Engine Spacing??

[VCinRI]

I was wondering what some of the pros and cons were for spacing outboards further apart.
I Believe the norm is 28" on center but see other boats with their engines spaced quite a bit further. I did see one 30' Grady that had to be close to 4' on center.

Any experiences?

Thanks,
VC

 

[seasick]

While there can be issues with spacing that is too close, spacing farther apart has different considerations. The design of the transom and hull may play a more important role in determining optimal spacing. Factors like prop height and it's relation to the hull as well as water flow throuh the prop are important. For example, a steep rise hull when making turns will lean to one side causing the opposite side to rise. If that causes the prop to be starved for water then thrust is lost and cavitation may be a problem. Handling may also be affected since the thrust vector (line of force) will be directed more off center to the keel.
I guess there is no simple answer other than to go with the manufacturer's guidelines.

 

[Marty grady 272]

The only positive that I can see is that there will be more and faster control when of the direction of travel, particularly when backing down for docking. The farther the motors are apart the more they can effect the line of motion with respect to the center line of the boat.ie the boat will turn faster. It might help also help in forward motion if the hydrolic steering were to fail. Other than that I have no idea as to the effect that would be found with respect to forward power/prop efficiency that was spoken to in the first response. Marty

 

[seasick]

The only positive that I can see is that there will be more and faster control when of the direction of travel, particularly when backing down for docking. The farther the motors are apart the more they can effect the line of motion with respect to the center line of the boat.ie the boat will turn faster. It might help also help in forward motion if the hydrolic steering were to fail. Other than that I have no idea as to the effect that would be found with respect to forward power/prop efficiency that was spoken to in the first response. Marty

A hull has a natural axis of rotation. The more aft of that point that thrust is applied, the greater the turning moment. Likewise the more forward the thrust the less turning moment. In an extreme example if the thrust were directed directly in line with the axis of rotation, the hull would move sideways and not turn at all
Of course the physics of what is going on in the real world is fairly complex and I am not a nautical engineer. I don't think you an say that increasing spacing will have the same effect across various hulls. It is not that simple.
Then again, I have been wrong in the past!

 

[megabytes]

When the Marlin was redesigned in 2004, the engines went from 26 to 34 inch centers. It definitely makes close turning much easier ( comparing to my 1997 282). As someone mentioned, the placement must ensure clean water without turbulence caused by strakes or other hull features.

I have heard stories of repowering old boats where the installers incorrectly placed twins. The motors would overheat due to poor water intake. Not good!