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Old 09-04-2008, 20:21   #31
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Originally Posted by Lodesman View Post
I assume the designer or previous owner(s) had some strategy in mind when they located the engines and batteries.
Evidently, the PO repowered a couple of years ago. He decided on repowering with larger engines. From 2 cylinder Yanmars to new 3 cylinder Yanmar diesels. This upgrade alone added at least 100 pounds of weight in the rear of each hull.

I can only assume that the battery banks were relocated aft at that time, since the 'stock' location is mounted to the aft cabin bulkheads. I'm sure it's current location made the repower more convenient at that time. Sean now has 3 T105's mounted almost on the transom in each hull. My thoughts are that the designed COG has now been modified.

I recommended he begin by moving batteries forward to the aft cabin bulkhead. A distance of about 2 meters. On these boats, it's difficult to move them into the cabin.
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Old 12-04-2008, 18:09   #32
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If you move the batteries and they are the kind that you add water to, don't forget to provide good ventilation to the outside for the battery boxes, due to outgassing of flammable gas. Also, take the advice already given about resizing the battery cables up, or you could start a fire from overheated cables. Use Blue Sea Systems to size the cables.
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Old 15-04-2008, 17:01   #33
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Quote:
Originally Posted by Lodesman View Post
David,

state that the righting moment drops as soon as that happens I couldn't agree less - IMO, the righting moment would be greatest at just the moment the CB shifts from somewhere between the hulls to the centre of the leeward hull. The GZ (righting arm) would be at its maximum - which ties into the spike in the graph you provided.



Kevin
I am glad that you don't engineer out masts and rigging sizes.
Or do you?
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Old 15-04-2008, 18:17   #34
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Point of maximum righting moment for cats

Richard Woods says-

stability article part 2 by Woods Designs
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Old 15-04-2008, 20:03   #35
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Originally Posted by Fard12 View Post
I am glad that you don't engineer out masts and rigging sizes.
Or do you?
Wow! Out of the blue you join the discussion just to take a shot at someone. At least that's what I think you're doing - since your post has no contextual relevance. What exactly, is your point?
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Old 15-04-2008, 21:58   #36
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Wow! Out of the blue you join the discussion just to take a shot at someone. At least that's what I think you're doing - since your post has no contextual relevance. What exactly, is your point?
David was correct in his post. The RM for a cat is calculated on its hull centerline beam and the windward hull just lifting above the water. This is at around 11deg’s.
A mono hull RM is calculated at 30Deg’s
Example an 11.5mtr cat with an effective beam of 5 mtrs a displacement of 5600kg and vertical height of center of gravity of 1.5 the RM used (90% of Maximum) would be 11186kg.Mtrs. At around 15deg the Total RM would be 11190kg.m. Therefore as you can see at 15 deg it has now 90% of the maximum. At 20 deg it is around 10000 kg.m at 25deg it is at around 8000kg.m.

As most of those here sail multihulls would know you can go from the penthouse to the sh.it house very quickly regarding the stability of a multihull.
Once that windward hull breaks the surface of the water things can get very bad very quickly as the boat will continue to rise very quickly.
I an recent race a boat went from just kissing the water with the windward hull to around 45deg in a fraction of a second. The main was dumped but not quick enough and there was enough pressure to the boat to continue its up movement on headsail alone. (It did not capsize but was close)
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Old 15-04-2008, 23:06   #37
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Quote:
Originally Posted by Lodesman View Post
David,

I don't see how a cat's CG would shift dramatically with heel? Unless large chunks of mass are moving around the CG should remain more or less in place. I agree the CB would shift radically as a hull lifts - but then you state that the righting moment drops as soon as that happens I couldn't agree less - IMO, the righting moment would be greatest at just the moment the CB shifts from somewhere between the hulls to the centre of the leeward hull. The GZ (righting arm) would be at its maximum - which ties into the spike in the graph you provided.

As others have said, the graph leaves something to be desired - as a lot of cats have positive stability up to or beyond 90 degrees of heel. Regardless, it can be seen from the graph that a typical cat should have a greater righting moment than the maximum moment generated by a typical mono, up to about 45 degrees of heel. On a cat with 20' BOA, this makes sense - the GZ would be about 6', which would be about the same as the distance between CB and CG on a 40' ballasted keelboat at 90 degrees of heel.

To Sean's question now - I've often heard the dogma of keeping the weight central to reduce hobbyhorsing, but wonder if there is any proof of this? If you look at a catamaran laterally (ie. from the stern) you see the bulk of the mass is generally divided equally between the two hulls, therefore set at the extremes of breadth, with very little mass centred. Yet I have not heard anyone complain about cats rolling excessively (hobbyhorsing sideways?). Personally, I think concentrating all the weight in one area puts greater stress on the structure in that area, whereas judiciously spreading the weight around will not change the CG, therefore will not affect stability. That said, I also think you have to work with the design of your boat. Sean's boat appears to be deeply rockered with the CB just forward of the longitudinal centre. I assume the designer or previous owner(s) had some strategy in mind when they located the engines and batteries. I also assume a sailboat should normally be trimmed by the stern at rest, to offset the forward pitch imparted by the thrust of the sails. I think if Sean experiments with 'moving loads' he should start by putting them as far forward as possible, to see if that has the desired effect. If so, a more permanent solution would be to relocate the batteries closer to the CB. JMO

Kevin
Kevin,
The CG does not shift with respect to the vessel unless you have a free surface effect from water or fuel moving around in a slack tank(s). For a typical sailing catamaran with tanks that do not extend very far transversely, free surface effect is virtually nothing. The CG can and does move with respect to the CB as the angle of inclination changes. Sorry, my mistake for not being very clear.

David
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Old 16-04-2008, 05:08   #38
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Rick is right here. I have new Yanmar 30hp engines, 6 heavy Trojan batteries, fuel tanks, a RIB dinghy and 8hp outboard all back where the cockpit is.

Amidships, I have a couple water tanks (55 gals each) and most of my gear and tools, food and stuff.

Forward, I have little weight in the staterooms, but have 120ft of BBB 3/8 chain, and a couple of 50lb anchors.

Here is a better diagram of my hull shape.

Seems like my CG (fore and aft) should be amidships, but it doesn't feel like it is. The bow reaches skyward when I'm motoring into serious chop and the stern just sits there. Still trying to sort out of this is a "catamaran thing" or a "my boat" thing.



Quote:
Originally Posted by rickm505 View Post
Evidently, the PO repowered a couple of years ago. He decided on repowering with larger engines. From 2 cylinder Yanmars to new 3 cylinder Yanmar diesels. This upgrade alone added at least 100 pounds of weight in the rear of each hull.

I can only assume that the battery banks were relocated aft at that time, since the 'stock' location is mounted to the aft cabin bulkheads. I'm sure it's current location made the repower more convenient at that time. Sean now has 3 T105's mounted almost on the transom in each hull. My thoughts are that the designed COG has now been modified.

I recommended he begin by moving batteries forward to the aft cabin bulkhead. A distance of about 2 meters. On these boats, it's difficult to move them into the cabin.
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Old 16-04-2008, 08:02   #39
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Fard12,

I misinterpreted David originally - and now see what he was trying to say. I still disagree with some of this:
Quote:
For cats, the center of buoyancy shifts radically to leeward as the weather hull leaves the water A cats center of gravity also shifts dramatically as it heels. Once the CG passes over the CB then thats it, your over.
At rest the CB and CG are vertically aligned; as a cat starts to heel the CB will move relatively quickly towards the leeward hull and CG is virtually static. At the exact point that the ww hull leaves the water, the CB will have moved laterally as far as it can go and be centred in the lw hull. Let's say this is around 11 deg. - obviously CG will not have moved very much from its position at rest. The GZ will be at its maximum - since gravity very seldom changes, the RM will be at its maximum at this point. This is what I said previously, and looking back at the diagram provided by David, it says the same thing: "RM is max when windward hull is just flying". I guess you should also hope that Marchaj doesn't engineer sailing rigs then.



Quote:
A catamarans righting moment drops dramatically after the weather hull leaves the water.
I see now, what David was trying to say, was "that from its maximum RM as the weather hull leaves the water, the RM decreases steadily with angle of heel."

So clearly if you apply a heeling force that exceeds the max RM, and continue applying that same force, the vessel will capsize - this is no different for cat's or mono's. Remove that heeling force, and the cat will quickly return to upright; although not at its max, the RM is still massive in a cat up to or beyond 45 deg of heel. And the RM is still positive in most cat's up to about 90 deg, and in some cat's beyond this. A heeling force that exceeds RM must be maintained to capsize a cat - momentum alone should not do it - the example you gave of the racing cat not capsizing after heeling more than 45 deg only proves my point.

Nothing I've said here, changes anything I said previously. If you think I've made a factual error, then please address that point directly - I would be glad to discuss it with you. We'll get more out of the forum if you avoid the cryptic pot-shots.

Kevin
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Old 16-04-2008, 09:49   #40
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Kevin,
Cryptic pot shot? I never meant any sort of disrespect and I apologize if you took any of what I said in that way. That is not the type of person that I am. All I am trying to do is explain something that Sullivan asked in a little more than just basic detail...and not insult or offend anyone in the process.

I think we are thinking along the same lines Kevin concerning a catamarans stability.

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Old 16-04-2008, 10:05   #41
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I'm confused. This RM stuff while interesting, has what to do with hobby horsing?
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Old 16-04-2008, 10:29   #42
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RM = righting moment, just got done with a class that had a section on stability (which means I half way understand this discussion)
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Old 16-04-2008, 10:33   #43
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RM = righting moment, just got done with a class that had a section on stability (which means I half way understand this discussion)

Oh, I understood the discussion, I just couldn't find relevance.

How about this.

If we were to attempt to understand why his bow rises under power we would have to first imagine a boat which is perfectly balanced, defined as where weight is equally distributed throughout the length of the boat.

For arguments sake the boat is sitting on perfectly flat water. A large trawler generating a 5 foot wake passes by the stern of the boat. As the wave hits the stern, it lifts. As the wave travels under the keel all parts of the boat along the keel would lift the same amount, The COG would be longitudinally in the center of the boat. If on the other hand weight is not evenly distributed and the boat is stern heavy, the same wave would lift the stern less than it would lift the bow and the COG would appear to be closer to the stern than the bow. Carry this to the logical conclusion and if the COG has been moved aft to where the motor mounts are, when power is applied I think the bow would rise.

As far as hobby horsing is concerned. I seem to remember that the rocker of the keel design has a lot to do with this. Catalacs have a lot of rocker. I'm defining rocker as a line drawn on paper along the keel of the boat. On Catalacs the bow and the stern are shallow and the center of the keel is much deeper in a deep "V".

If Sean were to move weight forward on the boat I think it would affect both the bow rising under power and the amount of hobby horsing he is describing.
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Old 16-04-2008, 10:40   #44
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Rick, not throwing aspersions on your level of knowledge, just giving an indication of mine.

Bill
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Old 16-04-2008, 10:50   #45
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Rick, not throwing aspersions on your level of knowledge, just giving an indication of mine.

Bill
Oh, I'm fine with your post. I just wanted to get the discussion back to Sean's hobby horsing issue. Check your PM
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