You're ignoring a key factor in the CG equation! It's not only the absolute weight that's important, it's WHERE it's located, that determines the boat's CG location! The weight of that 4 cyl engine (I think your I/O weight numbers are about right) is probably centered at least 12-18" forward of the transom, while the weight of an outboard will be centered 6-12" AFT of the transom!
I specifically asked Carl Moesly how the overall CG location compared on the I/O's vs. the outboards on the 20' Seafari. He said the CG is further forward on the I/O's, and this is in comparison to the outboard he designed the 19/20' hulls for, the I-6 Merc, which weighed less than 300 lbs! So how does that CG difference affect the performance of the boat? Here's a couple of examples:
CSC member Tiny had an I/O Seafari and sold it after he bought Fr. Frank's OB Seafari with the 90 hp Optimax, which weighs 375 lbs. He said the I/O model rode noticeably flatter and softer than the OB model; I went for a ride in the I/O boat and confirmed that it planed at 12 mph WITHOUT trim tabs! My Seafari would do that with a 300 lb outboard and WITH trim tabs. When I installed a 30" bracket with a 427 lb motor on it, my min planing speed jumped to the low 20's! This was totally unacceptable for offshore use because the 20' hull is relatively light and starts to go airborne over 20 mph in seas of about 3'! In those conditions, you can't even use 150 hp, let alone 200 or 300! I had to add a Doelfin and 4B prop, both of which reduce top speed, to get my min planing speed back down to 12 mph!
Also you should know that those quoted 4 stroke weights are DRY weight! Once you put a couple gallons of oil the crankcase, lower unit and tilt/trim system, I'm pretty sure that F300 will weigh over 600 lbs! The motor weight issue on the 20' hull has been discussed many times on here, and the consensus of the experienced guys is that it's a very efficient hull designed for lightweight motors, so lighter is better especially for a good ride, and around 400 lbs is about the heaviest you want to go for best all around performance! The Seafari has more weight forward so can handle a heavy motor better than the CC's, but you can compensate, up to a point, by moving the console, gas tank and batteries forward to help balance a heavy motor.
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