Ernest Scribbler writes:

My 77 Thunderbird rode no rougher than any equivalent
conventional hull:

http://blizzard.zmm.com/thunderbird/starboard.jpg


Nice boat. Looks like it's in great condition.

According to one of the photo captions, the stainless steel
prop reduces the top speed by 5-7 mph despite both props'
being 14.5" diameter and 21" pitch. Why would two props of
identicial specification have such different performance
characteristics because of one being steel and the other,
aluminum?

My father's 95-hp 1966 Mercury outboard, mentioned in an
earlier post, had a two-blade aluminum prop. I remember
my father telling me that a three-bladed prop would've
given our 16-foot Crestliner day cruiser a lower top
speed, but would've had more pulling power (like for
towing another boat, or pulling water skiiers).

I can certainly understand why a three-bladed prop would
give a boat more pulling power than a two-bladed one. That's
intuitive. But why would a two-bladed prop offer greater
speed? I'd have thought that the more blades, the greater
a boat's pulling power *and* top speed.

It occurred to me that maybe the explanation had something
to do with more blades rendering the prop less efficient by
creating more cavitation, but it seems like that would have
a detrimental effect on both aspects of performance.



Geoff

--
"The sky was low and heavy, like the brow of a retarded child."

"Geoff Miller" wrote
According to one of the photo captions, the stainless steel
prop reduces the top speed by 5-7 mph despite both props'
being 14.5" diameter and 21" pitch. Why would two props of
identicial specification have such different performance
characteristics because of one being steel and the other,
aluminum?

It's the other way around, the stainless is faster. I believe it's primarily
because the blades of the aluminum prop are much thicker (in order to get
sufficient strength from the weaker material) and this reduces its
efficiency.

(Geoff Miller) wrote in news:TvydnR2K2
net:

http://blizzard.zmm.com/thunderbird/starboard.jpg

The note says the aluminum prop is the slower prop. It is normal for a
steel prop to have faster top speed than aluminum, all other numbers being
equal, because steel allows for thinner blades and thus less drag. The
steel prop may have slight efficiencies due to shape as well, such as rake
and cupping, which are easier to do with steel and seldom seen with
aluminum.

On Fri, 16 May 2008 14:49:06 GMT, Reno wrote:

(Geoff Miller) wrote in news:TvydnR2K2
:

http://blizzard.zmm.com/thunderbird/starboard.jpg

The note says the aluminum prop is the slower prop. It is normal for a
steel prop to have faster top speed than aluminum, all other numbers being
equal, because steel allows for thinner blades and thus less drag.
Yes
The steel prop may have slight efficiencies due to shape as well, such as rake
and cupping, which are easier to do with steel and seldom seen with
aluminum.

Aluminum is easy to machine, not true of stainless steel. Bronze is
easy to work with, and has long been a favorite material for props.
Stainless is quite good, aluminum is cheap.

Casady