Confused with hull design and engine HP..

Discussion in 'Powerboats' started by SoNew, Nov 18, 2012.

  1. seadreamer6
    Joined: Oct 2012
    Posts: 29
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: helena,mt

    seadreamer6 Junior Member

    Just read an article on Honda's new 1.7 l diesel. A little over 100hp. Any thoughts on using that in a boat?
  2. Mark Main
    Joined: Jun 2016
    Posts: 1
    Likes: 0, Points: 1, Legacy Rep: 10
    Location: Fort Lauderdale, Florida

    Mark Main New Member

    I used Dave Gerr's formula and calculated for SHP (shaft horsepower) and increase the 5% for the BHP (brake horsepower) per the description. 110% to 120% could also be used as a rule of thumb increase.

    D = Displacement Lbs
    SL = Speed Length Ratio, 1.34 can be used or calculate using: speed knots / Sqrt(LWL feet)

    SHP = ((D × SL^3)÷10.665^3)
    BHP = SHP x 1.05

    The calculation assumed 0.333 = 1/3, which later became cubed when solving for SHP; this alters the result slightly, if you want to use 0.333 instead then replace the ^3 with ^(1/0.333)... SHP = ((D × SL^(1/0.333))÷10.665^(1/0.333))

  3. Easy Rider
    Joined: Oct 2009
    Posts: 920
    Likes: 46, Points: 0, Legacy Rep: 732
    Location: NW Washington State USA

    Easy Rider Senior Member

    As an example of small boat power my Willard 30' is 8 tons and has 40hp (5hp per ton)

    She's overpowered but not by much. Most of the Willard 30' boats were sold w a 36hp engine but were over propped so really only had 33hp. Never heard anyone complain about too little power. And the boat would do fine (even in heavy weather) w 30hp.

    But this is a full displacement hull with no semi-displacement ambitions. So for a real world number to shoot for IMO 4hp per ton seems perfect.

    Can't see the hull well in the pics but it looks flat w a little rocker. If the rocker is minimal I'd see it as a SD hull w a little angle of attack. Or a dirty FD hull at 4 knots. So I'm thinking 6 to 12hp should be fine. Just my opinion. Can't really go wrong unless she gets overpowered and overdriven.

    Speaking of opinion if this little boat were mine I'd put a 6 to 10hp outboard on her. Lots more space in the boat and LOTS less vibration. And if I was building her I'd take the previous suggestion and stretch two feet ...... or 3'.
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.