I/O's vs OB's

['84Offshore]

I have an '84 Offshore I repowered in my driveway for ~$3500 (300+HP).
Granted the I/O's are a bit more maitainance but I like them.........

It's a simple SBC and easy to work on.

Probably burn <10gal/hr at 30MPH...........3600RPM

Any thoughts?

 

[cgmiller]

At 3600 rpms you are going to have the secondaries open in the 4-barrel carburator and you will be burning more than 10 gpms..closer to 16 would be my guess. From my experiences running 3 boats with single inboards, 1 with twins and a 24 Rampage with a 260 hp 350 mercruiser coupled to an Alpha One outdrive. A 350ci engine will burn 10 gph at 3000 rpms. My Rampage ran best at 3200 rpms and burned 12 gallons per hour, but it only ran around 20 knots (22 mph). When it was real calm, it would run about 21-22 knots at 3200 rpms. It was real sensitve to waves and current. At 3600 rpms, maybe it would have run at 25 knots..granted it was a lot different boat then a 24 Offshore...bigger, heavier and a different hull shape. I had a 330 hp efi crusader in a 26 Shamrock and at 3200 rpms, it burned 14 gph and the boat topped out around 28 mph at 5000 rpms..but the full keep adds a lot of drag to the hull.

 

[JeffN]

Hull shapes are all so different I don't think one can extrapolate between different models with a great deal of accuracy. I run a 496 Merc. I/O in my 25' Kingfish. Running at 26 to 27 knots at 3400 rpm and I burn an indicated 14 GPH. I top out at 44 mph. Wider beam than the Offshore and heavier. I have found my fuel ecconomy with this and the previous I/Os that were in the boat to be very good. I really have no idea what outboards that yeilded similar performance on my boat would burn.

 

[jeffump18]

My '85 Seafarer 227 has a '95 Merc 350 I/O. 3100 rpms gets me planing at about 28 - 30 mph (without trim tabs). Don't know the fuel consumption, but overall, very happy with gas usage. Replacement cost of I/O much better than a comprobable outboard. I've noticed manufacturers aren't using I/O's any more. Much easier for them to throw an outboard on & save interior space. Except for the name & a few other changes, the inboard is just a modified car engine, easier to work on & comprehend how it works & troubleshoot. Outdrive is a different animal, but take care of the engine & it should be o.k. (unless you hit a friggin rock!) :-|