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Old 01-27-2016, 06:59 PM   #1
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Placement of heavy items - low or high?

Going to be adding scuba tank filler (air compressor) to our new boat (yes, we have purchased a new-to-us trawler! pix and details after we take possession). I have several options for placement of this 130# object, the two most obvious being the lazarette and in the covered area under / forward of the upper helm station on the fly bridge (under all the instruments - it's very roomy in there).

The lazarette location would put the compressor right about the water line, but isn't ideal for a couple of reasons.

The upper helm station might be great - but what about the weight way up high like that? Intuitively, it seems you'd want to NOT have heavy items high up. But I remember someone (probably here on TF) talking about the metronome effect - that a metronome slows down as you slide its weight up the stalk. In fact, I think someone referred to some boats that have a big, heavy "mast" primarily for anti-roll stability.

You guys are a lot smarter about this stuff than I am - so please, opine away!
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Old 01-27-2016, 07:07 PM   #2
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130 pounds isn't enough to worry about. I have that area jam packed with stuff and other weight up on the flying bridge. never even noticed a difference. At least 130 pounds on a smaller boat.


That is only the weight of one smaller person...so just keep one person off the flybridge in rough condition...like you wouldn't anyway.


If you can run the hose down to the main deck OK...but would you have to lug bottles up to the bridge?


I would shoot for the lazarette and make it work is I could and use the other space for lighter stuff that may need better ventilation or stuff more commonly used up on the bridge.


Currently at Patrick AFB till Sunday AM, is the boat nearby?
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Old 01-27-2016, 07:49 PM   #3
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psneeld,

You're at Patrick? Oh, man, that's just up the road from us! But we're out of FL this week, up in Indianapolis. And the new boat is still in Ft. Lauderdale, where we bought it. (Getting a new bottom job, all the way down to the steel.) While you're at Patrick, see if you can find a 42' Grand Banks Classic named Sabbatical II - that's my cousin Jan and her husband Hank. Great people, if you run into them.

"That is only the weight of one smaller person...so just keep one person off the flybridge in rough condition...like you wouldn't anyway." - That really brings it into perspective!

I believe we would be able to run the fill hose down to the deck - not real interested in toting tanks up and down a ladder for every fill. (If we can't use that long a hose, that will put the kabosh on that idea.)

The main issue with the lazarette is that the boat is a tad aft heavy, so I don't want to start adding weight as far back as the lazarette if I can put it somewhere else.
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Old 01-27-2016, 07:55 PM   #4
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Thanks...just buy a bigger anchor and more chain and put it in the lazarette...

I'll see if I can find your cousin.
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Old 01-27-2016, 07:56 PM   #5
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The lazarette location would put the compressor right about the water line, but isn't ideal for a couple of reasons.

One of those reasons in my book would be the proximity of the exhaust from engines and genny to the intake for the compressor. I am assuming (and yes i know what happens when we do that) you would need the genny running to operate the compressor creating a potential disaster under water if things are not just right. Would it be possible to run a remote air intake for the compressor up to the flybridge or another location away from any exhaust sources?

John
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Old 01-27-2016, 08:20 PM   #6
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The lazarette location would put the compressor right about the water line, but isn't ideal for a couple of reasons.

One of those reasons in my book would be the proximity of the exhaust from engines and genny to the intake for the compressor. I am assuming (and yes i know what happens when we do that) you would need the genny running to operate the compressor creating a potential disaster under water if things are not just right. Would it be possible to run a remote air intake for the compressor up to the flybridge or another location away from any exhaust sources?

John
There is an option for a remote intake, but I'm not sure I want to cut a hole in the deck to utilize that option. And I see your point about taking in air from the engine room / lazarette area - an exhaust leak from the genny would contaminate the air in the tanks. Maybe that flybridge location - far away from all exhaust sources - would be best. When we actually take possession, I'll be able to assess all possible locations. For now, I think the point of this post has been addressed - 130# on the flybridge doesn't seem likely to contribute significantly (if at all) to roll.
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Old 01-27-2016, 09:04 PM   #7
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I'll never forget the day I was tooling up A1A in the transam and a harrier came over the road just in front of us. The guy literally turned the plane toward us as he went over to the landing strip.
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Old 01-28-2016, 07:04 AM   #8
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Brian, I put my 550 lb dinghy on the upper deck on longer passages and cannot feel any real difference (I also don't go out in 10 ft seas). I agree with PsN and wouldn't worry about 130 lb.
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Old 01-28-2016, 01:18 PM   #9
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Congrats on the purchase! Post pics when you can!

I have a slightly different approach to weight distribution.

Boats get heavier the longer you own them. Stuff accumulates. No single item is going to make a difference. But I think there's value in considering the relative weight of everything as you stow it, or in this case, mount it. Over time, those few pounds here and few pounds there do add up.

As for sucking exhaust into a dive compressor, I'll leave that one for someone who's done it before. I'm only speculating, but I suspect it can be addressed without putting the compressor up on the flybridge.
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Old 01-28-2016, 01:35 PM   #10
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Brian, I put my 550 lb dinghy on the upper deck on longer passages and cannot feel any real difference (I also don't go out in 10 ft seas). I agree with PsN and wouldn't worry about 130 lb.
That is correct, you cannot feel the difference. But placing heavy items above the center of gravity of your boat does degrade stability, guaranteed. And you have no idea what the stability limit is before or after placing your heavy dinghy on the roof. As long as your boat doesn't roll over when you step aboard, it seems safe. But you do not know the limit of that safety, and it is less with the dinghy on the roof.

If there is any choice at all, the wise choice is to place added weights as low as possible in the boat. Then perhaps they will be adding to stability instead of reducing it.
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Old 01-28-2016, 02:10 PM   #11
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If there is any choice at all, the wise choice is to place added weights as low as possible in the boat. Then perhaps they will be adding to stability instead of reducing it.
Totally agree. And Tad, would not 130 pounds added 10 feet above the CG require an additional 1,300 on foot below the CG to maintain initial or ultimate stability?
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Old 01-28-2016, 02:17 PM   #12
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I would think that ensuring clear air by mounting it high is far more important than the 130lbs of weight. Thats less than the average person, it shouldn't be an issue I would think.
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Old 01-28-2016, 02:23 PM   #13
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I would think that ensuring clear air by mounting it high is far more important than the 130lbs of weight. Thats less than the average person, it shouldn't be an issue I would think.
The average person will not be up there when boat is rolling at say 30+ degrees.
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Old 01-28-2016, 02:28 PM   #14
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You guys are way overthinking this....

Yes the lazarrete can be done as there are filters...look at the gasoline powered hookas. My PO used one on the swim platform and after deck when rough when he taught scuba in the Bahamas for years on this boat.

130 pounds is NOTHING. A hardtop with enclosure plus a couple cases of water stored up there are probably 130 pounds...who wouldn't add those things to the flybrige of a new boat?


Now I'm not saying ignore weight distribution and add lots of stuff up there...heck, removing an older open radar array and replacing it with a new lightweight radome on the mast would offset that stability in all likelihood....


Yes a heavy dingy on a 47 Bayliner/49 Meridian top deck is a bit of a deal....My buddy just had to shift his chocks around up there as his 12 foot Boston Whaler jet boat was causing a significant list.


But 130 pounds? Some TFers carry that much more weight than my cruising companion...what...don't let them up on the bridge?
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Old 01-28-2016, 02:55 PM   #15
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Those that would install a 130lb thing on the FB probably has done a dozen other things just like that. Who's to say the boat isn't top heavy to begin with? It's just not very smart to add weight up that high. And if it was my boat I'd take the whole FB off. So I represent the other side of the "sure who cares" crowd. At least the OP's think'in about it.

As to the contaminated air for diving just mount the thing in the laz or the cockpit and run a hose or tube to a fresh air place .. perhaps the FB?
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Old 01-28-2016, 03:23 PM   #16
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I'm far from an engineer, but doesn't pressure drop as a pipe's length increases? that may be an issue with mounting low and running a tube up that someone who understands this way more should address before the OP counts on it to breathe.
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Old 01-28-2016, 03:25 PM   #17
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Considering the boat probably was designed to have up to a certain number of people up there, just reduce that number by one. Not all that difficult to figure out.

Another way to see if your stability has been affected too much, take the boat out in a decent seaway and time you roll. Add 130 pounds to the bridge and time the rolls. If the time increases significantly, especially in the hang time at max roll, not a great idea. If just barely noticeable, don't sweat it. That was the way we determined when too much ice was accumulating topside on the USCG cutters. I have also used it successfully on smaller commercial vessels.
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Old 01-28-2016, 03:56 PM   #18
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Totally agree. And Tad, would not 130 pounds added 10 feet above the CG require an additional 1,300 on foot below the CG to maintain initial or ultimate stability?
That's a good thought but no, it won't require 1300 lbs to offset the higher added weight.

The reason for that is, 130 lbs will change the boat's flotation (sinking it) by a fraction of an inch. But 1300 pounds will sink the boat (depending on it's size and shape) perhaps 2". The center of all underwater volume (B, or center of buoyancy, CB) will move up, closer to G.....And the whole relationship changes, which is why I have software to keep track of all this.

Suffice to say that some weight added down low would offset the weight up top, the exact calculation of how much is complex and depends on the size and shape of the boat.
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Old 01-28-2016, 04:02 PM   #19
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psneeld-- seems to also be an excellent way to determine when its "pucker" time
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Old 01-28-2016, 04:06 PM   #20
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Tad...would you say that the average boat represented here has the ability to take a reasonable live and dead load beyond being completely empty and the boat better off without the flying bridge?


I would hope that most boats owned here had a designer better than totally incompetent and that the seats and storage areas on the flying bridge (unless overstuffed or loaded with lead bars) can handle a half dozen people and a couple hundred pounds of gear without being dangerous. Rough seas and people probably shouldn't be there anyway for personal safety more so than stability.


Reasonable?
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