Prop Selection for Production Boat

[DivineFocus]

No, that would be the solution for cavitation on propeller blades that are excessively loaded. Ventilation is atmospheric air that is sucked into the front (low pressure) side of the blades. This happens when the blades are close to the surface. Outboards have an anti-ventilation plate over the propeller. On inboards the problem is when the propeller is too close to the stern. The bottom of the hull serves the same purpose as the plate on an outboard.

I understand ventilation vs cavitation phenomenon. But assuming hull modification is impractical the prop is the only lever to try to mitigate the problem.

 

[gonzo]

I understand ventilation vs cavitation phenomenon. But assuming hull modification is impractical the prop is the only lever to try to mitigate the problem.

If the propeller is ventilating, a change in pitch or cupping is not going to fix the problem. The shape of the blades, with a straight trailing edge, looks like a surface piercing propeller. Can you get some photos of the boat running?
That would make an analysis easier.

 

[philSweet]

Your performance notes of the Yanmar are confusing. You said 3200 max rpm, but then you said you hit 3500 rpm and could have gone higher.

With the old motor, was there a dramatic onset of ventilation at some point? did the rpms spike without any additional thrust? What about with the new setup? Can you post an RPM vs speed trace with your new setup. Include throttle position and fuel flow rates if you have a display for them.

 

[DivineFocus]

If the propeller is ventilating, a change in pitch or cupping is not going to fix the problem. The shape of the blades, with a straight trailing edge, looks like a surface piercing propeller. Can you get some photos of the boat running?
That would make an analysis easier.

The blade shape of the wheel in the photos is typical of ACME propellers. They all have virtually the same blade shape. Perhaps because the started out specializing in props for wakeboard/ski boats. They are CNC machined so the cross sections vary with variable pitch along the radius of the blade. Their stated pitch is what they calculate as the "average". I'll try to get photos of the boat running this coming week. I might be able to convince my bride to stand on a rock and shoot some photos as I cruise by. What sort of photos. Showing running attitude, wake, or... ? At cruise speed, WOT?

 

[DivineFocus]

Your performance notes of the Yanmar are confusing. You said 3200 max rpm, but then you said you hit 3500 rpm and could have gone higher.

With the old motor, was there a dramatic onset of ventilation at some point? did the rpms spike without any additional thrust? What about with the new setup? Can you post an RPM vs speed trace with your new setup. Include throttle position and fuel flow rates if you have a display for them.

Sorry for my poor use of terminology. I should have said the rated speed it 3200 rpm. Apparently Yanmar does not use an electronic governor as I was able to reach 3500 and wasn't at the limit of the linkage. So I assume it would have gone higher. I was in the breakin period of the engine so only pushed it up there briefly and didn't linger long enough to tab through the variables on the panel and record the data.

It's the same wheel with this engine and the previous one. And no it doesn't seem to lose grip through the rpm range. I have another identical wheel except 15 vs 14 in pitch. I'll be running it this week and will gather more data. Plus I'm past the break in period on the engine so will be able to run it a bit more at the top end of the power curve.

I've attached rpm, speed, fuel burn data as well as the theoretical power curves for the engine. I only have GPS speed data so the attached data reflects average speed for multiple data points. Also for my slip calculation I assumed that the 0.105 cup equates to 2 inches of effective pitch so slip is based on 16 in pitch. I didn't input percent throttle/load data into the spreadsheet and my raw notes are out of reach in my log aboard the boat. Will certainly collect more data this week including the high end of the rpm range.

 

[gonzo]

Perhaps because the started out specializing in props for wakeboard/ski boats. They are CNC machined so the cross sections vary with variable pitch along the radius of the blade. Their stated pitch is what they calculate as the "average".

All propellers have variable pitch. The radial speed varies with the distance from the center, therefore the advance speed varies too. Also, if you can over-rev the engine the propeller is not properly sized or is ventilating. For example, too small of a blade area will cavitate and create a bubble which in turn will let the engine turn too high. Ventilation can happen with a properly sized propeller. Otherwise, the pitch is too low.

 

[Ad Hoc]

.... The radial speed varies with the distance from the center, therefore the advance speed varies too. ....

You need to be careful not to mix and match definitions here.
Va = speed of advance, is the velocity of the water, in-way-of the prop...relative to the ship speed....owing to boundary layer, shaft brackets etc etc....ie the water velocity at the blades of the prop.
(Taylor and Froude wake)
Vr = calculation for dynamic pressure, when working out the cavitation number, which is related to the the velocity at a given radius, usually 70% of Rmax

 

[gonzo]

I stand corrected. I mixed radial with tangential speed. At the same radial velocity the pitch has to change to match the advance speed. Further, it is not a direct relation but rather complicated since the flow is not linear. Propeller design is really complicated and its own branch of engineering.

 

[philSweet]

Good trace. Nothing jumps out at me. It looks like the prop is doing it's job. If the slip stays around 10 knots absolute as you get over the hump you are fine as far as prop diameter is concerned.

It looks like the engine will accept the 15 pitch prop as long as you aren't hitting a brick wall at 8.1 knots. If you can get past 9.5 knots at 3200 with the 15" prop, it should be a decent match. If you can't get to 9.5 knots, buy a big dock box and empty some stuff out of the boat. Dock boxes are probably the cheapest performance boost you can buy. Let us know how it goes, but I don't see anything to worry about.

Check to see that you can slip it from forward to reverse and back without the engine idle dropping too low. Going from reverse to forward with all that cup could be a bit of a struggle, but I expect you are okay there too.

 

[baeckmo]

Ouff Phil, you are forcing me to divert from the celebrations...... If you plot the measured fuel flow on the nominal flow diagram, you will see that the consumption is deviating from the nominal from about 2000 rpm and getting increasingly lower with higher rpm. At 2700 it is 2,9 gph versus ~3,5 gph, and at 2800 it is 3,3 versus close to 4, the later indicating a power output of about 59 hp instead of 72 with a prop designed for the engine's max operating point at 110 hp/3200 rpm.

This deviation is typical for a prop in disturbed flow with incipient cavitation (here probably a mix of "real" vapour cavitation and gas/ventilation as noted previously). Here, the propeller is operating with very high propeller loading (a useful measure is the value for {Power/speed^3/diameter^2] in consistent units), making it very inefficient, even without the extra burden of bad inflow and ventilation. The operating conditions would have called for a Kort nozzle if it were a commercial vessel.

The problem is what to do to improve things; a sobering excercise, so I'll be back later........

 

[philSweet]

@baeckmo

... with a prop designed for the engine's max operating point at 110 hp/3200 rpm.

The prop was sized for the original motor 150 hp @ 4000 rpm. I think the fuel numbers diverging from the published Yanmar curve are because the current prop was intended for a motor that made the same torque at 4000rpm as the new motor makes at 3200 rpm, but with the boat going a lot slower. We'll see if the 15" prop absorbs more power, or not.

 

[baeckmo]

I use a calc program that comes very close to the Wageningen series, and when feeding the gnome with your measured values for speed, rpm, fuel flow and the resulting real power, the values show that the wake factor is ~0,8, meaning that the average inflow into the propeller is 80% of the boat speed through the water.

By calculating the thrust values up to the 8,1 knots measured, I get a rough value for the hull resistance, showing that with the 110 hp Yanmar and the prop alternatives possible, you would have a maximum speed of just shy of 11 knots. Operating this engine at 3500 rpm with the 17"x14" prop means it is running on the fuel stop curve. I'd estimate it will produce something around 106-108 hp there. As noted by PhilS this prop is too "light" for the engine.

Based on numbers above, you should aim at a 17"x16" screw with the same blade area as you have now. This will allow the engine to work with slightly higher effective pressures, which is better for the combustion, avoiding "glazing" of cylinder walls and keeping the turbo clean. If you look at the power curve, you see that the max power is delivered already at ~2900 rpm. Aiming at a max rpm around 3000-3100 will give you better cruising performance.

Considering the low inflow velocity, the effective angle of attack on the blade leading edge is on the high side already with the 14 pitch prop. Thus it would be better to use this prop and add a "coarse" cup to it, rather that going for a 16 pitch with no or small cup. The point here is that the blade leading edge should operate with a low angle of attack in the present case, where you have a combination of disturbed inflow, high thrust load and possible ventilation from behind.

With the propeller aperture available, there is little to gain by increasing diameter to 18", which would increase the risk for blade frequency vibrations. As said, the best option for performance improvement is a big dump box...... the hull is overweight for its length and the underwater shape is bad. Your propeller efficiency now is about 38% while cruising at 8 knots. The higher pitch will not be radically different efficiency-wise, but much better for the engine.

 

[montero]

What did the propulsion and hull designer have in mind?

 

[comfisherman]

Almost every prop gilnetter in the state, half the small seiners and a Hodge podge of other boats are over weight with a prop stuffed in a deep tunnel. The prop pimps up and down i5 in Washington state sell the lions share, as our state is rather easily serviced with shipping from Washington.

My personal favorite for my deep tunnel overweight boat is a bit less pitch and about a +2 cup. It was a combo that came on a boat I bought 15 years ago, and it did better than no cup and slightly more pitch when both gave similar loading and rpm.

My current is a veem, but the nakashima hi skew brought in by Michigan, teiginbridge, or similar high skew props all seem to be pretty popular. Would think the prop shops in anacortes would have a fair bit of ranger tug experience as well considering how many are floating around that neck of the woods.

 

[DivineFocus]

Thanks for all the replies, folks. Sorry I fell silent but I've been on the water most of the elapsed time.

...We'll see if the 15" prop absorbs more power, or not.

I did run the 15" wheel this past trip and gathered additional data. The increased pitch did draw more power and boat speed increased. Still exceeded rated speed at WOT = 3350 vs 3200 design. Data from both wheels follows. Sorry I didn't realize I used different rpm at low end:

17x14 0.105cup

17x15 0.105cup