Does someone have a good explanation of how the counter-rotating dual prop setups work?<br /><br />Do they turn fewer RPMs at wide open than a standard single prop?
i may be way off, but i think they operate exactly the same. one (usually the port engne) turns standard. the other simply rotates in an opposite direction to negate the torque of the other. keeps the boat from chine walking and listing too much. engines should be normal, putting out there rated hp and torque. the gearing is just reversed in one unit.
DDboater<br />The engine turn the same RPMS but not sure about props. Better fuel mileage is what I like and ofcourse no prop torque. Vovo recommends aluminum props. I got so use to useing prop wash to turn boat in tight places would take a lot of getting use to. Better hole shot and fuel mileage.
Thanks, Boatist<br /><br />Mercs' propaganda says the Bravo 3 offers a gear ratio of 2.2/1----does that mean the prop is turning two revolutions to every one engine revolution?<br /><br />Also, are both props engaged and spinning under power at all times?<br /><br />Did Volvo come up with the original design on this type of drive?
DDboater<br />I am not a expert on this but I think you have it backwards. 2.2 Engine RPMs to 1 Prop RPM.<br /><br />Yes both props turn anytime in gear.<br /><br />Yes I belive Vovo first to have and for sure before Mercury.
ddboater,<br /><br />Yes, both props turn in opposite directions. This is accomplished in the gear case.<br /><br />The engine turns 2.2 rev's per 1 rev. of the props.<br /><br />Volvo did pioneer this technology for the pleasure boat market.
I think I am beginning to understand, please bear with me.<br /><br />So the Bravo 3 props are spinning at about 1363 rpm with the engine running at about 3000 rpm, whereas on my rig, the prop is spinning at about 1829 rpm when the engine is running at 3000 rmp.<br /><br />Is this how the fuel economy is gained? What are the other advantages/disadvantages to higher gear sets?
ddboater,<br /><br />It really has nothing to do with the dual props, it's all in the gear ratio. You can buy a Bravo-3 for several Mercruiser engines (that's why they offer it in several gear ratios). If you chose two drives (say an Alpha 1 vs. Bravo 3) with the exact same gear ratio, and attached it to your 5.7L, the props will turn exactly the same. However, in my experience, the dual prop will give you a much better hole shot (but will be slightly slower on the top end).<br /><br />Cheers
The efficiency of the dual prop is less slip and more thrust transfered to the water. With two propellers you have twice as much surface area with three blades in contact with the water at all times. They like to run with less trim and less bow lift, that is why you lose a little top end speed. For a high speeds the Bravo I can not be beat.
DDboater<br />I agree with your prop 1363 rpm for 3000 engine rpms. I think I would call that a lower gear as prop turning lower rpms at same engine rpms. You can get with at least 3 different gear sets.<br /><br />The gain in fuel mileage is have less slip with 6 blades pushing. Also with single prop will have steering torque that you have to correct with a engine trim tab, this forces water to the side instead of back and causes drag. The dual prop best for large heavy boats.
Thanks for all the good info.<br /><br />My goal this year it to enter boating conversations with my friends and discuss only things I know something about.<br /><br />I continue to learn loads from this site.
Master Tech,<br /><br />So what's the Bravo II all about?<br /><br />It seems like you mostly hear about either the Bravo I (single prop) or Bravo III (dual prop).<br /><br />Buoy-10.
Buoy-10,<br /><br /> The Bravo II is the same upper housing with a much larger gearcase. It will swing a 20" propeller. Works good on heavy boats under 45 mph. and deisel applications.
