Twin inboard to single i/o repower

Discussion in 'Powerboats' started by goodidea28, Aug 19, 2008.

  1. goodidea28
    Joined: Sep 2006
    Posts: 5
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    Location: New Hampshire

    goodidea28 Junior Member

    I have a 28' Seabird sportfish (twin to Bertram 28) powerd with twin 225 chrysler inboards coupled to crusader v-drives (1.21-1) with 14/14 props. I am looking to repower with a single 330hp big block and a bravo drive. At present cruise is about 22 knots with top speed of about 28. Does any one know if I would get similar performance? The boat wieghs in around 11,000 lbs, I'm figuring around a thousand pounds shaved off with the repower along with being able to adjust the outdrive angle while under way which should be more efficiant. Has any one done this or could offer any input?
  2. Jango
    Joined: Aug 2005
    Posts: 519
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    Location: Mid Atlantic

    Jango Senior Enthusiast

    With 330 HP and a 2.00 gear, Top speed should be 30 knots with cruise about 24 - 25 knots. 10,000 lb with 18 x 18 prop.

    Make sure you reinforce your transom before mounting outdrive
  3. TollyWally
    Joined: Mar 2005
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    Location: Fox Island

    TollyWally Senior Member

    For what it is worth,
    I am making an assumption that you are refering to a more or less standard output BBC. I have such an engine. The drag racing boys over at claim that those stock 330 hp Chevy motors are good for maybe 280 hp on a dyno on a good day. My BBC maxes out at maybe 210- 220 hp based on fuel burn numbers.

    I am not naysaying any aspect of this proposed swap; in fact I'm VERY interested in how it may turn out. But I am perhaps introducing a note of caution about plugging advertised horsepower claims into number crunching equations and tranferring the results into the real world.

    Any chance of getting some realistic performance numbers with similar sized boats powered with similar engines. I have pretty accurate instrumentation and am constantly amazed at how slow my boat is in the real world. My hull is 28' overall, 24' waterline, 9' beam, estimated 8000#( factory number 6500#) BBC, 1.52:1 gear, straight shaft, 17/15 wheel.

    I top out at 22 kts, cruise at 18. I am averaging a bit better than 1.35 kpg on a good day.
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