Dave, except for wintertime runs, you can safely operate without the preheaters. I used to keep a small heater in my engine room so the ambient temp remained about 60 degrees.
Otherwise, a wolverine magnetic heater works well to keep the engine warm.
Peter- Marine Insurance Guru at Novamar Insurance Group (206-350-5051) & tuna fishing addict!
1989 52' PT Overseas yachtfisher