Propeller pitch & outboard performance?

[Tim Good]

How much difference would changing the propeller on an electric outboard which doesn't have a gearbox?

Why I'm asking: I say this because I changed the prop on my 3.5hp which has just the right pitch to allow me to plane in my Avon tender at 12-15 kts... It has just enough torque to get up on the plane and then boom im off and I can throttle back.

So... now I'm using a Toqeedo 1103 I'm wondering.... it clearly has a lot of torque. You can feel out it can push through waves and wind. But it's little two blade prop with very little pitch doesn't seem to generate more than 4kts regardless of the power. It just feels like I'm only about 15% away from getting up on the plane but there isn't enough speed in the prop.

Would a different propeller on an electric outboard, with no gearbox, benefit from a different prop or am I totally kidding myself here?

 

[Hot Property]

Assuming the two outboards produce the same power then in theory a re prop would get you close to your original performance.

Propping is a black art at the best of times ( as I step over the many that I have bought in my garage......).

Propping an electric outboard may prove just as difficult. Good luck!

 

[rogerthebodger]

What you need to do i look at the motor power /torque curve.

And the propeller power curves

Superimpose the two for the motor and the prop dia and pitch to match the two

 

[duncan99210]

Do Torqueedo produce props in varying pitches? I have a feeling that their props are designed to fit their motors so finding a prop to fit may be the biggest hurdle.

 

[Mikedefieslife]

You saw you feel about 15% off getting there. That could be right since the Torqueedo has almost exactly 15% less power than your 3.5hp 2 stroke.

 

[lw395]

The question is, is the outboard able to rev enough?, or is does the prop have too much pitch and area, preventing the engine from developing its best power?

Power developed by an engine is torque x rpm x a constant.

Thrust developed by a prop is efficiency x engine power divided by boat speed

So, when the boat won't go any faster, either you need more power, or:
If the prop is revving above the motor's max power point, you need a bigger pitch or diameter or blade area
If the revs are below the max torque point, you need a smaller pitch/diameter/blade area.

Or you can increase efficiency or reduce the drag of the boat.

The Torqeedo 1003C, allegedly 'comparable to 3HP petrol' has 480W of propulsive power. 1HP is 748W or so. Go figure.....
Its static thrust is 68lbs or 303N. 480W at that thrust suggests a speed of 1.6m/s or about 3.2knots, suggesting the thrust must be dropping off before your dinghy gets on the plane?

 

[QBhoy]

Not sure about the answer to your electric motor issue...but by god...I’ll buy your 3.5hp from you in a flash. 15 knots ? You should get that thing on the big boat instead, haha.

 

[Tim Good]

Not sure about the answer to your electric motor issue...but by god...I’ll buy your 3.5hp from you in a flash. 15 knots ? You should get that thing on the big boat instead, haha.

I know it’s quite funny. People we need notice we have a 3.5 and then later they see me whizzing around and wonder if I’be been hiding a away a big 6hp somewhere. Anyway I’m only 60kg and with a big bag of shopping it tips the balance and I struggle to plane... so clearly my weight is just on a margin of its abaility.