Speed Boat Centre of Buoyancy

Discussion in 'Boat Design' started by Styrofoam, Jun 2, 2012.

  1. Styrofoam
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    Styrofoam Junior Member

    Hi again BoatDesign.net,

    Maybe a dumb question but I wont know until I ask. Just been reading up more as per usual and this centre of buoyancy is fairly simple but what no one really tells you is where it should be. The closest I got is slightly aft of the CB at waterline length.

    Doesn't really help me much, so my question is, what is it for a planing hull speed boat? For simplicity I got these figures out of Delft!Ship HydroStatistics report:
    LOA of 9.834m
    LOW of 8.586m
    Midship Location of 5.018m

    My design in Delft!Ship says I got a Centre of Buoyancy of 3.757m (-14.68%).

    Now am just plain lost. The Centre of Buoyancy of 3.757m fits what I've been reading, so from what I can gather I should design all the components to fit and keep the boat balanced at this point. Engines, fuel tanks, water tanks etc. as close the Centre of Buoyancy as possible and balance the rest out.

    My question is, is this correct and if so how is it reached and does it factor in that I have a planning hull?:confused::?::confused:?
     
  2. Ike
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    Ike Senior Member

    You are talking apples and oranges here. I think you are mixing Center of Buoyancy and Center of Gravity. The center of buoyancy is calculated for when the boat is in displacement mode. This doesn't apply when it is on plane. When it is on plane a very small part of the boat is supported by buoyant force and that part is all far aft.

    With planing boats you have a conundrum. When the boat is in displacement mode you want the Center of Gravity over the Center of Buoyancy, and everything nicely balanced. But in planing mode the center of buoyancy moves down and aft and you also want the CG to be far aft over the center of buoyancy in planing mode. So what do you do?

    Well if your main concern is speed you design for the planing mode.
     
  3. Styrofoam
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    Styrofoam Junior Member

    WOW, that cleared up a lot Ike, thanks for that. Well this is a planning hull. 16.5% Deep V with a deadrise. So that answers the question pretty much where I need the weight.

    But after reading your post for the 10th time now (and am probably over thinking this) what do you mean exactly by "over" as in the same location or prioritized?

    But by shifting the weight aft, you will obviously change the CG drastically and also CB but not to as much of an extent as the CG.

    But how far aft do I want the CG and CB to be, I could drop the 2 engine and propulsion unit all within 2 meters of the transom with a total weight of 1.5 tonnes and the fuel and water on the 3.757m CB. It all seems logical to me but I got one of those feelings that something isn't right and that the boat will be cruising with its trim on max to keep the bow down all the time.

    But then again, outboards got the engines even further out (although admittedly they are lighter). Confusing myself again with Kiwi and Pineapples now.

    Just going over some pictures and I noticed this, the engine is no where near as far back aft as they could be placed yet it is a planing hull with the engines from my estimate placed roughly on the CB.
     

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    Last edited: Jun 2, 2012
  4. Ike
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    Ike Senior Member

    That depends on a lot of factors, length, beam, amount of Vee, weight and placement of engines, what type of propulsion (I/O, straight I/B, vee drive, and so on) the trim angle, and wetted area on plane. it's a real balancing act.
     
  5. Mr Efficiency
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    Mr Efficiency Senior Member

    Watching an old malibu-style board rider hanging ten, you will quickly appreciate that trim of a vessel subject to dynamic forces behaves very differently to one that is not. There is a fair bit of flexibility as regards lengthways positioning of the COG in a planing boat by reason of the dynamic force being greater at the leading edge, you shift weight forward, the leading edge goes forward, and the trim does not change to the degree it would in displacement mode, or not moving. The more lift the hull generates relative to the weight of the boat, the more so this applies. Planing cats can be more touchy because the leading edge doesn't alter that much with weight shifts, and the lifting area is typically less than a mono.
     
  6. Styrofoam
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    Styrofoam Junior Member

    That makes more sense to me. But boy it sure does sound like a balancing act. From what I've been finding out and reading so far, doesn't seem like there is a formula other than trial and error. Or am I mistaken? Hope I am... is there a secret formula or software?

    My search terms are just giving me the basic volume buoyancy and calculating the CG and CB but not actually any further or information on how to generate the correct CG and CB depending on the variables mentioned. Not many people talk about this which is unfortunate in helping me get to an answer :(
     
  7. Mr Efficiency
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    Mr Efficiency Senior Member

    Needless to say, the COG will be in the same vertical line as the COB at rest. From my experience, COG about a third of the boat length forward of the stern as a rough rule works out OK, further back than that and you will have porpoising and squatting problems, too far forward you will have excessive wetted area. If there were hard and fast rules, all such boats would have gravitated to the same shapes, but you are balancing a lot of sometimes competing priorities.
     
  8. Styrofoam
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    Styrofoam Junior Member

    Look like thats the closest I can get. Just saw theres some fancy software that could help out but looking at how to use them is complicated along with a rather scary price tag only to find out it wont do what I want.

    Based on the 1/3rd rule of the LOA it seems reasonable based on what I've seen. Will have to stick to that for now unless something else comes up.
     
  9. CDK
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    CDK retired engineer

    COB for a planing vessel is dynamic and assumed to be in the center of the wet surface. The faster you go, the smaller that surface becomes, so the COB shifts aft with increasing speed.

    Where it is at rest is not really important. Very fast boats sometimes look ridiculous because all the weight is near the stern, yet at speed they maintain a 5-8 degree angle.
     
  10. kach22i
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    kach22i Architect

  11. Styrofoam
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    Styrofoam Junior Member

    Yeah, its dynamic for sure. I got that much, I think that's why am getting rather confused.

    Slightly off topic though but the COB stated in Delft!Ship, that is the position of COB whilst the boat is stationary I assume.

    Obviously the more weight that goes onto a boat will raise the waterline. Is there a nifty but simple to use software that can calculate the boat waterline and how it will float at what pitch stationary as you add weights to different sections of the boat? Would help a lot for me to design the deck of the boat and where I can place all major components. But obviously if not that can be done by hand, provided I can figure out how the hull will float unladen.
     
  12. messabout
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    messabout Senior Member

    Complicating the issue a bit more.....The seemingly forward placement of the engines in the picture has a reason. That would be shaft angle compromise.
     
  13. Styrofoam
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    Styrofoam Junior Member

    From what I can see, the picture there is taken showing the rear side of the engine, so it looks like the transmission is on the bow side of the engine then reversed back down to the props.

    At least that's what I can see, but regardless they could simply just use drive shafts so they could enable the ideal placement of the engines.

    But from what I gathered so far, its just about impossible to figure out where you want the weight to be without a ton of experience along with trial and error so guess am stuck with the general rule of thumb which is 1/3 from the stern. Worse comes to worse if its soo bad I could always move things around or just make another hull to get more experience. But thats still a long way down the line!
     
  14. Alik
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    Alik Senior Member

    Normally one should choose optimum LCG and LCB for given speed and hull proportions, and only then design hull shape and arrange weights to match those LCG and LCB. It actually is a spiral process, but You have started from the wrong end.
     

  15. Styrofoam
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    Styrofoam Junior Member

    I read up on that and the process is a cycle until you reach the desired results. Unfortunately, I sort of skipped some steps and saw a design that I just fell in love with. So The design and hull shade is just about fixed. As for the speed, what ever 2 300hp Volvo Penta IPS can pump out which from what I gathered is pretty fast provided your not trying to push a brick through water.

    There's plenty of free or cheap software available to develop your hull and design etc. but rather non in the department of how to fine tune your design. Got some programs downloading and hopefully that will yield some more answers.

    Another problem is that its almost impossible to find information on how a boat will react with a pod drive as conventionally its either a stern drive outboard inboard or jet drive. But from what I can gather, pod drives react similarly to a jet drive.
     
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