Goat Island Skiff and Freeship Results

Hello ,
this is my first post here, I'm building a Goat Island Skiff. Now, in order to register the boat, I must submit to the maritime authorities the boat's plans and "a stability study of the vessel that supports its seaworthiness under different sail trimming conditions." The designer does not provide this latter requirement, so I used Freeship, managed to draw the hull, but I’m not really sure if the results it gives me are accurate.
I'm struck by the shape or outcome of the curves the program shows me, and by the fact that it doesn’t provide any information for the stability diagram or the results.
My question is: is the model properly drawn/defined? Is that the reason for such strange curves? What am I doing wrong—or is everything actually fine? Can I extract from this the information I'm required to submit?
Anyway, I understand I’m asking a lot. But if someone would be kind enough to give me a hand and guide me a little, I’d be very grateful.
I’m attaching the Freeship model and some results.
Best regards.

Mario

PD: cant upload the .ftm file

 

I don't know if the results provided by that app are correct or not, but what I am sure of is that the studies you have shown us are not sufficient to request the approval of a boat.

 

Seems to me that your authorities who demand such statistical data are not acquainted with the dinghy sailing world. What would be the registration requirments for a kayak or canoe.?
The GIS is a great little boat. In the real world, it does not need approval from political types who do not know a halyard from a rudder hole in the ground.

I do hope that you can cut through the political hogwash and proceed to build and legally enjoy your well proven little boat.

 

Freeship can give stability data, but you need to define sections and waterlines properly. Also make sure your center of gravity (CG) and load conditions are set, or the output won’t mean much.

 

In France, for the stability and buoyancy of sailing dinghy, even through an amateur building, you should comply with the norm ISO 12217-3, alas to buy (and expensive, 199 swiss francs), only the preview is free :
https://www.iso.org/obp/ui/en/#iso:std:iso:12217:-3:ed-4:v1:en

In short, you have several main options, among which :
- option 7 : Capsize recovery test, that you should do in situ (calm water and no wind authorized) and report on.
or
- option 6 : 3 main requirements that you can prove by computation, inc. minimum free-board, buoyancy of full flooding, wind stiffness test.

This last requirement (wind stiffness test) looks like the demand you face :
- a formula for the heeling moment in function of heel angle is given by the norm, to use with your sails area and center of effort, your daggerboard depth, and a wind force of 6 m/s (i.e. category D for sailing up to force 4 wind)
- on your side, you have to compute the righting moment in function of heel angle, in loading conditions with including a moment due to the crew hiking posture limited by a formula also given by the norm (roughly it corresponds to only one person sit winward at max beam)
- the 2 curves should cross for an heel angle lower than the one of first downflooding.
- in case of , you can prove that with a reefed mainsail (which reduced the heeling moment above)

sorry to not given more details, because of ISO copyright.

 

For your information, our local builder of GIS (Arwen Marine) claims on its web site that (I tranlated) :
« The Goat Island Skiff is equipped with watertight compartments. According to regulations, it is approved in Category C for two adults and 4 adults in Category D (provided it is registered and has the appropriate safety equipment on board).«
Arwen Marine - Goat Island Skiff https://www.arwenmarine.com/BateauxGIS.html

 

Thanks everyone for your responses. In the end, what I did was present the problem to an AI and see what it told me. I was surprised. While I was defining the characteristics of the ship, it asked me questions and recalculated some values, after which it gave me a report that I found very good. I'm sharing it here in case you're interested. Best regards.

 

I didn't read the whole report, I stopped at the description of the loading conditions. Are you sure a VCG of 0.5 m is reasonable for the crew?

Edit: I just read it's above waterline. VCG from keel is 0.675

 

yes, I look in freeship hidrostatics with care and abve all it say V/LT CG's, VCG is 0.22 m. When it recalculate everything it say the results are better... of course.

 

I haven't thoroughly studied the results you show us, but frankly, they seem lacking in rigor to me, not to say they're outright erroneous. It's acceptable to calculate GZ values for small angles by multiplying the initial GMt by the sine of the angle, but starting at, say, 7 degrees, this calculation is incorrect. Therefore, the GZ curve is erroneous.
The only conclusion that can be drawn from this study, assuming the VCG is correct (my opinion is that the CoG is too low), is that the initial stability is high. It's not possible to know if the stability and buoyancy conditions required for this boat, for example, by the ISO standards for small craft, are met.
Try to conduct a more detailed and rigorous stability and buoyancy study. To do this, start by calculating the total weight and position of the CoG as thoroughly as possible.