I replaced my old Martec model 778, Dia and pitch 12 X 5, 2 blade
folding in 2008 with a Martec Mk III Eliptec 2-blade folding,
non-geared. However I do not believe Martec is in production any
longer. One of the big concern at the time was drag when racing and
hull clearance. It was my understanding that two things happen with
insufficient hull clearance - vibration when going forward and walking
while backing. There was a formula that I can not locate for hull
clearance, my however 12" dia prop has a clearance of 8", which if I
recall was right at the minimum.
Since my C&C35 Mk2 has an offset propeller (not on centerline), I found
the J/105 Technical Committee evaluation of folding propellers
interesting. On Fireball the output flange to the propeller shaft is
marked with a red lines at the 180 deg. apart, to show when the
propeller blades are aligned horizontal to prevent a blade from drooping
at low speeds. I gave up jumping overboard in New England waters to put
a rubber band on the prop before racing a long time ago. I have copied
the evaluation below:
"The J105 has always been delivered with a 15 inch Martec folding prop.
The TC has analyzed whether the following replacement props are
permissible (all 15 inch): Flex-O-Fold Standard and Racing versions
(geared), Gori Standard and Racing versions (geared). Martec (geared),
Max Prop (feathering).
Most of these props are "geared" props, which means that the blades are
connected and open up in unison. Arguably, this feature provides an
advantage over the non-geared Martec since with the Martec, particularly
in light air and choppy conditions, a single blade could potentially
open up. On the other hand, owners of standard Martec non-geared props
are of course free to rubber band their props – although in cold or
polluted waters this may not be an alternative in practice.
Flex-O-Fold claims on its website that its product has somewhat less
drag than the standard Martec. They reference a Cruising World test
report from 1995 to back up their claim, although the test involved an
earlier version of the Flex-O-Fold prop with smaller blades and,
presumably, less drag. Another independent test report they reference
from 1997 indicates that the Flex-O-Fold and the Gori folding props have
approximately the same drag, which coupled with the Cruising World
report would indicate that the Gori, as well, may have less drag than
the Martec. Both these test reports indicate that the Maxprop has
significantly more drag than the Martec, Flex-O-Fold or Gori.
Notwithstanding these reports, Martec claims that its standard prop is
“the lowest drag sailboat propeller anywhere”, but Martec does not
provide any independent test results to back up that claim. It is
unclear which versions of the Martec were tested in these test reports.
It is important to note that *the prop on the J105 is angled to the flow
of water along the hull, thereby exposing the blades to the flow to a
much greater extent than if the flow were parallel to the shaft* – the
way the props were positioned in the tests. These issues make the
foregoing test reports of dubitable validity for our purposes.
The Technical Committee obtained samples of each of the Flex-O-Fold,
Gori, Martec geared and Martec non-geared props in order to resolve the
conflicting test results and manufacturer claims. In side-by-side
comparison it was clear that the standard non-geared Martec presented a
significantly smaller "bulk" to the flow of water than the geared Martec
or any of the Flex-O-Fold or Gori options, largely because the geared
props have much larger hub diameters (to accommodate their gears) than
the standard non-geared Martec. Hence, the resistance of the geared
Martec, Flex-O-Fold and Gori props will be somewhat greater than that of
the standard Martec. In conversations with the top US representatives
of Martec, Flex-O-Fold, and Gori, each of them confirmed that for a
J/105 application, the most significant drag factor among the folding
props being evaluated was the size of the hub, and that their respective
geared hub props would have more drag than a comparable size non-geared
Martec prop.
The TC has therefore concluded that all these geared blade props are
legal substitutes for the standard Martec prop provided they have a
diameter of at least 15 inches. In addition, based upon the above test
reports, the TC is of the view that the Maxprop feathering prop also has
somewhat more drag than a standard Martec of the same diameter and,
therefore, is a class legal prop for the J105 as long as its diameter is
at least 15 inches. The TC expresses no view on props other than those
listed above."
Don Kern
Fireball 12708
C&C35 Mk2
Bristol, RI
On 10/31/2018 4:34 PM, Charlie Nelson via CnC-List wrote:
If your Gori is sized properly based on the engine, transmission and
clearance (all likely given your performance), you probably have the
lowest drag propeller available.
Tests years ago on propeller drag in a water tank(published in
Yachting magazine) showed that the Gori had the least drag of any ~ 20
brands that were tested. You can probably get better performance
with others in terms efficiency under power (reverse or forward), but
you would be hard pressed to get a prop with less drag when sailing.
Charlie Nelson
1995 C&C 36 XL/kcb
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