Wednesday, June 15, 2011

Propeller Install

     This boat was originally powered, or I should say underpowered, by a pair of 6.2L Mercruiser smallblocks. The factory props were 17 inch diameter by 19 inch pitch 3 blades with these engines, with a 2:1 reduction in the transmissions. The optional 8.1L engines were available for this boat at the time, and used the very same shafts and 2:1 transmission ratio, but the prop diameter was increased to 18 inches, with a 22 inch pitch, and 3 blades.

     As many of you know, I have been taking great pains to reduce drag, improve the weight balance, and increase the efficiency of this vessel. The hull has been faired and smoothed, the struts and rudders have been profiled for reduced drag, the trim tabs have been modified, etc. etc. A big player in this effort is the propeller selection, and I have been researching the available options for many months before coming to a decision. The debate between the merits of 3, 4, even 5 bladed props will never end, and I don't expect to settle it here. But for the type of boating I engage in, my final prop selection is 4 blades. I have no interest in top speed performance. Instead, my goal is utmost midrange and cruise efficiency, along with secondary goals of smooth performance, low vibration, and good performance around the docks at very low RPM. These parameters led me to ACME Propellers -

     Briefly, these props are not made like conventional wheels. They are fully CNC machined. This manufacturing method yields precise control over the blade shape, not only of individual blades, but between blades themselves. The result is an extremely balanced, smooth running propeller. For all intents and purposes, the blades are effectively identical. These are props with optimized blade thickness and profile, that maximize efficiency, minimize drag, and run with minimal vibration.

     Normally, when moving from 3 blades to 4, the pitch is reduced slightly to allow the engine to reach rated RPM while spinning the extra blade. However, ACME has experience with 340 Sundancers powered with 8.1L engines, and after discussing with them the changes made to this boat's running surface, appendages, and balance, they specified a prop with an additional inch of pitch - these are 18 x 23. (Yes, I was shocked too, but they are the experts.) It does make sense though, when you consider the precise nature of the prop, and the accompanying drag reduction and efficiency increase such a manufacturing method lends to the prop. Anyway, the proof is in the pudding, so if any adjustments are necessary, that can be determined after actually running the boat in the water. ACME can then adjust the wheels based on actual data for my boat.

     I'll have more prop install details in the next post, and also some other information about shaft alignment. 


  1. I'm interested to know; are you able to install/remove the 4-blade props with the rudders installed? I have Michigan Wheels (4-blade) and the rudders have to be dropped in order to remove the props. Thanks!

  2. I can remove and replace my props with the rudders installed. Also, I can remove and replace my spares (a pair of re-pitched Teignbridge 4 blades, now configured as 18 X 22.5). Unless your Michigans have very long hubs, you should be able to pull them without removing the rudders. What interferes?, Is it the prop hub, or the puller itself? I fabricated my own custom puller, which I keep on the boat for emergency use. Let me know - I could make you a puller too, if you need it.