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I had exactly the same line of thinking when I put a Cummins BTA in my last boat. I wanted access to the full range of throttle, and deliberately underpropped it a bit. I think the decision should be dictated by the use case, including worst-case scenarios. There is no universal right or...

I've used three different props differing in pitch in my current boat over 15k miles, and have ECM supplied data like fuel consumption, % load, etc. As far as I can tell there is no difference between them in terms of fuel consumption. Fuel consumption/speed has not changed even though fuel/rpm...

Just for fun, and relating back to the OP's question, what happens if you add pitch and keep the same rpm? Max rpm is now 2200. You'd have to refer to the engine curve to get maximum power at 2200. Let's conservatively say 100 hp. Using Gerr's formula we can say that at 1750 the engine is at...

Show me a diesel - any diesel - that is overloaded at 1/3 rated hp and 1/2 rated rpm. All I need to see is the manufacturers power curve. I don't think you'll find one. Similarly with cooling. If an engine is rated to shed X amount of heat at full rpm are you suggesting that at 1/2 rpm it...

I don't understand why you keep talking about planing boats. This is a trawler forum. With non-planing boats the rules like Dave Gerr's apply reliabltly. With planing designs those rules go out the window. The concerns are truly different between the two types of boat.

From Dave Gerr, https://www.gerrmarine.com/Articles/enginepowercurves.pdf NOTE: If you don’t have any engine curves available, you can estimate that the propeller power at various RPMs as fol- lows: 90% of max RPM = about 68% of max rated engine power 80% of max RPM = about 48% of max rated...

Good advice. I'd add that finding a boat that's actually being used the way you expect to use it goes a long way. An awful lot of stuff that you might not consider is related to equipment rather than boat type. Robust electrical and fuel systems, dinghy, ventilation, storage, spare parts...

Yep. And if you're living aboard storage becomes critical. I think in general these types of boats are not designed for long-term liveaboard, particularly off the dock. A big lazarette is really nice when you want to bring your scuba gear. I can see the case for a sport fisherman. You're...

I've got 135 hp. Not many out there with less. Anyone contemplating a maximum cruise at less than 3 gph per engine is in the same boat ?

Maybe you should take another inch off!

I think you've just made my point. This entire discussion is in the context of cruising trawlers at less than hull speed. My case is hardly unique.

Yes, I've had extensive experience with my boats being both under and over propped. And being the type of person I am I've measured results carefully given my tools and compared them to the theoretical results. I'm fully aware of what manufacturers recommend and why those recommendations are...

It is in a trawler. If for some reason you've got got an engine rated for planing applications you can cut from 80% to 70% as your upper threshold. Whatever. The story is still the same. Define a safe upper rpm and everything below it is safe.

I'll try to explain why the distinction matters. On a planing boat the prop demand is highly dynamic, and is not progressive through the operating range. The engines are often working their hardest getting the boat on to a plane. On a non-planing hull the prop load is progressive...

So you're 700 rpm over propped?

Yes, but the consequences of overpropping on high powered planing boats can be dire in ways that don't apply to non-planing boats. That's why I'm trying to make the distinction. It's far less critical in a trawler. Agree 100%. I'd pay a lot of attention to getting it right if it were my...

I generally enjoy slow travel up stream. Get in close to the bank with an eye on the depth. It's more interesting than going downstream IMO. What area are you asking about?

That's great advice for sport fisherman. But in the trawler world it's malarkey. I invite you to take a prop curve for a displacement hull - any boat, any diesel engine, any prop - and show me where the engine is overloaded at less than 200 rpm under maximum achievable rpm. Load demand will...

One more time: this only applies to planing boats. It is completely irrelevant to the OP's concerns and any discussions on propping full displacement boats cruising at 1.2SL. I thought we'd resolved this earlier...

Again, this only applies to planing boats. In a displacement hull with fixed prop the mid-rpm torque capabilities of a diesel don't matter, because the full available power in that range is never needed on a continuous basis.

I think that any talk of accelerating through a range is specific to planing boats, and so not applicable to discussion in displacement boats like the OPs.

So what's your motivation for changing anything? You can certainly depitch by a couple of hundred rpm, but that means your new boat speed at 1800 is whatever you get now at 1600. If you want the same speed 2000 rpm is your new cruise. Whether that's good or bad for the engine health or fuel...

If that's the speed you always run I'd agree. No need to pull the props just to repitch.

I chatted with the broker and walked around that boat in the yard two years ago, and it's been for sale since. Definitely stout, but basically a home build on a work boat hull.

Yep. There are several good facilities in Iuka. My boat is in TN now and i was chatting recently with someone who had moved home marinas and his rates went up because it's in MS instead of TN. It's all relative I guess... not much danger from hurricanes but tornados can cause massive local...