OK, I have heard enough.
I was hoping that one of our self-described nautical experts would have explained this by now.

When the hull of a boat sits lower in the water as the boat speed increases it is called "squat" or "hydraulicdynamic draft increase"(see http://translate.google.com/transla...ute/ima/suh.htm&prev=/search%3Fq%3D.

The US Corps of Engineers refers to it as "squat" in http://www.usace.army.mil/publications/eng-regs/er1110-2-1457/entire.pdf
Guidelines for channel design and construction.

The most comprehensive discussion I could find on the subject is in "Mathematical and Computational Techniques for Predicting the Squat of Ships
Tim Gourlay Thesis submitted for the degree of Doctor of Philosophy
THE UNIVERSITY OF ADELAIDE Department of Applied Mathematics February 2000
http://216.239.51.100/search?q=cache:cxU1248uzoYC:academic.amc.edu.au/~tgourlay/theweb.ps

I first learned of this phenomenon many years ago in an article about an accident the Queen Elizabeth 2 had in 1992 near Cuttyhunk, US. The pilot knew how much water the QE2 drew at rest. They took a high speed shortcut across a portion of the bay which should have had minimal clearance. But due to the "squat" at speed, she scraped bottom and caused several million dollars in damage.

Okay I'll bite

Due to the low displacement compared to the length of the hulls the accelleration of the flow is mild or even low when compared to far more bulky hulls like freighters. Also the relatively large distance between the hull and the bottom of the body of water where cats sail make the bridge and channel example not very applicable to catamaran hull design. Naturally, these effects are always present but they are often neglectable small.

The depressing of the hulls that is sometimes encountered is far more the result if the induced downforce of the rig then by the suction on the underside of the hulls.

This induced downforce is the direct result of the lift force acting on the sail being rotated downward a bit as the result of the heeling of the platform.

On a certain 16 foot platform a 10 degree heeling of the boat will induce a 26 kg's = 57 lbs downforce when the crew of 150 kg;s is doubletrapped and lifting it's luff hull out of the water. This is about 10 % of the total crew + boat weight.

Often it is very difficult to compute the results of accelerated and decellerated flow around a body and I certainly haven't got the tool at home to do these kind of heavy numerical computations. However, I would be surprised if the suction would produce a larger downforce than the tilted rigw which can easily be 57 lbs.

And there you have it.

Wouter

i heard or read somewhere that tall ships sometimes were sucked under from not being able to reef sail fast enough. their speed created some downward force upon the hull of the ship, just like a sail creates lift. and there you go, instant submarine!

Maybe that is what happened to the R Boat that sailed to the bottom of Lake Erie -- I always wondered how that could happen. An R is a long, narrow-beamed, fast, racing monohull (old design) with a very deep keel. This happened at night during a race from Cleveland, Ohio to Put-in-Bay, Ohio. The crew had no warning at all that it was going to happen -- the boat just suddenly sailed to the bottom. One crew member was asleep below, and he escaped the cabin after it was underwater and swam up the mast. None of the sailors' reports about the incident at the time used the word "sank" -- all said, "sailed to the bottom." Weird.
I should add that this boat has a lot of sail area. I don't remember the circumstances regarding the wind at the time, whether a squall came through or what, because this was maybe 40 years ago.

I have expericenced this two times. Once on a mono hull under spin. The boat was going so fast it just kept getting lower and lower in the water until it looked like we were a foot below the surface. At that point the water crashed in on us. The other time on my TheMightyHobie18 off the wind the same thing happened.

You can talk about theory all you want on this subject but what I saw happen was the water being (displaced) pushed away from the front of the boat caused the rear to sink down until the drag caught up and slowed the boat.

The boat that you mention Mary most likely didn't have the cabin closed up when they went down.

Mike

Very interesting. So the same forces that cause a Hobie 18's bows to be forced down are also the forces that cause a big, heavy monohull to keep going down, and down, and down because of the combination of sail area and that big lead thing below? Makes sense about the cabin not being closed. I'm sure that sailors who do races around the world are a little more savvy than a bunch of fun sailors doing a short-distance race after drinking all evening.

Funny you mention the R-boat sinking ... We had one go down in the early 90s here in Toronto while racing. Friends of mine where on board and all survived. The boat was sailing off the wind when a rogue wave got them from behind, filled the **** wich then flooded the hull on the second wave. The R-boat is a low freeboard open ****/hull design similar in looks to a metre boat or is it meter ??. Boat was resurected within a couple of days I think.

I used to spend a lot of time on monos until I saw the light! I raced a Soverel 33 a bunch. We raced against a Hobie 33 and a Beneteau first 10 which I also raced on ocassionally. We saw similar things happen on all three boats.

I think after looking back on it, it was a combination of things that happened. We would surf down the face of waves, as the bow would push into the next wave ahead, it would slow down. The sails would continue to push the boat forward, but it couldn't go forward anyfaster because of the wave. In more sever cases, as the bow would go under, the rudder would start to lose bite (loss of surface area and flow). With all the water on the wide bow, it has a hard time going in a straight line. The boat will often keep going straight with the sails and water holding the bow down, but some times will go into a death roll. The deathroll usually brings the wave crashing in from the stearn and windward side. The sudden loss in speed results in the stearn wake catching up and the transome sinking slightly. So... water comes in from all sides.

w/o spending a lot of time thinking about it, I would think it impossible to completly displace all the water around a hull and make it sink. F=m*a.... for the water to be pushed away, something has to do that (the hull), so the hull is pushed back up by the water. If the hull does not push the water, the water does not move, so the hull stays where it is. Conservation of energy.

my thoughts

Will R

ok, but does this really happen with non-displacement boats such as cats?

TIME OUT!!
This discussion has wandered off into irrelavent issues.
The Weasel even had to do one of his juvenile trig calculations in a sad attempt to impress us.

There are many issues relavent to the depth of a hull while in the water. Some large some small.
My post was about one and only one issue - SQUAT.
That is the dynamic effect of a displacement hull riding lower in the water as related to the speed through the water.
It is generally a small factor when related to beach catamarans but can be important to large or heavy
boats.
Sorry Dave, but beach cats are displacement boats the vast majority of the time. On rare occasions they skip across the water as a planing hull. We have all skipped rocks across water, but I wouldn't call them planing hull shapes.

While not exactly correct, you can think of it this way; A displacement hull wants to ride at its normal waterline or displacement. When the hulls are dynamically pushing a bow wave it forms a trough. The hull still wants its waterline to match the water level next to the hull. It therefore has to sit "deeper" or closer to the bottom to achieve the same displacement. If the boat suddenly slows, the trough would collapse and the water rush in towards the "lower" gunwales.
Fortunately this is not a worry for beach cats.

sma said:

While not exactly correct, you can think of it this way; A displacement hull wants to ride at its normal waterline or displacement. When the hulls are dynamically pushing a bow wave it forms a trough. The hull still wants its waterline to match the water level next to the hull. It therefore has to sit "deeper" or closer to the bottom to achieve the same displacement. If the boat suddenly slows, the trough would collapse and the water rush in towards the "lower" gunwales.
Fortunately this is not a worry for beach cats.


That's a pretty good description of the forces in general associated with this event.

Lets move on Did ya hear the one about the .......

The weasel !

Or

Dutch Doofus (The other name you have given me way back then)

As if I care about your opinion.

No wonder I'm told that our local hobie fleet is struggling.....

You have such a way of putting things in perspective sam.

I have been reading his lies, exagerations, insults and bull for several years.
You weren't here when he regularly condemned and critized the U.S., especially U.S. sailors.
You weren't here when he repeated argued that the U.S had very little to do with winning World War II and that we didn't deserve any thanks form the EU.
He is still the same person who said, on 9/15/01, that the attack on the 9/11 was "way out of proportion" as if we deserved to be attacked, but not that bad.
He has been much more reserved in his criticizm of the U.S. since he found willing dupes for his f16hp scam overhere.
He is the person who stated on 10/05/01 that it would "save you alot of time" by looking at another website instead of "waiting for my photo's to be developped.", but then on 4/16/02 he states "I have the plans and they are real. I'm hoping to finish my boat this summer" and he has not announced the completion of his boat YET, but he continues to proclaim himself an expert on boat tuning and construction.
He has been feeding the people a line of crap for over a year.
He is legendary for his making totally incorrect pronouncements, like this one, about things he knows nothing about.
I don't like liars who take advantage of people for their own self gratification.
A snake is a snake and it will never be anything else.

P.S. The definition of "perspective -
the interrelation in which a subject or its parts are mentally viewed ; also: POINT OF VIEW b : the capacity to view things in their true relations or relative importance

Obviously the "point of view" of a 50 year old is different than that of a twenty sonething. I wonder how much crap you will put up with in 20 years.

Having not been here that long I've wondered why Wouter took a bashing from time to time. Thanks Sam for clearing that up for me.

Mike

geez sam, if you want to trash some one, go to the other forum. this place is a lot more adult
that that.

I've found Wouter to be of great aid when it comes to technical aspects of my boat. I'm sorry that you feel his comments were out of line, but like they say, opinions are like assholes, everyone has one and they all stink. Let go... believe it or not people will disgaree with you, I'm sorry that your 50+ years of life haven't taught you tolerance.

... Or at least tact ...

propaganda!!!

you are worse than the channel 28 news. your comment "He is still the same person who said, on 9/15/01, that the attack on the 9/11 was "way out of proportion" as if we deserved to be attacked, but not that bad" is completely out of context. here is the actual statement wouter made:

"I'm known to have my disagreements but this attack is way out of proportion and is an indication of a group that has lost their humanity completely.

My condolences to all, it was shocking to hear this news and indeed all over the world people were stunned and disgusted. All over the world our thought were with the casualties even as far away as on the little Greek Island I was on when we heard this dreadful news.

Wouter"

it sounds to me that wouter was giving his condolences to us not siding with the terrorists.

mary and rick, is there a way that i can block posts from this guy so i don't have to waste my time and bandwidth? i have come to the conclusion that his opinion is worth neither.

Perhaps it's time to re-do the statistics in the top post "How old are you?".