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Marine stress and corrosion on underwater bolts

 

My 8 meter sailing boat has a mild steel bulb keel with a drop keel inside which pivots on a mild steel 1/2"bolt. The bolt has corroded and I intended replacing it with a SS bolt I have been advised against doing so and also advised against using high tensile steel. However I have been given no reasonable reasons. Can you assist me in this matter. I am also having difficulty in locating mild steel bolts in imperial sizes. High tensile are available ,but are they suitable?

Ken Davidson
- Norfolk, UK

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Ken--

Not to experienced with Marine applications for bolting hardware but I can shed some light on a couple of things. Mild steel bolts come in a lot of grades (grade 8 being at the high end and grade 5 the lower end). These grades indicate a lot of things: bolt strength, amount of torque they will withstand, how much they can be stretched (during the torqueing process) and shear strength. SS bolts allow more corrosion resistance because of the higher Nickel content of the base metal. SS bolts are more prone to "galling" during the installation and removal process and should have a high quality thread lubricant painted on BOTH mating surfaces before joining the threaded parts. High tensile strength bolts can have a tendency to "work harden" if they are subjected to a lot of motion/vibration. They can break at that point because they are more brittle than the standard mild steel bolting. Not to sure on how to advise you without knowing the weight of the part being suspend! ed and how much motion/vibration it might encounter. Bolt strengths and grades should be available from any reputable fastener manufacturer. Let me know what you find out.

Bill Miller
- Shinnston, West Virginia, USA

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January 11, 2008

I think the point of the advice from you sailor friends is being missed. Every mechanical system has a failure point, intentionally engineered or NOT.

Imagine you are at hull speed with the spinnaker up and having a wonderful exciting ride. Blamo! You hit a solid rock! Do you want this pin to shear or do you want to rip the bottom out of your boat out?

Again, as far as corrosion is concerned this pin is may also be acting as a sacrificial metal. Replace it with stainless steel and the electrolysis will just move someplace else. In all probability a place much harder to repair.

Accidents happen. At the Isthmus on Catalina Island off of Los Angeles, California USA there is a submerged rock that everyone knows about. Well almost everyone. It is an isthmus which funnels and amplifies the wind to a large degree. A 30 foot boat of the light cruiser type, Catalina 30, left the dock, hosted all sails, and set about to scream out of the harbor. They hit the rock at hull speed. There is a only a thin coat of fiberglass covering the keel. At high impact the lead does not bounce but deforms and grabs onto what ever has been hit. The impact threw the shipper through the wheel and its post snapping it off. He hit the bulk head so hard his collarbone was crushed. His wife below was thrown so hard into a bulkhead that she received a compound fracture of her arm. The mast failed and launched forward. The shipper is is now unconscious in the cockpit, the crew is on the cabin sole bleeding and in great pain, and the boat is sinking fast. Fortunately they were only a thousand yards out and a small armada of boats came to the rescue both the crew and boat. Double fortunate that in every group of sailors there are almost always a few doctors. In some exotic remote foreign port the story would of ended differently.

I'm way into tech but when it comes to the sea I always listen very carefully to what the old salty dogs have to say.

Sail Fast
Sail Fair
and above all
Sail Safe

Gordon Krum
Marine Handyman - San Diego, California, USA