Need to repower a 30' displacement cruiser

Discussion in 'Gas Engines' started by Jim Phoenix, Mar 15, 2010.

  1. Jim Phoenix
    Joined: Mar 2010
    Posts: 3
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    Location: Tacoma, WA

    Jim Phoenix New Member

    My 1947 30' LWL Monk has a Chrysler Crown M7 (110hp) that has died and I want to replace it with a modern engine and transmission. The boat is registered as 9 tons displacement, but I think it actually displaces about 12,000 pounds in current configuration.

    I looked at the range of 75 hp diesels, but they are too pricey for my budget. I'd like to get a gas engine and tranny installed for $10k or less, but I may be dreaming. I see there is a GM base inboard 4 cylinder rated at 130 to 140 hp, would that be a suitable engine for this boat?

    Looking at the various hull speed/HP requirements calculators out there, they say I only need about 70-90 HP to move the boat at 8 knots which is plenty - a rough calculation to be sure, but I'm thinking a 130 hp 4 cylinder with the right transmission and correct prop should be fine - what am I missing?

    I have no idea what type of transmission would fit - a down angle looks best as the Crown is currently a direct drive straight installation with the engine angle up about 10 degrees or so. I ran the Crown at about 1800 rpm to get 7.5 knots. It would easily go above 2100 rpm, but the boat never went faster than about 8 knots maybe, typical for a displacement boat. The prop seems well matched to the boat is only about 3 or 4 years old.

    A six cylinder inline would be nice, but a V6 would be much more difficult because of the existing single exhaust would have to be modified. Does anyone make a new 4 or 6 cylinder inline complete package that would be suitable for this hull?

    What kind of transmission, and more importantly what gearing would be suitable?

    I just contacted a local company that does repower and I expect they will get back to me soon, but if it costs more than $10k, I may have to find an alternative. I certainly don't want to repair the Crown (failed con rod), it's been a good engine, but it's heavy (1000 lbs), noisy, leaky, has an ancient transmission with a giant Johnson bar and is not very reliable - it seems. The rest of the boat is in excellent condition with new decks, perfect hull and modern electronics, and we love her!

    Any comments or suggestions welcome.

    Jim
     
  2. gonzo
    Joined: Aug 2002
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    Location: Milwaukee, WI

    gonzo Senior Member

    A 3.0 GM wil give you 140HP with less weight. It is a very common engine. A VelvetDrive will bolt right on to it.
     
  3. Jim Phoenix
    Joined: Mar 2010
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    Location: Tacoma, WA

    Jim Phoenix New Member

    Thanks Gonzo, much appreciated.

    Jim
     
  4. TollyWally
    Joined: Mar 2005
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    Location: Fox Island

    TollyWally Senior Member

    It would be perhaps a little difficult to get the marinization parts but a 225 Dodge slant six would run forever. It's a long stroke motor, perfect for a displacement cruiser. Another sweetie would be a big 300 inch Ford six. I've got one in an old work truck, the torque is all in as soon as you let out the clutch. Where do you keep your boat?
     
  5. Jim Phoenix
    Joined: Mar 2010
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    Location: Tacoma, WA

    Jim Phoenix New Member

    Hi Wally,

    I know another 35' Monk with a 300 Ford six in it and that looks like a nice motor. I keep the Monk at Hylebos Marina. I'm looking at a Yanmar 4JH4-HTE now, so kinda going the other way. A diesel would be very nice.
     

  6. gonzo
    Joined: Aug 2002
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    Location: Milwaukee, WI

    gonzo Senior Member

    The 3.0 has been marinized for decades. The advantage is that there are off the shelf parts for them
     
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