Power Required

[Guest625101138]

Originally Posted by Asleep Helmsman

I wanted to find out what it would take to get a top speed, in terms of motors, in case the need arose to outrun unfriendly divers or something, but the crusing speed will be around 2 to 3 knots. Acceleration, and sterring within confined space, is going to be important though, so the extra power will be nice to have.

The motors are perminent magnet digitally controlled bruchless DC, that only use as much power as they need, so designing for extra top speed doesn"t severely reduce batery life.

What you asked for in the original post has nothing to do with what you are now proposing. I can now see why you were thinking of 25 to 30mm props.

The power estimate I provided relates to an open water prop of 300mm diameter. If you go to little jets you will at least double the power required.

This video shows what 200W can do on a 120kg boat using a 15" open water prop. Drag at 10kph is around 50N, similar to what you would have with a streamline 40kg hull at 8kts (no ports or ducts) but rudder and elevator. The motor is a PMSM weighing 59g and is geared 12:1.
http://www.boatdesign.net/gallery/da...11JE_10kph.wmv

If you are going to use small ducted props then the 500W I gave you for the 30mm prop will be more like it; providing you can set the ducts up not to interfere too much with flow and add extra drag.

Rick W

[Asleep Helmsman]

Originally Posted by Rick Willoughby

What you asked for in the original post has nothing to do with what you are now proposing. I can now see why you were thinking of 25 to 30mm props.

The power estimate I provided relates to an open water prop of 300mm diameter. If you go to little jets you will at least double the power required.

This video shows what 200W can do on a 120kg boat using a 15" open water prop. Drag at 10kph is around 50N, similar to what you would have with a streamline 40kg hull at 8kts (no ports or ducts) but rudder and elevator. The motor is a PMSM weighing 59g and is geared 12:1.
http://www.boatdesign.net/gallery/da...11JE_10kph.wmv

If you are going to use small ducted props then the 500W I gave you for the 30mm prop will be more like it; providing you can set the ducts up not to interfere too much with flow and add extra drag.

Rick W

It was a typo on the 25 mm I always thought 12 inch props. We did some work on props a while ago and discovered long narrow make for better efficiency.

My intension is to use large props with ducts. Now if it turns out that all they do is keep fishing nets out of the props, then I guess that's what they'll do.

[Guest625101138]

Originally Posted by Asleep Helmsman

Actually from the vey beginning I was thinking multiple trust. I just wanted to establish a baseline. 8 knots single propeller is a great starting point. But we are looking into hydrogen fuel cells and whatever the most advanced lithiom/whoknows batteries so functionallity and energy conservation are somewhat in conflict.

As you said fuel consuption reduces with the cube of speed. So the question is will a ducted 12" X 12" prop with a 250 watt motor attached to it pull harder than one without. And at 4 knots, which one will answer an all stop command from the computer faster.

I cannot imagine how you will fit multiple 12" props but if you do manage to do it without dramatically increasing drag then there will be a slight advantage in bollard pull of a ducted prop. You still have a lightly loaded prop. The blades will stall in both cases. Ducting will give you about 7% extra bollard pull at 250W on 12" diameter. Of course the advantage gets greater as you reduce the prop diameter.

You should think of ducting solely as a guard and maybe steering if you want to avoid the rudders and elevators. Keep as thin as possible consistent with protecting the prop and taper the leading and trailing edges to reduce drag.

The most effective sudden stop mode using a prop will be achieved by slowing the prop to avoid the blades stalling. This will be far more effective than going hard astern. Large ships use the helm to effect a crash stop as the turning absorbs a lot of the energy.

Rick W

[Asleep Helmsman]

Originally Posted by Rick Willoughby

I cannot imagine how you will fit multiple 12" props but if you do manage to do it without dramatically increasing drag then there will be a slight advantage in bollard pull of a ducted prop. You still have a lightly loaded prop. The blades will stall in both cases. Ducting will give you about 7% extra bollard pull at 250W on 12" diameter. Of course the advantage gets greater as you reduce the prop diameter.

You should think of ducting solely as a guard and maybe steering if you want to avoid the rudders and elevators. Keep as thin as possible consistent with protecting the prop and taper the leading and trailing edges to reduce drag.

The most effective sudden stop mode using a prop will be achieved by slowing the prop to avoid the blades stalling. This will be far more effective than going hard astern. Large ships use the helm to effect a crash stop as the turning absorbs a lot of the energy.


Rick W

I totally agree; we have complete control over motor speeds and applied torque. The mechanical systems are all 100% integrated with the on board computers. So we can do testing and find out exactly how much negative power to apply or to create an exact come to stop routine, within the power range of the motor.
They sample at 2.5mz and are accurate to 1/8000 of a revolution. Several hundred times the necessary control for this application. Although it is useful for programming complicated maneuvers, and for making fine adjustments to course and attitude.

[Asleep Helmsman]

When you say stall from not enough load, I take that to mean that a smaller prop would be more loaded and therfor less prone to stall?

But would require more power to opperate at cruise speed?

[Guest625101138]

Originally Posted by Asleep Helmsman

When you say stall from not enough load, I take that to mean that a smaller prop would be more loaded and therfor less prone to stall?

But would require more power to opperate at cruise speed?

You are confusing two conditions. The prop is lightly loaded because it has low power applied to a large area.

Any blades stall if they are spun too fast with respect to the oncoming flow. If you rip from ahead to astern the blades will be in a stall condition with a high angle of attack because the flow is initially in the wrong direction. They are not as effective as operating with attached flow.

A smaller prop will stall more readily because it has heavier loading.

Rick W

[Joakim]

Originally Posted by Rick Willoughby

The most effective sudden stop mode using a prop will be achieved by slowing the prop to avoid the blades stalling. This will be far more effective than going hard astern. Large ships use the helm to effect a crash stop as the turning absorbs a lot of the energy.

This is not true for a typical marine propeller. It may be true for a very low DAR propellers.

All "normal" marine propellers have higher drag locked than free running at any rpm. Stopping the propeller will cause a drag of about 0.5*rho*Cd*Ap*V^2, where Ap is the projected blade area and Cd is ~1 (depends mainly on P/D).

So at 8 kn a 12" 30% projected area prop will have drag of about 200 N, but that will drop to 50 N at 4 kn and then you can certainly get more by spinning the propeller backwards. If you have several of these (2, 3 or even 4???), you have a very good brake by just locking them, but this brake looses its power at low speeds and you need to spin them backwards.

Here are some measurements and theory: http://strathprints.strath.ac.uk/567...ints005670.pdf

[Guest625101138]

Originally Posted by Joakim

This is not true for a typical marine propeller. It may be true for a very low DAR propellers.

Conventional marine props stall at low AofA and are close to useless for regenerating. The Cl as a turbine is about 0.04. When you are not bound by cavitation you can go to thicker blades with higher aspect. With a cambered blade it will not be as effective regenerating as powering but it will give better braking turning to regenerate than stopped as the sub slows down. For the foil I did the original analysis on the Cl as a turbine is 0.8 - around 20 times better than a marine prop. The limit will be the regenerative capability of the motor not the stalling of the prop. If it is generating 240W at 8kts then the drag at 70% efficiency would be 87N. If you had the regenerative ability to get near the Betz limit the drag would get up to 400N at 8kts.

The prop I based the calculations on has a BAR of 12% and projected area somewhat less. So even with a 250W regen limit it will still be better than the stopped drag.

Rick W

[Asleep Helmsman]

I'm so confused.
It changes prop design according to watts used. It seems like you should input a need and it would tell you the required power.

[Guest625101138]

Originally Posted by Asleep Helmsman

I'm so confused.
It changes prop design according to watts used. It seems like you should input a need and it would tell you the required power.

What is "it"?

Rick W

[Asleep Helmsman]

Sorry Rick,
JavaProp.

I have been thinking that the idea is to try different wattages untill you get similar props for different speeds.

[Guest625101138]

When you use the "Design" function Javaprop notionally gives the best prop based on the set design conditions and the chosen blade profile.

The sequence is to go to the Options page and set parameters for water(density 1025, viscosity 0.0000013), the next is to go to the Airfoils page and select foils and their AoA for what you want to do. For your case start with the E193 100,000 Re# at 1 degree angle of attack.

Go to the Design page and set the prop diameter, number of blades, rpm, hub diameter and the design speed with the corresponding hull drag.

You then press Design It. This will be close to the best prop for you design conditions.

If you want to see what the conditions are at a different speed you need to adjust the rpm on the design page and go to the Multi-Analysis page to see how it performs. As a first approximation if you want to see the power for half speed, halve the rpm (DO NOT PRESS DESIGN IT), go to the Multi-analysis page and Analyse. You need to scan down the table until you find half the design speed and check that the drag is 1/4 of what is was at the design condition. If it isn't repeat the process making small changes to the rpm.

I have a more functional Excel based model that does all this automatically and allows for all the variations in parameters and endless variety of foil sections but it reflects a few years of unpaid development that I do not want to give away.

If you do a few screen dumps of what you have set up I can advise if you are on the right track.

You need to arrive at a blade that is robust enough to take the forces. No good designing blades that have a chord of 10mm for example. You should not go under 25mm chord at the root for the sort of thrust you need to design for.

Rick W

[Asleep Helmsman]

Thanks Rick,

I have a few pieces of unpaid software as well.

I actually did changes the options, but I was changing the speed and RPMs and of course it was not working without substantially reducing wattage, kind of a trial and error thing.
I'll try using the multi page

[Asleep Helmsman]

Originally Posted by Rick Willoughby

The attached image is one I had milled. The P/D is about 2 - quite aggressive pitch for easily driven boat with similar drag/speed to what you are considering. This prop has efficiency around 85%.

It took a few attempts to add enough meat for tool passing from my design but it worked out well. This one is aluminium and was linished by hand to remove the tool marks rather than going to fine finish on the mill. That saved a lot of machine time.

Anyhow it shows you what a high efficiency marine prop looks like for these low Re# applications.

Rick W

If you have a minute I have a couple of questions about this prop, if you please.
How did you determine the overall shape, at the edges, from the hub to the tip?
It looks like the cord ratios are constant from the hub to the tip. Is that true?
If true, what was the reason?
On the Java Prop site he list foils with large ratios for use close to the hub. He mentioned more efficient at the higher attack angles, I can see that, also the “thicker” blade towards the hub would be stronger.


Sorry, I expected the image to follow the quote, it was a picture of the aluminum prop you showed earlier in this thread.

[Guest625101138]

Originally Posted by Asleep Helmsman

If you have a minute I have a couple of questions about this prop, if you please.
How did you determine the overall shape, at the edges, from the hub to the tip?
It is the result of optimisation of efficiency within constraints of strength and ability to actually shape the shape.
It looks like the cord ratios are constant from the hub to the tip. Is that true?
The blade gets thicker at the hub for strength but loads are very low so it does not need to be very thick.
If true, what was the reason?
On the Java Prop site he list foils with large ratios for use close to the hub. He mentioned more efficient at the higher attack angles, I can see that, also the “thicker” blade towards the hub would be stronger.
Thicker is really for strength reasons. The inside of the blade does very little work.


Sorry, I expected the image to follow the quote, it was a picture of the aluminum prop you showed earlier in this thread.

See inserted notes.

Rick W