Rule Interpretation RI-04-02

RI-04-02
REPLACEMENT PROPELLERS (2004-11-19)
Q1: Is it permissible to use a prop other than the standard 15 inch Martec non-geared folding prop?
A1: Yes, certain other 15 inch props are permissible.

Class Rule 1.2 states that " Except where variations are specifically permitted by these rules, J/105s shall be alike in hull,. keel, rudder . Similarly, Class Rule 1.3 states that "All yachts competing in one design or class sponsored events, shall comply with standard specifications published by J/Boats, Inc. and these class rules. No alterations or modifications are permitted unless explicitly permitted by these rules". On the other hand, it has been a long-standing view of the TC that the intent of the rules is not to legislate hardware brands. If the piece of equipment functions as intended, then it is permitted provided it does not provide an undue advantage over standard equipment. See RI 00-03, RI 02-15.

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.

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