My boat is also 3.5T dual axle.

Kept in a shed so the sun doesn't get to the tyres, but they get a pounding on cornering.

I also tow quite once a year to Denham and a few times a year to Jurien, but there would be minimal wear from that.  The wear is from the hard cornering reversing in and out of my garage.

The last tyre I changed went out of shape and was causing a bit of vibration...probably lucky it didn't blow as I was doing a Jurien trip at the time.  It was pretty worn.

Also check your insurance if you are covered after 5 year old tyres. A family member has classic cars insured through An insurance company and was told he is not covered if he has an accident with old tyres. 

 Use Light Trucks, not car tyres. 5 years is a good rule of thumb. Any cracks visible, past their use-by date. Storage out of the sun helps, but does not remove, age-related problems. If you blow one "for no apparent reason" , replace the lot. Or risk a blowout. Just another cost of boat ownership, it adds up. And yes, tandems are very hard on their tyres ( and rims) when tight manoeuvering. The nimber of people I see chuck full lock u-turns towing tandems, when,  with a bit of thought it is not necessary, beggars belief. A case in point is the Kalbarri ramp. You can come down the approach road and do a full lock u-turn to line up for reversing. or you can come in from the other way , and only a 90 degree turn is necessary. Watch your tyres, or someone elses, on a tandem u-turn. 

 Learnt my lesson coming out of Dunsborough where I had a blow out on boat trailer tyre that had probably 80% tread but was over 6 years old. For a half decent light truck tyre at around a hundred bucks it's a no brainer to change them every 5 years as Ranmar says.

Yeah sounds good mine are 5 years old now with 90% tread so I’ll change out. Most blowouts are due to overheating from under inflation. What are pressure are people inflating to? If carrying close to max weight should inflate to around 60 psi for LT tyres 

 Yep , run close to max psi on longer runs

 it will quote max load at a certain PSI. heavily loaded, run that. But always run a bare minimum of 40PSI cold, even lightly loaded. Obviously , will depend on the weight of BMT.

Not directed at you specifically, but it surprises me that people will spend $300-$400 a pop on their 4wd tyres and get a new set every 60-80,000kms but are happy to put the cheapest garbage they can find on their boat trailer and then ignore them for a decade.  Not to mention that with braked trailers you are actually relying on your trailer tyres to be doing their thing with braking performance.

No idea what your rig is but at 3.5t it's got to be worth $80-$100K at least.  Buy a decent set of LT tyres, and replace them every five years.  What's the annual cost work out to be?  $200?

 When l worked  in the trailer service industry,  we would tell clients your tyres are a certain age and are starting to crack. We recommend you look at replacing. Nah theres heaps of tread left.

This is the result on Mothers Day this year. Went to check tyre pressures at Service Station. One valve stem inner popped out whoosh. Thought oh well worry about that in a minute go and do other side, whoosh exactly the same thing happened

Not very happy. So much for a river cruise and lunch somewhere.

 lol   so true 

just talked a m8 into 3 new tyres - 1 spare

his was on the boat since his dad had the boat - about 15 yrs minimum

he put a new motor on -i had to take it in to get fitted and refused to take it without new tyres

saying that though -those tyres were so hard, wasnt rubber anymore- probably would have lasted another 50 years with no wear 

now the other bug bear is trailer bearings- when last have u checked them 

 Bloke at the local tyre place told me they arn't allowed to sell tyres over 6 years old, and sunlight will stuff them by hardning the rubber.

Also was told its law that the spare tyre on the back door tyre carrier , must be covered, for this reason. couldnt find much about it on the net.

will look harder later.  But yes, cheaper to spend money on tyres than a lawyer when the insurance doesnt want topay up.

4 x Brand new 8 ply 700kg load rated light truck tyres, fitted to the boat trailer, ballanced, pumped up to 45psi, $320 inc..... I've spent a lot more on pissy nights. Its peace of mind for easily the next 5 - 6 years. 

I also find a spray of cheap tyre shine every 6 months or so conditions the tyres somewhat, and prevents cracking/aging to a certain degree. 

I also agree, 50+psi for the long trips, and never under 40-45psi at all. 

Doesn’t the spray on tyre shine reduce the longevity of rubber? So I’ve heard