Galvanised steel boat trailers - whats the go?

[Twosheds]

I've just completed a partial rebuild of my Dunbier boat trailer after finding it had serious rust issues in the cross members. It got me thinking about how quick 'modern' boat trailers rust out. I started paying more attention to other trailers parked at the ramp and noticed some real oldies with the galvanising still in fairly good condition.

So what has changed? Are they using different hot dip galvanising processes (ie cheaper) or am I just imagining that the good old days were better.

Regards
Twosheds

[Noelm]

There is various things that have changed, first up, the use of very thin walled square/rectangular tube, much easier to work with, but very prone to rust, then there is quick and cheap as you can get galvanizing, add to that a lot of really cheap Chinese rubbish on the market that is even worse than the Aussie made stuff. If a trailer was to be made out of C section, or even I beam, properly sand blasted and galvanized, it would last 30 years like the old Brookers and so on, just a sign of the times, my trailer was made sometime in the late '70s and is still OK, big C section......

[Giveitacrack]

I just repaired my 10 yr old Redco.The back 500mm of the main chassis rusted through and the bottom and the side walls were nearly rusted through .I wash it thoroughly flushing the channels,tilt to drain, treated internally with Lanolin spray etc. The only thing I think that killed it was for the last two years it has been outside (not near the sea) whereas it was garaged for 8yrs since new and no corrosion.I think most trailers rust from the inside out.

I cut off the affected area, mitred a new gal piece to get more weld and a stronger join,plated the joint top, bottom and sides, then prepared and treated the bare areas with Cold Gal. I expect surface rust to come back at the join some time because of the gal layer being compromised when cut and welded.

[Noelm]

The cold gal will rust to bits in no time in salt water unless you clean it off and redo it every time you use it.

[Giveitacrack]

The cold gal will rust to bits in no time in salt water unless you clean it off and redo it every time you use it.

Yep your right around the welds.It will require maintenance,but cheaper than a new trailer.

[gazza2006au]

im not a rich person so ive owned a lot of old second hand boats in my time, i currently have one sitting in my yard i checked the rego papers and its a 1973 model and rust has just started on the middle roller mount it looked to have been contagious actually the roller mount rusted out and it looks like the rust flakes may have fallen on the frame an compromised the gal coating beneath it

still looks in good condition

[scottar]

Combination of sh*t quality steel, poor prep work and thinner galvanizing would be my three main suspects . See the same with steel work boats. Look at some of the old tugs around - 30 - 40 years and still going strong with a bit of regular maintenance. Compare this to a modern tug built with inferior quality steel - continuously chasing your tail to stay on top of the rust.

[Darren J]

I believe the longevity of the HD gal relates directly to the thickness of the coating. Example there are Australian standards specifying thicknesses required for structural steel sections, dependent on exposure to atmosphere etc. If I recall correctly (someone correct me if I am wrong) standards for structural are around 300g/sqm. However for projects where you wanted to to last, it would be specified at 600g/sqm. Point being, thicker lasts longer. There is more sacrificial coating before steel is exposed to the atmosphere.

However the reality is you would not know the gal thickness applied by most galvanisers. Thicker means more zinc, so I only assume more cost, and thus an incentive not to apply more. I am not actually sure in the process how they achieve a thicker coating, maybe someone can elaborate???

I would say the primary reason for "early" rust would be thin gal coating first. Until its gone the steel remains protected, irrespective of wall thickness.
Then secondly, (particularly inside the sections where they are not seen), the thinner wall sections would mean less time before the apparent rust has removed enough structural integrity that you find a hole in the box section, often poked from the outside of an apparently sound chassis rail.

I know of older trailers that got 10 years of regularly use, stripped and re-dipped BEFORE the gal coating was really gone, and got another 10 years. I guess it is just maintenance, like changing an anode on a motor.... Typically it does not happen when the coating is gone on most trailers but.

[Giveitacrack]

You have to ask if a sacrificial anode would work on a trailer? Over to the metallurgists and "rustologists" if any Guru's can answer.

[Noelm]

Only to a minor degree, to work, a sacrificial anode must be in some sort of electrolyte, like salt water, so, for the amount of time the trailer is in the water, it will achieve nearly nothing.

[Twosheds]

Thanks for the replies fellas. I used 2mm wall thickness RHS for the rebuilt sections and coated all the interior of the RHS with tectyl after it was hot dipped. I guess I will have to wait 4 or 5 years to find out how effective this is.

Regards
Twosheds

[tunaticer]

Closed form (box tube) trailers unless fully sealed will rust out internally before externally.....that is fact.
The interior of the box section will retain salt residues even after the best washing practices......the humidity is constantly high inside the tube for a long time after the external surfaces have dried.

If you want to preserve your trailer seal all openings in the box section and put several gallons of old sump oil inside.....downside of this is if you develop a crack in the trailer you will drop oil everywhere.

If you want a trailer that will last for multiple decades get an open form trailer made (C channel or I beam). have it constructed to avoid any pooling and lamination areas as well as being easy to hose off every trip. Get it hot dip galvanized or sandblasted and tar epoxied.

[mitc69]

on xmas i saw a poly plastic trailer on airlie beach ramps maybe that might last....

[Dignity]

If you do some welding clean the area till you have bright steel then hit it with a zinc stick, you will need a gas torch, one of those cheap plumbers ones that have a high temperature flame and use a cigarette lighter might do the trick although you will need a few refills, I have a small torch that I connect to the gas cylinder that I use. Make sure it covers well then when cold hit it with some tectyl or similar compound.

If you really want to do it on the cheap and use cold gal over spray it with silver paint, this will seal it and look good as well. I have done it over the tectyl mainly due to the colour masking rust, more visible if it works it's way through.

[tunaticer]

However the reality is you would not know the gal thickness applied by most galvanisers. Thicker means more zinc, so I only assume more cost, and thus an incentive not to apply more. I am not actually sure in the process how they achieve a thicker coating, maybe someone can elaborate???

Hot dip galvanizing is almost an art.
It works like this......the thicker the coating, the colder the base material is allowed to heat up....also leads to a less good bond between the zinc and the base metal.
The longer in the bath, the hotter the base metal becomes and more zinc can run off during the initial cooling process resulting in a thinner application but more heavily bonded with the parent metal.

Think of it like hot and cold knives into butter.

Electro galvanizing is opposite......the longer in the bath the thicker the coating but at restrictions on sizes to be dipped need to fit the bath.

Most hot dip galv baths in Brisbane are capable of 1.8m wide x 2.4m deep.....most are a minimum of 10m long.