Righting moment

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Highlander
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Re: Righting moment

Post by Highlander » Sat Nov 19, 2016 12:29 pm


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kadet
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Re: Righting moment

Post by kadet » Sat Nov 19, 2016 12:36 pm

Bertil search the previous posts this has been done before with limited success. With everything from custom daggerboards to just filling the stock one with lead shot.

Bottom line is it's a :macm: what you are trying to achieve is akin to sticking mag wheels on a lawnmower and expecting it to mow better :)

The water ballast does not kick-in till 15 degrees heal and the hull shape is designed for 15-30.

What is the purpose of your mod? To race? Sail more upright?

You may find you spend a lot $$$ and achieve very little.

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sailboatmike
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Re: Righting moment

Post by sailboatmike » Sat Nov 19, 2016 1:58 pm

I will reiterate what I have posted before in regards to adding weight to the keel.

Adding an additional 200Kg in a bulb to the end of the keel would only go the further handicap what is already a VERY heavy boat for its size and class when the amount weight of the water ballast plus the fixed ballast of the M is added together, plus the additional drag incurred by having a bulb on the end of the keel.

The M already carries around 650Kg of water ballast PLUS an additional 127Kg of fixed ballast, giving a total of around 775kg or around 1800lbs, compare this with other boats of its class and style (8 meter trailer boats) which in general only carry around 350Kg, so one is already carry about the equivalent of 4 extra passengers.

To enhance the performance one would need to calculate the amount of water ballast that could be removed to keep the righting moment the same and yet reduce the weight.

Off the top of my head I would think a reduction of around 300Kg of water ballast capacity could be offset by adding a 200Kg bulb

Now the issue becomes, how do you equally reduce the tank capacity to get the required amount of water into the tank without upsetting the balance of the boat?

Second to this, one of the issues of using bulb keels is they tend to make the boat a bit tender until the keel starts to do its thing, so early heeling would only get worse rather than be reduced

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Highlander
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Re: Righting moment

Post by Highlander » Fri Nov 25, 2016 10:32 pm

Well u could always shorten the mast & get a longer boom & go with a bigger "longer" but shorter main sail or go to a different rig , that would maybe give u less heel , not sure how that would effect the balance of the boat but I believe someone on this site has already done this rigging mod

J 8)

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Ixneigh
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Re: Righting moment

Post by Ixneigh » Sat Nov 26, 2016 7:11 am

The M is a fine boat for what she is but shes not a racer. Trying to get more speed by any method other then good sails, clean bottom and proper trim is a losing proposition. Sell the boat and get something else. And remember butchered boats have no resale value. Ive done alot to mine but it can all be returned to stock.

If you are bent on better ballast id fill the tanks with lead, fresh water, and extra batteries before id go with modding the board trunk. Also: Rudders. I think making them stiffer and a few inches wider would help. I hardly ever have all the board down. Lastly and bulb or wing on the board will require detailed water flow study.
Ix

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cmagnus4
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Re: Righting moment

Post by cmagnus4 » Sun Nov 27, 2016 2:29 am

Bertil, have you seen this extreme modification? https://650anz.com. :)
Claes

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Tomfoolery
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Re: Righting moment

Post by Tomfoolery » Mon Nov 28, 2016 2:22 pm

What's useful to keep in mind is that the water ballast, or permanent ballast, or a weighted (bulb or fin) keel, doesn't act alone, as a separate component. It's just a part of the boat. The center of mass is a theoretical point in the boat that represents the total mass and the location where, if you passed a pivot (or axle) through it and supported it, in any of the three axes, the boat would not rotate about it under gravity.

Ballast and keels and masts and such are independent components of the boat, with forces and moments that are reacted internally to the overall vessel, but the complete vessel doesn't know or care about the bits - just the total mass and center of mass location and, of course, the center of buoyancy, where the vertical force the water exerts on the hull can be mathematically 'concentrated' as a simple force reacting against the hull for stability purposes.

Having said that, adding fixed ballast on the keel, and removing some water ballast to keep the total mass the same, will invariably cause the hull to sail flatter, all other things being equal. The center of mass is moved downward when heavy ballast is added low (or lower than the CG of the ballast water) and water is removed to keep the weight the same. Or even if the water isn't removed, but that would make the boat heavier for no gain, which isn't a good idea.

First sketch is a cartoon of the vessel with high center of mass, and center of buoyancy lined up with the CoM, sitting level. Heel the boat, and the center of mass remains the same as the boat heels, but the center of buoyancy moves as the hull shape in the water changes (form stability), and the horizontal distance between them is the restoring moment arm while the two forces (boat gravity force and countering buoyant force) is the righting moment. Dimensions shown are illustrative only, of course.

Image

Adding heavy ballast low in the hull, like the keel, and removing the equivalent weight in water (and reducing the size of the water tank so it doesn't move around) lowers the vessel's center of mass. The righting moment arm for a given heel angle is greater, so the balance between the wind force on the sails and the water force on the keel against the righting moment results in less heel for the same conditions.

Image

Water ballasted boats are going to be more tender than heavy keel boats with the same hull and rig size, shape, and total weight simply because their center of mass is higher, all other things being equal. But you can't treat the water ballast or keel as a separate entity except in calculating the center of mass of the boat, as it's all part of the same lump.

Just sayin'.

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BOAT
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Re: Righting moment

Post by BOAT » Tue Nov 29, 2016 8:42 am

The speed of the boat is limited to it's hull speed - when sailing the boat is a displacement hull like any other and is limited by it's waterline - the boat has a hull speed of about 7 knots -

You not going to add any speed to the MAC by adding weight - that's will just make it SLOWER. You can get more acceleration, more horsepower, but not more speed than that maximum hull waterline displacement unless you can add enough horsepower to get the boat out of the water so it will plane. No matter what you do the boat will stop at around 7 knots. Even with multiple sails you get no speed unless you can overcome displacement (plane) and WEIGHT is the wrong direction to take to overcome displacement. The fastest anyone reported on the M boat under sail was the guys up in Canada that sail in heavy winds with spinnakers - going down wind some of those guys got an M boat to go over 8.8 knots by planing the hull downwind under a kite with NO DAGGERBOARD. The lightest boats went the fastest in those conditions. There are stories from back in the 1980's of people exceeding 11 knots in older MACS going downwind under a kite with the ballast tank dry but it's a very dangerous thing to do. Adding more weight to the keel will not make the boat faster - just the opposite.

As for the righting moment - as Judy said that calculation is used to gauge the proper size for the rigging - well, just take a look at the rigging on the MAC! It's pretty clear that there is very little need for a lot of hardware! The MAC even as lightly rigged as it is remains over rigged for the sailplan - the boat is too light to put a lot of strain on anything - the existing rig can lift the boat out of the water because the boat is so light. A lot of the "righting moment" on the M boat is just the wind spilling out of the tip of the sails because the boat is so light and heels over so much. There is a point where the mast is leaned over so far the sail just can't catch any wind - that is your "righting moment" as far as the M boat is concerned. Adding ballast to the keel would indeed improve that situation - but not add any speed.

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Highlander
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Re: Righting moment

Post by Highlander » Tue Nov 29, 2016 12:49 pm

On a :macm: u can exceed over 10.5 knt,s wing on wing down wind "100Jib & main" surfing down 12ft swells running with the tide :) :wink:
Q1 & I did this on his boat on the St Lawrence QC :D

J :P

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Highlander
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Re: Righting moment

Post by Highlander » Tue Nov 29, 2016 1:05 pm

Bertil Rafting wrote:I asked that question because i am putting an extra keel 200 kg connected to the dagger board to get more RM. I am using a keel and not a bulb to also get appr 15 % extra sideresistance,
which a bulb does not give (better than the bulb on Black Pearl).
Of course also have to build a new strong enough daggerboard, and also make enforcements inside the DB-trunk.
-I need to know the RM before my modifications as above to be able to calculate the new RM to see if the mask and schrouds is strong enough.
-Thanks for the tips I got from you.
If u want to stiffen up ur rig & make it stronger u can always do my rig mod
http://i844.photobucket.com/albums/ab1/ ... 56a38b.jpg

http://i844.photobucket.com/albums/ab1/ ... 5qbnlu.jpg

added a 3rd set of masthead shrouds , second set of spreaders , backstay on a swivel & traveler and at least one more forestay this will allow u to fly ur spin from the masthead & fly multiple head sails :)

This mod is not recommended if u do a lot of trl sailing just for wk-ends as it does increase rigging time but if u r retired like me & have lots of time N/P , if slipped or moored most of the time its a no issue :)
http://i844.photobucket.com/albums/ab1/ ... z83y2m.jpg
Just Sayin that,s All
J 8)

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Tomfoolery
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Re: Righting moment

Post by Tomfoolery » Wed Nov 30, 2016 7:12 am

BOAT wrote:A lot of the "righting moment" on the M boat is just the wind spilling out of the tip of the sails because the boat is so light and heels over so much. There is a point where the mast is leaned over so far the sail just can't catch any wind - that is your "righting moment" as far as the M boat is concerned.
At the risk of sounding like a techno-weenie ( :| ), spilling air is actually reducing the heeling moment, which allows the boat to stand up higher, thereby reducing the righting moment. The two are exactly balanced when the boat is traveling steady-state (holding steady heel angle). Increase the wind force (which also increases the opposing water force) and the boat heels more, which reduces the projected sail area a bit, and also increases the righting moment from increased heel angle and weight vs buoyancy (see previous sketch), until they balance again.

The other half of the equation:

Image

Obviously the aerodynamics and hydrodynamics are more complicated than the simple sketch (like sails hanging to leeward, inducing yaw), but the concept is still sound regardless of the internal details that lead up to lumped element models like Center of Pressure and such.

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Re: Righting moment

Post by BOAT » Wed Nov 30, 2016 7:59 am

Why do you guys do this to me :cry: the guy wants to add ballast to the boat! That means more beef in the rigging.
I never get scared heeling over the MAC - because I know it can't flood before the wind falls out of the sails and I also know the rigging is already stronger than anything the wind can throw at it - the boat is just not heavy enough to snap it's own rigging. The stays will not break - the boat will just lay over before is will ever break a stay. Now, you take a heavy keel boat and start to heel it over 30 degrees and I get VERY SCARED - because I know at that angle the rigging is loading THOUSANDS of pounds on the hardware and there is a good chance something is going to break. I sailed a lot of keel boats and I have seen a LOT of them break because of shock loads on the rig. It's very scary knowing your rig is not strong enough to lift the boat out of the water because the boat is too heavy - that means in a high pressure situation, the rig will break before the boat gives way. That is a "bad" thing - that's where "righting moment is critical. On the MAC - not critical because the rig is too strong for the weight of the boat - the boat will just lay over.

A heavy boat with a heavy keel is harder to push over - but the heavy boat is also deeper in the water and it's gunnells are closer to the surface of the water so when the "normal" sailboat boat DOES heel over it's a lot more dangerous:

Image

The first boat above is a heavy keelboat - very stable and heavy - sits in the water deep - but look at the same boat heeled 30 degrees - it's ready to start taking on water over the side.

The second boat is light on the water - floats high, and the freeboard is much harder to overcome when it heels - 30 degrees is not even close to the gunnells - it takes over 65 degrees to swamp that boat but hey! guess what! That boat CAN'T EVEN GET TO 65 degrees! You know why? Because at 50 degrees the sails are leaned over so far they can't catch any wind! There is no way to blow the boat over any further because the wind can't get into the sails at that angle!

That's what I mean when I say the "righting moment" is sort of pointless on the MAC - now the idea of adding weight to the MAC keel (daggerboard) would indeed make it harder to lean over - but it also makes the boat HEAVIER so now it sits lower in the water and puts more strain on the rig. This is why I think the original question of righting moment that was asked is a legitimate question - I think he wants to know at what point he is adding so much weight to the "righting moment" that it requires him to beef up the hardware. I think Highlander noted some options.

If I were to add that kind of weight to the boat I too would want to know if the rig could take it. I also would worry about heeling at 30 degrees with the extra weight (will I swamp because I am lower in the water?) All things to consider. The ultimate deal would be to SHIFT existing weight from the water ballast to the end of the keel - if you could do that the rig would also need more beef - but what's the point? All it does is make the boat lean over less - it does not make the boat any faster and is absolutely does not make it any safer.

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Tomfoolery
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Re: Righting moment

Post by Tomfoolery » Wed Nov 30, 2016 8:29 am

BOAT wrote:Why do you guys do this to me :cry:
:D :D :D Not trying to tweak your tail, BOAT. Just clarifying for anyone interested in the balance of forces on a sailboat, and the fact that ballast (of any kind) is just part of the vessel. I was focusing more on the equilibrium, and less on the resulting forces when shifting the Center of Mass downward, with or without adding weight. :wink:
BOAT wrote:That's what I mean when I say the "righting moment" is sort of pointless on the MAC - now the idea of adding weight to the MAC keel (daggerboard) would indeed make it harder to lean over - but it also makes the boat HEAVIER so now it sits lower in the water and puts more strain on the rig. This is why I think the original question of righting moment that was asked is a legitimate question - I think he wants to know at what point he is adding so much weight to the "righting moment" that it requires him to beef up the hardware. I think Highlander noted some options.
This is all true - adding ballast in any way that lowers the Center of Mass of the vessel will increase the loading on the rig under given wind conditions (assuming same sail trim, etc.). And is why the OP was asking for info in the first place, though calculating rig forces is not going to be a simple thing. Scaling them up is simple to do, but if you don't know where you're starting, then scaling up an unknown value doesn't do you any good.
BOAT wrote:If I were to add that kind of weight to the boat I too would want to know if the rig could take it. I also would worry about heeling at 30 degrees with the extra weight (will I swamp because I am lower in the water?) All things to consider. The ultimate deal would be to SHIFT existing weight from the water ballast to the end of the keel - if you could do that the rig would also need more beef - but what's the point? All it does is make the boat lean over less - it does not make the boat any faster and is absolutely does not make it any safer.
No argument from me, though it's debatable whether or not you can sail marginally faster if the heel angle is reduced, assuming the weight is kept the same. But using permanent ballast increases the trailer weight, and may make it difficult to launch/haul is it has to sit higher on the trailer, at which point you'd might as well get a different boat, with adequate rigging. And a bigger tow vehicle to pull it. :P

We're having a violent agreement, in other words. :|

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Re: Righting moment

Post by kurz » Wed Nov 30, 2016 9:24 am

Boat, why the black pearl from BWY is so much better sailing than a normal :macm: ?

I don't think 100kg lead in the daggerboard lets the mac sitting so much deeper in the water. It's like a added person on deck.
And if you empty the ballast you will even have less weight...

To make the rig stronger is not a big deal, by the way. Just go to 5mm instead of 4mm...

I think heavy keels have many real good advantages, and thats the reason that all blue water sailors have a big loaded keel, inclusivly the mac65/70...

the only reason I know is that that :macm: has a light keel is to trailer easily and motor fast and get the daggorboard up easy and cheaply :D

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Re: Righting moment

Post by BOAT » Wed Nov 30, 2016 10:12 am

Pearl is not better. in 5 knots of wind 'boat' will beat her easy. I'm a lot faster in light winds. 'boat' is very light. There are places where a light boat is better:

it's all about location:

Image

Here are the winds as I write - 50% of the time along the coast your at less than 5 knots of wind in the green areas - why rig for 30 knot red and yellow areas? here on the Pacific side you don't see those 30 knot winds until you get over 100 miles out unless your up north - and there is nothing out there unless you go all the way to Hawaii. But here on the Pacific side down south all the goodies are within 80 miles of the coast - the channel islands, Catalina, Baja - the Coronado Islands - Sea of Cortez - you can cruse for thousands of miles and years on end and never really spend more than 20% of your time in anything over 30 knots!

If I were to venture off across the Pacific into those red zones (and be aware - those red zones are ALWAYS THERE - there is no getting past them) then I don't think I really have the right boat to do it in comfort - the 70 MAC would be great - it has submarine windows and it's designed to go under water when the waves crash on it - but here on the coast where the wind is less than 5 knots 50% of the time? (all night and in the morning hours) - I would just be a big slug going no where most of the day!

If I added more weight to the keel on "boat" it would be nice in the afternoon when the wind goes 16 to 18 knots but all the rest of the time I would be cursing the extra drag on the boat. If i were up in the North Like Highlander I might want that because up there it's ALWAYS blowing, but down here it would just be a hassle - it's not needed.

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