Propeller shaft torque?

Javaprop is a great tool though it might look tricky at first sight, particularly if you are not used with prop therminology and math.
If you are looking for a something simpler yet pretty useful for your needs, you can use Surfbaud's "propcalc" spreadsheet. I'm enclosing it here:

 

I have had a few refugees in my time . It is always good fun and never boring!

Rick, what can I say! This is marvellous stuff and after weeks of trying to get a handle on this I am making progress at last although my brane herts a bit.

I have downloaded Javaprop and seem to have it running but am stuck at the first page, when I try to calculate a prop it sends me a message to the effect that I am too dense! Could I ask you to do screenshots of the data you started with to see if I can get to the same end result?

Regards

Arnot

 

And again! Many thanks, I have downloaded the spreadheet and it seems to do more or less what I want. Along with the comprehensive explanation of propellers on the Javaprop site I seem to be almost there. I just need to play around with both approaches a bit and probably sleep on it...

Regards

Arnot

 

Arnot

"...It seems to be for calculating power at a given speed once you already know the torque, in my case it is the torque I want to know.."

That was the reason for asking my Qs above....to obtain the torque, more information about the boat is required. Since it is not brand new, 'things' are not working as they used too, plus we have no resistance data, hence the questions to reverse engineer the answer.

 

First thing to do is to go to the Options page and set the parameters for water. The density is 1000 and the viscosity is 0.0000013. The default values are for air.

Now you have to select a suitable foil. Go to the Airfoils page and select all four foils to the E193 at Re# of 300,000 and each of the four points at a 2 degree angle of attack.

You now need to design the prop. Make the settings to suit your prop. To get the 16" pitch I selected diameter 0.55, rpm 1000, blades 3, Spinner 0.1 velocity 4 and power 25000. As it happens this is identical to the design conditions you originally provided and will give the 16" pitch (means the 22 x 16 prop should get you to 8kts if it can get to 1000rpm). You have now designed a prop that should look similar to yours. You can look at the blade size in the geometry page. If you think your blades are bigger or smaller then you can adjust the design AoA to alter the blade size but this will not make a huge difference to the end result.

To replicate the data I provided you, go to the Multi-Analysis page and hit the Analyse command. This will give the data for 1000rpm.

If you want to look at other rpm then you need to change the rpm in the design page but DO NOT hit design. Just go back to Multi-Analyse and press Analyse. You will get the data for the new rpm.

If you have any trouble then just keep posting until you understand what is going on. I will be off to sleep in a while but will be back tomorrow.

Rick W

 

Arnot
I have done plots of the previous data, which will show more clearly what is happening. The plot includes a 25kW limit curve.

The intersection of the limit curve and the respective prop torque demand curves is the upper limit of torque that you will be able to supply to the props with a variable speed drive powered by a 25kW motor.

For the 22 X 22 prop it is 305Nm and for the 22 X 16 it is 260Nm. I expect these would be conservative as well because you will suffer hydraulic losses.

If you play around with the foil profile and the design angle of attack you can get slight variations. My initial stab at the Re# is a bit low as well. You can get this data from the Single Analysis sheet. I did a check with the MH116 foil at Re# 500,000 and AoA of zero and can get slightly higher peak torque values of 270Nm and 330Nm for the respective prop pitches. There is not much change in foil L/D with Re# between 500,000 and 1,000,000.

Remember the foils modelled in JavaProp are perfect shape. The profile of the leading edge is critical in determining stall characteristics. Sharp leading edges will stall earlier and are therefore incapable of absorbing the torque nominated here. So all this data should be taken as a worst case. In practice any imperfections in the blade will reduce the performance.

To get more accurate data you would need to carefully measure the prop and model it more precisely. It is very unlikely that the values will be much different to the numbers above and, if anything, will be lower.

Rick W

 

Many thanks for all your help Rick.

I have managed to get Javaprop working with the help of your instructions and I am trying to more fully understand the relationships between the different variables. It is opening up my mind to the complexity of the problem and it is rebelling slightly!

I now more or less understand what is going on and can see where the 300Nm comes from and agree that this is a good basis from which to specify the hydrostatic components.

One of the benefits of hydrostatic drive in a relatively slow boat and engine like mine is that with the benefit of constantly variable drive it will be possible to set the engine speed where I expect the cruising or manouvering optimum and then use the swash plate control to feather in the drive to the propeller.

The question I now have to tackle is what ratio or maximum transfer to go for. Do I go for a 1:1 ratio and use the swash plate angle as the main means of control or do I go for a 1:1.5 downspeed and use the engine speed control as well? The former will allow me to run the engine more slowly which sounds nicer and uses less fuel, the latter gives more torque.

All the hydraulic pumps I have been looking at are rated for use at speeds my engine could never achieve and the performance graphs tend to start about where I stop. I am waiting for some information from the pump manufacturers about slow speed being a problem but at the moment it looks good.

Next week I am going to try out a boat similar to mine with such a system installed which should also help.

Regards

Arnot

 

If you are going to the expensive of installing a full hydraulic system with speed control it would seem you would want to get the benefit of the fine speed control. With the swash plate you should also be able to get shaft braking and smooth reversal. These may be significant in operating the boat safely.

I doubt that there is anything you can do with the hydraulics that will reduce fuel consumption. Even running the motor at steady rpm the slight gain in motor efficiency will be lost in the hydraulic losses.

I am not a fan of hydraulics but for your application they might provide better control. They need to be well engineered with good fittings, short pipe runs, good filters (best is HP filter but usually not practical) and full flow pressure relief. Make sure the system designer is aware that the hydraulic drive might have to slow an overrunning load. Preventing the prop from freewheeling may be very effective in slowing the boat quickly.

With hydraulic systems I like to see design pressure no more than 3000psi although most of the components are usually able to handle more.

You could reduce pump size by using a belt drive but then there is more complexity.

Will be interested to see what you end up with.

The key to understanding your design issue with the prop is that the blades stall if the angle of attack is too high. This is exactly the same condition of a wing in an aircraft. It is the blade stalling that limits how much torque the prop can actually absorb. So peak torque does not normally occur at zero speed unless the prop has been designed for it such as a tug.

Rick

 

Arnot,
Yes, Bolinder was a Swedish company. It seems they had about 80% of the world market for fishing boat motors in 1920. I guess they lost the market to more weight-efficient motors.

I've been thinking about your problem, and had to check my intuition with the rules of thumb in Dave Gerr's "Nature of Boats". As I understand it you want to maximize the push especially at low speeds and you also want to use the propeller for braking. The 16" pitch should give you 8 knots with 40% slip at 1000 rpm. I don't think your 33 hp will give much more than 8 knots (give or take a knot) in a 72' boat regardless of propeller. Your existing propeller seems to be a correct choice.

If you were to go for 22" pitch, you would have to work at a lower rpm. A lower rpm also means that you need a larger diameter for the prop, so your 22" diameter would be a little small for 33 hp at 700 rpm. I don't see an advantage in going up in pitch.

In your position I would keep the propeller and dimension the hydraulic motor to provide enough torque for 33 hp for shafts speeds from 700 rpm and up. As you and Rick have calculated this means 300 Nm.

Normally I would like to have the engine running at a speed where the noice and vibrations are as small as possible even if this means sacrificing a few hp, and dimension the pump for this.

Erik