Changing engine from Mack to Cat

Discussion in 'Downeast Engine Room' started by Kemosabe fishing, Aug 13, 2019.

  1. Kemosabe fishing

    Kemosabe fishing Member

    Joined:
    Feb 7, 2019
    Posts:
    6
    Likes Received:
    4
    Hello friends

    I am looking for a little advice. I have a 1989 Duffy. I use it for northeast canyon fishing. Mostly 80-100 miles offshore. The boat came with a Mack e7 630 hp engine. Which I was told should turn 2300 rpm but only turned 2150 rpm at full. I am taking this engine out as I have had trouble with it and replacing it with a more efficient lighter engine. I am considering the cat c-9 503 hp which turns 2300 rpm. My question is will this be enough power to achieve the desired 17 kts at 80 percent load ? And should the zf 1.5 to 1 gear work for the new engine? Considering they operate at similar rpm. Im looking to make this change quickly but would really appreciate your input. Thanks
     
  2. WoundUpMarine

    WoundUpMarine Captain

    Joined:
    Jan 1, 2012
    Posts:
    3,424
    Likes Received:
    1,594
    Occupation:
    1st Engineer, Seadrill Americas
    Location:
    Boothbay, Maine
    Boat Make:
    26 Duffy, 14 Holland
    How size Duffy are you running, and what speeds were you making with the Mack? What size wheel are you turning? Lots of information needed to help you out with your repower.
     
  3. djmarchand

    djmarchand Captain

    Joined:
    Jul 24, 2012
    Posts:
    540
    Likes Received:
    189
    Occupation:
    Retired engineer
    Location:
    Litchfield, CT / Punta Gorda, Fl
    Boat Make:
    Atlas Pompano 23 outboard
    To get an idea of what a new Cat of 503 hp will do at 80% load (I assume that you are speaking of 80% of max power not 80% of wot rpms) or about 400 hp, run your Mack engine at about 1,950 and see what it will do. The speed should be approximately comparable to the Cat at 400 hp.

    David
     
    BillD likes this.
  4. Diesel Jerry

    Diesel Jerry Captain

    Joined:
    Oct 25, 2015
    Posts:
    4,273
    Likes Received:
    2,195
    Occupation:
    Marine Diesel Mechanic and Systems Technician
    Location:
    South Freeport, ME
    Missing many factors here. Length of boat, weight of the boat, weight of the engines, and etc.
    As a general rule I never suggest going down in hp on a repower. You are spending a lot of money to pull an engine out and put a new one in. If it were me I would consider going with the same HP or more.
     
  5. Kemosabe fishing

    Kemosabe fishing Member

    Joined:
    Feb 7, 2019
    Posts:
    6
    Likes Received:
    4
    The boat is 35 ft long. The boat weight is approx 18 thousand lbs. The 12 volt c9 is approx 2100 lbs. the Mack was a few hundred lbs more. The prop is 26 inch x 29 pitch which I’m sure will need to be changed with the new engine. Also the gear is zf311a 1.5 to 1. The gear is new and I intend to re use it for the new engine. I used to cruise at 15-16 kts at 1650 rpm with the Mack. I am looking to cruise around 16 kts at 1800 rpm with the cat. Any help would really be appreciated. I’m looking to get this going very soon. Thanks.
     
    capecodlobster likes this.
  6. traditions

    traditions Captain

    Joined:
    Oct 9, 2011
    Posts:
    1,168
    Likes Received:
    326
    Location:
    mid coast maine, muscle ridge channel
    Boat Make:
    38 Holland
    My advise is to get a different reduction
     
    Dslpwr likes this.
  7. snowstorm

    snowstorm Senior Member

    Joined:
    Jan 21, 2012
    Posts:
    153
    Likes Received:
    19
    Location:
    maine
    Boat Make:
    28 ellis
    2300 sounds about rite in neutral with no load. in gear under load most will run 150 rpm less. the mack is 12l its a converted truck motor just like all the others. i would fix the mack
     
  8. PatriciaLynn

    PatriciaLynn Senior Member

    Joined:
    Jul 18, 2017
    Posts:
    250
    Likes Received:
    112
    Location:
    Cape E
    Boat Make:
    Repco
    A QSC8.3 @600hp would be a good fit and would be right around 2000lbs all in. This engine spins up to 3k, so you would definitely need to pull some pitch
     
  9. Hammer Slammer

    Hammer Slammer Senior Member

    Joined:
    Sep 7, 2011
    Posts:
    347
    Likes Received:
    130
    Occupation:
    Builder
    Location:
    Plymouth, ma
    Boat Make:
    31 Duffy
    My buddy built a 35' with a c9 in it not sure of the gear or ratio, ( cape island built it) and it went ok 18 knots or so but was pretty thirsty. That Mack is a hell of a engine I actually have one in one of my trucks. I am willing to bet you will need a lot more power then the c9 to compare to the Mack.
     

Share This Page