Mad Props!

Why does the engine only go to 2400 RPM? What RPM should we get to – it's a newly rebuilt engine? Maybe the propeller is too big.... Yep, that's it!

We didn't come around quite as quickly as that sounded. After reading our engine specs, talking to the mechanic and reading a section of Nigel Calder's Cruising Handbook, we came to the conclusion that our 18” dia. X 12” pitch propeller was way too big for our 35', 19,500 pound full keel cutter with a newly rebuilt 39 HP Yanmar. The formulas in the Cruising Handbook are reprints and adaptations from Dave Gerr, published in two books: Propeller Handbook and The Nature of Boats. The numbers crunched down to a 15” dia x 9” pitch with 5 blades – WHAT? Sailboats generally have 2 or 3 blade propellers, 5 is crazy. This is where Nigel mentions you have to do a little real world fudging of the numbers, so we figured maybe a 16x9 or 17x8 3 blade would be good. The new problem was finding one used, because a new one would cost $600-$700 and we weren't even sure if our numbers were right.

The first one we tried was a 2 blade 17x8. It barely moved the boat at 4.5 knots in flat calm seas and it made a lot of noise (turbulence that Nigel describes). Next we tried a 18x12 3 blade, the same size as our original, but with much less width in the blades – it performed about the same as the original. We borrowed a 14.5x8 from a propeller shop in Ft. Lauderdale and it was much better – the sound was smooth, the engine could reach maximum RPM, but, it only pushed the boat at 5.5 knots. A lot of people we talked to thought we should be happy about 5.5 knots, but according to our calculations the hull speed was around 7 knots. We kept looking and Keith kept diving to swap props. The local prop shop came up with a 16x8 3 blade that we “borrowed” by leaving a $320 cash deposit. BINGO! We hit 6.5 knots (close enough), it was smooth and we reached max RPM. One last dive and we were done! We are keeping the 2 blade one as an emergency propeller.

Side-Notes
Even though $320 was the most expensive used prop we looked at, it is still half the price of a new one!

Keith did most of the underwater work free-diving. He used SCUBA once, but just thought it was easier without setting up all the gear. In the pictures you can see he had a bucket suspended with all of the parts and tools he would need. The “prop-puller” is a $29.99 pully-puller from the auto parts store.

Tip when using a pully-puller: loosen the shaft nut, but don't take it all the way off, so when the prop breaks free: the prop, puller and shaft key don't go tumbling to the bottom... Oops!

Your engine should be able to achieve its maximum rated RPM (3600 for our Yanmar 3JH2E), if not you are overworking your engine and causing damage to it. The common problem is “over-proping”. We never even thought about this on our first boat, but since we spent $5,000 on this rebuilt engine and installed it ourselves, we want it to last. That encouraged us to crunch the numbers and find the right propeller (or the closest possible).