Semi Displacement Power

Discussion in 'Outboards' started by fritzdfk, Nov 19, 2010.

  1. fritzdfk
    Joined: Jul 2004
    Posts: 45
    Likes: 2, Points: 8, Legacy Rep: 32
    Location: Alaska

    fritzdfk Junior Member

    I am looking at a 26' semi-displacement boat with twin 50hp Mercury 4-stroke outboards. The outboards are not the Bigfoot model. I am not interested in more than displacement speeds. The 50 hp Mercurys running at 2240 rpm will push the boat at 6 knots burning 1.4 gallons/hour. This is decent economy. I wonder if better economy could be had with twin Yamaha 25hp high thrust 4-strokes at 6 knots. The smaller high thrust Yamahas turn the same diameter props as the bigger Mercurys. One disadvantage however is that the smaller Yamahas are not EFI.
  2. apex1

    apex1 Guest

    There sounds something wrong to me.

    My 28ft semi does 14 kn with a 4cyl. 75hp Yanmar diesel, and 6 kn with about 25% - 30 % power. So, I would assume (not knowing your boat), you should at least do the 6 kn with just one of those Merc.s and still have some reserve.

    Anyway, you should be better away just selling one of your outboards and call it a day. (maybe a better matching/adjusted prop will help further?)

  3. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 494, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    I'd have to agree, that boat of yours must be pretty heavy to require 100 HP to do 6 knots. I have a 27' boat, weighing 4,500 pounds, with 185 HP that does 37 MPH.

    What make, model and year boat is it? In fact, some quick calculations and assuming a 5,000 pound boat with marginal hull form and moderately high drag and prop slip, you should still be running in the high teens, if not the low 20 MPH range.

    To answer your question, as a rule a single will have better economy then a twins.
  4. Joakim
    Joined: Apr 2004
    Posts: 892
    Likes: 52, Points: 28, Legacy Rep: 422
    Location: Finland

    Joakim Senior Member

    It doesn't! The engines are running only 2240 rpm and take 1.4 gallons/hour. 1.4 gallons/hour equals to something like 15 hp.
  5. apex1

    apex1 Guest


  6. kireodkaneo
    Joined: Apr 2017
    Posts: 6
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Ohrid.Macedonia

    kireodkaneo Junior Member

    need opinion!

    Hello to everyone.

    I have a semi displacement steel hull and weights about 14000 lbs.Using a marinized Mercedes motor that has aout 80 hp, and its reduced 2,97 : 1.
    The propeller is not calculated well so my top speed is 7 .5 knots.

    Anyway i am thinking to use outboards (twin or one).I have a reasons for that. our lake we don't have marina, so pulling the boat out is a risk, and expensive operation.
    2.i want to reduce the draft of the boat (1.2 meters).
    3.go faster when needed to go safe shore.

    I plan to use 2 outboards with 80 - 115 hp. What will i get?
    in addition here are some pictures.
    thx 2007_07_13#09.JPG
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.