Pocket Drive - modifications - photos

Isn't there a fair concern that adding air could make things worse? As in even less good water for the props? Curious what @baeckmo would say here..

sorry to doubt it, but certainly too much air would be bad; so one must wonder if some air helps or if it is all a linear fail...just making conversation here

 

I'm mostly curious why hull tunnels at all. That's a lot of expense and complexity, to make something that seems to perform poorly. Certainly surface props could have been installed much more simply, with better outcome and less cost.

 

Nope, it's not that simple. Cavitating and/or ventilating props behaviour differs radically from submerged ones (even when submerged...). When ventilated, the low advance ratio (J) region is characterized by low thrust and torque coefficients, often taking a "jump" up at about J~(P/D)/2 and above; VEEERY ROUGHLY! A short, fat, heavy hull with a huge hump resistance is the very last candidate for surface props!

 

I was involved in a fast catamaran ferry build in Norway that used one of the first integrated propulsion packages from ServoGear. That concept has the CP propeller and rudder carefully integrated with a hull tunnel. It took several iterations of tunnel geometry mods, some of them quite subtle, and propeller design changes, but when it was finally sorted out, the vessel performance was excellent and met or exceeded the levels guaranteed by ServoGear and the shipyard.

That experience gave me a good perspective on how sensitive such designs could be and how slim the margin between success and failure. Working with Don Blount later on optimizing the tunnels and props for a fast monohull only added to that perspective.

 

@DogCavalry The opposite of a Sea Sled... well, accept for the heavy part... <wink>

 

A couple of reasons a) less draft and b) a lower shaft angle

The lower shaft angle changes some parameters
1) lower horizontal thrust vector, So the shaft angle wrt to the hull PLUS the bow up angle when planing (optimized for minimum drag) (which normally falls between say 4 to 10 degrees up, depending on the effective
deadrise) will affect the horizontal component of thrust. (prop thrust x Cos of the angle= horizontal thrust)
2) in order to keep the thrust component to the lowest angle wrt to the water, the other alternative is to move the engines further forward ( perhaps affecting trim and the center of lift) or installing V drives to keep the engines near
the stern

I have often wondered if the prop could be operating say at 4 - 6 degrees of bow up and and maybe 8 degrees of shaft angle, making a total in the 12 degrees off the 90 degrees with the direction of travel, would affect a propellers
efficiency. Certainly with outboards and IO's the thrust line can be trimmed for optimum efficiency, in or out. (efficiency defined as best mpg for a given speed)

Bayliner quite a few years ago built many hulls with tunnels, 32 feet and above. They used relatively large radius rounded chines and tunnels. Performance with this combination was not that good and a company in Seattle
developed some molds to glass in square chines onto the sides. This increased the effective width of the chine resulting in more lift aft.

While the dynamic pressure at the rear of a planing hull is less longitudinally than toward the stagnation line, the tunnels did take away quite a bit of lift from the stern and made these hulls run with a high bow up attitude and quite far away
from optimum trim angles

 

SP's are thought of as very high speed items. And have correspondingly tiny props. I wonder to what extent the poor low speed performance is due to small props. How much does their performance curve resemble the performance curve of a fully submerged prop, also optimized for high speed? Hickman's original patents were for work done at 10-15 knots.

 

Yes, there are many ways to get it wrong, but only one to get it right...and sometimes none! The fluid movement involved with tunnels has quite a lot in common with waterjet inlets. It's said many times before; fluid mechanics is not well suited for intuitive treatment......

...and Barry, think I've got some papers on inclined shafts down in my sediments. If you are interested I can copy and send if you give a mail adress.

 

Just looked it up. Top fuel drag boats, with up to 10 000hp, have 11.5", 29cm props. What would the performance curve of a fully submerged prop at that size and power look like? They also use 6 speed transmissions

 

I realize that all my questions are annoying/ obtuse/ distracting irrelevancies, but Paul Kamen's 2005 paper said some very interesting things.