Outboards for a catamaran

[Ado]

I'm aware of the suitability of the Yamaha 9.9s, I've used the for over 20 years. However your assessment of 40% less power astern is of great help, thank you. Could you please reply with the pitch and number blades on your props.

 

[AngusMcDoon]

Remember that best pitch depends on gearing. I believe the Yamaha's are geared lower.

My PDQ is about 8600 pounds and will motor ~ 7.8 knots. Should be well about 6 knots.

The stopping power of the twin Yamahas compared to the stopping power of a Tahatsu is considerable, IME, since I have had both. Like 40% different. Other that pitch, rpm, and diameter, is the fact that Yamaha props are designed to vent the exhaust away from the prop in reverse, greatly reducing ventilation. This is perhaps the thing that sets the Yamaha motors apart in reverse.

I don't think that anyone doubts that the Yams are the best for the application. The problem is, however, that if the extra long leg is needed, they are no longer made. I've had both as well. The Tohatsus are an adequate, if not ideal, replacement. We have to live with what we can get.

 

[Chiara’s slave]

The Tohatsu final drive is the same as the Honda. Meaning fine pitch is needed.

 

[thinwater]

I'm aware of the suitability of the Yamaha 9.9s, I've used the for over 20 years. However your assessment of 40% less power astern is of great help, thank you. Could you please reply with the pitch and number blades on your props.

11.5 dia x 9.25 pitch, 3 blade is the spare I have. They also vary some by year. But remember that the Yamaha props are GEARED LOWER than the other motors, that are just dinghy motors with longer legs. Lower rpm = more static thrust with the right pitch. And the Yamaha hubs have an exhaust reverser in them. You can tell that they are getting a good bight, because they do not over-rev in reverse.

The PDQ uses long shaft, because the motors are in nacelles in the cockpit, probably 10 feet forward. They also do not plunge up-and-down as far in the position as transom mount.

I'd give some though to revising the engine mount. Might not be that hard.

 

[Chiara’s slave]

11.5 dia x 9.25 pitch, 3 blade is the spare I have. They also vary some by year. But remember that the Yamaha props are GEARED LOWER than the other motors, that are just dinghy motors with longer legs. Lower rpm = more static thrust with the right pitch. And the Yamaha hubs have an exhaust reverser in them.

Even given the gearing, 2.95, that seems a tad coarse. 9.25/12 x 5500/2.95 x60 /6000 (pitch in ft x prop rpm x mins per hour /ft per mile) gives over 14kn. Allowing for slip, it’s a bit coarse.

 

[Ado]

I've attached a photo showing the outboard arrangement. The 9.9s are shown in the down position, pulling on the lift strings raises the outboards up the riser tubes and simultaneously rotating them through about 60 degrees lifting them will clear of waves. The system is light and easy to use and has worked well for the 35 year life of the cat.