I am working on a design for a hydraulic pod drive for a small displacement cruiser. I purchased plans for Tad Roberts' North Coast and redesigned the accommodations and drive. I have studied some of the available information on propeller efficiency and hydrostatic drive efficiency and have decided to pursue a steerable, hydraulic pod drive. I have attached some renderings of the plan. I have just recently submitted the pod design for quotes for the castings...no cost figure yet.
- The pod drive allows me to place the prop far aft allowing for a 50% increase in prop diameter and a corresponding decrease in shaft speed. According to research done on small fishing vessels (Berg, A. 1982. Full-scale experiences with propeller re-installation on a purse seiner. Paper presented at the International Conference on Propulsion for Small Craft. London, Royal Institution of Naval Architects) a 50% increase in diameter resulted in a 30% reduction in fuel consumption...a 40% increase in range on the same amount of fuel. The original design calls for an 18" prop running at 1200 rpm. The pod drive makes possible a 26" prop at 625 rpm (the prop sizing is via an online propeller sizing tool). The increased prop efficiency more than makes up for the losses in the hydraulics.
- Other advantages of using a hydraulic drive
- (as opposed gears) the rotation of the prop is not coupled to the rotation of the steering...no lever arm as with a right angle drive
- complete control of prop speed when maneuvering in-shore via a variable displacement hydraulic pump
- properly sized (for the vessel and the prop), the engine can run at constant speed at its most efficient
- the design incorporates a commercially available "pod drive" from Hercules Hydraulic in the UK inside a bronze casting. Drive Pods http://www.herculeshydraulics.co.uk/index.php/products/drive-pods
- the pod shaft rises inside a composite tube formed with the hull that terminates 1' above waterline...no seal leakage problems as with typical sail-drives
- The shaft seals in the hydraulic motor will always be under positive pressure from the column of oil from the tank sitting 4' above the location of the seal, 2' above waterline...no water ingress due to worn seals...the motor seals are further protected by the double shaft seals of the Hercules pod and a sealing o-ring at the pod to motor interface...the bearings of the Hercules pod are sealed as well.
- By using non-conductive hydraulic hoses the pod drive can be electrically isolated from the engine and better protected from corrosion due to stray currents
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