Power vs hull displacement

Discussion in 'Fiberglass and Composite Boat Building' started by alesserfate, Feb 11, 2018.

  1. alesserfate
    Joined: Feb 2017
    Posts: 8
    Likes: 0, Points: 1, Legacy Rep: 10
    Location: British Columbia

    alesserfate Junior Member

    Hi folks,

    From year to year I've been fine tuning the center of buoyancy and weight of my 26ft fiberglass deep-V powerboat. I have added a few things to it over the years such as solar batteries and a hard top, and found a sweet spot where it planes out the easiest and has the most stability in rougher waters by having to move some weight of the accessories around.

    My hull is a planing hull, so with enough speed it will eventually plane out. I noticed when I bring a lot of gear with me, the engine does work harder and a little bit hotter on longer cruises. More specifically I noticed that if I have the weight further forward, that's when the engine runs more bit hotter, and is also when the forward portion of the hull sits about an inch deeper into the water.

    To conclude this, I was wondering whether the total amount of water displaced by the hull, would require more power to maintain the same speed when on the plane ? And whether if I moved some of this weight further back, that when it's on the plane it would displace less water, and the engine would have less load on it?; since the hull further towards the transom is more flat (less deep-v), and therefore generates more hydrodynamic lift at the same speed. Please correct me if I'm wrong.

    Any input would be appreciated, and apologies in advance for my lack of marine engineering knowledge. Thanks in advance.
     
  2. Ad Hoc
    Joined: Oct 2008
    Posts: 5,663
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    Location: Japan

    Ad Hoc Naval Architect

    HI alesserfate,

    A hull, no matter what type of hull displaces only what it weighs. Thus if your hull is a box barge if you add more weight, it sits deeper in the water. If your hull is an aircraft carrier, or an F1 powerboat, more weight added = sits deeper in the water.

    If the hull has not changed, adding more weight will increase your resistance roughly pro-rata of:

    Power(new) = Power (old) x displacement (new)/displacement (old)

    You can't escape the fact adding more weight will require more power.

    Since your hull is a Vee planing hull, other factors can influence the powering. Such as, the optimum location of the LCG. You have half done this yourself. So, take onboard say 1 large tin of water of say 50 litres (kgs) each. Place it as far aft as you can....do a run at several rpms, from min to half to max....see what speed you get at each rpm setting. Then repeat by moving the weights say 1.0m fwd...repeat as before...then 1.0m fwd again and repeat..and so on.

    You will then get a plot of speed v LCG distribution.

    This will tell you where your "sweet" spot will be...roughly.

    Then if you can repeat again, but using 2 x 50kg tins...now you'll get the same as above, but with varying displacements. See if the 'sweet spot' moves.

    Once you have this data, you can make several assumptions about the performance of your boat.
     
    Last edited: Feb 19, 2018
    alesserfate likes this.
  3. alesserfate
    Joined: Feb 2017
    Posts: 8
    Likes: 0, Points: 1, Legacy Rep: 10
    Location: British Columbia

    alesserfate Junior Member

    Thanks a lot for your reply, @Ad Hoc . I appreciate it. I will try and see what I can do.
     
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