Impeller size

[LinekinBayCD]

I repowered last winter going from the original 1986 Westerbeke 46 (with a bad cylinder) to a new Westerbeke 55 in my Cape Dory MS 300. I'm kind of surprised that the impeller on the 55 is only a small fraction of the size of the impelller from the old 46, maybe only 20%-25%. The old impleller from the 46 is larger in both diameter and width.

Do the water pumps on more recent engines operate at higher RPM's than older models? Does that account for the reduction in size? I was thinking that a larger engine would need at least as much cooling as a smaller older model.

[r.furborough]

There are a number of factors involved, not just relating to the impeller size or pump volume. It also includes the heat exchanger and its efficiency....

[klem]

As said above, there are several factors at play.

The first thing to look at is the amount of energy going into heating the block. Modern engines tend to do a better job of converting the engine in the fuel into useful work rather than heat in the block and exhaust.

What determines the amount of energy taken out by the cooling water is the temperature increase in the water and the flow rate. If you have a better heat exchanger, it will increase the temp more requires less flow. In addition, the design of the cooling system could be getting much more efficient flow requiring a smaller impeller.

There is also the possibility that the previous impeller was much larger than necessary. On engines with thermostats, you can run a very large cooling circuit which is bypassed by the thermostat most of the time.

As long as it isn't running hot, I wouldn't worry about it.