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Impact Screws

December 2015:

As a component of our ongoing continuous improvement pogram we have been been researching ways to reduce wear patterns developing over time with particular emphasis both on Yanmar SD20 Saildrives which have instant engagement " dog " type clutches and higher powered units that will have commensurately larger blades fitted.

In both the above cases the force per unit area between the reverse roller and  blade root will be at its highest, focused on the portion of the reverse action where the roller is closest to the blade bearing.

We needed a solution that primarily offered metal to metal contact - thus eliminating any potential wear pattern developing on the Zytel™ blade root but of course required other attributes as well. Low mass - so as to not affect feathering, no potentail for crevice corrosion - as would occur with a sheet metal type addition, commonality with both K3 & K4 unit blades, secure attachment over time, simple to install using a common jig for left and right handed units and favourable economics to retain the value package of the unit.

The photo below shows what we have termed an " Impact Screw " fitted to a right handed K4 unit with Tri Rollers. Note the upper surface of the screw is set down flush with the  blade root so the Tri Roller will transition smoothly as it slides initially forward on the  blade root to the point of maximum pressure - then aft to the trailing edge of the blade root as the last 100º of the reverse action of the boss is completed.

The screw initially was a  Pozidrive # 8 gauge x 1/2" or ~ 13 mm  C/Sunk Self Tapping in SS 316 positioned so as to not interfere with the pitch screw .

Extensive bench testing was undertaken to check friction loads  and wear patterns including making Tri Rollers out of AB2 to compare friction of AB2 on SS 316 vs SS 316 on SS 316. We then fitted these to local vessels to ensure in service utility replicated bench testing.

With over 200 installations and high time units over 12 months of service including live-aboards with far higher than normal engine hours and not a single reported problem we have now made this modification a standard in all builds going forward.


A high time user with a Gardner Diesel and now 500+ engine hours since installation of a K4 with 19.50" blades has and very slow engagement speeds just reported near zero wear on the blade roots.


With the availability of  # 10 gauge square drive screws in SS 316 - we switched to these as they have  an increased Ø - which increases contact area while not interfering with the pitch screw which is very close at the center of the blade. It is not intended that these would ever need to be removed so they are inserted with  272 Loctite™. The close up photo below shows a K4 blade with 10g Impact Screw.