Do as You want, but...
All technical lit., all theory, says the bigger prop is better, by about
pwr2 / radius, in efficiency.
So does most anecdotal evidence.
Yet...
It is also quite true that most-all current props/drivelines are very
inefficient.
Compare a motor pulling with a rope from a tree on-shore to a prop, and
a huge difference is observed.
Usually, 100-300%, imho.
Thus, mostly, vast differences in power use/efficiency are observed with
ICE, with electric propulsion, with multiple very different prop setups.
The big differences are because any single part of the multi-stage
equation can have a major effect, giving rise to all sorts of
optimisations, which are quite true, right, and work, but are based in
improving a basically flawed system.
A friend setup a huge engine of small power, in a very heavy
displacement craft, like a missisippi riverboat.
70 tons displacements, 24 m, steel.
About 100 hp power, 2000 kg in mass from the engine , from a train
locomotive.
Vs the old ICE engine the boat was hugely overpowered, and the prop
would froth / freewheel effortlessly.
In practice, the engine was used at 10% or so of power, worked extremely
well, used very little power.
One tank of gas = all summer.
Imo, for what it´s worth, the last thing You want to do is cut down the
prop D.
Reduce rise, if you want to or need to, with current propeller.
A bigger prop is much better.
Slower rpm at prop is better.
My opinion/understanding, is that props should be about double-triple
current diameters, per boat size.
Should run at about 400-600 rpm.
Should have very low rise, about 1/4 typical, or about 10" D with a 2-3"
rise, or 1/4 or so, vs typical 1:1, 1:0.8 - 1:1.2 or so.
Subs use things like this.
Tugs are like this.
Marine engineering, physics, textbooks, industrial practices support me
on heavy slow-displacement craft,
although not applicable to semi planing stuff or fast or light boats.
A sailboat is a slow-displacement craft, imo, ime.
An ideal perfect prop driveline has zero turbulence, zero wake, zero
bubbles, zero slip, zero losses.
A prop at 1200 rpm will always have more turbulence/bubbles than one at
400 rpm.
(but due to the many effects involved, in some cases a 1200 rpm prop may
be better.)
There are many effects involved ... stick/slip, sticktion, caviation,
boundary-layer conditions, area/radius/force/effiency effects, etc.
My point is:
Our current props are, typically, extremely inefficient (ice, typical
boats).
This is mostly due to history re: us gas prices.
In the US, Gas was cheap, efficiency was immaterial, power was marketed
as desirable.
The us is == 50% of all leisure boats in the world.
Thus, what us marketers successfully sold became accepted yacht wisdom
over 1-2 decades.
In 1920-1930, a commercial 30m trading ship, one of the first ones using
engines, might have had a 150 hp engine.
300 metric tons.
The naval architect rule for decades was 1/2 hp per metric ton.
For decades, small sailboats with "beta marine" 12 hp engines, 10-12 m,
worked fine, all over the world.
With tiny inefficient props and drivelines from the 1970s etc.
Because it is almost zero-marginal-cost vs desirability for a yard,
(majority of market 10-12 m) sailboat/yacht engines hp grew from 12 hp
to == 50-100 hp, over 40 years.
Mostly because of (US) marketing advertising more hp for "storms etc".
At the same time, prop efficiency grew not at all, or little, because
significantly bigger props = more draft = "more risk" in marketing, less
"gunkholing", "less accessibility".
And because the market, ie us buyers, pay almost nothing for gas in
sailboats/yachts, so it is not a concern.
Personal anecdotes:
I have been in major storms 4 times.
In no case has motor power been relevant. Reliability, yes, very important.
Hurricane aftermath off africa (11 m sailboat) as a kid, near survival
conditions off tunisia (F7-8) in a baia azzurra 24 m powerboat (offshore
deep v), training at crouch sailing school off essex in force 9, and
"securite-securite" offshore menorca 2 years ago single-handed.
I came in to the port in menorca, because I was tired after 2 days, and
saw that the other option was 2 nights more at sea - where I would have
headed off about 50 miles, to be safe.
In hard conditions ... but a sailboat lightly loaded surfs.
It was a judgement call .. I was fine .. but might not have been. It was
scary. And dangerous.
I have lots of experience with scary and dangerous, and zero desire to
acquire more.
(not saying it was the right choice. Have done the other choice, backed
up, as a dive team leader, multiple times. I don´t mind being seen as
party pooper or pussy, if I must.)
With some, any, working motor power (electric power) in a sailboat
practically all storms are survivable, with minor problems.
I consider vomiting in the hull, not eating for 3 days etc. minor problems.
Unpleasant, disgusting, but not dangerous, for anyone with training and
mental aptitude.
A bigger engine will not make You less nauseous or more safe.
Often "bigger engines" make less-experienced people take foolhardy
decisions since they have a less-than-real appreciation of natural
forces vs ship capacities.
No boat, ship, yacht can actually resist any sea state appreciably,
unless it is a nuclear carrier, and even they may take upto 29% lists,
according to members who served.
On 11/05/2017 20:44, king_of_neworleans wrote:
>
> So let's say I cut one of my 14x10 props down to 12", go with a top
> sustained RPM of 1000, stick with the 48v bank, what Lynch motor do
> you think would best suit my needs? I dont even see HP rating on their
> site. They really have very little info there. I don't know anything
> about all that newton stuff. I guess they aren't really trying to
> capture the US market.
--
-hanermo (cnc designs)
Posted by: Hannu Venermo <gcode.fi@gmail.com>
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