Prop tip velocity

We agree on that. It all depends on the intended use of the engine, on the boat characteristics and on how does one define "advantageous".

For long cruises (which, as I understand, is your case) a bigger engine at 75% will give longer life, though not necessarily a better fuel consumption, due to weight penalty (as shown below). The last factor will depend on the boat size, weight and on the hull type and shape. For short uses on fast boats which work at close-to-max power a smaller engine is still be advantageous, due to reduced weight.

Take two cases:
1) Volvo D3-170 (170 HP @100%)
2) Volvo D4-180 (de-tuned, 180 HP @100%)

D3-170 consumes 35 l/h to give max. power output of 170 HP (0.21 l/h/HP). It weighs 301 kg with transmission.
D4-180 consumes 33 l/h to give max. power output of 180 HP (0.18 l/h/HP). It weighs 546 kg with transmission.
Hence, in terms of pure fuel consumption, D4 appears to be better. I said "appears" because a weight difference of 245 kg is important, and it might change things completely. A higher weight increases vessel's resistance and can compromise the trim. Both factors can more than offset the initial advantage of the bigger engine. So, the real advantage will depend on the overall system "boat+engine".

Besides that:
D3 weighs 301 kg and occupies 0.61 cu.m, giving 0.56 HP/kg and 279 HP/cu.m
D4 weighs 546 kg and occupies 0.67 cu.m, giving 0.33 HP/kg and 269 HP/cu.m
Hence, in terms of Power/Weight and Power/volume ratios, D3 wins hands-down. A higher power/weight ratio translates into lower boat resistance, power requirements and propeller loading for the same speed. Higher power/volume ratio translates into smaller engine room and more space for payload and other uses.

So, I would say, each engine to it's own application and a multifaceted approach is required during selection.

Cheers

 

My application is a barge tug. Max velocity 7 knts. Max efficient barge velocity 4.5 knts.

I have graphed some data on tip velocity vs prop diameter. Guess what?: exponential curve downwards as diameter increases. The data came from calcs, which I will double check. If the calcs are robust I can now work out reduction box ratios for any given diameter and engine RPM. Very useful. The data was built for motor yacht max speed 9 knts.

Why use tip velocity? Because if I go a bigger or smaller prop I need to know if I should change the gear ratios.

The aeroplane boys have already worked this out as important and say the their best is 0.8 -0.9 speed of sound.

 

Re Diesel motors: They have governors. The issue is not as simple as it appears. I will comment tomorrow. A few extra KG is not going to worry a Tug. The fuel consumption figures you show are at 100% output. This is not practical. I will comment tomorrow.

Thanks for the interesting discussion

M

 

Ok, then it is clear that hi-speed example doesn't apply to your case. +/- 250 kg weight doesn't change things for you. And I know you don't want to make engine work at 100%. The above example serves for comparison purposes, between two different-size engines of very similar power. I could have as well find two 220 HP engines and let them both work at 75-80% power, to obtain 170 HP cruise power. For example, a D3-220 and a D4-225. It wouldn't change the final conclusions - you can check it out, if you want.

Cheers

 

Airplane propellers don't cavitate like boat propellers do, so their calculations are not really applicable. What do you mean by "exponential curve downwards as diameter increases"?

 

Any particular reason not to learn how to size props, etc by "forward" engineering since the methods are available? Two references were provided in response to a related question of yours on the http://www.boatdesign.net/forums/props/help-prop-calcs-using-planing-hull-tug-47035.html thread (posts 15 & 17).

 

For aircraft props the tip speed is limited to Mach 0.8 to 0.9 or so because at higher speeds shock wave formation near the tips will increase the torque required to turn the prop and decrease prop efficiency. Shock waves don't occur with boat and ship props.

 

Michael:

Propeller tip speed is indeed an important propeller sizing constraint, but not in the way that you want to use it. It is one metric used to predict when cavitation will begin to meaningfully cause problems. It is never something to design to, but is a constraint that is used as a check after you get the RPM and diameter that gives you the best performance.

Having said that, I am not surprised that you might find a scatter that looks like some line (e.g., "exponential"). This is an accidental outcome of looking at common applications and their propellers - but this should not be confused with an optimum design lane. So, as has been suggested, size your propeller using methods that have been suggested, and compromise only if tip speed is excessive.

Don MacPherson

 

'Any particular reason not to learn how to size props, etc by "forward" engineering since the methods are available'?

Why not do both? - Get as much info from industry as possible, then compare with calculation. This all should result in bell curves. Decide where in the curve you want to be. Then and only then should we go out and spend $10,000 - 20,000 on a prop. Its all very well saying 'trust the calaculations' but everything changes when we have to go out and spend money.

We design workboats. These props a big, heavy, and expensive. You cant bring a box home for experimentation. Also: mine is a real-world scenario in a developing country (India). They have access to a limited type of power unit, gear ratios and props.

If calculation alone was that great suppliers of small power systems would not try a range of props on a new unit (which they do) to get the best performance for the client. Notice how little information they give on their prop performance?

I only used the aircraft example to demonstrate that tip velocity is a very useful parameter not as an example relating to water.

Note that a 10% increase in diameter results in a 31.4% increase in tip velocity. The data I have suggests we should decrease tip velocity at an exponential rate in relation to an increase in diameter. Logically, the way to do this is through gear ratios.

The next step is to bring in pitch. Over the next weeks I will post some graphs to demonstrate some interesting relationships. I need more data. The key to statistics is heaps of robust data.

Don: I have made very clear in a number of ways that I propose tip speed as an indicator of where any given configuration lays in relation to the norm - not as a parameter to design to. How many times do I have to repeat this? Reverse engineering is an important and legitimate process. I can give many example of manufacturers running around after industry to ascertain what works best. Shearing combs is one where the shearers, heating, bending, and working with sandpaper have driven design - not the designers.

Cheers

M

 

There is definitely a trade off here between several performance and design variables. Generally for tug boats, push boats and things like that the tip speed should not be much greater than 110 ft/s. But of course this assumes many factors such as tip clearance, shaft angle, etc. Cavitation is also generally a limiting factor, so it very possible to be in the situation where in order to get the cavitation to a manageable level, the tip speed needs to be increased. But I believe that if you were to look at the majority of successful river towboat applications you will find that the tip speed is 90-110 ft/s range, so this is certainly a good starting point.

 

Doing both is worthwhile. Have you been through the "forward" engineering calculations using a standard method already and have a design from them, or do you plan to do the calculations using a standard method once you have a preliminary design from "reverse" engineering?

If the shaft and prop rotational speed (rpm's) remain the same when the prop diameter increases by 10%, then the tip velocity increases by 10%, not 31.4% (assuming tip velocity is tip velocity relative to the vessel to be exact).

For the tip velocity to increase by 31.4% when the prop diameter increases by 10% the shaft and prop rotational speed would have to increase by 19.5%. (1.10 * 1.195 = 1.314)

 

Thank you John. I am not a lone voice in the wilderness (not that I care 

The data I am using at the moment relates to max efficiency of props on high resistance displacement hulls with max speed 9 knts

It shows: 1m diameter 122 ft/s
1.5 m diameter 87 ft/s

Going on your recommendation, which I suspect is sensible, subject to all the other factors, we should be looking at a diameter somewhere between these 2

I do have some hull resistance figures on this particular design and will integrate them shortly.

Cheers

M

 

Hi David
Yes of course you are right. What I was getting at is that is that we have to be conscious of tip speed as it increases at a rate of 3.14 x the increase in diameter. It would be easy to forget this and go over the 110 ft/s John refers to.

Just my way of seeing it.

M

 

Tip speed increases at the same rate as the increase in diameter, not 3.14 x the increase in diameter.

Distance the tip travels per revolution is 3.14 x the diameter, but that is true for all diameters. So the ratio of the distance the tip travels per rev on two different propeller is the same as the ratio of the diameters. The difference in distance the tip travels per revolution is 3.14 x the difference in diameters.

Here is an example of the ratios: From post #2

Tip speed in m/sec

= Prop diameter (m) / 2 * Shaft speed (rpm) * 6.28 radians / 1 revolution * 1 min / 60 seconds

= Prop diameter (m) * Shaft speed (rpm) * 0.0523 (radians/revolution*min/sec)
​

Initial prop diameter 0.75m, Shaft speed 500 rpm

Tip speed (m/sec) = 0.75 * 500 * 0.0523 = 19.61 m/sec​

10% larger prop diameter 0.825m, Shaft speed remains 500 rpm

Tip speed (m/sec) = 0.75 * 500 * 0.0523 = 21.57 m/sec​

21.57 is 110% of 19.61. The 10% larger prop has a 10% higher tip speed at the same shaft speed.

You might want to work through an example or two on your own.

 

David: this is not rocket science. if a prop is doing 1 RPS. and we increase the diameter by one foot we increase tip velocity by 3.14 FPS. That is my point

And: if the prop is doing 10 RPS the tip velocity increases by 31.4 FPS