Draft Measurement Question?

Herb 3 · Post by **Herb 3** » 25 Dec 2003, 02:36

I have a question on IOM Draft measurements. Instead of the use of a water tank, can the following be used to measure the same?
This way one does not have to haul a water tank around, where as a simple measuring tool can be used instead.

Thanks,

Herb

Post by **Chairman** » 25 Dec 2003, 14:48

Hi Herb

Speaking personally, I think this is an excellent idea. The International Marblehead class already has a "water-free" device it calls a "draft restriction gauge". Details from the PDF file of the Marblehead class rules, downloadable from the ISAF-RSD Web site, http://www.radiosailing.org/pdf/Marbleh ... 20revA.pdf, which looks like this:

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The IOM ICA Technical and Measurement Sub-Committees are looking at this and seeing if and how it might be adapted for IOM measurement. I think it would be very useful to get rid of the flotation tank!

Steve Landeau · Post by **Steve Landeau** » 25 Dec 2003, 23:48

I think you'll need to have "verified" floatation marks on the boat (like the old IOR rule). Somehow the event measurer will need to know the real waterline for a jig like this to work. If we can ever get to the point to where we have authorized builders, the hull draft can be automatically approved for production boats, then it is very simple to measure keel draft with a jig.
Without proof of waterline, it will be difficult, if not impossible to use a jig to verify total draft.

Post by **Chairman** » 31 Dec 2003, 23:03

Steve Landeau wrote:Without proof of waterline, it will be difficult, if not impossible to use a jig to verify total draft.

Certainly, it would be more or less impossible to have a simple draught gauge, as per the M rules, yield the same draught measurement as a flotation tank for all possible designs. The International M class rules have dispensed with total draught in relation to a waterline as measured in a flotation tank. Instead, the M rules now only deal with draught which is limited by a gauge. So the M draught is now controlled in a way that doesn't use the hull waterline as such. As a result, some existing designs could find their fin is a mm or two short of the permitted maximum, and others might find their fin is a mm or two too long, depending upon the exact details of mid-hull beam and fullness in the bilges.

ole_peder · Post by **ole_peder** » 14 Aug 2006, 22:35

This issue has been discussed before.

1. Marblehead draft, The marblehead doesn't have restrictions on hull dept which for an IOM is 60 mm. Not all M's has a keel at maximum draft which is if I remember right 770 mm.

2. It is more or less impossible to measure draft without a tank on an iom whithout affecting the design parametres. The closest way to do this is defining a waterline since more or less all IOM's floats with the stern and bow kissing the water or the stem some mm above.

A longitudinal measurement jig as FIN are eksperimenting influences the design of an IOM the least.

A transvers measuring jig will restrict the waterline with which will influence the designs more.

In both cases the rules must be adjsuted accordingly.

Again I will strongly advise that a waterfree measurement must be carefully handled

Post by **Chairman** » 22 Aug 2006, 14:27

Just a personal thought for discussion.. There has been much comment about the complexity of the IOM rules. This thread provides the oppertunity to discuss a rule change to simplify measurement. Just set a maximun fin depth. Our class rules suggests our hull design is free apart from length and draft. so remove draft and add fin lenth from bottom of hull.
any thoughts
Greg Willis

awallin · Post by **awallin** » 22 Aug 2006, 17:17

bfox1 wrote: That sure sounds like a simple solution and is consistent with the manner some other classes use to control draught.

However, in this and other discussion there seems to be a lot of concern over doing it that way and a feeling that the hull draught itself needs to be controlled. Not being a designer or student of design for any kind of boat, I don't understand what the effect of a deeper draught is. If that was explained then it might be a way to either stop the talk about a dry measurement system or clarify why one can work if we do as you say.

with an unrestricted hull depth, there's a possibility to make a very narrow fin-like 'hull' which is very deep. You could put your boat corrector weights at the bottom of this deep(say 150mm) hull.
then you could put a maximum length fin on that hull...

so, I think not restricting hull depth will result in 'rule braker' boats with funnily shaped very deep hulls.

Anders

Steve Landeau · Post by **Steve Landeau** » 22 Aug 2006, 17:43

I like this discussion, because the topic has merit. Also, we have 2 years to figure it out since our agenda is set for this cycle.

I think a draft jig touching stem and stern with a set number (could be 60mm) for draft and then overall 420mm will work. It will likely show some current boats to be illegal because many current boats do not put their stem and stern on the waterline. We could either grandfather existing boats (too many I think), or increase the draft number to the deepest known draft from stem/stern measurement known that is currently legal. That would take some research, but would be the fairest method. My guess would be that it will change to around 62-63mm.
This would be changing the draft maximum rule to make measuring easier. As a result, there will be very slight new allowances for new designs, but I don't think they would have any more advantage than different designs have now.

awallin · Post by **awallin** » 22 Aug 2006, 18:11

Hi Steve,

I'm not sure I exactly understand what you mean, but if simply measuring from a point somehow defined as stern and bow, then, would it not be fairly easy to build a "rule-braker" by having the boat float with the stern and bow very deep in the water. See attached pic.

by "rule-braker" I mean a boat that would be legal under a newly proposed gauge-rule, but would be significantly different from, and possibly much faster than, a current IOM.

Steve Landeau · Post by **Steve Landeau** » 22 Aug 2006, 18:24

No, my idea is still measuring hull draft, I'm just not sure that all current boats will pass a 60mm check when measured from bow and stern (some show a waterline a bit below). I agree, you can't just measure 420. We still need to measure 420 as max, but we may need to adjust the 60 rule to allow current designs to pass when measured from bow/stern rather than waterline.

Marko Majic · Post by **Marko Majic** » 22 Aug 2006, 21:21

VCinfocomms wrote:so, I think not restricting hull depth will result in 'rule braker' boats with funnily shaped very deep hulls.

Anders

Hi Anders,

I disagree...

I would be very surprised if you saw very much change in the basic IOM hull shape if the 60mm canoe-body draft restriction were to disappear... I think you might see some hulls going down to 70mm or so - but I doubt you would really see something radical...

Of course, without actually building a "funnily shaped very deep hull" or, more likely a series of them

, there's no way of telling for sure... And, since the 60mm CBD restriction does not seem overly onerous to designers, builders or owners - probably best not to mess with it...

Marko

P.S. If the 60mm was abolished, I suppose that the D2.2(b) section would have to be "reinforced" to prevent "skegs" as part of the hull (in which to stick corrector weights) - which would be one way to exploit the extra depth without significant negative effect on the overall hull shape...

Gyula · Post by **Gyula** » 23 Aug 2006, 20:10

Hi
I know that Naviga isn't very popular here

but may I have a word?
In our rules there is no other draft restriction only the depth of the fin measured from the bottom of the hull is limited to 38cm, and the main section of the hull, where the fin is attached, has to be a convex curve. I have seen only a very few boats that has a deep hull (80-90mm), and those has no visible advantage of that.

Regards

ralph kelley · Post by **ralph kelley** » 24 Aug 2006, 15:37

I know we cannot address this immediately but is it not time to visit the question as to the need for the 60 mm restriction.

What Gyula said about the Naviga approach makes a lot of sense to me.

1 - Very easy to measure. Just takes a tape. Stops all the work and energy putting into making tanks, deciding how to set up tanks, looking for qauge quality non-water measurement devices and all the like.

2. The convex requirement eliminates weird design approaches

It does not restrict the overall draft, but as Gyula has pointed out in their experience, there is a practical limit to the draft of the boat such that there are no weird designs and no design breakthroughs. That might be a sticky point that needs to be resolved.

As for the question of more V shaped hulls versus the common canoe shaped hull, is it not clear, from models as well as real craft, up to Volvo's & America's Cup craft, that a canoe body with a max fin and bulb combination is the best?

We do have folks in the class with the computer modeling software to compare V shapes to canoe bodies for speed potential. It does not take long to evaluate alternatives so hearing from these experts would provide a lot of information on the subject.

The question of unlimited draft is the big unknown.

Ralph

We do have

Marko Majic · Post by **Marko Majic** » 24 Aug 2006, 17:45

Gyula wrote:Hi
I know that Naviga isn't very popular here but may I have a word?

Hi Gyula,

I'm sure I speak for everyone here when I say that the discussion(s) on this forum are open to all R/C sailing enthusiasts - although experience in one metre long crafts (which you obviously have) probably helps your credibility...

Your experiences with the Naviga E-class (E - right?) are fascinating as it is almost like an "IOM alternate reality"... Two classes that are obviously closely related (having read some of the E-class rules some times back, I can't help but thinking that Naviga "borrowed" some dimensions from IOM class rules but packaged them differently in their own rule writing) - yet, probably nearly impossible to reconcile (at the class rules level - although, I imagine that modifying an IOM or E-class boat to legally measure in the other class could be fairly easy - at least making an IOM into an E-class)...

Gyula wrote: In our rules there is no other draft restriction only the depth of the fin measured from the bottom of the hull is limited to 38cm, and the main section of the hull, where the fin is attached, has to be a convex curve. I have seen only a very few boats that has a deep hull (80-90mm), and those has no visible advantage of that.

That is quite interesting - unlike our own discussion here which tried to gauge the positive impact of the deep hull based solely on the advantage of putting the corrector weights deeper (which, on an IOM, in the best case scenario may amount to 200g or so) it is obvious that under Naviga rules (similar to the US1M CR verbiage) deeper hull places the entire weight of the ballast deeper! And still, as you point out, there doesn't seem to be a decided advantage to going extremely deep! This is very interesting to learn (although not altogether surprising - mildly surprising I guess

).

Naviga's "convex curve" restriction for the midship section is basically what I was referring to in my previous post as "reinforcing section D2.2(b) - i.e. a way to prevent moulding a skeg (a deep thin blade - like the beginning of the fin) as a place to put the corrector weights (without taking a penalty of unfavourably deep hull). Personally, (with only 100-200g at stake in the IOM class) I don't think you would see too much of that - but it's best to close the loopholes that are obvious and can be anticipated... Of course, under Naviga rules something like that is more or less critical - since it affects the position at which the fin is measured (so there's an awful lot of incentive in that scenario for a skeg).

However, this discussion is rather "academic" as I don't really sense an impetus within the IOM community towards abolishing the 60mm mark... As I said, I personally don't believe it would make any impact on the competitive IOM landscape (certainly can't see it making existing designs "obsolete") - and I also think that it would make life easier for the home builders who want to start dabbling in hull design or experiment with "porting" free designs from other classes (like US1M or Marblehead) since it would remove some potential headaches (I mean - if your keel is too deep you can cut it down, but if the hull you built over last several months is slightly too deep - what do you do?).

However, in the grand scheme of things, the benefits ("relaxed" home builders and slightly easier event measurement) are not that great and while I personally don't believe the impact would be measureable - others would disagree (rightfully so) - and the risk (of me being wrong

) is certainly there (and that alone may outweigh the meagre benefits)...

Cheers,

Marko

RoyL · Post by **RoyL** » 24 Aug 2006, 22:37

Marko: Very well reasoned post. I think it is likely that changing the class rule to an overall depth requirment would probably not obsolete all existing boats. However, that would only work until, inevitably, someone would find a way to make a fast, deep, skinny boat and then everything would fall apart.

Overall, I think we should just stick with the tank measurment system and provide some guidelines on how tanks should be set up and used.

Roy Thompson · Post by **Roy Thompson** » 07 Sep 2006, 18:02

Barry,
As far as I know there is no defined proceedure for using the water tank for draught measurement, OK?
We are not profesionals, and the variety of designs of watertanks (those that I have seen) would not all allow a set proceedure.
That said, I hope that all event measurers use a bit of common sense when doing the floatation measurements.
The use of a (longitudinal) gauge to restrict hull/total depth (not draft please since I believe this is related to a waterline, which without water we can't fix) has been discussed in a number of threads for a number of years, and some prototypes have been tested in various parts of the world but nothing more has been done about it........
Is the tank a problem? I believe so. In some countries they don't even have one, and in others there is only one or two. It's use is as you quite rightly say, somewhat haphazard due to both the designs and set ups of the tanks )and maybe even human nature).
It will take someone with bigger b... than me to implement such a 'radical' change in the rules......

Good luck.