Drag Graphs by Type
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The question of drag comparisons between the various types of propeller is not as simple in real world situations as first impessions would indicate.
A Kiwiprop™'s drag will be virtually identical to the Max-Prop or other 3 bladed feathering unit.
However this was measured in the tank at MIT tank where the shaft was parallel to the water flow.
Because the Kiwiprop™ blades always follow the streamline for each blade -
drag will always be the same under all sailing conditions.
A two bladed folder's drag will be determined by the streamlines of the projected area of the blades when folded
which will not be zero. Projected area totally dominates the drag equation and should be minimised to reduce drag.
If you have an un-geared folding unit then the lower blade will drop down and offer increased drag,
particularly at lower speeds. Again his is dependent upon the position of the join which ideally should be vertical.
Now we have to decide if the 2 bladed unit is folded with the join vertical or horizontal.
With the join horizontal - the projected area relative to the streamlines at a shaft angle of say 20º
will be significant - less if the join is vertical and higher if un-gearded so the lower blade falls under it's own weight.
Now you need to consider on the wind and off the wind.
On the wind there will be leeway which delivers projected area to the blades when vertical.
Off the wind - then this component of drag will go away.
So on balance we believe that over all sailing conditions the drag of a Kiwiprop™ will be no greater
than a geared two bladed folding unit and less than an un-geared unit.
Another graph ex Yachting Monthly that summarises drag of vessel and propellers by type:
This graph shows how hull drag increase dramatically as speed rises with commensurate increases in propeller thrust.
Drag at 6 knots is 70 Kg but increases to ~ 130 Kg at 7 knots. This illustrates how difficult it is to obtain marginal improvements in motoring from any engine / propeller drive train.
