Re: Prop Recommendation: I have all the data
Swiss, to the normal person what you are saying appears to be correct, but that is not what is actually happening. If you will first look at the Real Speed Chart, you will notice that the Painted Steel 17" and the Performance 16" track almost exactly parallel together, in RPM and Speed, which implies that they are theoretically the SAME Pitch. But as the Performance 16" reaches 4,000 RPM the Slip goes to 7%, which is about the normal Slip for a stainless prop running at WOT RPM on most boats. At 5,050 RPM the Performance 16" is now at 3% Slip and the prop at this stage does NOT have enough thrust left from the torque and horsepower of the motor to develop any more RPM, because of the geometry and surface area of the prop. But yet the Painted Steel, as a semi cleaver prop has 700 RPM left it can turn to 5,750, because of the geometry and much less surface area design of the prop, as well as it is moving less mass but yet it is enough to push this boat at this weight with the mass of water that it is moving. It requires less thrust from the prop, and thus less torque and horsepower, as well as it is finally reaching the full 70 horsepower from the motor at 5,500 to make it able to attain almost the full RPM rating of the 70 horsepower at 6,000 RPM.
There are many different theories on prop slip in normal planing boats, my opinion is when you get to below 7% or less Slip you have too much blade surface area, diameter and/or geometry for that particular boat, motor and prop combination, unless you normally carry a heavier load in the boat. At that point you need to change blade surface area, either by geometry or Diameter to increase your RPM and speed, which is usually easy to do with prop Diameter. If you will look, there are a good many props that as the pitch goes up the diameter of the prop comes down. Most of the highest speed props revolve around the theory of a 2 to 1 relationship. Meaning that the pitch is twice the diameter of the prop.
Not that many people ascribe to this way of thinking, but it absolutely works because you are reducing surface area from ANY prop when you have a smaller Diameter and the same blade geometry. You have to remember that the hub for outboards and sterndrives is approximately 25% to 30% of the diameter of the prop. So if you theoretically have a 15" Diameter prop and the hub is let's say 5", for easy working numbers, then that means on that Diameter prop that the blades represent only 10" of that 15". If you decrease the Diameter to 14", the hub size does not change, so you effectively decreased the prop blades to 9", and theoretically with the same geometry prop you now only are moving 90% more or less of the water that you were moving with a 15" prop. And physics doesn't change, when you only have 90% of the mass(water) being pushed behind the prop then you will always have an increase in RPM because of the less thrust that is required to push that amount of water.
Sorry, I got sidetracked, I didn't really answer your question on Slip. After you pass about 7% Slip in most
normal planing hulls and stainless props it is eating up RPM that could be used for higher speeds. So in my opinion, when you show less than 7% Slip it is usually stopping you from reaching higher speeds that could be attained with a prop with less diameter and/or less surface area in the same or similar geometry of prop elements, unless you normally carry a much heavier load in the boat.
H
