Vhf is line of sight.. as you head over horizon the top of you aerial wont see the other aerial .. weather conditions play a small part also.. this is what we were told when doing VHF radio course. . 

 Is the squelch control adjusted correctly?  

 On the boat and have it vertical not sloped back .i have mine mounted ontop of my hardtop and get about 50km range on mine

Cooments as above. But did you specifically call the sea rescues or asked for a radio check from anyone.
That will give you an idea also. Always try nearest rescue and if that fails ask for a general radio check from anyone.

Squelch is tuned till the noise is just gone. Antenna is on the hardtop too and vertical. The only thing that I can think maybe affecting it is the radio is a Navman & the antenna is Gme and the thread pattern is different and doesn't pull in super tight.   

 "Squelch is tuned till the noise is just gone".

Try adjusting the squelch at the distance where it did not work as the incoming signal level will be a lot less than close in and the squelch level will overide the incoming signal,

Maybe ask the local sea rescue if they can do a power level/coax cable/antenna check or recommend someone with the proper test equipment. 

 Have you checked the power setting on the radio?  Bit of difference loading up the aerial with 5W vs 25W.  For me, 15nm is about as good as i get.  Concerns me a bit given i'm often out 18-20nm. 

 "and the thread pattern is different and doesn't pull in super tight"

 You need to resolve this problem. The biggest issue with radio comms is poor connections so this will not be helping. Do you mean the antenna doesn't screw onto the base well or the connector in the back of the radio is loose? Either way a base and lead is about $50ish so getting the right one will help. 

 Squelch was tuned out at sea.  

Radio is set on hi.

Pretty sure the connection at the back of the radio is the problem by the sounds of it. I still have the old connection which screws in tight can I join them or is that opening another box of worms?   

. 

 Kirky i was told by chivers not to join a few years ago when i asked the same question. 

 Can easily get Gero VMR from Easter Group but they have an antennae high up.
Discovered last year when I was having trouble that the USB iphone charger cut my range to about 1/4 when plugged into a 12V socket

Hi Kirky

I only get 7nm at sea talking boat to boat but can get sea rescue only just, at 23nm.  Would like to get better.  Currently running a 1.8 m aerial ontop of the rocket launcher.  I might have to go to the 2.1m aerial next but that is alot of aerial. 

I know it sounds a bit of a silly question to ask, but what channel are you using to call on.

If you call on VHF channel 73 instead of channel 16 (ch 16 is emergency and call channel) channel 73 is a repeater station and is located up in the hills and there fore at a greater height that the rescue groups base mounted antenna.

One of the things that gives you longer range is height as has been said above VHF is line of sight, although at times you can get a skip off the atmosphere that will allow you to communicate with other stations at great distances.

Below is an example of how to work out mathematically just how far you should be able to communicate with VHF with all things being in optimum operating level.

The range in nautical miles - that is, how far your radio can 'see' to the horizon is equal to 1.23 times the square root of the height of your antenna in feet.

So if, for instance your antenna height is 60 feet, the distance it can transmit before it runs into the horizon is ~ 9.5 nautical miles. (for you math challenged - or those not having a calculator handy, the square root of 60 = 7.74. Multiply that times 1.23 to come up with approximately 9.5.)

Now, keep in mind, the vessel to which you are transmitting also has his antenna above the water. So, let's assume, for example, his antenna is 18 feet above the water. He can transmit about 5.2 miles. You can add your range to his to come up with 14.7 nautical miles.

A land based station is usually much higher. An antenna on a hill on a tower might be very high, but for argument's sake, let's say it's 400 feet. So theoretically, he can transmit 24.6 miles. Add that to your 9.5 miles and you have about 34.1 nautical miles.

I hope this helps, as I have said with all the connections and antenna being in optimum (good) condition.

Sorry about leaving that bit out.

Australian spectrum Management Authority ASMA site lists the marine repeater as channel 83, I think this is a typo mistake as I always use ch 73.

Anyway the height listed for Turner Hill which is where the repeater is located is given as 365 metres above sea level.
So with that figure and the height of your antenna above sea level you can work out the theoretical distance of VHF.