Weight distribution on a 26 classic

[walt]

I posted the quote in sticky thread about the differences between the 26 foot water ballast boats. I will repost it here for discussion since that other thread is getting screwed up for its original intention. This has just a little to do with where you place bed but might be more interesting for how weight distribution in general affects the performance of this type of sailboat.

Quote from the sticky thread below
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I like the 26C rear berth myself. In the last few years of owning a 26S, I have been a little more focused on making the boat a better sailboat. These boats are light weight with rigs to match so keeping them overall light makes them faster, The weight distribution also likely has some influence and the models I go by for weight distribution are:

Model 1. CG should be in the middle of the boat for least horizontal acceleration of mass due to waves

Model 2. CG should be near the widest point of the boat. The ballast has its longest moment arm with the boats center of buoyancy when heeled at this point - giving the best bang of righting moment per weight

Model 3. When the hull moves through the water, it generates lift - even a displacement hull and if this lift is generated efficiently, it reduces drag of the hull. An efficient air foil shape creates its maximum lift in the forward part of the foil. The best place for the boats CG is near this most efficient place for the hull to generate lift.

Model 4. You want a smooth release of water from the aft end of the boat and maximum water line length so the transition from the bottom of the boat to the horizontal transom should be just above water line.

For performance, all of the above point to placing weight near the center of the boat or slightly forward. Beds tend to be "light and fluffy" so for a light weight boat like the 26 classics, are best to the rear and heavier stuff placed in the center of the boat or slightly forward. The rear berth of the 26S is best for weight distribution if the bed is kept relatively light weight (easy to do). .

And.. one thing that may really not be important except for piece of mind.. On these ungrounded mast boats, if lightning strikes the mast, it will 100% of the time arc from the bottom of the mast to the nearest water surface. Be careful about putting your head right under the bottom of the mast if you intend to sleep on the boat in an area where you may get lightning activity.

 

[BudGates]

Walt, I would agree with points # 2,3,and 4 but not #1 or your conclusion. Each one of your statements deals only the center of gravity. Although the CG is very important to keeping the boat level, it has little to do with angular rotation. The other issue that you need to be concerned with is the “moment of inertia.” Consider the cartoon below:

Both images have the same weight, the same mass, and the same center of gravity. The picture with the weights at the ends of the bar, however, has a much higher moment of inertia and thus takes more torque to rotate the weights (in any direction other than linear to the bar). When you add this to the boat’s natural buoyancy which tries to level out the boat once it pitches and rolls the boat will rock back and forth. The location of the weight will determine the natural frequency that the boat rocks. Moving the weight toward the center will increase the frequency. Moving the weights away from the center will slow the frequency down. This rocking will be pronounced when the wave frequency matches the boat’s natural frequency. The only way to avoid this is to either change the boat’s natural frequency (by adjusting the weight) or change the wave frequency (by change direction).

 

[walt]

what you said... is what I meant to say.. I agree..

 

[BudGates]

what you said... is what I meant to say.. I agree..

Okay, so you're saying that a higher frequency would be preferred for pitching and rolling? Conventional practice tends to support that so it's probably true. I don't get to the water often enough to make any judgment call myself.

How much moveable weight do you actually have? I find that after storing my anchors on the bow and enough things in the V-berth to counter the weight of the outboard and anyone in the cockpit there's not that much moveable weight left. The battery, head, and fresh water are already pretty much centralized from the factory.

 

[walt]

My two really heavy things (outboard and batteries) are at the worst possible places.. The outboard is at the very back end of the boat and the two golf cart batteries are at the very front of the boat. I put the batteries as far forward as possible from hearing on the internet that this is best for speed a long time ago and now Im not sure that is true. They may be a little better moved somewhat back towards the mast. However.. the weight of the batteries is in a good position for trailer tongue weight and with them forward, it leaves some room in that compartment under the Vberth for water (in one gallon jugs - I dont have permanent water storage on the boat).

One thing I like about the 26 classics is the motor transom. The weight of the outboard is not as far back as you have with a conventional motor mount and when the outboard is rotated up out of the water, the heavy part (motor) ends up even further forward. This has a down side when your in a slip.. the outboard prop end sticks out making the boat longer and you may have to pay $$ for this if you want to keep the prop out of the water.

Movable weight.. except for my home made ice chest (across from the head), everything is in plastic bins so movable. I think I got this idea from you but my Vberth area just holds a bunch of plastic bins (plastic bins dont weigh much). One bin holds dining stuff, one bin holds a Mr Buddy heater, etc.. I have one big plastic bin that defiantly gets moved day to night - it sits on the aft bed during the day, gets moved somewhere else at night.

FYI, with my weight distribution where I have most heavier stuff forward except the outboard, the drain in the back of the cockpit area allows some water to pool from rain when the boat is in a slip - because the front of the boat is slightly lower from heavier stuff forward. If there is water in the cockpit, it drains as soon as I motor or sail. A while ago I took some video of my transom release. My boat is probably slightly lighter than when this video was taken but still close. Looks like a fairly clean release and you can see a vortex swirl around the rudder as its generating lift.

 

[Jackdaw]

The model 3 statement is not correct. Lift is generated by pressure differental on opposite sides of a shape as it moves through a liquid. That means of course that the liquid (water or air) must be able to pass on both sides. This is never true for a hull. Hulls don't generate lift.

 

[BudGates]

The model 3 statement is not correct. Lift is generated by pressure differental on opposite sides of a shape as it moves through a liquid. That means of course that the liquid (water or air) must be able to pass on both sides. This is never true for a hull. Hulls don't generate lift.

True, the hull doesn't create lift in the classic sense but the movement through the water will create a higher pressure near the widest part of the boat thus raising it, ever so slightly, out of the water.

 

[walt]

The model 3 statement is not correct. Lift is generated by pressure differental on opposite sides of a shape as it moves through a liquid. That means of course that the liquid (water or air) must be able to pass on both sides. This is never true for a hull. Hulls don't generate lift.

I disagree (with Jackdraw). If the movement of the hull is able to accelerate water downwards, well.. that is a force vector and same as lift. If the hull is able to accelerate water downwards without dragging water along with it, that is a high lift to drag ratio.

An example.. we used to windsurf on boards that would not float you when not moving (sinkers). Once moving, the board would come up to the water surface and "float" you completely differently than when not moving. The bottom surface did this by accelerating water downwards as it moved over the water. The formula boards even went for highest lift to drag by having a very high aspect planning surface. Or.. water ski's, they dont float your weight at all when not moving yet support your weight at speed. They can only do this by generating lift.

If this is just an argument about the definition of "lift" and only an air foil can generate lift.. then that is for a different thread. What I am referring to is a downwards acceleration of water caused by the hull moving across the water surface. This creates a force in the direction the water is accelerated.. I am going to call that lift.. but dont really want to argue if this is just a naming convention..

 

[BudGates]

Very good explanation Walt.

 

[walt]

Just wanted to capture your quote

Hulls never accelerate water downwards. Water is incompressible. It never gets moved DOWN. That whole statement is nonsensical.

 

[Jackdaw]

Just wanted to capture your quote

Yea I deleted that post because I didn't want to get confrontational in this forum. But is you want to argue about water getting accelerated downward go right ahead. With enough speed, certain hull shapes rise out of the water (after moving some to the front and sides) precisely due to this NOT happening.

 

[walt]

Yea I deleted that post because I didn't want to get confrontational in this forum. But is you want to argue about water getting accelerated downward go right ahead. With enough speed, certain hull shapes rise out of the water (after moving some to the front and sides) precisely due to this NOT happening.

Hulls never accelerate water downwards. Water is incompressible. It never gets moved DOWN. That whole statement is nonsensical.

Dont like to argue.. so Ill just give my opinions again.

Water is not compressible but it is a fluid (at the temps we care about here). You can very easily accelerate water any direction you want. I can move my hand (not a foil, not a special planning shape) across the water surface and clearly accelerate water downwards. Or side ways.. even upwards. A prop accelerates water backwards to generate thrust. etc etc..

We are both just giving opinions here without references. My opinion is that some hull shape rising out of the water MUST have some force that caused it to lift up against the force of gravity. Seems pretty simple to me that this force is mass (water) *acceleration (with a vector component in the direction of gravity).

My opinion (no references, Im not an expert at all on this "who doesnt even need to give references" so I could be wrong but when a hull moves through the water, it will displace water downwards and that creates a force normal to the hull (just like lift is created normal to a foil). If this downward force can be created efficiently - ie, without dragging water along with it in the direction the hull is traveling, this "lifting force" works againts gravity and reduces the drag components associated with the weight of the hull - one of those components being wave drag (again..Im just giving my opinion - I dont have a reference).

I sail a 15 foot planning dingy and it never feels like it planes like a windsurfer does but it also often exceeds the theoretical hull speed by a significant amount (maybe 40%). When the boat is going fast, it is sitting higher with respect to the water surface than when the hull is not moving. Some force is required for this to happen and its pretty clear to me that the only way for to happen is an acceleration of water with a vector component in the direction of gravity (which is downwards). I also know that where my weight is on the boat affects the top speed and I think this is at least partially due to how efficiently the hull is able to create this lift (sorry, this is simply the best word to use here) without dragging (or accelerating) along water in the direction the hull is traveling

 

[Jackdaw]

Dont like to argue.. so Ill just give my opinions again.

Water is not compressible but it is a fluid (at the temps we care about here). You can very easily accelerate water any direction you want. I can move my hand (not a foil, not a special planning shape) across the water surface and clearly accelerate water downwards. Or side ways.. even upwards. A prop accelerates water backwards to generate thrust. etc etc..

We are both just giving opinions here without references. My opinion is that some hull shape rising out of the water MUST have some force that caused it to lift up against the force of gravity. Seems pretty simple to me that this force is mass (water) *acceleration (with a vector component in the direction of gravity).

My opinion (no references, Im not an expert at all on this "who doesnt even need to give references" so I could be wrong but when a hull moves through the water, it will displace water downwards and that creates a force normal to the hull (just like lift is created normal to a foil). If this downward force can be created efficiently - ie, without dragging water along with it in the direction the hull is traveling, this "lifting force" works againts gravity and reduces the drag components associated with the weight of the hull - one of those components being wave drag (again..Im just giving my opinion - I dont have a reference).

I sail a 15 foot planning dingy and it never feels like it planes like a windsurfer does but it also often exceeds the theoretical hull speed by a significant amount (maybe 40%). When the boat is going fast, it is sitting higher with respect to the water surface than when the hull is not moving. Some force is required for this to happen and its pretty clear to me that the only way for to happen is an acceleration of water with a vector component in the direction of gravity (which is downwards). I also know that where my weight is on the boat affects the top speed and I think this is at least partially due to how efficiently the hull is able to create this lift (sorry, this is simply the best word to use here) without dragging (or accelerating) along water in the direction the hull is traveling

I'm in violent agreement with everything you wrote here! ;^)

 

[walt]

And.. once again no reference..

Woops.. I didnt read that clearly.. so Im not sure what it means.. forums are good in one aspect as you get diifferent opinions from different people.. I could be wrong with everything I said.. its happened before..

 

[Jackdaw]

And.. once again no reference..

Woops.. I didnt read that clearly.. so Im not sure what it means.. forums are good in one aspect as you get diifferent opinions from different people.. I could be wrong with everything I said.. its happened before..

Means we're good. ;^)

 

[walt]

Relaxed.. blood pressure moving down.. too early to have a beer however..

 

[BudGates]

Too early for beer???? I don't understand that.

 

[Jackdaw]

Too early for beer???? I don't understand that.

I know. I was going to argue the point, but I just went and had a beer instead.

 

[walt]

Also somewhat interesting.. (maybe... LOL).

Almost all of my larger boat sailing (26 foot is the largest I have been on) has been on a water ballast boat but I did sail for one day on a deeper keel (shoal draft with centerboard which I like) and one thing I really noticed is that in higher winds, the deeper ballast keel boat really stiffened up once the heel got up to some level. The water ballast boat also somewhat stiffens up but not nearly as much. In light winds, water ballast seems to me to sail similar. The water ballast boat is still self righting (a good thing), just not anywhere as stiff as the deeper keel boat at higher heel angles. You can still sail a water ballast boat in higher winds (done all the time) but its why I think they are more work (or maybe more fun like a dingy) in higher winds and its also why I really like having a nice traveler on this boat.

Also, you "hear" that water ballast boats stiffen up after a little heel and this is true but the reason often given is that the water ballast gets lifted above the outside water level. I dont think this is true at all (an opinion again), I think it has to do with the boats center of buoyancy moving to the outside because of heel and hull shape and creating a moment arm that begins to lift the boats mass - thus creating a righting force. An interesting thing about this is that the high ballast location gets moved almost vertical for low heel angles. Since you have to act against gravity, this is nearly ideal for creating righting moment. This would indicate that a high ballast (ie, water ballast) gets the best righting moment bang for the ballast buck at low heel angles. Another reason to "try" and sail a water ballast boat at lower heel angles. The attached picture hopefully illustrates some of this..

 

[Jackdaw]

Walt,

That's a good analysis. At early angles of heel, it is the HORIZONAL separation of the two centers that create the righting arm. That why beam matters in initial stability as the center of buoyancy quickly moves outboard. Only when you approach the angle of ultimate stability does the initial vertical difference (created by mass way down low) between the two centers factor. This initial vertical separation is now almost horizontal (creating the arm), and keeps the boat from going over.