While a trailer may seem a simple thing that you pull behind a vehicle, there are forces at work with any moving object and when speed and weight are increased these forces can have deadly outcomes if not respected.
Many Spitfire outlets offer a boat fitting service and below are the procedures they follow when fitting a boat to a trailer.
First, lets have a look and understand the history and evolution of the trailer.
It is important to have a roadworthy trailer and one designed to do the job asked of it.
All Spitfire trailer designs are certified by independent Australian vehicular engineers before going into production
Trailers are one of the oldest vehicles and date back 3,000 years.
The first known trailers were used as chariots to frighten and run over advancing armies.
Over time the trailer has evolved from a war chariot to commercial use in various forms and today they travel on highways at speeds around and over 100 Ks an hour.
An unbalanced or incorrectly fitted boat trailer can be deadly
Through thousands of years of evolution, the main principal rule has never changed...
The weight and C of G ( Point of Balance with 10% on the tow ball) must be over the axles.
A low C of G helps towing performance and safety.
A low trailer chassis assists in loading at shallow boat ramps or on the beach.
For this reason Spitfire trailers use Torsion Axles
Torsion axle offer many advantages to a boat trailer.
Unlike the traditional leaf spring suspension which requires the weight of the boat to be carried on the trailer chassis ...and then boat and chassis weight is carried by leaf springs ... and these leave springs are above and carried be the axles.
All Spitfire s trailers use torsion axle cross members.
These axles are fixed directly to the chassis and reduce boat loading height by around 300 mm compared to leaf springs and this eliminate the sway stress that comes with a leaf spring suspension.
Torsion axle crossmember advantages.
1. The majority of the weight of the boat is directly on the axles.
2. The drawbar, like the bullocks in the bullock wagon, is merely pulling the axles along with minimum stresses on the chassis.
3. A low C of G gives more stable towing and minimises sway every time you pass an oncoming semi.
4. Lower entry height in shallow water.
5. A smoother ride with longer chassis life.
Place the trailer on a level plane. Spitfire 2021-5G Model trailers may have configurations as a Bunk trailer or a bunk trailer front side rollers or full side rollers.
After a hull inspection, side support, location and height will be determined.
The side supports will be positioned at the height and width to suit the hull and will be set 5mm to 10mm lower than the final desired height as this will ensure the majority of the boat's weight is resting on the keel and the boat sits upright.
Level up the boat and tighten the supporting legs.
You have raised the side bunks or rollers to ensure the the boat sits upright and level.
Secure the boat to the winch post with the winch post safety chain attached with some slack... this will be the final thing to adjust.
The objective is to have the C of G over the axles with 80 % of the boat's weight supported by the keel cross members and axles and 20 % supported by the side bunks..with 10% the total weight of trailer and boat on the tow ball.
Adjustment for this balance is by moving the boat forward or backward and in extreme cases of a bow very heavy or a bow very light boat, then the axles may need to be adjusted along the chassis.
It is important for safe towing and to avoid flex and so it important to set your boat up properly on the trailer with the trailer in it's natural alignment.
Correct winch post height adjustment is the key to keeping the trailer straight.
A WINCH IS A POWERFUL MACHINE... Place it too high or too low and it will pull the boat down or up and bend the chassis when you tighten it.
Place jack stands under the chassis about 1 metre in front of the wheels so the trailer is horizontal.
Wind the jockey wheel up just off the ground.
Now that you have relaxed the chassis and the chassis alignment in a straight and it's natural shape, you can position the winch roller to the correct height.
On fibre glass boats with a conventional half moon Bow Eye it is best to place the height of the winch roller drum, so the top is horizontally 15mm below the bottom of the bow eye.
Now that the C of G is over the axles and the Ball weight is at 10%, the winch bracket and winch post are the last to be adjusted to meet the bow eye.
Only at this time can the Winch Post and Winch Bracket be adjusted.
This is the perfect set ups for your boat, driving on.
Normally, when you winch a boat on, it is a important to have a horizontal pull of the winch strap from the Bow Eye to the Winch Drum
However today, with many people driving on, it is advantageous to have the bottom of the Bow Eye positioned about 15mm above the horizontal plane so that when the winch is tightened you have some slight downward pressure on the Bow Eye.
REMEMBER, A WINCH IS A POWERFUL MACHINE AND BE YOUR TRAILER STEEL OR ALUMINIUM YOUR WINCH WILL BEND THE DRAW BAR UP IF IT IS TOO LOW TO YOUR BOW EYE.
* The Bow Eye is above the Roller which means if you drive on too hard the boats bow will rise up on the roller and gently slip down as momentum is lost.
This is also important because every time you break you apply force to the winch post which is transferred to the draw bar and chassis.
* When you tighten the winch for travel the winch strap is putting slight downward and forward pressure on the Bow Eye so your boat is gently locked in and your boat won't move.
* In this set up configuration, if you encounter any rough roads that throw your trailer around, the winch strap and the winch roller can act as shock absorbers and absorb stress without transferring it to your trailer draw bar or chassis.
* This same set up can be achieved on Aluminium boats with a welded Bow eye.
* In the case of an aluminium boat, simply attach a D shackle ( With Lock-tight ) to perform the same effect. This is one place where a galvanised D Shackle is better than stainless, as it will grow together where as stainless tends to work loose.
The inherent design of a winch post is to resist rearward forces when a boat may be 2 or 3 Ton is being winched on to the trailer.
However, today many boat owners drive their boat on and this is fine if you understand the function of a winch post and use it accordingly.
There is a growing trend and for no apparent reason, to place the Bow Eye of the boat under the winch roller.
This completely changes the dynamic of it's opperation.
Instead of resisting backward forces as it was designed the winch post is being asked to resist forward stress.
The winch post is now being assaulted from the rear. It is now being expected to perform in a way that it was not designed.
The original design was to have the Roller under the Bow Eye which accepted forward impact as the boat could gently rise up and down in the event of hard braking.
Brakes are much more efficient today than they were 70 years ago and so now incorrect fitting of the winch post means every time you drive your boat on to your trailer too hard... and every time you brake you are applying forces that the post was never designed to accomodate.
For every action there is an equal and opposite reaction. Something has to give .. and will one day.
This will transfer stress to the Winch Post which then goes to the Draw Bar...it could lead to deadly results.
Do not do something, just because others do it... Think for yourself and consider the safest way to enjoy your boating and protect yourself and others when driving down the highway at 100 kph.
IF YOU TIE THE BOW EYE TO THE DRAW BAR... NEVER USE A RIGID CHAIN AND TURN BUCKLE... OR A TIGHT RATCHET STRAP.
ALUMINIUM TRAILERS FLEX WITH ROUGH ROAD CONDITIONS, JUST LIKE AN AIRCRAFT WING FLEXES IN TURBULENCE.
THIS FLEX DISSIPATES STRESS AND VIBRATION THROUGHOUT THE WINCH POST, DRAW BAR AND THE ENTIRE TRAILER CHASSIS.
MAKING A RIGID TIGHT CONNECTION TO THE BOAT AND TRAILER WILL NOT ALLOW THE TRAILER CHASSIS AND DRAW TO ABSORB STRESS OVERALL.
RATHER AS THE TOWING VEHICLE BOUNCES UP AND DOWN ON ROUGH ROADS, AS THEY DO, IT WILL CONCENTRATE STRESS TO THE DRAW BAR.
THE WEAKEST POINT OF A TRAILER IS THE DRAW BAR CHASSIS JOINT. THIS STRESS NEEDS TO FLOW UNRESTRICTED RIGHT ALONG THE DRAW BAR AND CHASSIS RAILS.
USE A MEDIUM WITH SOME ELASTICITY SUCH AS ROPE.. AND NO TRUCKIES KNOTS...IT DOES NOT HAVE TO BE SUIPER TIGHT.
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