Rob, I think we probably agree more than disagree on the BP vs jacket issue. But, there were several points in your comments that prompted some additional thoughts.
It is true, however, that a BP/W is generally negatively buoyant because of the metal backplate. This can make a difference to your total need for ballast weights. If this is really a big issue then you can also deal with ballast weight by changing the tank type. Aluminium cylinders are considerably more positive than steel cylinders, for example. In fact, your choice of tanks (and suit) will affect your stability in the water a lot more than your choice of BCD.
I fully agree, that cylinders are both an additional way of addressing ballast, and a factor is stability. In the context of the OP's original post (jacket vs BP), and his statement about traveling to Bonaire (where he probably won't be carrying steel cylinders if he travels by air, and where he probably won't easily find some for rent), I focused instead on the BCD issue. And, I usually use HP steels (100s) when diving locally (with a jacket or a BP) or travelling by car. I find that bigger steels (e.g. HP130s), and even HP120s to a certain extent, adversely affect my longitudinal stability in the water - I tend to rotate a bit (around the sagittal axis), as the weight of the cylinder seeks its natural position below the center of lift. So, I spend a bit more time in station keeping (finning) than I do with smaller steels or aluminums. But, the primary comment was about ballast. The issue, for me, is simple - why would I willingly choose to use a BCD that causes me to add
even more ballast, to compensate for its inherent 'floatiness', when I already have to add ballast to compensate for my own inherent 'floatiness', and the inherent 'floatiness' of my exposure suit (irrespective of what suit I am using), etc. I learned this lesson years ago when diving a drysuit with my Zeagle Ranger and an aluminum cylinder. I had to add so much lead that I actually damaged the material in the BCD weight pockets (which Zeagle repaired years later, at no charge). That experience led me to my first steel cylinder, and then to my first steel backplate. The net difference (reduction) in ballast requirements just in switching from the Ranger, an otherwise great BCD but very 'floaty', to a SS BP was considerably more than the gross weight of the BCD vs the BP. I don't want to add any more weight than I absolutely have to to be able to descend the first 5 feet, and to hold a safety stop with a near empty cylinder. So, why would I use a BCD that makes me do that. Other divers, who are inherently more negatively buoyant than I am, may actually benefit from a floaty BCD. Most divers I work with do not fall into that category.
b) this point makes no sense at all. If it fits, it fits. If it doesn't, it doesn't.
I wonder if this is either a misplaced comment, or a misunderstanding of my original comment. My point had nothing whatsoever to do with
fit. It was about physics, and physiology. And, to paraphrase your observation, I would suggest that 'the type of BCD is utterly relevant to this.' It is about aligning the center of weight in the BCD with the diver's center of lift - the thorax. What do we repeatedly tell divers about buoyancy control? Use your breathing, not your BCD, for primary control of your buoyancy underwater. The thorax (the lungs, specifically) is where the physiologic center of lift is. If we put the center of weight 'below' that point - e.g. on a weight belt, or in the vast majority of integrated weight systems available today, many divers will tend to end up in a somewhat foot low position underwater. That is simply physics - the center of weight and center of lift seek to vertically align themselves in any space affected by gravity, including water. Most of us can compensate for a certain degree of foot low effect with lighter fins, or subtle station keeping. I can do it, with reasonable ease. But, personally, I don't feel the need to be constantly station keeping, no matter how minimal the effort, to address this if I don't have to. Manufacturers have (belatedly) come to acknowledge this fact of physics, reflected in the growing number of BCDs (be it a jacket or back-inflation unit) appearing on the market with trim pockets placed higher on the BCD. Those pockets place weight where it should optimally be - more closely aligned with a diver's physiologic center of lift.
c) you will need less ballast weight with a SS backplate as compared to a jacket. However, the discussion about ballast weights is going to depend much more on the SUIT you're using and not on the BCD you're using.
I fully agree, that the suit makes a difference in the total amount of ballast. But, irrespective of what exposure suit, and associated suit ballast requirements, a diver uses - and going back to the comments regarding point a) - why would I want to strap on more ballast to compensate for the 'floatiness of a fabric BCD (be it a jacket or back-inflation unit)? Other divers might choose to do so, and their is their privilege. My comments outlined MY personal preferences. As a related example: I choose to dive a drysuit in certain environments. I pay a price to do that - I willingly strap on more lead, to accrue the benefits a drysuit offers compared to my 3mm wetsuit. I get no particular benefit from a fabric BCD - in terms of fit, comfort, or unique utility - that justifies added weight.
e) you might need a crotch strap on a BP/W for floating on the surface. Under water, it has virtually no function unless you're wearing 40 or 50kg of tanks that you need to keep from moving around too much. A jacket is a BCD intended and designed for one tank and no stages and the manner in which it is designed makes a crotch strap an unneeded feature. This is really a case of comparing apples and oranges.
Not at all. Rather, it is specifically about having a crotch strap vs not having a crotch strap (which, as it turns out, many jacket BCDs simply do not). I agree that the 'need' for a crotch strap is probably greater at the surface. And, I think 'need' is in part a matter of individual diver preference. But, that 'need', real or perceived, is the same at the surface for either a BP or a jacket, so why not use a BCD that is more likely to readily meet the need? And, if a diver is wearing a substantial amount of NON-INTEGRATED weight (e.g. on a belt), even with a BCD that may be fully adjusted (but still not fit well), there is a tendency at the surface for the BCD to ride up on the diver (or, perhaps more accurately, for the diver to slip down / out of the BCD). For me, a crotch strap can be a benefit at the surface. Underwater, even with single cylinders, there can be a certain amount of weight shifting, particularly with larger steels, and a crotch strap is also a benefit. Can a diver do without a crotch strap? Yes. As an example, I can and will if absolutely necessary, dive backmounted double steels (and deco bottles) without a crotch strap. However, I prefer to have one, so I choose to dive rigs that afford me the opportunity to easily include one. Call it need, or simply personal preference. It adds stability.
f) This will depend on the jacket. Personally I adjusted my BP/W ONCE in 2002 and haven't touched it since and I adjusted the straps on the jacket ONCE the day I bought it and haven't touched it since. The exception are the shoulder straps on the jacket that you generally need to tighten when you put it on before every dive. This is about as complicated as putting on the seat belt in your car. My advice would be that if you aren't able to tighten a shoulder strap or put on a seat belt by yourself, that diving might not be the sport for you.
The number of times a unit is
adjusted has little to do with whether it properly
fits or not. I will go a bit further, and possibly annoy some - I think this distinction may not be fully appreciated by a number of divers, and by a number of dive shop personnel. Cranking down on shoulder straps and waist straps does not necessarily result in a proper fit. And, that is another thread, altogether. But, 'adjustment' was not the point. Like you, I haven't adjusted my BP harness since I first put it together years ago - although I spent some time adjusting the unit initially to get what I considered to be a good fit. And, I use different BPs for drysuit diving and for wetsuit diving, instead of trying to adjust one harness to properly fit me under both sets of conditions. I also haven't adjusted my jacket since I first put it into service. Of the two, my BP harness
fits ME better. Remember, my original point was that 'I PERSONALLY prefer' a BP, and my comments reflected the advantages that I see, for me.
Over the years, I have helped, and currently I help, fit A LOT of divers in BCDs - students, customers, etc. In most cases, I have been reasonably successful, in a few cases I have not been, so I make no claim to perfection. And, while I have found that it is generally easier to get a better, snugger, more comfortable fit with a BP harness, than it is with a jacket BCD, it is NOT universally possible to properly fit everyone with a BP harness, or properly fit everyone with a jacket, or properly everyone with a soft back-inflate unit. The majority of jacket BCDs come in fixed sizes, and depending on how 'standard' a diver's abdominal girth, or shoulder width / thickness might be, it may be a bit easier to properly fit them with a BP harness than a jacket.
Again, this is over-reaching in the quest to find arguments that sound on the surface like they make any sense.
On this one we can disagree. I probably could suggest the same thing - that you are overreaching to make a contrary point. But, that doesn't serve the point of the thread very well, does it? The original comments do make real sense, both on the surface and below the surface. They may not be applicable to all divers, and it was not my intention to say that they are, nor to
argue that everyone should use a BP. Rather, they were composed so that others might understand the specific basis for my statement that, while I own and dive both BPs and jacket BCDs, I PERSONALLY PREFER a BP. Frankly, what another diver chooses to use or not use doesn't affect me. But, I am happy to share my experience with others - such as the OP, who specifically asked about something with which I happen to have a body of experience - in the hope that it will help them make a decision that suits them.
