Designing a small bay boat

Discussion in 'Boat Design' started by Shrimpkin, Jun 24, 2016.

  1. Shrimpkin
    Joined: Nov 2015
    Posts: 21
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    Location: Baton Rouge, LA

    Shrimpkin Junior Member

    I recently purchased a small 15.5' boat with a 5' beam powered by a 2003 Yamaha F60 4-stroke. I mostly bought it for the motor as the boat is very heavy for its size. The bottom is a double hull of 1/4" aluminum with 3/16" sides and a 1/4" boxed transom. It rides like a plow basically and has trouble getting over 30 mph.

    I've been wanting to build a boat for a while now and I run a fabrication shop with a few CNC machines (plasma and waterjet) and I have all the welding equipment I would need. I've also been dabbling with some 3D designs and then flattening out the parts to cut on the CNC.

    Here is a small scale model I made of one hull I designed:

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    I'm kind of trying to mimic an alumacraft bay boat with a CC. I'd like to try and get at least 18' length with around a 84" beam and power it with the 60 for now then upgrade later to something around a 90. I know there are a lot of other things to do to the hull to make it better such as strakes which I plan to do with some small angle and just tack it down the length. Same with a chine down the side, I could maybe contract someone out to bend it but I think I could also just use a piece of angle strapped down the side length of the boat.

    I guess I am looking for a little more in depth theory on hull design for this type of boat. My current design has a 12° transom angle and 12° transom deadrise. The transom height is 20" and the LOA is 18'. The keel has a slight flat spot in the back about 2' long then rises to a 1° angle until it hits a smooth radius up to the tip of the bow.

    Looking for comments/suggestions or just general knowledge on small CC bay boats. I estimate the hull would be around 700-800lbs finished without the motor.
     
  2. PAR
    Joined: Nov 2003
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    Location: Eustis, FL

    PAR Yacht Designer/Builder

    That's basically a garvey style of hull and the various parameters about hull design fill several books, which you might want to invest in. There are plenty of plans available for this type of boat, some with CNC cut files available, so you might want to look around.

    You transom deadrise doesn't appear to be anything close to 12 degrees, more like 5, maybe less (how are you measuring this?). This shallow a deadrise will permit the boat to get up on plane fast and accelerate quickly, but it will also pound unmercifully once you get over 30 - 35 MPH.

    Attached is a little boat I did recently and it has a 14 degree transom deadrise. It's nearly a perfect monohedron, so will do very well are high speed.
     

    Attached Files:

  3. Shrimpkin
    Joined: Nov 2015
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    Location: Baton Rouge, LA

    Shrimpkin Junior Member

    The deadrise is 6 degrees on either side so maybe it's a 6 degree deadrise? This boat is going to be used for in-shore fishing and maybe going out on some small lakes so it won't ever be in any super rough water. To compare I am going out in my 15.5' boat in the same places no problem and it doesn't do well AT ALL in chop.
     
  4. PAR
    Joined: Nov 2003
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    Location: Eustis, FL

    PAR Yacht Designer/Builder

    Dead rise is measured from a horizontal line across the transom, at the centerline. So if you have 6 degrees, that's how much deadrise you have. Low deadrise and flat bottom boats do poorly in a chop. The garvey shape you have will be slightly better than a flat bottom, but not much, because the bow is too bluff and the deadrise so shallow. If you want to "soften" the ride an appreciable amount, you'll need at least 10 degrees and a much finer entry 1/2 angle too. Of course, with these changes, she's not going to hop up on plane as quickly as a flat or low deadrise bottom will, so you may also want to add some strakes, the help "get 'er up".
     
  5. Shrimpkin
    Joined: Nov 2015
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    Location: Baton Rouge, LA

    Shrimpkin Junior Member

    Well compared to what I have now most anything would be an upgrade in ride quality. Just to give you an idea here is what I have. Keep in mind, 1/4" bottom and 3/16" sides with a 1/4" transom. It's crazy heavy for its size.

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    It's basically a garvey with a flat bottom and two pontoons. It beats the hell out of you and isn't very stable with 2 people walking around on it. Most of the fishing I do is inshore in the marshes and freshwater lakes and rivers.

    My plan was to add strakes just like I have on my current boat above. Just strap some angle to the bottom and sides of the hull, not only for extra strength but for handling and ride quality.

    How would a higher deadrise around 10-12 degrees and a sharper bow entry angle affect speed with my little 60 on my 18' bay boat model?
     

  6. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 474, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    You're asking questions that are difficult to answer easily, because there's so many possible variables.

    As a rule the more deadrise, the more power you need for the same speeds as a flat bottom, but the more comfort you'll have, once you're up and cooking over say about 25 MPH. If the hull form is monohedron, you'll have higher speed potential, as flat and warped bottom boats start to progressively pound the faster you go, eventually becoming unstable, if you try to push it faster. Unfortunately, the more deadrise the more displacement the boat needs and the more draft is requires too.
     
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