What Size Boat Trailer Do I Need?
The question we are asked most about trailers is what size boat trailer do I need. The short answer: the trailer must suit the boat’s length, and its ATM must be above the boat’s loaded weight plus the trailer’s own weight. This guide explains the three numbers that matter, lists the six trailers in the Sea Raider range by boat length, and works through an example.
Three numbers: boat length, loaded weight and ATM
Boat length
Length decides where the rollers or skids sit and how long the trailer needs to be, so each trailer is built for a band of boat lengths.
Loaded weight
This is the boat as it will be towed: hull, engine, full fuel, batteries, water and the gear you normally leave on board. Brochure dry weights are hull only and are often well short of the real figure once the engine, fuel and gear go on.
ATM, GTM and tare
As the Sea Raider owner’s manual explains, the compliance plate on the drawbar carries three figures. Tare is the empty trailer. ATM (aggregate trailer mass) is the fully loaded trailer – boat, engine, fuel, water, batteries and gear – measured at the coupling. GTM (gross trailer mass) is the loaded trailer’s weight through its tyres. The difference between ATM and GTM is the tow-ball mass, which should be roughly 8 to 10 per cent of the ATM and must not exceed the tow vehicle’s ball-load rating. When you size a trailer, the ATM is the limit: loaded boat plus trailer tare must stay under it, with some margin for the gear you add later.
Sea Raider trailers by boat length
| Boat length | Trailer | ATM | Axles and brakes | Price |
|---|---|---|---|---|
| 4.3m – 4.8m | 4.8m galvanised, ATM 750kg | 750kg | Single axle | from $2,195 |
| 4.3m – 4.8m | 4.8m galvanised, ATM 1200kg | 1200kg | Single axle, mechanical override brakes | from $2,795 |
| 5.2m – 5.7m | 5.7m galvanised, ATM 1400kg | 1400kg | Single axle, mechanical override brakes | from $3,583 |
| 5.2m – 5.7m | 5.7m galvanised tandem, ATM 2000kg | 2000kg | Tandem axle, mechanical override brakes | from $4,974 |
| Up to around 21ft (about 6.4m) | 6.9m aluminium | Rated to 1550kg | Dual axle, braked | from $12,000 |
| Up to around 8m | 8.2m aluminium | 3,490kg | Dual axle, hydraulic brakes | from $15,000 |
Prices are the current list prices for the trailer; options, registration and delivery are extra. Two 4.8m and two 5.7m trailers cover the same boat lengths at different ratings, so the choice comes down to loaded weight.
Single axle or tandem
A single-axle trailer is lighter, cheaper and easier to move around by hand, and it is the right choice for most boats up to 5.7m with a modest loaded weight. A tandem spreads the load over four tyres, carries more and is steadier at highway speed and on uneven ramps. In the galvanised range the 5.7m single axle is rated to 1400kg and the 5.7m tandem to 2000kg. Both aluminium trailers are dual axle.
Roller or skid
Rollers make launching and retrieving easier, especially on shallow ramps, because the boat rolls on and off with less of the trailer in the water. Skids (bunks) support the hull over a longer area and suit some fibreglass and flat-bottomed hulls. The galvanised 4.8m and 5.7m trailers are built as a roller or skid version; the 6.9m aluminium runs wobble rollers with adjustable side adjusters, and the 8.2m runs a multi-roller set-up. We set either up to your hull at Cleveland Marina, Brisbane – see custom-built boat trailers.
Brakes: what the weight requires
The owner’s manual summarises the national Vehicle Standards Bulletin VSB1: a trailer up to 750kg GTM needs no brakes; over 750kg GTM it needs brakes on at least one axle, which may be over-run (mechanical or hydraulic) up to 2,000kg GTM; over 2,000kg GTM it needs brakes on every wheel, controlled from the driver’s seat, and a breakaway system that applies and holds the brakes if the trailer comes off. That is why the 1200kg, 1400kg and 2000kg galvanised trailers come with mechanical override brakes, which need no controller in the tow vehicle, and why heavier rigs move to the braked dual-axle aluminium trailers.
A worked example
Say you own a 5.5m aluminium side console that comes to about 950kg loaded – hull, outboard, full fuel, battery, anchor, safety gear and esky. It sits in the 5.2m to 5.7m band, so the choice is between the 5.7m galvanised trailer with 1400kg ATM and the 5.7m galvanised tandem with 2000kg ATM. Add the trailer’s own weight to the 950kg and the single axle at 1400kg ATM leaves a sensible margin, so it is the economical choice at $3,583. If the same owner tows to the Sunshine Coast most weekends, or the boat is a heavier fibreglass hull of the same length, the tandem at $4,974 gives more margin and a steadier tow. A 4.5m tinnie with a mid-size outboard, by contrast, sits comfortably under the 4.8m 1200kg trailer, and a light 4.3m tinnie with a small outboard can use the 4.8m 750kg trailer.
Not sure of your boat’s weight? Tell us the make, model and engine and we will estimate it. Material is the other choice to make – see galvanised vs aluminium boat trailers. Registration, delivery and finance are covered on the registration and delivery and trailer finance pages, and a boat stone guard can be fitted to any of the six.
Frequently asked questions
Start with the hull’s dry weight and add the engine, full fuel, batteries, water and the gear you carry. If you only know the make and model, tell us and we will estimate it.
ATM is aggregate trailer mass: the fully loaded trailer, boat and gear, measured at the coupling. It is the figure on the compliance plate that your loaded rig must stay under.
A margin is sensible, because gear accumulates and fuel is heavy. Going far higher than needed adds tare and cost, so choose the next rating up from your loaded weight rather than the biggest trailer available.
When the loaded weight is near or over a single axle’s rating – in our range above 1400kg – or when you tow long distances and want the stability of two axles. The 5.7m tandem is rated to 2000kg.
Under VSB1 a trailer up to 750kg GTM needs no brakes. Over 750kg GTM it needs brakes on at least one axle, and over 2,000kg GTM it needs brakes on every wheel plus a breakaway system. Our 1200kg, 1400kg and 2000kg trailers have mechanical override brakes.
Still not sure which trailer?
Send us your boat’s make, model, engine and how you use it and we will recommend the right size and ATM. Enquire online or call the Sea Raider team at Cleveland Marina, Brisbane on 07 3477 9338.