Height spec help

Planning my first trailer. Height is critical. It'll be for a lightweight sailing boat. The boat is ~70kg + extras, maybe 100kg all up would more than cover it. It sits on a little launching trolley, essentially a triangle with wheels that is 2700mm long x 1700mm wide. The boat is long and low, so I can lift the bow of the trailer about 500mm safely before the stern hits the ground. Other people mostly use modified box trailers and lift the whole thing (boat + trolley) on top.

My question to those more familiar with building trailers is - say I bought some 13" rims and low profile tyres and some 3- or 4-leaf 45mm slipper springs from Martin's trailer parts - roughly how low will the back of the trailer sit (using 50mm SHS along the chassis rails). If I look at side profile pictures of trailers on an image search, they seem to sit roughly with the top of the rim aligned with the top of the chassis rail. Is that accurate?


Rough profile idea:

Attachment 287310

The idea is to have rollers on the back of the overhang tray, rollers on the back and along the top of the lockable box. Some small ramps to help it on its way. Lift the trolley nose onto the back of the tray and then winch it up like a regular boat trailer. But if that tray top height is too high I'll have to modify my plans to have a drop axle, and that's the motivation for this thread - do I need one when I don't want to pay for it if I don't need it.

Other ideas to comment on if anyone wants to correct me:

- 75 x 50 x 3mm drawbar
- 50 x 50 x 2.5mm chassis rails
- 50 x 25 x 2.5mm chassis cross members
- 50 x 25 x 2.5mm box frame/boat support
- 2._mm checkerplate lockable box
- 2._mm checkerplate overhang tray
- 45mm 3- or 4-leaf galvanised slippers
- 40mm square axle
- not sure what to choose in the way of axle, hubs, rims - I need to read up on the differences between Holden and Ford, although I came across some nice 14" Volvo 240 alloy rims with supposedly the same PCD as early Holden rims (5 stud 4.25"/108mm ??).

I feel I'm overengineering the chassis somewhat, but it'll be long so I'd rather be safe. The load will only ever be small - the boat, sails, gear, etc. The box so that it can be trailered somewhere for a weekend and stuff locked in it overnight or when out on the water.

Any ideas or advice gratefully accepted ...

Quote:

---

Originally Posted by Legion

Planning my first trailer. Height is critical. It'll be for a lightweight sailing boat. The boat is ~70kg + extras, maybe 100kg all up would more than cover it. It sits on a little launching trolley, essentially a triangle with wheels that is 2700mm long x 1700mm wide. The boat is long and low, so I can lift the bow of the trailer about 500mm safely before the stern hits the ground. Other people mostly use modified box trailers and lift the whole thing (boat + trolley) on top.

My question to those more familiar with building trailers is - say I bought some 13" rims and low profile tyres and some 3- or 4-leaf 45mm slipper springs from Martin's trailer parts - roughly how low will the back of the trailer sit (using 50mm SHS along the chassis rails). If I look at side profile pictures of trailers on an image search, they seem to sit roughly with the top of the rim aligned with the top of the chassis rail. Is that accurate?


Rough profile idea:

Attachment 287310

The idea is to have rollers on the back of the overhang tray, rollers on the back and along the top of the lockable box. Some small ramps to help it on its way. Lift the trolley nose onto the back of the tray and then winch it up like a regular boat trailer. But if that tray top height is too high I'll have to modify my plans to have a drop axle, and that's the motivation for this thread - do I need one when I don't want to pay for it if I don't need it.

Other ideas to comment on if anyone wants to correct me:

- 75 x 50 x 3mm drawbar
- 50 x 50 x 2.5mm chassis rails
- 50 x 25 x 2.5mm chassis cross members
- 50 x 25 x 2.5mm box frame/boat support
- 2._mm checkerplate lockable box
- 2._mm checkerplate overhang tray
- 45mm 3- or 4-leaf galvanised slippers
- 40mm square axle
- not sure what to choose in the way of axle, hubs, rims - I need to read up on the differences between Holden and Ford, although I came across some nice 14" Volvo 240 alloy rims with supposedly the same PCD as early Holden rims (5 stud 4.25"/108mm ??).

I feel I'm overengineering the chassis somewhat, but it'll be long so I'd rather be safe. The load will only ever be small - the boat, sails, gear, etc. The box so that it can be trailered somewhere for a weekend and stuff locked in it overnight or when out on the water.

Any ideas or advice gratefully accepted ...

---

Hi, a normal trailer hitch is about 13" or 330mm plus the 50mm for the chassis brings it up to 380mm, as far as wheels go I'd be inclined to set it up with the same wheels that are on the towing vehicle to save carrying a spare.
I feel that the springs that you suggest are to rigid for the weight you want to carry, where you get the springs from should give advise/suggest on what would be the best choice. You might want to work out the weight for the trailer to get a better idea. Instead of using checker plate for the box I would suggest going aluminium to keep the weight down and also to prevent rust. When the box is bolted to the chassis, put a thick layer of silicone in between to cut down on the electrolysis of ally and steel.
Kryn

Thanks Kryn. Slowly refining plans today so that I can get a more accurate weight spec.

- there's no way I'll put the same wheels on the trailer - I drive a Land Cruiser with 265/70 R17s and this is a lightweight box/boat trailer. I definitely would if I were building an off-road trailer but not this time.
- I was leaning towards checkerplate because it's about 20% cheaper than galv sheet of similar thickness (2.1 non-galv checker vs. 1.95 sheet) from where I was thinking of buying it.
- I was leaning towards steel (didn't even consider ally) because I thought I'd form the box by laying down a few welds and then seal between the welds with silicon. I don't have a TIG or MIG setup, just a stick inverter.
- I found the section in VSB#1 that mentions coupling height - between 320mm and 420mm so that gives me some idea. My trailer plans are tentatively 5m long (the boat hull is 4.45m long) and it seems very low if underneath a 320mm coupling the 75mm drawbar sits only 245mm off the ground for ~3.3m and if the overhang at the rear is 320mm for ~1.7m behind the axle.
- the springs are the lightest ones that Martin's sells. I thought they'd do. But I'll come up with a weight spec as mentioned so I get the most suitable ones.

** Don't you hate it when you type an extensive reply, and you hit the wrong button and lose it...!! ** :((

I made a road trailer for my brother-in-law for his Impulse and beach trolley. My biggest issue was its centre of gravity for towing. The beach trolley is set up such that its wheels are a little behind centre for ease of dragging it around by hand, so setting it up on a trailer meant that for the boat to have towball weight, the beach trolley wheels had to be on top of, or in front of the guards. Eventually we set it up so that the beach trolley wheels are removed once the boat and trolley are part way on, then the beach trolley axle sits hard against the rear of the guards. Having a jockey wheel and a spare tyre mounted up the front then adds a little bit of weight to the front.

The road trailer a basically a conventional trailer chassis (all be it a little longer), 39mm round Galv. axle, 3 leaf Galv. springs, Galv. HT hubs, made with 75 x 50 Duragal RHS for the drawbar, and 40mm Duragal SHS for the main chassis. This is then covered with ply over most of the trailer (this allows the trailer to be used to carry other things when required), then teflon strips are screwed across the trailer at several points, allowing the beach trolley to be slid along the trailer by one person. We positioned and centred the boat and beach trolley once complete, then screwed several positioning blocks along the diagonals of the beach trolley so that as you pull the boat and trolley along the road trailer, it centres itself. And once the trolley axles touch the guards, it won't move any further forward. Larger plywood blocks were also screwed to the top of the positioning blocks to lock the beach trolley from lifting once fully forward.

Thanks Yonnee.

I did a bunch of measurements over the weekend.

- the boat trolley has about 1500mm boat overhang behind the axle. <edit - the following is wrong - I'm not sure where I read it but not in VSB>VSB#1 indicates proper boat trailers can't have an overhang > 150mm. While this isn't a proper boat trailer, it sort of is - I'd hate to get whatever the trailer equivalent of a yellow sticker is for violating the 150mm rule. Despite this rule, plenty of other boats stick out the back of their trailers. Often it's just a 8' x 5' box where we all help the owner lift the boat on top.</edit>

- a random set of 4-leaf slipper springs I enquired about at the trailer shop have 140mm of free camber to the top of the spring from the eye. This puts the top of the rear of the chassis at about 420mm with 600mm diameter wheels. I'm tempted to just spend the extra $100 and get a 75mm drop axle but I'm concerned about ground clearance.

- Land Cruiser ball centre sits at ~460mm, so I'll probably have to have a raised drawbar section. I'm not sure whether it's best to put a short raised section at the front closest to the coupling or if I can get away with a drawbar that is higher all the way along except underneath the chassis.

- I'm pretty set on having a 4m chassis with the back 1.5m pure flat tray and the front 2.5m box/boat rest and tie down area.

- there will be a lot of chassis cross members. Do I tie the drawbar into every single one or just the front one and front spring hangers?

- still deciding between 1000mm or 1500mm of free drawbar in front of the box. Somewhere on there I would like to fit a winch post/mast rest. The mast needs to be high enough so that the bottom of the mast can sit in the boat and the top is high enough that it clears the vehicle but doesn't clip trees if possible. Mast height spec is 7626mm but I'm not sure if in sailing talk that means from the hull bottom, hull top or what (i.e. not sure how long the mast itself actually is).

I want it so a teenager or two can load the trailer alone. The common large box trailer works, but works best with three strong people or more. That's annoying when everyone's running around packing/unpacking their own stuff.

I saw a great trailer similar to what I'm thinking on the weekend for another boat. I'll keep you all updated as plans and building progress.

I'm not sure what you mean with the 150mm rule...?? The VSB1 has been my 'bible' for over 15 years, and I can't recall that dimension. The rear overhang of a trailer refers to the distance from the centre of the axle to the back of the body of the trailer. This can't be greater than the front overhang, or more than 3.7M. The overhang limit of the load you put on/in a trailer varies from state to state, but all goes out the window with a boat trailer. Because technically, as soon as you put any boat on any sort of trailer, it's legally becomes a boat trailer. And as such, the deal with a boat trailer is that the boat itself becomes part of the trailer, including all dimensions, overhangs, and can even be used for mounting lights on.

With my B.I.L.'s trailer, we made it as long as the rear of the Impulse to give the boat itself a little protection at the rear.

Yeah, I was wrong Yonnee. I can't find that in VSB1 either. I did read it somewhere over the weekend but I'm not sure where. Regardless, as you say, if it's not in VSB1 it's not a law.

I've gone over 400 different ideas in my head to achieve what I need within the constraints of the boat/trolley + storage box. Latest idea is to chop the chassis behind the rear spring hanger. Box extends rearwards to that point. Near the top of the box weld on two stub axles. Have two "break back" ramps on either side of the box, rotating about the stub axles. Then the rear height of the chassis is irrelevant and I just have to rotate the ramps down to load the boat on. Like a double decker car carrier. I'll sketch up a picture later. Probably needs a new thread.