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Catalina/Capri 25/250 Sailor's Forums

General Sailing Forum

Forestay Strength

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GaryB
Master Marine Consultant

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USA
4304 Posts

Initially Posted - 09/27/2010 : 11:47:09

I'm trying to find out what the breaking strength of a 1 X 19 3/16" forestay is. I looked on the WM website and found some 304 stainless wire with a 940# breaking strength.

Does that sound right for a forestay??

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GaryB
Andiamo
'89 SR/WK #5862
Kemah,TX

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islander
Master Marine Consultant

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USA
4024 Posts

Response Posted - 09/27/2010 : 12:50:47

Found on WM
<blockquote id="quote"><font size="1" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote">Home > Sailboat > Mast Hardware > 1 x 19 Wire > 1 x 19 Brite-Stay Wire, 3/16" Wire Dia., 4000lb. Breaking Strength | Share Searches related to 1 x 19 Brite-Stay Wire, 3/16" Wire Dia., 4000lb. Breaking Strength: 1 x 19 wire, navtec 1 x 19 wire, 1 x 19 Brite-Stay Stainless Wire, Type 316, 1 x 19 brite-stay stainless wire, type 302/304, stainless cable 1 x 19 1 4, LANDMANN Mast Hardware, SEAFIT Mast Hardware, Wire Sailboat Hardware, 1/4 twist sailboat hardware, 3/16 twist sailboat hardware<hr height="1" noshade id="quote"></font id="quote"></blockquote id="quote">

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Voyager
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5378 Posts

Response Posted - 09/27/2010 : 19:08:54

I'm not a mechanical engineer, but I know that breaking strength is tested by pulling the material along its axis (securing one end and pulling on the other end).

From my mis-spent youth, when we used to set up Zip Lines between our tree houses, I also learned that by placing a steel cable between two end points (trees) and pulling on it perpendicular to the cable, you can exceed its breaking strength using much less force (like 2 x 100 pound teenagers!)

Moral of the story -- when you exert longitudinal force, the cables are very strong (2 tons), but if you push or pull the cable perpendicular, it doesn't take much to snap it.

Something about a hyperbolic function approaching infinity, as I recall from calculus.

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GaryB
Master Marine Consultant

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Response Posted - 09/27/2010 : 19:56:41

Thanks guys! I knew 940lbs didn't sound right.

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OLarryR
Master Marine Consultant

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3467 Posts

Response Posted - 09/28/2010 : 03:44:08

A number of years ago, there was a guy in my area that posted his experience when his Catalina 25 (believe late 70's vintage) slipped off the stands while the marina personnel were lifting his sailboat with the travel arm. The boat tilted forward and his forestay/mast hit the travel arm putting significant tension on the forestay. The forstay held and ripped up the deck from the bow and back about 1 foot. He posted the photo on this Forum but it may be too far back historically to pull it up - His bow looked like the open mouth of a shark ("Jaws"). So....the forestay is at least strong enough after many years of use and suspect with no significant corrosion to pull up the deck of the sailboat. No one wants their deck damaged but at least it proves the forestay after many years was up to the task functionally.

The fact that the deck/bow area is reinforced with a SS fastener plate, I would think that a force much greater than 940lbs would be needed to cause that damage....maybe double that number ~ 2000 lbs. If the forestay can handle that load, then it will not be the weak link in the system. Inorder for it to consistently handle that load from day one and for many, many years, then the wire has to be designed to withstand mechanical (fittings and any forces other than longitudinal) and corrosion effects thru it's lifetime. If it's designed with a factor of safety of 2 to account for these other factors, I could see where a wire with a breaking strength of 4000 lbs makes sense. A wire that starts off with a max of 940lbs breaking strength could over time have considerably less strength and would soon be the weakest link.

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skrenz
Captain

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351 Posts

Response Posted - 09/29/2010 : 15:30:51

Remember though, that even though the breaking strength of 3/16 is indeed around 4000 #, these are purposely over engineered. I ran the calculations on the Cat 25 based on wind speed, displacement, length, etc and the fore aft moment is much less than 4000#. I did this to build my own adjustable aft stay system. I ended up specing all the parts to handle close to that kind of load but this was probably overkill.

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