# Freship and stability again

Discussion in 'Stability' started by Kudzu, May 16, 2009.

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### KudzuJunior Member

I design kayaks. I am not trained, just found I have a knack for this. I have been using Freeship for a while and it works very well for what I need. I can relate the software's Cross Curves to seat of the pants feel. I would like to be able to take that information and be able to give a 'stability factor' to each boat.

I have read and understand Steve Killing's method and would like to apply it.

I have read past posts and seen the charts of how FreeShip calculates but I am not seeing how to determine M or is it N in Freeship. I used trig on a daily basis but it has been years. Maybe I am just overlooking the obvious, but I am not seeing how I can take the FreeShip output an work backward to determine the metacenter. Am I just overlooking something or is not possible?

If I can't do this from that data that's OK. It is just something I would like to do, but it's not worth a lot or effort.

2. ### Guest625101138Previous Member

You need to produce the cross curve for the kayak loaded to the specified displacement. That will include the hull weight and the 250lb load. From this you can obtain the value for KN at 15 degrees. This is the length of the righting arm.

You now need to calculate the height of the centre of gravity above the keel. This length is usually termed KG. The overturning arm at 15 degrees is therefore KG*sin(15).

The righting moment is the displacement multiplied by the difference between the righting arm and the overturning arm.

For a person seated in a kayak the height of the CofG of their body is very close to the height of their naval; typically a bit under 1ft.

Rick W

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Kudzu

"...but I am not seeing how to determine M .."

#### Attached Files:

• ###### metacentre.pdf
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### KudzuJunior Member

I had to edit my post. Looks like I had some bad info. I looked a little more and I found this description form the source, not second hand.

The determination of the metacenter is not easy without a computer program and to do a physical test on an existing boat in the water is very difficult, so it is easiest to have the designer determine these numbers.

Once the metacenter at 15 degrees has been determined, the stability factor is simply the metacentric height in inches divided by 12.24 (for kayaks use 7.50) times 100.

I think I have enough info to calculate this. Thanks for the help. I have been looking at this for a couple of hours and now I find out I had bad info!

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### KudzuJunior Member

Update. OK, I give up. I have worked on this for hours. I simple can not see any way to determine the metacenter height at 15 degrees from what Freeship gives you.

I have charted out all of the dimensions I have. Used this chart and I have come to the conclusion your can't just use trig to calculate it and the program doesn't give this to you.

6. ### Guest625101138Previous Member

I have attached the screen dump for the cross curve for the default hull in Freeship. Does your version produce Cross Curves like this?

You can see on this chart that KN*sin(theta) is 0.587m at 15 degrees. (Actually I used the data table to get the precise number.)

You should be able to do the same for your hull.

You could also roll the hull to 15 degrees but you would need to mirror your half hull and remove the symmetric function.

Rick W

#### Attached Files:

• ###### Freeship_Screen.png
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### KudzuJunior Member

To answer your question yes, I can do all that. Pretty familiar with all that FreeShip will do. Not everything, but most.

Now a confession. I am an idiot! I have thought about this too long. When I read your answer I was thinking "So what? How does that help me?" The it ocurred to me what I was doing wrong. I was thinking the metacenter height was always measured vertically, relative to the water line. I just realized is always measured on the center line of the boat! Regardless of the angle of heel it's still measured on the center line.

So I had this measurement all along and I was thinking I needed something else. Where is the icon for all red faced and embarrased??

Thanks for all the help. I feel so stupid right now!

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kudzu

Did you not understand the sketch I attached?...since that is the same as the one in your post #5?

Ok, so you understand how to calculate it all now...yes?

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### KudzuJunior Member

Now I do. I followed the drawings, both of them but I was just misunderstanding that one aspect.

I have it now and have my stabilty factor calculated on all my designs.

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Kudzu

Good...and no need to feel embarrassed. We all have to start somewhere and better to ask and explore and understand why. Otherwise you'll end up like many, who just press buttons on the PC to run the program which does all the number crunching and they wont know how to interpret the results correctly nor the theory behind the crunching or if the results have been crunched correctly.

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### KudzuJunior Member

Thank you. I am having a ball and the more I know the better boat I can produce. I was surprised once I calculate the numbers on my three boat at how well what I read lined up with what I felt.

My latest boat, Curlew has a stabilty factor of 93. In the information Steve says a boat of 92 will be uncomfortable for most new paddlers. I knew then I had done it right. While I love this boat I agree a newbie would think he was going over. All my other boats worked out the same. Even the one I can barely keep upright.

Now to figure out Godzilla!

12. ### Guest625101138Previous Member

There is a slight difference between Godzilla and Freeship when you are considering a stability constraint.

You are able to set the minimum value of GMT in Godzilla as a constraint. This is not the same as KMT used in Freeship.

For Godzilla the default CofG is taken at water level. Hence when you are comparing KMT given in Freeship with GMT set in Godzilla you need to allow for the draft.

As an example, if you want a KMT of 0.35m then you will need to set the minimum GMT at 0.25m on the basis that the optimum draft is going to be approximately 0.1m. After the first optimisation you might need to iterate the minimum GMT to get closer to the required KMT based on the determined draft. One or two iterations will get very close to the target KMT.

Post #25 on this thread might give you a leg up in getting going with Godzilla:
http://www.boatdesign.net/forums/design-software/racing-canoe-19110-2.html

Rick W

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### KudzuJunior Member

Good discussion there. I have played with Godzilla and my problem has always been getting the input file to run. I always seem to have a problem with it.

The last boat I Godzilla'ed, while I am sure it made a faster designt, I didn't like the end results. There were couple of thing I wanted in the boat and it changed those totally. So I finally decided to just do it my way and was pleased with the outcome. I am sure there were ways to change the Godzilla output to restrict some of the changes but I was frustrated and needed to get that one built.

However I am playing with a racing/go fast fun kayak design and since there is no restrictions it would fun to see what Godzilla could do with it. Looks like it's going to be a rainy day (again) so that might be this afternoons project.

Plus I just converted to a spiffy new fast computer so it will make doing this much faster!

Again, to everyone, thanks for the help. I really appreciate it!

14. ### Guest625101138Previous Member

I usually start with just displacement and the intended power level to see what a completely unconstrained hull looks like. For a pedal boat 150W at the cranks results at about 120W on the hull. The design speed ends up being 3.3m/s. For a displacement of 95kg the unconstrained hull is 7.2m long with a beam of 275mm.

Obviously this hull would not be suitable as a canoe. It is not much different to a single scull. However it gives a starting point so you can see what certain constraints cost.

I did one pedal boat hull with a stability constraint for 130W at the pedals. It had a length of 6.2m and WL beam of 400mm. It worked as predicted.

Rick W

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### KudzuJunior Member

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