Propellors & Cavitation

E

Ed Price

Guest
----- Original Message -----
From: John Friese
To: amphicar-lovers@yahoogroups.com
Sent: Monday, June 30, 2003 12:40 PM
Subject: [amphicar-lovers] Re: Wanted - Propeller


--- In amphicar-lovers@yahoogroups.com, rogtwo@a... wrote:
> "I particularly like the idead because the rear edges of a bronze prop
> can be cupped which would help reduce Amphicar's high speed cavitation
> problem a lot."
>
> Cavitation???
>
> With the large blade area of the propellers and limited horsepower
of our
> Amphi's, cavitation seems most unlikely. Cavitation occurs when
there is a local
> low-pressure area on part of the propeller blade. These low
pressure areas
> are in turn created by heavy blade loading due to little blade area
trying to
> push too hard. With two propellers, our Amphis probably have twice
the blade
> area of a typical 50 hp outboard motor.
>
> What indication have you seen that there is cavitation? Cavitation
is almost
> always diagnosed because it creates pitting of the propeller blades.
This
> pitting would be on the low-pressure side of the propeller (that
would be the
> side you DON'T see when you look under the back of the car). The
pitting is
> often worse near the root of the blade. Have you seen such pitting?
>
> Roger
> White '63
> Seattle
>
> PS - I'll be away for 2 weeks, so I can discuss it more when I get back.

Roger,

Both my Amphicars have significant cavitation when operating at high
speed (3000 RPM and over). It is understandably less when passengers
are in the back of the car. This cavitation is inevitable because the
props are well above the bottom of the boat and at speed the water
won't follow the hull line up the into the tunnels, thus the props
tend to grab the air. Years ago, when I was racing outboard boats, we
ran the engines with the props significantly above the rear of the
boat and cavitation was impossibly high unless we cupped the rear edge
of the prop. I don't really know why this works so well but my
experience says it does. A bronze or brass propeller would also have
a thinner blade cross section and I suspect that would improve
propeller efficiency too.

John Friese


Remember that the prop shafts are only turning at 1/3 engine RPM, so the
propeller RPM might be around 1800 RPM maximum. And, if you cruise at around
3000 engine RPM, then the props are only turning 1000 RPM. That's slow by most
any marine standard.

I doubt that we could get into a more passionately argued topic than marine
propeller design. The number of blades, pitch angle, leading / trailing edge
geometry, blade area and diameter are just some of the design variables. OTOH,
Amphi props operate under relatively simple conditions. Not only is the power
transfer fairly low (remember the declaration of marine horsepower being under
10 HP?), but the propeller shafts are parallel to the direction of travel.
Further, an Amphi changes its pitch angle a only little when underway (even at
full speed, Amphis do not "climb out of the water"). Amphis are displacement,
not planing hull boats.

For the diameter of an Amphi propeller, I suspect that we could turn much more
than 1800 RPM without cavitation. I used to have a cruiser that had twin six
cylinder, 100 marine HP engines. Each engine turned its propeller at 1:1 engine
RPM. IIRC, the props were 12" pitch diameter; a bit bigger than an Amphi prop.
Typical cruise RPM was around 2400 RPM, and full throttle was around 2800 RPM.
And I never found any propeller (or adjacent hull area) evidence of cavitation
damage.

True, the cruiser's bronze props were beautifully curved. The Amphi's props look
a bit strange, almost primitive. The geometry just looks too simple to be
efficient. OTOH, I think fine tweaks in the Amphi prop design are likely to be
wasted, since the major design factors in Amphi water performance are dictated
by Amphi's weight, boxy design, low horsepower and displacement hull.

There is one area where efficiency might be improved. (Notice I said efficiency,
not speed. Speed is hopeless; but miles per gallon may be helped.) In general,
when you move a hull through the water, any turbulence is an indication of
wasted power. And Amphis sure do make a turbulent, bubble-filled wake. If you
can reduce your wake turbulence, then operating efficiency will increase.

Now I'm not a mechanical engineer, but my guess is that Amphi props are not
"starved" for water due to the shape of the hull (and the slow maximum hull
speed). But I do think that the flow near the props is way too turbulent, and I
suspect that the cause is the rear wheel and wheel well. I think that the wheel
and wheel well allow the prop to "suck air", causing the prop to be working in a
spongy mix of air and water. Since air is compressible, some power is wasted in
simply compressing air and blowing it aft into the turbulent wake.

I would love to see some underwater photos of an Amphi cruising by the
photographer. (Hint to you Florida amphibians; that would make a nice summer
project! <g>) Maybe a shot of an Amphi going directly overhead. That would
easily show what's going on with the flow (plus it would make for some really
nice pictures).

About a year ago, I had speculated that perhaps the propeller "shields" were
originally intended to improve the water flow past the props, by blocking any
wheel well induced turbulence. Comments from the group left me unconvinced about
the utility of the shields, although most felt that the shields were more of a
debris trap and increased the likelihood of propeller damage. Maybe we still
don't understand the real purpose of those barrier vanes.

As to tweaking the Amphi propeller design, the available space pretty much
limits us to the existing pitch diameter. And since the existing props have four
blades, going to five or more seems a bad idea. OTOH, the shape and area of the
blades can be changed. It might be interesting to see what a larger area,
three-blade prop with much steeper pitch would do. (It would load the engine
harder, so maybe max RPM would be limited. But then, an acceptably good cruising
speed might be met at considerably lower RPM, resulting in lower cruising engine
noise and better miles per gallon.)

See how much fun speculating on propeller design is? BTW, does anyone know of a
web site that has simulation or analysis software for propeller design?


Ed
El Cajon
67 Rust Guppy


[Non-text portions of this message have been removed]
 
N

nelson625@aol.com

Guest
Ed-
Regarding your mention of "remember the declaration of marine HP being
under 10 HP" - to what are your referring? Is this somethere in the Amphicar
Specs that the HP available to the props is less than 10? I ask because at one
local lake, they limit "boats" to some fairly low HP - I'm not sure just how
low, but it is posted. I'd be glad to learn that our craft have only 10 HP to
the props that I could support with printed specs. Vic Splash Nelson - 67
Split Personality


[Non-text portions of this message have been removed]
 
A

amphipoda

Guest
Regarding brass propellers & cavitations?

I agree brass propellers would be very cool to
look at. I might even buy into the weakest-link
keyway concept (which is pretty darn clever).
But I'm trying to imagine being the guy who
has brass propellers and brass balls big enough
to test that keyway concept. Sort of like the
first guy to volunteer for a circumcision...
"I'm gonna do WHAT?!" Of all the possible
outcomes only one is the preferred keyway
shear without harm to tranny, props, or shafts.
At nearly $5K per tranny it doesn't seem feasible
to test, but maybe I'm missing something. I
imagine even the brass props would be fairly
cost prohibitive for such research, and so are
the nylon ones for that matter. Still, I have to
admit brass props would look mighty awesome.

I don't profess to understand the hydrodynamics
of propeller cavitation, but I do believe Amphis'
essentially boil water rather than achieve the 8 or
10 knots of speed the engine is capable of. I had a
friend who actually did a quick computer model
of an Amphi hull & props configuration and ran
it through a hydrodynamic simulation. He told
me with only slight hull modifications to channel
increased volumes of water into the props that
Amphi could perhaps do 10 knots... but he also
said that such speed might pose other problems
(i.e. excess vibration) that could potentially damage
the transmission or worse. Still, I have to admit
going 10 knots an hour would be mighty awesome.

For each risk there is a potential reward... and
the potential for needing some very expensive
replacement parts. Reckon I'll just stay stock
and just marvel while the fast brass Amphis go
zooming past.

Amphipoda
'64 Turquoise
San Diego, CA
 
A

a_colo_native

Guest
<SNIP> He told
> me with only slight hull modifications to channel
> increased volumes of water into the props that
> Amphi could perhaps do 10 knots...

I was wondering about that too. I was thinking that maybe some sort
of horizontal "wing" of sorts mounted below and ahead of the props,
angled slightly upward to direct more water into the props would be
the ticket. Maybe I'll try that and see how it works out.


About 3 years ago I had checked into brass props. First off they were
$250 each! But the guy did say he could "polish them up shiny-er than
a dime in a goats ass". I am not experienced enough to determine just
how shiny that may be. ;) I'll just take that as purty durn shiny!

John
<{{{=<
 
E

Ed Price

Guest
Vic:

You can find a table of land and water horsepower in the Club's Members'
section:

http://www.amphicar.com/members/amphicar_horsepower.htm

Ed
El Cajon
67 Rust Guppy

----- Original Message -----
From: nelson625@aol.com
To: amphicar-lovers@yahoogroups.com
Sent: Tuesday, July 01, 2003 9:28 PM
Subject: Re: [amphicar-lovers] Propellors & Cavitation


Ed-
Regarding your mention of "remember the declaration of marine HP being
under 10 HP" - to what are your referring? Is this somethere in the Amphicar
Specs that the HP available to the props is less than 10? I ask because at one
local lake, they limit "boats" to some fairly low HP - I'm not sure just how
low, but it is posted. I'd be glad to learn that our craft have only 10 HP to
the props that I could support with printed specs. Vic Splash Nelson - 67
Split Personality


[Non-text portions of this message have been removed]


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[Non-text portions of this message have been removed]
 
N

nelson625@aol.com

Guest
Thanks Ed. Will do. Vic with the '67 "Split Personality" near Daytona


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