Lengthened 1990 Hakvoort LRC; opinions please?

Marmot did not confuse anybody that understands what CG is really about, your earlier post stating it moves forward (away from added weight) and your latest post stating it moves back towards the weight confuses everybody.

That poor CG does not know if it is coming or going.

Yes the handling will change but there is a big difference between CG and trim.

I built many commercial fishing vessels and due to the requirements of their licenses which required them to be a measured length from the rudder stock to the forward perpendicular a lot were built with a lot of hull aft of the rudder(s), a registered 10 meter or under fishing vessel could have an overall length of nearly 14 meters. Yes the handling sucked in a following sea but that extra length was required for their gear.

The Uniflite / Chris-Craft 42 Double Cabin MY actually benefited from the 6' Factory Cockpit Extension without changing the Running Gear. The extra planing surface was a plus and the new boat was as a 48' Cockpit Double Cabin MY, just functioned much better.

Then I understand completely. The result will be whatever it needs to be. The measurement is made from one point in one version of the vessel, and from a completely different point in the second version of the same vessel. In an absolute sense, the GG moved aft of where it was, due to weight. Relative to the new transom, it moved forward (if the extension is aft). And you are both "right".

I think this way of counting backwards from a 0-station is inherited from when drawings were with ink. Today when most of us use a computer, it takes one second to move and rename any position or number of positions so you get a proper drawing also after the adjustments. Since we are so far off topic anyway, here are pictures of original drawings I got last week for my oldest boat, designed in 1930, delivered in 1931...

Capt. J - If AMG deserves the reputation he seems to have in the industry, I don't think he needs someone else purporting to know "what he means" when he publishes his thoughts. Please do not ever extend that courtesy to me. Especially when you immediately prove that you do NOT know what AMG is thinking, in light of the fact that he is clearly "not interested in counting backwards" and you are willing to allow that one can account for the extension by "using negative numbers". What as been conclusively demonstrated, by AMG himself, is that he changes the location of his starting point for the two CG calculations.

It is not "SPLITTING HAIRS" to determine whether one's frame of reference is the same as someone else's, and your frame of reference is to "measure the CG from the new transom". Well gee whiz, no wonder the CG moved "forward". If you moved NYC 1 mile east, it is farther from Seattle, but closer to London. So is it farther, or closer?

In the specific case being discussed, the CG moves aft from its original position IF MEASURED FROM THE SAME STARTING POINT, and moves forward IF MEASURED FROM A STARTING POINT UNRELATED TO THE ORIGINAL STARTING POINT. How surprising. Wonder if the starting point influences the conclusion? Hmmm....

Additionally, the nature of the point being argued in this discussion is not one that falls into 95 to 98 % level of confidence. 100% of the time AMG is right, within his system, and 100% of the time, Marmot is right, within his system. The two systems are incompatible for the purposes of labeling the movement of the CG "forward" and "aft", which is fine, since no one agreed (at the top of the discussion) which system was to be used.

Well, since my name has been dragged back into this ...let me remind you of the statement that aroused my interest.

Sorry skipper but all aft extensions move G aft of its original position. The Universal Truth of G applies in every instance.

And as far as trim, some extensions add buoyancy and trim the bow down, some don't. A competent NA will work it out, they know about the Universal Truth of G as well as the effect of B.

There seems to be a lot of trim adjustment going on today but it isn't moving G forward of its original location.

And with regard to the post about which reference point is used, that is irrelevant. The only thing the reference point changes is an arbitrary number that has no meaning as far as stability and trim is concerned. The location of G before the extension is where it is. After the extension it will be further aft of where it was. No matter how you measure it, the physical location of G is a physical fact. You can play with those numbers all you like and get away with it on a little skiff because you can pretend anything you want. You can't do that with a boat that requires a stability book because physics and math cannot be fudged to satisfy some personal need.

A boat that has been retrofitted with a hull extension aft has its G moved aft. Period. That is the point that a few have yet to grasp, and for some reason need to continue to claim otherwise.

When a professional NA who is skilled in trim and stability calculations considers such an extension he will base all stability and trim calculations on the existing references in order to determine the effect of that extension before it is constructed. No NA is going to change the reference point after the fact in order to achieve some preconceived result. It is not semantics or any other manipulation of ideas to state the unalterable incontrovertible fact that the original G moves aft with the addition of an extension to the stern.

Sorry Marmot, but your truism does not apply in this context. Consult a NA...

Hmmmmmm

Thought using binary, that 10 + 10 = 100


Let's check with the Internet:

Binary Calculator

Simple typo, yet I'm absolutely certain that it will negate my primary point completely in the eyes of many.

For those that believe, no explanation is necessary. For those that do not, no explanation will suffice.

I picked binary on purpose, because the actual, absolute, scientific "right" and "wrong" of this is indeed binary. How you fall into one camp or the other depends on whether you are willing, while making two separate measurements, to change your reference point.

And before you or someone else points out the obvious, 10+10=20 in all bases >2 and <11. Which kind of makes my point from the opposite angle - two arguments can both be right, even though the frame of reference is different.

Too funny.


Did you go to school in Sweden?


P.S.- Simple typo or brain fart?


P.S.S.- Just kidding! Just kidding!

So I will offer you this case, as the hull extension will be done at the boatyard.

Instead of the typical multiple boat stands you have successfully balanced the boat dead level to the yard surface by locating the exact LCG and supporting it by a single point capable of withstanding the load.
Now add your hull extension, including everything necessary to outfit this proper yacht. You never moved this single balance point - what happens next? She tilts aft and will probably come to rest on her beautiful new transom bottom edge - right????? Computer or hand drawn, no difference.

We are just talking physical gravity here, F= ma stuff, not hull shapes/volumes/displacements/trim, etc

Simple typo. I am not in the habit of retreating from my published positions, which is why I press so hard when others do.

Not educated in Sweden, but I daresay there are some places to be educated that are worse than Sweden, based on the viewpoints presented in this thread.

Glad you found my post too funny.

Just kidding, just kidding.

I thought I'd give this a try, and I intend to make this very simple. Marmot is correct about the location of the center of gravity after the hull extension being aft of the original center of gravity before the hull extension. I will use the following example to illustrate:

You have a rectangular piece of HY80 steel of uniform density that is 10 meters long. (You may use any material of uniform density you want, but as a former submarine sailor, HY80 is special to me.) You label one end of the length of the rectangle "Bow" and the opposite end "Transom." The center of gravity is located 5 meters from the Bow and 5 meters from the Transom. (If anyone doesn't accept that last statement, then you need to educate yourself on center of gravity.) It's Wednesday night and you don't have anything else to do, so, you decide to add a 1 meter extension of the same uniform density HY80 to the Transom. The after-extension center of gravity is 5.5 meters from the Bow and 5.5 meters from the Transom. This shows that the new center of gravity has moved aft of the original center of gravity, because the original center of was 5 meters from the Bow and the after-extension center of gravity is 5.5 meters from the Bow. It,also, allows for AMG's system of measuring before-and-after-extension centers of gravity from their own transoms. The after-extension center of gravity is in fact farther away from the Transom than the before-extension center of gravity. And, yet the after-extension center of gravity is aft of the before-extension center of gravity. I can't make it any simpler. Center of gravity follows the addition of mass and retreats from the removal of mass. Period.

You are correct in that it is very simple. And both of your conclusions are correct. Which is why both Marmot and AMG are right, within their respective frames of reference. They just have different frames of reference.

This is true to both you and Marmot BUT. If the center of gravity is measured from the transom, and the transom becomes 10' longer, then the measurement of the new COG will still be further foward from the transom (new) in feet and percentage to OAL, than it was before if a 10' cockpit is added. As far as I knew, COG was always measured in distance from the transom, is it not??????

You and Marmot are not right, because this all started with you all responded to my intial post about how the extension moves the COG foward from the new transom and how it effects the handling of the vessel and negatively impacts it's ride. Which was directly anwering the OP's questions about extensions and how it effects him, not where on earth the boatyard is going to block the vessel and all this other arguing about symantics. You also went into this charade with a total thread drift, of which the OP gained nothing from.

It is a bit late in the game to try and change the goal posts, so to speak.

If I measure it from the boatyard gate I might get a really big number but the location of the new CG is still aft of the original CG and the boat on which the extension is attached is still heavier at the stern than before the extension. It will certainly NOT as you very explicitly claimed, become bow heavy because weight added aft moved the CG forward.

It may or may not trim down by the bow. That is another calculation altogether and one that is also requires the use of B and post extension G to determine.

If you had been reading the posts or any of the links provided you would be aware that is most certainly NOT the case. As they say, "the Devil is in the details" to which I might add, "ignore them at your own peril."

Anyone familiar with stability calculations knows that measuring the new CG from the boatyard gate does not change the location of G relative to its initial point.

Anyone who is even fleetingly knowledgeable about stability calculations knows that to determine the effect of any change in weight or buoyancy resulting from events or material changes, the location of those events or changes must be measured from the initial location of G. Anyone who moves the reference point is lost from the start ... a condition that has been made abundantly clear in this thread.

Read this:

With all due respect to another member of the forum, and with as much civility as I can muster ... I call BS to both of those statements. You never once mentioned transom. The transom was never brought into the discussion until someone else felt it was necessary to explain away the fact that it was a number, not the actual CG that moved forward ... and that number is based on a point that didn't even exist previously. As far as calculating changes in stability of the already existing vessel is concerned, that number is meaningless.

Some of the comments here bring to mind an old saying attributed to Abe Lincoln about when silence might be appropriate.

That is good to read, it makes posting this stuff worthwhile and is one of the reasons this forum exists.

Keep in mind that where stability is concerned, and no pun intended, opinions don't hold much water. Stability is a science, it is based on mathematics and physics.

Ask if there was an inclining experiment performed after the extension and what the results were. If you buy that boat and need some assurance that it is stable and safe to operate the way you want to operate it, or if you are willing to operate it within the limitations imposed by the new conditions, have an inclining experiment performed and a stability book developed. Learn how to read it and use it. For a boat that size, the "book" will be pretty small and not very complex.

There are more than a few NAs in San Diego, spend an hour and a very few bucks to meet and ask questions and get real answers. He or she will quickly show you where opinions stop (subjective appraisal of ride qualities) and reality intrudes (roll this far and the boat will keep rolling) and what the terms you have read here mean to you and your boat.

Stability not difficult to learn but, as you have read so far, it is an element of boating that is outside the margins of most small boat operators and even designers. It becomes critical when moving to larger boats that offer a wider range of loading conditions.

J, you continue to miss the boat, although I feel honored that you have determined that Marmot and I are the only two people in existence that understand Newtonian Physics - that sophomore engineering class has now paid off in spades.

Its is funny how the symantics that you describe only apply to everyone else in the universe but yourself, those rose colored glasses must be truly "*****in" in CA speak.....

Let me explain it in another way for you - when you hook up your trailer carrying your ride-on mower to the hitch on your favorite F150 or equivalent, did you just move the center of gravity aft on your pick-up or does it not count because we are not measuring from the back of your trailer rig????

Or we can use your own example again - the ORIGINAL LCG is 20' forward of the ORIGINAL TRANSOM on the Hull without the Extension. Now, the owner wants a 10' cockpit extension. So now the EXTENDED LCG is 25' forward of the EXTENDED TRANSOM, but guess what, refer it back to the ORIGINAL TRANSOM location and it is only 15' forward of the ORIGINAL TRANSOM, which is AFT of the ORIGINAL LCG, how the f@#$k did that happen????

Stop right there and don't get into Flotation/Displacement/Archimedes/Trim, etc., no more but/if/maybe/could have been, just try to grasp it before you speak and muddy the waters once more.