Bulbous Bow Design

Discussion in 'Boat Design' started by Guest625101138, Jun 24, 2007.

  1. Boston

    Boston Previous Member

    ya Ive been looking into that as well for my build
    Im not so sure about pitch dampening but I have several designers notes ( which I am not at liberty to share with the group unfortunately ) on there canoe bottom designs and they both claim a distinct improvement at slower speeds and a smoother ride at hull speed using flat bow sections in the chine. also a fairly sharp angle on the bow entry

    my two cents
    B
     
  2. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    Frosty

    Attached is some research we did, admittedly on catamarans.
    The bow shapes investigated there was very little in it, especially when comparing U to V shaped bows. Overall, not much effect or gain regardless.

    Adding a T foil will increase drag, it shall also increase slamming, as does the "flat bottom" as noted above. This does not decrease pitching, just gives the appearance of doing so, whilst making the ride much harder, ie higher accelerations.

    Since it is the accelerations, not just the amplitude that is the governing factor of "effectiveness". T foils have very little added mass (unless you go very large..but that would slow the boat down so much one wouldn't do this as it is totally impractical, not to mention the added weight), which is the key to "shapes", since one is interested in increasing the added mass competent of pitching. The pitch period is directly proportional to the A(55) added mass inertia.

    The best way, is the old fashioned way, increase the waterline length.

    Some of our research attached for info.
     

    Attached Files:

  3. Boston

    Boston Previous Member

    Add have you ever looked into an inverted T keel for a semi-displacement power boat that is larger for and aft and skinnier in the middle. Have yet to see one anywhere but I was kinda thinking that it would dampen pitch acceleration. Sorta creating resistance in the vertical plane while offering minimum resistance in section

    cheers
    B
     
  4. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    In a dynamic mode, assuming at speed, if set up correctly they can affect pitch acceleration by dampening. The 'foil's can be tuned, but, it depends upon the hull form. If the hull form has poor pitch stability to start with, the amount of lift required, to dampen the pitch, requires very large foils, which slows the boat down and adds weight etc etc...in which case, you may as well go the whole hogg and have a hydrofoil, ie lift the hull above the water!!

    This is why adding fins/foils to a SWATH is so effective. The pitch stability can be very easily tuned.
     
  5. Frosty

    Frosty Previous Member

    What I am trying to do with my 44 foot cat 500Hp is to improve on speed and to steer well in following seas. My bulbs were very badly designed, they were fared in so that they were facing downwards. This made bow steer and was not nice in following seas.

    I modified this by destroying the upper part of the faring that had a downward force. This improved this tremendously and put a knot on top.

    It is possible there is some more improvement yet to come.

    The manufacture fit some with and some without, they are not telling why.

    A boat of this size in my opinion should have a shallow sharp entry.

    The bulbs are cosmetic but quite large, fairing say 6 feet down the hull. Ide like to remove them but I dare'nt.

    Im hauling out In Thailand Satun in mid Jan. I shall be making some more adjustments to rudders and alter the bows.

    I want the hull to lie as she wants to lie without any influence from the bulbs. Speed is always 20Kts,--thats all I drive it at.
     
  6. Ad Hoc
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    Ad Hoc Naval Architect

    Following and quartering seas are the ones to watch for, as with everything in design, it is all a compromise, there has to be a quid pro quo somewhere.

    Without looking at the lines and shape of your hull, difficult to say....but reality is, you're not really going to be able to do much, other than increase the waterline length, is the best option. You can do this in the form of "a bulbous bow", but with minimal buoyancy, so like a knife on edge....i suspect the BBs you have, from your description are nothing more than a "marketing tool"....does nowt for you.
     
  7. Frosty

    Frosty Previous Member

    BB's a marketing tool!! i think your right I wish I dare remove them. Thing is, the boat floats flat at the moment and I dare not put one more kilo of weight forward. When she is planing (semi planing) she is at a nice say 5 degrees and i have a feeling those "diving" bulbs are still affecting her in a downwards trend.

    As I have mentiond earlier in this thread I have thought of fitting 2 NACA 63412 foil wings on the bulbs.

    Or encouraging air just at the bulb the same way as stepped hulls work, In other words a half tube bring down air to the bulb to approx 1 foot below the planing water line. prelimanary experiments running up and down the dock with said half tube suggests that it will work and will pull air down to 1 foot quite easliy at running speed.

    Ive never seen anyone do that before.
     
  8. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    the fact you modified the upper part and had benefit, just indicates, a marketing tool. Since I'm sure she would run just as well without them.
    Then, if you were not happy and still wnated, add a long thin section of bow fwd....but without data, hard to say for sure. But that would be my call....remove them!

    You're running at high Fn's...way beyond the range of BBs being effective for pitch dampening anyway.
     

  9. brian eiland
    Joined: Jun 2002
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    brian eiland Senior Member

    BULB (BULBOUS) BOWS on Multihulls,.... Why?

    I've been working on some postings to go on the discussions of 'anti-slamming bridgedecks' utilized aboard multihulls (HERE), and concurrently I ran across:
    1) this photo of the bulb bows that were added (after launching) to Pacific Harmony (below)
    2) Frosty's attempt to modify his bow bulbs for better performance http://www.boatdesign.net/forums/fiberglass-composite-boat-building/bulbous-bow-addition-39199.html

    It occurred to me that we do need to distinguish between the design of, and purpose of use, of bulb bows on multihulls verses those on monohulls.

    The use of bulbous bows really originated for the most part in the tanker vessel fleets of the world. These vessels are basically long slender monohulls that operate for extended periods at very near the top of their 'displacement speed'. Naturally they don't try to 'plane' these vessels as that would be very uneconomical. But as they approach their top displacement speed they are operating in a single wave length....that is a wave crest at their bow, and one at their stern. The vessel itself spans this single wave length. If the height of the wave crest at the bow could be reduced, then the vessel might gain a fraction of additional speed with no additional engine power needed to 'climb' over that bow wave.

    Numerous different bulbous bow shapes have been tried on tankers and displacement power yachts (trawlers, etc), where the basic premise was a roundish (spherical) front leading shape followed by a concavity that 'sucked' the surface water down rather than allowing it to 'climb' the ship's flared bow. In some cases the bow bulbs could effectively reduce the height of the bow wave by ½ at best. In general the bulbs ended up being only effective at 'specific vessel speeds', the displacement hull speed of the vessel itself.
    NOTE: You do not see many (any) viable bow bulbs utilized on planning monohulls....just not appropriate as these vessels really need to add more power so they can really climb over their bow wave and plan off.

    But along comes multihull vessels, and suddenly the appearance of some bow bulbs on a few of them. Why? Well you say they are 'displacement boats' aren't they? Yes, but they are exaggerated slender hulls that don't conform to Froude's rule. They don't end up traveling in a single wave length like the monohull, and they don't need to 'climb' over their bow wave for better efficiency.

    So why add bow bulbs to multihulls? They might be added for totally different reasons, and therefore might assume a really different shape. Primarily bulbs would be added to correct for a deficiency in buoyancy in the bows either from the 1) original design, 2) builder modifications to the original design, or 3) subsequent modifications to the vessel during its lifetime.

    A prime example of this need for the addition of bulb bows would be aboard the vessel Pacific Harmony.

    Some quotes from various forums:
    Malcolm Tennant....
    It also needs to be recognized that the "super skinny" hulls of the Pacific Harmony are not a function of the original design but were caused by the increasing of the LOA from the original designs 72' to the final 86' without increasing the hull beam. This resulted in a hull beam of around 1.4m/4' 6" ,whereas if it had been designed from scratch to be 86' it would have been at least 1.8m/ 6' in beam. So any problems ascribed to the ultafine hulls are actually attributable to the increase in length rather than the basic design. We certainly do not usually use such an extreme fineness ratio as all our data indicates that it is just as counter productive as making the hulls too wide.”

    Tennant again...
    “However John Winter also has a point when he says that extreme fineness ratios on displacement hulls are not necessary and have some associated problems. Certainly if the fineness ratio is too high you start paying a viscous drag penalty. Particularly at higher speeds. Pacific Harmony had a very high fineness ratio. However it should be realized that it was not actually designed with this high a ratio. During construction it was lengthened to 86 ft from the original as designed 72 feet with the concomitant increase in the fineness ratio. The displacement also increased which meant an increase in the size of the engines from 600 to 800hp.”

    Pacific Harmony with bulbs.jpg

    Pacific Harmony before bulbs.jpg

    This is a pretty good photo of the bow bulbs that were added to Pacific Harmony. These are my observations about their shape:
    1) I like the V'd shape added to their top surfaces as this would decrease the tendency to push the bows down.

    2) I question the lack of some concavity on the upper sides where they join the hull. I believe some concavity here would 'pull down' some of that bow wave that is going to be jammed into the bridgedeck underside.

    3) I believe the bottom sides of these bulbs are too flat, and will slap the waves as they rise and fall meeting oncoming waves. Many bulb additions to pleasure craft have reported this 'slapping tendency' of their 'cylindrical shaped' bow bulbs.


    Finally I will just voice my preference for traditional flare in the topsides of vessel bows for both beauty and reserve buoyancy purposes, rather than the addition of bulb bows. Also make the vessel a little bit longer in the bow than you intended, BUT don't fill that volume up with ANYTHING other than perhaps a collision bulkhead. A little extra length in the vessel doesn't cost that much, and a pretty bow is worth it.
     
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