With the boat back in the water the most pressing issue was to get the propeller pitch right. Farron Peffer at Beta Marine was adamant that we should strive to get the max RPM at wide open throttle (WOT) within 50 RPMs of the designed 3,600 RPM. He explained that even though we might never operate at max RPM having the pitch and diameter of the propeller correct would insure the engine was operating in its sweet spot at any RPM and would not be working in an overloaded capacity which could shorten it’s life. The other thing Farron mentioned was you want to make sure you can achieve the full 25 HP if necessary and you can’t do that if you can’t attain max specified RPM when under load. Last, he said it is better to be slightly over max designed RPM than under.
Throughout the engine installation project I was in regular contact with the very professional Nina von Bergan at FlexOFold. She was patient and quickly responded to my many questions. I reached out to her as soon as we got the boat into the slip and she said they were determined to help me get the propeller correct for the boat. She was surprised and maybe a little skeptical that we were 600 RPM off their calculations. She asked me to confirm the reduction gear ratio. Which I did. We had it right–2.6:1.
At the same time Farron at Beta was also surprised by the low RPM. He told me to check the max RPM in neutral. The new engine literature came with a test sheet from the factory that said the engine should make 3,800 RPM not under load. I checked and we could only make 3,450. Farron was not having it. He said the Beta engine tachometer is not right. He was very confident the engine would reach specified RPM in neutral. “Go to Harbor Freight and purchase an photo optical digital tachometer for $35.” So, I did. I used the included reflective tape and place a small square on the fan belt hub. At wide open throttle the photo tachometer read 3,789 RPM. So the engine was fine but the tachometer was off. Farron said that the tachometer can be adjusted with a small device that plugs into a connection in the back of the tach. He said he would mail it to me free of charge.
In the mean time to get the propeller blades exchanged by FlexOFold the blades could only be in the water for eight days. So, with the photo tachometer in hand Gayle and I took the boat out and made a test run. With the engine cover off and Gayle at the helm and throttle I took RPM readings on the engine at various speeds to coincide with a chart I made up. The bottom line is that at WOT the actual engine RPM was 3,280 RPM (vice the 3000 RPM as indicated by the Beta engine tachometer) so we were still low but only by 320 RPM vice 600.
I contacted FlexOFold with the new info and Nina responded right back. She informed me that one inch less in diameter equates to change of about 250 RPM and one inch change in pitch to about 125-150 RPM. She recommended we go from a 16X12R to a 15X11R. I discussed this with Farron at Beta and he agreed. His comment is that FlexOFold is a very competent propeller manufacturer with a solid reputation. He reminded me we should strive to get within 50 RPM but in a perfect world being a little over is better than under. I told Nina let’s go with the 15X11R. She said to leave the hub on the shaft and just remove and mail them the blades. If I paid for the second set of blades they would mail them to me ASAP and refund the amount when the original blades arrived in Denmark. So, that’s what I did. The blades were $580. There was no added shipping cost.
I donned my wet suit and hookah and got into the murky water at the marina. We have a fair number of alligators where we keep the boat but they have never bothered the divers who regularly clean boat hulls. American Alligators are not normally dangerous to people though there have been a few attacks. But I still asked Gayle to keep a lookout while I was in the water. I took a five gallon bucket and put a two pound weight in the bottom. I flooded the bucket and hung it over the propeller shaft to catch anything I might drop. I put my tools in the bottom of the bucket. The visibility was about one foot. Leaving the hub on and only removing the blades made the job a whole lot easier. All I had to do was removed two SS Allen head bolts and pull two 12mm SS pins and the two blades would come right off. I removed one at a time and the job was done in about three minutes. Nothing to it.
About five days later the new blades arrived. I got back into the water and performed the same steps in reverse. Again, it was a simple task. An important note–there is a pre applied thread locker on the bolts. It’s not a regular thread locker. It is applied to the threads by FlexOFold on land and the bolt can then be applied under water. The thread locker will activate and harden in 24 hours. Regular thread locker (Lock-Tite for example) will not harden underwater. Nina sent me two new bolts (no charge) with the thread locker already applied. I have done a lot of research on this and I am “pretty sure” Lock-Tite 248 will work the same way. However, if you attempt to apply regular blue or red lock-tire like #243 on the bolts then install them underwater the thread locker will fail and you will lose your blades. You can however use regular thread locker to install the bolts on land as long as it cures before you launch the boat. So, do your homework.
With the new propeller blades installed we took the boat out for another test run. Our target was to achieve 3,600 RPM at WOT. Using the photo tachometer we ran the engine under load and at WOT we achieved 3,612 RPM. Perfect. We ran the engine at various RPMs and checked our speed against a Garmin GPS 76. We ran across the current and with the wind on the beam. 2,200 RPM equated to about 5.3 kts. 2,500 gave us about 6.1 Kts and WOT gave us about 7.2 kts. My cruising speed goal was 5 kts so it all worked out about right.
A few days later the electronic calibrator sent to me by Beta Marine arrived in the mail. It took maybe five minutes to punch in the correct number to calibrate the tach. Farron told me for my engine the code should read 10.4. When I activated the screen the code that was highlighted on the tach screen was 11.3. He said a higher number equates to a lower RPM reading. So, the tach was off from the factory. With the new code pouched in I tested the engine tach against the photo tachometer and it was dead on.
The engine installation is now mission complete.