Looking at Cummins 4BT, but comes with 2.57:1 gear and 23x26 prop-building 25'planing

Discussion in 'Props' started by Northeaster, Oct 10, 2013.

  1. Northeaster
    Joined: Sep 2011
    Posts: 265
    Likes: 8, Points: 18, Legacy Rep: 45
    Location: Eastern Canada

    Northeaster Senior Member

    Hi folks,
    If someone could give me the coles notes version of how (in what order) you are supposed to pick prop, gear and engine I would appreciate it...
    In the meantime, however, there aren't alot of good used marine engines available locally, in the 100 -150hp range.
    Missed out on a Cummins 4BT with a 1:1 gear.
    Now, found a Cummins 4BT (150hp at 2800rpm) near me but it comes with a 23" x26" prop and 2.57:1 gear.
    I am building a 25' aluminum Glen L Double Eagle, which is a hull that is supposed to go smoothly from displacement to semi D to full planing.
    I will build the inboard -shaft drive version with full keel.
    Disp is 4200lbs but I will be lighter as only doing CC for now at least - no cabin.

    Below is performance estimates and one builder has same boat with a 4BT and 1.5:1 reduction and gets 25kts.

    Another builder has a Mercruiser 3.0l, around 80 -100 hp , 2:1 gear, 16" prop and gets over 20kts and good economy.

    Is there anyway this 2.57:1 gear and/or large 23x26 prop can be used efficiently on this boat??

    6 knots 10 SHP
    9 knots 25 SHP
    13 knots 40 SHP
    18 knots 70 SHP
    22 knots 105 SHP
    27 knots (*) 150 SHP
    31 knots (*) 200 SHP
    (*) Attempting to obtain these speeds with the Inboard Version, due to the deep skeg, may cause excessive fuel consumption and may not be practical.

    If not, Could I expect to sell them (without waiting for years) and roughly for how much??
    Any help would be appreciated. Engine comes with above plus SS 16' shaft (diam 1.5")- now cut in half..... and engine panel, exhaust hose, muffler, thrust bearing for $3000 firm. Have VIN but don't know year yet, or hours.
    Last edited: Oct 10, 2013
  2. Northeaster
    Joined: Sep 2011
    Posts: 265
    Likes: 8, Points: 18, Legacy Rep: 45
    Location: Eastern Canada

    Northeaster Senior Member

    Nevermind the larger prop folks. I thought a somewhat larger prop might fit, but looking at the plans, it says the max diameter is 16".

    I tried running the numbers through an on-line prop calculator, and It suggested a pretty large prop, to go with the 2.57:1 gear - so,if I can only fit a 16" prop, I guess that gear is no good to me, unless the used market (looking for them) is good.
    If anyone could advise me to what my chances would be of selling the gear and/or 23x26 bronze prop, and roughly what they might realistically fetch, I would appreciate it!
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