Not an easy place to find!
Scott: I have a 1985 H'31. We mounted ours under the v-berth as close as possible forward of the bulkhead. This location also gives you some time to react if you are in shallow water. You just need to keep in mind how much cable you have. By the time you route the cable under the floor and then to where ever your control box needs to go you could end up with a mighty long run. The only thing I found bad about this location is that nothing loose should be in the compartment that could damage the transducer. You might think about glassing in a box around to to protect it and then you could put a cover over it. Do you know what brand you are going to use?
