Heeled Waterlines and Calculating Righting Moment

[John Owles]

It is understood that DS was never designed for asymmetrical hulls or symmetrical hulls in a heeled condition. However, there seems to be a workaround which I have been playing with and, as I understand it so far, should be able to provide the data to calculate Righting Moment.

The initial routine seems to be:-
Turn off both sides
Mirror Hull (check Connect Existing Points)
Rotate to desired angle of heel
Note the new draft at dwl
Enter new draft into Project settings (to retain Displacement)

So far so good.

Has anybody had anybody had a go and got the hang of this?

Having managed to keep the displacement to the correct figure, I am struggling to find VCG and TCB. Probably because the old brain cell is wanting a break.

[Icare]

:woohoo: … Hey! And what about about adding a 90° heading change?!?
Wouldn’t the longitudinal stabibity calculation be used to calculate the lateral stability?

I assume the results won’t be absolutely reliable …I guess that’s why the D.S. team always refused to recommand this software for multihull/asymetric ships designs; But it could give a broad idea… for SCALED models only.
As soon as a life is involved, a better method should be used. 😉
😉
Oh! And by the way: “Hello Jayoh!“.
… My brain cells must be as tired as yours :whistle:

[John Owles]

Hello Icare, Thanks but the Longitudinal stability calc won’t give the information that I am after I’m afraid, its going the wrong way.

I am designing another 14-15ft open sail and oar boat which, it is hoped will be built over this winter. While I have much experience of these types of boat, along with their larger cousins, it helps when designing the sail plan if I have a reasonable indication of the transverse righting moment. This information helps to match sail area, C of E and crew weight to the hull. The final calculation to finding the righting moment is actually very simple (The horizontal distance between the centre of gravity and the transverse centre of bouyancy). The tedious part is finding CG and TCB at varying angles of heel – a lot of drawing, measuring and calculating for each chosen heel angle. My inherent laziness leads my search for a workaround to encourage DS to give me the numbers.

I do agree with your comment concerning lives being involved, when designing ships and sailing cruisers etc. However, as this is about small sail and oar boats, where, in the end result, stability is hands on and all about crew weight against sail area and hull shape. This is also where a lot of the fun of sailing is had.

[Sharat Jami]

Dear Sir,

I understood your discussion, the procedure elaborated by you is perfect, but the results you asking cannot be found on free version of Delftship, it can be found in much primitive version called Freeship (Absolutely free to use GPL)….

TCB can be seen in design hydrostatics…..

But VCG is not a hydrostatic property it depends on weight, you have to tediously compute each and every detail of material you are using…

I hope i have resolved….

[John Owles]

Hello jsharatchand,
Thank you for your input. You are correct for the free version, however, I am using DS Pro. Freeship does provide the results but it is nigh on impossible to port DS files into Freeship.

You are also correct about the need for weights and their locations and I have now discovered VCG, LCG and TCG. For this I now have a work around which gets me reasonably close to the results that I am looking for. Use layers to separate all components and enter thickness and weights for each layer. As this is a dinghy so crew weight is a vital component, I add two boxes, with symmetry unchecked, to represent helmsman and crew. These can be moved to any likely location on the boat, according to the point of sailing that is being simulated. VCG and TCG can then be found in Design Hydrostatics. Using a spreadsheet the data can then be used to calculate a reasonable approximation, from which to assess the sail area.

I use TurboCad to draw the sail plan and find the resulting CE of the sails and another spreadsheet to calculate sail balance.

[Robert Holme]

I have had some success porting between Delftship and Freeship by exporting/importing as parts.

HTH Maryak

[Sharat Jami]

Dear Jayoh,

It is possible to export DS to freeship by exporting as parts,

Or else you can purchase inclined hydrostatics extensions from DS, if you want to avoid hassles between two softwares.

[John Owles]

I have the latest FreeShip v 3.5 and unfortunately I have not had any success with importing as parts. I am now trying Archimedes, which accepts my export from DS but is struggling to complete its calculations. I may well end up purchasing the inclined hydrostatics extensions from DS, it would probably be the sensible way to go.

[Author]

Posts