Prop Selection for Production Boat

[DivineFocus]

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.

Thanks so much for taking the time to do this. Your calculations are pretty darn accurate. I just finished testing the 17x15 prop and at WOT was getting 3350 vs 3200 target rpm. So 17x16 should put it about right on 3200. And as you can see from the data above max speed was just over 11 knots but that is the average of only a couple of SOG data points so could easily be off by a knot or so. I was aware of wake factor but didn't realize how significant it could be. So my real slip numbers are substantially higher than the zero slip I've been looking at. According to Gerr slip should be running more in the 30-40% range whereas I'm running 50-60%.

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.

This is what I was thinking as well i.e. approach angle already high and only gets worse with higher pitch. I wonder if this is what the original design had in mind by specifying the original heavily cupped prop.

With pump design approach angle has a significant effect on efficiency and in the extreme causes cavitation on the high flow end and recirculation/cavitation on the low flow end. And in the end a propeller is simply an axial flow impeller. But in pumps we have a constrained flow path which is more easily understood and manipulated.

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.

Wow that's a rather pitiful efficiency number. The boat is really beautiful above the waterline but they certainly lost there way on the hull design. When I run the numbers for SL per Gerr's book changing the weight by +-500lbs only changes top end speed by about 0.3 kts and hull speed by about half that amount. I do see the difference in performance at the end of a week long cruise when we're light on water and fuel. But there's not much more to be done to reduce displacement via optional gear aboard. I did get rid of 160 lbs this season by switching to LiFePo4 batteries.

So I suppose it just is what it is. No magic levers to pull. As the old saying goes you can put lipstick on a pig but it's still a pig. I do have access to an 18in wheel which I'll go ahead and test. It will be an interesting exercise regardless of outcome. The wheel also fit's the lower pitch/higher cup discussed above. We'll see what happens...

Thanks again for the effort you spent on your response.

 

[DivineFocus]

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...

Good point.

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.

I've got just the ticket then. I've put my hands on a 18x13.5x 0.120cup that I'll try this week. I hope I have a similar experience to yours.

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.

Interestingly I've yet to run across a Ranger owner that's running anything other than factory supplied wheel. I've talked to a couple of shops(Mukulteo an Tacoma) and neither one of them seemed to have experience with the boats. They both just recommended I get my wheel tweaked to 17 pitch.

 

[DivineFocus]

Just returned from several days on the water running a four blade 18x13.5/0.120 cup. The 18 in wheel(10 percent tip clearance) didn't produce and significant noise/vibration. There was a bit of near synchronous noise at extreme low and high rpm but it wasn't enough to be problematic. Performance was marginally better then the previous run with the three blade 17x15/0.105 cup with WOT at 3300 rpm vs the 3350 with the other wheel and 3200 design. It may be confirmation bias but it did seem like the larger diameter wheel punched into head seas/winds a bit better. If I assume the cup results in an effective pitch of 16 in then the slip is also somewhat less than either of the previous wheels.

The boating season here is short. So my plan is to run the four blade wheel the rest of the season as is. Then I may send it to a shop to increase the cup a bit more.

 

[DogCavalry]

So how about modifying the hull to add the lower half of a Kort nozzle?

Asking for a friend

 

[montero]

So how about modifying the hull to add the lower half of a Kort nozzle?

Asking for a friend

Why do you think it's a solution ?

 

[comfisherman]

Current prop is getting close to perfect on the engine load and rpm, honestly from this point its just small tweaks to fine tune.

Deep tunnels and over weight is a common combo up here, about 15-20 years ago there were some articles about Mississippi truck able tugs that had half nozzles in tunnels. Naturally several guys had them built for retrofit on boats up here. It seemed to have benefit in a small bandwidth of speed, usually heavy and slow. At least two shapes of nozzle were tried, the downside was in maneuvering. Remember the first one I tried, was like parallel parking a train. Could help with a pivot rudder and thruster but added system complexity for a small amount of very specific improvement.

Probably just best to do some minor tweaks to the current setup and run as is.