Flat bottom skiff

Discussion in 'Boat Design' started by Laut, May 4, 2020.

  1. Laut
    Joined: May 2020
    Posts: 3
    Likes: 3, Points: 3
    Location: Indonesia

    Laut New Member

    Hi everyone,

    I'm building a 390 cm long skiff, 97 cm wide, inspiration taken from Bote Rover/Solo skiff/powerfisha stealth.
    I will fit a 5hp outboard on it.

    This small boat will be used for coastal exploration and fishing. I live in Jakarta, Indonesia, and want a boat small enough to drop on the top of my car. I already have a kayak but looking for something with a bigger range.

    Construction is EPS foam core (15kg/m3), full sandwich with 2mm cork skin, 4oz glass between cork and foam, probably 3x4oz glass + 50 cm width carbon UD strip bottom/deck, all epoxy of course, under vacuum. I've used the same construction for windsurf, surf and paddleboards and it's quite strong. Weight should be around 20kg without outboard/bracket. Construction is underway, now reinforcing the bracket area with HD foam, cockpit deck cork skin to follow.

    Outboard will be fixed with a dismountable bracket, bolted to the skiff via 2 US fins boxes.

    First 2 m of the hull is completely flat, going to a pinched vee at the nose.

    My question is the following: i'm afraid that due to a very flat bottom the boat will have a tendancy to slide and not track properly when powered. I'm thinking of adding 2 strakes on the bottom of the first 2 m of the hull. I know that strakes are aimed firstly to lift the hull, my main target here is to have rails in the water increasing the hold so I'm really not sure it will help... side advantage would be to limit abrasion of the hull when pulling the skiff on the beach and help the tracking when paddling without motor.

    Any thoughts? I should maybe leave the bottom as it is, any advice would be great. If I put strakes, what would be the best shape? U or V? Thinking of 2cm wide x 1,2 cm high strakes, HD foam.


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    Dejay likes this.
  2. bajansailor
    Joined: Oct 2007
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    Location: Barbados

    bajansailor Marine Surveyor

    Welcome to the Forum Laut.

    Have you done any rough calculations as to how much buoyancy you have in this hull?
    And have you added up the total weight that you will put on the boat - including yourself, the outboard motor, some fuel, a paddle (in case the O/B motor stops working), and some supplies?
    Where are you hoping that it will float with everything on board? Maybe halfway up between the bottom and the deck edge?
    Re your additional strakes, I think that I would more favour 'U' shaped rather than 'V' shaped - you could even put them on edge, rather than flat.
    It might also be useful to have a strake on the centreline forward of the well for the outboard motor?
    As this will be where the first impact will happen when the hull bottom meets the beach?
  3. messabout
    Joined: Jan 2006
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    Location: Lakeland Fl USA

    messabout Senior Member

    I repeat the question that BajanSailor asked. Have you calculated the buoyancy and running lines of the boat at its operating displacement? You have given away, unnecessarily, some forward bearing by carving so much hollow into the bow. The balance of the boat, if flat bottomed, will be an advantage in making the displacement more nearly suitable. Have you calculated the operating center of gravity and/or center of buoyancy when the boat is loaded with skipper and other items? it appears that the driver/skipper will be forced to sit farther forward than optimum. The hollow bow sections might make that possibility more problematic. .

    The boat is a bit narrow at 97cm for a power boat of that length. What is the purpose of the different heights of the bottom when viewed from the transom? The design that has the motor moved forward ahead of the transom has some advantages. In this case it appears that the length of the "squat boards" or cut out mid section, is farther forward than conventional.

    The pictures reveal that you have done a most workmanlike job of this project. It is artistically done.

    Please let us know how the boat behaves when you wet her bottom. This is an interesting project that has captured my attention.

    Best of luck.
    bajansailor likes this.
  4. Laut
    Joined: May 2020
    Posts: 3
    Likes: 3, Points: 3
    Location: Indonesia

    Laut New Member

    Thanks a lot for your comments, very appreciated.

    Pictures may be a bit misleading, the lenght of the aft cut out section is less than 75 cm for a total lenght of the boat of 390 cm (sorry for the metric units I'm french...). This is more or less what I have seen for similar skiffs on the market so I just copied it...
    Total volume will be around 460 L.
    Total weight should be around 135 kg (75 kg pilot + 20 kg outboard + 25 kg hull + 15 kg various). I didn't do very precise calculations but based on preliminary ones on excel water line should around 6 to 7 cm heigh from the bottom at mid lenght for a total of 20 cm hull thickess (20 cm at mid lenght, 18 tail, 25 bow). Small continous rocker on the bottom aft and towards the bow (less than 3cm aft & 2,5 cm bow side). Depth of the cockpit is 4,5 cm, I wanted to lower the centre of gravity based on my experience with paddleboards.
    I expect to drive it with the bow slightly up (see vids of Bote Rover for reference). I will have to sit quite foward obviously and fit the outboard with a tiller extension.

    I'm still not sure of which outboard I will put on this, I hesitate between 3,5 HP and 5 HP (both 2 strokes). 3,5 HP is lighter (13 kg VS 20 kg), but may not be sufficient to reach planning speed.

    See below picture of the hull without distorsion.

    Last edited: May 6, 2020
  5. KJL38
    Joined: Jun 2008
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    Location: Tasmania

    KJL38 Senior Member

    To get it to turn you will need to use your body weight otherwise it will continue in the same direction sidewise.

    To see some boats built in a similar way google "Avon descent powerboats".
  6. tlouth7
    Joined: Jun 2013
    Posts: 282
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    Location: Cambridge, UK

    tlouth7 Senior Member

    With the minimally rounded chines and the "internal" chines formed by the sides of the outboard cut-out I think you might get better tracking than you think. Certainly you might as well try it as is before adding strakes.
  7. Len
    Joined: May 2020
    Posts: 8
    Likes: 0, Points: 1
    Location: Florida

    Len Junior Member

    Hello everyone,

    Regarding the questions about buoyancy and running lines, is there a good source for running these calculations available? We are in the late stages of building a similar design but haven't run any calculations yet and we would like to reduce the trial and error process as much as possible. Thank you for any help that you may be able to provide.

    Best of luck with your project Laut!
  8. bajansailor
    Joined: Oct 2007
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    Location: Barbados

    bajansailor Marine Surveyor

    Welcome to the Forum Len.
    I think it would be wise for you to start a new thread on this Forum to ask your questions - if you can also post any sketches and photos that you have of the boat you are building, this would help a lot.
  9. gonzo
    Joined: Aug 2002
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    Location: Milwaukee, WI

    gonzo Senior Member

    How is the outboard mounted? The transom or bracket should have an angle of around 14 degrees. Also, if the outboard is mounted at the forward end of the gap, it will be too far forward to turn the boat effectively. Strakes will not help. A skeg, or pair of skegs will help the boat turn. On the down side, they will probably also make the boat heel (incline) outboard which will tend to throw the crew overboard.
    Joined: Jun 2018
    Posts: 24
    Likes: 10, Points: 3
    Location: Sharpsburg, GA

    PROPGUNONE Junior Member

    There isn’t much bow up front. I helped design (as in offered what I wanted and had someone else draw it) a similar skiff, but capable of a much higher load. Getting it to turn at low speed is going to be tough with that much torque from the sponsons. At moderate to high speeds I have no issues, even prior to sharpening my chines or adding spray strakes. I dont think strakes will help you, but a sharpened aft length of chine might help. Just not much buoyancy up front with that bow shape. I’d figure out your loads nice and carefully if you didn’t do the math ahead of time.

    You can see my version of the soloskiff/whipray crossbreed on Instagram @PROPGUNONE

  11. Laut
    Joined: May 2020
    Posts: 3
    Likes: 3, Points: 3
    Location: Indonesia

    Laut New Member

    A quick update of the project:

    Skiff is nearly completed, only remaining a few details.
    Final weight is a bit over my expectations, around 22kg, but the boat is still transportable under the arm like a paddleboard (i put a specific sup handle).

    Outboard bracket is fixed on the hull with 2 US boxes (normally used for mast/fin windsurf boards). It is fully dismountable (bolts and T nuts).

    Outboard will be a tohatsu 5hp 2 strokes.

    I am away for one month so testing not before September...

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