Trailer Design Question(s)

[Fascadale]

Hi

I am trying to design a trailer with which I can bring ashore (up a slip) my boat and then move it around a yard. It will not be street legal, no electrics or brakes. Really a cross between a launching trolley and yard trolley.

I have two "double wheel" axles, and a 20 ft about 12 cm heavy box section beam with a towing eye at one end.

The boat is an Invicta 26, long keeled, drawing four feet of water and weighing about 4 tons. There is a drawing and photos of an Invicta here

Two questions

Where on the beam should I put the axles, together under the centre of the boats gravity( Plan A) or one axle just five feet from the towing eye and the other at the far end of the beam (Plan B) ?

If I spread the axles to the ends of the beam (plan B) the centre of gravity would certainly fall between the axles and the trailer would be more stable but would it be manoeuvrable in the yard, would it turn ?

If I go for (Plan A), the axles together under the centre of gravity I think it would turn better BUT how do I find the centre of gravity.

Would the positioning of the beaching legs give an indication of the centre of gravity ?

Or am I missing the point completely and asking the wrong questions,
(never designed or built a trailer in my life before)

Thanks

Paul

 

[Lakesailor]

Some piccies here http://www.lakelandimages.co.uk/keels/ of trailers and trollies. They were taken for the keels so may not have a lot of information relevant to you.
I think you need to be sure before you commit your boat to moving around on a trolley. It only has to fall off once.

 

[brianrunyard]

By "two "double wheel" axles" I assume no steering, therefore if you want to tun you need to get the axles as close together as possible. Even close coupled twin axle trailers don't turn easily.

 

[cliffordpope]

Close together is best in terms of manouvrability, but not too close. At all costs you must avoid the possibility that the trailer might tip up, eg when being pulled up a slope, and crash the back of the boat on the ground.
Even with the wheels close together, you are limited in how tight a turn you can do because of the scrubbing effect on the tyres. As I discovered the hard way, tubeless tyres are not suitable for trailers because if the beads are pulled away from the wheel rims, all the air escapes.
Ideal is a long-wheelbase steered trailer. That will be highly manouvrable, and the CofG is not critical because it will always be somewhere between the wheels.
Bear in mind that although the trailer might have a strong backbone, it is a moving platform, so the side supports must be capable of taking the weight of the boat eg on corners or inclined ground.

 

[VicS]

Road trailers, if double axled, have the two axles as close as possible to each other (close coupled) however for a yard/ launching trolley I would strongly recommend that you should not go for a close coupled arrangement but make a proper two axle trailer with one axle steerable for manoeuvring in confined spaces. The front axle from a small truck should provide all the necessary parts. A close coupled trailer is difficult to manoeuvre in tight spaces (With a small one you can wind the jockey wheel down to take the weight off the front wheels) but a non steerable one that is not close coupled will be impossible.

You should talk to the yard concerned, they may have some strong views. The yard were I am based dislikes most owners own trolleys and prefer to use their own trolleys even though that means blocking the boats off and reloading them again every autumn and spring. There is one person however who has a superb home built and steerable trolley for a Vega which is pretty similar in size and shape to an Invicta. A bit bigger and presumably heavier.

 

[bendyone]

I made the trolley below, its a bolt together design to fit in the back of the car. Will carry 3200KG but more with larger wheels. Towed my Hunter 26 on a smooth surface with no problem.

 

[William_H]

A few people here have keel boats on trailers. A once a year trip home type trailer. The style would perhaps suit you.

The backbone of the trailer is a U shaped channel into which the bottom of the keel sits. Obviously the channel must be just wide enough for the bottom of the keel. The draw bar attaches to this. It can be in line but having the channel lower and draw bar higher is better.
Some have an axle arrangement on a lever with separate springs which means the wheel hubs are above the bottom of the keel.
If you have an axle right across then the keel must be above the axle. With a clearance if you have springs. My trailer has a cross axle but the axle has a step in it so axle is lower than the hubs which attach to stub axles.

At the back of the trailer a cross member leads out to angled support posts which come up above the gunwhale. One set of side supports appears to be enough at least for smaller boats.

Now for road use they tend to have a post at the front to attach the bow to. This presents real problems however as the bow in relation to this post rises and falls from water to dry land. The trailer is at a slope on the ramp while the boat is horizontal and as you haul it out the boat tips stern down to settle into the trailer. So while you want to tether the boat onto the trailer the bow is not the best place to do that.

Initially the boat will hit the trailer with just the front of the keel in the channel and the back of the keel floating above the channel. It can be difficult to get the boat in place before hauling out. This problem is worse with a steep ramp but can be helped by having the front of trailer (channel) as low as possible. ie aligned to the bottom of the keel when the boat is floating.

I suggest a steel rod guide around the front and sides of where the keel goes at a height of at least 40cm. This can be a stop to pull the boat against and at the back will stop the keel slewing away from aligned with the channel.

The only problem with this kind of trailer is that the bottom of the keel is not accessible for A/F unless you jack the hull up. This is often not worth worrying about. A few groundings will take the paint off anyway and/or give it a good clean.

I think you worry too much about where the cofG will be. The middle of the keel would be a good place to assume. I am inclined to suggest 2 pairs of wheels close together. You will find that the actual fore and aft location of the hull is not very controllable so allow for plenty of weight on the draw bar if you can. You can adjust the front of keel stop rail if necessary with experience.

At least for the first try out you need to have a mask snorkel and wet suit so you can see what is happening under water before haul out.

PM me if you want to discuss olewill
There are jinkers around clubs here that have the steerable front wheels which seem OK but a bit complex.

Lastly beware the rust in your square steel tubing. Put lots of oil or similar in there. good luck olewill

 

[Avocet]

I'd be inclined to bin one of your axles and get a steered axle from a scrapyard. If you get a beam axle from something like an old Sherpa van or an older Transit, it will be a relatively simple matter to make the front wheels steer (crudely) and then you'll get a stable, steerable trailer.

Alternatively, depending on what sort of surface you want to tow it over, you might find some very heavy duty castors that would handle 4 tons between them.

 

[Lakesailor]

Castors could be the answer. Dolly, the oldest mechanically powered vessel in the world has just been lifted on a specially designed cradle that has massive castors which are specced to cope with being run on only 3 assemblies as it's moved about over not quite level tarmac. The cradle itself cost many thousands of pounds. However the structural integrity of this old lady needs to be assured.

 

[TheBoatman]

I've built a number of trailers over the years and the best solution is to connect the axles to the centre beam by using "U" bolts. That way you can adjust the axle position to get the best balance point.

Most modern trailers come with the ability to adjust just about everything and they achieve that by using "U" bolts (or similar).

Peter.

PS
If you've got a close coupled trailer than I serious recommend that you fit a jockey wheel so that you can lift the first axle off the ground when trying to move the trailer around with no boat on it.