T-Pain Is On A Boat: Re: [Electric Boats] Props for EP

Wednesday, December 16, 2015

Re: [Electric Boats] Props for EP

Exactly.
I fully understand why most dont drop the driveshafts, and increase prop size.

In your case, you cannot, probably, do much.
Or, it might be, that a same size prop but much lower pitch, might work better.

The dynamics with electric drive are totally different to diesel or gas.
Electric motors, if very well driven (modern controllers), are pretty much insensitive to rpm and load vs efficiency.
Thus, its perfectly possible, that a 14/4 prop might be "better" and much more efficient - if the prop itself is efficient at the common range usually used.

A prop made for ICE use is not likely to be efficient in such a way, imho.
They are not looking for high dynamic range, as the system cannot deliver it.

Basically, because a modern electric drive is pretty much perfectly efficient at 300 rpm, 1000 rpm and 3000 rpm.
In fact its still efficient (no heat) at 1 rpm, you just have little torque, relatively, in the same nominal drive.
(Actually, you usually have 300% extra, for some time (peak torque on servos and VFDs, short-time-rated, 3 secs to 30 minutes).)

As long as power in == power out, heat out == zero to low, there are no-to-low losses.
This applies to modern traction drives (servo drives), == VFD/3-phase motors, and thus potentially to marine main motivators, ie yacht engines.
The dynamic range re: efficiency is potentially 10-50x higher with (really good) electric drive, than any gas/diesel/geartrain combo.

My pov has long held, that the actual, "right" choice is approx 150-200% greater diameter, or approx 300% area, at 1/3 the rpm, at propeller.
And with, potentially, huge efficiency increases with electric drive.
There is a great deal of evidence for this.

1. Early 1900x, commercial ships in 200-500 gross tons size, used about 100 hp engines.

2. A friend had a 24 m steel displacement boat, 70 metric tons.
It was grossly overpowered with a 100 hp very-high-torque engine (from a train engine) he put in.
(Normal use was 10-20% of power available, give or take).

3. Early rule of thumb was 1/h hp per ton of displacement for commercial use.

History, marketing, yacht sales:
The situation was distorted by $$, since about 50% of all yachts worldwide are made/used in the USA, thus USA == the market.
For a long time, (1950-2000, give or take) power and ICE engines were really cheap in the USA - thus efficiency was not a priority, in the US yacht market.
US gas prices are about half of everyone else in the (OECD) world for historical reasons.

Marketing led to the preference for "more power" without any real need of actually using it efficiently (or not much at all, often).
There was little financial incentive to do so, and great commercial pressure to market "more power".
Thus customers got used to looking for "more power".

The yacht industry works on what sells - rightly - and not what you need, technically.
Otherwise, providers cannot stay in business.

And yes, I know about the drawbacks, scaling, lengths, reserve, wind, engineering, etc etc etc.

Yet, a larger prop is a "better reserve", vs a small prop and triple the HP, in major storm/wind/wave conditions.
It will hold much better, and give more push (albeit slower).

Hope this is not boring of offending people ...

On 16/12/2015 18:53, biankablog@verizon.net [electricboats] wrote:

Since many of us who have converted to EP are limited to which prop parameters can be changed. For example the original 3 bladed prop I'm using is 16 inches in diameter has a 14 inch pinch. Like many others I can not increase the diameter but, could change the pitch or number of blades

It is working well but, I don't know if I could do better. Think there is a real need for a program geared to EP systems to find the best‎ prop parameters for each boat with EP. Sounds like a great engineering grad project. :)

Sent from on board BIANKA

http://biankablog.blogspot.com