Inclining Experiment - Back Against the Wall?

Just received July /August 2015 Ship & Boat International and there's a note from Andrew Blyth regarding a paper by RJ Dunworth, Back Against the Wall. The paper examines the assumptions behind traditional Inclining Experiments, principally that the vessel is wall sided near the waterline and M doesn't move during the Inclining. Small/Modern hulls often change shape a lot around the waterline. The paper offers a new method for evaluating results taking this change in M into account.

The paper is here.....

ttp://www.rina.org.uk/p/1/IJSCT%20-%20Back%20Against%20the%20Wall.pdf

Anybody else aware of this or already dealing with it?

 

Tad, I'm not able to open or download the link. Is it in a "members only" part of the RINA website?

 

Try it now David?

 

Link is working. Thanks.

 

Very interesting, thank you, Tad.

I've read the paper once and have to do it some more times, because of my poor English.

 

Tad, there is another problem: methods of inclining experiments on water do not provide accuracy for small craft say up to 15m length.

 

Can you tell us more about the reasons inclining tests are not sufficiently accurate for small craft?

 

The reason is simple: there are accuracy requirements built into inclining test methods, those are specified by IMO and followed by Classification Societies. Say, the displacement of the series of vessels should be within 2% and CG within 1% otherwise inclining test should be conducted for every vessel. This accuracy is not achievable on small craft where realistically possible accuracy of WL identification is lower than 1cm.

I would say for boats below 10-15m (or less then 1m canoe hull draft) stability assessment methods/criteria based on inclining in water should be avoided.

 

I would say larger than that.

We did a series of 6 off 27m patrol boats (24m Lwl)...we had to weight them all, client requirements, yet when we did the inclining each were different because, as you noted the level of accuracy that is possible diminishes on smaller vessel. The systematic errors can amount to a lot..which ultimately influences whether Flag/Class accept the inclining or not and thus whether you end up having to do a stability book for each sister vessel. Luckily we argued our case and won.

But. Going larger, we did a series of 45m high speed catamarans....in all, 13 off, even these the systematic errors resulted in margins exceeding the 2% and 1%....at the time Class insisted on a unique stability book for each, where there were no differences between them other than the readings for the inc expt.. Painful.

But on another series of 50m high speed cats, i successfully argued (same class as before) that the systematic errors meant it was impossible to satisfy. Was a relief after the previous mess.

So, in reality i could say even up to 45m, it is still problematic; depending upon the hull form and its LD ratio too. Heavier vessels it is easier to comply than lighter ones, for the same length.

 

Agree, it depends how You read the draft. Say, if painted load marks are used 5cm apart, there will be no accuracy for <45m vessel.

For catamarans, inclination test is not accurate at all due to subtraction of large values resulting small value. Say, ISO12217 does mention that for catamarans with high GM should not be inclined on water. Though societies surprisingly do require such tests.

 

2AdHoc
Since I understand that You do not design small craft under ISO but rather high speed craft, this info might be helpful. This is the extract from ISO12217-1, Annex E in 2013 version of standard. There is no point to incline catamarans, we always use this ISO as an argument to Society when they ask for that.

The vertical position of the centre of gravity (VCG) can be found using any of the following methods:
a) an inclining experiment in water (see 3.5.6), the results being corrected to the appropriate displacement
condition;
b) an inclining experiment in air using a known length of suspension and moving weights transversely (as in
water), the results being corrected to the appropriate displacement condition;
c) calculation based on the calculated mass and centres of gravity of individual components, raised by an
addition of 5 % of (F M + T C ).
Method a) shall not be used for boats with a metacentric height greater than 5,0 m, since inclining experiments in water for such boats are liable to significant inaccuracies.

 

Alik

b) not so easy on a 200+ tonne catamaran!
c) Done this endless times. BUT...it doesn't always satisfy Flag/Class. It really depends how mature the Flag state or Class society is and whether they accept this as an acceptable alternative, given the difficulties in trying a).

 

b) for catamaran will just generate discrepancies over those required by IMO.

This is what I am saying every time, those 2% and 1% requirements are just creating the confusion and are only suitable for large commercial ships. They should be dropped! We as small craft designers always suffer from such built in figures, and once we try to educate the surveyors (and some our colleague designers, will not point the finger) we are under attack of conservative/ignorant criticism.

 

Thanks Alik and Adhoc for your thoughtful comments. Sounds like maybe the discussion should address discrepancies between calculated and inclined cg's? With an eye to establishing reasonable expected margins of error?

I can see that a calculated cg could be more accurate in a new build with good oversight and complete drawings. My problem is establishing some (even if vague) stability data in old, existing boats for which there are no/few drawings, built of wood or fiberglass with unknown density, and ballasted with cement and scrap of unknown amount and density. I am aware that my figures are crude at best, thus I build in safety factors, such as reduced downflooding heights or not including deck structures.

 

SNAME is currenfly working on a set of guidelines for in-air inclines as a combined project of the Small Craft T&R committee and the SD-2 panel.

Lead author is Todd Hiller at MARAD.

Draft is in review.