electric boat, proper rpm and propeller

Hello everybody, this is my first post, i am nikolas from greece and i would like your opinion on my project. I have a 2.6 meters g.r.p. boat and i am going to make it electric. After a few thoughts i am thinking to use a 2 hp inboard electric motor which will give about 6 knots to my boat. My problem is the correct selection of rpm(i am thinking about 1500 rpm) and the correct propeller diameter and pitch. Your help will be very useful to me...

 

Hi Nikolas
If you have a picture of your boat, main dimension and the intended total weight I can do a drag calculation and performance estimate.

Have you got a motor in mind? Have you considered a commercial electric outboard?

The most efficient propeller will have a high aspect. Do you have draft constraints? It is unlikely that you will get an efficient prop off-the-shelf for this application but it is not hard to make a good prop if you can weld stainless steel.

Rick W.

 

I do not have a picture at this time, but you can see a simple drawing of the boat at the attached file below, also i think that the total weight should be about 250 kgs with me and the other stuff inside(motor, batteries,etc). For the motor, i have attached another file, where you can see the motor i am thinking of(http://www.leeson.com the low voltage one(24V). Also, i am going to "builld" the dc controller alone so i do not want to use a commercial product. But as i told you i need your advice, and calculations for the proper rpm and propeller if it is possible because this project is my university diploma thesis. I am studying electrical and computer engineering at patras university, Greece, thats why i am going to make the controller alone and thats why i need the math calculations i told you, unfortunately we do not have a naval engineering dept at Patras to take advice . I am looking forward to your response.

 

Nikolas
If it is a university project then you should be doing learning as much as you can about the design.

For a start you can Google JavaProp and run the prop design Applet. You have to set thw Options page for water. Density is 1000 and viscosity is 0.0000013.

You then go to the Airfoil page and set the foils to E193 at Re100000. Set the angle of attack at 0 degrees.

Now go to the design page and set some prop parameters. Start with a diameter of .3m", rpm 1000, spinner .03m, power 1500W, speed 3m/s (6knots). Then press design. You can play with the paremeters to find the combination that gives best efficiency.

I will do some numbers on the boat drag later today and send you a curve. This will show if 6kts is possible with 2HP.

Rick W.

 

Nikolas
I have attached an image of the hull and a chart showing drag versus speed. It is heavily loaded. The drag is very high for a small boat because of the shape of the stern and the load it is carrying. 250kg is a lot for a 2.6m boat.

The hull as you have drawn is made for planing so is not ideal for only 2HP. It would probably plane with this power level if it was much lighter.

Anyhow you will find it will almost do 6kts with the 2HP so it should meet your objective. I can modify the shape if you think it is not a good comparison. Send a photo if you can get one.

If you have trouble using JavaProp then make a post with your problem.

Rick W.

 

Mr Rick thank you very much for your help...i am going to use the java applet you told me, and soon you'll have news from me...also i would like to tell you some things. I know that 2 hp is not enough for a planing boat, at 10 knots savitsky method gives me about 5 hp, so i decided to reduce the speed at 6 knots because more hp means more Ah which means more batteries and much more weight for my boat and you already told me it is heavy enough. About the weight maybe it will be a little lower about 200-220 kgs. Also my boat is about like you draw it, tomorrow i' ll probably send you a photo of my boat. Also when you say drag what exactly do you mean? I am going to post my news soon. Thank you very much again.

 

The drag on the hull is a result of water friction/turbulance at the surface of the boat and the wave energy required to move the hole in the water that supports the boat. The values on the chart provided are in Newtons. The drag is dominated by wave drag.

The data produced by Javaprop will have a thrust figure in Newtons. You have to iterate the result from JavaProp to match the drag for a given speed in the chart I have provided. Each iteration will require the velocity setting to be changed in Javaprop. Speed in knots is almost twice the value of m/s.

This hull is intended for planing so is not ideal for displacement mode. With reduced weight and adjusting the trim so it is down at the bow it might be possible to lower wave drag.

You should be able to get prop efficiencies around 80%. It is not practical to make blades that have a chord smaller than say 50mm to take the thrust loads. Also the Reynolds number is more like 300,000 so use the E193 300,000 data.

I have a more flexible and precise prop design method and if you want to actually make a prop then I will provide a design. However JavaProp will enable you to understand the variables and gives a reasonable result. These props are high efficiency for low loads not like a normal outboard propeller.

Rick W.

 

thank you again, i will work on it and soon i'll inform you about my results...

 

by the way mr rick, have you any way to calculate the displacement of the drag you send me if you know that the height of the red area is about 10cm?

 

Yes.

To support 250kg the back of the hull is down 17cm if the keel is horizontal.

When it is at 10cm at the stern is the keel horizontal or inclined with the bow down.

Rick W.

 

At 10 cm it is horizontal, but when it is about 10 cm in the sea, i have not put any batteries & motor, show the weight should be about 150 kgs. If you can make two calculations 1st with 250 kgs and 2nd with 150 kgs as i told you, you would help me very much...thank you again.

 

Mr Rick here are some photos of my boat...maybe will help you for the better comparison you told me, but your comparison is very good too i think...

 

It should be noted that the bottom looks by the photos to be very ruff, which will effect the drag calculations. Spending a day or so sanding the bottom and painting it will produce better performance results. I just thought I would mention it before you loaded it up with batteries and mounted the motor. You are probably going to find that there is not going to be sufficient clearance for the diameter prop you need for maximum efficiency; at least not at an acceptable thrust angle. I am interested in hearing more about what prop you are going to use.

 

Nikolas
I have revised the hull shape slightly based on the photos and reduced the displacement to 150kg. It needs to have a draft of 115mm for this.

The predicted drag is much lower as you can observe in the attached chart.

Rick W.

 

Nikolas
I noted that the hull has an outboard bracket. You should consider making an electric outboard as this will be a lot easy to set up and do subsequent work with.

All you need is a small right angle gearbox. There are many manufacturers. The nest I know of are Mitrpak:
http://www.mitrpak.com/
You can get version with 2:1 reduction. This would allow running a big diameter prop to maximise efficiency. It also means you can just mount the outboard in position once you have water deep enough. Could make it tilt like a conventional outboard if you want to get more complicated.

The outboard also avoids the need for a rudder. Just swivel the outboard to direct thrust.

Rick W.