Propeller size for 1976 Grand Banks Alaskan

[Gail Duym]

Hi,
Inquiring if anyone knows the correct/fitted wheel (propeller) size : diameter and pitch of a 1976 American Marine 53' Grand Banks Alaskan powered with twin John Deere 640D 6 cylinder/120 h.p. diesel engines?
The current wheels were changed by the previous owner to oversized wheels, thus not allowing the engines to turn up to the suggested 1900 rpm's. Current max rpm's now obtained 1550 - 1600.
Hard on engines unable to get to their power band. Any help would be greatfully appreciated to get the current condition back to "square one".

We are in the process of sanding,filling & painting the bottom of this old girl, and would appreciate any information!

Thanks,
Kevin and Gail

 

[Cigatoo]

Necessary propeller pitch changes as the boat changes such as added weight. Best bet is to get a propeller shop involved. The “size” of the propeller may be fine. Maybe pitch needs adjustment. Before doing anything you first need to confirm that the engine(s) can reach rated wide open throttle while not under load. IE in neutral. Previous owner may have governed things down. If you don’t follow the necessary steps you may be wasting time / money. Take the time and get a professional involved.

 

[Comodave]

Agree on professional help. Get the best prop shop in your area to help you.

 

[koliver]

Your boat is deliberately over-propped.
Does it reach your proposed cruising speed? Is it ever going to plane? Certainly not with those engines.
Will your engines get better fuel economy at the lower rpm?
If you were to reduce the pitch, thus to increase the rpm at cruising speed, would the extra remaining rpm, if any, enable you to do anything you can't already do?
Do your engines run at a temperature that allows the thermostats to open?
Youo say this is hard on those engines. In what way? In whose opinion?

I will suggest that your former owner wished to save engine wear and fuel, by increasing the pitch on the propellers, thus reducing the rpm at cruise. This has the effect of reducing WOT rpm too, but if you never want to run at WOT, nothing is lost.

I swapped out a pair of 150 hp engines for a pair of 200s, 21 years ago. Within the first year, I added pitch, bringing my cruising rpm down from 2700 to 2000. Since then I have added roughly 2500 hours usage, exactly none of it at WOT. According to my mechanic, who recommended the added pitch, I am in no danger of hurting my engine and can continue to expect this pair to outlast me. Oh, my fuel consumption dropped measurably with the reduction in rpm at cruise.

 

[utclmjmper]

Check on those engines a little deeper and you will find that 1550 to 1600 is right in the best torque band and and near the best horsepower band.. Couldn't be better for fuel economy,, If you need the extra RPMs for some other reason,, then be prepared to pay more for it.. ( Just because the engines have reached there max RPM with these wheels,, doesn't mean you should advance the throttles any farther than necessary to maintain that RPM,, that would just be dollars going out the tailpipes.>>>Dan