Paul,
We have converted many boats the size and dimensions of your S&S 33. I would suggest that any systems from a quality marine vendors would perform the same. Note that a number of builders around the world built the S&S 33 and they were each a little unique.
Some observations about the boat and the propulsion set up:
Your propeller is likely over sized in diameter as there were racing regulations that benefited a 16inch diameter over a more conventional 14 inch diameter and since it was a folding propeller there was no sailing speed loss. That is what they installed back them. The 10 pitch is right for the 20hp diesel.
If you have a shaft installed with a steep angle just behind the keel, there is a significant inefficiency to your system. If it is further aft at less incline than your system would be more efficient.
A 10kW system should perform much like your 20hp diesel. This is the Speed and Power curve that we see for your boat with a 10kW (8.5 continuous) :
Kts Watts
2.1 288
2.6 480
3.3 960 what a 2kW genset would push the boat continuously
4.1 1920 best speed to get reasonable range with modest battery pack
4.7 2880 what a 4kW genset would push the boat continuously
5.2 3840 what a 5kW genset would push the boat continuously
5.6 4800 This is likely the power draw in a 10 to 12kts headwind
5.9 5760
6.8 8160 Can support hull speed in calm waters (8.5kW continuous)
9800 Max power for a 10kW system (48v X 200amps)Hull speed 6.58
Our projections also show that based on the assumptions you would be able to push into a 10 to 12kts headwind for about 2 hours with a 14.4kWh battery bank using AGM batteries and a little better with LiFePO4 batteries. In calm seas the same battery pack would push the boat at 4kts for over 5 hours. The rule of thumb of double the power for every knot is very predictive.
To your question of will an inexpensive genset mounted on the deck (all the portable systems must be) push the boat, the answer is yes. The issues is for how long before their low cost system would fail is a guess. They would likely be a good temporary solution for the occasional longer range cruise and serve you well. Be prepared to sail and be careful of the fuel for the motor as gas varies and has a shelf like.
There are many obvious benefit to having an electric auxiliary motor on a sailboat. In all aspects it is better except for one, range. The reality of electric propulsion today is that when removing a diesel and having the same weight in as weight out, you will see a significant reduction is range. Range for an electric with a lead acid based battery system (AGM or TPPL) would be very much the same as having a 1.5 to 2.5 gallons of diesel for a diesel powered boat. You would likely increase the like for like to about 2.5 to 3.5 if installing Lithium batteries.
So the question is, would you leave your slip/mooring/anchorage with the limited range of Electric. If yes, than go electric. If you are comfortable to leave your slip/mooring/anchorage with the knowledge that you have a non-marine genset that would expend you range at modest speeds then, again electric is a good solution. If sailing plans include significant motoring due to time constraints or skill, than you may want to spend the money and live with the diesel until battery technology allows you to go electric.
Mike, Electric Yachts of California/Pacific
Posted by: mike@electricyachtssocal.com
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