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Boat Hull Design

  1. Nov 26, 2014 #1
    So I go into a debate this morning with a co-worker about a boat hull design.

    He is stating that if we create tunnels in the bottom of the boat that run longitudinally and are capped on the bottom but open on the ends they will trap water and act like ballast tanks thus adding stability when the boat heels. My argument is that since the water is not trapped like a typical ballast tank the water will just move out of the back of the tunnels as the boat heels, and that the water inside the tunnels cannot be considered trapped because the tunnels are open on the ends, thus allowing equalized pressure in the tunnels as the boat heels. I would think that you will see a slight resistance in the boats heeling acceleration because you have more surface area interacting with the water but since you are not containing the water within the hull by leaving the ends open I would not believe it would do the same as having a ballast tank inside the hull at approx. the same location.

    One last note this argument only exists when the tunnels are under the waterline.

    I drew two different pictures. One showing a standard boat hull and one with the tunnels. The argument is not about when the boat is running in a straight line but when the boat is standing still and just heeling from side to side.

    I am hoping someone can explain to me the physics of what is happening to that water inside the tunnels. And if I am looking at it from the wrong point of view.

    Thank you

    Hull with no Tunnel.JPG Hull with Tunnels.JPG
  2. jcsd
  3. Nov 26, 2014 #2

    jack action

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    The way the case is presented, I agree with you with the same reasoning. You are just changing the shape of the hull and the water in the tunnels is still in the sea and cannot act like ballast inside the hull.

    Although, since the hull will sink deeper (as you draw), depending on the shape of the hull, the center of gravity of the boat might become lower with respect to the center of buoyancy, thus increasing stability (In the image below, the boat condition could change from the one the right to the one on the left by adding the tunnels).

    But, to argue for your co-worker again, if you design the end of the tunnels properly and add an air duct between the two tunnels, you could get a system of external stabilizer tanks that counteract the roll motion of the boat.
  4. Nov 26, 2014 #3
    Thank Jack,

    That makes sense to me.

    So now what if we look at a situation where we have a boat with tunnels vs. a boat with tunnels but the tunnels are "capped" at the bottom.

    So when sitting still in the water both tunnels are filled with water. Now when you heel to the port or starboard, will the tunnel with the cap on the bottom heel slower because the water is trapped. If so can you explain?

    Thank you

    Tunnel with no bottom.JPG

    Hull with Tunnels.JPG
  5. Nov 26, 2014 #4


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    Those tunnels will help as ballast, in much the same way as a standard rocker stopper - a common item on boats, used specifically to calm rocking.


    Anything that creates friction to the rolling action of the boat will tend to limit it ,such as a large volume of water having to flow slowly out of the tunnel mouths.

    The design is not very efficient, and I suspect that you will have secondary unpleasant motions, which might actually be just as bad. (eg. what hpapens when the bottom rights itself, and a ton of water flows into the side that id dropping?)
  6. Nov 26, 2014 #5


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    It will because, until the water fully exits the tunnel, it is acting as ballast. The small exit area means that it will take time to flow out.
  7. Nov 26, 2014 #6


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    I have a question: why leave the tunnels open at all?
    Why not open them, let the water flow in and then close them? You now have free ballast. Ballast reduces heeling.

    This is exactly how water-ballasted boats - such as a MacGregor - work.

    http://kobernus.com/hunter260/water_ballast/water_ballast_index.html [Broken]
    Last edited by a moderator: May 7, 2017
  8. Nov 26, 2014 #7
    Thanks Dave,

    But the MacGregor system works by pulling a plug on the transom and letting the ballast tank fill with water. Then you put the plug back in when you have filled the tank. Basically filling a contained tank inside the boat with seawater, and since this added weight is below the boats VCG it lowers the boats VCG.

    In our discussion the only difference is that you add the plate to the bottom of the tunnel. And his argument is that by adding the plate you are trapping the water, and adding weight to the boat outboard and below the waterline thus lowering your VCG.

    My argument is, that you are not "trapping" the water and lowering your VCG. I see it that you are adding resistance by basically adding a fin or surface area which causes more resistance in the water during heeling moments.

    Also the reason why the back and front of the tunnels are open is to allow any water in the tunnel to move freely through the tunnel when moving forward.
  9. Nov 26, 2014 #8
    Sorry one other note to answer the mail about why we don't close the tunnels. This is a planning craft boat like a speed boat. So any water added as ballast weight will reduce the speed of the boat.
  10. Nov 26, 2014 #9


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    It is not the trapping of the water that lowers you VCG; it is the volume of water your hull displaces. And your hull displaces less water than a hull without tunnels, yet it weighs the same.

    So your hull design, with its tunnels, will automatically have a lower VCG that it would without the tunnels.
    To conceptualize this, imagine increasing your tunnels until they are 90% of the volume of the hull. Now your hull will sit with a mere few inches of gunwale above the waterline.

    Why? That simply means more friction.


    There Ain't No Such Thing As A Free Lunch. You want ballast, you're going to have a heavier boat.

    Putting holes in your boat will not reduce friction,; it will increase it. Any surface area in contact with moving water will create drag. Moreso, if the water is constricted, such as in a tunnel.
  11. Nov 26, 2014 #10
    Understood, but is there a difference between the two pictures above where one has a plate on the bottom of the tunnel and the other doesn't.

    Well if placed correctly, you can reduce the friction of the hull on water but getting them positioned where when on plane the tunnels are out of the water.
  12. Nov 26, 2014 #11


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    If he wants a fast boat, putting scoops on the front is sure not going to get him up on a plane.

    If he wants stability while at anchor, why doesn't he just use a sea anchor rather than compromising performance? (Rhetorical question: because he fancies himself an inventor with a new idea.)

    In fact, it sounds like a Mac is EXACTLY what he wants. You don't leave the water in when going fast. You simply open the stopcock on the stern, and the inclination of the accelerating boat drains the tanks automatically. When you stop, the tank refills and you're stable again.
  13. Nov 26, 2014 #12
    If you can do it with the hull of the boat, it becomes passive.

    Who wants to keep a stability anchor on boats that you have to worry about putting over the side of the boat and then pulling back in.

    With that said do you think he is thinking about it correctly, is it "trapping" the water. Or is it just adding more surface area to push against the water.

    Thanks for the discussion.
  14. Nov 26, 2014 #13


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    See last post. I updated it.
  15. Nov 26, 2014 #14
  16. Nov 26, 2014 #15
    I guess, I know it works. It is proven but I don't agree with his interpretation of why it works.

    I do not agree that is trapping the water. I do not see it as adding to the weight of the boat, thus lowering the VCG. I see it as adding additional surface area that push's against the water.
  17. Nov 26, 2014 #16

    jack action

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    I'm no expert on boat design, but I'm pretty sure that if the tunnel theory was adding stability, it would have been used already; And it doesn't seem to be the case.

    But you are putting a doubt in my mind.

    I guess the weight of the water would add an anti-roll moment that have to be resisted, because water cannot come out. But it doesn't affects the CG location though. But since the boat has to sink deeper because of the reduced volume, it might be at the same level depending on the hull design.

    But if your boat tilts enough that the tunnel goes above the waterline, then you loose everything fast.
  18. Nov 26, 2014 #17


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    It would only be "instant" in proportion to how large your exit aperature is.
  19. Nov 26, 2014 #18


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    I have thought of an alternative that is a mix of both.

    Eliminate the port on the front, keep the port on the stern.

    When your boat is sitting still or moving slowly, the stern port - which is below the waterline - will keep the bilge full of water.
    As you accelerate to a plane, the boat will tilt up, and the water will automatically drain from the bilge.

    It eliminates the dangerous instability of a scoop of the front, and it fills and drains automatically.

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