electric boat, proper rpm and propeller

Mr Rick, i attached the files from javaprop, as you can understand i have many draft constraints because i have the boat, i am not going to get another one, i can not put very "big" in hp motor because it requires more batteries which means more weight and cost, so i have to find the best propeller and rpm selection with my requirements, with these requirements i tried to get an efficient enough propeller from javaprop. Also my propeller is a little big for me and the draft constraints i told you, so it my be about 3 cm less, also i have attack angle at 0 degrees and Re# 300000 E193, as you told me. The next thing i am going to see what propellers there are out in market and i am going to select the more efficient. I am waiting for your opinion for the propeller and the rpm. Good night from Greece...

 

Nikolas
I checked what you have done and it shows you have a good grasp of JavaProp. You might get a bit faster than 3.5m/s but what you have should be conservative if you keep the weight to 150kg

The prop with 0 degree AofA has blades with large chord so you will probablyy have trouble finding ones like this. I have increased the AofA to 2 degrees and the chord is more reasonable around 60mm. This increases slip so the pitch goes up to 207mm. Efficiency is slightly better as well.

So you should look around for a propeller with a diameter of 250mm(10 inchehes), pitch of 207mm (say 8 inches) and maximum chord around 60mm (say 2.5 inches). It does not matter much id it is 2 or 3 bladed. If three bladed the chord could be a bit less.

If you can find a propeller similar to this you can recalculate just to make sure based on exact measurements.

Do you have facilities to weld stainless steel flatbar that is 50mm wide and 4mm thick. If you do I can give you a procedure to fabricate the prop. This is backup if you cannot get the prop you want. However you should have no trouble getting a 10X8 prop that is very similar to what you need.

Rick W.

 

Mr rick, i would like your opinion also if i get a smaller one propeller, about 22-23 cm with another pitch because of my draft constraints, would that be a big problem?

 

Once you find a propeller that will fit the you will need to recheck its performance. As you reduce the diameter you will find it harder to get a propeller and the efficiency will drop off. It will still wotk but thet boat will go slower than it would with a larger diameter prop.

Rick W.

 

ok mr Rick, soon i will be back with my news for the propellers i found in market, also i would like your opinion for the rpm, about 1500 is ok, how can i be sure about this? is there any mathematical calculation for this?

 

If you can only fit a small diameter prop then 1500rpm is OK.

If you do trials using JavaProp you can easily assess the impact that variation makes.

I believe you have determined that you do not want to shroud the prop. In this case I think you will find a prop around 400mm diameter running at 700rpm will give the best efficiency at around 250N thrust and 3.5m/s. Experiment with JavaProp to test this. You then know how little power you need to meet these conditions. For example a 400mm diameter prop running at 700rpm would require 1090W.

As 400mm is too big you need to set the diameter to what you can fit in. If that is 220mm then set this in JavaProp and test different RPM to see what gives the least power. I have attached a screen dump from JavaProp with these conditions. You can see it will require 1330W and the pitch is just over 8 inches. The efficiency improves slightly if the prop is run at higher RPM but you start to get very small pitch and you will not be able to buy a suitable prop. You should be able to find a 9X8 prop. The chord does not matter too much. Also it will not make much difference if it is 3-bladed.

Once you find a propeller you then check with JavaProp to see where it will operate.

Rick W.

 

Thank you very much mr Rick, about the start of the new week i wll inform you about the propellers i have found. have a nice day.

 

Hello again...i am coming back with my news. Mr Rick, i got information from different marine stores, one of them also gave me for free a 9x10 3-bladed,used aluminium propeller, witch may be good for as. Also i can have whatever bronze propeller i want for different diameter and pitch(that i think is very good). But one of the technicians told me that it is possible the 2hp electric motor can not move the boat because of the big diameter(i mean the 8 or 9 or 10 inches, we have calculate), and told me to see for smaller one propeller(like the outboard 2-2.5 hp, but these have different rpm from mine). My opinion is that with a bigger propeller i can get better efficiency(java prop, am i wrong?These are my news from the stores, probably i will build a new propeller with my requirements in pitch and diameter. Is it possible to use the free one propeller i told you i got? Also i would like your opinion for the two cases, in the rpm selection for the 9x10, and also your opinion on rpm and pitch for a 8 inch bronze propeller for a total weight of my boat about 200-220 kgs, as i think it is.Good night from Greece.

 

You will get better results with the largest diameter you can fit providing it is not overpitched. The 9X10 might be too higher pitch but it is worth a try. You can determine how it will operate using JavaProp.

For the heavily loaded case you are still better off using a large diameter prop to the maximum you can fit but you need a smaller pitch. Use JavaProp to work out the pitch based on the initial drag curve provided. Will probably be a 9X7 if you can get one.

The smaller diameter props will reduce efficiency but will still work. See what you can get and use the software to check it out. If you have trouble doing this then let me know.

Rick W.

 

Nikolas
I have looked at the suitability of the 9X10 prop with the boat in heavy displacement at 250kg. You could expect around 4kts. This is on the basis that motor is current linited and you are running at 1.1kW at 1100rpm. This means you will be running at rated current if the motor is designed for 2HP (1.5kW) at 1500rpm. The prop efficiency is down to 48%.

If you could get a 9X7 prop then you can get full motor power and run at 1500rpm but speed only gets to about 4.5kts.

The message is to try to keep the boat as light as possible. There is a point of diminishing returns from the perspective of loading it up with big batteries. It is not an efficient hull for good performance with low power once weighted down.

If you can keep the total weight down to 150kg then you could expect to get 8.5kts with your 9X10 prop. Drag at this speed is 235N and the prop efficiency is 73%. So the 9X10 will be a very good match if you keep the weight down. 8.5kts is an impressive speed for a low power electric boat. You might have to adjust your position in the boat to get is to trim to gain this speed.

Rick W.

 

thank you very much mr rick for your advice, tomorrow probably i am going to order the motor, for the propeller selection i will decide int he weekend and i will work with javaprop, i will let you know about my decision, have a nice day.

 

Will,

You speak about building a stainless propeller as if it was pretty easy to get a good working high efficiency propeller, is it so? I would be very interested to know how I can most certainly weld stainless flatbar, I work in a metallworkshp, I running our digitalmachines like the abrasive waterjet and bending machines.
I´m also thinking about putting an electric motor in my boat, but its a bit lager then Bairachtn´s boat, its 13.5m*2.55 meters sailboat.

Would of course be better with a folding or pitchable propeller, but maybe a fixed is the only way to get good charging when sailing?
Maybe I could even build a pitchable propeller, maybe something like the Kiwiprop, but that could be locked.

Will, how do one shape massive stainless to the airfoilsection one want? Have any good advice? Would be very interesting to know more about this propellerbuilding technique you mention!

Best Regars
Robin Larsson

 

Robin
I weld stainless steel with stainless welding rods. It is not much different to welding mild steel. The props I make are lightly loaded. If you want a prop for a 13.5m sailing boat you should be able to buy a suitable prop.

For my solar-wind boat I have been looking at a Perm Motor PMG-132 motor running at 48V with a 2:1 reduction drive with a Sevcon 4- quadrant controller. The gearbox will be a ZP3 2:1 box running submerged but with a lubricating oil head. It will be able to be dropped in and out so is easily serviced and does not have to be in the water its entire life. I have operated smaller right angle drives submerged without any problems.

It would be possible to make a heavy stainless prop but finding a suitable hub could be difficult.

Rick W.

 

Rick,
Thanks for your answer!
The workshop I work in weld alot of stainless, thats most of what we do, build primarily windows for yachts So that is no proplem. Could probably build the hub to, if I wanted to.
But as you say, shouldnt be to hard to find a suitable propp. I´m thinking that a Volvo Penta 3bladed folding could be nice. Maybe modify it so I can lock it in extended position.

The thing is that I really want to be able to charge good when sailing, with a regenerative controller. I´m thinking about using a Perm 132 with a Alltrax 7234 controller, or maybe that one doesnt have regen? Maybe a Millipak would be better? But I want a 72Volts system, to get as much power as I can, when I want to.

I was thinking that maybe I could use the bottompart of my Penta 110 Saildrive leg, just rebuild the top, and connect the motor verticaly, with just a reductiongear of some sort. But maybe it would be to unefficient when sailng+charging? It would be much easier to rebuild the saildrive, than to cut away alot of the skeg to get a good position on a shaftmounted propp...
Already have the hole and such for the saildrive.

I have more questions, and its hard to get good answers about electric drives here in Sweden, sailors here are extremely traditional... Should write I new thread about it, but not now, need to sleep, its midnight here

Regards Robin

 

Robin
The sail drive leg is a really good option. I had a look at the Yanmar legs:
http://www.yanmar.com.au/marine/sail_series/sail_series.htm

You will see the SD20 leg should be a good choice. I would be interested in the price if you get that information. The gear ratio of 2.6:1 should match the Perm PMG-132 reasonably well. It does 3480rpm on 72V so you would have 1340rpm at the prop. This would enable a fair 2-bladed prop - something around 70% efficiency would be possible. The prop would be 18 x 11 assuming the boat takes full power to do 4m/s (say 8kts). The motor has good overload capacity so you could run a bit harder into a head wind if you needed.

You could get prop eficiency up over 80% if you spin it slower. Something like 800rpm with a 2ft prop but you will waste some of the gain in the added gearing.

The Sevcon 4-quadrant controller should work well with the Perm PMG-132. I believe it will provide good charging once the boat is doing a few knots. You should be able to set the regeneration current limit to prevent stalling the prop. I am not sure if this controller can be set up to prevent overcharging the batteries. You may need some extra control.

I have a prop fabrication procedure if you wanted to have a go at making a prop but if you are going to buy a sail drive then you should be able to get a nice 2-bladed prop to suit. I am not sure if a folding prop would work with regen it might want to close up.
http://www.australpropeller.com.au/fixedyacht.htm

Rick W.