GB 42 - Lehman 120s - Prop Pitch

[SeaPuppy]

This old sailboat driver recently purchased a beautifully restored 1979 Grand Banks Europa with 120 HP Ford Lehman engines having a reported 3700 hours per engine. Engines start and run smooth with no significant smoke and clean engine oil analysis. Max RPR at full throttle (under load) is 2200 rpm. Engines overheat above 2000 rpm. Speed of 7 knots at 1400 rpm. Bottom is clean and smooth. Props are clean with no dings. Props are marked 24X18. Boat is equipped with stabilizers which were in operation at time of test run. Mechanic says boat is over propped (sounds reasonable to me.) We are going through the cooling system with one new water pump, hot tank heat exchangers, replace engine and transmission oil coolers, etc. (to eliminate cooling system as part of the overheating problem.)
Question: Based on the above data, does it sound like we are over propped and, if so, what would you repitch the props to?
Kirby (Seapuppy)

 

[SCOTTEDAVIS]

Max rpm is quite a bit under the proper RPM for the engine, so yes they are over-propped based on that, assuming the engines are healthy and set up for and getting full fuel flow.

The speed at 1400RPM sounds about right, 16" might be just the ticket but make sure everything else is good before re-pitching the wheels.

I have a Ford/Sabre 120 and have a 22X16X4 wheel and turn 2450 out of a max of 2500RPM but the difference in transmission ratios makes it hard to compare.

Boats get heavy with stuff over time so if you are running a SD hull can she be put of a weight loss program?

 

[RT Firefly]

Greetings,
Welcome aboard.

 

[psneeld]

I have single Ford 120 with a 22X17 prop and another TFER runs his 40 with a 20X16 with 2:1 reduction.

Neither of us even remotely overheat at those mid range rpm and I never had it go warm even when I ran at 2200 back when I first bought her.

Hard to imagine a twin overheating that bad...but it is definitely running big, steep props on those 120s.

 

[Marin]

Our GB36 with two FL120s was built with three bladed props, both 24" in diameter, one at 18" pitch and one at 17" pitch. This was to compensate for the different final drive ratios of the BW Velvet Drive transmissions, one of which has an extra gear in it to counter-rotate the prop.

I suspect the props used on the GB42 with the same engines were similar.

A previous owner replaced the three bladed props with four bladed props at the same two pitches. On the advice of the prop shop we used, we had the pitch of both props reduced to 16." The prop shop said that, as a general rule, the pitch of a four-bladed prop is usually set one inch less than the pitch of the same diameter three-bladed prop.

This engine is "happiest" running in the 1500-1800 rpm band. However, it should not overheat when taken to WOT if the cooling system, raw water pump, coolant pump, thermostat, etc. are functioning correctly.

 

[Ski in NC]

If you are not going to try to run above hull speed of about 8-8.5kts, the engines will not be overloaded and you can leave props as is. Just understand that the speed range above hull speed will overload the engines, and they don't like that.