It Depends....
I assume her 27 is from the factory with the OB well in the lazarette. If so, and you don't want to be cutting the hull, you'll need a 2 stroke if you want to be able to tilt the motor up out of the water. The powerhead on the 4 strokes are too tall to tilt into the transom well. Which ever longshaft you use, (mine is a 20" if I recall) you'll need to tilt it up to the first or second spot on the shallow water kick-up to allow the OB leg to clear the rudder. If you don't, the lower unit bumps the rudder under when you throttle up, and prevents steering (not a good thing!). Even if you get a 2-stroke, take measurements of the OB and the opening in the transom. I replaced my '91 Evinrude 9.9 2-strk with a 2000 Johnson 15 2-strk specifically because they are supposedly the same engine (the 9.9 is a de-tuned 15). With the changes in the motor over 9 years, I still had to do some minor cutting in the lazarette. Make sure it'll fit and tilt up. The 9.9 is just about the right power. Both the 9.9 and 15 push the boat at 5.5 knots at about 3/4 throttle, and about 6 wide open. (Hull speed is 6.2.) The 15 allows me to pull a dink or push into 15-knot headwind without losing much speed, but burns about 1.3 GPH, compared to about .8-.9 on the 9.9. The gain in power isn't worth the loss in fuel economy. Get an electric start, and rig remote controls. Then you've got most of the advantages of an inboard, without the cost or complexity. The electric has a small (mine's a 6 amp) alternator to charge the battery and run lights. I understand an 8HP will fit more easily in the well. But with an 8, you're a little underpowered for motoring longer distances or into wind/chop, and the OB will run at higher throttle level. If you only use it to get in/out of harbor, that may not be a problem. Good luck!
