How do you compare electric motor power to diesel power?

More on torque. Units of torque are a distance multiplied by a force; for example foot lbs.

1 lb of force applied at the end of 1 foot lever results in 1 ft lb of torque at the fulcrum of the lever, which is also 12 in lb of torque and 192 in oz of torque.

 

I was using this formula or attempting to. think I see my problem. Power is lbs ft per second and RPM is revs per minute. Need to convert one or other to same time period.

 

yup gotta find all your common denominators and then a few rules of thumb. 1% loss for any gear, belt, chain, bearing, turn in your system including all the stuff in your transmission. Huge amounts for the prop. Could be anywhere between 40 and 90% efficient depends on a lot. So try a prop calc to get minimum slip and required HP. Might make all this a lot easier.

cheers
B

 

thanks Boston. The trolling motor has the prop on motor shaft so no belts, gears, nada. I've looked at prop calcs but they only go up to 4 blades and are primarily aimed at planing speeds and high horsepowers.
Do you know of one includes 5 blade props and motor HP 1 to 3 Hp?

 

The one I use is called Propcalc and does have consideration for planing hulls

but a trolling motor is never going to get you up onto plane. Just doesn't work that way. Your gluing two fiberglass hulls together, adds weight to remodel to that extent. Also might not be safe to punch that thing up to planing speeds unless you throw full length stringers in it = more weight. Sounds also like you started with a displacement hull anyway so not real conducive to planing. Hate to be a stick in the mud but horses for courses is a major rule of thumb around here.

displacement speeds are hands down the most efficient, might want to calculate for that instead. Planing forms are a whole other animal.

Cheers
B

 

Boston
I never ever dreamed of planing this boat! Least of all with trolling motors! Hahaha.
My point was the prop calcs didn't work for me. Because they were aimed at planing boats. Mine is displacement.
I've towed all over the world at 7 knots. I like 7 knots. I'm a 7 knot captain.
The most I ever expect from IntrepiDos is 7+ knots motorsailing.
I'll look at Propcalc. Thanks

 

ah I misunderstood, my bad

 

JavaProp is a wonderful calculator for displacement hulls.

Check it out, it gives a lot of information and is very "tunable".

-Tom

 

I downloaded javaprop 2 days ago. Can't make it work. No matter what values I enter, even tiniest, it says "Model failed, try unloading tips)
Can't find instructions how to use it. Do you know a link for a tutorial?

 

In addition to the inefficiency of the prop, you also need to take into consideration the efficiency of the electric motor. If the electric motor is being run at the design speed and power it will have its rated efficiency. When you run the electric motor at part speed and high power, the electrical current is higher and the losses are greater. The winding losses are the current squared time the resistance of the windings. So the losses are a square of the current ratio. If the motor has a 10% loss at its rating point, if you increase the current by 20% the losses go up by the square of 1.2, or to put it another way they go up 44%, or to about 14.4%, it adds up fast. I'm simplifying things here, but you get the idea. You can't assume that the motor efficiency is a constant, it isn't.

If you calculate the thrust you have (or do a pull test to verify what it takes to drive your hull) and then compare your measured current to what thrust you are producing you are going to see that the prop and motor efficiency isn't very good.

 

I have done the most important tests. I have run the boat with the Minn Kota motors, Hydrofan props, and Propguard Kort nozzles.
The props and nozzles create a lot of drag under sail. I estimate they rob an average of a knot and a half of my sail speed.
It takes very little power to overcome this free wheeling drag. Motorsailing, a mere 12 amps increases my speed over sail alone by 2 kts. one & half knots of this is eliminating the drag because the props aren't free wheeling. And I'm getting about half a knot extra push. I get all this 2 kt benfit because the sails already are providing speed and power. It's sort of like when a planes engine screams in a dive. Gravity is pulling plane down, air is rushing thru prop that isn't result of the props work, so unused energy of engine over revs prop. The sails are doing for me what gravity does for the plane. Hopefully my boat won't dive!
I tested at 60 amps sails furled or doused, motor only. I got 5 kts. I don't intend to motor only, except docking and departing, and don't plan to do that at 5 kts. I just wanted to know what I'd get if I was becalmed and had to get somewhere.
There are 4 ways to reduce propeller drag. 1. don't have one. just pure sailboat. 2. folding prop. 3. CP prop. 4. Motorsail.
The last is my choice in options.

 

Sounds like the tip velocities may be too high. Try a smaller diameter or lower RPM. There is a huge amount of info available on JavaProp, I can't recall how you access it, but persevere, it's worth it.

-Tom

 

thanks. I also downloaded propcalc as recommended by Boston. Doesnt calculate 5 blade props. otherwise gives same figures i calculated

 

thanks for trying Hoyt
I found the same MT link couple days ago.

http://www.downloadatoz.com/multimedia-design_directory/javaprop/tutorial.html

Customize airfoil with this tool.
There's no tutorials from our Editors for JavaProp.

 

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