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  1. #1
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    50hp 4cyl Merc vs. 70hp 3cyl Merc vs. ??? - Weight vs HP - Opinions??

    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

  2. #2
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    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

  3. #3
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    Have you checked out the 2 cyl Tohatsu/Nissan 70 hp engine?

    Light and strong.
    Markus' Performance Boating Links:
    www.toastedmarshmallow.com/performance

  4. #4
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    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.

  5. #5
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    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
    "12" Super Lite Tunnel (11') "88" 25 Yammy twin carb "BANANA SPLIT"
    "77" Hydrostream Viper "87" 140 Rude heavy modded w/15" mid, Bobs nose and lwp "DANGER ZONE"
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    "Too much is never enough" Keith Richards " Dreams become reality via hard work and perseverance" G.A.Carbonneau

    "This coming from an old man that strapped two bananas together, hung a motor on it and calls it a boat" XstreamVking

  6. #6
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    Just get a 3 cyl OMC and move some weight to the bow.
    Azure AZ259, 8.1

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  8. #7
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    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.

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