It is my understanding that there are a number of ways you can affect the distance a radio is able to transmit. Apologize if I don't have the correct terminology but you can do it by increasing the impedance in the wire between the radio and antenna, 16' of coaxial vs, 500'. Anyone have a number/idea of what length would drop the broadcast range down to a tenth of a mile at 25 W broadcast?
Or can this also be acomplished with attenuator as orginally posted and if so what size?
Increasing impedance isn't the term I think you are looking for. What it sounds like is that you want to induce so much loss in the cable that there isn't enough signal to be received anywhere other than within a few feet.
There are some things you have control over, and some you do not.
Yes, longer lengths of coaxial cable will increase signal loss. The amount of loss depends on:
Exact cable type.
Cable length
Frequency (156.800 in your case)
You could use enough cable to drop the signal down low enough that it wouldn't radiate worth a crap. You could technically plug right into the back of another radio and have a nearly closed loop.
600-700 feet of cheap RG-58 coax would result in enough attenuation that a 25 watt radio would put out somewhere around 0.009 watts at the far end. That will probably be safe for the receiving radio, but it's not a great idea.
There are companies that will sell 1000 foot reels of RG-58, and it'll run you a few hundred dollars. Then you'll need to install connectors, and you'll need to do it right so there's no leakage.
But there's things you do not have control over, like the sensitivity of the receiving set up at the radio site. While you may be able to attenuate the signal enough to make this work, any leakage from the cable (there will be some) would make it possible to be received.
You also don't have control over other boaters that may be close enough to hear all this, and may call it in to the Coast Guard.
Other option is to buy an RF attenuator that would let you connect the radios together, but it's going to cost you about as much as all that coax and it is still not without risk.
That's about as far as I'm going to go with this. Maybe someone else will feel like linking you to specific products.
Personally, I think what you are trying to do here is well meaning, but you are ignoring several knowledgeable people that are trying to tell you this is an exceptionally bad idea and it's very irresponsible.
What you are doing runs the risk of triggering a system that will deploy a lot of people and assets to respond. None of those people do that job without risk. The amount of money wasted on fuel and manpower comes from somewhere. So, essentially you would be putting first responders lives at risk and possibly wasting taxpayer dollars rather than just show a You Tube video, or ask the USCG if they have a better way of demonstrating how this works.
