Is it apples and apples OR apples and oranges?

Given two otherwise identical boats, one powered by a Mercury 300XS the other powered by a 350 MAG MPI (300hp).

Which will be faster??

Apples and oranges. Outboards weigh alot less per HP.

Outboard would spank it by a long shot

Torque is the key and is what moves the boat. The 350 mag will have a lot more torque than the OB. The outboard has a lot more RPM range though. In rough water, normally the stern drive will push through the waves better. The OB is normally not quite as deep in the water and it will have less torque to push through the waves better. Single OB's are good for light fishing boats. Single sterndrives for run-a-bout ski boats (torque for pulling skiers).

Using my hull as an example..........

A 300x, twin 200's and a 496HO (425 HP) all run very close to the same speed.

Like already mentioned, the weight is a big factor.

You really have to compare a smaller, higher power I/O to the outboard.

Generally speaking, that 350 mag / 300 HP will outlast the outboard by a long shot. More cubes per HP generally means longer life. But, more cubes per HP means more weight and a slower boat.

There was a point in time where an outboard needed less hp to match a stern drive in speed. Possibly 75 percent as much. Now that everything is surfacing and jacked to the moon they are closer but OB are still a little faster.
Jim

Outboard by far. I like the sound a V-8 makes with thru hulls vs an outboard tin peck sound, but when you figure the weight difference and compare weight to HP ration the outboard will always be on top in performance. I think my 240 Mercury weighs 341 lbs, thats everything, top to bottom, and could be made to weigh as little 300 if it was put on a diet.

Can't deny the power-weight ratio, especially in boats. Outboard will win every time, although the I/O will have an edge in rough stuff, again weight is the factor here.

when you load an outboard boat with fuel and people it is going to show its true colors real quick. An I/O with the same weight and fuel added will not lose the mph that the outboard loses.

A high torque, high hp 2 stroke will come close to an I/O in characteristics but a 4 stroke no chance.

As far as maintenance I will take an outboard any day. Less lower unit repairs, no headers to leak, no chance of reversion, etc.

Problem is to achieve the speeds we like in the size boat we like to be comfortable in the transom does not have enough room to accomodate the hp needed. In some cases the stacks of outboards on the back effects the performace of the boat, attitude of the boat, etc.

Remember the gyro effect a stack of outboard motors have all turning the same direction (motor not prop rotation) stacked on the transom of a boat.

2 CC 33 Powerplays, one with 300X's 73 mph, the other with 502 MPI's 415hp per side 76mph.