Check Your Lag Bolts And Engine Mounts

[boatpoker]

Originally Posted by maxingout

Since Exit Only is a catamaran that does not heel, it kept the engines from moving around most likely. There was never any unusual vibration to clue me in to the state of the lag bolts.

Every time you shift to forward and every time to reverse loosens lag bolts a little bit.

[Jim Cate]

Originally Posted by maxingout

Since Exit Only is a catamaran that does not heel, it kept the engines from moving around most likely. There was never any unusual vibration to clue me in to the state of the lag bolts.

Dave, as I understand it you haven't been sailing much in the past period, so maybe the bolts were still ok the last time you seriously taxed them, and they've been sitting quietly decomposing while your back was turned.

But a good catch anyway you examine it!

Jim

PS Those bolts are sooooo wasted... I wonder if there could have been any leakage current into the soaked timber to hasten the decay? Seems a stretch, but they look a bit like fittings that have been subject to stray current corrosion.

[boatpoker]

Originally Posted by Jim Cate

I wonder if there could have been any leakage current into the soaked timber to hasten the decay? Seems a stretch, but they look a bit like fittings that have been subject to stray current corrosion.

Stray current corrosion is highly unlikely as the lags are isolated from the engine by the rubber (unless they are rigid mounts).

It is almost certainly crevice corrosion which will also accelerate decay of the wood.

[Jim Cate]

Originally Posted by boatpoker

Stray current corrosion is highly unlikely as the lags are isolated from the engine by the rubber (unless they are rigid mounts).

It is almost certainly crevice corrosion which will also accelerate decay of the wood.

BP, I lack your wide experience, but the shape of some of those screws is not similar to examples of crevice corrosion that I have seen. Perhaps the one that appears full (+/-) diameter but sheared off is, but not the ones that have wasted into a skinny needle shape.

And is crevice corrosion a threat to mild steel? Not trying to start an argument here, but trying to learn!

Jim

[Alan Mighty]

Originally Posted by Jim Cate

BP, I lack your wide experience, but the shape of some of those screws is not similar to examples of crevice corrosion that I have seen. Perhaps the one that appears full (+/-) diameter but sheared off is, but not the ones that have wasted into a skinny needle shape.

And is crevice corrosion a threat to mild steel? Not trying to start an argument here, but trying to learn!

Jim: remember your earlier comment that those wasted lag bolts reminded you of what happens after stray current corrosion?

You were right then. But Boatpoker is also right now when he calls the process crevice corrosion. And Boatpoker has backed his identification of the process with a wonderful paper, by A J Baker of the Forest Products Lab of the US Dept of Agriculture, which I'd not seen before.

Crevice corrosion is just about a situation in which there is a big difference in concentration of some chemical (such as oxygen) from one end of a confined space (usually the outside world) to the other end (such as the bottom of pit in stainless steel, a stainless steel to stainless steel junction, a mild steel bolt in saltwater soaked timber, or a brass or even bronze bolt in saltwater soaked wood).

Because of the cheapness of 316 ss, cruisers are used to crevice corrosion in ss.

The older generation of boaters in wooden boats with mild steel, bronze, and brass fixings were used to the other cases of crevice corrosion.

And, as A J Baker's paper brilliantly points out, in the iron lag bolt penetrating into seawater soaked timber, the crevice corrosion effectively creates a electric cell: the head of the bolt surrounded by oxygen becomes the cathode; the shank of the bolt denied access to oxygen and surrounded by chloride rich water becomes the anode (and the seawater soaked into the timber is the electrolyte). The result is genuine electrolysis. So real stray current!

Big thanks to Boatpoker for posting the pdf of A J Baker's paper. Worth much more than one of those icon thingies showing respect (particularly since I don't know how to make those icon thingies).

[Alan Mighty]

Originally Posted by Alan Mighty

The result is genuine electrolysis. So real stray current!

Having written that and thought about it, I'm not sure that the process is "electrolysis".

It's probably just galvanic corrosion between the head of the bolt (in the air) and the shank (soaking in salty water with lots of chloride ions, a low pH, and no oxygen).

As Baker pointed out, the head of the bolt acts as the cathode and the shank as the anode.

Anyone who can define what "electrolysis" really is (as opposed to how it's usually misused)?

[jimbunyard]

While stainless bolts and nuts through a metal bracket is probably the most preferred method for longevity and reliability, many millions of engines have been fastened to motor mounts secured to wooden stringers with galvanized lag bolts...as the original post suggests, if that is the case for anyone's boat the message would seem to be that these type installations should be subject to a yearly or so inspection.

If the wood into which the lags are screwed is dry and competent, if there is anywhere 5200 has a use, this is it; fill the hole with it and coat the screw with it before screwing it in. It's messy but can be cleaned up with paint thinner or mineral spirits. Use washers made from the same material as the lags (notice the stainless washers on the galvanized screws in the pictures) to help avoid setting up the 'battery scenario' described above...

If the holes themselves are compromised but the wood is dry, and there is enough width in the mount base, the old holes can be filled with epoxy (or even 5200) and the mounts rotated enough to fasten them into a new section of timber, drilling the appropriate diameter and depth pilot hole first, filling it with 5200 and screwing liberally coated, stainless lag screws in to secure the mounts.

If the stringers are comparatively thin and the wood is not wet, the existing holes will have to be dealt with. Many possibilities here, drilling or coring out the bad parts and filling with thickened, mill-able epoxy, and then drilling or tapping for stainless machine screws comes to mind (or inserting waxed screws into the unhardened epoxy before hand, though this takes more chutzpah than I have.)

If the wood is wet, obviously you've got a bigger, likely structural problem that has to be resolved first...

[Jim Cate]

Yep, an interesting bit of practical research, and it certainly shows that electrochemistry is at large in damp wood/metal interfaces. The loss rates in his experiments were not so high as to convincingly explain the wasting in Dave's bolts, but I guess it is a viable story. Well beyond my pay grade to evaluate that! I too appreciate the posting of the research article.

BTW, did you note that the research assistant who prepared the samples was named Harley Davidson? What sort of parents would do that to a child?

Cheers,

Jim

[NYSail]

When I replaced my mounts on my first boat, a Cal 34-lll many of the bolts were so bad they just broke. The wood was still solid. Had no room to move engine so I used s hole saw, drilled them out and refilled the holes with new plugs epoxied in. PITA.... of yes pretzel access only working around engine! But it worked and all was eventually well... good luck with replacement.

[Fiveslide]

That's a common problem I've run into. I can't imagine the number of these little runabouts and skiboats that have engines that aren't really bolted down to the engine bed, I've pulled some out with just my fingers. The boat owners are generally too cheap to put new stringers in, so the problem is usually ignored, good thing engines are heavy.

[boatpoker]

Originally Posted by Alan Mighty

Having written that and thought about it, I'm not sure that the process is "electrolysis".

It is definitly not electrolysis
Electrolysis Mythology

[Exile]

Originally Posted by boatpoker

It is definitly not electrolysis
Electrolysis Mythology

Very helpful piece boatpoker, and well-written. Bookmarked!

[puffcard]

Think about the force it takes to move a 20,000lb boat into a head sea at 6kts. All that load is totally carried by the motor mounts. Could you tied a line around your motor and tow your boat at 6kts with it?

[a64pilot]

Originally Posted by Jim Cate

BTW, did you note that the research assistant who prepared the samples was named Harley Davidson? What sort of parents would do that to a child?

Cheers,

Jim

Jim,
my brother in law’s last name is Ball.
So what did he name his daughter?
Crystal.
Think this child will change her name as an adult or insist on a nickname as she gets a little older?

[flyingfin]

Check your engine mount bolts every year to ensure they have not vibrated loose. Every five years, pull the bolt out halfway and recaulk with LifeCaulk or 4200. If the bolts spin and never tighten, you have rotten stringer. Dont just put em back with 5200 and try to sell the boat !