Jeremy,
In advance, I apologize for the "wall-O-text". And, please consider that the views and ideas expressed are my own and may not sit well with others that peruse this forum.
A significant amount of my experience and knowledge are focused around the needs of my conversion with respect to outboards, motors and controllers, and props. I believe there are others that frequent this forum that are far more versed, and the engineering types, (armchair and practicing), have all the rules of thumb and pat answers you may be seeking. I can tell you that in my humble experience, that the increase in output is a significant factor for each inch of diameter you are able to use in the prop. I see many visitors trying to "cheat the system", by attempting to use too small a size of motor, drive unit, and battery bank. Years ago, My father-in-law, an electrical engineer, and practicing electrical contractor explained some elements of what he called "practical electricity." Here in I will most assuredly get a plethora of engineering types that will contradict, quantify, and exceptionalize what I am about to say. But if you will remember that I am talking about the "practical application, and not the exceptions or the extremes, my statements are true. It is this: In a range of use, it does not take any more power to turn a given prop if one uses, say a 5 KW motor, or a 10 Kw motor. Albeit an engineering fine point that they will talk about mass, inertia, resistance, etc., the truth is, that in the real world, you put either in your boat, and both can function almost exactly identical to the smaller one. However, If you bought the smaller one because it was computed to be the smallest to still do the job, you will find over time, that you will be sadly disheartened. By adding the slight bit of extra capability, you will see over time, that the motor will run cooler, last longer, and work easier than the smaller one. This results to you in a longer lasting,quieter running system.
Here is a proven example: I recently sold a home that had a water system designed for 3 families to share a common well. The engineered plan had allowances for drought, "party days", down time for repairs, and well recovery allowances (cone of recovery). The system had storage tank plans for each home owner to pump off the well to fill their family tanks for 4-6 hours of an 8 hour segment. This gave the well 2 hours times 3, or 6 hours each day to recover. This was well within the capabilities of the well, and was far more than any one family could use in a day.
The first family that used the system followed the plan, and we all thrived, and the well worked great. That family moved away, and another family bought the property. They were instructed by both me and the engineers that had designed the system. The new family did not like the answers they were given and "felt" that they could push the system beyond design. Instead of sharing the well at the designed rate, they decided to commandeer the 3 share, and then, to press my family to give up our rest time to allow this greedy family to pump beyond specs. They pimped endlessly for 5 weeks straight, day and night. One morning the neighbor ran to my door franticly to exclaim, that "my crappy system" had failed. We discovered upon inspection, that the foolish neighbor had pumped the well dry, and then burned up the pump.
Here is another with relation to gensets many in the boating world are using: This has to do with the typical gas-powered "construction type" generators that I hear about "strapped to the decks" of their beloved sail boats. Here are the facts. This type of genset is designed around the target runtime of 2,000 hours. Some get slightly more, and some less. But the engine manufacturers have determined that this is the ideal time for a 3600 RPM engine, (same as lawn mowers). That equates to 90 days at 22 hours a day.
In the off-grid housing market, this is a norm for these "throw-away" systems. Some get 85 days, and some get 95 days, and then the new off-grid homeowner cusses the supplier for "selling faulty gensets." If the homeowner does his or her proper due diligence, they will be looking for an 1800 RPM, or even the harder to find 900 RPM gensets. These are much larger but last exponentially longer that the first ones.
The same is true with your boat drive system. I suggest you find a 10, 20, or even 40 HP outboard to convert. Or in a case like mine, look into the sail drives coming of age. Install a 10K, 20K or 25K motor, with an equivalent controller, and the system will be many times far more reliable. And you will be far more satisfied with the money you will only have to spend one time, instead of several times, and have to "pack a spare because they are so cheap". In the case of my tug, it was originally designed for a single 22", slow turning brass screw, driven by a 10 – 30 HP diesel. Through my research, it have determined that twin 14-16", slightly faster props will, ... theoretically produce equal to greater output that the original design. And in an effort to reach that size of prop, I opted for customizing a pair of 85 HP Merc lower units from salvaged O/B motors from my local marina. They are prevalent in my area and parts are not hard to find. And given that they were originally designed for 80-140 HP, the 25K motors will turn them without exception and everything has a comfortable margin of safety.
Most people that convert an O/B discover, an air-cooled motor doesn't need the water impeller and that reduces the torque requirements by as much a 1/4 HP or several amps. I would suggest you use the largest prop that will physically fit on the shaft. I have seen a lot on props, and the Dave Geer book on props is a good primer if you are serious about making this all work. I have noticed that most start with something less than a "square prop." That is to say, the pitch is most always less than the diameter. (The slang term here, "square prop" refers to a prop that has a pitch equal to the diameter, I.E. 14 X 14.)
Bottom line, if the motor is correctly sized, your best thrust is a larger, slower prop.
And lastly order a Sierra Catalog and study the gears in the back. Some O/B makers offer different ratios depending on if it is to be a ski motor or a fishing motor. When using the typical pod-type trolling motors, you are more likely to burn up a cheap motor, or internal regulator, than if you go with a more robust system like the several e-Boat suppliers will get you. Yes, the trolling motor is just a hundred bucks or two, but you then need to be prepared for the failure.
On your side note, Torpedo motors are a short-life designed motor. Think about it. They are made for a couple hundred yards, full out. Then they are shrapnel fodder. Reminds me of the aircraft tyre salesman that said his tyres were the best,... "Guaranteed to last you to the end of the runway!" He never added that the typical runway is NEVER more than 2 miles long.
enjoy,
Dan
ozarkcustomboats.com
Sent: Monday, December 5, 2016 12:24 PM
Subject: Re: [Electric Boats] Re: Any experience or ideas for newbie electric conversion on Newport 16 sailboat?
Thank, Dan, for the numbers on gear ratios. That sounds like an outstanding conversion. I'm curious to learn more about it.
This sizing of conversion components is a interesting puzzle. What effect does a larger prop have. I just met someone who used torpeedo motors on his boat. Yes, torpeedo, not Torqueedo.
Jeremy
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Posted by: "Dan Hennis" <dhennis@centurytel.net>
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