Understanding Displacement / Stability Relationship (Not GM)

The boat does appear a reasonable candidate for what you propose, and there is always the option to add ballast in increments if it rises too high in the water for your liking, you would lose both waterline length and beam taking away the keel, neither of which helps the cause, but as you say, it is relatively flat bottomed. It gives more scope than a deeper more rounded bottom.

 

My plan is to add ballast if needed. If I end up using lead based batteries that is 300 to 500 lbs right there.

 

That might be the best plan, experiment with ballast. You might find it doesn't need an equivalent amount, the boat does appear to have reasonable form stability and wasn't over-reliant on the keel.

 

I think from a DXF file it schould be possible to export the hull shape in a STL file. This can be transformed in a specific 3D grid of cartesian coordinates (CSV file) describing the hull surface. If you are interested in it and ready to deliver the CSV (I would state the specifics before), I would upload the file an you could work with your hull and look for its answer. Not limited to small angles of heel, you could change displacement, move weights on board. This is the site with another hull (click button "English" top right):
Schwimmlagen eines Bootes http://www.bootsphysik.de/boot25m.php

 

I got Dan's permission to share the file. Thanks for any help you could give!

 

Instead of a keel, relatively thin or V bottom shape outriggers / amas could provide similar function. And add as much stability as you'd want without ballast.

 

I agree that amas would work. But I need this to be trailerable. I'm doubtful that it would end up working well without a ton of work.

 

Yes this is probably not the easiest solution making it demountable or foldable.

 

Hi Jeff, I know you have had a lot of sound advise from those who have previously commented, but I thought I would put my two cents in. This question all depends upon whether you are interested on immediate upright stability, or dynamic stability, where larger angles of heel can be expected. Also, it largely depends on the initial displacement of the vessel as the shape of the previously reserve buoyant portion of the hull, now immersed, will have a large bearing on whether the GM increases or decreases with draught. At increased draught relative to very shallow draughts we would generally observe an increase of stability, as you correctly mentioned. However, when we come to draughts more representative of the deeper "operational" range, the effect is often the opposite as "Heimfried" correctly pointed out. Generally as the draught of a vessel increases, so too does the fullness of its shape, therefore, the shift in the centre of buoyancy decreases and also the righting lever. I have conducted some sensitivity with regard to variations in draught, KG and weight which you can observe in the attached and its effect on GM (for a passenger vessel). I should also say that by reducing the displacement of your vessel, you are increasing the reserve buoyancy and also freeboard, both of which are highly important from a safety perspective, so as long as you do not negatively affect the KG in a significant manner, I believe your changes will make your boat safer. She might be a bit tender, but probably safer. That being said, if you are going to remove weight from your boat, best not to do it from below the KG unless she is empirically stiff.

 

Starting a similar project. San Juan 21. Last picture is the goal.

 

Welcome to the Forum @ChrisLG

Re the re-fit project on your San Juan 21, would you like to start a new thread about it, perhaps in the Fibreglass boatbuilding group on here?
Fiberglass and Composite Boat Building https://www.boatdesign.net/forums/fiberglass-composite-boat-building/

 

At this point most of my questions are about stability and safety. Where/how can I get a “lines drawing” for the SJ21?

 

Here’s an idea. Use the original keel. Unfasten it from the pivot. Secure it in a position where it is 4” deep and horizontal. Cut the top of the locker. Glass it all in top and bottom. I’m thinking this will be needed to ensure optimal ballast.

 

You could take the lines off the hull yourself - but that will take some time and effort.
You set up a grid system, with vertical and horizontal reference datums, and measure the offsets to the hull from these datum lines.

But once you have a lines plan, what do you intend to do with it?
Will you digitise it, and put it into a stability program?

Rather than hijacking this thread further with your questions, it might be better to start your own thread?

 

Was thinking it would help me to be more scientific about ballast. I’ll start my own thread. Thanks!