electric boat, proper rpm and propeller

http://www.thameselectric.com/ for information.

Pericles

 

@ Man Overboard in the bottom there is only g.r.p. box with very small weight...i'll let you know about my propeller.

Mr Rick i have thought about it, but putting and outboard motor, probably will give to much weight at the bottom of the boat, also i have something else in my mind, such us the attached photos below...From your experience,what you say about 1500 rpm, and a propeller about 20cm diameter. Tomorrow i am going to work with java prop and i will post my news...Thank you for your help, it is very useful for me.

 

Nikolas,
To add to what Rick has said about outboard motors, it might be easier (and cheaper) to convert an old outboard motor. I am sure you could find an old 2 or 5 horse with a shot motor in Greece, No?
Check out this site:

http://www.psnw.com/~jmrudholm/etekoutboard.html

 

i will have this at my mind, but with this i change my first plan(inboard motor), and i have worked a little bit on this, anyway every thought from anybody is welcome here.

@ Pericles, thank you for the information...

 

Nikolas
The reason I suggested the outboard over an inboard system is the design flexibility it gives you. It will not present a stability isssue particularly if you have a couple of batteries in the bottom of the boat.

I think you will surprise people with the performance that can be achieved with the small boat and electric motor if you keep the weight down and make an efficient propeller. So if best performance is important then you need to design accordingly.

With high efficiency propellers you get high bending loads in the blades if they are run at an angle to the flow. This is because they operate it a very small angle of attack and having an inclined shaft results in the blades alternating between thrusting and braking as they rotate. This give high vibration and will flex blades and shafts to fatigue failure.

With an outboard there is no need to put a shaft through the hull and all the complexity of leaky shaft glands. Also you will see with JaveProp that a big diameter prop will be more efficient the a smaller diameter prop. With a large diameter, the prop will be on a severe angle, which will make the problems noted above worse.

Do you actually intend to build the drive unit or are you only required to submit a design?

Rick W.

 

yes i am going to build the control system

 

Do you intend to actually use the control system in an operating boat? Meaning will you need to acquaire a motor and propeller?

Rick W.

 

Mr rick, the pictures of the boat i sent you, is the boat we are going to make electric, in a few days or weeks i am going to buy the motor, propeller, batteries and the other stuff to start building the boat, thats why i need your help, the control system is going to build after january, and we expect about june to be able to use the boat, i have a question for you, with savitsky method i have calculate Re about 5*10^6 to 6*10^6, is the Re values suitable for my application?

 

Nikolas
The Reynolds number is a function of speed and length. What you have for the boat at a given speed does not apply to the propeller.

When you use JavaProp you can find the Re# for different sections of the propeller in the "Analyse" page. You use this iteratively to select a suitable foil section. JavaProp has a limited range of foils loaded. I have many more and can help you when you get to this stage.

You need to be aware that you will have difficulty buying a propeller that will give best efficiency. You need to see what you can find commercially but it will be better if you can actually fabricate the propeller.

First see what you get from JavaProp and I will check. Then you can look for a commercial prop that comes close to your design. This is the best place to start because you then know what is possible and you can assess the compromise in getting something that is not ideal.

Rick W.

 

shrouded rotor, square tip, what it means mr Rick?

 

Nikolas
I have attached some screen dumps from JavaProp to give you an idea of what to expect. You can see one of the columns in the Analysis page has Re for different positions on the blade.

If you spend some time adjusting the airfoil angle of attack and other parameters like speed you should be able to produce a propeller with better than 80% efficiency.

So see if you can get to that stage and then we can discuss options.

Rick W.

 

because if i tick, shrouded the efficiency goes about 80%

 

You can alter the settings to see what affect they have. Most boat propellers operate unshrouded. Some are shrouded in what is referred to as a Kort Nozzle. This reduces edge losses from the blades and improves efficiency.

If your propeller has low efficiency to start with it could make a big improvement. If the unshrouded efficiency is high then the improvement from shrouding is not as great.

The gain in efficiency is offset to some degree by the extra drag from the nozzle so this needs to be taken into account if you want to shroud. Nozzles are most often used in high thrust, low speed applications where tip losses are high. At higher speed the gain in efficiency does not offset the extra drag.

If you load the boat to 250kg then you might benefit from a nozzle. With a load of 150kg the open prop should be better because you could expect to do better than 6kts.

Rick W.

 

here are some results for Re# E193 300000, and 0 degrees,what you think?

 

Nikolas
Try some larger diameters and slower rpm.

You now have to tie in the thrust produced to balance the drag for the speed from those curves I supplied.

Do you want to design for the 150kg case or the 250kg case.

Have a play with it and see what you work out. If you tick the shroud then you have to think of making a nozzle for the prop to operate in. You can Google Kort Nozzle to see what they look like. This will allow you to get good efficiency from a small diameter prop.

Also you have to make allowance for any mechanical losses in you system. So rather than designing for 1500W maybe you should drop to 1400W.

I am going to bed no so will look for your next set of numbers. Also look around to see what propellers you can get.

Rick W.