Ken wrote in part: The bottom line presently is that fuel economy seems to be roughly the same twin versus single (or, at least it appears to be within 5%> I really don't think that for a long range boat it makes much difference if you have one or two engines as long as the hull and engine configurations were designed for them. Other variables will change the range and fuel burn much more that this factor in the real world, IMHO. <In a single engine boat I'd be running 1,600-1,700 rpm to maintain the same speed. I prefer less vibration.> This isn't really the case unless the engine selection is poorly matched to the usage. Larger lower RPM engines that produce the power required may in fact turn slower at cruise than two lower HP lighter higher RPM ones. Again this depends on the designer/engineer who decides the power selection, prop size/pitch and reduction ratio. Few boats seem to install full isolation/free floating mounts and drive systems. Doing so can eliminate 90% of the vibration from the shaft, prop, and engine/trans. Swan Song's DD 6-71 can be moved an inch or so in any direction on its very soft/tuned mounts. We have an EMSS drive system from the boys up in Maine http://www.evolutionmarine.com/ that takes all the propeller thrust and allows the engine to be free fore and aft and thawthships. We also just had our 38 X 28 three bladed prop run thru the Prop Scan machine and brought up to Class S specs. We'll see if that further improves the little vibration we did had and helps the speed/economy. Few propellers are at this spec as new. A good topic for another thread ;-) Good system design & engineering from the get go should really mitigate the concerns about single vs. twins in the areas of fuel burn and vibration. Let the choice be driven by things like: Do I wish to maintain two vs. one engine? Do I have better access with one vs. two engines? Am I comfortable with the "all in one basket" approach of one engine? Does the one prop vs. two expose the running gear to more potential damage from foreign objects in the water? etc, etc. As always YMMV....ours does :-) Dave & Nancy Swan Song Roughwater 58