How to Select a Better Propeller
Topic: Gear Ratio and Propeller Pitch Selection Guide.
Question #1: How do I select the correct gear ratio and propeller combination when upgrading my current product?
Response: There is no basic answer to this question while the reality is that nothing replaces having good data to review when making selections. No matter how much estimating and calculating is done, nothing will replace having hard facts on how the existing boat performed with a known setup in known conditions. Therefore here is the approach we recommend following.
Data Collection of the existing setup
1. Record the gear ratio and serial number of the current engine and sterndrive.
2. Record the pitch of the current propeller.
3. Note the condition of the engine. If you know the engine is not currently making the correct level of power this affects the data going forward.
4. Record the maximum speed along with the maximum rpm achievable with the existing setup. Is this recorded with a GPS or the standard speedometer on the boat that may not be 100% accurate?
5. Note how the boat is loaded when recording this data. Is this the normal loading condition for the boat? For example do you normally operate the boat with only 4 of the maximum rated 12 passengers on board? Do you normally fill the fuel and water tanks every time you use the boat? You have to make the choice of how you want the “performance penalty” of weight to be factored in. It could be that you want to make your choice based on running in the worst case loaded condition even though this happens very rarely, simply because it will allow you the opportunity to run at this worst case loaded condition when it happens without damaging the engine. Otherwise if you set the boat up for maximum performance/speed being configured around the lightest case loaded condition (which happens 99% of the time) you will get the best performance however if you do operate in a heavy loaded condition repeatedly or for an extended period of time it may cause damage to the engine.
Use Recorded Data in Propeller Calculator to Select New Setup
1. Now that we have good recorded data telling us how efficiently your particular boat hull goes through the water with a given horsepower engine, and a given gear ratio sterndrive and propeller pitch we can determine the best all around setup to go with.
2. Engine manufacturers provide the maximum rpm range to target when selecting your propeller. If you choose to small of a propeller you will go over the maximum range. In contrast if you select too large of a propeller not only will you probably have a poor hole-shot but if you cannot achieve the lower side of the rpm propping range you will likely damage the engine.
Other Important Information and Helpful Links
1. It is important to understand that the gear ratio of the sterndrive is similar to your coarse adjustment on a micoscope while the propeller pitch is more like the fine adjustment. If you select the wrong gear ratio there will never be enough fine adjustment range in propeller pitches to get you to the correct setup. You always want to select the gear ratio that puts you into the middle of the propeller range so you have direction to move, be it up or down in pitch range.
2. You always want to go with the highest gear ratio sterndrive & largest propeller. Most boaters think they will get more speed by going with a lower ratio setup because it is closer to 1:1 and therefore will turn the propeller faster. The reality is that the slower turning, larger propeller is more efficient because it yields less slip and therefore more efficiency.
3. Normally 1” of propeller pitch equates to about 150 – 200 engine rpm when dialing in a propeller.