The greencat has a neat arrangement. Would be interesting to know how much their pods cost by the time they were installed. The only significant difference with what we have is the low BAR prop with relatively larger diameter. This gives us higher prop efficiency. With clean hulls and calm conditions the prop efficiency is 72%. The outboard prop was getting around 40% efficiency.
The drive unit is not much deeper than the main beam when raised. When the lw unit is hard up against the deck there is just a single blade that protrudes about 300mm below the beam and is toward midship of the beam. The prop blade is only slightly deeper than the well in the deck.
The motors get just over 90% efficiency through most of the range we will use. For motoring we will run lw at 2.5kW and ww at 1.5kW to make best use of the installed battery capacity - 5kWh on lw and 3kWh on ww. Motor losses on the lw side will be 250W. The motor box will not get much hotter than water temperature due to all the immersed plate on the fairing and the motor is fitted with an air cooling fan as well as the conduction from the base plate. We do not have thermal protection on the motor but we will check motor case temperature once we do some motoring.
Prop clearance is adjustable but the props cannot hit the hulls because the motor housing prevents travel.
We have set the props quite deep to avoid ventilating in waves. You would not want the hulls to be deeper than the props. Bottom of the prop is almost 900mm below surface. On this boat the rudders provide the crash stop in shallow water - they are about 1.5m deep. The props are nylon and have some compliance plus a shear pin. They can take a bump particular with the stretch in the restraining rope. Owner plans to carry at least one spare prop. Each prop cost USD33 plus postage so a trivial expense if that is all that gets damaged.
Rick
On 23/12/2012, at 12:19 AM, LucD wrote:
Until your solution this http://www.boatdesign.net/forums/hybrid/greencat-605-retractable-electric-propulsion-44542.html seemed to be the best solution yet. It is not the best price. It would be interesting to hear your take on this as your reasonings are at least as interesting as the results you show.
As the arm is locked anyway when the prop is in operation I suppose there would not be an adverse effect pivoting the arm sideways at a forward angle making sure the prop would end up in front of the beam instead of remaining under the deck when pivoting straight sideways.
Are you concerned at all about overheating the engine as it is not submerged or do you count on the heat conduction of the aluminum housing extending in the submerged shaft covering foil?
The difference in the props pulling vs pushing the boat would be interesting.
What is the clearance between tip of the props and the hulls when in operation?
Would it be worth it to design the draft of the hulls to surpass the draft of the props to protect them from floor strikes or would you rather extend the shaft cover downward below the prop diam?
Rick Willoughby
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