Dimmer for a 12V 50 watt halogen

For this type of application, I'd suggest using a 'switch-mode' controller circuit. This type is highly efficient - very little waste heat is generated. There are has some different 'varieties' depending on what type of switching system is used - they have some subtle pros and cons. A basic Pulse-Width Modulation (PWM) switching unit will probably suit your application.
About the only drawback of this type of unit is the generation of high frequency 'noise' / emissions - most of this will probably not cause you any concern and can be shielded or filtered out if they do cause concern. As you want to use it in a harsh environment, sealing the unit in a small, fully enclosed metal box will prevent most 'interference' from occurring and give you a good degree of water/salt ingress proofing as well.
You need not specifically look for a light dimmer to suit your application, a DC motor controller will do the same job quite satisfactorily. There are many self-build and pre-built kits available for this type of application; Google should be able to point you in the right direction.

Hope this lot helps you somewhat - please let us know how you go.

CJ

 

Unless it's run close to its maximum rating then you can cast it. The heat will still find it's way out, but mixing aluminum oxide in the resin will improve cooling.
Using a kit that has a high rating and small heatsinks might even give you the opportunity to run it without heatsinks at all. Try it out like that before casting.
Electronic speed regulators (for electric R/C models) are often rated for 10's (& even 100's) of amps while still being non-heatsinked. Multiple low-resistance transistors is the trick.
Potentiometers like the one in the picture can not be cast however, it'll have to be replaced with a sealed type.

 

Just get a rheostat at a higher power rating. A welding supply store will have these available and I have seem last many years in outdoor enviroments. I have used them on similar applications and not had any heat issues running 100W DC 12V draws. They are much simpler and cheaper than any PWM circuit I think would be.

 

why use an halogen a banch of 250kmcd can do a better job and less power then halogen by far