# How to measure the stability of a dinghy on the sea

Discussion in 'Stability' started by Frank Le, Jan 22, 2009.

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### Frank LeNew Member

We're doing design project in University. Our group need test and compare the stability of 2 dinghies but the only place we can do it is on the sea. And that sea is relatively windy. Is there any method available?:?:

We are thinking of letting the boats float on the sea, where the wave will perturb them. 2 inclinometers, which are attached on the boats and connected to a data logger, will be used to acquired data of tilt. We'll try to compare the graphs of data of the 2 boats. We're not sure this method will work or not.

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The stability of the boat is influenced by the total weight and where the center of gravity occurs with respect to the waterline. If you are to test dinghys, then you will have variables that relate to position of crew, position of items that may be in the boat, etc. You will no doubt need to set up some columns of variables and tabulate according to whatever variables are in effect at the time of testing. You may also need to consider sea state while testing.

The classic methods for calculating stability does not involve inmersing the vessel in water. I rather like the emperical method that you contemplate but the classic method is quicker and drier. Find the center of bouyancy (CB) of the boat. Establish the height of the center. Now find the center of gravity (CG) of the boat and it's contents. Calculate the location of the CG with respect to the CB at various angles of heel. You can then construct a graph that describes righting moment ( or the lack thereof) at any angle of heel. At some degree of heel the CG will coincide with the CB when viewed in the cross sectional graphic. That is the point of no return. Further heel angle will put the CG farther outboard of the CB and capsize is a certainty. Dinghy sailors are quite adept in shifting the CG by moving their own weight in the appropriate direction. Movement of weight will further complicate your stability calculations of course. I am sure that you can readily find stability calculation methods more thoroughly described in numerous books on the subject.

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### TeddyDiverGollywobbler

Testing a dinghy without a crew is pointless. Look for a couple of scandinavian tourists (preferably males) in the downtown on and offer them a free fishing tour. Give them free beverage. Make observations from a safety of dry land..

There's a lot of variability to be expected...

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### likiSenior Member

Based on my experience with university studies the following might work:

ISO 12217-3 standard defines downfloodind and offset load tests that a small craft has to succeed for its crew limit defined by manufacturer .

You could follow the test procedure with weights or test persons and define the maximum allowed load for both of the dinghies and see that the test results follow what you would expect based on the hull form. Simple evaluative approach but enough to write a paper.

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### Frank LeNew Member

Actually, we did some calculation of the stability (CB, CG, etc.). Since the structures of the boats were a bit complicated for exact calculation, we made some assumptions with respect to the shapes in our calculation. We perhaps need more study on the topic to calculate stability of those fluid dynamics shape. We have also done some testing to find the metacentric height of prototypes of the designs. However, the client, who proposed the project to our uni, didn't seem to be satisfied with just simplified calculations and prototype testings . He asked us to test the stability on the real boats.

Therefore, we're looking for an experimental method to find out stability indications such as CB, CG, angle of heeling at different moments or anything quantitative and scientific to convince him. However, the real boats are at a sailing club and we're not allowed to take them away . The only way is to test it on land or on the sea. Our experimental method used on the prototypes is a static test, which is can't be used on the windy sea. One of the group member then gave the idea of the perturbation method in my first post. However, its feasibility is still questionable.

Edit: Wow, it took so long for me to write the reply. So many other replies have come out. English is not my first language. Haizzz, still struggling with it.

I'll check the thread of the aircraft carrier and the ISO 12217-3.

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### ancient kayakeraka Terry Haines

I think your first proposed method could work but is prone to errors which you must try to control. Obviously it can give only provide a comparison not actual values. If you know your client's situation you should try to judge what is required.

For example the client may represent a boat manufacturer and already has accurate stability data for the two hulls, which might show thaat the two are similar. However, if this data is from static calculations it will not allow for the damping effect of a hard chine hull (one with edges) compared to a more rounded hull.

If a comparison is judged sufficient, then for each boat you will need to load it to its design displacement, ensure that the load is secure and reproduces the center of gravity of human occupant(s). If people are used they must be prevented from moving during the test.

If hard data are needed, however, you will need more control over the experiment. You will have to provide an accurate heeling moment and eliminate the influence of waves. An alternative method, however, would be to measure the dynamic behaviour of the hulls, such as their rocking period and time to damp down the rocking to, say half. That makes us of the waves and eliminates the need for accurate measurement of heeling moment. it also has the benefit of providing actual physical values for stability that can be compared to other hulls.

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### IkeSenior Member

Why don't you do a simple inclining experiment. You can do this in a pool or calm water. You do not need a person. You can subsitute weights for the person. You then simply place known weights at a given distance off the centerline. Measure the heel angle. From that you can calculate righting momenst and metacentric height.

If you are just trying to compare two or more boats, do the following. Place weights on one side of the boat as far outboard as possible until water comes aboard. The amount of weight it takes is your comparison. This is how the US and the EU test small boats to determine maximum persons capacity. See http://newboatbuilders.com/pages/load.html

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### Knut SandSenior Member

Page 11-12 here have some formulas, its in the major knowwn language Norwegian...
http://www.marin.ntnu.no/havromsteknologi/depot/tema-hefter/stabilitet.pdf

To determine the COG for a dingy hull, lift it in one rope with an front down angle (some 20-30 deg will do), see it 90 deg from the side, the COG of the hull is somwhere along the extended imaginary rope, normally in the center of the boat (some tank/ equipment may be worth considering). Mark this imaginary line with a marker/ greasepencil.
Repeat the liftingprocedure, but now with the aft down, mark the imaginary line. Straight behind where they're crossing, there is the COG. A start... Right above here, Ika has mentioned the rest I believe.

And then, any person sitting in the boat have their own COG, the complete COG is pretty easy to determine. I would not use all kinda scandinavians as Teddy suggested, Finns, Norwegians , Danes tend to float about and be rather unstructured in behaviour, under some well known chemical influence we dont take instructions too easy... . The instructors will need to have the patience of an angel, not too many of those around. Maybe some Swedes will do...?

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### ancient kayakeraka Terry Haines

Don't have any experience of Danes, Swedes and Finns but based on a sample of one, cats would be easier to control than a Norwegian.

(Sorry Knut: consider it a compliment)

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### Knut SandSenior Member

could it be anything else??

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### Knut SandSenior Member

Back on track:
http://www.marin.ntnu.no/havromsteknologi/

Go to "illustrations"

1:30 minutes into the video, there will pop up some formulas, and explained by animation, I believe around 4-5 minutes theres something about internal tanks, difficult to get if youre not into Norwegian... But quite a lot of the video will be understandable even without any knowledge of Norwegian.

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### Frank LeNew Member

Thank everybody

For the client, comparison is good enough.

Our problem is that we don't have a static place to test the boats. We're not allowed to bring the boats away from the club and the club don't have any big water tank. Testing can only be carried out on the sea or the dry land. We're checking the video, formula and the feasibility of above testing methods in our conditions.

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