Resizing a prop for a small electric outboard.

[Matthew Dunk]

Hmmm… the speed issue needed to be resolved so I grabbed the phone and went for a spin. If Navionics on a Samsung is to be trusted, and assuming the PWM speed controller is linear, I got better results than I expected.

In flat windless conditions, just me in the Highfield, fully charged bike battery…

Full power, 3.4 knots.
Half power, 2.1 knots.

There’s a “sweet” spot (quietest) around the 2/3 mark on the potentiometer which gave me 2.8 knots.

Next task will be to measure current. A bit harder, my multimeter maxes out at 10 amps (it’s more for electronics work), but I’ve got some nice 30 amp shunts in a box somewhere which I will dig out and attach.

I’ll phone around some outboard wreckers and see if I can source a cheap spare prop and have a play, might be simplest approach.

Pretty damn pleased about this project, I really didn’t expect it to go so fast.

 

[Matthew Dunk]

Now I am thinking that if the original engine did run at closer to 6000 rpm then the prop is CLOSER to correct pitch than I first thought…

Had to go back and think that one through a bit.

 

[C. Dog]

Most outboards set 5500 RPM as the WOT maximum RPM, but they have to manage up to about 8000 momentarily or they will be exploding when the prop ventilates. For future reference, there are loads of electric trolling motors discarded in this country because of failures not involving the motor, already sealed and a roughly streamlined housing, though many folk are placing foils over the submerged part of the shaft on small outboards to pick up a bit of performance.

 

[Matthew Dunk]

Most outboards set 5500 RPM as the WOT maximum RPM, but they have to manage up to about 8000 momentarily or they will be exploding when the prop ventilates. For future reference, there are loads of electric trolling motors discarded in this country because of failures not involving the motor, already sealed and a roughly streamlined housing, though many folk are placing foils over the submerged part of the shaft on small outboards to pick up a bit of performance.

I considered a trolling motor, but frankly, the ones I tried were utterly gutless. Not surprised, they were all 12 volts, mostly less than 250 watts. Also, for all the efficiency gains, I still read plenty of posts from people who’ve lost their outboard to water intrusion into the motor housing.

But mainly, if I’d bought some ratty old trolling motor I’d then have had to spend even more money on a lithium 12 volt battery of at least 30AH. (lugging some FLA lump of lead in an out of the dinghy would defeat the whole purpose.). The joy of my little design is I could make use of the multiple pre-existing ultra light 10AH 36 volt batteries we carry for the electric bikes and in the process get a little more power without the problems of handling big currents. And it has worked way better than I’d expected.

I think the scope of the actual question I asked is a bit beyond this forum, maybe trial and error from here, if I can find a cheap spare prop. Given the current performance it would be an exercise in curiosity only.

 

[C. Dog]

Your original question was regarding "limiting RPM"; and that is IC thinking, steam and electric work a bit differently. The diagram below may help you understand that. There are also some videos of a competition in the states for printed props on an electric test rig. Here is the first.

 

[Matthew Dunk]

Your original question was regarding "limiting RPM"; and that is IC thinking, steam and electric work a bit differently. The diagram below may help you understand that. There are also some videos of a competition in the states for printed props on an electric test rig. Here is the first.

No, my original question was how to reduce the amount of propellor, not “limiting rpm.”

And I already know about the power curve difference between IC and electric. I remarked on that early on.

And no, I don’t want to go with steam.

Enough now, there’s nothing more to be gained here. Someone posted an interesting formula for calculating the appropriate diameter. I think I’ll play with that formula for a bit, see what it suggests I should do. Plus I’ll take a side trip on learning whether shortening the chord depth or reducing the diameter is likely to be a better approach. (Since 3D printing a prop, no matter how clever, is not economical or rational in this context.) And then I’ll see if a spare prop can be obtained cheaply enough to be worth the experiment.

 

[C. Dog]

Good luck.

 

[Matthew Dunk]

Good luck.

Thanks. I’ve been pretty lucky so far. Here’s to hoping my luck holds. (But I’ll bring the oars. Just in case.)

 

[Rumars]

Based on the speed results you are indeed very slightly overproped, but getting it right is going to involve more data. The Highfield 3.1 has a waterline lenght between 8-9ft, the displacement speed should be 3.8-4kn.
Your outboard leg has gear reduction, motor rpm isn't prop rpm, and you must know what it is. You can find that out by determining the original outboard model the leg comes from, or you measure it directly.
For example the current 4 stroke Mercury 2.5hp has 2.15:1 gear reduction and the standard prop is a three blade 7"x7.14" pitch prop.
You don't need to cut the prop down, there are replacements on the market. Or you can simply call it good enough and use it as it is, it's not like you can row faster.

 

[Matthew Dunk]

Based on the speed results you are indeed very slightly overproped, but getting it right is going to involve more data. The Highfield 3.1 has a waterline lenght between 8-9ft, the displacement speed should be 3.8-4kn.
Your outboard leg has gear reduction, motor rpm isn't prop rpm, and you must know what it is. You can find that out by determining the original outboard model the leg comes from, or you measure it directly.
For example the current 4 stroke Mercury 2.5hp has 2.15:1 gear reduction and the standard prop is a three blade 7"x7.14" pitch prop.
You don't need to cut the prop down, there are replacements on the market. Or you can simply call it good enough and use it as it is, it's not like you can row faster.

Yes, I do think I can call it good enough, but I’m a tinkerer and I know I’ll always be wondering.

However, the fact that I’m not reaching hull speed with the current setup does not necessarily mean I’m over propped. I think it’s VERY unlikely that 350 Watts, no matter how efficiently applied, is enough to get the boat to hull speed with my 6 feet of bulk on board.

(I will check the reduction ratio when I can, but I’d bet the ratio on the four stroke would be a lot higher. Can’t imagine them revving that thing to 5500 or 6000 rpm as has been suggested for the 2 stroke version.)

 

[portacruise]

No, that’s my response to someone so anxious to prove what they know that they didn’t actually read what I’d written.

What does that statement have to do with anything, even if it is true?

I guess I'm just more pragmatic, and focused on my approach which is likely to get me help when I need it, regardless of personalities..

I learned a lot more from my competent, arrogant, demanding and even obnoxious College professors, compared to the extremely popular ones that gave out high grades- with their Cracker Box courses.

But of course, that may not be true for everyone!

Good Luck!

 

[fallguy]

I have a small 2.5hp motor on a 16’ boat. My primary beef, well, 2nd actually, is the prop is too small for any low rpm work. I like trolling at say 2mph, but the prop won’t support that in a headwind, so I can only troll downhill.

if your use case is to achieve top speed, then I’d shop for a smaller prop before chopping one up that is running at 2700 against 3600 max…It sounds pretty close to ideal to me

my 2.5hp motor has no room for a bigger prop; did not look real hard at pitch options