quote"><font size="1" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote">How do I know what the probability is of mast failiure?<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">
Not really that likely if there are no cracks developing, dents or other nasties. If there are, well...
Not really that likely if there are no cracks developing, dents or other nasties. If there are, well...
If only there was a simple answer!
Masts have two key enemies:
1. Fatigue
Sail on one tack and the winward side of the mast is in tension (being pulled)whilst the leeward side is in compression (being pushed). Change tack and the situation reverses. So for a point on your mast, it is pulled then pushed each time you tack. This repeated pulling then pushing eventually causes failure. The stress range (maximum - minimum) controls the life, which for al. alloys has an absolute maximum of 10 million stress reversals (cycles) bu in practice will be rather less.
A cycle is certainly every tack but on the sea could be every wave.
Driliing holes in your mast can increase the local stress range by typically 300%, so don't do it unless you have to.
2. Compression
Your mast is a long strut and the rigging is trying to push it through the bottom of the boat. The longer the strut the lower the compressive failure load. The tighter the standing rigging, the larger the compressive pre load and the greater the risk of mast buckling. Look up the luff groove using one eye. If the mast is 'S' shaped, the rigging is too tight and mast failure more likely.
When trapezing up wind, the weight of the crew is relieving the load in the windward shroud, so is not necessarily a bad thing. Off the wind, the crew moves back and introduces some fore~aft bending but possibly no worse than the mainsheet load close hauled.
So, sail on a pond in light winds and don't drill holes in your mast and it should last a long time.
Mark
Masts have two key enemies:
1. Fatigue
Sail on one tack and the winward side of the mast is in tension (being pulled)whilst the leeward side is in compression (being pushed). Change tack and the situation reverses. So for a point on your mast, it is pulled then pushed each time you tack. This repeated pulling then pushing eventually causes failure. The stress range (maximum - minimum) controls the life, which for al. alloys has an absolute maximum of 10 million stress reversals (cycles) bu in practice will be rather less.
A cycle is certainly every tack but on the sea could be every wave.
Driliing holes in your mast can increase the local stress range by typically 300%, so don't do it unless you have to.
2. Compression
Your mast is a long strut and the rigging is trying to push it through the bottom of the boat. The longer the strut the lower the compressive failure load. The tighter the standing rigging, the larger the compressive pre load and the greater the risk of mast buckling. Look up the luff groove using one eye. If the mast is 'S' shaped, the rigging is too tight and mast failure more likely.
When trapezing up wind, the weight of the crew is relieving the load in the windward shroud, so is not necessarily a bad thing. Off the wind, the crew moves back and introduces some fore~aft bending but possibly no worse than the mainsheet load close hauled.
So, sail on a pond in light winds and don't drill holes in your mast and it should last a long time.
Mark