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G.D) Tracked Boat Trailer


The problem:

Getting a boat (not mine!) along the top of a shingle bank, down a steep shingle slope (up to 45 degree slope), over stone-strewn sand, soft mud and sand banks to the sea. And getting it back again....
The tried and tested 'usual' way to acheive this is with inflatable rollers that have to be continuosly picked up from behind the boat and passed forward to the front, keeping up with the movement of the boat. The problems with this are:
  • It is difficult to keep the rollers evenly spaced and the boat comes off the rollers and scrapes on the shingle;
  • It is hard work - all it needs is a chap with a drum and throwing rollers forward could be a scene from Ben Hur!
  • The boat rocks from side-to-side on the rollers and eventually falls off.
  • Rollers are very difficult to use when they are wet and slippery.
  • The boat has to be turned around at the top of the shingle.
Other methods seen on the beach include lengths of gas-pipe, greased railway sleepers and, recently, attaching 80m of rope to the back of an old tipper Transit ("Tarmac your drive, mate?") on the road at the top of the shingle and driving off at 30 mph.

These all have disadvantages, including damage to the bottom of the boat and Transco coming along and nicking their bits of pipe back.

This is the sort of damage that can result from a boat coming off rollers onto shingle.

I am inherently lazy and, to quote the advert: "There must be a better way". 

Definitions.

The weight of the boat, engine and fuel, etc., is approximately 250 kg.
  • The maximum slope is approximately 65 degrees.
  • Approximate pull of winch: 3,400 N

Fig 1: Boat on 'Version 1' trailer.

 

Version 1:

The single wheels where removed from the trailer and in their place were fitted four-wheel bogies, which pivot about the original axle. Two smaller wheels were added at the front to take the small nose-load.

The soft tyres run at low pressure (5psi, 35 KPa) to spread the weight of the boat. The bogies allow the wheels to move and stay in contact with the ground.

The tyres on the forward wheels of each bogie are slightly lower pressure than the rear tyres so that the front tyres tend to dig in less than the rest and the bogie tends to ride out of any soft ground.


Fig 2: Bogie detail. This image was creatd in Pro/DESKTOP from PTC.

This works well but the handle at the front makes it difficult to float the boat off forwards during launching. The transom of the boat takes a pounding from the waves during retrieval and the boat can fill with water.

To compare the effectiveness of various solutions, the ground pressure or the contact area can be used. They are the same thing really, but it just helps to visualise the differences sometimes.

With a working pressure of approximately 35 KPa (the "Loading"), the contact area can be assessed as follows:
Load per wheel = 250 x 9.81 / 8 = 306 N


Fig 3: Deflected tyre.

Area = (p.a.b. )

= 306 / 35000

= 0.0087 m2 per wheel.

 

Version 2:

The most recent update is to run the wheels inside a track system constructed from roller-shuttering that was originally the rear shutter of a large trailer. De-constructing what was trying to be achieved, it could be stated that:
  • If the boat is only ever going up and down the beach in a straight line, why need to turn it round at either end?

  • If water is breaching the transom whilst it is being towed out of the water, why not leave the boat facing the waves?

  • 'Normal' launching trolleys have handles at the front that make launching bow-first difficult - are the handles really needed?
By cutting the roller-shutter up into 30 cm lengths it could be looped back onto itself to make a track. Each element, or 'link' of the roller-shutter is shaped in section like so:

Fig 5: Link detail.

The roller-shutter is aluminium extrusion and the linking mechanisms give each link considerable strength in bending. Each link is held in place with washers pop-rivetted into the end of the extrusion to stop relative movement between them.

The tracks (of forty-seven links) are looped around the wheels of the bogies detailed above and held in place by sections of shaped plastic extrusion that run between the tyres, thus:

Fig 6: Tracked bogie

Track tension is effected by the tyre pressures (higher pressures = larger diameter = more track tension) and the links are sealed with low modulus silicone sealant to prevent the ingress of sand and grit. It is the silicone in the joints that is causing the top of the track to 'hog' in the picture.

It is pretty impossible to predict the distribution of load over the lower portion of the track without some impressive calculations / modelling, since this distribution is a function of track tension, ground profile and solidity, but an assumption that 40% of the track is taking the load gives:

Area = (width * 40% * axle separation) / number of wheels

= (0.3 * 0.4 *1.3) / 4

= 0.039 m2 per wheel

i.e. approximately 4.5 times more area (less "loading") than the wheels and tyres alone.

At 7.7 KPa (250 * 9.81 / [8 * 0.039]), the loading is 1.13 psi, which is approximately 8% of that of a Challenger II tank!

The tracks offer other advantages:
  • They 'lay down their own road' and the tyres are not forcing their way through shingle.
  • The leading edge of the assembly, and the lower loading, presents a less inclined slope for the leading wheels to ride up and over.
A pair of these is fitted to a dedicated, bespoke launching trolley. This trolley is design to be completely immersed during launching and retrieval, with the boat floating off forwards, and also being able to float back on during retrieval, with the bow facing the waves. The trolley, boat and all will be winched up backwards without turning the boat around at any point.

A series of longitudinal rails will be surrounded by foam.

Fig 7: Main framework under construction.

 

Fig 8: Trailer almost complete. Just awaiting diagonal struts and front wheels. Foam is 25mm thick 15mm pipe insulation, held in place with big, thick, black , monster cable ties.

 

Six metres of 3.2mm steel cable from one shackle to close to each bogie. This is for two purposes:
  • Tension is applied close to the bogies, minimising the bending in the beam between them (tension applied to one point at the centre of the beam would act with the drag of the bgies to impart a large bending moment).
  • To help pull the trailer in a straight line. If it goes out of line, one length of cable tends to go slack and the increased tension in the other acts to pull it straight.
 
The front wheels cost £3 from eBay and are lightweight trolley wheels. They are so lightly loaded in use that they are nother more than skids.
 

 

 

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