Picking the perfect prop...........
Most RTR boats come with a propeller that will work in a wide range of applications. Changing the prop will almost always yield a performance increase but most times at a cost of less total runtime, due to higher amps.
When searching for the perfect prop the hobbyist/racer has to balance four important factors.
- Runtime: An increase in prop size will generate more amps(most times), which will reduce runtime.
- Top Speed: We all want more top speed, but sometimes the cost is poor handling and reduced runtime.
- Heat: The faster we go, the more heat in the components. (batteries, motor & ESC. Sometimes this can be controlled by the runtime. For example, OSE has customers that run giant props for 2-5 seconds for high-speed drag runs. They would never run for a few minutes without burning something up.
- Time to plane: The ability of the hull to get up on plane will vary with prop design, less physical prop "tongue" will reduce drag and sometimes increase top speed. But most times at the cost of less take-off speed.
So how do we find the correct prop? There are a few thoughts on how to do this.
- Copy a friend's setup: This will get you close but it's not always foolproof. No two boats are set up the exact same way.
- Experience: this obviously takes time to learn. No one starts out with it, so asking guys on forums and online social media sites can help in this area. Sometimes someone else has already done testing, so a bit of research online can yield the answer.
- Testing: Now, this is the most time-consuming, and a bit expensive if you don't already have the props. But this will yield a faster boat, with fewer burnt-up parts and most importantly the boat characteristics that you would like. So here goes........
TESTING Method:
I suggest starting with a smaller prop and working upward in size to the optimum size. On a RTR boat this is easy, get a prop that's the next size up from the stock prop. For example, a if the prop is a 50mm prop, get a 52mm prop in the same ratio. Never make a huge jump, putting too large of a prop an electric boat on the first test will lead to burnt-up ESC, Motor, or Batteries. For a boat that you have built, contact the hull maker, they should have a starting point for you.
Remember, bigger is not always better in every case, this is particularly true with high RPM motors(35k-40k total rpm range). To run at the best efficiency, a high-rpm motor needs to spin to that rpm. Too large of a prop will not allow the prop to reach its best RPM efficiency range and will cause heat build-up in the ESC, Motor or Batteries. On the flip side, if you put too small of a prop on the boat, you will end up with a boat that doesn't get on plane due to the propeller not providing a large enough thrust cone to move the weight. Here are some examples of prop diameters with boat sizes.
Start with a 1.4-pitch ratio prop. An example of one is the Octura X4 series(x442) and OSE cnc 1.4" props(4214250).
18-20" length range, a prop in the 27mm diameter.
21-25" length range, a prop in the 30mm diameter.
26-30" length range, a prop in the 38mm diameter.
31-36" length range, a prop in the 42mm diameter.
37-42" length range, a prop in the 47mm diameter.
42-50" length range, a prop in the 55mm diameter.
50" length range, a prop in the 55mm diameter.
60" length range, a prop in the 60mm diameter.
This testing requires you to test for current draw and heat build-up in the motor, ESC, connectors, and batteries. If this isn't done correctly, you can burn up your parts or worse yet burn up your lipo cells. Causing harm to yourself or fire damage to the surrounding area.
Measuring Temps: Measurements should all be taken immediately after a run. Measuring even 60 seconds after will not yield accurate results.
-Motor temps should be in the 120-130F range. Measure inside the vent holes to get inside the motor. Measuring the water jacket or Motor housing is not accurate.
-ESC temps should be in the 100-130f range.
-Connectors, measure the heat of the connectors. High heat in the 160f plus range will mean the connectors are not adequate enough OR you are drawing more amperage than they can handle.
-Batteries, batteries should be in the 110-140f range. Higher heat can mean that the batteries can not handle the amperage required to power the system OR that they are not the correct c rating or mah. It can also mean that they are being drawn to low. WARNING, any pack that's puffing/expanding beyond its original size is a bad sign. Puffed packs are packs that are not handling the system well and could lead to a fire. Dispose of them properly.
This method will work well with lipo chargers that will provide mah (milliampere-hour) that was charged back into the pack after a charge. Most decent lipo chargers will do this these days.
Step one, fully charge your lipo pack.
Step two, run the boat for a couple of laps at full speed for an exact measured amount of time. 30 seconds is usually good.
Step three, quickly recharge the pack and take note of the amount of mah put back into the pack.
Step four, click here https://www.offshoreelectrics.com/conversion.php and plug those numbers into the first table on that webpage. While you are there, take a look around the page for other useful formulas listed there.
Things to remember
- Moving the COG (center of gravity) forward or backward will help to get on plane quicker, which also changes handling characteristics(that's for another article). For most monos and catamarans 30% forward of the transom is a good place to start. To check the COG put your left and right finger 1/3 of the hull length forward of the transom and have the boat balance in your fingers.
- If the boat will not come on plane, and the motor seems to be at full RPM the prop is cavitating. You may need a larger prop with more thrust to get on plane.
- A well-balanced and sharpened prop will always spin at higher RPM allowing a higher top speed!
- Always monitor motor or engine heat! Testing on a 50F day will yield different results on a 90F Day.
- Different water conditions will achieve different results. When comparing test results from a different day make sure the water and wind conditions were similar!
The points I have touched on are only a small sample. I hope to add as I have time and think of additional thoughts. But there are many other topics on the forum about these ideas and methods. Check it out.
Important links, click to follow them
Tool to make some top-speed estimates!
IMPBA Racing Association
NAMBA Racing Association
OSE Forum
