50hp 4cyl Merc vs. 70hp 3cyl Merc vs. ??? - Weight vs HP - Opinions??

[Steve2ManyBoats]

I have a 17' Sea Ox center console I restored last year. It floats in 5" of water - just what I need to get into and out of my shallow water dock. It's not a big 17' boat - only 6.5' beam. I replaced the transom, stringers and deck and made the boat self bailing. However, being a small self bailing boat, it's sensitive to transom weight.

I currently have an early 80's 4cyl 50hp Mercury on it. I've always loved these motors. They run smoothly and last just about forever. However, I need a little more power on this boat.

I've done quite a bit of searching for light 70hp motors. The 3cyl 70hp Merc motors from the 70s (not the heavier 3 cyls from the 90s) are the lightest I can find - approximately the same as the 4cyl 50s - around 190 lbs. Both blocks also use the same mid and lower unit. I could try to find a decent 3cyl 70hp Merc and swap it on my current 50 mid/lower with trim.

However, I've learned the hard way that hp ratings can be all over the map.

Here is the question - does anyone have any experience in comparing 4 cyl 50hp Mercs vs. early 3cyl 70hp Mercs on the same boat? Are they really significantly more powerful? I realize both of these ratings are before 1984 after which motors were supposed to be prop rated.

The only other motor I can find close to the weight of these motors is the older 70hp Yamaha at 229 lbs. Too bad the 3cyl OMCs are a bit too heavy at 250 lbs (20" with power trim). Those things are everywhere, cheap and durable.

Opinions appreciated.

Thanks...Steve2ManyBoats

[pointer]

Experience with 4 cyl 40 Mercs and 3 cyl 70s of the 70s vintage back in the 80s. Did not have much luck with the 70 triple, it was rebuilt more than once in a span of 5 yrs, likely 50 hrs per season. The electrical gave trouble as well - rectifiers. May have been all operator error, several people ran that 70. I do remember the dealer considered the triples time bombs. Hopefully someone has better experience with them. The 4 cyl ran for years without issue. I took it off the boat we used to own last summer as the current owner went ETEC. It needed a coil.

pointer

[Markus]

Have you checked out the 2 cyl Tohatsu/Nissan 70 hp engine?

Light and strong.

[Steve2ManyBoats]

Yes, I did check out the Nissan/Tohatsu 2 cylinder motors. They seem to have a good reputation for reliability and are supposed to weigh about 207 pounds - about the lightest 2 strokes available. However, at least around here (mid-Atlantic East Coast US), there are very few available used and very few sources of parts. If I could find a used one in good condition not too far away, I'd be interested. But I haven't found any.

Thanks...Steve2ManyBoats.

[olboatman]

I had a 77 70hp tripple on my 18ft CC dive boat. The problem they had was with the cooling system as I found out the hard way 17mi from shore! They had a water routing pproblem that caused the top cyl to run hot....the reason they had the "time bomb " rep. If you kept a new pump empeller in them they would hold up longer. Could be why ya don't see many now. BTW I motored back from 17mi out on two cyl.s with the upper plug removed!!! Gary

[gotboostedvr6]

Just get a 3 cyl OMC and move some weight to the bow.

[Steve2ManyBoats]

Wish I could, but I built this boat light and simple. There is no weight to move, Battery and fuel are already in the center console. Due to my shallow water dock, I need to keep the draft to an absolute minimum so I really don't want to add ballast if I can avoid it.

Thanks...Steve2ManyBoats.