Multihull in Fast Flowing Water

Discussion in 'Multihulls' started by Fanie, Jul 2, 2009.

  1. apex1

    apex1 Guest

    Chris I did know it will happen..........................

    there is no such argument, and if, it does not belong here!

    valid for all boats except the rafts you like to bring into diskussion everytime again.
    Those rafts are neither Boats, nor Catamaran vessels, they are just simple floating devices! And they go uphill on a truck when finished the job.

    No doubt cats have a advantage. But that was´nt the question here.

    You are right on the money here, rafts have nothing, absolutely nothing to do with this issue.
    And btw a air matress is not a monohull either!

    You should have put in your name here instead of Teddy´s!

    This is quite nice to notice, so, you should have learned your lessons. Now it is time to leave your one way point of view and to bring the results of the claimed background into the game!

    What was the threads title? River craft? This is NOT another Cat vs Mono thread

    Just some final comment and I am through with it.
    Although I said there are no significent difficulties with the cat Ferries here, one has to look closer to understand there could be some!

    The Bosphorus is a wet towel compared with several real nasty waters, but even here, the Cat ferries do not serve the stations where they had to pass the wirlpools, the mono´s do, every day. And it is easy to understand why. If the current "catches" one hull of a cat while the other remains in relatively calm water, the boat gets turned in a second.
    There are three bad spots here, mighty enough to turn a big bulker in a moment (we had two such accidents in the past few weeks, one handysize bulker of about 40.000tonnes the other a RoRo of 22.000t), all three spots are daily crossed by the monohull ferries, the Cats stay away a min. of 150 meter!
    Questions?

    Do´nt come again please with such BS like "shop talk".
    Your questions and information are biased.

    Regards
    Richard
    btw. actually I started a river cat project in Asia...............
     
  2. TeddyDiver
    Joined: Dec 2007
    Posts: 2,589
    Likes: 125, Points: 73, Legacy Rep: 1650
    Location: Finland/Norway

    TeddyDiver Gollywobbler

    So much in one reply Chris..
    So, where's the argument for fuel efficiency, Teddy?
    It's more a matter of hull form, not about the number of them, right?
    I see cats as having splendid potential for precise steering with twin engines. Yes, but having a need to do that precise steering continuously is not what I have in mind when
    So, what does a person do who has never done some river boating? How do they make a proper decision as to type and potential without some kind of properly derived data?
    Either believe someone who has or make one's own mistakes..
    If raft's have nothing to do with this issue, Teddy,
    I dont' care what they are called but drafting down stream has some obvious restrictions compared to "real" boating..
    You're going to experience some very tough sledding, there, Teddy
    You wellcome!
    Teddy, you suggest that you have river boating experience in your commentary
    Well, I'm not trying to sell my plans or boats to anyone, and I trully like multi's, so please if you don't believe me you are wellcomed to make visit here and run some testing.. I'll charge 40e/hour (+gasoline) including 2 identicall river boats and engines to use either as a mono or as a cat, and you might find out too the answers to the following too..
    What makes you think that a cat can not simply park side-to the dock and/or shore just as a monohull does? What makes you think that the only method for doing so is to take it bow-on? In the situations when bow landings are preferred, what makes you think that the cat's hull side exposure is any greater than is a comparable monohull?[/QUOTE]
     
  3. Chris Ostlind

    Chris Ostlind Previous Member

    Apex Richard finally gets up on his hind legs and gives it a go. Bravo, my man!

    This quote of yours... Just look how easy it would have been to deliver just such a set of comments, without fanfare, without getting all huffy. You are asked a versy simple question, "Why, in your opinion, are monohulls more effective in highly turbulent waters than are catamarans? Instead of calmly offering-up a nicely worded response such as the one shown above, you sought to obfuscate and dodge the issue with derision and smoke screens.

    The question was sincere, it needs to be answered time and again and not neatly tucked away just because it's more comfortable. If you are really in service to the folks who use the boats in which you have a hand, then you will ask questions like this over and over.

    You want to know now why I pursued it as I did? Because I knew that eventually, you'd have to come straight up to the question, not wishing to look like poorly. Now that you have come correct, it can be put aside.

    And just to clue you in, Apex R... not all of the river cats are trucked from place to place. Some of them go back up stream and yes, my friend, they behave just like boats in whatever fashion you wish to describe them. Of course, you already knew that and were just being obtuse for the fun of it.

    best regards,
     
  4. Chris Ostlind

    Chris Ostlind Previous Member

    So, where's the argument for fuel efficiency, Teddy?

    It's more a matter of hull form, not about the number of them, right?

    Actually, Teddy, it's both.

    I see cats as having splendid potential for precise steering with twin engines.

    Yes, but having a need to do that precise steering continuously is not what I have in mind when

    I would think that any big turbulent river would put a premium on precise steering. All the rest of the time, the twin, outboard engine setup allows one to throttle back, enjoy power source redundancy and not spend a cent more for the powerplant aspect of the boat. If any boat has only one engine it will eventually fall victim to the issues of lack of control on the river. The river will then take the boat wherever it wants it to go and it may not be conducive to safety, nor joy for the occupants.

    So, what does a person do who has never done some river boating? How do they make a proper decision as to type and potential without some kind of properly derived data?

    Either believe someone who has or make one's own mistakes..

    Or they ask lots of serious questions and further ask that some form of verifiable proofs be presented. In a world overflowing with test results for all manner of objects and behaviors, one would think it a fairly simple task to provide a set of data to support an argument not universally accepted


    Teddy, you suggest that you have river boating experience in your commentary

    Well, I'm not trying to sell my plans or boats to anyone, and I trully like multi's, so please if you don't believe me you are wellcomed to make visit here and run some testing.. I'll charge 40e/hour (+gasoline) including 2 identicall river boats and engines to use either as a mono or as a cat, and you might find out too the answers to the following too...

    Teddy, please accept the statement I made above to Apex R when I said,
    "The question (I asked) was sincere, it needs to be answered time and again and not neatly tucked away just because it's more comfortable. If you are really in service to the folks who use the boats in which you have a hand, then you will ask questions like this over and over."


    My best regards,
     
  5. TeddyDiver
    Joined: Dec 2007
    Posts: 2,589
    Likes: 125, Points: 73, Legacy Rep: 1650
    Location: Finland/Norway

    TeddyDiver Gollywobbler

    So please then you tell me what benefit should a cat have over a mono in this regard if their hull forms and displacements were identical?
    Like you said it yourself ""The question (I asked) was sincere, it needs to be answered time and again and not neatly tucked away just because it's more comfortable. If you are really in service to the folks who use the boats in which you have a hand, then you will ask questions like this over and over."
     
  6. johnholland4
    Joined: May 2009
    Posts: 12
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Toronto, Canada

    johnholland4 Junior Member

    question

    I want to ask about loose-footed mainsails, pros and cons. Also a question about a very peculiar boom I have. I'm new to the site. Is this the right place to ask? If not, How do I initiate a question? Thanks, John Holland
     
  7. TeddyDiver
    Joined: Dec 2007
    Posts: 2,589
    Likes: 125, Points: 73, Legacy Rep: 1650
    Location: Finland/Norway

    TeddyDiver Gollywobbler

    You choose appropriate subforum and start new topic with your question.. unless you don't use "search" in the menu bar and read some of the multiple threads of the subject.

    Wellcome to forums :D
     
  8. apex1

    apex1 Guest

    You´re going to waste my time mate!
    This thread wasnt started by a shoolboy, be assured he did understand our comments before you chimed in.

    Thats barefaced.
    Are you a retired teacher or is it just the attempt to teach?

    No, maybe you, I will not. Once a subject is clear I am able to accept facts!

    Not really Herr Oberlehrer! I would have been as comfortable staying quiet, as I did in the aformentioned thread. Because I do´nt like your arrogant attitude!

    It would be interesting to know how you would achieve that!? If you would be so nice to read my post, you would have mentioned that I said "RAFTS". Not river cats.

    And just to clue you in, Herr Professor, these rafts are ALL trucked back uphill!


    Well Chris actually its more a matter of hull form, which can be far different from a mono, due to the fact that the hull must not provide some duties the mono has, so both are right! Or at least none is wrong..........

    Regards
    Richard
     
  9. Chris Ostlind

    Chris Ostlind Previous Member

    Here's some light reading for you, Ted.

    I know that if I were to tell you these same things, you'd be off on another tangent just to mess with me, so...

    Before you read the article below, perhaps a quick look at the qualifications of the author will demonstrate an appropriate level of credibility to the argument. http://www.bloomfieldinnovation.com/about.html

    Catamaran Resistance and Trim - Stuart Bloomfield www.bloomfieldinnovation.com

    Resistance is the term applied to the force that resists the forward motion of a boat; in other words it's the sum of the forces from the air and sea opposing the forward force provided by your propulsion system that you have to pay the fuel bill for. Resistance of ships is a complex topic and this article will concentrate on the specific area of resistance related to the trim of a vessel which is of great importance to the case we will consider, that of the 40' to 80' catamaran travelling at typical fast cruising speeds (10-25 knots).

    Resistance can be broken down into a number of components: air resistance, skin friction (viscous drag, which is parallel to the hull skin), wave-making and other residual components (which are perpendicular to the hull skin, i.e. changes in pressure around the hull pushing back at the forward part of the hull and pulling back at the aft part of the hull). Air resistance is typically less than 10% of total resistance, skin friction is a major component (especially at lower speeds) and is proportional to wetted surface area (which is reasonably constant for non-planing hull forms), wave-making resistance can vary greatly depending on a number of factors including trim and residual resistance is relatively minor.

    Wave-making resistance is represented by the wave pattern around a vessel and is proportional to the energy required to produce the waves, in other words the bigger the waves the more power you have to use.

    As a boat moves through the water it has to push the volume of water where the hull will be out of the way, in doing so the pressure in the water will increase and at the surface this increase will cause the level of the water to rise, the momentum of the rising water will cause it to overshoot its equilibrium point and therefore it will bounce up and down (forming a wave) until the energy has been dissipated.

    Wave trains around a vessel are made up of two types of waves: divergent waves (moving diagonally away from the sides of the hulls) and transverse waves (moving in the same direction as the boat). Both wave trains are influenced by the change in volume, width and depth along the length of the hull. Divergent waves are predominantly due to changes in the shape of the hull at the waterline (e.g. the bow entry, the stern and any shoulders). Transverse waves are influenced more by the shape of the keel and the trim of the boat. Conversely the wave system can also influence the trim.

    A couple of critical points control how the wave trains behave and in turn how they influence the trim:

    The entire wave system moves forward at the same speed as the vessel.

    Wavelengths of waves are a function of the wave speed, gravity (which is fixed) and water depth (which is deep). They get longer as speed is increased.

    When the wavelength of the transverse wave train becomes the same as the length of the ship there are wave crests aligned with the bow and stern and a trough amidships, this speed is known as the hull speed and is independent of hull form or whether the boat is a monohull or catamaran; this speed is 10 knots for a 55' waterline. As a boat increases speed above hull speed there remains a peak at the bow and the wave lengths increase resulting in the hull effectively having to continuously climb up its own transverse wave, this is sometimes referred to as the resistance hump because the rate of increase in resistance with increasing speed is greater at this speed than slower and faster speeds; as the speed increases further the longer wavelengths mean the hull is effectively closer to the peak at the bow and the slope of the water surface around the hull starts to get flatter (less steep near the crest) again.

    So how does all this relate to trim and why are catamarans so much better than monohulls?

    The job of the hull designer is to produce a hull that will push the water away from the bow at just the right rate so that the overshoot we referred to previously is minimized resulting in the smallest possible waves size, and the reverse at the stern. One of the primary factors influencing this challenge is the displacement length ratio, which is a measure of how heavily loaded a hull is per unit length. This is where catamarans have the advantage over monohulls because each hull only has to push half as much water aside as a similar monohull. A second factor, the length/beam ratio, also has some effect on the wave generation; again catamarans have a significant advantage over monohulls which have lateral stability requirements that often dictate wider hulls for safety. Catamarans pay the penalty in wetted surface, often having greater wetted surface than an equivalent monohull, so at speeds below hull speed monohull designs can have a resistance advantage, whereas at speeds greater than hull speed catamarans are generally a lot more economical.

    Planing hulls operate in a similar fashion for both monohull and catamarans; they produce significant wave resistance but use the dynamic pressure of the water hitting the hull bottom to raise the hull out of the water thereby reducing wetted surface area. Planing hulls will always trim as they effectively act as the lower face of a hydrofoil or wing in the water; the trim is there 'angle of attack' which is required for lift; as speed increases trim will decrease to maintain pitch equilibrium. For our purposes we are primarily concerned with semi-displacement (also know as semi-planing and marketed as displaning) hulls which are more efficient than planing hulls at speeds below four times the hull speed.

    As mentioned previously there is a strong interaction between trim and the transverse wave train. This can form a positive feedback system (more trim leads to larger waves leads to more trim…) resulting in significant increases in overall resistance if not handled correctly; for example several degrees of trim may increase resistance in the order of 100%, meaning you will use twice as much fuel to go the same speed and achieve half the range, not to mention the cost of those expensive engines. Optimal trim (for minimum resistance of well designed semi-displacement catamarans) is normally slightly bow down (less than one degree), but level trim is normally quite close to optimal and there can be other negative effects (on handling) associated with bow down trim. This (zero trim) often initially feels unnatural to sailors used to the feeling of a trimmed boat and it can even feel like the bow is trimmed down at first even when the decks are flat. The primary way to control trim is with a good hull design, but additional devices like trim tabs can offer good improvements even for well designed hulls. The things that work against a design are heavy (short) hulls, hulls spaced closely together (where the wave trains from the two hulls combine) and wide hulls (low Length/Beam ratio), sometimes these limitations can compromise the ability to minimize the transverse wave resistance.

    A well designed and trimmed boat will have almost no transverse wave train, which can be clearly seen as a flat wake (no rollers behind the boat). In fact looking over the stern of the boat is a very effective way to tune the optimal trim setup.

    Resistance is a very complex subject and we have only just scratched the surface but hopefully it has helped you understand some of the fundamentals of how and why different hull shapes and configurations are better suited to particular applications and you will know how to set up the trim on your boat so you are not burning fuel to tow the ocean behind you.


    Stuart Bloomfield www.bloomfieldinnovation.com

    The above is but one of hundreds of documented pieces of data derived opinions on the matter. Our own Richard Woods will be able to similarly qualify the writings you have just seen with perhaps his own take on the matter.

    When I asked you for some data to support the argument for monohulls over multihulls in a turbulent river scenario, it was this type of researchable and verifiable data driven information for which I was looking. Since you wanted to know about fuel savings for a big river cat, I give you the following as an example: http://www.allamericanmarine.com/pdf/AAM_fuel_effieciency.pdf

    Scroll down to the comparison of the cat to a comparable monohull for river travel. I don't know about you, Ted, but being able to give a client a $360,000 fuel cost savings sounds like a pretty big deal to me. Apex, of course, spends that much on Cuban cigars every year, so what does he care when it comes to $360K ;-)
     
  10. Chris Ostlind

    Chris Ostlind Previous Member

    Fun stuff, Apex. I'm going to go out on a limb here and assume that you have exhausted your repertoire and that we can move on?

    Good

    .
     
  11. apex1

    apex1 Guest

    Not really but I do´nt see any valuable information we could further provide on the subject.

    Bye
    Richard
     
  12. Fanie
    Joined: Oct 2007
    Posts: 4,603
    Likes: 174, Points: 63, Legacy Rep: 2484
    Location: Colonial "Sick Africa"

    Fanie Fanie

    Geezzz I wish. With all the chicks around off late I really woul not mind going back :D

    I'll go out and play in the stream between the bull sharks. If something odd occured I'll let you know. Leaving in a bit ;)
     
  13. TeddyDiver
    Joined: Dec 2007
    Posts: 2,589
    Likes: 125, Points: 73, Legacy Rep: 1650
    Location: Finland/Norway

    TeddyDiver Gollywobbler

    Chris, please read the question carefully before you give írrelevant information. Or actually, there was one point, the same one I tried to point you earlier but you missed it somehow. " One of the primary factors influencing this challenge is the displacement length ratio, which is a measure of how heavily loaded a hull is per unit length."
    Comparing different hull shapes doesn't apply here. Neither does the multi/mono issue.
    BR Teddy
     
  14. TeddyDiver
    Joined: Dec 2007
    Posts: 2,589
    Likes: 125, Points: 73, Legacy Rep: 1650
    Location: Finland/Norway

    TeddyDiver Gollywobbler

    Without "negogasting" your source I (without any degree in naval engineering) can name a lot more qualified Yach Designer being in Americas Cup projects etc and who doesn't know a **** about resistance thou he propably thinks he does so.. :cool:
     

  15. tspeer
    Joined: Feb 2002
    Posts: 2,319
    Likes: 295, Points: 83, Legacy Rep: 1673
    Location: Port Gamble, Washington, USA

    tspeer Senior Member

    I've not tried to sail in a small, fast river. I've sailed in larger bodies with tidal currents and in a slow-moving river (Thames). In both cases, it's mostly a matter of wind relative to the water. A multihull is no different than a monohull in that regard.

    Once case where I was single-handing a 24 trimaran and was able to compare with a larger monohull cruiser was going up the Saratoga Passage next to Whidby Island in Puget Sound. The bottom drops off steeply along the shore, so it's possible to have 40 ft of water only a couple of boat-lengths from shore. It was wind with current, so out in the middle the wind relative to the current was very light. The monohull was tacking back and forth, making very little progress.

    I short-tacked right up the shoreline where the current was less. This did two things for me - the current wasn't setting me back to leeward as much, and the wind relative to the water was higher. I would tack away from the shore until I started to feel turbulence on the rudder or the wind speed seemed to drop a little. Then I'd tack back in, going within a boat-length or two from shore before having to tack again.

    As I approached a narrow point of land, the tacks became shorter and shorter to stay out of the rapid current funneling past the point. After I passed the monohull, they started to come in closer to shore, but then chickened out - probably because they were afraid of running aground (my boat had a centerboard and kick-up rudder, but there was water for their keel). I made one attempt at getting around the point and failed to make progress against the current, so had to go back into shore and make even smaller tacks right up the very end of the point. Then I was able to shoot around the point and into the bay on the other side, where the current dropped off again.

    In general, the current will go to the outside of a bend. At points of land, the current will be stronger right at the point. So you can use these facts to your advantage by getting in tune with the rhythm of the land, hitting the points and tacking for the point on the other side if the current is in your favor, or hugging the shore to ride the back-eddies if it's against you. A multihull that can sail in shallower water may have an advantage for this kind of sailing. But a monohull that can tack often without losing speed has its own advantage. A catamaran that can't tack easily would be out-paced by the monohull when hem-stitching the shore.

    Powerboats have to gun their motors for the simple reason that they want to penetrate the current. If you are making 10 kt and you have a 4 kt current against you, you are only making 6 kt over the bottom. By going 13 kt, 30% faster, you will make 9 kt over the bottom, a 50% increase in speed-over-ground.

    Sailboats are no different. Sailing in a current can be like being on an aquatic treadmill. You're making good speed but going nowhere. It's always discouraging when you look at the bow-wave of a navigation buoy in a current, and realize you're losing the race to the buoy. Of course, that's when you'll actually do better by anchoring than continuing to sail.
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.