More Corroded SS Bolts

[Brian B]

2010 25SC, just over 1000 hours.
I was missing a bolt head on one of the bolts that holds the skid plate to the keel forward of the prop. The boat is on stands for a bottom paint job so I thought it would be a good time to address the issue. There are 7 bolts that hold the plate on. In removing the bolts 3 bolt heads separated from their studs as I began to turn the socket wrench. I was able to remove the plate then I was able remove all the threaded studs with a vice grip except for one, the metal just crumbled even with the keel. These are 3/8 -16 bolts. 6 are 1 5/8" L and 1 is 3 1/4" L.
I went to W.M. for new bolts but no one seems to know what grade S.S. they sell.
The old bolt heads are marked S30400. From the web: UNS S30400. 304 Stainless Steel Bolts are austenitic stainless steel bolts manufactured from alloy containing chromium and nickel. Stainless Steel 304 Bolts are used for corrosive environments.
Should I be replacing these bolts with 316 Stainless?

 

[HRowland]

Hi Brian,

Here is an interesting article. Looks like 316 is better.

https://www.passagemaker.com/channels/s ... -corrosion

Howard

 

[Matty]

Great article. i just had a stainless steel counter made for grill area. We live on the inter-coastal so I asked this very question. The fabricator told basically what is on the article. His original quote was for #304 and I had him upgrade to #316. I have a feeling I will be glad I did.
As they say "the only thing saltwater does not destroy is saltwater".

There are also sprays (XPS lube by sea-doo) out there that are said to slow the process of corrosion.

 

[Irish Mist]

I would suggest if maybe it would make sense to just replace say every 4 years regardless of how they look. I will need to look at mine the next time the boat has a dry bottom. I suspect everyone eventually will need these replaced.

 

[BB marine]

Marine Fasteners are 18-8 or 316 stainless steel. 304 SS is used for washers , backing plates, railings, trim tabs, and other fittings. If you have Stainless steel showing signs of rust,or corroding it most likely is not 316 SS. I have a feeling Fluid Motion uses 18-8 fasteners and a lot of 304 SS. If you see fasteners rusting or corroding make sure to replace them with 316 SS fasteners.

 

[CapKeith]

Here is a great source for ss bolts (particularly 316 stainless): boltdepot.com. Actually, all types of fasteners, very quick order response, and US sourced.

 

[NorthernFocus]

...I went to W.M. for new bolts but no one seems to know what grade S.S. they sell...
Should I be replacing these bolts with 316 Stainless?

This subject keeps coming up. There should be a "sticky" thread on the topic.

You can tell whether fasteners are 304 vs 316 by using a small magnet. 304 IS slightly magnetic, 316 is NOT.

Yes, any fasteners that are normally exposed to salt water or that stay wet continually should be 316ss. 304 is OK for using in the cabin, engine spaces, etc where it may occasionally be exposed to salt water, condensation, etc but is normally dry.

 

[YukonRon]

You can tell whether fasteners are 304 vs 316 by using a small magnet. 304 IS slightly magnetic, 316 is NOT.

This is normally the case, but, be careful using the magnet test. The process of forming the fasteners can sometimes impart a small amount of magnetism into the stainless steel which was originally non-magnetic before the cold-forming re-aligned the metal. The magnet test is not always reliable.

 

[Brian B]

Thanks for all the info, I'll be searching for 316 stainless to replace the original bolts. This should be another maintenance item as boats age. From my observation the bolts that I just removed looked fine until I started to remove them then the heads separated. The corrosion was in the area between the bolt shoulder and the skid plate, so backing the bolts out about a 1/2" should show any signs of corrosion going on.

 

[BB marine]

viewtopic.php?f=19&t=8969

I had a concern about the keel shoe bolts early on. When I was painting the bottom I used a different primer on all under water metal. I Was prepping the Keel shoe and noticed the bolts were not installed straight and the keel shoe didn't look sealed properly. I posted my concerns on Tugnuts. Ronnie responded and assured me it was all ok. I still ended up removing all the fasteners to confirm that sealant was used. There was sealant but no sealant was present on the bolts. Sealant was applied to the washers in the bilge and the keel shoe. The Keel shoe was spotty with sealant. I removed each bolt replaced it with 316 SS 3/8-16 bolts nuts and washers ( I worked at a refinery, because of corrosive environment 304 SS is not used, all fasteners must be 316 SS. This was my resource ). I coated the shank of each bolt with Life caulk sealant, the complete shoe, and washers in the bilge. I coated the Threads and shank of the bolts that threaded into the Brass insert located in the Keel extension with Life calk and was able to thread them in straight and the bolts tightened up fine. At the time I was doing this work to a boat that was still under warranty. This was not a warranty repair, it was a repair of substandard workmanship. Quality control!

 

[Stevan47]

Concur with you all: 316 is the only grade to use. Accept no other. Regards to all.

 

[irwinengineering]

Let's refresh this thread with some recent findings:

We have a 2011 R-27, new to us three years ago. It's been a fantastic boat for us here in Ventura, lots of trips out to the various Channel Islands.

The boat was hauled recently for bottom paint. As this was the first time it had been out of the water since 2018, it was a good time to upgrade a few things and to check on other things. We installed trim tab drop fins, a new underwater light and a Divers Dream Mini stern zinc. It was noticed that there were several screw heads missing from the swim step support struts, the trim tab mounting brackets and the stern thruster.

All of these fasteners were removed, most in rusty little pieces. The most frightening set was the stern thruster bolts. Not one of the six was still a fastener, the only thing holding it in place was the sealant, scary.

Bolts and washers were replaced with 316SS, nylock nuts in 18-8 to avoid galling. All fasteners were sourced from McMaster Carr with good quality, fair prices and fast delivery.

If it has been a while, and your boat is out of the water, I would strongly suggest checking on the underwater fasteners. I shudder to think of how quickly the boat would sink if the stern thruster peeled off the hull.

fair winds,
Brendan

 

[ohioan55]

I didn't see Monel mentioned and is an excellent choice when corrosion stress cracking could be an issue. It is a top choice in Naval construction for this reason.

 

[knotflying]

Just a clarification. The exterior screws on the rear stern thruster only hold on the exterior shroud. The thruster arm on the exterior is held in place by interior bolts and there is supposed to be a gasket and sealant between the hull and assembly. Hence I believe the only loss would have been the shroud and possibly the oil thruster prop destroyed and possibly the shear pin snapped.

 

[irwinengineering]

The screws in question were the six 1/4" ones that held the thruster to the stern, they were the only things other than sealant holding the entire assembly to the hull. It was a major pain to get down in there, remove the thruster motor and replace all six. However, the peace of mind will last a lot longer than the bruise on my ribs from hanging down in that rear compartment.

 

[irwinengineering]

For visual reference, here are some photos.