Understanding Longtail Props
Rule #1: Know your RPM's
You need to know your RPM’s = Revolutions per minutes.
This is accomplished with a tachometer.
Rule #2: Set a Goal
What do you want to accomplish?
Speed? Load Hauling? Weedless?
Revolutions per minute = RPM’s
Revolutions per minute (RPM’s) is how many full rotations of the crankshaft occur in your engine, in one minute. In simple terms for longtail mud motor use = how fast your prop is spinning. It is a 1:1 ratio for engine crankshaft rotation to prop rotation.
Torque is the force, the muscle behind the spinning.
Engines with short stroke crankshafts will gain most of their torque while reving higher RPM’s, while engines with a longer stroke will gain more torque at lower RPM’s.
High RPM’s: Low Torque
When you want speed, you need high RPM’s.
- Standard props operate best below 4,000 RPM’s
- Hurricane props operate best at 3,600-5,000 RPM’s
- Bullet props operate best at 4,000-7,000 RPM’s
- Racing props operate best at 6,000 RPM’s and greater.
Moderate RPM’s: Moderate Torque
To stay weedless, the prop needs to spin fast enough to throw the weeds off the prop, but have enough torque to continually cut through vegetation and the occasional branch. That’s why we designed our own Weedless prop that helps with both cutting through and clearing vegetation from the prop one rotation after the next.
Hauling a Load
Low RPM’s: High Torque
Prop wise, when carrying a heavy load, you want the less pitch on a prop. Our Standard props handle heavy loads very well. The great thing with our Standard props is that they come in multiple diameters, which allows you to really dial in the correct prop size to the load you’re carrying.
Prop Size to RPM Target
Now to actually start putting things together to understand the science behind picking the best prop for the job.
Figure out what your main goal is to determine the ideal RPM range:
- Going fast – highest RPM’s your engine can handle.
- Cutting through vegetation – 6.5″ or 8.75″ Weedless prop
- Hauling a heavy load – Peak of the torque curve (normally ~2,500-3,000 RPM’s)
Going faster – Let’s say your engine governs out at 3,800 RPM’s (governor stops the engine from spinning faster). The target RPM range is 3,750. This way you know the engine isn’t governing out.
Your first test is with the 8″ prop. You run it wide open throttle to see how fast you can go. You get back to the ramp, check you tachometer and see your max RPM was 3,800, and your max speed was 16 mph.
So you move to the next size up which is an 8.5″ prop. You run it again and your tachometer reads 3,700 with a max speed of 18 mph.
In this case, the 8.5″ prop would get you the speed you were looking for as long as it was easy to handle. If you go to a bigger prop, like the next size up which would be a 9″ prop, you may get higher speeds, but it may become harder to handle and your max RPM’s will drop to a level which wouldn’t be optimal.
Carrying a Load – For example: Say you had a hunting trip. You went up river with all your hunting gear and knew your correct prop size was 8.5″. You knew this because your RPM’s are around 2,500 RPM’s where the peak of the torque curve is for your engine.
You get to your hunting spot, have a successful hunt, bag a deer, and start to head back down river. You now have a greater load in your boat and your engine is lugging at around 1,900 RPM’s with the 8.5″ prop.
You need to get back into the torque curve of your engine at 2,500 RPM range by going down in prop size. You change the prop out to an 8″ and at full throttle, you are now reaching 2,450 RPM’s which is close to your peak torque curve target.
- High RPM’s: Increased RPM’s = Faster Spinning Prop = Increased Speed
- Pitch over Diameter: Pitch is your friend when it comes to speed, not diameter.
- SPS Weedless Prop: The best weedless prop in the longtail mud motor market.
- Experiment: Know which props perform best with your unique loads.
- Standard Props: Best load performing prop is the Standard Prop.
With different hull designs, engine manufactures, weight distribution in the boat, and kit sizes; there are many variables that can affect prop selection.
The Standard props that come with your kit is a great place to start and will work in many situations. From there you can increase or decrease the prop diameter or pitch (Performance props) based on your needs. Our props are affordable, so that allows you to experiment.
Another great resource is our Performance Stats Page where customers have shared their experiences and results. See what Swamp Runner Owners are running. See what boat they have, the engine, the kit, hull weight, load, prop size, and MPH.
Use all this information as a guide to help you dial in your longtail so it performs the best for your unique set up.
Be willing to experiment and test, and share your experiences with other owners on our Swamp Tribe Forum.