Catamarans High Speed Blow Over - Causes & Solutions

Discussion in 'Hydrodynamics and Aerodynamics' started by kidturbo, Sep 11, 2013.

  1. kidturbo
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    kidturbo Junior Member

    Hi All, new member but long time viewer here requesting a casual discussion on the causes and possible solutions to blow overs in high speed / high powered cats.

    The offshore racing and pleasure craft world has seen a big jump in the top speeds of catamarans over the past 10 years. 140mph is now considered mid-range speed, accessible to anyone who can afford a new performance boat. Along with this speed increase has came the increase in blow over related accidents, recently with headline grabbing tragedies in both classes.

    The discussion of cause and effect is my only goal here, not placing judgment on any design or use of the equipment. It's a well known fact that cat's, hydroplanes, and other tunnel designs have a much higher risk of blow over vs a V-hull. My question is what reasonable design features could be changed or added to lower this risk without degrading performance?

    Simple stuff like wings, flaps, airfoil dumping options and such that could be implemented on existing craft or into the new molds. With computer technology at today's level, it seems like the marine industry has missed the boat when it comes to solving this very old problem.

    I'm sure this is an old topic to many of you, however it's a "too hot to touch topic" on another popular boating forum at this time. So please bring those of us just joining this discussion up to industry speed. I've long felt this site is better suited for those seriously interested in solving a problem than others, so please prove me right.

    Thanks

    -K
     
  2. kidturbo
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    kidturbo Junior Member

  3. rwatson
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    rwatson Senior Member


    Huh ? Low risk with no performance loss - are we looking at chaging the laws of physics here ?

    Your concept of simple is different than mine - as soon as you say computers and salt water you are in deep, for a start.

    Wings, flaps etc are all extra weight, so how do they not decrease performance, as well as adding severe complexity. ?
     
  4. Ad Hoc
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    Ad Hoc Naval Architect

    In a nut shell, it is the usual design "compromises".

    Speed v stability. For greater stability one requires control surfaces. These surfaces add drag, which slows the boat down. Thus the quest for speed is dictated by the risk and known risk of such 'blow overs'.

    You can sum this up simply by this:

    Would you have a much more stable and thus safer cat running at 100knots, or one that can run at 140knots but the risk of loss of stability is high and requires a highly skilled helmsman and even then no guarantee?

    Which boat would you race in?
     
  5. Mr Efficiency
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    Mr Efficiency Senior Member

    Life is difficult when you travel at break-neck speeds, in any medium. Besides blow-overs, when there are waves to contend with, cork-screw rolling is a real possibility.
     
  6. kidturbo
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    kidturbo Junior Member

    Thanks for the replies.

    All good points, however I believe you'll find the performance boat industry is quite willing to focus more on safety. If for no other reasons than most owners like to insure their very expensive toys. 3 of the 4 videos I posted above were pleasure craft, in open course events, not racing boats. These high speed incidents are certainly drawing attention to the down sides of this hull design.

    On simple designs, lets look at the roof flaps used by NASCAR. Zero user input required, airflow change over the roof causes them to deploy. A very light, KISS design that has proven extremely effective at preventing lift when the car spins. That's what I call a good simple solution. My ideas down this line lean towards dumping the air flow to stall the foil lift.

    On the compromise with wing and canard surfaces, sure these might add some weight and drag, but remember we are talking of adding a few pounds onto tonnage. Used correctly they could easily increase performance, just look at F1 racing to see this principle in action. The engines used in these boats are making 1100-1500hp X2. The new turbines are even higher. A little extra drag, not a problem..

    To the point of computers, electronics and salt water. These boats all have no less than half a dozen onboard ECU's from the factory. One more for stability control would easily find it's place. I feel this is ultimate long term solution, once a proven designs can demonstrated, it will become a standard. Humans couldn't control a flying wing aircraft like the B2 bomber when it was introduce, but adding a computer makes it look easy.

    Reason I consider this a "simple solution" is I fly RC choppers for a hobby. The gyros used today are amazingly small, fast, and accurate. One the size of a quarter can keep the tail rotor of a 3' long RC helli pointed in the same direction while performing 3D maneuvers at some incredible G-forces. So how hard could it be to transfer that kind of technology to a couple flaps or canard on a boat? I'm not proposing boats fly, just how best to stop them from flying.

    What's concepts are out there already that I've missed?
     
  7. Ad Hoc
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    Ad Hoc Naval Architect

    Emphasis is on the "simple".

    A car travels on a fixed and level and thus know response surface and the winds in a similar manner. A boat travels on a constantly varying surface, wind driven and swell driven waves often with a constantly changing wind pattern (no obstructs close by) and so on. All this makes any control algorithm very very complex indeed, very!!. Owing to the speeds involved it also requires super fast responses to the every changing conditions. This adds weight too as a slow actuator compared to one with a response rate below 100hz is not light nor cheap.

    Not saying it cant be done...but it is not "simple" at all.
     
  8. Mr Efficiency
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    Mr Efficiency Senior Member

    After watching an offshore championship race dominated by big tunnel cats, my reaction could be summarised by the words, "these b@stards are mad" ! No wonder safety capsules are fitted, wouldn't make me feel safe though.
     
  9. kidturbo
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    kidturbo Junior Member

    Yes that's the biggest problem I see with finding a solution like the NASCAR roof flaps in this situation. Being passive, then autonomously deployed. The airflow under these boats is constantly changing, would need to be at least partially activated off angle of attack.

    Two of the incidents I've listed above involved boats catching "prop roost" in the tunnel area from a lead boat at high speed. Just replacing some of that air with a jet of water seems to increase the lifting pressures enough to start the rapid bow climb. If you watch that footage frame by frame, [other good angles of both situations are out there] you realize how little time it takes to go from stable lift to being blown over.. No way a human could react fast enough to deploy an edge that counteracts those forces. So any feasible design needs to be autonomous.

    But this doesn't have to be overly complicated either. The roof flaps came about back in the mid 90's after several nasty wrecks involving flipping race cars. "We developed them so NASCAR would not slow the cars down more," said the devices' primary engineer, Jack Roush. << A self taught engineer who solved an old problem, thus avoiding further speed regulation in the sport he loves...

    This quest to find a solution to blow overs is nothing new. Maybe a sizable bounty would help spark interest? Here is one idea from a few years back I found interesting.



    Personally I love the concept, but due to the speeds I'm talking about, would probably just placed the rockets under my seat :) However the idea does allow the boat continue on, which is a route I propose also. And often there is friends onboard with ya at 150mph..

    No argument with ya there... And many of these guys are very dedicated to their goals. Less than 1 year after that big 50' Mystic listed above was dancing on her a$$ in Canada, resulting in a hole you could walk thru, she shattered a speed record in Missouri.



    As you might of gathered money is not an object here, cause it's no fun if ya ain't around to spend it. So call up your old engineering school buddies and invite them to the topic.

    :cool:
     
    Last edited: Sep 12, 2013
  10. daiquiri
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    daiquiri Engineering and Design

    Ok, so where can we find some data about boat type from the opening post (if it is some case in question), something to start from?
    Dimensions, weight, position of CoG, and other physical characteristics which might be relevant? I won't even dare to try asking for principal moments of inertia...
     
  11. Petros
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    Petros Senior Member

    Kid Turbo,

    welcome to the forum. Power boats have never interested me, so I do know a lot about their design or performance. but those spectacular airborne wipe outs have made the news regularly and as an engineer with former extensive aerodynamics experience I am always struck by the way these boats behave once they become airborne.

    It seems you have some very knowledgeable and competent people designing the hulls, the problem I see is that when the hull becomes airborne, even if just inches above the water, it is no longer a boat, but an aircraft. As an aircraft they are designed very badly, they become pitch unstable and overshoot their stable angle of attack, stall and flip over backwards (and hit the water stern first).

    This might be "normal" for a speed boat, but this is completely unacceptable behavior from any aircraft, including these boats, which become aircraft once out of the water.

    It seems to me that all of this can be avoided if they were designed to maintain pitch, yaw and roll stability when airborne. That way if they get in the air, they stay pointed in the direction they were at when they go into the air, and as they loose speed will land on water in a stable, upright and controllable attitude.

    It would also be best if this can be done in a passive way, with just the shape of the hull (and perhaps a few strategically placed fins or spoilers), so that there are no mechanized control devices, electronics or moving parts to malfunction or fail. That is the main disadvantage of the drag device design that you posted the link to, when it deploys when the hull goes airborne, it is too complicated and likely to malfunction, it also means you either slow down and lose, or loose the ability to stay upright after it deploys once.

    With a careful aerodynamic design on the hull it seems to me it might be possible to make the boat maintain its attitude once airborne, and when it returns to the water surface ("lands" on the water), it is still controllable. All that needs to happen is to alter the design of the hull so the CG is ahead of the aerodynamic center of an airborne boat hull. Not sure if that will interfere with its performance when in "power boat" mode, but any stabilizing aerodynamic forces will be dwarfed by the forces of the hull on the water. and if not, it might be possible to deploy some simple spoilers or flaps to keep the angle of attack of the hull such that it will land on the water right side up and pointed in the right direction.

    What I see happening is once the hull looses contact with the water, besides the initial inertia forces, all of the forces on it are aerodynamic. It is pitch unstable, the angle of attack increases and it generates lift that continues to lift the boat higher over the water. Than the hull stalls (aerodynamically) and the lift is lost. And it starts falling back toward the water, but with the aft CG (relative to the aerodynamic center) it causes the stern to sink faster than the nose. And it flips completely upside down long before it can touch the surface of the water.

    If there was no aerodynamic lift, it would drop back to the water surface fairly quickly, not giving it enough time to flip over back wards. So it seems killing the lift on the hull would help, but if the lift is beneficial to the speed on the water, than killing the lift is not beneficial, so it means you have to design the hull to generate aerodynamic lift, AND be aerodynamically stable. That is not that hard to accomplish, fold a paper airplane and adjust the trailing edge and the CG location, and it will sail across a room and gently land. This crude paper airplane demonstrates more stability in the air than these very powerful and costly speed boats. No computers or moving parts required, just the correct shape to maintain stability.

    Why is it that no one has considered the aerodynamic stability of the speed boats once they are airborne? It seems and obvious problem because these kind of accidents (sometimes deadly) are well documented and very dramatic, and always make a lot of press.

    Or am I missing something about the shape of the hull that is essential to a speed boat on the water, that makes it a death trap once airborne?
     
  12. daiquiri
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    daiquiri Engineering and Design

    Oh come on, Petros! This phrase surprises me, considering that your remarks are usually very rational. I hope you don't really believe that these 100+ mph boats are designed by laymen in the field of both aerodynamics and hydrodynamics and that you (or me, for that matter) know their job much better than them? ;) Let's be serious, we are talking here about design teams with big experience, big brains, big balls and a fair budget for research.

    So, to answer the above question - of course the aerodynamic stability is being given a lots of considerations. Actually, it is one of the primary considerations in the design of this type of boats.
    But just as everything in this world was made to exist and work inside it's operative boundaries, so these vessels are.

    The first obvious fact to start with is - they are planing powerboats, not airplanes. As such, they have a series of hydrodynamic requirements which have to be satisfied, in order to make them efficient and sufficiently seaworthy. Hull shape and rigidity, power-train, mass distribution - just to cite some.

    Second, they are racing powerboats - so speed is the main discriminant in their design. Therefore, the position of their center of gravity will be dictated by the requirements of planing efficiency and stability, not by the requirements valid for the stability of aircraft. And the two are fundamentally (should I say - extremely) different. Consider just this - their center of gravity is grossly at around 30% of the LWL from the transom. At the same time, their instantaneous aerodynamic center is anywhere in between 40% and 75% from the transom, depending on the height from the water and whether the craft has lifted out of GE. And plus, there is the contribution to the lift from the propeller and the rudder. So, it is a radically different case from the airplane.

    Finally, they extensively use the ground-effect (GE), that's where their speed comes from. However, the aerodynamic behavior of vehicles operating in GE mode is quickly and dramatically changed once these vehicles get out of it. That's where most of troubles come from, once the angle of attack increases over the maximum limit for longitudinal stability.

    Just to give you an idea, this is how the centers of gravity and pressure vary on a similar vessel as the height from the ground increases:

    TH.jpg

    Red dot is the CoG position, green rectangle is the CP in ground effect, magenta is where the CP can travel as the boat moves with 6 degrees of freedom.

    So, no - it is a slightly more complex case of stability analysis than that of an aircraft.
     
  13. tomas
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    tomas Senior Member

    Hi daiquiri.

    To my untrained eye, I do not see evidence in shape features of these boats that this aerodynamic stability consideration is included. Could you post some photograph examples where it is present? (Edit: beyond the ground-effect consideration)
     
  14. daiquiri
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    daiquiri Engineering and Design

    They are given stability considerations inside their operative boundaries. Which is the ground effect mode. Once they start flying, they are outside the box.

    Since airplanes were mentioned for comparison, a blow-over is somewhat like a deep-stall situation of an airplane. It is outside the design boundaries and the recovery is very uncertain, often impossible - even for an airplane which was designed to be perfectly stable inside its operative flight envelope.

    Cheers
     

  15. tomas
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    tomas Senior Member

    So that means anything, active or passive, added to deal with "outside the box" issues will affect the performance, the inevitable compromise that Ad Hoc wrote about. Then the question becomes, it that acceptable to this particular high-performance enthusiast community? So far, the answer appears to be "No".
     
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