Changing props to go slower

Have owned and still own a couple 5.9 and 6.7 based engines, albeit none with that pump... have seen that pump on cta/8.3 lower hp motors but never fiddled with them.

Having had those blocks in a backhoe, several skiffs, multiple trucks, a generator, and one small boat.... they all would idle. In hp settings from na 110 to the upper 300s, with stanadyne pumps, p 7100 pumps, vp44s and the like. All idled, didnt mater if they were 2850 top end or 1800 top.

So my hunch is find someone who does pump work near you and get that issue sorted. The one ive done the most work with is the p7100 engines and we have one setup with an idle near yours as its on a jet and its the engagment rpm that the impeller primes best. At the lower 750 idle it would require a rev to prime the impeller.

From there you can see the effect of a couple hundred idle rpm, then you can play with a prop calculator (gerrs manual, or even the web based victoria prop calculator) to see what pitch and diameter would get your speed goals.

If it requires to much prop adjustment, that would cause some serious upper end and normal cruise... thats what trolling valves are for.

Trolling valves can either be a pita to install or really straight forward. On modern zf gears like a 220, parts are readily available and reasonably priced... at least for the mechanical valves. On my last one we did a 24v trolling valve integrated into the electronic controls, the valve and associated electric elements cost darn near as much as the gear it hooked to. On twin disc gears it can be a bit harder to source parts, but often the legacy gears have solid enough parts to re use even old parts. We did a 509 from the 70s and put a mechanical troll valve from a junkyard boat 509 from the same vintage. Still rocking and 50 years old.

If your gears are orphans.... as some of the zf models can be... that might push me towards the more exotic options.