Reply To: WOT RPM speed to weight relationship.

[Steve Lewis]

Joe,

To summarize the ideas that Tony, Rob and I have presented, From an Engineering Theory Perspective, It does not matter what your transmission ratio and propeller diameter and pitch are, a load will be presented to the propeller on your boat. That load will be transmitted up the shaft to the transmission and to the engine which will be asked to provided HP and Torque. The load presented to the propeller, and by extension to your engine, will vary infinitely based on the “Ship Resistance” which is the sum of all of the conditions that contribute to that “Ship Resistance” which is previously referred to as “Load” . That means that your engine(s) will be asked to provide power in direct proportion to the load presented to the propeller.

Tony will always recommend the ideal propeller setup so that the varying load presented to the engine will ALWAYS be less than the engine manufacturer recommends based on the published curve. Burn less fuel at every RPM that is presented on the “Propeller Curve”. I agree violently 🙂 Long engine life will result, as long as you maintain the engines as they should be maintained. That, my friend, is an entirely separate set of requirements.

Your base question, however; is fundamental. The answer is fundamental as well. Larger Load equals Larger Fuel Burn. Full Stop.

It does not matter what the gearing and props are. You can read about “Hydrodynamic loads on Propellers” from all sorts of different sources. I spent an hour this morning Googling this topic and read some papers from MIT and other engineering sources. Those papers, though very advanced, can be distilled down to the point I have been making all along. It does not matter what the mechanical advantage is for your boat. A larger load presented to a propeller will result in a larger fuel burn rate. The only things that will vary will be the extent to which that load varies, and by extension, the extent to which the fuel burn will vary. The resulting speed will be directly proportional to the sum of the variables in this complex equation. Yes I am an Engineer.

My 2 posts were an attempt to provide real world examples and proof of these concepts.

Cheers to all. The Admiral and I are going to get a Christmas tree this weekend. Happy Holidays to all!

Steve