Seafury Marine Surface Propulsion Systems
Seafury Marine Surface Propulsion Systems

About Surface Drives

What are surface drives, and how do surface drives work? Put quite simply, surface piercing propellers operate half in and half out of the water in the planing wake region of a boat just behind the boats transom. Having the propeller at the surface level reduces drag and reduces the vessels draft, making the surface drive suitable for many applications where the owner is looking to reduce fuel costs, increase speed and create a more efficient vessel.
There are advantages and disadvantages for all propulsion systems for water craft. But for planing and semi-planing vessels, surface propulsion has several distinct advantages over its competitors in the propulsion marketplace.

Firstly there are great advantages in not having any appendages such as a propeller and shaft underneath the hull of the vessel. Having no appendages reduces the drag on a vessel (up to 15% drag co-efficient), therefore making higher speeds possible with less horse power required. Less horse power equals less fuel consumed, making running costs a lot more economical than a conventional drive. 'Surface propellers virtually eliminate drag from all of these sources, as the only surfaces to contact the water are the propeller blades and a skeg or rudder.'

Then there is the issue of cavitation. Cavitation is the rapid formation and depletion of air bubbles produced by the turbulent flow of a spinning propeller. With surface propulsion, half of the propeller is above the surface, and the surface blades create a super-ventilated environment rather than one of potential cavitation. This eliminates the risk of propeller damage due to cavitation, providing for less maintenance issues over time. The surface propeller as opposed to a conventional propeller, operates in a super-ventilated environment and at no stage suffers the problem of cavitation. 'The surface propeller effectively eliminates cavitation by replacing it with ventilation' 'Although the flow over a superventilating propeller blade bears a superficial resemblance to that over a supercavitating blade, most of the vibration, surface erosion, and underwater noise are absent.'

Of course, having a propeller that sits at the water's surface produces a shallower draft on a boat, enabling great advantages over their conventional drive counterparts, making surface drives perfect for shallow water applications.

Surface drive propellers are larger than conventional propellers for the same sized vessel. This is beneficial when undertaking close quarter handling in that the propeller at that stage is completely submerged and the operator has the advantage of greater thrust forward and aft. 'There is virtually no limit to the size of propeller that will work. The designer is able to use a much deeper reduction ratio, and a larger, lightly-loaded, and more efficient propeller.'  

And then, of course, there is the 'trimmable vs non trimmable' aspect. As writes renowned Naval Architect Paul Kamen "Fixed systems, on the other hand, do not require the hydraulic cylinders and associated pumps, control devices and high pressure plumbing. Furthermore, fixed systems are often designed to work with conventional solid shafts and stern tubes, rather than the more complex universal joint drivelines found in articulated systems. It should also be noted that articulated systems should not be relied upon to control vessel trim angle. Trimming the drive up and down will have only a small effect on vessel running trim, and separate trim tabs or other devices may still be desirable."

There has been much talk in the past of the poor reversing performance of surface drives, but years of research and development have seen significant developments in this area. Seafury Surface Drives require a 45° transom to counteract this problem, thus producing superior performance when going astern, when compared to conventional drives. If it is a retrofit, a mock transom can be installed, creating the same effect. Seafury has also optimised it's propeller design to create maximum thrust and further enhance astern performance.

Overall from a surface drive, you can expect greater fuel efficiency, zero cavitation and shallower drafts, and in some cases less maintenance and exceptional astern performance, making choosing a surface drive the smart choice.

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