Lengthened 1990 Hakvoort LRC; opinions please?

There is quite a bit of word play going on but none of it comes from me.

The movement of G in response to movement of weights is not a belief, it is an unassailable fact of physics.

Apparently, many members do not understand that extending a hull aft adds weight to the hull aft of its original CG. That moves the CG aft.

Buoyancy has nothing to do with the location of G unless, as I repeat, flooding is involved.

Here is a start, based on vessels most closely approximating yachts.

http://www.imo.org/blast/blastDataHelper.asp?data_id=27389&filename=A267(VIII).pdf

Just because you choose to measure the location of the CG from the new transom location does not mean the CG moves forward, it only means the number of measurement units from the new transom to the new (but still further aft) CG has increased.

CG is what and where it is, changing the point of reference does not change the physical location of the CG on that vessel, it only changes the number you choose to use to describe it.

There is a reason why alterations are based on a measurement from a point or perpendicular that has not changed. LCG is measured by builders, class societies, maritime authorities, and navies by either the forward perpendicular, the midpoint between perpendiculars, or the aft perpendicular.

When a competent designer moves a perpendicular he will use the other perpendicular or the original midpoint because the distance from the new one is meaningless when compared to the previous measurement from that point.

For purposes of stability and trim you must calculate the CG of the extension itself and use that as the basis for the location of the new CG, exactly as you would for any other trim or stability calculation. I understand that small boat operators or designers don't have a requirement to produce or reference a stability book but that doesn't change the fact that the physics are the same for a 300m box boat as they are for a 15m runabout.

Your sketches illustrate that fact very clearly.

Since this discussion revolved around a cockpit extension on an existing hull, I would keep my stations in their original position on my Lines Plan Drawing (aft perpendicular is station '0' in this case), and number the stations for the cockpit extension accordingly (minus).

I would not go to the trouble of renumbering them as a weight study would have already been done with the existing stations and revising the weight study would save time with this approach, especially for a custom one-off exercise.

Respectful question

I'm not interested in dusting off my undergraduate minor in physics to take sides here. However, this last post makes it clear to me that you guys may be talking past one another, and not really disagreeing, due to the lack of an agreed-upon reference datum.

So with enormous respect for the both of you, I ask AMG: Why would the station that was previously labeled as "0" no longer exist in the extended boat? You took no matter out, the molecules making up the material at station 0 are still there, and they were not transferred to another form. Station 0 cannot have ceased to exist, and after reading your posts for the past 8 months or so I don't think you believe otherwise.

I therefore must conclude (correct me if I am wrong) that by "but your station '0' does not exist in the extended boat" you mean that it can no longer be called station 0. If that is the case, then the reference datum has obviously changed. I submit (again respectfully) that if the reference datum changes during the experiment, then the pre-experiment results and the post experiment results are no longer directly comparable.

Hence you guys may both be right, within the framework through which you are viewing your respective presentations.

My question is: why can you not continue to use the previously labeled Station 0 as the datum?

Yep, and if the number from the LCG to the transom has increased more than the number to the bow, we call it a shift forward...

This adds to my sense that you guys are not actually disagreeing, when the reference point (or definition, perhaps) is taken into account.

You can call it whatever you like, the fact remains that the CG is further aft of the bow than before the extension was added.

Adding an extension aft does not make a boat "bow heavy" and it does not move the CG forward.

I am in total disagreement with anyone who thinks, claims, or believes that adding weight aft will move the CG forward.

And if it is even further from the transom than before the extension, the LCG has moved forward. Which is what Capt J said and you questioned.

The LCG must of course have a relation to the complete hull after an extension, in mm, inches or percent, and in our example the extension is empty, why the LCG moves forward. Fill it with lead and it certainly moves aft.

Ah, I see now. Your extension is weightless. Silly me, I just don't keep up with the new building materials like I used to.

I guess it is weightless on paper, the problem is we are, or were, talking about real boats built of real materials that in this universe have weight.

And in this universe, contrary to what so many want so much to believe, G moves toward added weight. Period.

The CG may very well be further aft of the bow of the ORIGIONAL HULL. BUT, you now have a new hull that is longer, so now the CG in percentage of length from the transom is going to be further foward. The old hull is now a distant history.

If you have a 65' Hatteras MY and the origional LCG is 20' foward of the stern. You now extend the vessel to 75' with a 10' cockpit extension, the center of gravity might very well be 27' foward of the stern (as you're not adding much weight with just a cockpit), so the CG has now moved foward when you consider the new Overall length and to be right it would be around 22' foward of the stern to equal the LCG balance it started with. You have to then try to add weight to the stern.....usually with fuel and water tanks (but you lose that weight as you use them and run the boat, and the LCG moves foward as you drain fuel and water from that area of the stern.) to try to get back to the same LCG as where you started from in percentage of length......Usually a manufacturer tries to keep the fuel tanks on most yachts 100'<, where they will change COG the least when drained and not effect ride as much as possible. This is not the case with the extension as you either have too much fuel back there and too much water, or too little depending on where you're draining fuel/water from. Also since the tanks are so far stern, they create a huge swing in the the trim of the vessel foward or aft.

Buoyancy - the power of a liquid to make someone or something float.

Buoyancy = Upward Pressure

Weight = Downward Pressure





So, what happens when a 10' extension is added?

Do I continue to enjoy lunch or

Do I continue to enjoy lunch, with a better view - bow lower?

If you added a 10' extension to the stern, left the fat guy in the same place, the boat would then be level and even slightly bow heavy

It's quite simple really.

If that 10 foot extension is added behind the existing transom, the CG will move aft due to the added weight of the extension. The CB will move aft as well which will create an arm that will trim the bow down and the diner will have a better view forward. The increased buoyancy will also reduce the draft of the entire vessel, and if designed by a competent NA, the vessel will sit level on its marks.

The bow could not possibly become heavier since the CG has moved aft. It will be trimmed down if the length of the arm between CB and CG has increased but the bow (all on its lonesome) can not gain weight or be trimmed down by adding weight aft.

Added buoyancy aft can trim the bow down in relation to the stern but it cannot add weight to the bow. Adding an extension to the hull to obtain buoyancy aft also adds weight aft. Read the following"

CG moves toward added weight. Period.

Of course, but since you added length also, the COG would be further foward from the transom (measured in distance or percentage to the new transom) on the overall length of the new hull. That is precisely what you're failing to understand.

Longitudinal Center of Gravity = That point at which the combined weight of all the items that constitute a ship's weight are considered to be concentrated; usually stated as either aft or forward of the middle perpendicular or the midship frame.

With an aft extension, the midship is moved aft, most likely more than the LCG, why the LCG is moved forward in relation to the midship. End of story.

I guess you completely missed the points PacBlue made.

Changing your personal reference point doesn't move G. That can only happen in the world of 2 dimensional paper boats.

The discussion is about real boats floating in real water with real material added to the real stern.

Real NAs use existing reference points when they calculate the effects of weight changes on CG. They do not move the point of reference to defend an indefensible position.

G moves toward added weight. Period.

P---ing into the wind

Marmot,

You are forgetting about the fat kids, burn a few gallons of fuel and add one, burn a few more and add another but then where are they moving from.

You could always do as one client did and add a hot tub in the newly installed cockpit, fill it up and if needed add a fat kid to the water to increase the weight or a skinny kid to decrease the weight.

Use the angle of the dangle from the aft perpendicular which varies by who you are talking too, transom, centerline of the rudder stock or the forward edge of the rudder stock then measure to the forward perpendicular which again moves around if you are on fresh or sea water or if there are fat or skinny kids on the bow.

PS. I will stick to sailing around on a boat or ship that you have designed as I can not find many of those great boats Carl talked about to check out the stability.

Thanks YJ,

I don't design boats but if I did they wouldn't violate any of the laws of physics that govern this universe.

Right, but you need to buy a book and learn about LCG before you start...

AMG - any chance I can get an answer to the question I posed to you in post #63?

I think I have given the answer many times by now, the extended hull is what should be considered after the extension. And the center of gravity in the extended hull, is forward of the center of gravity in the non extended hull, by all means of measurements. Simply because the added part is lighter than the rest of the hull. This is why Capt J elaborated on how some load it with fuel tanks that makes the LCG move back and unfortunately fwd again as the fuel is used.

How anybody can see this as against the laws of physics is beyond me...