Prop pitch...what a difference an inch makes!

So I hit a rock and replaced my prop. I wanted a spare so I contacted Waterline Boats and Scott Helker told me that they had settled on 24X19 for the latest builds. Took the old prop off and it was stamped 24X18 but also had XXed out 16 and 17. Cupping was also XXed out. Looks like it was rebuilt/repitched a couple of times.

Previously I was calculating about 6.5knts at 1490 with 2.5 g/hr. Now I'm getting 6.5/1350/2.5 and 7.0/1470/2.5!!!

Can this increase in performance and efficiency be due to an inch of prop pitch?? Or was the old prop a total Frankenstien??

Ooops.

That second series of numbers is off. Should be 6.6knts @1350rpm with 2.0gal/hr burn.

It looks like your original prop was repitched twice from 16 to 18 inches, right? Repitched props are always slightly less efficient than a prop built for the new pitch. Also your new prop is pitched even more, letting the engine operate at a slightly better fuel burn.


Both of these factors are probably contributing to your improved performance.


BTW you could have edited your first post and it would have been much clearer.



David

My props were 25x18 and ran 7 mph @ 1900. Repitched to 25x22 and now do 7.5 @ 1500. Big difference in mpg.

"My props were 25x18 and ran 7 mph @ 1900. Repitched to 25x22 and now do 7.5 @ 1500. Big difference in mpg."

There are folks that are terrified of matching the engine and prop to the boats cruising requirements.

Glad you have the usual good results.

With a more aggressive pitch on your props the boat speed will increase. But at higher RPM you are taking the life out of the engine. Also you will probably not be able to turn up to the specified WOT.

It is also possible that the prop was properly adjusted , with the blades properly matched. There is a lot more to prop efficiency than just pitch and diameter.

"Also you will probably not be able to turn up to the specified WOT."

The engines in most cruising boats will not stand 100% power for very long.

AS long as the engine is with in its designed operating range , no harm or extra wear would be expected.

Since many engine mfg. relate engine life to fuel consumed , lowering the engines GPH use is excellent.

"Also you will probably not be able to turn up to the specified WOT."

That is the interesting question. Have you been able to run the boat at manufacturer's specified WOT rpm? If you can't get there you are likely overpropped. See many TF threads and many many posts re pros and cons of being overpropped.

If an engine that is over propped were to fail during its warranty period, it is possible that the engine manufacturer would deny the warranty claim. So be aware of the pitfalls of over propping just to save a fraction of a gph in fuel used.

I believe that if an engine rated at 3,000 rpm is over propped to only reach 2,600 rpm then you are ok if you never run it above 2,000 rpm. But if you run it at 2,400 rpm you are overloading it and it will wear faster and fail sooner.

David

I wish I would have purchased the new prop when I bought the boat. 2000 hours driving the boat with .5 gal/hr savings and diesel at $4 during that time would have paid for two props!!��

"What a difference an inch makes!"


Oh man! There is such a good joke waiting there... But then, this is a family forum.

A look at most HP/Fuel use/theory prop power requirements that engine converters post will show that about 10% reduction in RPM will solve most any over load issue.

For almost every engine 10% or 300 RPM pullback from WOT RPM is enough.

IF engine converters or mfg were more honest they would publish the engines fuel map , so the best choice of long range cruise , and fast cruse could be decided and the tranny and prop selected before the boat is assembled.

Obtaining WOT of a usually oversized engine in a displacement boat may be fine for the engine seller , but is a useless goal for the boats owner/operator.

Thanks for all the informative posts! I hope I never have to run at WOT as my boat is a single screw trawler. After 1500 rpm I see a very small increase in speed for a LARGE increase in fuel consumption and this fat old girl will never plane.
Read an article by Tony Athens who is a bit of a Cummins mechanic on “lifetime” or “10,000 hour” engines. Along with some other recommendations based on 24/7 commercial boats, was to run my engine at 1500 rpm maybe up to 1800rpm. WOT on my boat is in the 2100 range.

Practical prop vs. unrealistic WOT

In replacing the 4-bladed prop on my 32 Apollo, single screw with a Perkins Turbo 6-354, I worked through prop calculations with several mechanics and The Prop Shop in Mulkiteo, Wa. Similar to several other posts here, the prospect of my semi-displacement hull achieving a top WOT were close to none as after 14-15 knots the boat is trying to lift it's own weight out of the way. At its practical speed of 8-10 knots it handles 8 knots at 1100 rpm, with a clean hull and a Michigan Wheel four blade 22 X24 pitch, and fuel consumption of 2 US gallons/hr. Dropped into gear at idle it makes 4 knots, without a drop in engine rpm. The loading on the engine to achieve a 18 knot WOT would require far less prop pitch and only lead to higher fuel consumption at a speed not appropriate to the boat hull and engine/driveline loading.

Just last night I was reading some articles on "Passage Maker" regarding diesel engine life and maintenance. Maybe do a search there regarding calculating engine life for more information. It's not just about getting best MPG figures. Over propping has it's downside including dirty engines at best and lugging causing damage that could be worse.

I have only owned and operated one diesel and its a smaller Yanmar operating at higher RPM than the larger Trawler engines. As I am looking to sell my sailboat and retire to the Trawler life I am interested in this discussion.

Keep it going