Question on prop for min mud motor

Mark W

Mark W

Well-known member
Need a littel help. The mini mud motor is still just shy of working the way I would like it to work. Rather than buying a "torquier" motor, I thought I would try to trim the prop a little bit first. the current prop is a T-10 from young's Prop. there are no specifications on pitch and the only information I can get on other props from them is that I should try to cut this prop down a bit. It is currently a 5 3/4" counter rotating diameter prop.



Taken from antoher site and posted by Rob:

According to Dave Gerr's propeller handbook and doing the math, a 3" prop will let a weed whacker turn to its full rated rpm. Here's some math for y'all. Suppose your boat can be driven to 4.75 mph with 1 hp, which I'm assuming a weedeater makes at 7000 rpm. All these are for 2-bladed props. Here's what the calcs tell you:

7000 rpm (1:1) - 3.12" x 1.92" prop (not much prop surface area here)
3500 rpm (2:1) - 4.73 x 3.81" prop (Already WAY better)
2800 rpm (2.5:1) - 5.41 x 4.81" prop
1750 rpm (4:1) - 7.17" x 7.7" prop (this is the Lil' Indian ratio)
1120 rpm (5:1) - 9.37 x 12.03 prop (getting steep, I wonder if it would start well from a standstill)

Notice how the 4:1 ratio gives you close to a "square" wheel - e.g. diameter and pitch about the same? There's nothing magical about this, but it is an indication that you're somewhere in the reasonably efficient range, ASSUMING those numbers are indicated by the formulae. Arbitrarily using a given prop will get you the problems that others have found, like stalling or lack of thrust.

So, my question, how much should I cut this prop down to get the motor to work? Appreciate any responses.

Mark W
 
A little at a time. I would start with a 1/2", try it out,then another 1/2". when you get close do 1/4" cuts. I'd hate to see you take it all at once and have to start all over again. I know it doesn't take much difference in size and pitch on a regular outboard to make a big difference.
 
Seems to me the unwritten rule of thumb is an inch of pitch is worth two inches of diameter. Doesn't help here unless you figure out what pitch you have.

I have forgotten what gear ratio you have but a 5 3/4" dia seems a bit large for a 1hp motor. As Lee stated take it down a bit at a time until you are happy. I know the prop guy we use has a program and his experience to go on but I would hazard a guess that this would be uncharted waters for him. I'll ask him anyway.

What rpm and gear ratio are you looking for?

I'll look at my 2 hp outboard to see what it runs for a prop.

Eric
 
That is the problem with this design, there is no gear reduction and I am not running a clutch either. I have heard that you only run at 1/3 - 1/2 throttle on these motors with the Youngs Prop T-10. My experience and the experience of others say that if you try to go full throttle, you will bog out and die. this does happen. The ideal solution would be to run a 3:1 reduction from the 7,000 rpm motor that I have. I cannot find a reasonable gear reducer that can be modified to fit these types of motors. Staton Inc has a 3.1:1 reducer but at $129, it more than doubles what I have into this motor.

I can almost get it to go, it is just a little shy of working well. I can run the prop about 3/4 in and the boat goes OK. When I do get the whole prop in and it "bites", the boat takes off. If I could only get this to be a full time event.

I do have eyes on a 2.5 hp motor that would probably take care of things but I would like to see if I can get this to work all the time with the whole prop in the water.

Appreciate the help.

Mark W
 
Without the converter, when you jam the motor into the water it would act like pushing a weedeater into a big clump of grass. Can you lower it gradually while picking up speed so the torque didn't overcome the motor? I wouldn't think running at a constant 7k rpm would be good for the motor.
 
One other thought. Make a wooden prop. Laminate several pieces of wood together and carve the shape you think it should be. Pitch and diameter. When you find the right combination have the wooden prop cast in aluminum.
 
Without the converter, when you jam the motor into the water it would act like pushing a weedeater into a big clump of grass. Can you lower it gradually while picking up speed so the torque didn't overcome the motor? I wouldn't think running at a constant 7k rpm would be good for the motor.
Click to expand...


That is how the motor is run. You kind of get it going out of the water and slowly lower it into the water. It works well with 3/4 of the prop in the water and it will go for 100 feet or so with the whole prop in the water, then it dies. When you run the motor in the water, you are never at full throttle. It just doesn't work well that way with the prop partially or all the way in.

It is so close I can just about taste it.

I like the idea of the wooden prop. I don't know if I would have an aluminum one cast. What would be the cost for something like that any idea. The bigger motor (almost 3X the horsepower of current motor and more torque) would cost me somewhat slightly north of $100. I'm guessing, and maybe wrongly, that that much more HP and torque would get me to the promised land.

The project that keeps on giving.

Thansk for the help.

Mark W
 
Mark

Since your heart is so in it, I have some birch plywood left over from my airplane prop business. It is thick enough for a mini prop. Send me your shipping address and I'll send you a piece big enough for a couple of experiments. Casting out of aluminum isn't so hard. Getting the hole straight with the axis of things might take some doing.

Mark F. Cheney
 
Hey Mark -

Thank you for the kind offer. I'm not sure if the homemade prop is the way I am going to go yet. The only reason I'm thinking this is that I don't think I am talented enough to carve one.

I'm going to trim the one down that is currently on the motor and see how it works. If it doesn't, I'll be emailing you my address. Or, if you would hurry up and get that mini mud motor done, I could just buy myself one.

Thanks again Mark for the offer, it is appreciated.

Mark W
 
to even venture a guess. His first question was "Why?" hehehe.

Checked the 2hp Johnson and the 3.6hp Scott Mac and they have a 6.5" and 7.25" dia respectively for props.

The 3.6 has a 1 to 2/3rds ratio. The 2, I couldn't tell.

Sorry I couldn't be of more help.

Eric
 
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