help for the underpowered?

Discussion in 'Boat Design' started by kapnD, Jan 3, 2003.

  1. kapnD
    Joined: Jan 2003
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    Location: hawaii, usa

    kapnD Senior Member

    Im wondering if I can obtain faster/more economical trolling speed by adding to the length of the boat to accomodate a prop pocket.
    Boat is 30'x8'6", pretty much a hard chine lobsterboat- narrow deep entry warping to a 12degree transom.
    Powered by 115hp Perkins through V-drive, it seems to drag heavily in the water at 6-10 knots, prrecisely the range where I typically troll.
    The hull is quite paralell in the aft 6-8' would be easy to cut and add 3-4' as I do have acess to the original mold.
    I can see that adding length will help lift the weight of the engine and help the boat get on top of the water, but my concern is will the added drag negate any benefit obtained?
    In the present configuration the motor is working quite hard to get just 8 knots and 10 or 11 is full speed, just almost on plane.
    The prop pocket will allow me to eliminate the heavy ancient v-drive and turn the motor around with a short straight shaft, should gain some efficiency there too.
    Any thoughts/ideas/advice welcome! Mahalo!
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  3. yipster
    Joined: Oct 2002
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    Location: netherlands

    yipster designer

    hello kapnD

    i'm not a naval architect but think this is a normal hullspeed. 1.34*sq.rt of WL length is froudes wave law for displacement boats, that isnt fast, but dont think that 115 horses get your boat out of the hole planning. you better read some more on the forum here to get a better idea. offcourse adding lenght to the boat is adding WL is adding -a little- speed in the formula

    hope to have been of help
  4. Tom Lathrop
    Joined: Oct 2002
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    Location: Oriental, NC

    Tom Lathrop Junior Member

    Sorry to say Kap, but I expect you are out of luck in getting that boat to plane with 115hp. If you are wanting it to run more efficiently at the 6 to 10 kt range you mention, I think that is also not in the cards. The only way to have a powerboat with a planing hull run easily in that range is to make the bottom loading (lbs per sq ft of bottom contact area) very light. My guess is that your boat will not qualify as light enough to accomplish this.

    As was said, adding length will improve the low end speed some, but not as much as you want. The only powerboats that run well at the intermediate speed range are true semi-displacement types, very light ones, multihulls and very long and slim types.
  5. kapnD
    Joined: Jan 2003
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    Location: hawaii, usa

    kapnD Senior Member


    Thanks for the input, guys.
    Didnt think there was much hope, just trying to avoid the expense of repowering mostly.
    Am still interested in prop pocket idea though,in order to improve shaft angle and decrease draft.
    how long does pocket need to be to work efficiently?
    how much clearance around prop?
    how much of prop can be buried?
  6. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    I'm not sure what kind of use your boat gets, so my answer may be way off. I have built and installed swim platforms that are closed in the bottom. They effectively modify the water exit. It may not increase your maximum speed by much, but it will decrease your fuel consumption significantly. I can help you with more specifics with some photos.
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    Guest Guest

    I know it's an old topic, but let me say that has a wonderful article in their "Insider" section about cavitation plates that might help you. :)

  8. Peter_T
    Joined: May 2003
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    Location: Gulf Coast

    Peter_T Junior Member

    You did not state the draft and displacement, neither the actual conditin wetted surface, bottom surface angle i.e. chine angle important for speed hulls.

    From Yipster, he quoted a speed/sqrt(L) ratio of 1.34. This is before getting into transition to planing. Planing benefit requiires to reach like 1.75 for the transitional zone and will plane on surface when you reach 2.7 and over. To get planing you must a planing hull and best with steps to keep the spray water out of reach of the top sides.

    You quoted that at speed 11 knot, is works hard but like a plane. It is possible that the rush of water had work to lift the bow a bit. Extending the hull length is great on displacement hull. This will move forward pressure forward, increasing the length reduces the speed length function. In your case, you may also consider to add some length by extending the bottom shell at the transom. Just as building a step platform for swimmers will offer some credit in shifting the aft pressure point.

    During the speed trial, did you load up the vessel to have proper propeller immersion. Sometimes, you think of cheating by running as light as possible. It is the other way, to seek the optimum running condition. Running with stern trim usually get some speed gain. Excessive trim will have bow pitching problems.

    The craft is considered to the right length on the waterline, when running at the service speed, with a good wave pattern as not to have the trough at the stern. When this happens, significant hull length change can correct the problem.

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