Lengthened 1990 Hakvoort LRC; opinions please?

Longitudinal Center of Gravity (LCG)

or

Longitudinal Center of Buoyancy (LCB)


Skip to the bottom of link (Longitudinal Stability):

Longitudinal & Transverse Stability

It depends if the extension is watertight or not...

This was offered in post #38. Apparently not read by everyone.

Thank you for explaining your understanding of stability and trim.

Let's go back to the grade school basics.

Every object has mass which we measure as weight. Every object has a point where, if we were to attach a lifting device, it could rotate freely in every direction without any force to restrain it. That point is called the center of gravity. When we talk about boats we refer to that point as G.

Boats are more or less free to move in 3 dimensions but for them to be of practical use, we like to keep them upright. In order to help those who design and operate those boats do so safely and for captains to know the limits of loading and movement before something awful happens, we describe G by two distinct terms. Not because they are different things but in order to avoid confusion.

Because the major and most critical motion of a boat is in the roll or longitudinal axis, along the line between the bow and stern, we concentrate most heavily on that axis. Since we like to keep the boat upright as much as possible, we try to make sure that there is some force available to rotate it back to level. The "axle" on which the boat rotates is the vertical position of that line and we call it the vertical center of gravity or just G for simplicity.

We could call it quits right there because G is G, no matter how else anyone might want to describe it, there is only one center of gravity on any object.

But, because a boat also rotates along its lateral or pitch axis, we like to have a figure for that point so we can keep the bow and stern more or less where we want for best performance. That point is exactly the same as where G is located but for calculating stuff that captains need to know, we measure its position from a fixed and permanent point along the line between the bow and stern. It might be a bulkhead at the bow or stern, or it might be halfway between the two. It really doesn't matter because G is G is G.

Any changes to weight added or removed will move G. If we add weight, G moves toward the added weight. If we remove weight, G moves away from the location where weight was removed.

For transverse stability calculations (roll) we spend a great deal of time and attention looking at where G is as far as its height above the keel or away from the center of the beam. If G is too high, the boat can capsize, if it is too low the passengers get sick and the crew looks for another boat. If G is too far off to one side walking on deck is difficult.

We spend a lot less time looking at where G lies along the line between the bow and stern because it isn't quite as critical. But, just like there is a "righting force" created when a boat rolls and the center of buoyancy B moves toward the center of the submerged volume to create a "righting arm" that forces the boat back to level, the same thing happens in the pitch axis.

B or the center of buoyancy is the geometric center of the underwater part of the hull. It moves around a lot in bad weather but G never moves unless weight is added or removed.

How does all this apply in the case of a hull extension at the stern? Very simply, the material used to fabricate the extension adds weight (no matter how little or how great) aft of G, which as we all know now is the center of gravity both vertical and longitudinal. VCG and LCG are the same place in the universe. They can be measured differently and illustrated by different numbers but they are always exactly in the same place.

When calculating the effect of changing weight, it is necessary to know the location of the CG of that weight. If we place a helicopter on the upper deck we measure the distance of the helicopter's CG from the vertical and longitudinal CG to determine the arm, the amount of force that adding the helicopter's weight applies to the hull to produce a change in roll or pitch.

When a naval architect considers an aft hull extension, he will (or should) calculate the weight and CG of the extension as a separate component, just like that helicopter. Then, when the extension is attached and the boat re-launched, everyone involved will know how it will sit on its lines. Since weight and buoyancy has been added aft, a new stability book will be probably be required if one already exists because the LCG has moved aft and the VCG has probably moved downward.

Trim may have changed due to a longer arm between G and the new center of buoyancy B and trim may have to be adjusted to compensate.

You cannot have a vertical CG in one place and a longitudinal CG in another. That violates every law of the universe. You and I cannot be in two places at once and neither can G.

G - VCG and LCG moves toward added weight and away from removed weight. Period.

Adding an extension aft of G will move G aft, and in the case of a cockpit probably lower. Period.

Could it be that the Center of Gravity has moved forward...

Not unless the extension was forward of G.

G moves toward added weight. That is a universal truth. Challenge that if you like, but it is a fool's errand.

You keep saying this, and there is a tad of truth in it since the LCG moves a little less forward depending of the weight of the extension. But let´s say the LCG was in the longitudinal center of the original hull and you want it to be in the longitudinal center of the extended hull, you probably will find that the LCG must come back, as it by magic is forward of the longitudinal center after the extension.

Refer to the Universal Truth.

Believe whatever you like or need to but there is only one reality. G is where it is and it follows the Universal Truth.

Banging your head

Marmot,

Do you think others on here have the capability of reading links or doing there own google search, they don't need to listen to what you are saying.

The same search that you may have read I did also, very simple G moves towards added weight and G moves away from removed weight.

Banging your head into a wall or p---ing into the wind springs to mind when reading the replies trying to justify there stance.

I hoped that other members would research this for themselves and would have chimed in but with only very few exceptions that has not been the case so I respond to what I see posted and to questions that are addressed to me.

I assume that the reason people are reading this thread is because they are interested in its content.

I am fully aware that I am p***g into the wind but I believe in letting others defend their position even when it is scientifically unfounded. If nothing else it lets others see that "all that glistens is not gold" and it can be an entertaining diversion from more demanding tasks while at the computer.

Isn't that why nearly everyone is here?

Cg

I had looked up various web sites to confirm that after 45 years of sailing around on those big steel boxes and plastic tubs I was not senile after all. Then reading the replies to your postings I even thought that I must have lost it all together because how could a Captain and yacht designer be so wrong.

So this morning I spoke to the naval architect who designed the extension on a very famous Feadship (colored green).

I asked him first off where or what did he use as a reference point, his answer was the rudder stock.

I then asked him how does he reference the stations, his answer was rudder stock would be "0" and then each frame would be 1,2,3 etc going forward.

I then asked him would he or did he change the reference point "0" when the extension was added or planned, his answer was NO. "0" is "0" and the forward stations would keep the same numbers and the new stations in the extension would be -1, -2, -3 etc, (minus one, minus two, minus three etc).

I then asked him which direction would the center of gravity go from its original point without an extension to a point after the extension.

Sorry Marmot, you are correct, lol.

His answer was, the center of gravity would go in the direction of added weight, low and behold the CG would go aft towards the added weight of the extension.

Thankfully he proved to me that I am not senile nor loosing it and also proved that some on here should stick to driving them and others should maybe look at how certain European boats are designed.

PS. that certain green boat did not have any kind of tanks added in its extension.

You asked the wrong questions if you wanted to know if the LCG was forward or aft of the desired position after the extension. If he prefer to call the rudderstock 0, it is just a matter of presentation, he must still include the extension. This is one of the most silly discussions I have ever seen...

Silly

Talking of "silly", I have no idea why you can not accept the views of experienced marine personnel or the opinion of a very highly regarded naval architect.

Go look up the information that various members have listed links too, CG moves towards weight added, is this "Lost in Translation"

When you add a cockpit stern extension to an existing hull, the center of gravity does move aft. But since the cockpit is lighter per linear foot than the rest of the existing vessel, it doesn't move far enough aft for the boat to perform properly or ride properly, because it creates a lot of stern bouyancy for the weight of extension added.

When you then look at the overall length of the new hull (extension and old hull), the LCG is much further foward from the new stern, than it was on the old hull, when measured from the new transom. This is then counterbalanced by adding a fuel tank(s) and water tank(s). But then as you fill or drain those tanks, it shifts the LCG all over the place. I'll give an example.

I ran a 65' Hatteras that was extended to a 75' Hatteras which I will use as an example. The 65' Hatteras had 2 fuel tanks a 525 (called foward tank), and a 650 gallon (called midship), these 2 tanks were in-between the center stringers midships in the boat, the foward tank butted up to the midships tank. Adding or draining fuel to these tanks did very little to the CG or LCG and therefore ride/trim of the vessel because they were right around the natural or dry CG of the vessel with no fuel in it. It had a 350 gallon water tank that was much further aft. The generators only could draw fuel from the center tank.

They added a 10' cockpit, and in that cockpit they added a 350 gallon water tank that gravity fed to the factory 350 gallon water tank (meaning the cockpit one drained first), and they added a 650 gallon fuel tank that went from stbd hull side to port hullside to add enough weight to counteract the positive bouyancy the cockpit extension added.

Here lies the problem....When they do an extension, they don't move any of the origional tanks around in the hull so they're all in the wrong spot fore and aft compared to the new CG so they have a much larger effect as you fill or mostly drain them.........You have 2 engines sucking fuel, from either 1 or 2 tanks. If you ran them off the foward and center tank (they're no longer midships anymore because the hull is 10' longer), the boat would get more and more stern heavy and eventually really start to lose speed. If you drained the cockpit solely, the boat would get a little faster but really bow heavy and push all over the place and the boat wouldn't track straight. Or you drained cockpit and bow.......No Matter how you did it, it was a VERY FINE balancing act to keep the boat trimmed right, so that it both handled safely and comfortably and you didn't also lose 20 % in speed. The difference of 30 gallons either bow or cockpit meant when the weight got too far off was 1000rpms 8.5 knots or 10.3 knots, cruise 15.8 knots or 17.2 knots......you could then figure out what way the weight needed to be shifted and literally valve the other engine that way (both supply and return) and in 30 minutes of running pick the speed up again. Almost all yachts with cockpit extensions suffer this fate. I've run at least 50 extended yachts, whether a factory extension but mostly an aftermarket one. Mostly Hatteras' and Browards, but a few others mixed in.

I never said the NA gave the wrong answers, I said you were asking the wrong questions. But I can understand if Marmots play with semantics have got you confused...

Marmot has taken this thread way off base from the thead and my post was in line with lengthened yachts and what you experience with lengthened yachts including the one the OP is asking about. My post initially went on to descibe what an owner would see/feel underway while owning an extended yacht and now we have 5 pages of nonsense and technical stuff that most people including the OP won't understand all playing on symantic BS. He can sit there and argue different formula's and this and that, but at the end of the day it's not going to change the way a stern extension negatively effects handling.

One thing I must add, the extension on the 65'/75' Hatteras I mentioned really made that boat a yacht and added an incredible amount of functionality to it. They gained a larger FB deck with area for chairs/lounge chairs, a built in full size propane grill, grilling station- cutting board, sink, refrigerator, icemaker. They then had a real aft deck seating area with large freezer and sink, and a cockpit as well as a lot larger fuel and water capacity.

No sir, you have never addressed my specific question, so I'll rephrase it and ask again: In post 60, you state "OK, but your station '0' does not exist in the extended boat...". I'm asking why a previously identified reference datum no longer exists? Where could it have gone?

You can keep it and count backwards if you like, but as I said, the extension is now part of the boat and should be included when you do the math.

And that doesn't sound like an answer, but like a dodge, much like the "I'm not saying the NA gave the wrong answers, but that you asked the wrong questions", and much like the very post that I'm asking about, given that it started with the word "OK", as if you agree with the previous point, but then had to explain why it had to be wrong.

Let's be direct about this one very small, very specific part of your position. You stated that Station 0 no longer exists in an extended hull. I asked where it went. You won't specify where it is, but tell me I can keep it. I would argue that to hold on to it I need to know where it went, so I would indeed like to know. Where is it? It either exists or it does not.

If it exists, and you no longer use it as the reference datum for the calculation of the new (post extension) CG, your math will be flawed just as much as if the extension were left out of the calculation, simply in a different way. If it exists, and you relabel it to support your argument, then no one will successfully argue that you are wrong, because you won't be, under your system, even though it is not the system everyone else is using. (This is what I believe has actually happened). Finally, if Station 0 actually does not exist (we extended the boat, not shortened it), something has confounded the law of conservation of matter.

So, are you now backing away from your earlier position that Station 0 no longer exists in an extended hull?

I can understand someone saying "10 +10 = 20, that is a universal truth". I can also understand someone following that up with "No, 10 + 10 is 200", and really believing that they have stated a universal truth, and they have, as long as we understand that the first speaker is working in base 10, the second in base 2. I'm simply trying to identify whether you are working in a different base than Marmot, which would be the case if you are arguing that pre-extension CG is calculated from one reference datum and the post-extension CG needs to be calculated from a different reference datum.

What he means is it's irrelivant because Station O has no effect on what it used to be. The point of relevance is no longer the same point of relevance as it used to be. You could on the extended hull relabel the new transom station 0, or the origional station O as O and the extension as negative #'s. Either way the new hull with extension is the new hull in entirety......You measure the CG from the new transom.

Let me phrase in really layman terms so all of you people can understand it. In 95-98% of all cases, a hull extension is going to impact and DEGRADE the ride of the vessel and it's seakeeping abilities. Move the propellors and rudders 5-10' foward on an un-extended hull and tell me what that does to the ride and handling.

I really wish everyone would stop SPLITTING HAIRS.

This discussion reminds of an intellectual melt-down. I said that you can keep it, but I am not interested in counting backwards. I start from the new transom as this is much easier to use as a reference point in the new hull. And the idea that Marmot is right because the LCG is closer to your station 0 after the extension is just nonsense. The real boat has a real extension and the real LCG is now further away from the new transom than it was from the original transom. It has moved forward and on that all can agree, even you and Marmot.