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Posted by on Nov 11, 2010 in Outboard Engines, UK, US |

Outboard Expert: Yamaha SDS Prop Hub

Innovation takes the "clunk" out of shifting gears at Yamaha.

In January I wrote about the Yamaha Shift Dampener System (SDS), a feature of the new Saltwater Series II props offered for the 4.2-liter Offshore outboards, but felt that that report was incomplete because Yamaha didn’t have a good photo of the hub design, and I didn’t have a chance to take one at the media event. But now I have that image, because Yamaha brought a propshaft with the basic SDS components attached to a test session it hosted last month in Georgia. If you didn’t quite get the SDS premise before, these photos might help.

Yamaha SDS components include the keyed washer in the foreground and the splined rubber damper, shown here in place over the prop shaft. The damper mates to the shaft with a splined bronze insert, and is press-fit into the prop. It’s been removed from the prop here for clarity.

Yamaha SDS components include the keyed washer in the foreground and the splined rubber damper, shown here in place over the prop shaft. The damper mates to the shaft with a splined bronze insert, and is press-fit into the prop. It’s been removed from the prop here for clarity.

SDS serves a purely aesthetic purpose – to eliminate the “clunk” sound that is the take-up of freeplay, mostly in the shifting mechanism and bevel gears in the gearcase, when the motor is shifted into gear. The SDS hub features a splined rubber damper mated to splines within the stainless steel prop hub. The rubber damper, which is a softer material than was used in previous prop hubs, acts like a torsion spring and allows the prop to rotate about 3/8-inch before engaging a keyed washer against slots in the prop hub. When thrust is initially applied, instead of banging against the driveline the prop now sort of gently “snugs up” to the prop shaft. No more clunk.

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When assembled, the keyed washer fits into slots in the prop tube. When the motor is shifted into gear, the prop twists against the tension of the rubber damper until the keys on the washer engage with the slots in the prop hub, creating a solid connection between the prop shaft and the propeller.

As I mentioned, this is a cosmetic enhancement. Yamaha says SDS does not enhance gear-train durability. Outboards have always smacked into gear, but as the motors have become quieter and quieter, that clunk has sounded louder and louder. These motors are supposed to be refined. Motors that go clunk don’t belong on a $250,000 boat.

This illustration shows how the SDS keyed washer engages the prop hub.

This illustration shows how the SDS keyed washer engages the prop hub.

For now, Yamaha is offering SDS only on the Salt Water Series II in 13-inch to 23-inch pitch, which were introduced with the new 4.2-liter V6 motors but can be used on previous Yamaha V6 models with the separate purchase of the washer and some other hardware to match the prop. Yamaha says SDS will become a feature of its other prop lines in the future.

Charles Plueddeman