Catalina 30 TRBS #2889 "Northern Light"

Replacement for Universal 5411 Thermostat

The rest of the modifications.

Over the years I've had a few thermostats that lasted less than two seasons. The price tag (around $50 at the time) was quite high. In May 2019 the Catalina Direct price is $108.60!

A few years ago I decided to find a less expensive exact replacement. If that failed I was going to machine a new thermostat housing to accommodate a reasonably priced automotive thermostat. (Yes, I know "automotive" is not marine-rated, but the only thing that is marine-rated is a fish. On second through, every thermostat is marine-rated...they all sit in water.)

After some searching I landed on the web site. I found some thermostats that looked similar enough to be hopeful. I called them and the sales rep was quite helpful. Since iBoats doesn't have detailed specs for thermostats he offered to put me in touch with the manufacturer, Sierra. A call to Sierra to request a spec sheet came up empty handed because they "simply go by application." Doesn't someone, somewhere, have design documents on these thermostats??? However, I was able to get some key sizing information from Sierra so I went back to iBoats and ordered one that looked right.

The result? It's an exact mechanical fit (1.390" flange diameter) and exact temperature rating (143°F), and is used in a number of Johnson, Chrysler, and Mercruiser applications. In other words, it's a marine thermostat.

The old thermostat had two holes drilled in the flange that allow a small flow of engine cooling water to get to through the thermostat at all times. The total hole area amounted to two 0.125" diameter holes so I simply drilled two 1/8" holes in the new thermostat. I believe these bypass holes accomplish two things:

  1. They provide some water into the exhaust flow even when the thermostat is closed. The stock plumbing for this engine on the C30 has the exhaust water from the engine fed into the muffler. A totally closed thermostat during warm-up means no water to cool the muffler.
  2. They prevent totally blocking pumped water flow (while the engine is cold) which would result in back pressure at the water pump. Back pressure in the water pump tends to push water down the pump shaft, through the oil seal, and out the weep hole in the pump. That's too close to getting water into the engine oil for my comfort.

Observation showed that the two 1/8" holes may be too large because at idle the engine never really heated up. I closed one of the holes. Subsequent observations with an IR thermometer show that both thermostat housing and exhaust manifold settle at something like 144°F regardless of throttle setting, so the thermostat is clearly regulating to its temperature.

It's part number 18-3557, $30.49 (May 2019), at, 800-914-1123.(The image of that thermostat on their website appears to be a generic photo.)

By the way, it seems that sediment accumulates in the lower section of the thermostat housing. I remove the bottom plug and drain the housing annually.