Block heaters?

[Fishbucket]

Looking at stick on block heaters that stick to the oil pan.
What wattage (for each motor) is sufficient for yanmar 4cyls?
Also would the fact that the oil pan is 1/4" thick aluminum defeat the purpose, sort of acting as a heat sink?

 

[tunafishhkg]

The Wolverine pan heater works great and I think 250W is the smallest for engines and all u would need . Alum would have no affect and it heats the oil and radiates upward to keep condensation off engine during the winter which is why its on 24/7. I have used mine on my volvo 74c 450hp 24/7 10 months at a time, going on 12yrs now and still going as I type.