Unstable after installing new battery bank

It sure looks otherwise ...

Quote below slightly modifed for clarity. The (whatever) is code for keel.

You are welcome to show the proof of your claim. Show us the numbers that indicate placing those batteries symmetrically outboard will change the roll period or amplitude.

As others have suggested, the misinformation and lack of basic stability knowledge listed in this thread is not only misleading but some of it is flat out dangerous. Just do a google search on GM Stability Measurment or Calculation and you can get some good basic info in a matter of minutes. Stability is measured in GM which is the distance from the vertical center of gravity to the metacentre.

Good luck with this thread if it turns out that one of the participants is moving the reference datum in the midst of advancing his/her viewpoint.

This is a good thread. Let's keep it civil and keep discussing the issue. Now without running the numbers it seems to me that the amount of roll or heel that a 200 lb person stepping onto a 50,000 lb can cause is going to be 99% dependent upon initial stability of the hull form. That is how much heel is required to restore the CB/CG equilibrium.

How fast it will achieve that equilibrium and how far it will overshoot will be affected by relative location of the added weight to the center-line. So that may play a factor in perception.


I may have to pull out my copy of Dave Gerr's The Nature of boats to run the numbers properly.

You would be far better off to input the hydrostatics of the vessel in question into a real stability program rather than depend on hearsay and boating mythology for answers.

It is physics and math, not a subjective appraisal based on a general text for recreational boaters that will speak with repeatable and reliable voice in these matters. Ask Mr. Gerr to run the numbers. I expect them to show exactly what I suggested in post #17 ... any difference will be so minute as to be not worth mention. It would probably be lost in the "noise" of wind, skin friction, and weight of mooring lines and gangway.

Really? Can you please send me the hydrostatics of the vessel in question? And the exact displacement at 10% fuel/water and 90% fuel/water. No. So I guess we can never solve the problem or even come to a conclusion without hiring a naval architect before putting some extra batteries in a 40 foot boat.

You are a brilliant guy in some ways....

Funny, I have not seen you provide one piece of reliable information or a solution or explanation to the OP's issue, just a willingness to be critical of posters comments, choosing to lable as "hearsay and boating mythology" which is quite amusing coming from someone who obviously does not display a lot of small craft knowledge. Next you will expect someone to produce a Stability Booklet or Letter or Class Certification, etc. as if that has been a commonplace event in the recreational boating world for vessels of this size these past 60 years.

You can only work with math as you have most likely not had to either ballast or add significant equipment to a small narrow boat after the fact, you must not have any experience based event on this topic to rely on, otherwise you would have a better response than just show me some calcs. I have had to do it quite a few times before for a host of reasons, so even though you would net a zero transverse moment with weight positioned directly on centerline , you have to consider the effects of the net zero SUM of moments for two equally placed transverse weights (Port/Stbd) further away from the longitudinal centerline. Mass distribution does matter. Other posters have already stated the same. NA's use this practice routinely, especially when there are no adverse stability issues to address upfront.

Y0u want calculations on roll period? I would prefer to cut to the chase and do a quick roll test on the boat, preferable before and after, as I have done on others as my proof. If you want physics, think about adding a concentrated weight directly along the axis of rotation versus adding two masses of the same weight much further from that axis. But how about providing a solution based response instead?

Part of that brilliance is knowing that without having the real world data on that boat I am not so foolish or arrogant as to tell anyone he has ruined his boat, needs to move weight outboard or anything else.

Read my posts, they make it very clear that I believe people who think they can determine the stability of a boat without knowing anything about it are very foolish. And as another member posted, might even create a hazard by posting conclusions about area in which they have very little competence.

Considering you know absolutely nothing about my small boat history that is rather an arrogant and insulting comment. Having owned (and refitted) small boats for most of my life and having written and taught CG approved stability classes for engineers at a marine engineering college I think I know enough to know when to keep silent about conclusions based on a total lack of data.

Why on Earth would you think that? Is it just a throwaway insult or because you lack relevant input to the topic? If you have something to contribute, please do so. Otherwise find someplace or someone else to attack.

Making assumptions without having the data to support them seems to be a consistent problem for you. I prefer to work with data when it comes to stability issues since stability and trim is relevant to safety and economy of operation. Winging it might work for you but as a professional in the industry I prefer to work with facts. Intuition has its place in the art but not the science of of stability and trim.

If I had the data, the hydrostatics of that hull, or performed a series of inclining experiments in the various configurations I would provide a solution. Lacking such data if I told the OP what to do I would just come across as an internet "expert" who makes a lot of claims but can't support them. You are the one who told the OP how to correct what he feels is a problem, I didn't. It is up to you to provide proof of your claim and validate your advice which is, by your own admission, based on a total lack of information about the boat and its stability. Rather than "cut to the chase" I prefer to work through the process and acquire a solid base from which to propose alternatives.

When I read posts that indicate the master of a vessel charged with the safety of his passengers is completely unaware of such a critical component of stability as free surface effect, or when someone who knows absolutely nothing about a specific vessel tells the owner how to manage its stability I am quite happy to point out that information from those posters should be carefully vetted before applying any advice they provide. Stability seems to be a particularly ripe area for the adherents of nautical mythology and pseudo science.

It is preordained. If Rick steps onto a thread, instability follows. By my calculations, the roll period is now zero. That's how much tolerance I have left...

For years, I've defended knowledge. Now, I must side with harmony. Relationships can take years to build, but they can be torn apart in minutes. The same is true with forums when someone creates a hostile, unwelcoming environment. I can't count the number of complaints I've fielded over the years, nor the number of people who feared being chastised by you, Rick.

In the best interest of our community, your membership is being suspended.

This thread is closed.