Thread: Boat/box trailer for lightweight sailing skiff

I got as far as calculating all the lengths required but there is significant work involved and I'm looking for easier designs.

One that I'm quite interested in is like this:

Renegade trailers, ground lowering trailer, low loader trailer

I can do this. My constraints match quite well with the idea. It would look like this:

lowering_trailer.png

The top would be essentially a flat plate that the boat ties down onto (on top of its dolly trailer). A long thinner box in front of the axle for sails, booms, rudders, keels, etc. A shorter box at the back for bags of gear, more easily accessible when loaded. A hollow section in between the boxes for the pivoting axle to retract up into. I did some calculations today and I could easily have it so that the bottom of the frame sits on the ground without hitting anything when the pivot bar is around 45 degrees.

The box is about 400mm high from top of top frame to bottom of bottom frame.

A couple of gas struts to soften the action and a winch to assist lifting the trailer off the ground, like the Kiwis do.

One thing I'm curious about, from you trailer builders, is the frame thicknesses. I have three frame thicknesses to worry about:

- the bottom of the box frame structure.
- the top of the box frame structure (probably just the two grey rectangles - the front triangular section can be a lighter extension).
- the pivot subframe structure.

My initial thoughts, worried about the dynamic load required, was 75x50x3.0mm all round. But it seems overkill to have the top and bottom of the boxes, and the pivoting frame all so thick. Makes for a heavy trailer. I would prefer to go with 65x35x3.0mm or even 65x35x2.5mm if I could get away with it. Maybe the box frames lighter, where there is multidirectional support, and the pivot frame 75x50x3.0mm since it's standalone and has to maintain its integrity.

To my untrained eye it looks like the Kiwis are using 75x50, but they're also loading cars on theirs.

Remember, besides the weight of the trailer it will only have a payload of 150kg max, more like 120kg max (that's 70kg boat and more than enough for all gear).

I can explain further if you have questions.

Any advice appreciated.

To explain the pivot point since I was a bit vague - I'd weld a second axle on to the leading bottom corner of the rear box and the rear of the pivot subframe would be welded on to those bare flangeless hubs that you buy to make swing-out spare wheel carriers. More than heavy duty enough.

Or, rather than welding a 40mm square axle directly to 2.5mm or 3.0mm thick RHS I'd probably weld either a whole length or several shorter lengths of 50x50x4.0mm SHS to the leading bottom corner of the box and have the axle running inside that, and weld the axle to that.

Came up with a structural drawing today while I had some time. Here are some views.

Top view:
top_view.jpg

Side view:

side_view.jpg

Oblique with labels:

oblique_view.jpg

Another oblique view:

oblique_view2.jpg

The big issue with this, is that the steel I'd planned on using would put the weight up around 475kg steel alone (add 2 x axles, wheels, lights, tow coupling, etc, etc).

Revising somewhat I can get it down to a minimum of about 313kg but then the structural integrity comes into play. Luckily most of the weight savings are in the box walls, so I can safely keep the frame strong but lighten up the box, although I'd probably keep the roof check plate.

Here are the weights, with the first few rows corresponding to the labels on the diagram above.

Steel_weights.jpg

The maximum weight obviously came with the thickest strongest frame and check plate all round. If I dropped down to 1.6mm sheet metal (I will probably stick weld it together and that's about my limit) then I can minimise the weight of the box. Probably need to factor in some 25x25 SHS bracing for the box lids as well, no matter what I choose. I kind of feel that I'm talking to myself here, but comments welcome anyway.

Revised weights calculation:

revised_steel_weights.jpg

Interesting trailer design.
The thing I can't quite work out yet is the suspension. It's doesn't look like a beam axle, because as the front of the pivot frame rises, you can see the the body section lower towards the ground way beyond the centre of the wheels.

And out of curiosity (not so much an issue in your case), there's no tandem... which means you're limited to a single axle carrying capacity.

The Kiwis are using stub axles. You can make it out from some of their pictures and videos.

http://www.pintotrailers.co.nz/files...241a9a3513.JPG

It looks like a custom job, possibly with custom leaves too.

They have a tandem too, e.g.:

Renegade Trailers Tandem Low Loader for V8 Supertourer - YouTube

Because I can get away with a box at the front big enough for sails and a box at the back for gear and a void in between, I can use a beam axle with leaf suspension mounted on the pivot frame and the axle retracts into the void, no problem.

I was thinking that the pink part that I have labelled as "structural subframe" doesn't actually need the strength I thought it did. For some reason I thought it was supporting the pivot point axle, but the pivot point axle is supported by the pivot frame. So if I have a strong drawbar and a strong pivot frame (75x50x3.0mm each), the box structure can be relatively weak. Trimming the skeleton with this in mind I've got it down to 363kg.

Another interesting variation from someone else:

Road-king45-RRD.jpg

I guess an eye on the front of the leaf, and the rear gets pulled into place and then you lock it in with a pin as well. You'd want solid mounts for the pins, for sure.

If you do a google image search for "lowering trailer", there are plenty of examples out there, all doing the same basic thing.