Using a larger leg is not necessarily a 'need' decision, but an availability decision. If you design a motor from ground up, you make different decisions than when re-purposing what you can find. With a few exceptions, all lower legs will have the same components to rotate, the loss in efficiency from moving more massive parts would be the loss in efficiency of a larger than necessary choice. There are so many potential factors in a decision involving re-purposing something for a specific need, it would be unreasonable to impose arbitrary limitations. After all, isn't it about the journey? If nobody made poor choices this would be a boring forum!
On 04/19/2017 12:05 PM, oak oak_box@yahoo.com [electricboats] wrote:
Scott,
I think one of the primary motivations of using a larger leg is so that it can spin a larger prop.Gas engines tend to spin at higher speeds so they spin smaller props faster; electric boats go at slower speeds that are more efficient with larger props. Hence, the thought that by using a higher power lower unit, you can fit a larger prop, and just spin it at slower speed.
John
From: "Scott Masterson smasterson2@gmail.com [electricboats]" <electricboats@yahoogroups.com>
To: electricboats@yahoogroups.com
Sent: Wednesday, April 19, 2017 12:29 PM
Subject: [Electric Boats] Re: My Electric Boat Conversions
My name is Scott Masterson. I have been performing marine conversions for years. My own personal projects are a 22ft Pontoon Boat, a 17.5ft 147HP Bowrider and a 9.8ft inflatable. Here are some videos if you are interested:
Here are my thoughts regarding the correct lower leg and prop for electric marine propulsion.
First, I am curious as to why people are convinced that they need a larger lower leg to accommodate a motor that is smaller in power than it is designed for. I understand the concept of electric motors providing max torque at revolution one and speaking as a guy who has "done that", let me assure you that all you will doing is creating a load on the motor beyond what the power curve of the motor's design. Simply put, most motors are designed to deliver peak efficiency and power at 3,000 RPM. If you put too big of a lower leg and prop that is not designed to that motor's max power, but two to three times it, all you will do is over tax the motor and it will become less efficient and most likely overheat before it can deliver it's rated power.
What you need to do is to match the lower leg to the continuous power rating of the motor. So if you have a 3kw motor, (4hp), you need a 4hp lower leg.
Scott
-- The greatest art is to shape the quality of the day ― Henry David Thoreau
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