210 Cummins RPM

And the ideal cruising RPM on a 88 GB 32 with a very well maintained 210 Cummins w/ 2650 hrs is?

Wot neutral= 2800 WOT in boat 2600 fast cruise 2400, eco cruise 1850-2200.

Least if you get those numbers you are propped right. That is from memory for A 5.9 BT 250...not much difference. Drive it, feel it and then look at the tac, the boat will tell you.

Welcome aboard!
Depends on what "ideal" is your priority. Fastest cruise speed, most economical fuel use, or maybe a little of both?
A couple things you will need is an accurate tachometer and way to measure fuel use.
A digital photo tach is less than $50 new and should be a standard tool for anyone with an older, non-electronic diesel. Make a simple table and record photo tach verses dashboard tach readings. The dash tach is usually driven by pulses from the alternator. 10 percent error is not uncommon.
Fuel use is harder to determine. There are plenty of threads on figuring that out.

I cruise between 1400-1600. This gives me 7.5knots and uses around 1.5 gal/hr.

My boat is likely somewhat lighter at 20k lbs than the GB

Thanks, always a lot of different answers with extraneous info.....New boat for me and I’ve been at 1900 rpm

My last boat, a Marine Trader 38 Double Cab with 210 Cummins @ 1650 did 7.3 Knots at 2.3 GPH. That was averaged over 6000 nm.

For the easiest long range cruising a speed of the Sq rt of the LWL works well.

This speed is in Knots

Multiply by .9 for a heavy fat boat or 1.15 for a light sleek boat.

Whatever RPM is required to maintain this easy cruise speed will be comfortable over the long run and about as efficient as the boat can be.