Selecting the Right Propeller

Selecting the right propeller can be a challenging and mysterious task. We understand the frustration, so we’ve created a one-stop resource to give you the key things you need to know, so you can be confident in your purchase.

Will my motor run the Roost propeller?

This is the million dollar question!!! The Roost is a high-performance shallow-water propeller, but nobody likes a one-trick-pony. That’s why the Roost was designed to be versatile so it could be used on a wide range of motors/boats, and in a wide range of conditions. Even if extreme shallow water isn’t your thing, you will still find the unique blend of hole-shot and top speed to be advantageous.

HP Requirements- The Roost is designed to operate with a wide range of engine sizes. We recommend the Roost for use on engines of 23 HP or more. However, because you can adjust the gear ratio on surface drive mud motors, lower HP motors may be able to run the Roost as well. For those running upgraded engines making higher than stock HP, we recommend taking the standard approach of adjusting your belt ratio to fine tune performance to your particular engine tune.

Gear Ratio- The gear ratio used in your motor plays a significant role in your motor’s performance. If you’re running the stock gear ratio on your surface drive, you will most likely have sufficient power to enjoy the Roost. However, some motor manufacturers have changed the stock gear ratio they use over the years. Some of these changes over the years have been substantial and can be quite different from the ratio new motors are shipping with today. If you are running an older motor, you may want or need to adjust your gear ratio to get the most out of the Roost. ***If your motor is currently lugging or bogging with a conventional 10 pitch mud prop, you will need to adjust your gear ratio to run the Roost***

Engine RPMs- The RPMs your engine runs at full speed will vary depending on your specific boat/motor setup, as well as the weight and weight distribution in the boat. We like to compare the Roost to a common 12.25x10 conventional 2-blade mud propeller, since that is a very common propeller that people run. In testing on ultra light boats, the Roost caused little to no loss in RPM (50 RPM or less) compared to the 12.25x10 mud prop, but produced higher speeds. On larger/heavier boats, the Roost ran about 300 RPM lower than the 12.25x10, but also achieved and maintained higher top speeds. In the tests we’ve conducted, regardless of HP or boat size, the Roost provided better response to throttle inputs and more controlled RPMs during hole-shots. The Roost has less slip than conventional mud propellers. If you have too much weight forward in your boat, the Roost may prevent your engine from attaining full RPM and max speed. Conventional mud propellers mask weight distribution problems because their high slip allows them to attain full cruising RPMs even though the boat is not moving at full speed.

Boat Hull- Hull design plays a tremendous role in the performance of your rig. This goes for both speed and shallow water capability. There are so many boat manufacturers and models of boats, it is simply impossible to test them all. The best gage here is if your boat already runs well with a conventional mud propeller, it will almost certainly run better with a Roost!

Tips and Tricks

Trim- Chances are the first thing you will notice when running the Roost is how you trim your motor for maximum performance. Since the Roost has less slip, it doesn’t need to be submersed as deeply as conventional mud propellers to get on plane and to hit max speed. If you are used to trimming down deep to get on plane, you will most likely need to trim up to your running height or maybe slightly higher when starting out with the Roost, so the engine can build sufficient RPMs. When starting out, you will likely need the propeller mostly submerged. Once you are on plane, a good rule of thumb is to run with about 50-60% of the propeller in the water. Every boat gets on plane and planes out differently, so you may have to try different trim settings until you find out what your boat likes the best.

Altitude- Altitude has a significant effect on the power output of your engine. Higher altitude results in less engine power and lower altitude results in more engine power. The Roost was developed and tested at an altitude of 4200 ft above sea level. When referencing our videos, please refer to the description to learn the specifics such as the altitude, boat, motor, HP, etc used in that particular video. We put this information in the video description to help give you context so you can get a better idea of how the Roost performs in various situations.

Engine Break-In- If your engine is brand new, then engine break-in may be a short-term consideration since this will resolve itself with use. Engine break-in can have a significant impact on how well an engine can spin a propeller. All Roost Marine propellers are designed for optimal performance with a broken-in engine, since that’s where the engine spends most of it’s life. During the first 20 hours of operation most engines will have noticeably less power compared to once they are fully broken in, which means the engine could struggle to spin the Roost at first. Once an engine has more than 20 hours, it will start to feel more powerful, and should turn the Roost without much difficulty. Even though a large portion of the break-in is accomplished in the first 20 hours, most engines will continue to gain power for some time. During the development of the Roost, some test engines continued to have perceivable increases in performance until they had 80-100 hours. It is debated exactly how long it takes to fully break-in an engine, and the effectiveness of break-in depends almost exclusively on how the end user treats the engine during the first 20 hours. Since we aren’t an engine company, we’ll leave those details to the engine gurus. Individuals who run a very large/heavy boat or an engine that is on the low end of the HP rating of the boat hull, are most likely to find engine break-in a limiting factor. If you find your engine struggling to spin the Roost, we recommend removing all excess weight until the engine has 20 hours on it. After that, begin adding the weight back. Most engines will have good power by 30 hours, if they’ve been properly broken in.

Terminology

Blade- The wing-like surface which extends outward from the hub. This is the part that pushes and controls the flow of water in order to generate thrust and propel a boat forward.

Hub- The center of the propeller which has a mounting hole to be used for installation onto a drive shaft.

Diameter- The diameter of a propeller is measured as an imaginary circle who’s center lines up with the center of the propeller, and who’s outside circumference lines up with the tips of each propeller blade. Every company measures the diameter slightly differently, so it is common to find that the diameter you measure yourself is slightly different from the advertised diameter. This is also done for simplicity in the sizing designation. It keeps the numbers short and easy to read and say. For example, “12x11” is much easer to read and pronounce than “12.245 x11”. Roost Marine uses the simplified sizing designation. When we measure the diameter of our propellers, we measure the effective diameter, which is slightly less than the tip to tip diameter.

Pitch- In the most simple terms, pitch is the measure of how steep the blades are angled. The greater the pitch (higher number) the steeper the blade angle. In more technically accurate terms, pitch is the distance in inches that a propeller will move forward in one complete revolution without any slip. For example, an 11 pitch propeller will move forward 11” in one complete revolution. Again in the most simple terms, this ultimately equates to speed potential, but it’s not as simple as selecting the highest available pitch. Pitch is only one piece of the puzzle. The most important thing to remember about pitch is that it is measured and experienced differently. The pitch that a blade measures at, is not necessarily the pitch it will run at. This is due to something called slip (see below).

Measured Pitch- This is the value of the pitch which is obtained by pitch measuring tools and equipment. This measurement is usually expressed as the total pitch of the blade, meaning the lowest point to the highest point at a particular radius. The measured pitch is generally what is advertised and marked on a propeller.

Effective Pitch- This is the value of the pitch, except the measurement is derived from a mathematical calculation instead of tools. The easiest way to think about effective pitch is it’s the equivalent pitch a propeller operates at on a particular boat/motor with a particular load. Because propellers slip during operation, a propeller will almost never operate at it’s measured pitch. For example, a 10 pitch propeller may only move the boat forward 9” every revolution. So even though the propeller measures as having 10” of pitch, it operates as though it had 9” of pitch with zero slip. Boat speed and propeller RPMs are used to calculate the actual distance the propeller moves through the water in one revolution. Effective pitch is not a fixed number. It is impacted by propeller RPM, propeller geometry, trim depth, boat speed, boat weight, wetted surface, aerodynamic drag, and other factors. Ultimately, effective pitch is used to calculate slip, which is used to gain insight into how the propeller is functioning.

Slip- This is exactly what it sounds like. Slip is the opposite of grip. In car tires, it’s a bad thing, but in propellers it’s a necessary thing. Slip is what lets the engine rev up in order to make sufficient power to push your boat. Getting the right amount of slip in a propeller is based on how you intend to use the propeller, and can be made to change depending on how fast the boat is moving. Too much slip and your boat will be slow, waste gas, may not get on plane, and in some cases may result in the propeller completely loosing grip (known as “blowing out”). Too little slip, and the engine will bog severely, struggle with heavier boats/loads, and may never get on plane. Conventional mud propellers have very high amounts of slip so they can build sufficient power in mud, which is much thicker than water. This is also why conventional mud propellers struggle in shallow water where soft mud is not available. The Roost has lower slip which allows it to perform even when mud is unavailable. Slip is expressed as a percentage and is calculated as: Slip= 1- (effective pitch / measured pitch)

Grip- This is a term we use to describe the propeller’s ability make the most of the water it has to work with. Grip is not readily quantifiable, but it is an important term to understand. Propellers that have good grip resist over-revving of the engine, make sufficient amounts of thrust even when water is scarce, resist blowing out, and move forward with an effective pitch very close to the measured pitch.

Aeration- This is when air mixes with water and makes its way to the propeller. Aeration is not inherently good or bad, but can have a significant impact on propeller performance for better or worse. The effect that aeration has on propeller performance is based on propeller design. Propellers which are designed for full submersion (no aeration) can experience severe loss of performance when air is introduced. This is commonly experienced as a sudden increase in RPM accompanied by a significant decrease in thrust. It is also possible to design propellers to tolerate and even prefer aeration. Surface-piercing propellers are one type of propeller that are specifically designed for optimal performance in aerated water. Propellers that require full submersion or have a low tolerance of aeration are fine when there is plentiful deep water, but for use in extremely shallow water, aeration is common and often unavoidable. Selecting a propeller that not only tolerates, but excels in aerated water is essential to getting optimal performance in shallow water. And that’s why we built the Roost.