weight of prop vs. the outdrive.
I purchased 2 props (both alumininum) to reprop with.
The guy agreed with me that if I hit stuff; it's better to destroy the prop than
the outsdrive -- but for performance, a ss is the way to go.he said the weight issue is silly because it's underwater and doesn't weigh as
much and that no one would use ss if it wore out the outdrive faster. So -- Here I am; for curiosity sake; asking you if the weight of a ss prop
really does wear out the drive faster than an aluminum one. -j |
Josh
Jun 8
|
| Oh please. |
trainfan1
Jun 10
|
| I thought 4 blade helped lift the stern. |
Josh
Jun 10
|
| It might, but any prop, 2, 3 or 4 blade, with more blade surface area,
will keep a porpoising condition at bay with higher trim or jack plate
settings. Rob |
trainfan1
Jun 11
|
| Good post. I can't help thinking that you would get better gas mileage with
a more efficient ss prop. Notice I said mileage not performance.
One blade prop? You mean like an auger? |
Jim
Jun 11
|
| Semi-interesting discussion. But, essentially meaningless because propeller
selection is impossible to pinpoint except through trial and error. Any
number of computer programs can select the "perfect" prop for your boat
engine combination except they won't except by luck. They'll get you close
but perfection is only acheived by trial and error. Furthermore, the
perfect prop for one set of conditions will not be the perfect prop for all. Do you want perfect hole shot performance? One prop. Do you want maximum
speed? A different prop. Do you want optimum economy at lowest RPM? Yet
another prop. Whatever prop you choose will be a compromise and provide
perfect performance for only one, if any, operating condition. All we can
do is strive for a prop that provides good all around performance and
economy. As to the original question from the OP.... as Clams pointed out, it's about
enertia. |
Butch
Jun 11
|
| That is another consideration. The difference is going to be negligible
over the range of the fuel capacity of a recreational boat(18'-21' w/ a
27 gallon tank being on the large side). To make an accurate comparison, you will need a selection of props, a
GPS, & fuel flow meter(and of course your tachometer). > One blade prop? You mean like an auger? PWC jet pumps typically have 2 or 3 overlapping-blade
impellers(different dynamics - the water is captive) almost resembling
an auger, but I meant an actual 1 blade prop! That's why it is so hard
to balance. 1 blade makes less ancillary disturbance/interference in the water than
2 or 3 or 4. Props are always a compromise of blade thickness, materials cost,
balancing ease. Not to mention blade area, shape, rake angle, linear,
regressive, or progressive rake, overall pitch, cupping profile, hub
hydrodynamics, leading & trailing edge profiles. I have several props that purport to be the same, they never perform the
same. Aside from a few props that are CNC machined from billets, you
would be hard pressed to find two identical wheels from a production
run. They're almost like snowflakes. The lower the horsepower & speed, though, the less the differences are
noticeable. Rob |
trainfan1
Jun 11
|
| Do you notice a difference between performance in saltwater vs fresh? |
Short
Jun 11
|
| Why? Because you get your panties in a bunch when dissagreed with? It's
theory, and I have enough practical pratice (in the real water, not fixing
propellers) to dissagree. All things being equel, you can drop a pitch when going from aluminum to SS,
because the SS blades are thinner and cut water better. Perhaps aluminum flex works against fact that in some theoretical
high-horsepower environment - but not in day to day operation with normal
horsepower ranges.
-W --
CC Marine
cc-marine@(spamblock)earthlink.net
Lake Hartwell SC
"Where the fish are always biting, and the swimmers are always nervous!" |
Clams
Jun 11
|
| Errrrr, I meant go UP a pitch. - not drop a pitch. <sigh> --
CC Marine
cc-marine@(spamblock)earthlink.net
Lake Hartwell SC
"Where the fish are always biting, and the swimmers are always nervous!" "Clams Canino" <anon@nospam.com> wrote in message
news:Dagbi.19343$Ut6.9935@newsread1.news.pas.earthlink.net... > All things being equel, you can drop a pitch when going from aluminum to
SS, |
Clams
Jun 11
|
| HEY I am merely the messenger. That's what the leading prop rebuilder in
the US told me 15 years ago. I have no reason to doubt him.
|
Steve
Jun 9
|
| It's not MY theory, and it's not theory. thanks for the input. I remember
now why I left this group 3 years ago.
|
Steve
Jun 9
|
| It's an old wives's tale. Try to duplicate your aluminum prop "flexing" in your shop and report
back with the results. AND, there is always a prop rebuilder better than your guy. Or my guy.
"Leading prop rebuilder" is a pretty brash & open-ended statement. Rob |
trainfan1
Jun 9
|
| Steve, it's not even not theory. You're disseminating false information, that's all. Rob |
trainfan1
Jun 9
|
| oh really? well take it or leave it then. why would a person put out false
information if they didn't work for the government? |
Steve
Jun 9
|
| well he serves just about the entire Midwest. I rekon that says something
for his work.
|
Steve
Jun 9
|
| Dang you take things way too personally -- you're the one that brought up that
if flexes that much. sorry to have gotten your panties in a bunch |
Josh
Jun 9
|
| I doubt everything anybody tells me until I prove it to myself. I can't
believe you just believe what people tell you! ps: Based on that, cavitation and boiling won't happen as much if you use a
stainless steel prop either..... well, that's a bad example cause its partly
true since those issues are caused by bent props, nicks etc. |
Chuck
Jun 10
|
| I saw a bunch of high speed film one time of different types of props
and the engineering professor's film illustrated that they don't
"flex" as much as "vibrate". The more tuning the props went through
(like for roundness, blade weight and shape, leading edge rounding
[you'd be surprised at what that does to a marine prop]), the less the
vibration. I asked several prop machinists including the guy over at Ocean State
who does a lot of prop work in New England (including big wheels for
cruisers/fishing trawlers, etc.) - they all pretty much said the same
thing - not enough to matter in the sense that a aluminum prop warps
enough under load to change pitch. The consensus seems to be that the real difference is weight, shape
and more material related issues - stainless props are more uniform
and balanced than aluminum, they are tougher and less prone to nicks
and gouges from minor debris and, in general, don't degrade in salt
water as fast as aluminum. For fresh water use, there really isn't much of a difference besides
looks. As to thre blade/four blade - I've noticed a heck of a difference
between three and four blade props of the same size and pitch. I'm
firmly in the four blade camp. |
Short
Jun 10
|
| well considered thoughts to the readers of rec.boats:
Both do, but aluminum will bend up to about 4 times as much..... Check the values for "modulus of elasticity:" http://www.engineersedge.com/manufacturing_spec/strength_properties_non_ferrous_metals.htm
http://www.engineersedge.com/manufacturing_spec/strength_properties_non_ferrous_metals.htm >> and looses some pitch. Stainless does not.
>>
|
Gene
Jun 10
|
| The chart you are referencing gives (), or -0- for shear modulus
of elasticity on cast aluminum alloys until brass is introduced. There
is some tension, of course(as in an application as fastening hardware). Aluminum alloy props are very rigid & brittle as the chart shows. Rob |
trainfan1
Jun 10
|
| well considered thoughts to the readers of rec.boats:
Lemme try again... I screwed up the links:
http://www.engineersedge.com/properties_of_metals_strength.htm
http://www.engineersedge.com/manufacturing_spec/strength_properties_non_ferrous_metals.htm MoE for die cast aluminum is 10.3; for stainless 28+. Your assumption that () = 0.0 is silly. If that were so, cast
iron and/or lead have no yield point.
|
Gene
Jun 10
|
| It appears that you are quite knowledgeable on the subject. What are your |
Jim
Jun 10
|
| It's not the weight per-se, it's the inertia of the static ss propeller mass
when you shift into gear from neutral that wears the dogs a little faster
than aluminum. Once it's actually spinning all is pretty much equel. -W |
Clams
Jun 9
|
| fwiw ... I was advised by the dealer to step down in pitch e.g. 23" to
21" when going from aluminum to SS. Never looked into it further, but
a step down would imo reduce stress on the outdrive. |
Jun 9
|
|
Odd.... because WOT RPM's actually rise a bit going to SS. -W |
Clams
Jun 9
|
| Another argument to choose Volvo-Penta Over Mercruiser. Rob |
trainfan1
Jun 9
|
| The reason for the step-down recommendation is because the aluminum flexes
and looses some pitch. Stainless does not.
|
Steve
Jun 9
|
| Stainless props also typically have a more pronounced cup to them, which is
another good reason the drop in pitch is usually warranted when moving from
aluminum to stainless. |
RG
Jun 9
|
| The pressure created by the prop pushing the boat is orders of magnitude
greater than anything you could attribute to spinning the weight of the
prop. The different between aluminum and ss would not be noticable. "Josh Assing" <XjoshX@jAssing.com> wrote in message |
jamesgangnc
Jun 9
|
| Prove it. Rob |
trainfan1
Jun 9
|
| flexes - I think that's generally accepted; but flexes to the point of loosing
2" of pitch? I find that hard to believe. On this theory; I'm giong to be way under propped as I had a SS 19 and I went to
an aluminum 17; so on your theory; this will be like a 15 pitch SS.... |
Josh
Jun 9
|
