Variable Pitch Surface Drive?

[PetterM]

Is the world ready for a new Variable Pitch Surface Drive?
With low or no appendage drag, high propulsive efficiency, shallow draft and good low speed maneuverability, the combination of surface piercing and variable pitch should have a considerable advantage over conventional solutions, provided both few challenges both technical and commercial challenges can be overcome.

List of Variable Pitch Surface Drives:
Yellowfin VSD (made quite a few non commercial units, but never made it into production)
FRANCE HELICES (only made one?)
Servogear AS (made one installed on a patrol boat in Norway)
CPS- Drive (made two test units installed on one specially designed catamaran in Norway)
Stormfagel (still going, only one made so far in Sweden)
Escher Wyss (7 blades, used on a SES in Norway, don’t know of any other installations)
Have I missed any out?

[PetterM]

Am I the only one interested in variable pitch surface drives?

[baeckmo]

Yes! The performance gain is not compensating for the "loss of simplicity" (=increase in cost). This is what determines market success.

[PetterM]

the performance gains can be very significant though. A well designed variable pitch surface piercing propeller can run at 80% efficiency. Combine this with good acceleration then and maneuverability then it should be interesting?

[apex1]

I would recalculate that Petter! The market is always right when it comes to "cost / gain" comparison.

[baeckmo]

Originally Posted by PetterM

the performance gains can be very significant though.

That is questionable from a functional standpoint. You must remember that a ventilated propeller, as well as a supercavitating one, has an operating characteristics completely different from a submerged, non-cavitating propeller! What is holding back the surface piercers is a combination of facts:

1/ The majority of vessels where spprop might be applied are planing boats with short, beamy hulls. Typical for these is a very high hump drag; about as high as the cruising drag at about 30 to 50 % of the cruising speed.

2/ In the fully vented speed range (ie the advance ratio up to ~60 % of design) the spprop thrust factor kt is constant, independent of pitch. This means that in that operating range, there is little improvement in thrust, no matter how you change the pitch (or rpm for that matter).

3/ With reduced pitch setting, the blade tips will operate base-ventilated (as if they run in the high advance/low load region). The low advance speed thrust from the important high relative speed tip region is thus reduced further.

Combine the propeller thrust characteristics (2 and 3) with the drag characteristics of the major target group in the market (1), you find that there will be problems with the hump region. There is only one solution to that, no matter P/D ratio, and that is propeller working disc area.

As long as you have enough thrust (=area) to pass the hump in fully vented condition, cruising and top speed will come without fuss at base vented operation with high efficiency.

The "natural" hull shape for spprops is in fact the slender hull, either mono or catamaran. With a slenderness ratio (LWL/Displ^0.33) above ~6, the hump is no longer a problem, either in planing or displacement mode. It is an unfortunate irony that the spp in "modern" boating minds has become so completely associated with high speed applications, when it is actually very well suited to medium speed operation.

Proof of this is work at former Denny Shipbuilding (?), who produced a number of displacement vessels with surface propellers back in the 1920:ies and were planning a new development programme as late as 1963. Another proof, known to most "forumists" here is Frosty's catamaran with excellent performance due to the characteristics of hull and propeller working together.

So, until we have blade profiles for ventilated operation, that have continually rising thrust with reducing advance ratio, the controllable pitch surface propeller is simply not adding enough technical value for the market target, to warrant its success! There are simpler solutions to the problem!