How Does a Surface Piercing (Boat) Propeller Work?

[d3mm]

This is a propeller on a boat, that is half in, half out, of the water. They provide more speed on light, fast boats.

If you require more information, google "surface drive" or "wave piercing propeller". Companies like Arneson and Seafury manufacture these. Wikipeda has only 3-4 lines on it, and it says nothing. http://en.wikipedia.org/wiki/Surface_piercing_propeller

Why are these 'better' than conventional propellers? Is there an equation for it that I could enter numbers into to see?

They say that there is less drag from the propeller shaft, and less cavitation due to the churning of the water, but how does it produce more thrust with only half of the contact area? They are larger than regular propellers but I still don't see.

 

[sophiecentaur]

I don't know much about these propellers but I can say that your comment that 'only half the contact area' will affect the thrust, as a matter of course, doesn't necessarily follow.Of course, if you take a conventional propeller and try to operate it half out of the water, it won't work as well but these new propellers have a different area, pitch and shape. The way that all propulsion systems work in a fluid is to push a mass of water backwards at a high enough speed. The pitch of the screw makes a huge difference. You can work out the speed of a conventional boat through the water with an underwater screw, given its pitch and its revs - when below the design speed, it performs very much like a screw in the water (with a bit of slippage). In the case of these surface peircing blades, there must be a lot more slippage but, as long as the design pushes the right amount of water backwards, it will provide the required thrust. All those Google links describe the advantages (but remember, many of them are posted by manufacturers and enthusiasts). It may not really be as good as they suggest and you need some good, unbiased, comparison tests.
One disadvantage that immediately springs to mind is the performance in swell (pretty well all the time at sea, aamof). The risk of the engine over-revving and losing drive when the prop comes out of the water is greater when the screw is already right on the surface.

 

[pbuk]

At high speeds the biggest issue in propeller design is cavitation. Most cavitation effects are negative but a supercavitating propeller turns cavitation into a virtue by creating a continuous vapour band on the leading face ("suction side") of the propeller thus dramatically reducing friction, and also ensuring the cavity is maintained beyond the trailing edge thus avoiding the problems of cavitation which are mainly associated with collapse of the cavity (vibration leading to inefficiency, noise and fatigue and pitting). However they only achieve supercavitation at high speeds and the design necessary to achieve this makes them very inefficient at lower speeds so they are only used on craft required to operate at a sustained high speed (say 50 knots+) or on model craft where the hyrdodynamics require a faster rotating propeller which cavitates at a lower forward speed.

A surface piercing propeller achieves the same advantages as a supercaviting propeller by sucking air into the propeller stream (ventilation). Again this is normally a disadvantage, but careful design can achieve a continuous (air) vapour band on the leading face over a wider range of speeds than the water vapour band in a supercavitating propeller. Also the propeller can be designed to operate more efficiently at lower speeds in "normal" (unventilated) mode.

The elimination of shaft drag is a secondary advantage, which for a planing vessel can be highly significant as the shaft and rudder are the only parts of the vessel in contact with the water for a significant proportion of the time. This is true to a lesser extent for a hydrofoil vessel.

These effects are very complicated and defy analytic solutions (cavitation and ventilation are chaotic effects); prop manufacturers use empirical data to complile data tables for specification of the appropriate propeller for a given range of operational parameters.

 

[sophiecentaur]

Do they have some servo arrangement to keep the prop in the right position?

 

[pbuk]

Do they have some servo arrangement to keep the prop in the right position?

No, this would not be feasible* and is not required because

• a planing craft can only achieve high speeds in relatively flat water due to the increasing stress of impact with wave crests as these become higher
• the most significant force to overcome to maintain speed is friction of water against the hull; when the boat "flys" off a wave crest the propeller loses forward drive, but the water friction force reduces to zero
• the shape of the hull and transom creates a "trough" of relatively constant profile for the propeller to spin in - the surface of the water at the propeller is below the normal surface

* the required reaction time together with the need to transmit the whole power of the engine (say 1000HP) to to propeller via a rotating linkage of varying length submerged in salt water are considerable engineering challenges

 

[d3mm]

These effects are very complicated and defy analytic solutions (cavitation and ventilation are chaotic effects); prop manufacturers use empirical data to complile data tables for specification of the appropriate propeller for a given range of operational parameters.

Thanks for reply. I was afraid of this. More black magic and design by trial and error! Turbulence is hard, isn't it? :-)

Do they have some servo arrangement to keep the prop in the right position?

Some do, correct trim would obviously be vital to correct operation.

"The propeller shaft is vertically adjustable, allowing selection of the optimum thrust angle for varying load and sea conditions --- from http://www.arneson-industries.com/page.php?type=products&id=drives"

As you correctly say, rough water is a problem but these drives are generally used on inshore racing boats, like cigarette boats. I can see you are British so you would call that RCD Category C (which would probably still be a bit rough for the average cigarette boat). I saw one once in St Kats (on the Thames) once.

 

[gmar]

I understand the necroing old posts is bad but I found this via Google and I can answer it. It was not answered.

Advantages:

* One less mechanical coupling meaning lighter weight and efficiency, which is important for a lightweight, high performance vessel.

* Reduced drag from propeller shaft and rudder.

* Higher propeller RPM without cavitation.

* Shallow draft (depth of propeller below water) allowing shallow water operation.

Disadvantages:

* Increased noise and vibration.

* Poor performance in heavy seas.

Despite what some (overly traditional) sailors will say (and there are a lot of those) the system is tried, proven and tested on racing powerboats and miltiary/customs interceptors.

 

[sandhammaren]

A surface-piercing propeller is not simply one running half in and half out of the water. It is any propeller where the blade tips or any part of the blades pierce the water surface each revolution. This eliminates cavitation and reduces drag. All racing props are surface-piercing.