radio interference

hi we have just fitted a car radio and a fishfinder to our boat. but when we turn the fishfinder on it will interfere with the radio. is there anyway that we can have the two working togeher.

Separate batteries are often the only answer, but it depends on the type of 'interference'. A commonly reported interference (interference is usually assumed to mean stray radio-frequency signals) is actually nothing of the sort - it's caused by something like a sounder, which draws it's power in bursts, pulling down the battery supply temporarily as it fires each burst of sound. (Ever seen the headlights on a car dip when somebody has too big a stereo fitted? - same effect. They often have a seperate battery or a sodding huge capacitor fitted across the power supply lines to take the surge out of the lines) Try a bigger battery - ie one with a higher Amp/Hour rating to see if that eliminates the problem. If not, try separate batteries.

There are a number of other solutions, but it'll depend if the problem really is related to an 'unstable' power supply or genuine radio interference. Does your radio have a cd or cassette built in? Is this affected the same way or is it only radio reception that's affected? Post up a better description of what's happening and I'll see if I can steer you right....

thanks for the halp sandman

Im going to try a tractor battery in it first and maybe two seperate ones then. its a tape player thats in it but i havn't tried it yet. i will post back next week with more info and let you know how i get on. thanks again.

nice one Sandman, bigger battery done the trick :D only thing now is when the boat is going slow the fishfinder is fine but when we open it up a bit its not able to read the depth! :( any ideas?

nice one Sandman, bigger battery done the trick :D only thing now is when the boat is going slow the fishfinder is fine but when we open it up a bit its not able to read the depth! :( any ideas?

Your problem with the sounder at speed is probably caused by cavitation - air bubbles passing under the transducer. If you can get it a bit lower down on the hull, it should solve the problem. I'm making a guess here that you have a transom-mounted transducer, so it needs to be clear of the propellor turbulence as well. If you can't get it to work right, the only solution then would be a through-hull transducer. If you post up a pic or even a good description of where the transducer lies in relation to the waterline and propellor, we might be able to advise further.....

I've done this a few times on my boats but i think this time i have finally got it right. Your transducer should be as close to the center of the hull as possible while offset from the propellor by 3 inches. this means your prop and transducer should have a 3 inch gap of hull space between them. On my last boat i had horrible problems with this but now it works very well most of the time. It is important not to have it too close to the prop as it can cause cavitation as stated above and apparently this can cause damage to the prop shaft over time...

Just coming in on this, I have a new finder ordered as old one needs an unmerciful whack to get it to work and when it does theres a lovely half inch clear gap running down the middle of the screen :? . Now, the old transducer is situated on the extreme rhs of the transom, relatively low down. I had fierce trouble with the thing picking up "fish" when travelling at any speed at all. Screen would be full of stuff but on stopping, of course nothing was showing. Tried adjusting sensitivity levels etc to no avail. I read with interest Lukes comments on positioning. I would have previously thought that the further away from the prop the better but advice is to leave a gap of 3 inches from prop. Just to clarify this as I dont want to be drilling numerous holes to get it right! How low would you set the transducer on the transom? As low as is possible or a bit higher up or what? And if im reading it right, have it offset about 3 inches off the line of the prop? Thanks in advance lads.

Your problem with the sounder at speed is probably caused by cavitation

ya that sounds like it. it is a transom mount transducer. the instructions said to keep it 15in offset from the prop and using the template the transducer would be about 3mm below the hull. max we kept it offset was about 12in and in fear of breaking it when the boat is beached we rose it to about 2-3mm above the hull. looks like we cant do much till it comes in. were going to try dole fins on it anyway maybe this might solve, if not id say that will have to lower it.

thanks for the help :)