Mike G wrote:
To refresh. I've got an "85" OMC 1.6L seadrive with a 13 1/4 X 17 prop
on it that needs replacing.

The nice folks at West Marine were kind enough to send me a huge catalog
full of things I can't afford but, since replacing the prop is a
necessity that was one of the things that got some serious attention.

My question relates to the ProPulse composite material, adjustable pitch
prop. I'm figuring on dropping over there sometime next week to take a
look at one and the Michigan Wheel Vortex but until then.....

West lists the flex of the composite as about the same as the aluminum
prop so I figure that and the reduced weight over an aluminum prop is
going to be a plus. Don't bother with that little item, you won't change
my mind. I've been working on my own vehicles, cars and motorcycles,
since I was fourteen and it doesn't take a rocket scientist to know that
the less hardware you have to sling around the more the horses get
translated into power output.

So, anyone with any experience with these props?

Further, from the chart I will either have to sacrifice a 1/4" on the
diameter or gain 3/4" since the props appear to come, as far as my needs
go, in only 13" or 14". What can I expect in operation if I sacrifice
the 1/4" or go for the 3/4" gain? Assuming I keep the 17" pitch that is.


Here's a thanks up front.


The composite props probably do flex a little bit but aluminium boat
props don't "flex" to the extent commonly believed. Indeed anyone with
any experience of aluminium will know that aluminium props, particularly
the thinner tip areas, would have a very very short life if the blades
were "flexing" back & forth on every revolution, what?? 3000 times a
minute at WOT??? snap:-)

The major reason stainless props perform better than alloy ones is the
blades' profile is thinner for the same strength, i.e. much stronger
(yes, yes & heavier:-)) material for a given volume allows thinner &
better finished blades. The extra weight of the material is even
compensated a little by needing less volume to achieve the same or
better strength.

Composites are light for a given strength but not very dense (i.e. big
on volume, well under 2 tonne a m3) so they lend themselves to
applications where weight to strength is paramount & volume is not as
important; say aircraft, race car or yacht rig components.

Aluminium alloy is also light for it's strength but again not very
dense (i.e. big on volume, only around 2.7tonne a m3), as above, good
where weight is a priority or as in this case corrosion resistance vs
strength vs ease/cost to mass produce, volume comes a long last:-).

Steel, stainless or otherwise, is heavy for it's strength indeed
composites can easily exceed the strength of steel but always when
compared weight vs weight (mass actually before you yell at me Del:-))
but steel is very dense (i.e. low on volume, around 7 tonne a m3) so
despite is relatively low strength it can achieve a given strength
target in less volume, so when weight isn't as critical a consideration
but least volume for a given strength is; then steel can & oft does win.

A boat prop is like most things moving in a fluid & trying to achieve a
driving reaction by displacing the fluid beit a boat prop or an
aeroplane wing, in that they consume power in two ways;
(a) the first is the usual parasitic surface drag caused by the object
being forced through a fluid, beit a boat or aeroplane hull or a boat or
aeroplane propeller. Clearly anything that can be done to minimise this
primary "drag" saves power like; smooth surfaces (i.e. a shiny stainless
prop vs painted or pitted alloy surface) means lass surface drag. The
next thing to minimise primary drag is to use a shape that will least
resist flow over/around the surface (i.e. sharp entry, minimum
thickness, the faster the article is traveling through the fluid the
more critical this is) So seeing water is about 600 times more dense a
fluid than air & boat props are moving through is a huge blade speeds,
this primary drag is a very significant consumer of HP & it is totally
wasted HP because it doesn't contribute to thrust.
(b) the second drag is the drag created by the actual displacement of
the fluid the article is being forced through (i.e. the angle of attack)
the higher the angle of attack, up to a certain point, the greater the
displacement of fluid & therefore the greater the secondary "drag" or
consumption of HP. This is the drag that does create thrust & ensuring
the fluid is displaced at an even pressure over the entire surface is
important to efficiency, so propeller blades are twisted such that every
part of the blade sees the same pressure over most of the blade's surface.

So composite blades might flex a little but alloy blades do only a very
little, what does matter is the surface finish & thickness of the blades
for a given strength & this is why diam for diam, pitch for pitch the
stainless props perform better, because they waste less of the available
HP overcoming primary drag, this means more of the available HP left for
secondary drag which, if the blade wing or whatever is properly shaped,
is translated into thrust or lift.


K