Prop efficiency v/s prop diameter

I think that propeller speed v/s boat speed affects the idle boat speed. Lets say you have a direct drive gear and another identical boat has 10-1 reduction. I think the 10-1 reduction boat's idle speed will be considerably slower than the direct drive. This is admittedly a very extreme example but sometimes extremes will make it obvious what small differences fail to show.

Could it be that you have a 1.5-1 gear? My W30 has a 2.57-1 ratio and has an acceptably slow idle boat speed. And I idle at 900. I have the idle set a tad high so I can get underway soon w/o any danger of my engine quitting while shifting gears. The engine idles nicely at 700 when warm but I could back out of my slip and possibly into another boat if I set it at 700 because the engine may quit while shifting gears.

koliver wrote;

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Eric:

I don't get what you are driving at. I think boat speed at idle has nothing whatsoever to do with gearing.

I have always related idle speed to full speed. For Example, if I have a boat with a full speed of 9 knots at WOT of 3600 rpm, and you have a boat with a full speed of 25 knots at WOT of 3600 rpm, I will expect your idle speed at 750rpm to exceed my idle speed at 750 rpm by the same ratio, 25:9. So if my idle speed is 3.5 knots, I will expect yours to be 9.7 knots.
This explains the guys in the fast sport fishermen who have to go in and out of gear to get below the no wake speed when coming through the marina.

FF wrote;

"The thrust required to move a boat at a set speed does not change because of prop diameter.

Producing the required thrust at slow (under 10K) is always more efficient with the largest diameter that will fit under the boat that it can be geared for..

This assumes the number of blades and blade width is also optimized.

As an example a ride on a tug (HUGE PROP and frequently 6-1 gearing) will show the engine is slowed very little to cut speed while free running .

Going from 10K to 5K does not require half RPM, just far less thrust.

Run a bunch of props thry Skenes to see how Diameter is king of thrust efficiency."


As I think I've said before I'm quite certain that FAPP the bigger prop/lower gear combination will always produce greater thrust at slow speeds and full load. The higher speed smaller prop will always have less thrust ..... at WOT at rated rpm. But I think that at some point of reduced rpm they will become equal and w a further reduction in speed (prop speed) the smaller faster prop will have more thrust and present more load to the engine. The right propeller thrust curves would reject or embrace this theory.

I'm not sure of the statement that boat speed at idle has nothing to do with gearing. Boat speed has to do with the thrust being generated by the propeller(s). The slower the prop turns the less thrust is generated, the slower the boat goes.

The transmissions in our boat have two different output ratios because of the extra counter-rotation gear in the starboard transmission so I'll pick one, which is 2.1:1. So the prop is turning roughly half the rpm of the engine. So at our idle speed of 600 pm the prop is turning at about 300 rpm.

If our boat had a final drive ratio of 1.5:1 at 600 rpm the prop would be turning at about 450 rpm if I used my calculator correctly. So, faster-turning prop, more thrust, higher idle speed through the water.

Altering the pitch of the prop blades does exactly the same thing in terms of the thrust developed. Reduce the pitch, reduce the thrust for any given rpm including idle, reduce the boat speed.

And I suppose that installing a prop of greater diameter would, if the number of blades and the pitch remained the same, increase the thrust, too, and thus the boat's idle speed through the water.

Since it's usually not practical to change the transmission or its gearing, the easiest way to change the thrust is to re-pitch the prop. That's why we had our two four bladed props pitched down a few years ago, one by one inch and the other by two inches.

We did not do this to reduce our idle speed through the water-- the previous idle speed had not posed any problems. But one effect of our pitch-down was to reduce our boat speed at idle by a little bit.

This all seems pretty basic and logical to me, and while it's not something a boat owner can change, at least not easily, the transmission gearing most definitely has an effect on idle speed.

Basically what I hear from others is that the thrust curve is the same for all diameters and speeds of propellers.

I think they are not the same. The thrust curve for lower speed larger dia props should be more curved ... less flat. The curve as it climbs to max should be steeper or more vertical as it approaches maximum thrust and that thrust will be greater than a smaller and faster prop. But the thrust for the big dia. slow speed prop may be greater than the faster smaller prop at any speed ... not just at the top end. But I personally think not.

I think the thrust curve for different diameter and speed props is different. Does someone actually know? In the meantime what are some of your opinions? Opinions are usually more interesting than facts.

Does someone actually know?

Skenes has published the prop tables since about 1960.

Gentlemen
The bigger the prop diameter, the more efficient is the boat at idle speeds. This is because the prop "digs" more water no matter the speed at which it is turning. To have a big prop, we need to have a high gear ratio.
My boat for example is the second boat of the same design. The first boat had a 2.5:1 ratio in the gear, turning a 26” x 16” prop. Mine is 3.96:1 and it will turn a 34 x 26 prop. Both have the same engine.

The higher the gear ratio, the bigger and slower will be the prop. Yet more efficient in low/idle speeds.

Take the professional tug boats for example. They need to have big props to have thrust/efficiency at idle speeds. That’s why they have very high gear ratios.

On recreation Trawlers, is ideally good to have high gear ratios with bigger props to be more efficient @ low speed, which translates in fuel economy. In a displacement hull, there will be differences in RPM to achieve a certain 1.0 x Square root of LWL. If we have two boats exactely the same with the same displacement one with a 2.5:1 gear and the other 4.0:1 gear, with properly dimensioned props, the second will need less RPM to achieve the 1.0 x Square root of LWL.

This is a proven fact

Portuguese

"This is a proven fact"

At full load.

Most commercial tugs and trawler have slow turning engine, low HP and large diameter props. Many of the 60 ft older trawlers are set up similar to the Eagle. The Eagle DD 671 is 165 hp with a 3 to 1 reduction, 38” diameter prop, 26 pitch and 4 blades, max speed is 10 kn at 1800 rpm. However, the idle speed is 500 rpm, 3 kn but we push/churn a lot of water. I think the diameter and pitch is important to push a the most water at low HP/rpm. So a lot depends also depends on the engine rpm and HP.

However, for a high rpm and HP engine pitch is biggest factor. For the run about, I have several props all the same diameter but different pitch depend on the use of the boat. For water sking, where quick come out of the hole with a 200+ lb slalom skier, we use a 17 pitched prop with a cup, so the engine does not bog down, 2500 rpm with in seconds, but the cup grips the water and top speed 35 mph at 3500 rpm vs, a 21 pitch prop, little cup, which is a dog out of the hole and bogs down the engine, but top speed is 40+ mph at 3500 rpm. So my experience with a high hp and rpm engine the pitch is the variable not the diameter.

If you want to max the performance than be prepared to spend some money as it’s not cheap and sort of trial and error. I have no idea what WOT is on both, NOR do I care. You should be looking at setting up for your intended use and/or max fuel economy, not necessarily WOT?

Phil Fill said:

Most commercial tugs and trawler have slow turning engine, low HP and large diameter props. Many of the 60 ft older trawlers are set up similar to the Eagle. The Eagle DD 671 is 165 hp with a 3 to 1 reduction, 38” diameter prop, 26 pitch and 4 blades, max speed is 10 kn at 1800 rpm. However, the idle speed is 500 rpm, 3 kn but we push/churn a lot of water. I think the diameter and pitch is important to push a the most water at low HP/rpm. So a lot depends also depends on the engine rpm and HP....

Click to expand...

Phil: How do you define low hp? Don't most tugs have a much larger power to tonnage ratio than recreational or commercial vessels?

For what it's worth, I posted the results of some crude diameter versus thrust calculations in the "Fuel mixture/load" thread. By the way, Gerr's "bollard pull" formula is T = 11.87*(SHP*D)^.67 Select a horsepower as a constant and change Diameter (inches) to compare Thrust output. Not exactly an efficiency number...but perhaps useful for discussion purposes.

Portuguese, All,

I don't have a problem w the fact/concept that bigger props are more efficient but it actually, if taken far enough can be false. There comes a time that the increased thrust form a larger prop consumes more power from friction of the blades moving through the water that it gains from it's size. There also will come a time (of size increasing) whereas all the power of the engine will be required to turn the prop without any pitch at all.

Read the OP or ...
As you descend a percentage in rpm from max you will loose a percentage of thrust. I don't think this percentage is fixed for all props. I'm saying that if you descend 50% or your rpm (from max) lets say w a given prop you'll have 27% of your max thrust w a big prop. With a smaller prop that develops less thrust at max as you descend to 50% of it's max rpm you may not have the same % of thrust that the larger prop had. You MAY have 29% ... 2% more. But that may or may not be more than w the big prop. But at some point it may be more. So running a low gear ratio and big prop WILL be w higher thrust at full load but MAY not be at cruising load. My Willard has an 18" dia prop but there is room for a much bigger prop but I've never seen a 30' Willard w a bigger prop. Probably wouldn't have cost them (Willard) much more at all to max out the available space and install the biggest prop that would fit. I'm think'in they MAY know that it's just as efficient to run a smaller prop at low loads. I put a 19" prop on my Willy but it was 1000rpm down from max (over propped) from too much blade area ... it was a 5 blade. See pic. I'm thinking there may be a good reason they didn't use a larger prop.