Wall thickness for mild box steel ?

kando · 15th May 2008, 07:26 PM

G'day mates,
It's been a while since I've been around here...over a year and a lot has happened in that year, some good and some not so good but that's life.
If you Glen, seqfisho Wizard, get to read this, I hope things are going good for you Glen mate and I bet you're giving the camper trailer a workout.

I still have the Disco TDi and it'll do me, it's never missed a beat or let me down, it's economical and reliable...who could want anything more from their vehicle!
One problem we all have living in Australia is this, when we take-off on a trip, we can take the caravan, trailer, camping trailer, boat or trail bikes but we can't take them all, whereas if we had the same road laws as Americans, we could tow three or four items behind our vehicles!
In the states, it's not unusual during holiday time to see a vehicle pulling a caravan, a boat and perhaps a trailer with a couple of dirt bikes on-board...A proper little road train and the driver can do that quite legally on his standard drivers license! Anyway, if we tried that here in Auss, we would be in clink faster than a bunny down a hole being chased by a fox!

Now I like to go fishing from a boat when I can get away but I can't tow a caravan and a boat at the same time, so to get around this, I bought myself a V bottom 3.3m inflatable boat! The idea is I can pack the inflatable and the 15HP motor in the back of the Disco, hook-up the Pratline and take off on a trip.
There are a lot of great caravan parks around Auss' but they are not all on the water, quite a few are not even close to water...river, lake or sea! So I went looking for a trailer for my inflatable, something I could dismantle and stow away in the van, or the back of the Disco, then when I got where I was going, I could unpack it, assemble it, hook it up to the Disco and take my boat to the water for fishing... and I found just what I was looking for in Ballarat!
The trailer was all aluminum constructed, apart from the stub axles, 4.2m long when assembled and stowed it was only 1.4m, weighted in at 46KG and is legal in all states of Australia!
The one thing that put me off was the wheels...They were only 8”! I would like 10 or 12” wheels!
So I've decided to build my own inflatable boat trailer that I can dismantle and stow in the back of the Disco for the long haul, then reassemble it when I get where I'm going. But! I'll have to work in mild steel because I don't have the tools or the jigs to work with aluminum.
I plan on using 45mm x 6mm round tube for the axle with 50 X 50 for the tow bar but I'm not sure what wall thickness I should get, I was thinking of getting 50X50X1.5mm, the objective is to keep this as light as I can, easy to assemble and disassemble...but I've been told that box steel with a wall thickness of 1.5 will be a waste of money and will not be suitable! The most weight there will ever be on this trailer is 140KG, that's the inflatable boat, the 15HP motor and a full fuel-tank!
If any of you trailer builders can give me some advice on wall thickness I would appreciate it.

Cheers mates,

Bill

**100I** · 15th May 2008, 08:01 PM

My non professional opinion is for 1.6mm in around 75x38 would be plenty strong enough

**Blknight.aus** · 15th May 2008, 08:19 PM

mate you could do that with just 1.6mm 50x50 box section

I assume your just going to make an axle cradle with a singe beam draw bar or something more complex? how rugged does it have to be?

as an idea if you have dual spares on board you could save weight and effort by utilising the spares as the trailer tyres. IF you running them as lightly as you claim if you have sufficiently strong rubber fitted you could probably tow the boat on them dead flat if you needed to replace a tyre on the car.

**AJSLRD** · 15th May 2008, 08:46 PM

Kando,

I suggest you go a little thicker in wall thickness for all your structural members were any loads are present. Use the lighter sections every where else. Remember that the lighter the wall thickness the harder to weld. The last thing you need is for it to break in half when traveling down the freeway. One other thing to think of is your boat isn't very heavy and if you get this to light all it will do is bounce all over the place. Check out all the boat shops and look over what they sell and get ideas from that.

Allen

**LandyAndy** · 15th May 2008, 08:54 PM

Hi Kando
look into portable boat trailers,Ive seen them advertised,exactly what you describe.
Have a look and copy.I wouldnt mind betting there are heaps on Ebay!!!
Andrew

**100I** · 15th May 2008, 09:05 PM

Originally Posted by AJSLRD

Kando,

I suggest you go a little thicker in wall thickness for all your structural members were any loads are present. Use the lighter sections every where else. Remember that the lighter the wall thickness the harder to weld. The last thing you need is for it to break in half when traveling down the freeway. One other thing to think of is your boat isn't very heavy and if you get this to light all it will do is bounce all over the place. Check out all the boat shops and look over what they sell and get ideas from that.

Allen

1.6 should pose no problem with a MIG.

Soft springs & soft tyres

kando · 15th May 2008, 09:31 PM

G'day mates,
Thanks for your ideas, the alloy trailer I looked at is a real neat job but I don't like the 8” wheels...It's the only thing that puts me off the trailer.
I plan on building my trailer the same way but I'll use steal in place of alloy.
Here is a picture of the alloy trailer...there's nothing welded, everything can be unbolted for storage then quickly reassembled when you get where you're going.

Here is a rough drawing of what I plan for my steel trailer, this is where the axle and beam/tow-bar meet, they will be held together by two u-bolts and plates. There are no springs and I'll be using 12” Mazda wheels and hubs...or perhaps 10” Mini wheels and hubs.

A mate of mine says if I plan to build the trailer with everything bolted in place with stainless steel bolts, I should go with 50 X 50 x 2mm Duragal High tinsel steel for the axle and beam/tow-bar. The only items welded in-place are the two stub axle housings, one to each side of the axle.
I'll go make some inquiries about Duragal steel tomorrow but I would appreciate your views on using Duragal High tinsel steel...My mate reckons it's bugger to weld...What do you think?
Thanks again for taking the time to share your ideas.

Bill

**Blknight.aus** · 15th May 2008, 09:45 PM

thats a bit of overkill...

take a frame like that make the axles 3 sections
make the draw bar in 3 secttions

you need the guards on one of each section of the axle (which is an normalish axle cut i half

you make up a t section that these slide into and bolt to

the draw bar slots into the T and is pinned and bolted

bolt on your spare tyres and air them down to about 10psi

put the boat on upside downand tie it to the mud guards

have a shade cloth tarp made up that hooks onto the front of the mud guards runs under the boat + a frame and comes up and over the hull to keep the stones off the front end (think of an upside down canopy and your on the right track)

attach a light bar to the transom and run its lead over the top of the boat.

If you have to carry the outboard you modify the T piece so that its carry points are on the T piece of the whole thing.

If that doesnt make sense I'll do some quicky TD for you.

**slug_burner** · 15th May 2008, 10:35 PM

Originally Posted by Blknight.aus

thats a bit of overkill...

take a frame like that make the axles 3 sections
make the draw bar in 3 secttions

you need the guards on one of each section of the axle (which is an normalish axle cut i half

you make up a t section that these slide into and bolt to

the draw bar slots into the T and is pinned and bolted

bolt on your spare tyres and air them down to about 10psi

put the boat on upside downand tie it to the mud guards

have a shade cloth tarp made up that hooks onto the front of the mud guards runs under the boat + a frame and comes up and over the hull to keep the stones off the front end (think of an upside down canopy and your on the right track)

attach a light bar to the transom and run its lead over the top of the boat.

If you have to carry the outboard you modify the T piece so that its carry points are on the T piece of the whole thing.

If that doesnt make sense I'll do some quicky TD for you.

agree with a part welded construction to provide ridgidity at the joint instead of solely using U bolts to hold it together because if you do not triangulate the axle is just as likely to go a'skew with just a bolted construction unless you want to carry a plate around instead.

The reason you have a boat is to go fishing, to go fishing you need rods, bait etc, when you come back the boat is going to have fish and guts in it. If the boat is upside down the rods will fall out

and the fish will be all over the road.

Fishermen use their boats to carry the fuel tank and all the other stinky bits like bait fish and guts. If you are staying in a place for a while you don't want to be assembling and disassembling everything everyday.

best if you keep the boat right side up.

good idea to get that shadecloth happening to contain the load if the boat only has low sides and down the sides and under the front to stop the stone damage when travelling on unmade roads.

go with the 75 x 38 it will provide you with a bit more resistance to flexing if the 75 face is oriented vertically, particularly for the longest bit the draw bar.
1.6 sounds ok, just ensure the axle is just behind the CoG to provide a bit of weight on the draw bar/ball

don't know about the duragal given the model you are copying is made of aluminium I doubt that you need that high a tensile strength.

my two bobs worth

**Blknight.aus** · 15th May 2008, 11:01 PM

Originally Posted by slug_burner

agree with a part welded construction to provide ridgidity at the joint instead of solely using U bolts to hold it together because if you do not triangulate the axle is just as likely to go a'skew with just a bolted construction unless you want to carry a plate around instead.

The reason you have a boat is to go fishing, to go fishing you need rods, bait etc, when you come back the boat is going to have fish and guts in it. If the boat is upside down the rods will fall out

and the fish will be all over the road.

Fishermen use their boats to carry the fuel tank and all the other stinky bits like bait fish and guts. If you are staying in a place for a while you don't want to be assembling and disassembling everything everyday.

best if you keep the boat right side up.

good idea to get that shadecloth happening to contain the load if the boat only has low sides and down the sides and under the front to stop the stone damage when travelling on unmade roads.

go with the 75 x 38 it will provide you with a bit more resistance to flexing if the 75 face is oriented vertically, particularly for the longest bit the draw bar.
1.6 sounds ok, just ensure the axle is just behind the CoG to provide a bit of weight on the draw bar/ball

don't know about the duragal given the model you are copying is made of aluminium I doubt that you need that high a tensile strength.

my two bobs worth

its not a tinny its an inflateable and Im guessing that its also a soft floor... and based on this line

Originally Posted by kando

The idea is I can pack the inflatable and the 15HP motor in the back of the Disco

Im also guessing that the back of his disco will now be empty so the fuel tank esky and sealable bait bucket can go there.

If you cant find room for a set of rodsnreels in a rover your either crap at packing OR your carrying too much to start with.

Im taking compactness and lightness when folded as being more important than what it can carry..

theres nothing to stop you from putting the boat on the right side up if you want to But IMHO your better off carrying the soft floors upside down on trailers with some protective mesh ESP if your in close country as it helps prevent thrown up stuff puncturing the or the floor from rubbing through on the trailer if it starts to flap.

What I did forget to mention is that for the muguards you need to line them with a marine carpet to protect the pontoons from the metal of the guards.

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