Offshore 30' ish class development

Thanks for the warning, but I think it was fine. However I have padded it out a bit to make it clearer.

There has also been quite an interesting discussion on SA.

http://www.sailinganarchy.com/forums/index.php?showtopic=46061

I hope Class 9.50 will learn the lessons from Class 40, so they don't have to go through the same development pains.

I totally agree with the need to limit stability to break the 'more RM', 'more speed', 'more cost' spiral. Without this there will always be the temptation to either scrimp on the hull structure, use the more expensive materials (in Class 40) or use more expensive design time to optimise the layup so the weight saved can put more lead in the bulb.

I think in Class 9.50 the class organisers should use three different methods to calculate realistic scantlings and then pick a minimum hull (without keel) weight. I know splitting keels and hulls for measurement weighing is a pain, but there is another way. Remember that we are not really concerned with hull weight, but in ensuring there is no advantage to be gained saving weight in the hull structure to add to the bulb (because all up displacement is controlled). An answer therefore is to limit bulb volume, which is easy and cheap to check.

If there was a maximum total volume allowed in the lower 1.5ft (500mm) of the keel and bulb together, it would still allow experimentation with shape. To check this (and one I use when backward engineering yacht stability), you construct a water tank (or use an old bath for 9.50's) and fill it with water. Then get the travel lift to lower the boat into the tank to the required depth, collect the overflowing water and you know the bulb volume, and therefore it's maximum possible weight, and therefore keel loads on the hull structure.

I think in Class 40, the use of the RM90 test alone as the only check on stability, means every design must go for Bmax. I like the idea of the water ballast /beam trade off in the 9.50 as this would appear to give other beam options.

 

Huh? I read that post at Leos site the opposite the first time. Sorry?

I like the ballast beam trade thing too. Of course that will be modeled for max RM and a typical beam will be established.


I think in Class 9.50 the class organisers should use three different methods to calculate realistic scantlings and then pick a minimum hull (without keel) weight. I know splitting keels and hulls for measurement weighing is a pain, but there is another way. Remember that we are not really concerned with hull weight, but in ensuring there is no advantage to be gained saving weight in the hull structure to add to the bulb (because all up displacement is controlled). An answer therefore is to limit bulb volume, which is easy and cheap to check.

Not to forget the upper RM limit. That will render using weight unwisely--well unwise. I thought I would put the weight down low too but you might as well make the boat strong! Right? or am I misunderstanding something?

I thought the rule was unfinished but now I am not so sure. Have they announced these rules as final?

 

Yes, more RM is always faster. So if the boat has a minimum legal displacement, the greater percentage of that weight you can get in the bulb the better.

I think you're right. Perhaps this is how they plan to keep check on excessive RM. But the wording is a little obtuse:

"The boat in loading condition is heeled at 90° and kept in this position with the aid of a strop passed around the mast at the level of the band at the top point of the mast. For boat whose mast height is 15.50 m, the tension exerted on the strop must not be lower than 140kg, while if a tension higher than 190kg is applied the yacht must not have any positive righting moment anymore."

Even the French version is not a lot clearer: "et à une tension supérieure à 190 kg le bateau ne doit plus présenter de couple de redressement positif." which to me is "and with a tension greater than 190kg the boat must no longer present a positive righting moment."

Perhaps needs an email.

Are rules ever 'finished'? As can be seen from Class 40, rule reviews are an inevitable fact of life, even with the most straightforward constraints. But it must help, for as many people as possible to analyse any proposals for possible glitches before gallons of epoxy get mixed. The meeting next month certainly seems to offer the chance to pass on suggestions and I appreciate your efforts kicking this around with me. I have a couple of clients who are very interested in these boats, but want clarity about the real 'nature of the beast'. Too many early adopters of new classes have been left 18 months in, with either the only boats in the 'fleet' or very expensive prototypes for everyone else's more competitive Mk 2s.

 

"The boat in loading condition is heeled at 90° and kept in this position with the aid of a strop passed around the mast at the level of the band at the top point of the mast. For boat whose mast height is 15.50 m, the tension exerted on the strop must not be lower than 140kg, while if a tension higher than 190kg is applied the yacht must not have any positive righting moment anymore."

That has puzzled me too. I think I understand what is said. That will warrant that the boat at 90º of heel will have a minimum RM, but also a maximum possible RM.

Initially that was not making any sense to me, but after you Guys were talking about the need to control RM for costs, that began to make some sense. If you put a limit on RM and have a minimum weight, then you have a lot of latitude in the way weight is distributed, I mean bulb and boat weight. You don’t have any advantage in putting more weight in the bulb (and make the hull lighter) if the max RM is going to be bigger than allowed. That will give stronger and more inexpensive boats.

What I don’t understand is why measure the max RM at 90º of heel. It would seem to me that the Max RM should be measured at 30º of heel, and that at 90º, the more, the better (for safety). Perhaps because it is a lot simpler to measure that at 90º of heel. What do you think?

 

Cool rule and good conversation.

As to the concern of the bulb weight/structure of boat trade-off , the rule is effictively putting a type of limit on the bulb weight by limiting the max RM in the 90 degree test. It would seem unecessary to also limit the volume of the keel.

Also, the comment regarding the classe40 beams is not entirely accurate, while most production 40's are built to Bmax, several one off boats have been built to beams less than 4.5 m. The O&C designs for example (4.15m). I had the opportunitly to discuss this topic specifically (Bmax) with Allen Clarke before the Rhoute du Rhum in regards to their entry Bollands Mill. According to Allen their (Merf Owen and Allen's) take on this (based on VPP's, loads of experience designing similar boats, and the full power of their two enormous craniums) was that with the restricted sail area of the 40's (underpowered RELATIVE to Open 60's) the speed you lose in the light due to the increased drag of wider beams is not offset by more power in heavier air as more power is not necessarily available (SA is restricted). In looking at the numbers, I would expect the same to could be said for the 9.50

 

The most significant change to the 2007 40class rule was also the introduction of a max RM at 90º.

Red=new

302. 1) 90° TEST
This test is aimed at proving that the boat is capable of righting itself from the broached position
with empty ballast tanks.
It must be completed in the presence of a measurer substantiated by the Class 40.
When heeled at 90 degrees the loaded boat (see 201) is kept in this position with the aid of a strop passed around the mast at the level of the band at the top point of the mast, that is 19m (see 208 of the present rules). The tension exerted on the strop must be a minimum of 220 kgf and a maximum of 320 kgf.

 

I have clarification of Paragraph 302 back from Charles Bertrand which confirms that they are merely looking to enforce an upper and lower limit of RM90 as a way of regulating bulb weight as RM90 is predominately governed by the position of the CofG.

So pretty much as we thought. However I would still suggest that the Archimedial measure of bulb volume is not much more involved, especially as the boats have to be lifted to be weighed.

Vega - you have to remember that the Righting Moment is a couple dependent on the position of the GofG, the Cof B and the Displacement. Here they are only trying to get a rough measure of the CofG. Assuming all the boats weigh the same (min rule displacement), we can take displacement as a constant, and at 90 degrees the Cof B becomes (almost) independent of Beam, so the only variable is position of CofG. To get an idea of the position of the CofG at any other angle of heel, including RM max, we have to know the shape of the boat in order to work out the position of the Cof B.

It's a working methodology that relates boats with a high degree of similitude to a known norm. I'm sure there are some on these fora who would like to nit-pick through the premise, but you have to remember that it goes hand in hand with a class requirement for meeting the Stability Requirements of RCD Cat A.

Bobo - My comments regarding Class 40 being driven towards Bmax was just a thought in response to the new (2007 rule) imposition of an upper limit on RM90. I appreciate some existing boats were less beamy, but now bulb weight has effectively been controlled, it will be interesting to see if new designs can still make narrow pay. I shall have to wait and see as I have enough to do exploring the 9.50 rule.

 

Cay, what Charles Bertrand said confirms what I have thought (and said) about the finality of Max RM at 90º of heel and also why they are going to measure it at 90º (it's the easiest way).

But you have said that you were trying to develop two models, one of them substantially narrower, and in that case that rule is going to be very disadvantageous to the narrow one. The narrow boat can have a lighter hull and more weight on the bulb, but its superior AVS will provide a RM curve, slightly softer at the first 30º of heel (less RM) and a less inclined curve till the AVS. That will translate at more RM at 90º of heel.

To respect that limit (the same RM at 90º), if the boat is narrower, you will end up with a boat with less initial stability, if compared with a beamier boat. If we want to control the boat's capacity to carry sail the RM that should be effectively controlled is the initial one, not the final one.

Not a big problem, providing all the boats have approximately the same beam, but not the best methodology to obtain what is intended, even if the easiest one. Charles Bertrand also said: "Providing the yachts are similar in terms of dimensions and rig weight... which they are usually" and I agree that if this is the case, there is no problem at measuring Max RM at 90º, but not for a narrower boat.

About fixing the bulb weight, I believe this formula gives more freedom to design a boat, preventing the temptation to make weaker hulls, to put more weight at the bulb. Of course, if those values are adequate and about that I don't have a clue

 

What you say is true, and was the reason I felt that a similar rule in Class 40 may change the arguments against the narrower designs that have been produced.

However, I think in Class 9.50 there is a need to evaluate different beams because of the provisions of Rules Paragraph 207.02. This ensures the righting lever component of the water ballast remains constant as beam is reduced. Or in other words, the amount of water ballast allowed, is proportionally more in the narrower boat.

I don't absolutely know which way all this will pan out when all the trade offs are resolved. My gut feeling is that the linear ballast compensation won't automatically compensate for the reduction in form stability.

 

I'd put money on that gut feeling.

 

So I presume you have all seen that the upwind sail area has been limited to 80 m^2. How does that look on your sail plan?

 

This is a pretext to post a nice movie and I am sorry if it goes a little out of the thread, but I believe that the sailors that will jump on these boats are going to be mostly the ones that come from this kind of races(and it is really an interesting movie ).

Those are boats about the same size that can go fast but needs a crew. I believe the 9.50 will go faster, without a crew. I guess that there are a lot of young sailors very excited about this class.

Take a look at the video, it’s from the "Tour de France". There are a lot of good sailors on these series, as you can see.

http://blogs.lexpress.fr/aularge/archives/2007/01/en_attendant.html

 

80m^2 (861 ft^2) of plain white sail is the figure I have been using.

It's the one that gives the SA/Displ ratio of 43 upwind (but as I said earlier NOT calculated in the conventional way by using simple triangles, becuse that seems unrepresentative with modern square topped mains and large roaches).

The spinnaker adds about another 90m^2 (968 ft^2), although I haven't seen if this is limited.

While we're talking sails, the class limit is 7 including a storm jib. What would your seven be? If the inventory was lowered to 6, what would your inventory then be (including the storm jib)? Is there any worthwhile saving?

 

I beleive you can call the trinquet a storm jib if it is orange. (at least in class 40 you can)
The question of course would be wrapped around whether to have two spinnikers or just one. If the sails did not have to be registered and I could change them from race to race...it would vary with the race. Upwind, light airs, down wind and heavy air will make the suit look very different.

 

Vega
Cool Movie...that looks like some serious fun.