Trailer operation and performance

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.

Spitfire procedure for fitting a boat to a boat trailer

Basic principals of trailer dynamics

Basic principals of trailer dynamics

Basic principals of trailer dynamics


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


Basic principals of trailer dynamics

Basic principals of trailer dynamics


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.

Evolution of the trailer

Basic principals of trailer dynamics

Centre of Gravity over the axles


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

Centre of Gravity over the axles

Advantages of torsion axle cross members

Centre of Gravity over the axles


Through thousands of years of evolution, the main principal rule has never changed...

The weight and C of G must be over the axles.

A low C of G helps towing performance.

A low trailer chassis assists in loading.

for this reason Spitfire trailers use Torsion Axles

Spitfire Torsion axle cross members..

Advantages of torsion axle cross members

Advantages of torsion axle cross members


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 suspended leaf springs .. and the springs carried be the axles....Spitfire AASS standard model trailers use torsion axle cross members.

These axles are fixed to the chassis and eliminate almost all of the sway stress that comes with a leaf spring suspension.

Advantages of torsion axle cross members

Advantages of torsion axle cross members

Advantages of torsion axle cross members


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 load with minimum stresses.

3.  A low C of G gives more stable towing.

4. Lower entry height in shallow water.

5. A smoother ride and longer chassis life.

Spitfire fitting procedure.


1. Preparing the trailer.

Spitfire place the trailer on a level plane.  Spitfire AASS Model trailers are designed as a modular trailer and  may have configurations as a Bunk trailer or a Bunk trailer with keel  rollers, a Bunk Trailer with a ladder rack keel roller system or a full multi roller trailer.

Some keel rollers are adjustable so they will be adjusted to their lowest position.

positioning the side bunk or roller supports.


2. Check and measure

After a hull inspection bunk location and height will be determined.

The bunks  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.

These are then tightened 

adjust the boat


3. The boat is then placed on the trailer

The side bunks or rollers are then raised to ensure the the boat sits upright.

Many Spitfire boat trailers come with a front roller, however in most cases the design of the bow is such that this roller and bracket are not needed.

The boat is secured to the winch post loosely with a safety chain.

Adjusting the balance


The objective is to have the C of G over the axles with 80% of the boat's weight supported by the keel cross bunks or keel  rollers  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.

The winch post is the last thing to be adjusted

Winch Post Adjustment


Correct adjustment of the winch is one of the most important and neglected point of Boat fitting

Now that the C of G is over the axles and the Ball weight is at 10%, the  winch bracket and winch post are adjusted  to meet the bow eye. The strap needs to draw on a horizontal plane.

To achieve this, adjustable jack stands  are placed under the chassis and the weight taken off the jockey wheel. This will allow the chassis to relax in a straight unstressed position.

Only at this time can the winch post be adjusted.

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Final adjustment

In the case of this image, the winch bracket would be adjusted down and the winch post moved backward to align the winch strap in a horizontal plane with the chassis,  which is already now horizontal and unstressed.

Because of this fine tuning, the winch is in the perfect location and there will be no upward or downward forces on the trailer when the boat is locked in it's final position.

This fine tuning ensures that the trailer and boat are as one with each other and with no opposing forces or strain.

Consider a Spitfire boat fitting.

Draw bar bracket


Forward collision safety bracket

Spitfire trailers have an adjustable draw bar bracket. 

This bracket is designed to allow a chain  to connect the the boats bow eye to prevent the boat lurching forward in case of  a frontal collision.

A chain with a turn buckle which only needs to be hand tightened to secure the boat.

Do not use a ratchet strap on this as it will put unnecessary forces on the trailer adjustment.

Winch Safety Chain


Never rely on a winch when towing or loading a trailer

The job of a boat trailer winch is to position the boat at the forward point on the trailer.

* There is a safety chain winch bracket on the winch post.

* Always connect the safety chain in case of winch failure.

* Never let anybody stand behind a boat being winched up.

* Keep winch gears lubricated.

* Inspect winch straps and hooks prior to every use.