Calculating ballast for a motorsailer

I am seriously loathed to wade into this endless pointless debate with a poster that doesn't understand the basic of what they are discussing. However, fro the sake of argument.

100% incorrect. G is the CoG. it cannot be any more simple.

You are, as always, conflating several issues and assuming them to be one and the same, they are not. Any rigid body that rotates in a simple cyclic manner does so about a centre. This centre is its centre of gravity. If it did not it would not be in equilibrium and merely spin out of control. It is simplified into a 2D plane since motions are terribly complex with all 6 degree of freedom. There is not such thing as pure roll or pure trim. Because the volume and WPA changes.

A vessel is considered to be a rigid body. Thus upon rotating in a cyclic manner it rotates about its CoG.

Place this rigid body, or vessel, into water. If one were to produce a means to rotate the vessel, by an external force, it would rotate. But this time it would not rotate continuously as it would do in air. The rotation about the CoG remains, but now, there is a force opposing this rotation, called the restoring force. This restoring force is created by the buoyancy of the floating object, in this case the volume occupied by the rigid body or vessel. Thus if the vessel is disturbed from its initial equilibrium position it returns back to this same position. Given certain conditions which i will ignore for the sake of brevity!

So, when a vessel is disturbed and moves it rotates about its CoG. But and this is the important part, its response when floating has a restoring force imparted to it that has its centre of this restoring force located in various positions depending upon the nature of the motion and shape of the underwater volume.

But it does not change the centre of rotation of a rigid body only its manner of response. A rigid body is essentially a lumped mass and always rotates about its CoG.

A floating body has volume and is not related to its CoG. Its stability is measured by the immersed volume and waterplane area, functions of the shape, not the CoG.

The volume and shape of waterplane area is referring to a condition of stability and a means of measuring it. That is all. Thus it is “customary” to consider the rotation about the LCF or M, for trim and heel respectively. But only as a means to calculate the restoring force and its ability to provide a measure of a vessel being stable or unstable. Thus the centre of the restoring force it is not the centre of the rotation of the vessel. Only the centre of the rotation of the restoring force!

That is why in post #11 the BM indicates a rotation and the reaction - the buoyancy force - at an inclined angle acting trough an assumed location, M, the metacentre. This is the restoring force only, nothing else.

To summarise, a rigid body with a fixed CoG, that has its CoG below the location of the metacentre noted at small angles of inclination, M, (it returns to upright) but, it will behave differently to one that has its CoG at M (it remains stationary at the angle it resides at). and again to one that has its CoG above M (it will keep heeling and capsize – unless at loll). In each case the CoG remains the same, but the response is very different. The rotation is a measure of where the centre of this restoring force acts on the rigid body and thus creates the righting moment.

 

You bore me, Ad Hoc.
From "Principles of Naval Architecture"(SNAME)


From "Introduction to Naval Architecture" by E.C. Tupper, BSc, CEng., RCNC, FRINA, WhSch

End, for me, of the current discussion.

 

And again..no comprehension of the the difference between centre of flotation and centre of gravity. Shocking ignorance, but as always the usual trolling to divert and misdirect.
Just a waste of bandwidth....as always.

Not surprising when you don't even know what the difference is between KM and GM.
Your ignorance on the subjects at hand is there for everyone to read on every post you make. But hey ho...that's your prerogative. Your ego always gets the better of you....

 

Annode, do you have any photos or drawings of the vessel(s) that you are considering for conversion into a motor sailer?
If you can obtain the Stability Booklet for the vessel that would help a lot.
(I am assuming that it would have been a requirement for the vessel to have had a Stability Booklet when she was a commercial fishing vessel).

An older, more traditional type of hull shape (for example like the double ended timber trawlers in Britain - and especially the Zulus and Fifies) would be more suitable for conversion than one of the 'rule beater' type of trawlers that tend to be very beamy and deep draft (ie very boxy) to get the maximum space within a given length.

 

What a waste of everyone's time. I'm not a naval architect, I'm a mechanic engineer with a 3 digit iq, but it is tragically obvious that Ad Hoc is a larping phony. Less of a Naval Architect than I am. If a floating object rotated about centre of gravity, metacentric height would be zero. Righting moment would also be zero.
Consider vthe case of a high ballast ratio object. CogG well below the water line. Object heals, rotating about the CofG. Object now completely submerged.

Even the article Ad Hoc quotes states motion of CofG, not motions about CofG.

And the infantile obnoxious behavior! Honestly, I'm genuinely embarrassed.

Folks please, remember we are friends, hoping to advance our Noble art.

I just opened the account because I have a serious question, and I'm deeply saddened that my first post is diminished to this extent.

Dave Gerr, are you out there?

 

DC, please do ask your serious question - it might be better though to start a new thread.
And we shall all see if we can offer any suggestions re an answer.

 

Excellent. So you can answer the following simple questions. to enlighten everyone:

Interesting.
Can you explain this statement please.

Can you clarify what you mean by high ballast ratio..... and.... how this relates to a waterline?

Sounds a sensible proposal.

 

That is gracious suggestion, Bajansailor. I will do so.

 

Bajansailor, TANSL, I have done so.

Bajansailor, TANSL, I have done so. Ad Hoc, this thread was started but Annode because they had a question. I won’t waste any more of his time with your infantile pissing match.

 

Gentlemen pleeeeeeeeeeease.... sheesh....

This reads more like a political debate than science.

This is not rocket science. I have a masters in engineering too, so lets stick to the engineering and subtract the snark and egos.

Eddison tried to convince the world that AC was dangerous because he held the patents to DC by electrocuting animals
Jan. 4, 1903: Edison Fries an Elephant to Prove His Point https://www.wired.com/2008/01/dayintech-0104/
He was an inventor and an engineer that could not embrace a better idea ego?

Salieri hated Mozart.. ego?
Antonio Salieri - Wikipedia https://en.wikipedia.org/wiki/Antonio_Salieri#Relationship_with_Mozart

What I love about science is its objectively provable and repeatable. I am trying to understand the basic engineering principals of boats. I accept that there may be differing truths that approximate a floating bodies motion with 6 degrees of freedom and that performing an integration along the length of a curved surface is the only accurate way to calculate the force exerted by the water moment by moment, but I am looking for a reasonable approximation.

So just pretend I am a 4yo, and lay it out step by step like this:

Nice. Thank you.

Guys I am just a simple 4yo. Please define any letters used in these explanations fully!

Thank you for the .pdf. BM is Boat moment force?

G? M? all these letters need a full discussion so we can all get on the same page.
Pretty please with sugar on top )

>Annode, do you have any photos or drawings of the vessel(s) that you are considering for conversion into a motor sailer?
Just picture a commercial trawler with a canoe stern, made of steel about 80 to 100ft long and with a draft of about 8ft. A blue water motor boat. Not a fully rounded hull for sailing. It will never be efficient, but then neither were the wooden boats of old and they plied the trade routes for centuries.

 

Hello Annode,
Re your questions above, the BM is the vertical distance from the Centre of Buoyancy to the Metacentre.
If the Metacentric height above the keel is KM, then KM = KB + BM.

The GM is the distance from the Centre of Gravity to the Metacentre. It is a measure of the stability of the vessel. The larger the GM, the more stiff / stable it is basically.
We also have KM = KG + GM.

Re your trawler conversion, a canoe stern hull 80 - 100' long with 8' draft should still sail reasonably well I reckon with a decent rig on it.
This would be like a bigger version of a Zulu, but in steel. With 8' of draft, if you do need any extra ballast the odds are that some internal ballast might be sufficient.
Some more info about the Zulus here -
Scottish east coast fishery - Wikipedia https://en.wikipedia.org/wiki/Scottish_east_coast_fishery

 

I am still trying to understand how to calculate heel and thus ballast required to prevent capsize...

 

To calculate that meaningfully, you’d need the exact hull form, to calculate centre of buoyancy at every angle of heel. Then you’d need an exact location for centre of gravity to calculate how far the centre of buoyancy has moved in the direction of heel, so that you can then calculate how much righting moment there is, at any angle of heel. This is enormously difficult for an existing boat. Particularly when you want the answer “how much lead and how low.” You would actually get useful numbers by studying existing conversions, and comparable motorsailors. Really, what you want is the percentage of displacement as ballast. Then how low is; as low as possible, consistent with structural considerations.

 

Aaaah OK. Thank you for that explanation. Makes a lot of sense.
I was considering the structural/ballast implications of two mini keels like this:

The added benefit is the ability to dry out the boat and work on the hull without a haul out.
So the question becomes, how to determine the CofG vertically and longitudinally and approximate the Center of Buoyancy for different angles of heel. Given this kind of hull was designed for stability when craning 7tons of nets over the side, my guess is that it has a lot of heel stability (resistance) for small angles at the sacrifice of resistance at larger angles.
Also have not a guestimate as to the amount of heel that is reasonable for this kind of compromise hull

 

The maximum heel will be determined either by the loss of stability ( it will capsize), or flooding angle ( there are openings that will allow water into the boat); whichever is less.