Originally Posted by
Bill Van Steenwyk
Lets pretend the prop has NO FUNCTION as far as propelling the boat thru the water, other than the fact it is there in it's normal location, on the prop shaft, and in the water. In this visualization, propulsion is being achieved in this example by ANY other means you might imagine EXCEPT the propeller. Could be water pump, sail, oars, or any other type power that will move the boat forward through the water EXCEPT that prop on the motor.
Now imagine it raised on the transom so only a part of one blade is in the water at any one time.
Now imagine it lowered on the transom so ALL of the blades on the prop are completely submerged in the water at one time.
Which location of the prop (depth in the water) do you think will give you the best speed, all other factors remaining the same.
Same thing applies when the prop is driven by an engine and that prop is the sole propulsion for the boat. The more blades in the water, the more DRAG. It also may be easier to imagine the prop as a complete disk. The bigger the disk the more drag. If the disk has holes or slots cut into it, drag will be reduced. That is also the reason a jet airplane is faster (among others) is an airplane with a propeller generates much more drag than one without. Add more propellers (think 4-6 engine bombers) and more drag. In addition, the tips of the propeller reach supersonic velocity while the vehicle itself is much below the speed of sound, also leading to propeller inefficiency. That and drag is why a prop jet is slower than a pure jet.
There is much more to it than just this, just as air is much different than water insofar as the way it acts when moving through it with a propeller or any other object,and that is where number of blades, pitch, diameter and other factors enter into the equation. BUT one of the large things to effect a prop (or any other object moving thru air or water) is DRAG. This is also the reason that some still run props with only two blades when racing, although not a lot anymore. If water conditions will permit, a two blade will probably be faster (taking into a lot of other factors) than a multi blade prop, especially when you are running a surfacing type prop, is there is LESS DRAG with 2 blades than 3 or more. This is not always true, depending on handling problems you might encounter in rough water, etc. You can raise a multi blade prop higher out of the water and have the boat have better handling capabilities because of that word again, DRAG. Eventually you get to a point where handling and prop efficiency become more important than drag reduction, especially if you can not control the boat anymore because of lack of directional control which the lower unit/prop combination in the water give you.
Unfortunately there is no magic formula which will allow you to calculate this point. The only formula is testing and good record keeping.
Hope this helps.
Bookmarks