Electric Sculling Drive.

Rick,


Here in Europe it's hard to find a good propeller.
Can you give me some adresses i an contact for a propeller diameter12"
(0.305 m) and a pitch of 9 " (0.229 m ) with the highest efficiency at 400 revolutins/minute?
Up to now, i found efficiencies of only 63% in seawater of 25 °Celsius.
Thanks,
Lorejas,

 

The most important requirement for a propeller is that it is matched to the application.

If you give me details on the application like the hull shape, speed and power I can work out the best propeller and tell you what efficiency to expect. You then have to find or make a propeller that is as close to the best as you can get it.

By specifying the diameter and pitch you are likely to be condemning the design to sub-optimal.

I can provide detail fabrication procedure for low power props if you cannot buy a suitable one. You need to be able to weld to make it.

Rick W.

 

Rick,
I really appreciate your quick answer.

I have a 5 meter electric driven boat, deplacement 0.250ton, waterline of 4.5 meters. Electromotor has the highest efficiency at 400 rev/minute.
I'am trying to improve the autonomy of the batteries.
The 12/9 dimensions are the actual propeller.

Thanks for your cooperation.


Lorejas

 

Economic speed is likely to be under 5kts.

Have you got a photo of the hull?

If no photo then what it the waterline maximum beam?

Is it an efficient low speed hull or is it designed for planing with a flat transom?

If it has a transom how deep does it sit in the water at rest.

I need to be able to model the hull so I can produce a drag curve.

Rick

 

Hi Lorejas.
Welcome to my thread, your problem is not a sculling problem.
If you give me the HP or KW of your electric motor and tell me if your boat have narrow stern or wide flat stern I can tell you what propeller you need. I have working with propeller calculation for the last 35 years and I have my own program to make this calculation.
Kjell

 

kjell,

35 years of prop calc, would you be prepared to share your program with the world at large, or simply post a copy here, I promise not to tell anyone.

I have used the Propcalc that is from this site, but it does not necessarily agree with my physical use of props we use. It is close, but not exact, not that prop calculations are ever exact.

ta.

 

Hi Lorejas.
I f you have the PROPCALC program you are a lucky person. This is one of the best propeller program, based one the Bp-Delta method. 100% exact propeller calculations not exist. Manu time it is a question of interpretation. (If you have a violin that doesn’t mean that you understand music.)
Kjell

 

The Scull Drive can give better thrust thanks to its big flapping area and the possibility to a use higher AoA without the cavitations risk.
Kjell

 

Kjell, yeah mate but a player piano still plays music, so too does the program, so the ansswer is, that there is no answer.

Now when you recommend a prop to a customer, and he comes back sayong it is no good, what next. Have it repitched, diameter made smaller, who pays the slipping fees etc etc.

Come on fellas, this is very difficult. There is no simple answer.

 

Hi
It was not my intention to offend anybody with music example.
To calculate a good marine propeller is very difficult. Most new boats have the propeller badly calculated with the result of overheating engines and not reaching the offered speed. As Marine engineer it was my job to modify propellers. Before starting to modify you must understand how propeller works and know how to calculate them. The difficulty is to obtain the right numbers to introduce in the propeller calculator. The max efficiency for a marine propeller is 74%.
Kjell

 

Yeah kjell, it certainly is a difficult problem. We have been doing props here for the same years actually, all I can go on is what worked in the past for a certain type of boat and setup underwater, I have never found anyone that can be spot on from an untried design (not saying it cannot happen, just that I have never seen it). We seem to get ours OK as we basically do trawler stlye boats, and they really are very similar underneath.

My heart bleeds for you doing it as a profession! All the best.

 

Landlubber.
You are right. Many new bots are equipped with the wrong propeller. The main reason is that the naval architects don’t have the real weight of the boat and the engine power is not right. To be able to sell the boat they promises a boat speed that can not be archived when the boat is full loaded with water, fuel and personal equipment. With the PROPCALC program it is easy to verify if the propeller is right or not. For Displacement and Semidisplacement hulls the program is very accurate. For Planing hulls it is more critical do to different hull types. Knowing the LWL. weight and the SHP it is easy to verify if the boat is able to plane. Planing starts at SL-Ratio 2.9 – 3.0
Kjell

For any question about PROPCALC contact me at
kjellgood@gmail.com
You are welcomed.

 

I have done a video of the test to se how the scull drive works when you need to maneuver the boat.
http://video.google.es/videoplay?docid=-1656067526827705858&hl=es

Kjell

 

Kjell, It's good to see you are still alive and kicking.

We corresponded with Kim Prentiss down in AU.

Now, it seems israel is taking the lead with wingsails.

Hope you are doing well.

Mark Diener
Florida, USA

 

I wonder if this sort of drive will be a good idea (even with the energy efficiences people say there are) in ice: No superfast moving propeller to be damaged?