For the people trully interested in catamaran design here are some explanation to undo the damaged caused by Evans spin.
>WHO SAID IT WAS FROM THIS TEST? THE MARSTROM HAS BEEN FAMOUS FOR MANY YEARS FOR STRENGTH. WHY DON'T YOU KNOW THIS?
Than all hulls are famous as the hulls are mostly designed for stiffness and their strength is never a serious issue when not changing the general layout of the boat. Example : replacing the double forestay with a single one and bridle strop IS a change in general layout and endangers the hulls but putting a larger rig on it isn't/doesn't. Booth was wise not to change the general layout of the boat, by doing this he limited the increased loads on the platform and in some cases pretty much keep the loads almost the same. Mastrom hulls may be famous for strength but Booth made excellent use of limits and freedoms that a catamaran design allows. To such a degree even that the same thing can succesfully be done to pretty much any platform by a respected builder like Hobie, Nacra, AHPC, etc.
The only true exception to this were the daggerboards. Although the same argument between designed for stiffness and not for strength could be used here as well. However here the two are not as widely spaced apart as with the hulls and beams.
>>Is 2.5m wide, instead of 2.3m,
: Does not seriously increase loads on the platform.
I DIDN'T USE THE TERM "SERIOUS", YOU DID. BUT YOU AGREE THAT LOADS ARE INCREASED.
the increases are accompanied by equal magnitude reductions.
ONLY IF NOTHING ELSE CHANGES.
With respect to only increases in width I can say that this does not significantlly increase loads on a platform. By far the largest component contributing to the loads is the mainsheet tension. Therefor a huge increase in width still only accounts for a small resulting increase in loads. Again the problem is not loads as some still seem to believe but stiffness. An huge increase in width may not change the loads but does decrease the resistance to flexing. A good example here are the beams. On my own boat for example the mast foot load increased by only 1 % as result of the increase in width from 2.34 to 2.50 mtr. With this the loads in the beam stay pretty much what they were. However if I were to use the same beams than the flexing would have increased by 22 %.
Indeed, I introduced the term "serious" to indicate that looking at changes in strength is the wrong thing to do here even when loads do increase, in this particular case neglectable. In most cases it is far more important to look at the calc related to stiffness.
>>The mainsail is 15sm instead of 13.94sm,
; Doesn't matter ; righting moment determines the laod on a platform not the sail area.
WRONG! A SAIL IS SUBJECT TO INSTANTANEOUS LOADS FROM GUSTS AND OTHER SHOCKS. YOU NEED TO LEARN ABOUT "ACCELERATION". YOUR SIMPLETON STATIC CALCULATIONS DON'T DETERMINE MAXIMUM LOADS.
I never said that righting moment calculation excluded dynamics. If anything these same calculations hold true when using derivatives as well. Righting moment is in basis a set of relations around a fulcrum and this translates easily into a set of enertia relations around the same fulcrum when multiplying the terms with angular accelerations. What do we take from from this ? That the ratio between static loads is the same as the ratio between dynamic loads for the same general describtion of the equations hold true for both. Therefor if a gust would hit the modified design that has about 30 % increased loads than the dynamically induced loads would be about 130 % as well. If a boat can stand this increase in static loads than the same boat can stand the same increase in dynamic loads when assuming that fatigue related failing is not applicable. Of course this last assumption can be made when the boat will only be used for a relatively short time. Afterall all fatigue processes take time to develop and result in an eventual failing. However the point of this is that in this case the estimates based on static equations may be assumed to give a good estimate on the dynamic loads as well. This isn't always the case but now it is. Remarkable ? Well, not really as the static equation is al about gravity, a thing that is alternatively expressed as an accelleration itself ! So the static equation is already a equation that links an accelleration (gravity) to a particular catamaran design. Why would it not continue to make sense when another accelleration than gravity is observed ?
There is also one other point. The increase in sail area proposed is 15/13.94 = 7.6 % So a 96.4 % gust that hits the 13.94 boat results in the same angular acceleration as the 100 % gust that hits the 15 sq. mtr. boat. This leads in both case to the same loads in the platform (if only the sail area was modified). What does this say about the maximum loads ? Only that boat with 15 sq. mtr. encouters them at NEGLECTABLY lower windspeeds/gusts than the 13.94 boat. And this only hold when assuming that the mast doesn't flex sooner as well and depowers as result of the same increase in sailarea.A good designer will not design a boat and say that no gust will ever pass this or that clearly defined treshold. He will therefor use a large safety factor (in order of 200 %) to be absolutely sure that the boat will hold even when a dummy takes it out in 35 knots with gusts like bullets.
Actually this increase in sailarea is similar to only an increase the crew weight by 7 %. I don't think anybody serious gets nervous when sailing any given catamaran at 7 % more weight than is regarded as optimal or as was used as the design crew weight. Therefor why are we all getting screamish about an similar neglectable increase in sailarea ?
Again when flexing of the mast is taken into account than it is easy to see that the loads are highly decoupled from the amount of sailarea featured. If it wasn't then a 18 HT with 20 sq. mtr. mainsail would be noticably far more nervous than any other comparable catamaran with a smaller rig. The fact that this isn't the case means that the mast is adjusted to flex more and depower proportionally when the same gust hits. This of course implies that the loads are "depowered" as a direct result as well. Now what is the dominant factor in determining the right amount of flexing ? Moment of enertia. And to what is this moment of enertia directly linked ? Destribution of weight along a radius from a given fulcrum. What other feature is fully determined by this distribution of weight ? Precisely, righting moment. From this observation a simple rule of thumb can be derived. Loads are hardly determined by changes in sailarea but rather follow changes in righting moment. The last is especially so when the mast is adjusted to the increased sailarea so that the boat retains its expected non-nervous feel. As was done to the 18Ht and M20 masts.
>>The boat weighs 78kg instead of 73 kg,
; This is neglectable
NO DUH!!! THAT IS MY POINT. THE ONLY WEIGHT GAIN IS CAUSED BY THE ADDITIONAL EQUIPMENT.
BS. That was not your point. In your original post you commented on the famous strength of the Marstrom designs and than followed by quoting all the "force increasing" mods that its A-cat design withstood succesfully. My reaction to it was that this increase in boat weight is all but neglectable.
>>And of course it has a 15sm gennaker.
;Spinnaker does not introduce higher loads on the sidestay than when sailing upwind.
I DIDN'T SAY "SIDESTAY". SUPPOSEDLY, WHEN SPINNAKERS HAVE BEEN TRIED ON SOME A CLASS BOATS THE SIDE LOADS RIPPED OFF THE WINDWARD BOW.
That is right, I SAID IT. Actually you didn't say anything about stays or other stuff for that matter so what gives ?
Of course my answer was split in two points both adressing the neglectable effect that adding a spinnaker has on the loads of most catamarans. Of course one has to stay within the general layout to make the last comment true. More on this later. Point to take away is that adding a spinnaker does not load up the boat more when looking at beams and sidestays. That was part 1.
>>It does not load the hulls up more than a jib does on the same upwind course.
THE SIDE LOADS ON THE BOWS FROM A SELF-TACKING JIB ARE MICROSCOPIC COMPARED TO A SPINNAKER.
They are not. I also don't understand why you propel this false hood that I know you understand to be false yourself. The sideloads that are put on the hulls as a result of the jib are considerable. In the order of 75% - 90% of the tension that is in the forestay. This is because you need to have a single forestay and bridle setup when sailing with a jib. AND you must load up the forestay more to prevent the luff of the jib from sagging to much. Often this means loading it up more than the forestay of a cat rig or uni rig. If you do the calcs you will find that the forestay tension by far most determined by mainsheet tension. This is arguably the highest on upwind courses as here the mainsheet loads are by far the highest. This loads up the hull (the bow halve) enormously at the main beam. It is this system that leads to breakages.
Of course you will come back saying that YOU where talking about the BOWs, as the most forward tip of the hulls, and not the hulls underneath the mainbeam. However, I never talked about anything other than the hulls in the part that YOU reacted to. So who has English language reading difficulties ?
With respect to A-cat breaking off their bows under spinnaker the following :
-1- give me example, so we can check the truth behind this claim. (I know of not one example)
-2- If the bow section broke off between the main beam and bridles than the hulls were designed weaker because of the decision to go with the double forestat setup. In this case the A-cat can not feature any headsail. Not a jib (as my statement included) or a spinnaker
-3- If the bow broke off between the bow and bridle than moving the spi support wires back to the bridle chainplates most likely would have prevented the damage.
-4- All this is pretty mute discussion with respect to the M18 -> 18HT as the M18 was already build to take the spinnaker and so by virtue of righting moment limits would also take the larger 18HT spinnaker. With respect to the Hakans A-cat it all comes down wether the M18 hulls are indentical to the A-cat hulls or wether the M18 hulls were reinforced to take the spi. The M18 push rods as supports do alter things a little when compared to wire supports but if a A-cat can survive jumping of a wave at speed and then landing in the water bow first than it can take the spi as well. If it can take teh single forestay arrangement than it can take the spi as well.
-5- Pretty much the A-cats are the exception if anything. For any single forestay boat the hulls WILL take the spi. All the designs that use double forestays can be counted on one hand.
No matter how you slice it my statements hold up and stands. A spi can be added to all boats that have not made use of the double forestay load situation by allowing the hulls to be designed weaker. I think Marstrom does not fit into the last group of designers. If anything he has used the double forestay to increase horizontal stiffness of this A-cat hulls.
>>I haven't done the calcs on mast compression yet.
DON'T BOTHER, IT WILL BE WRONG.
How do you know ? Makes we wonder what kind of background you have to be able to know this. If anything I've put my money where my mouth is. Remember the boat that I was never building according to you ? These same calcs proved to be right for that. All I know of you is that you are a very mediocrs sailor without any engineering projects under your belt. So tell me again, please how you just KNOW that it will be wrong.
>>The hulls didn't break.
: The hulls are build to resist shock and point loads this make the hull alot stronger than necessary for the sailing loads.
NO DUH AGAIN!!! REMEMBER, I SAID "FAMOUS".
Than why does this "DUH" moment make Marstrom hulls famous and not all others ?
>>Only serious point of concern are the sidestay chainplates.
These do experience increasd loads to 170% to 200 %
WRONG, WRONG, WRONG!
Please elaborate. Or is this just your record hanging on the same piece of music.
>>The mast didn't break.
: It was a M20 mast ; it was designed for double trapeze spi use.
NO DUH THREE!!!
So what is the use of anybody writing down " The mast didn't break" ?
>>The daggers didn't break.
: This is interesting,
WHAT DOES THAT MEAN?
These see increased loads to again 170% - 200 %
Means that I think these components to be suspect when making large mods like this (M18 -> 18HT). Like they will break or jam when put under these increased loads. Are we sure that Booth used standard M18 boards ?
>>The rudders didn't break.
: Rudders are less critical than the daggerboards. so if the dagger hold ..
WRONG!! YOU CAN'T RACE A BOAT VERY WELL WITH BROKEN RUDDERS.
Less critical as in less danger of breaking. One expects the daggers to break first. So if they don't break than the same layup for the rudders can be expected to hold as well.
>>The hardware didn't break.
: Yes well, that is because for small open boats there is pretty much one line of breakstrength used on all boats from 14 foot to 22 ft.
WRONG!! THAT IS YOUR MOST RIDICULOUS STATEMENT YET. IF YOU ATTENDED REGATTAS YOU WOULD SEE THE DIFFERENT SIZES OF HARDWARE ON THE DIFERENT BOATS.
Yes, but do the breakstrengths differ much between them. Hint, do some catalogue checking. I for one have never seen even the smallest blocks fail on a catamaran due to loads. Which afterall have not increased that much by the mods as was just shown. So no wonder the gear is holding up. With respect to specialized hardware like ratchets and swivel cleats, there is often only one item in the range that is available for open boats.
>>I think the issue of strength has been resolved.
Yet but the issue of flexing hasn't.
WHO SAID THE MARSTROM HAD A FLEXING PROBLEM? EXCESSIVE FLEXING OF A COMPOSITE MATERIAL CAUSES EARLY STRUCTURAL FAILURES. WHEN HAVE MARSTROMS FAILED?
The issue of flexing AFTER the mods have been applied. Man Sam, learn to recognize the context in which a statement is made.
>>The boat was used for a couple of weeks max and it has not broken yet.
This is also interesting as indeed damage to beams and other stuff is nearly always the result of metal fatigue and low tension ruptures.
WHO SAID? WHAT ARE YOUR SOURCES FOR THIS STATEMENT?
I said that, got a problem with it ? Answer 2 ; various university level sources of mechanics of materials. My own background as a schooled engineer. Realisation that the loads in these parts (beams etc) are way below breakstrengths. Therefor failing mechanism can only be of the low stress kind -> ergo low tension ruptures and fatigue processes.
What is your basis to question my sources ?
>>PITIFUL, JUST PITIFUL!!!
You said it (about) yourself.
Wouter
Last edited by Wouter; 10/29/04 08:30 AM.
