I agree on everything You said ...
was just pointing out that almost always, a *much* bigger prop is the
right choice, for a displacement boat.
I would expect that the large prop, at lower rise, probably much lower,
would be ideal.
Tugs typically do so, at a rise of perhaps 10:2 or so, add diameter to suit.
The common boat props are usually about 1:1, +/- 20%.
These have evolved due to how the fairly inefficient ICE engines work..
Typically around 3000 rpm, give or take a thou.
and because almost all boats are sold in the USA, and in the USA gas was
very cheap, and still is.
About 50% of all recreational boats made globally, are made in the USA.
Thus, US marketing favouring HP, not really caring about efficiency (as
gas costs were not really material), led to engines much larger than needed.
Speed became desirable, acceleration on getting to planing, etc.
As almost all on this list know, hundreds of examples show that about
1/3 of electric engine == similar performance (2-5% less, perhaps) ICE
engine.
Almost certainly, a really good prop optimised for a given speed at 85%
hull speed +/- 2-4 sizes larger would make the electric engines quite a
bit better, still.
There are multiple seeming "sweet spots" in any combo of
engines/drivelines/props not optimised.
Thus, while an ICE engine "wants" to run at 3000 rpm, with 2:1
transmission, giving 1000 rpm at prop, typically, huge variations are
seen with electrics.
All numbers nominal, typical common numbers used today.
As I pointed out multiple times in the past, using en engine to pull the
boat via wire/rope will quickly show that most of the power used is lost
at the prop/driveline.
Ie the same engine will deliver double, triple or more power if using a
lossless system like pulling via a rope vs a prop in the water.
Of course, double the power is ususally == 1 knot more speed, more or
less, once you approach hull speed in small craft (under 20 m).
On 12/05/2017 07:16, 66b6dcd5b59507e7d751ea81382ea1f6 wrote:
> Hi Hannu,
>
> I think you have missed the point of this thread. There's no dispute
> that a large, slow turning prop is efficient. So it's not a question
> of do what you want, more one of do what you can.
>
> That is this thread is about direct drive related to the motors
> discussed and what prop can and cannot be fitted due to motor
> constants and the limitations in this situation they impose on prop
> choice, which decides (V) rpm vs (A) torque and ratios of one to the
> other based on prop diameter and pitch ratios.
>
> https://en.wikipedia.org/wiki/Motor_constants
>
> John R
--
-hanermo (cnc designs)
Posted by: Hannu Venermo <gcode.fi@gmail.com>
| Reply via web post | • | Reply to sender | • | Reply to group | • | Start a New Topic | • | Messages in this topic (28) |
No comments:
Post a Comment