Twin outboard separation

Hello, I'm designing a 25ft cc sportfish to be powered by twin outboards.
Are there any guidelines covering the horizontal separation between the engines?
What advantages/disadvantages come with increased separation? Any info would be greatly appreciated.
Regards,
Scott Seale

 

I am also building a 26 ft cc boat, how is your project comming along and did you come up with any answers for motor separation??

 

In general placing the outboards closer together might increase overall speed slightly (look at raceboat setups where engines are staggered to improve speed and balance of the hull) at the expense of maneuverability, especially when docking (where with the ob's apart, you can place one in forward and one in reverse to turn in tight quarters much like with twin inboards where you must do this, if they are very close together, this no longer works very well.) Just my 2 cents...

 

I suggest you contact the o/b manufacture to see what their literature suggests!
1. Consider the tie-bar you will use between the motors and type of steering, this and the manufactures recommendation may give the answer.

2. Positioning of the o/b's should take into a/c where the props will end up. Try to avoid positioning behind spray rails or going to far up the chine that may lead to ventilation. Consider also what the waterflow will do on turns - is it disturbed by the type of keel you have?

Richard Stimson

 

As has been said, the outboard makers do have there own recommendations. However these are usually on the safe side for spacing. For example Evinrude recommend 660 mm between their 150 D.I. models, which helps with access. We needed to fit a closer setup on a special DIVE 700 - see www.explorermarine.co.uk - and select DIVE button - Here we fitted at 580 mm centres but did have to have a special steering tie bar made up and the service access is tight for filter changes, control cables etc.

From a dynamic viewpoint, closer engines help with speed and high speed stability and this helps with straight line work with one engine switched off, however, more separated engines are great for low speed steering when used "ahead and astern" and on a vee hull the engines can be mounted higher on the transom as they are moved out.

Hope this helps Hugh Mattos C.Eng.M.I.Mech. E.
Explorer Marine