Multihull in Fast Flowing Water

[Fanie]

I've never sailed in fast flowing water ie like in a river.

How does a multihull behave in this ?

The motorised boats have a problem going upstream since they have to gun their motors to go upstream.

Logic tells me if there is wind a sail boat is going to have less of a problem, am I right and what can one expect ?

[TeddyDiver]

Of course it depends of the river but they have a turbulent water flow and that tends to turn boats (also mono's) suddenly to unexpected directions. With a cat this affect is more than disturbing..

[apex1]

Of course it depends of the river but they have a turbulent water flow and that tends to turn boats (also mono's) suddenly to unexpected directions. With a cat this affect is more than disturbing..

Just to add to that:
for this reason you do´nt see any cats on the Amazon river for example.
And for a given displacement and hull form there is no difference between motor and sailing vessels, both need the same power to go upstream.
On the other hand we have some sailing cats on the river Elbe in Germany and they do as well or weak if tide and the rivers current add against their heading.


Regards
Richard

[Fanie]

The amazon doesn't have wind if I'm not mistaken. Too many trees and structure ?

I agree the water drag on the hull would increase, but shouldn't a sail have an advantage over a prop in this case ? The sail propultion is out of the water while a prop is greatly affected.

In fast flowing water one should experience the same feeling or effect as when you sail fast, right, so steering may be more sensitive ?

[CatBuilder]

The cat will behave in a nearly identical manner to the mono, except that it will be pushed around slightly less by the eddies owing to the fact that it doesn't have a deep reaching keel.

As to your original question of motor vs sail, they behave identically with respect to the effects the current has on them.

Both cats and monos require a lot of extra power to the prop to overcome the current when motoring.

When sailing, you also require a lot of extra "power" from the sails to overcome the current. One thing that makes it different to sail up a river with a lot of current is that the strong current can change your apparent wind direction, sometimes very abruptly if you turn the boat in the current.

Source: Just spent a season in a river the USA's 2nd strongest navigable current.

[Richard Woods]

When I was last up the Amazon river (I've been longing to say that) it was very windy some days. One thing we didn't expect in the upper Amazon was to be affected by sandstorms, but we were.

We were on a 98ft motor trimaran - so there is a multihull connection.

Tides can run fast anywhere, here in the Pacific NW the strongest tides run at 15 knots, 17 at springs.

A friend with a 20ft monohull was becalmed in the Alderney race (part of the UK's Channel Islands). He anchored. But then, as the tide runs at 8 knots, he had to hand steer until the tide changed.

He said it was the "fastest" his boat had ever gone and at the peak of the tide the boat was out of control and broaching. Not really surprising, few 20ft cruising monohulls can sail steadily at 8 knots.

As others have said, it is the whirlpools that cause problems in strong currents. So multihulls don't fare too well in that respect

Richard Woods of Woods Designs

www.sailingcatamarans.com

[Chris Ostlind]

... for this reason you don't see any cats on the Amazon river for example.




Could you explain this a little more?

Thanks

[apex1]

It was all said above, turbulent waters aka whirlpools or neerstrom.
And as I remember we had that discussion already at the "Amazon river loadumup" thread!?

Regards
Richard

[Chris Ostlind]

It was all said above, turbulent waters aka whirlpools or neerstrom.

And as I remember we had that discussion already at the "Amazon river loadumup" thread!?




With all due respect, Richard, simply making a statement that says whirlpools, etc...., does not address the issues as they may exist for catamarans as opposed to monohulls in the same waters. For statements such as this, there would be some kind of data, some kind of substantive, provable issues, etc. if the statements have any veracity.

As you remember, the discussion we already had, also did not address any substantive, or quantifiable data. It was more of the floating commentary kind of stuff that didn't really get to any causal realtionship as it may apply to multihulls. As you know, one guy restating that which he has heard over and over does not make for a reliable source.

If you know something that you can put before us, I would love to read it and give you due credit for supplying the data. I'm not looking to bust your head on this, just looking to get to the bottom of why these kinds of statements are made so casually.

Way back when in that other thread, I indicated that if the problem is uniquely about cats in turbulent waters, why then do so many river running cats exist suuccessfully in rivers just as turbulent as the Amazon, if not more so?

These are both human powered, as well as engine powered setups, so the issues can not be neutralized simply due to the power source. Why do so many catamaran ferry boats operate in highly turbulent tidal waters, such as those in and around British Columbia if the type is so fraught with vulnerability issues?

Personally, after running a lot of big water rivers in the Western US, I have seen a remarkable number of cats of all sizes being used on a regular basis. These guys do not just a buy a boat because of fashionista statements. They get them because they work well and make money for the operator. The key word is money for the commercial river guys.

[apex1]

Chris please, before we go into a endless debate just forget about river rafts in this comparison as well as purpose built commercial craft.
We run Catamaran ferries here in Istanbul on the sometimes very violent Bosphorus, with a average current of 4kn and some tricky "corners". But have no significant difficulties compared with the monohull ferries.
Does that mean we can assume the same for recreational craft?
A cat in turbulent water is more difficult to handle than a mono. Et basta
And you know that!

Regards
Richard

[Chris Ostlind]

It doesn't have to be endless.

Just tell us why a cat is more difficult to handle in turbulent waters. (some supporting documentation would also be nice, if it's handy)

And since you agreed that cats are being used successfully in very turbulent and commerically busy waterways, perhaps a note, as to what may have driven the owners decision making when they, too, should be aware of the poor handling issues.

Where does the insurance business step in and tell them loudly... "cats suck in turbulent waters. There's no way I will write any paper on your enterprise."

[Richard Woods]

There are too many Richard's here!! And sorry, I only speak English, so don't know what "Et basta" means

Last time I white water rafted I used both a conventional raft and a catamaran raft. The catamaran was easier to manouver.

Having said that, in my experience, the two catamaran hulls can be caught in two whirlpools (one per hull) rotating in opposite directions. Which is why I said monohulls have it easier. Of course monohulls have other problems, but then no small boat is really ideal for use in strong currents (meaning over 10 knots)

Richard Woods of Woods Designs

www.sailingcatamarans.com

[TeddyDiver]

Just tell us why a cat is more difficult to handle in turbulent waters.

Once more.. Two hulls both having different and differing water flow means you need constantionous steering/propulsion adjustment to do. Doable but considering the fact that on rivers you don't need a cat due stability issues so you insted take only one canoe hull, maybe somewhat bigger, and have the benefit of reduced drag and mono-ease-of-steering thou it's a bit tricky still sometimes.
Data? It's obvious to anyone done some river boating.. so we don't need the data..
Rivers big enough to ships to navigate are a different thing. Deeper water reduces eddies, or they stay deeper, closer to surface causing them (the bottom)
Drafting and raft's have nothing to do in this issue..

[TeddyDiver]

Just one more thing to consider.. How you going to land on a bank when other hull stops on to shore and other one is having heavy drag in the deeper water.. cant' stop the propulsion before the boat is secured.. Sailing is not an option and propellers get damaged.. Ultimately you end up rolling along the shoreline every hull tip bounching against the rocks in their turn..

[Chris Ostlind]

Once more.. Two hulls both having different and differing water flow means you need constantionous steering/propulsion adjustment to do. Doable but considering the fact that on rivers you don't need a cat due stability issues so you insted take only one canoe hull, maybe somewhat bigger, and have the benefit of reduced drag and mono-ease-of-steering thou it's a bit tricky still sometimes.


So, where's the argument for fuel efficiency, Teddy? All that boat traffic that gets to go down river, has to go back up if they expect to do more work on said waters. Up river means propulsion requirements. Tell me about the relative cost per mile traveled upriver with a monohull and a catamaran?

As for steering ease, I see cats as having splendid potential for precise steering with twin engines. Can a monohull turn around in a very tight circle? When a cat applies power forward to one engine and reverse power to the other... guess what happens? Can a mono perform such a feat in close quarters without a huge array of independently rotating engine pods and very potent bow thrusters?





Data? It's obvious to anyone done some river boating.. so we don't need the data..


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? Word of mouth data is but one kind and remarkably susceptible to all the various foibles that can be sprung by the human mind.




Drafting and raft's have nothing to do in this issue..




If raft's have nothing to do with this issue, Teddy, then I guess we need to describe them differently than we do "boats". Is that your position... that rafts are not boats, as well? That they do not behave with the same constraints and design requirements that we place upon "boats"?

You're going to experience some very tough sledding, there, Teddy, if you want to put that argument before a body of naval architects.



Just one more thing to consider.. How you going to land on a bank when other hull stops on to shore and other one is having heavy drag in the deeper water.. cant' stop the propulsion before the boat is secured.. Sailing is not an option and propellers get damaged.. Ultimately you end up rolling along the shoreline every hull tip bouncing against the rocks in their turn..

Teddy, you suggest that you have river boating experience in your commentary. I'm quite sure, then, that you have knowledge of the interesting phenomenon in which a counter current develops along the shore of fast moving body of water? How about the inner shore of a turn in the river? Is the current as strong over on that side? How about the relative speed of water in the carved channel of the main flow as opposed to the speed of the water at the areas that are not main channel environments? Still the same speed of flow?

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?

All this stuff you guys are putting out here is convenient and it looks very much like there are no proofs at all save for something less than a rigorous testing process.

Apex Richard suggested that I know this stuff already. He's correct in that I have a great deal more background on the topic than the typical guy with a passing interest in boating. I get that Apex Richard would like me to stop stirring the coals in this cozy campfire topic. Unfortunately, there are way more interested readers here than guys who "supposedly know" why catamarans will not work on a river. There's a whole huge body of readers who never post and they come here to learn about boats. Some are experienced, some complete novices.

From my perspective and experience, I can't see a single substantive reason that a catamaran could make for a truly fine river craft. The fact that nobody has been able to produce anything more than shop talk against cats, further illustrates the problem.

How about we take the time to expand our thinking, rather than confine it?

[apex1]

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

So, where's the argument for fuel efficiency

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

All that boat traffic that gets to go down river, has to go back up

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.

As for steering ease,

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

If raft's have nothing to do with this issue, Teddy, then I guess we need to describe them differently than we do "boats". Is that your position... that rafts are not boats, as well? That they do not behave with the same constraints and design requirements that we place upon "boats"?

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're going to experience some very tough sledding, there, Teddy, if you want to put that argument before a body of naval architects.

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

Apex Richard suggested that I know this stuff already. He's correct in that I have a great deal more background on the topic than the typical guy with a passing interest in boating.

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!

I can't see a single substantive reason that a catamaran could make for a truly fine river craft.

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...............

[TeddyDiver]

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]

[Chris Ostlind]

...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!



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,

[Chris Ostlind]

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,

[TeddyDiver]

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.


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."

[johnholland4]

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

[TeddyDiver]

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
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

[apex1]

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.


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.

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


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.

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

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.

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!

And just to clue you in, Apex R... not all of the river cats are trucked from place to place.

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!


Actually, Teddy, it's both.

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

[Chris Ostlind]

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 ;-)

[Chris Ostlind]

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

.

[apex1]

assume that you have exhausted your repertoire

.

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

Bye
Richard

[Fanie]

This thread wasnt started by a shoolboy, be assured he did understand our comments before you chimed in.

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 ;)

[TeddyDiver]

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

[TeddyDiver]

perhaps a quick look at the qualifications of the author will demonstrate an appropriate level of credibility to the argument.
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:

[tspeer]

I've never sailed in fast flowing water ie like in a river.
How does a multihull behave in this ?

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.

The motorised boats have a problem going upstream since they have to gun their motors to go upstream.

Logic tells me if there is wind a sail boat is going to have less of a problem, am I right and what can one expect ?

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.

[Fanie]

Hello Tom,
Thanks for taking the time, although you're a bit late :D I'm back already.

I wasn't allowed to launch the tri on the sea. I'll follow this up with the so called 'authorities'. It seems the sea in SA now belongs to some and no one else.
I was charged R200 to launch the tri on the river which is rediculous :( There are no facilities so they just want.

So as for my original enquiry -

In the case of a prop, if the boat is capable of doing 10 kn and the water flows at 8 kn then of course the boat will do 2 kn. I'm fine with that.

However, a sailboat's speed is determined by the wind. Granted, there would be more drag on the hull, and upstream sailing would be slowing the boat some.

The only time the current would take the sailboat downstream would be when the hull drag becomes more than the sail force, but there should be an advantage.

The faster a sail boat sail the greater the aparent wind angle and you lose sail force (you can only sail so fast in a given wind). If you are slowed down mechanically then the force from the sails should be more than when you are at speed.

Does anyone know how the water drag on a hull increase if you keep on increasing the water speed ? I know it will eventually end up in planing or flying :rolleyes: but Freeship does not indicate much speed.

[Guest625101138]

Fanie
Here is something to ponder.

Wind over land is zero.

Downstream water flow is 10kts.

Are you able to sail upstream?

Rick W

[tspeer]

...However, a sailboat's speed is determined by the wind. Granted, there would be more drag on the hull, and upstream sailing would be slowing the boat some.

The only time the current would take the sailboat downstream would be when the hull drag becomes more than the sail force, but there should be an advantage.

The faster a sail boat sail the greater the aparent wind angle and you lose sail force (you can only sail so fast in a given wind). If you are slowed down mechanically then the force from the sails should be more than when you are at speed.

Does anyone know how the water drag on a hull increase if you keep on increasing the water speed ? I know it will eventually end up in planing or flying :rolleyes: but Freeship does not indicate much speed.

You need to look a the problem from a couple of different frames of reference. First consider the frame of reference of a person floating in an inner tube as the sailboat passes. The boat speed through the water, as seen by the person in the inner tube (or the sailor) depends on the wind speed relative to the water. This is no different than when sailing without current.

Next consider the speed of the boat as seen by an observer on shore. The speed relative to the shore is the vector sum of the speed through the water and the current. This is the same for the power boat and the sailboat.

What's different is the speed of the wind as observed by the person in the inner tube vs the person on shore. The speed of the wind experienced by the person in the inner tube is the speed of the wind seen by the person on shore, minus the current. If the wind is with the current, and at the same speed, there will be zero wind over the water.

You can see this at work in the example of short-tacking up the shore that I gave earlier. The current and wind were in the same direction, so the wind relative to the water dropped over the part of the channel where the current was greater. This is why the monohull had difficulty making any progress through the water. When I went close to shore, the current was less, so the wind over the water increased. This allowed me to sail faster through the water. It also meant that the set to leeward from the current itself wasn't as much, so there was a double benefit.

Another way I've found useful to deal with current is to imagine the water is stationary and fixed points, like buoys, are moving through the water. So if I need to round a buoy in an adverse tide, I imagine where I'd have to sail to intercept the moving buoy as though it were a boat steaming at the speed of the current.

It's all relative.

[DaveJ]

I might throw my 2 cents in as well. Some thing that i believe you guys are overlooking in the inertia of the boat. Lets see if i got this correct.

The river speed is 10kts and all things aside the boat is pointing upstream at 10kts hull speed in a straight line against the current, you could say the boat is at a state of rest, it has no energy. Now you want to change direction and turn, it will feel more responsive and turn better than lets say a boat doing 10kts on still water, because the boat on the river has no inertia, and doesn't have to deal with the forward inertia.

Now lets turn that around, the river is 10kts and the boat hull speed is 10kts down stream, so its forward inertia is at twice at 20 knts than a boat doing 10kts on still water. Now if it wants to change direction it has 20kts worth of inertia for only 10kts worth of steerage, so its going to be quite sluggish and can get very dangerous on very bendy rivers.

This is where twin engine over single engine will be at advantage, as you still have steerage even with no hull speed. Now as for sailing, my head hurts too much trying to figure that out, and its home time.

Dave.

[Fanie]

I think you guys are ab so friggin lutely right.

The wind is going to blow downstream at 10 kn and the current is going to be 10 kn. Marvelous sailing at 10 kn in 100% wind still conditions. Just think, I can sail at 10 kn side ways and having a braai aboard and the smoke would go streight up.

I never thought the wind would go with the stream, and while we were there it actually did at one stage.

I got my answers, thanks everyone.

[Richard Woods]

For a multihull in fast flowing water you may want to look here

http://player.video.news.com.au/couriermail/#1180586374

Richard Woods of Woods Designs

www.sailingcatamarans.com

[DaveJ]

yer, the south port passage can get very tricky sometimes. Just a question, in that instance, would it be better to do that with dagger boards up or down, i would think up, as would want the pivot point of the boat as far aft as possible and allow abit of side slip if the stern started to overtake the bow. Having the dagger boards down would give the pivot point around them, and if the stern started to overtake the bow, there would be no side slip, the wave would then have the leverage to swing the bow around until it is side on with the wave, and who knows what will happen then.

Dave.

[Fanie]

Hi Richard,
That was well done with that cat, one would expect nothing less from a multihull. Daggerboards up and you can surf. Why not. Just because it's not a hobie doesn't mean it has to be sluggish.

[Fanie]

That actually is a good example of how a cat would would run down a wave in high seas and where one would use a drogue to prevent it.