Pod/Hull extension design

Discussion in 'Boat Design' started by Connor Ferguson, Jul 20, 2019.

  1. Connor Ferguson
    Joined: Jul 2019
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    Connor Ferguson Junior Member

    I have a 23 foot aluminum pilot house, it has a 300 Verado on the back and sits super stern heavy in the water. The boat also has sponsons on the back half of the hull, and the gas tank is right at the stern, making the boat very stern heavy. It has 2 flotation pods full of foam so I’m assuming they aren’t helping. When not moving the boat sits so stern low that the scuppers are under water, it almost looks like it’s taking on water. While up on plane it’s dragging the back end of the boat, the flotation pods aren’t even out of the water. The boat was designed for light 2-strokes of the 80s.

    This fall I’m going to do a new pod or hull extension. It’s an expensive job but if it makes the boat run faster and more efficient then it’ll be worth it. However, I only want to do this once, no half *** jobs. Does anyone know of a naval architect that has experience with smaller crafts, or an engineer that would know what needs to be done. I know there are a lot of guys around that make great pods, I want to make sure it’s engineered to work with my boat. The first person I talked to suggested cutting off everything from the transom back and building a new larger pod that is an extension of the hull. No tapering or anything.

    My worry is creating new problems with the new pod. Any suggestions are much appreciated.
     

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  2. messabout
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    messabout Senior Member

  3. messabout
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    messabout Senior Member

    Connor the pictures show the main engine extended well behind the transom. Moving that big engine as close to the transom as is practical will mitigate the problem somewhat. You also seem to have a smaller engine on or near the transom. That makes the problem worse.

    Imagine a playground see-saw. Put too much weight on one end and that end tends to overwhelm the lighter weight on the other end. Moving the pivot point of the see-saw closer to the heavy end will let the thing balance. Moving the big engine forward will have the effect of moving the pivot point more to the favor of the boat. Think of the system of weights and distances as if was a system of levers.

    You would need some very large pods to fix the problem if the motor stays where it is.
     
  4. TANSL
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    TANSL Senior Member

    In the second picture the boat seems to have a correct floatation. May be the problem is not only about weights distribution.
     
  5. Ad Hoc
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    Ad Hoc Naval Architect

    It does look visually that the outboard you have is too large/heavy for the hull.

    So, the first thing you really need to do, before any cutting is do a simple weight and centres calculation, as alluded to above my messabout.
    But you need to do this for the whole vessel...itemise every item on the boat and use a datum point for the distance. Usually this is about midhips, but on small hulls like yours about the transom is better. From this you can work out the location of the LCG of the design. You then compare this to the hydrostatics of the hull. This will tell you straight away if there is a simple centres issue.
    Secondly, if you have an resistance data (which I doubt) for the hull. A simple look at the LCG chase of the hull will tell you if the centres are creating the basic problem you have. (I would suggest yes, but you need to confirm this before any cutting). Additional features like adding a fixed wedge may also assist or even simple interceptors.
    But once you run the basic numbers - before any cutting - you can see if adding an extension (simplifying extending the whole hull aft) will solve your problem and more importantly how much to extend it by - rather than just an assumed amount.

    For a naval architect, your best bet is someone close to you, so you can chat with them face to face. Two forum members here called TAD or JEH will give you what you need and they are very local to you.
     
    Last edited: Jul 21, 2019
  6. Mr Efficiency
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    Mr Efficiency Senior Member

    Without a stern view, it is hard to comment, but if the bottom of the pod is not continuous with the hull proper (seems likely), naturally it tends to reduce available lift back there. Your fuel tank being rearward is a bad idea, especially when full. You get a double whammy, heavy boat with the extra weight where you don't want it. A possibility would be to extend the hull on both sides, and leave the pod as-is, but so as not to interfere with the water flow into the prop.
     
  7. Connor Ferguson
    Joined: Jul 2019
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    Connor Ferguson Junior Member

    Thanks for the reply. The motor can’t come any more forward. When tilted all the way up, the motor is touching the transom.
     
  8. fpjeepy05
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    fpjeepy05 Senior Member

    Adding 50lbs of lead in the anchor locker is a lot cheaper.... Any amount of floatation aft of the transom will help if the motor doesn't move. Making a step between the extensions (port and star) and the hull as well as pitching the bottom of the extension up 1":10" should not alter the performance of the hull. Trim tabs can be added to push the bow down if you would like.

    To test what the static line will be with your extensions, calculate the volume that will be underwater (length x width x depth) x 2 (port and star). Then go to home depot and buy 4x8 1" sheet insulation board (the pink stuff) cut it however you need to get the same volume and secure it to the underside of the hull with a ratchet strap. The extension itself will have a little bit of weight, but it will also be further aft than the underside of the hull, these will cancel each other out a little. It won't be exact, but it will give you an idea if it is worth your time.

    Racket balls use to fit in our scuppers and were a lot cheaper than extensions to keep your feet dry when fishing, and easy to pull out in an emergency situation.

    If you want to test if the boat is less efficient because it is running too bow high... Run it with a guy with that kicker motor in his lap sitting on the bow pulpit and compare to him setting aft. If its faster with the guy on the bow then trim tabs or interceptors would help. If it's the same or worse, it's not worth your time.
     
  9. Connor Ferguson
    Joined: Jul 2019
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    Connor Ferguson Junior Member

    Thanks for all the input. There isn’t anything I can move forward for weight distribution. The fuel tank won’t fit under the floor anywhere else.

    Talking to a few people, it seems that a hull extension pod is looking like the best option. It will add a lot of floatation in the back, give me the ability to add trim tabs and a proper kicker bracket. It will also essentially push the fuel tank forward to closer to the middle of the boat.
     

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  10. fpjeepy05
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    fpjeepy05 Senior Member

    It looks like it already has an extension... Will you be cutting that one off? Either way that it a large change and employing a nav arch would be money well spent.
     
  11. Connor Ferguson
    Joined: Jul 2019
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    Connor Ferguson Junior Member


    Yes, everything behind the transom would be removed. Then a 1 piece hull extension pod welded on.
     
  12. Mr Efficiency
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    Mr Efficiency Senior Member

    Thanks for posting the extra pics. It seems the engine is not stepped back far from the bottom of the boat. It could be more a case of the COG being too far rearward. Tabs could help at cruise speeds. I don't think surgery is advisable until other avenues are explored. I'd suggest putting some blocks under the rear of the boat trailer, and carefully and slowly roll the boat back along the trailer, till it just starts to tip. Naturally ensure it isn't free to keep rolling back, and drop to the ground. Note how far the fulcrum point is forward of the transom, if less than roughly a third of the boat length, it does suggest stern-heavy.
     
  13. fpjeepy05
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    fpjeepy05 Senior Member

    Either way, Connor still needs a solution, because I don't believe returning the boat is an option.
     
  14. Mr Efficiency
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    Mr Efficiency Senior Member

    We need to understand the problem a little, before settling on a solution. The most obvious solution is trim tabs, if the COG is too far back, they won't help much till the boat is driven up to planing speeds, but if altering weight distribution is not practicable, they are at least a partial solution.
     

  15. Connor Ferguson
    Joined: Jul 2019
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    Connor Ferguson Junior Member

    Thanks for more replies. The boat is very stern heavy, I can lift the boat on the trailer off the ground from the tongue of the trailer. Moving the fuel tank forward isn’t possible.

    I spent an entire winter gutting and rebuilding the interior, I really enjoy the boat and just want to improve the performance.

    The existing pods are of poor design and an afterthought from the previous owner. I think trim tabs are a great idea but it doesn’t fix the bad pods.

    I’m leaning towards hull extension pods and trim tabs this off season.
     
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