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Building a tough 4WD boat trailer

Owning a 4WD gives you access to some of the most amazing places in the world. We’ve been travelling WA for many years now, and have seen a huge number of these spectacular places. We love the coastline, and with 12,000km of spectacular coast in WA, you’d be hard pressed not to.

One thing we always missed though was having a little boat with us, just to get out that little bit off shore. Spearfishing and fishing is a huge part of our holidays, and with all the gear you carry around, having a little boat makes it a real pleasure to get to the best spots. However, an ordinary tinny and boat trailer just wouldn't cut it where we go, so something had to be done about it.

Carrarang station

A boat makes places like this even better!

We wanted something light, easy to move and big enough to take out even when it was a bit choppy. Something you could drag anywhere the 4WD went, including Steep Point, Murchison House station and Kalumburu and Cape York one day.

Operation tough boat trailer

We were looking for a boat under 4 metres, and a trailer that was already well built and in good condition to set up. After trolling Gumtree for many hours, I came across an older stacer 3.75m boat on a brand new trailer that looked quite strong. We went and looked at it, and despite a few issues with the trailer that could be easily fixed, picked it up fairly happily.

So, what was wrong with the trailer?

For starters, the boat was sitting entirely on the slides, with a good 10mm gap off the rollers, which should take the majority of the weight. There were only 4 keel rollers, and the slides were built out of electrical unistrut, with 1.5mm thick uprights. Then, the rear angle supporting just the tail lights was 75x75x6mm, and only supported in the middle, allowing it to twist and fatigue. The drawbar was only 50x50x3mm RHS, and looked like it was very weak where it attached to the main chassis.

The outboard motor had no support, and the leaf springs were far too hard. Even if you jumped up and down on the boat you couldn't get the single 600kg parabolic springs to flex, and I found out they were installed the wrong way around too, with the slipper facing forward.

Tiny wheels on the tinny

Original tiny wheels and the wrong wheel track

4WD boat trailer

Single parabolic spring that was far too hard and the wrong way around

Weak drawbar

Guaranteed the draw bar would have broken here

What does a tough off-road boat trailer need?

We had bumped into a group of guys along the Gibb River Road in the past, who had just come from an epic fishing trip to Kalumburu with their tinny, set up to handle the shocking (literally) roads up there. I had a bit of a squiz at their trailer, and was impressed. Essentially, you need a bullet proof trailer that is going to take care of your boat whilst not falling apart. How do you do that?

I did a heap of reading online, and basically combined everything I read into this post. Hopefully it will make your lives easier!

Keel support

You want the majority of the weight of the boat to be supported by the keel, on the hull. The slides on the trailer touch the side of the hull and should be there to stop the boat from rocking and moving, not to take the weight of the boat. We went from 4 to 8 rollers, and made sure the boat's weight was almost entirely on the hull, and spread over a larger contact patch of rollers.

Boat trailer rollers

Lots of rollers to support the keel

Slide quality

The slides on the side of your hull should be strong, and stop the boat from rocking one way or the other. They can be adjustable, but it does increase the chance of them moving. It took a long time to get these the right height to match the rollers, but once they are fixed they aren't going anywhere. We removed the rubbish slides that the previous owner had made up, and replaced them with 50x50x3mm SHS.

Slides on a boat trailer

Solid, heavy duty slides

Quality suspension

Your suspension is probably the most critical thing on your boat trailer. It needs to absorb the bumps and corrugations as much as possible, so your boat doesn't take the brunt of the shock. It should be eye to eye shackle or independent suspension. Slipper leaf springs are not ideal for offroad use. Shock absorbers are beneficial, but not entirely necessary. We purchased 500kg rated eye to eye offroad springs. Initially I removed a leaf, but found it was too soft so put it back in.

I used a block of linatex (heavy duty rubber) as the bump stop, which works very well.

Clearance

Your clearance under the trailer's axle should be similar to your tow vehicle. You'd rather get hung up on your vehicle before the trailer catches. Consider the entry and departure angles too, depending on where you are planning on driving. However, remember that the heigher the boat sits, the harder it is to launch. Initially I had the axle under the springs, and the top of the tinny was miles off the ground; it just wasn't going to work.

I moved the axle over the springs, which drops the trailer by about 70mm, and although its still tall its not too bad.

Tie down points

We use a couple of the light duty ratchet straps, and we welded several tie down points on either side of the chassis. The boat should be firmly on the trailer, but not pulled down with so much pressure that you damage the hull over rough roads. Basically you don't want the boat to be able to move up and down. Something to bear in mind is the angle of the tie down's too; if you go from one side of the trailer to the other and ratchet it up over the boat, you can squish the hull inwards, which makes it more likely to crack.

Tie down points on the tinny trailer

Lots of tie down points

Suitable tyres

We were pushing to run the same tyres (285/75/16's) on the trailer as on our Land Cruiser for spares and convenience. This did cause some issues though, as they are massive tyres to have on a tinny. Second hand Goodyear tyres were purchased, and put on rims we had at home. The main issue was ensuring enough clearance was available between the top of the tyre and the mud guard, and then the mud guard to the tinny.

Running such big tyres is fantastic for rough roads; you can let them down hugely, and because of the lack of weight on the trailer they just work fine. For high speed gravel driving, we had the tyres at around 12 - 15 PSI. On the really rough sand tracks out to Steep Point, we let them down to 5 PSI and they bagged nicely, but not nearly as much as expected. The benefit of this is two fold; the trailer floats nicely on the tracks, but it also absorbs a huge amount of the vibrations and bumps through the tyres, without even needing to use the suspension.

4WD boat trailer wheels

Ample clearance and a matching wheel track

Strong chassis and drawbar

This is the heart of your trailer, and you want to be happy with it. Overall the chassis on our trailer was pretty good, but the drawbar was an obvious weak point, as was the rear angle bit that the lights were attached to. We welded in extra RHS to support the rear angle, and a second RHS length under the existing drawbar. The mount for the hitch was also removed, and replaced with a massively over engineered version.

Drawbar upgrade boat trailer

50x50 SHS layered together and braced with 6mm flat bar

Winch and mounting points

The winch plays a pretty important role while you are travelling; it keeps the boat in position on your trailer. The upright was replaced, along with the winch mount, and a brace installed to stop the whole post sliding along the drawbar. Ultimately this is the only thing that stops the boat moving forward and backwards, and you want it done right.

Braced winch support boat trailer

New upright section, winch support and brace

Solid mud guards

If a normal boat trailer is taken off road, the first thing to break is always the mud guards. The sheet metal mud guards crack easily over vibrations, and will let go very quickly. If you support them properly they are fine, but after searching for suitable guards to cover the big tyres and coming up short, we decided to make our own. 50 x 3mm stainless flat bar was used, and just bent around between 2 angles off the chassis, with rubber conveyor belt to seal it all up. Not the prettiest, but its simple, rust proof and solid as.

Mud guards on our tinny trailer

Solid mud guards and mounting points

Axle and bearings

The trailer came with a non standard size axle, and weird bearings/hubs. We ditched the whole lot, and replaced it with 45mm square axle and parallel bearings.

Wheel track matching

This is not an absolute must, but it is handy. If you can match the wheel track of your 4WD to the boat trailer, its worth doing. In our case, we were replacing the axle anyway, so it was just a matter of getting the right size one. With the -22 black sunraiser rims on the trailer, the wheel track is within 30mm of our Land Cruiser, which means it tows better and stays in the same ruts.

Offroad Hitch

Again, not something you 'must' do, but an off road hitch will make the trailer ride smoother (without the rattle from the towball), and does give you better ability for sharp angles, where a tow ball would just jam at maximum angle. Initially I picked up a chinese copy of the Tregg hitch from Martins Trailer parts, but I wasn't happy with it.

The finish quality looked suspect, and it wasn't stamped as they are supposed to be. Despite some resistance from the guys that sold it to me, and a desire to conserve the funds, I went ahead and swapped it for a genuine Tregg Hitch, and welded it on.

Genuine treg hitch

Chinese treg hitch copy vs the genuine one on the top

Welded treg hitch

The hitch all welded on

Nylock nuts x 2

Corrugations are the nastiest thing for trailers. Nylock nuts were used where possible, and doubled if the thread allowed. Where nylocks weren't available, we used 290 Loctite. You can't be having nuts vibrate loose on rough roads.

Outboard motor support

The back of your boat is put under huge strain when it has an outboard hanging off the back, and is bouncing up and down. The best way to travel is with the motor trimmed all the way down (which we could easily do due to ample clearance), so I made up a support that locks the motor to the back of the trailer, and a ratchet strap stops it from bouncing around. Linatex was used again to look after the motor, and its working perfectly.

Rough roads motor support

Chunky motor support for rough roads

Light weight

Our boat trailer now probably weighs under 100kg. With the boat and a bit of gear inside, I'd say the total setup would be under 400kg. One thing to be careful of is the amount of weight you put inside the hull; they break most often when people overload them! We carry a maximum of 60L of fuel inside, a spare tyre, a couple of empty eskies and a few other light weight gear; bulky stuff that doesn't weigh much.

Weight issues in a tinny

Easy to move around without too much weight

Upgrading the motor

Our tinny came with a 15hp Honda 4 Stroke, which initially I was excited to see. However, it would barely plane with just two people (the one at the front had to lean forward to make it get up and plane) and replacing the propeller/messing with hydrofoils just didn't seem economical.

I looked around for ages, and found second hand motors were only a few hundred less than a brand new motor with a 3 year warranty, so I sold the Honda and purchased a brand new Mercury Seapro 25hp (made by Tohatsu with a great reputation). I knew it would be louder and use more fuel, but wasn't too concerned about that side of it.

Outboard mercury seapro

The new Mercury 25hp 2 stroke

Steel and other trailer specifics

Drawbar - 50 x 50 x 3 SHS doubled on top and laminated

Chassis - 75 x 75 x 6mm angle upside down

Rollers - 8 x red boat rollers with stainless pins

Springs - 715mm length, eye to eye offroad 500kg rated

Axle - 45mm square

Hitch mount - 10mm plate top and bottom, bolted together and welded to the drawbar with the Treg hitch welded on top

The trailer is absolutely overkill for the weight of the boat and where it is taken, but I'd rather that than have it breaking on us in the middle of no where.

The mayden run

Asides from a quick beach run down to Preston one afternoon, the trailer had done little work before our trip up north. In fact, the outboard motor had only been run at the shop, and put back in its cardboard box. We headed out to Carrarang Station, and spent several nights there and a full day out at Steep Point.

Overall, the trailer was flawless. The only thing that broke was the stainless wire holding the mud guards back off the tyres, which we kind of expected to happen. I'll have to beef it up with something else.

We towed it everywhere, including some very rough tracks around False Entrance and in Murchison House Station, and were impressed with how soft the suspension was, and how well it handled.

How easy is it to launch?

You probably notice the boat sits up quite high, and that does make it harder to launch; there's no two ways about it. The only way around this is to fit air bag suspension, which I didn't want to mess with. On normal boat ramps and beaches where the water drops off reasonably, you can launch it easily. The boat is so light that with a few guys you can shove it off the trailer even with a bit of a gap between the trailer and the water and it just slides in.

If any water gets over the back the bilge pump gets rid of it quick smart.

On beaches where the water gets deeper very slowly, things are a bit harder; normally we unhitch it from the vehicle, and push the whole trailer into the water, then lift the drawbar up and slide the tinny off. Retrieving the boat is done in the same way, and we use a strap to pull the trailer up the beach to hook it back on the 4WD.

4WD boat trailer launching

Launching is relatively easy, although it does require a few people

Horrocks Cobia

A decent Cobia caught off Horrocks

Future trips

We are heading back to Steep Point in the next couple of months, and will be taking the tinny up with us. It will come to Port Gregory, and to the Pilbara next year. Wherever there is a need for it, someone will tow the boat. It makes life a lot more adventurous, and I don't need to be concerned about something breaking and causing us grief.

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