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Old 06-04-2010, 06:26   #1
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Mounting Two Outboards Instead of One

Does anyone know what the cummulative effect is of mounting 2 outboards instead of a single.

Thinking of Twin 9.9 HighThrusts on a larger sailboat. Lower RPM - Larger props. These OB's will accommodate up to an 14 inch dia prop at 11" pitch.

Effect of two props turning I don't think it's a straight linear relationship (everything is x 2)

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Old 06-04-2010, 06:34   #2
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If one will get you to hull speed then all two does is double your fuel consumption and your maintenance costs, except of course for having a backup when engine number one decides to take the day off. If one doesn't get you to hull speed then two will get you closer to it. The high thrust outboards turn the props relatively slowly due to their gearing, so if your boat is big enough you may run out of prop pitch before reaching hull speed.

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Old 06-04-2010, 06:46   #3
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Having two props versus one will definitely have a difference but I don't think it is what you are going to notice in the long run. The factors that really matter are prop diameter, pitch, blade shape, depth, and positioning relative to the rudder. Outboards by definition don't do a good job of putting the prop low in the water. In addition, they can be hard to maneuver under power because of a lack of waterflow over the rudder unless you actually turn the outboard which adds a level of complexity. Two outboards means twice the running gear in the water which will add to drag some and two props so you will double effects related to the hub, tips, etc.

Having two props means having two engines which means twice the maintenance and significantly more cost. In addition, you will need to find a way to hook up two sets of controls to your satisfaction. Two engines does give you redundancy (without much thrust) but maintenance will become a chore.

On most sailboat hulls, it will be extremely hard to mount twin engines in a satisfactory manner. Many modern transoms have a swim ladder or something in the center which won't allow an outboard to be mounted on the centerline. With two engines, you will end up with mounting off of centerline by definition. This will make motorsailing only possible with one engine at most times because the propeller on the other engine will constantly clear the water. There was recently a thread on one of the sailing forums where someone tried building a mount for a larger sailboat that would pull the outboard clear of the water and they gave up due to problems with prop depth and travel.

It will certainly work for some situations but it might be a real hassle.
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Old 06-04-2010, 17:52   #4
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If one is powerful enough no reason to have two, unless you have a power boat.

Getting a proper prop (better adjusted for slow speed / heavy displacement boat) does seem to work. I have seen people do it on small outboards too. Regular outboards' props are designed for fast ride in a light hull.

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Old 06-04-2010, 19:24   #5
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How about one electric and one gas?
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