Hull Inspection

[Bob Gray]

  Several days ago I had a hardware malfuntion and my mast went tru my deck just in front of the mast step. I checked the hull over and only found the hole tru the deck, the center stringer broken and about 10" of the top skin delaminated. The winds were forecast to be lite the next day so I made a really nice duct tape repair. I took the boat out the next day and went sailing in winds 5 kts gusting to 8. As I walked around my boat after a scrub race I found that one side had a bad crack in the mast step area. I took the boat home and removed the top skin and found two bulkheads broken loose and another crack in the skin.
   DN'sare very strong for their weight but they are fragile kind of like an eggs shell. With everything intact they are strong but violate the shell and there's not much strength left. I read once in an article about DN's a few years back that powered up , there's about 1000# of force on the mast step. Judging by the way the holes in mast hounds are pulling out of shape and pins distorting, this probably isn't far off. This year at the Gold Cup a friend of mine had his boat break in half for no good reason other then there some minor failure in the hull that went unnoticed. The critical portion of the hull is the top portion since wood is 5 times as strong in tension as it is in tension as it is in compression ( according to wood reference manual). We put these boats under incredible stresses and beat the living dayligths out of them. What we need to do every once in the while and always at the end of the season is to give them a complete inspection and fix and weak spot so you don't havea complete failure.

[Geoff Sobering]

... doubly good advice for anyone who sailed at the Centrals this year!   

Cheers,

Geoff S.

[Paul Goodwin - US 46]

While I missed the Centrals, I understand it was quite a ride by the end of the first day.  But... was it a tougher test than the first Gold Fleet race at the 2007 Worlds???

For the most part, I find a lot of modern DN hull designs suprisingly resilient.  Most of the hull failures I see can be traced back to more than one failure point.  Frequently grain runout in one (or both) of the side panels plays a big part, since the side panels carry all of the load in the hull.  The deck stops at the #4 bulkhead, so the side panels and and rails pick up all of the compression load at this point, and the transition creates a stress riser making matters even worse.

Bob, I suspect that the damage to your hull from the "hardware malfunction" was more than skin deep.  Most likely the mast step assembly had some structural damage, and one side panel may even have started to crack, but so small that it went unoticed.  The mast step assembly is very critical.  A good design can be surprisingly light and still capable of handling the mast step loads.  I copied a design from Jan Gougeon after taking a close look at how it tranfers load to the side panels - I believe it has alot of safety factor built in, and can sustain some damage (or less than perfect construction) and still function.

At one time I calculated static loads on a DN rig from mainsheet tension - that is I assumed the mainsheet was pulled in all the way but with no wind load.  The numbers I came up with were in the range of 1200 lb on the forestay, and 1400 lb on the mast step.  These number come close to the breaking strength of 1/8" SS cable, explaining the change to 5/32" wire for the forestay.  Work the numbers and you'll find these loads are well within the capabilities of the side panels, provided the load is transferred properly.  Excessive grain runout, or maybe a glue-starved joint, and you'll find your precious boat in splinters (thank God for epoxy!!!)

Paul - US 46

[Bob Gray]

Paul, The hardware malefunction on my boat was caused by the retaining bolt on a European mast step fixture vibrating loose and the mast sliding foward. My point with my boats failure was that the foredeck skin and side boards are very strong when intact but when something starts to go, the boat might break. Ask Jim Grogan.