Lengthened 1990 Hakvoort LRC; opinions please?

Wild extensions

I remember a Burger with a cockpit extension. The new hull bottom was raised (shallow draft) and had only enough buoyancy to hold (support) up its own weight. Boot never changed.

Another had a cockpit extension added. The props & rudders were moved back. With less shaft angle she picked up over 7 knots. I'm sure improving the length to beam ratio helped on that speed increase also.

Then the pig; Cockpit added. Required 2000 Gallons to hold her asp down. Rudders were moved back but not the wheels. 70' loa and no thrusters.
When the fabricator realized a problem, they increased the rudder size. 11 feet between the wheels and the large stabilizer fin rudders.

Designed correctly extensions are great. Poor designers use lots of lipstick.

How do you know a new ext will add hull speed? Thinking about swim platform for 62 striker. The boat squats a 5degree angle right now. Is an architect: engineer going to be able to tell?

Hi,

Get it designed and drawn by a NAVAL Architect -Note emphasis on the word NAVAL, it is quite different to someone who would design your house.

Once drawn up they can run it through a CFD Program and see how it will go.

I'm no where close to smart enough to offer the science and math to explain. However, thru my years, I have read, heard and experienced a narrow beam ratio (vs length) is faster.

Were on a 58 x 18 (FAT) Bertram MY. Still can't offer the math but all the cockpit extensions on this model have offered a small increase in speed. When we win the lotto I'll put an extension on.

Your Striker squats at the dock ("right now") or at speed. Every (old) Striker I've run does ride bow high.

Lets open another post if you need more from the forum.

I don't think so ... you might change the center of buoyancy (B) but adding the weight of the (aft) extension can only move the LCG aft. There is no such thing as displaced CG.

There is a change in B and LCB with immersion but it cannot have any effect on G unless there is flooding involved.

In my experience, that only occurs with submarines and sinking ships.

The change in longitudinal center of buoyancy might counteract the shift in LCG so that no change in trim is evident but there is no way in this universe that adding weight at the stern of a boat is going to move the LCG forward.

So you are saying there's no longitudinal CG on a floating object?

Well, it is not only weight, but also an extension, so measured from the new transom, the LCG is further away... forward...

Hi,

If it were at Frame 50 ( 30m from Rudder Post) and you extended the vessel 6m or 10 frames at the stern, would the LCG remain at what is now FR 60, move aft or move fwd?

So, Marmot measures from the bow

(LCG is still same distance away)

and is correct, too.

Hi,

Last time I looked vessels were measured from the stern, frames aft of the rudders have - numbers and those fwd positive ones.

No, I am saying the the longitudinal CG of any object - boat, aircraft, or any other object is unrelated to the medium in which it exists.

If you add weight aft of the existing CG, it will not move that point forward, it will move the LCG toward the added weight.

It does not matter one iota from which point the location of the CG is measured. The statement was made that adding weight to the stern moved the LCG forward and that statement is preposterous.

If that were true, you could keep adding weight on the transom until the stern was completely out of the water!

Anyone is welcome to show how adding weight aft of the CG can move it in the opposite direction ... good luck.

The LCG is a figure stating the distance from the lowest part of the transom to the CG. The statement was that if you extended the hull aft, you get a longer distance to the CG. Relative to the new hull length, the LCG has moved forward and in some boats they are adding weight in the extension to compensate for this, meaning getting the LCG closer to the same percentage from the transom...

Which means the CG is aft of the point at which it started. It doesn't matter how far or how little aft the transom was moved or how you want to measure the distance from the new aft perpendicular, the location of the LCG has moved aft of its original location.

The more weight you add to that extension the further aft that CG moves. It is a fact of physics and mechanics that the CG of any object moves toward added weight. It doesn't matter if it floats, flies, grows, or just sits there.

The LCG can be measured from the center of the vessel, midway between perpendiculars or from the forward or aft perpendicular. Where it is measured from does not change the physical location of the LCG one femtometer.

I think you guys are getting confused with the LCB and it's impact on the center of floatation or the point around which the hull rotates to affect trim changes. If you put enough additional buoyancy on the stern you will lift the stern and immerse the bow but that will not change the fact that the weight of the structure that produces that additional buoyancy will move the LCG AFT of its original location.

Unless the COG is a gearwheel that was not properly secured in the hold and broke loose when the bow trimmed down, that statement is incredibly absurd.

Of course the CG is aft of the point at which it started, but it can still mean the LCG have moved forward. This is why it may need to be moved further aft...

(We are not talking about the Hakvoort now, since it was extended midships)

Oh, for heavens sake, is this a game or something?

LCG is measured from either the midpoint of the hull between perpendiculars, the forward perpendicular or the aft perpendicular.

You sketch shows what I assume to be what you believe the new LCG (in red) will move to when weight is added aft. You show it moved forward of the initial LCG with the addition of weight aft of the original location. Surely you don't really believe that is possible?

If the concept is so difficult, take a pencil and suspend it with a thread at its LCGm add weight to one end and let us know what the end with added weight does.

You are trying to show the new location of the LCF (sometimes thought of as the center of rotation in the longitudinal axis) as being the LCG. A ship does not always rotate about its LCG. This is very apparent in those hulls with sponsons, extensions, or unusual underwater forms.

Isn't there a naval architect on this forum who can explain this?

I am simply amazed at how such a simple concept has befuddled those who are supposed to know a bit about ship stability, loading, and nomenclature.

I tried to explain it as simple as possible, so try again...

Just a cockpit adds a lot of bouyancy without adding a lot of weight compared to the rest of the boat which has a structure above it, per linear foot, so it changes the center of gravity or the balancing point of the vessel and shifts the balance foward, so the boat now rides bow heavy.

When you add a 10' cockpit, all of your real weight is now further foward both in feet and percentage from the stern, and the cockpit is lighter than the bouyancy it creates, so they try to conteract that by adding weight, usually a fuel and a water tank. But as you drain those tanks, you change the balance point of the boat or trim further foward. It's like adding a 18" afterplane to a striker SF, it is a simple plate that is the same angle as the rest of the hull, but makes the boat run a lot flatter and less stern heavy at cruise and adds speed because it just added 18" of planing surface.

Hi,

In an attempt to stave off unnecessary damage to nighties and the loss of a treasured toy when its thrown from the pram in a fit I have found the following article where those who can comprehend what they are reading should be able to answer all their questions some of which may well be owing by a misnaming of some of the essential components of the stability equation.

Longitudinal & Transverse Stability

BTW: The last time I looked the best shape for a wheel was round.

Thank you for illustrating the reasons why this subject is so confusing for some people.

Any amount of weight, forward or astern of the initial CG will move the CG in that direction. It doesn't matter if it is an ounce or a ton.

Adding buouyancy does not change the location of the CG. It only changes the trim. You could easily demonstrate this by fitting an inflatable bag under the transom. The weight of the bag is the same inflated or deflated, The trim of the vessel will change considerably when the bag is inflated. The center of rotation and the center of buoyancy moved but the CG has not moved one nanometer. "Bow heavy" is a misnomer in this case, it is based on the boat driver not understanding what floats his boat. You cannot change the location of the CG unless you add or remove weight in which case the location moves toward added weight and away from reduced weight which explains :

As weight is removed by draining fuel from the aft tanks, the CG moves forward. As weight is removed by burning fuel in the aft tanks, the buoyancy of the afterbody actually decreases as its immersion (draft) decreases. The combination of those two factors is what causes the boat to trim by the bow. Depending on the design of the stern and the capacity of the fuel tanks it is possible that with empty tanks, the stern could be completely out of the water. In that case the buoyancy negating weight argument just sunk in deep water.

Is real weight different in your world of stability and trim? When you add a 1" cockpit you add "real weight" aft of the initial CG. Like I wrote earlier, if your version were correct, you could raise the stern out of the water by adding more weight in the cockpit.

Buoyancy is a whole different animal than weight and CG. The only time the two are related are when the boat is sinking or a submarine is submerging.

Speaking of submarines. A submersible operating in mid water may be ballasted so that it is neutrally buoyant, that may be described by some as "weightless" but I can assure you that that submersible still has mass and the center of that mass is its CG. That submersible can be trimmed forward or aft by moving the CG forward or aft without changing buoyancy by moving ballast forward or aft. It can accomplish the same thing by inflating an external "bag" with air at one end or the other. That doesn't change the CG but it changes the center of buoyancy and the trim.

There is another whole world of difference between static and dynamic trim. Do you also believe that trim tabs move the LCG?

I think a few members might benefit from a good course on intact ship stability. Find a good stability book and study the thing ... look at the tank plan in particular and play with the numbers.

Sorry, but this time the only one confused seems to be you...