Initial Dinghy Stability

Discussion in 'Stability' started by serdar, Sep 9, 2022.

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1. Joined: Aug 2020
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serdarJunior Member

Dear Naval Architecs ;
As an electronic engineer I tried to design a small 3.8mt/1.3mt sailing dinghy using Delftship There is just hull now and I 3D printed (1/10) to make water trial. LCB is placed on 1.83mt from aft and LCF 1.675mt from aft.

Designed draft is 11cm with 160kg . 3D printed sample simulates 130kg, I expected a trim towards to Bow assuming LCF point will be trim center. But it is trimmed towards stem, opposite direction Is Delftship working wrong or shall I go back to my transistors...

Regards
Serdar

2. Joined: Sep 2011
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TANSLSenior Member

@serdar, Delftship will not find it easy to determine where the CoG is, which is what only you can manage, without changing the shapes of the boat, to obtain the desired trim.

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HeimfriedSenior Member

@serdar
I'm not a NA, just a remark.

The way you worded your question let me think you are not clear about the forces which are creating trim. Even at a certain hull LCB and LCF are not fixed values, they both depend on trim, heel and draft. In a given load condition the LCG is a constant (in boat fixed coordinates). Assuming no heel or list, if the LCG (force down) is not vertically aligned (in earth fixed coordinates) to the LCB (force up) , both of them are creating a turning moment which alters the trim until the forces are aligned and therefore the moment vanished.

You can look at a graphic showing the mentioned points. CoB dark blue, CoG red, CoF light blue.
Schwimmlagen eines Bootes https://www.bootsphysik.de/boot25m.php
(klick button top right "English")
Start condition is upright, trim = 0, heel = 0.
Let e.g. the sailor step in the boat and you see the the CoG shifting accordingly. The distance between B and G, creating the moment, will be visible. Clicking "find equilibrium" shows the new floating position with the CoG and CoB aligned again.

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4. Joined: Oct 2009
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DCockeySenior Member

To reiterate Heimfried's comments, the calculated LCB and LCF only apply to the draft/displacement and trim used for the calculations. LCB at 130 kg displacement will usually be in a different location than for 160 kg displacement.
TANSL also has a good comment. Where is the CG of your model?

LCF is only relevant once the equilibrium condition is determined with the LCG in vertical alignment with LCB.

With the vessel in equilibrium
if a small weight is added (displacement increased) at LCF the vessel will sink a small amount with no or little change in trim. With the vessel in equilibrium if a weight is added (displacement increased) forward of LCF the vessel will trim bow down a small amount. With the vessel in equilibrium if a weight is added (displacement increased) aft of LCF the vessel will trim bow up a small amount.

With the vessel in equilibrium if the CG is moved forward (no change in displacement) the vessel will trim bow down. With the vessel in equilibrium if the CG is moved aft (no change in displacement) the vessel will trim bow up.

Last edited: Sep 10, 2022
5. Joined: Aug 2020
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serdarJunior Member

Dear @TANSL ,@Heimfried , @DCockey ,

Thank you for your vaulable feedbacks . I realized CG and LCB alignment importance for equilibrium, that seems my problem.
Do you have any software advice (economical one ) just for hull design simulating GC, LCB and LCF same time with different trim angles ?

Regards,
Serdar

6. Joined: Sep 2011
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TANSLSenior Member

There are many naval CAD programs, some free, that calculate LCB and LCF of any boat. The position of the center of gravity can only be defined by you. Normally some tables are prepared, in Excel for example, with all the items that make up the total weight of the boat, with the position of the CoG of each of them. A General Arrangement plan will be very useful, once each item has been drawn on it, to determine its position with respect to a previously established coordinate system. With this you will be able to calculate the coordinates of the CoG. To obtain the CoG of the hull surfaces you can use the CAD program but little additional help can be found in this aspect (although it is already a lot). It is, above all, the personal work of the artist.

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serdarJunior Member

Dear @TANSL ,

Thank you, then I'll use Delftship for LCB & LCF calculation and model it using Solidworks with real material densities to locate COG... Then 3D print another sample for proof of concept... I hope that will work.
Regards
Serdar

8. Joined: Sep 2011
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TANSLSenior Member

But keep in mind that the position of the engine, fuel tank, passage, and various other items cannot be given to you by Solidworks.
I don't know very well what you are going to achieve with a 3D print, except to have a nice, decorative, object to place on the table. But if you like it, why not do it?

9. Joined: Aug 2020
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serdarJunior Member

Dear @TANSL , sure why not
I can design&model most of the required equpments on a boat with solidworks (calculates Mass Properties of any assembly automatically) , this is different task then modelling a hull.
3D print will just proove my initial stability of hull design with water test. I imagine there are more important several tasks in a sailing dinghy design, this is just the starting point of my learning curve which makes that hobby more attractive for me.
Regards

10. Joined: Sep 2011
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TANSLSenior Member

Allow me a question, what do you want to study, verify, clarify with the tests of your model?. Whatever they are, good luck with the channel tests of your 3D model. That is quite a complicated task, that of extrapolating the results of a model to the real ship. Good luck and keep us informed, please, of the progress in your essays.

11. Joined: Aug 2017
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DolfimanSenior Member

You can try my Gene-Hull Dinghy application, free (just need Open Office itself free), where I also adress the stability issue :
Gene-Hull Dinghy 3.0 version, and post-applications Stab and VPP in connection. https://www.boatdesign.net/threads/gene-hull-dinghy-3-0-version-and-post-applications-stab-and-vpp-in-connection.66426/

, and I also investigated the intrinsic stability of a sailing dinghy, proposing a scale from poor to very good stability taking into account the load (i.e. the sailor/the crew weight) :

12. Joined: Aug 2020
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serdarJunior Member

13. Joined: Sep 2022
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rodrimontiJunior Member

Hi Serdar, I can help you to load weights and test stability with the hull you draw in Maxsurf

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14. Joined: Jan 2017
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sailhandSenior Member

In delftship you can create small horizonral panels and move them about. You can assign densities to each of these objects to achieve the desired weight. This is a simple fast and efficient way to visulise effects on trim etc. You can move passengers, furniture, fittings, fuel, water, batteries etc etc in any direction and recalculate your hydrostatics until you bring your centre of gravity back to your centre of buoyancy. I find this visual representation more efficient when demonstrating the effect of the general arrangement on a vessel.
It is also very useful for adapting a vessel to a fixed general arrangement such as a workboat where the owner wishes to maintain the ergonomics of a particular work environment with specific equipment and machinery. I simply calculate the centre of gravity of each item and move them to the relative position within the hull on all three axis. Simple, fast, and easy to rearrange to suit different layouts.

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15. Joined: Aug 2020
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serdarJunior Member

Dear @sailhand ,
I tried that method but when I draw bulkheads , decks etc as intersectioned layers some Hydrostatic parameters (prismatic coeff., LCB, LFP even Displacement) goes somewhere else beyond galaxy
After your suggestions I tried each item as seperated (not intersectioned-connected) , as small spheres and it works, thank you.
Regards

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