Help with my little boat

Discussion in 'Boat Design' started by mydauphin, May 11, 2009.

  1. mydauphin
    Joined: Apr 2007
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    Location: Florida

    mydauphin Senior Member

    I have a 1953 Aluminum DURACRAFT V-BOTTOM BOAT. Its 16' feet by 5 foot beam in middle. The hull though a v-hull is more like a pointy teardrop. The stern is about 4 feet wide and its at max beam about 1/4 down from front. The chimes are rounded and it has a small 1 inch keel down middle.

    All said boat runs fairly well. It will plane with one person with 15 hp and putter along nicely on a 5hp. I recently install a 25hp and it change personalty of boat totally.

    Engine anti-cavitation plate is about two inches below water at rest and seems in ideal location on plane.

    It now planes with two adults easily but slowly at about 3/4 throttle and really works much better. It is no longer a dingy but a descent bay boat.

    The problem is this. At above 3/4 throttle, the engine/prop has makes weird grinding noises like cavitation. I know it is that because if I throttle down or get in better trim the noise goes away.

    One time at full throttle the boat was really flying but it lost all steering and I was afraid it was going to come around if turn engine too quickly.

    So here is my diagnosis and ideas.
    Boat was not designed for this much power and I am exceeding its design.
    That said will doing the following help:
    1. Hydrofoil fins on outboard, will they prevent some of that cavitation and also more stability.
    2. Add large trim tabs in back to extend boat transom and move planing area rearward.
    3. Change propeller to larger one more in tune with boat.
    4. Adding some kind of keel extension or larger skeg on outboard.
    5. Get a new boat like a skiff design that can handle power if I want to go faster.

    Your thought and ideas are welcomed....
  2. mydauphin
    Joined: Apr 2007
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    Location: Florida

    mydauphin Senior Member

    Oh No, no replies yet. I am so
  3. robbie2161
    Joined: May 2009
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    Location: Falmouth

    robbie2161 Junior Member

    I know this sounds crazy, but on my 11 foot Boston whaler I have a 15 horse power meaning at full power I do 22kts and at ¾ I do about 16.

    Well what you describe you hear seems to match what I used to hear on the Boston.
    I fiddled with it for ages I have flaps on it now and I was constantly adjusting the angle then testing it with no success, until one day the idea came to me to raise the engine so that the capitation plate was at the same height as the base of the transom and I’ve never heard a noise since, might sound crazy but it worked!
  4. mudman
    Joined: Mar 2007
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    Location: Madisonville, LA

    mudman Junior Member

    I had a 12 foot flat with a 25 with the same problem. The nose dug in pretty well and the motor pretty much pushed the back of the boat up. All that I did was set the trim bar on the engine and it did help, but I never could get it to run exactly right.

    I think that trimming the engine to the best running position, and setting the fin even with the bottom of the boat should work, but if it doesn't..........

    Some guys here love a 18 foot boat with a 30 hp. They say that the 16's don't lay right while on plane. I prefer a 16, but again, I cant keep the bow up. Length will help your problem. I've seen it.

    Another thought is to put more cup in the prop. I swear by that. I had that problem with my mudboat, not cavitating, but ventilating. Put a whole bunch of cup in the prop and problem solved (someone on this forum told me about it). It slows down your rpm, but certainly stops the ventilation problem. More cup and a little less pitch. Make sure that you are sucking in water from for cooling though before trying this one.

    I'd play with the trim and stuff before I get into the expensive stuff though.
  5. Jimboat
    Joined: Feb 2002
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    Location: Canada

    Jimboat Senior Member

    mydauphin - if you can post some pictures of your hull and your engine setup, i may be able to give some suggestions. or email me.
  6. mydauphin
    Joined: Apr 2007
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    Location: Florida

    mydauphin Senior Member

    Will post some over weekend when I go see boat... Thanks all

  7. Lt. Holden
    Joined: Sep 2007
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    Location: Western Massachusetts

    Lt. Holden Senior Member

    It certainly doesn't sound like it's overpowered with 25 HP; mounting height, weight balance/trim, propellor type and motor angle all factor in as others have mentioned. Some good photos, especially while underway would help some of our resident experts help you sort this out. When I was a kid (still am, I was just younger!), I had a Boston Whaler Sport 13' with a 33 HP Evinrude, fiddled with the above and ended up with a wonderfully quick and responsive canal and bay boat. Your problem is not terminal!
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