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MultiHulls General Discussions (MultiHull/MultiOne)
21

Thanks for the post Peter.

As one who you probably classify as “just don’t understand how multihulls work”, I find your “would-be-multi” sailors comment a bit startling. I thought it was pretty much understood that multi-hulls flip every once in a while and that most (the fast ones) are not self righting. And one further, that a multi would flip a lot more readily with a beginner at the helm.

Isn’t the idea to balance the craft of one hull. In my experience, “balance” is something that is difficult to perfect with full sensory feedback, let alone when driving by stick.

I actually appreciate Dick’s post. I said right up front that I wouldn’t be building one in the near future, but I enjoy learning about them.

Between you and Ernst I can’t figure out who belittles newbies (I am the only one) more in this thread. I think that is the one think Dick is not trying to do. Don’t worry though, I can take it.

Luke

22

Luke

If you read back through several posts on this forum you will find that I as well as dick and a couple of other people have spent countless hours trying to explain to monohull sailors how to help prevent a multihull from capsizing. We can now sail our boat’s in 30 knots true wind and have very few capsizes. There are no secrets it takes concentration and very good set up.

If you take the time to read back through some posts, you will also find that it was several monohull sailors that were considering the idea of trying out a multihull but couldn’t get away from the idea of capsizing then having to go out and retrive etc.

Multihull sailors are a bred to themselves, you either like them or you don’t.

I welcome newbies with open arms, just please realise that multihull’s will capsize and if you do try one realise that eventually you will have to get in the rescue boat and pick the thing up.

(just make sure that the rescue boat floats) (for WIS)

I hope that this explains my post a bit more.
Peter

23

<blockquote id=“quote”><font size=“1” face=“Verdana, Arial, Helvetica” id=“quote”>quote:<hr height=“1” noshade id=“quote”>Originally posted by Peter _Birch

(just make sure that the rescue boat floats) (for WIS)
<hr height=“1” noshade id=“quote”></blockquote id=“quote”></font id=“quote”>

Peter,

I am FULLY aware that a multi CAN capsize!!
I thought it was THE game…thats how I will see it, well at the beginning, try not to capsize it, but go to the LIMIT!!!

As for the rescue boat, I changed my mind, I also bought a rubber one, just in case, the rc one doesnt work as I want to.
So, I am READY, I got the rescue boat, Aspirine with the concentration[:-angel]…so its a 100% go for me…

Could I be disappointed?

Nah, no way…nothing get beat the few pics I saw of multis sailing on one hull!! These few seconds of high adrenaline cant beat a few minutes in a rescue boat!
Thats how I see things…

Now, for the other, I WILL take pics and make some vids when I get to sail THE monster!

So you will be able to see the whole thing, a total multi noob…with all!!Joy and tears (IF) No CG…

Then, I ll be one of the BAD multi guys…hehehe…I ll be the HOSTILE one again (or maybe just becoming…sorry for that [;)]

Again, the only thing was/is/will FUN…enjoy sailing, the rest doesnt even count…all these talks about how good Mr.#'s boat is…who cares? me?..nah…Anyway, the multi coming to me WILL BE THE BEST ON EARTH!!? Why? easy…'cause it ll be MINE![:-banghead]

My 0.02 Yen, Euro or whatever

Wis

_/ if it isn’t broken, don’t fix it! _

http://wismerhell.esmartdesign.com/index.htm

24

I happened to run across some prediction software that had been used to test catamaran hull wavemaking and found it to be interest. Basic surmise is that they compare the distance between hulls to the hull length to determine wave making and drag. The small white numbers (S/L=1, etc.) in each of the models represents the ratio of the hull distance (S) to the hull length (L) or S/L

<center> </center>

For comparison purposes, my 18 Square Meter big cat is 12 feet wide and 18 feet long (S/L = 12/18 = .666) and a standard beach cat (Hobie 18) is 7.5 feet wide and 18 feet long (S/L = 7.5/18 = 4.166) and if one were to build a 1 Meter catamaran of 39" wide x 39" long (S/L = 39/39 = 1.000) so looking above one can see that as the beam gets wider, there may be a tendency for less drag on the hulls. The company running these figures also notes that there are issues if the bow is in the water when at rest (static) and then rises … and also not considered was the wet/dry transom drag.

At some point one must also consider the center of gravity and where the center of effort is depending on rig and sail size.

The reason I post this is that if we can agree to the findings of higher wave drag as the hulls get closer together, I am wondering if the trimaran configuration where floats are closer to the main hull, would have an increased tendency to increase waves and their drag resulting (possibly) in a slower platform than a cat? Just wondering - any thoughts?

And yes, I realize a trimaran could weigh as much as 1/3 more than a catamaran simply because of the extra hull, thus causing it to be slower as well.

25

Dick,

That’s very interesting. I didn’t fully understand thou what you mean with the issues about rising bows. Did you find any more info about this?

Do you have a link where I can find more of these schematics?

Marcus

26

Marcus -

the idea is that some of the prototype testing didn’t take into consideration the difference of the bows in static versus sailing trim. Often cats go “bow down” in upwind trim, but if ballast (crew) shifts towards the rear, the bows can come up and the stem actually clear the water/waves. This could/would change the wave making properties. On the other hand, the new A Class boat designed by Morrelli and built by Performance catamarans has the radical raked bows (even more so than the German “FLYER”) and they are designed to go through waves. The difference in wave making is acknowledged by the testing company/software, but they have just recently started developing a multihull formula and benchmarks and don’t have a good handle on the results as yet. I just thought it interesting that most of beach cat designs aare “strangled” by the laws which cover how wide loads can be on our highways. Tornados (over here) are trailered to races on a titled trailer to keep them inside the traffice laws. My 18 Square must be broken down for trailering, and the A Class, becuase it isinternational by nature is limited to 7.5 feet wide for Europe roads. By keeping hulls close together, it seems we limit performance - and now with wave making theory added, it looks like it is even more important to go as wide as possible - given that sooner or later practicality takes hold.

For this one, I found under the Goggle search “Splash” or “South Bay Simulations, Inc.” Unfortunately, software is way to expensive for a home model builder - as is most hull design software. SPLASH has done a lot with monohull simulations that might follow along with 1 Meter style hulls. Take a look. Sorry but I did not bookmark the site, was just surfing for any new hull design software that was for other than hard chines.

27

Thank you Dick,

Now I understand what you ment.
Tornados are usually broken down over here too. But even if you build a Cat as wide as 1:1 it will only be usable for offshore races or speed trials since handling of such a wide boat around the triangle or in a fleet would be a pain.

Here is the link:
http://www.panix.com/~brosen/catamarans.html

Marcus

28

Well - my boat at .666 is getting pretty close to the 1:1 ratio, and it sure doesn’t “bob” around in the waves between races like the narrow boats do.

Yes it takes time to tack, and because it is fast beach parking at regattas isn’t an issue - usually one of the first to get back.

I did a C Class compare 25 feet x 14 feet and it comes out in the .56 range so with big boats not to much to look at and compare. I know when DL was discussing, he came up with a 1 meter long, but well over 1.2 meter wide theory - but it was to be foil-borne as well. I don’t think these mathematical comparisons work once hulls are out of the water and up on foils.

Interesting though !

29

Mmmh, my personal theory is, that the designers are building the big boats with a narrower beam 1. for stability reasons (huge loads on a long beam…) 2. for ease of tacking and 3. to avoid the force vector of the sail to point between the bows (-> nosediving)

If your beach cat gets stuck in a tack you can always use the rudders and the jib to get going again, but on a 30m long cat? But maybe I am to naiv on that. I’ve never sailed anything bigger than a tornado.

The only big cat I now that has a comparable width like a beach cat is Mediatis (0,833). all the others are somewhere between 0,5 and well below 0,6. So the C Class fits in there. I personally think narrow cats looks somewhat uncool… [8D], but they only seem to work that way…

On Mediatis the fact that the force vector is pointing between the bows might not matter for three reasons: 1. it has dynamic stability in the bows through its planing hull concept, 2. Parlier can always reduce the sail area on the windward mast to keep the force vector outside of the leeward bow. And 3. tacks are not an issue if you are going across the big pond…

Just my thoughts, please correct me if I am totally off course. I am sure I am one of the least experienced cat sailors on here.

Marcus

30

Peter,
Thanks for the clarification. Interesting point about the 30 knots true wind. If the wind is that strong around here, its usually very gusty. Not much clear air around here. Do you have the same conditions, or is it more steady where you are? If it’s blowing thirty though, I’m going windsurfing nevermind these toy boats. : )
Luke

31

Dick and Marcus,

That is a pretty interesting study with seemingly easy to interpret results. What they are measureing is called interference drag and it’s important with trimarans as well. It mostly comes into play with powered tris which have short amas (only serve for added stability). The placement of these amas along the length of the hull can yield positive or negative interference. I don’t think it’s so important with sailing tris because you want the amas as long as possible for stability and laterline length since the ama is actually the hull that provides the most displacement when sailing.

The other thing that I didn’t find mentioned (didn’t look real hard though) is that the interferance effect changes with speed (as the wave train stretches.) The froude number of 0.5 tells you that the a one meter long hull is moving at 1.57 m/s or roughly 3 kts. This is already above “hull speed” which occurs for most hulls at a froude number of .4 or V/sqrt(L) of 1.33. It seems to me that modern cats and tris operate at froude numbers greater than .5.

Also the differences between a modern multi hullform and a wigley hullform are significant and would affect the shape of the wavetrain produced. All I am really trying to say is that accounting for the differences in speed and hullforms from those used in these tests, the increase in drag from interference between S/L of .6 and 1 is probably minimal.

Maybe that helped, maybe it just muddied the waters.

Cheers

Luke

32

From a non-engineer, thanks for clarification.

33

Thanks Luke. That helped a lot…

34

Dont forget that adding beam from 0.6 to 1.0 ratio adds airdrag significantly

  • HJ

“Expertice is gained trough mistakes. However repeating
same mistake is not learning but stupidity.”