Critique my BP / wings setup

[Scubaroo]

Do you really think that the 8 versus 11 is going to make a big difference? With the 8, you could just string 2 x 4lb weights, for the 11, just add a 3lb between them, or make them 5's and see if you can get by without the extra pound.

I don't use a weightbelt, but given that so many divers in Monterey are lugging around weightbelts with at least 25 or 30lb on them, the difference between 8 and 11 seems somewhat trivial.

 

[Soggy]

Your requirements seem like you may be VERY close to the limits of a 27 lb wing. Why not just get the 36 lb? For single tank diving in the Boston area, that is what almost everyone uses...with good reason. The 36 lb wing will also be fine in the tropics...a bit big, but nothing to worry about...when it's almost empty of air, the thing is paper-thin.

 

[docmartin]

i agree with soggy and followed a similar logic in chosing my setup. the 36# wing creates no extra drag when not inflated to the point where the bellow design comes into play and at that point you presumably need the extra lift.
a 9# bp in warm water is very heavy. i need about as much weight as you describe in thick rubber. i got a 6# bp. because of the lack of bouyant padding etc. you need significantly less weight with the bp/wing setup. with a 3mm i am actually somewhat overweighted with the 6# bp and i carry no additional weight. with 9# i'd be crawling on the sea floor.

 

[DiverAnderson]

I had ruled out the 36# wing because it was significantly more expensive and someone mentioned that the side gussets cause failure within a year. I expect these reports of failure are overrated but still I was concerned. I like the idea of a really small wing, especially after my experience with the Ranger which was like diving with an uninflated Zodiac on my back. I will call a Boston area LDS and ask for advice.

I am now convinced a 9# plate is too heavy and I will het the 6#. Thanks for the advice everyone, this has been most helpful!

 

[Soggy]

DiverAnderson once bubbled...
I will call a Boston area LDS and ask for advice.

I believe the only Halcyon dealer in the area is Northeast SCUBA in Chelmsford, MA. SCUBA Shack in Rocky Hill, CT is also a dealer and is 1.5 hrs away from Boston.

 

[DiverAnderson]

Maybe I'm thinking of buoyancy wrong. I thought that if properly weighted, at the beginning of dive I would be a little heavier than neutral in order to account for the fact that the tank is lighter at the end of the dive and you want to be neutral at the end of the dive. Then the wing is only inflated to account for the wetsuit compression which is significant but not even half of 27lbs. So I'd expect I'd never inflate my 27lbs over halfway unless I was at the surface swimming out or if I removed my BP in which case the 27lbs lift would certianly float a 6# BP and 7# channel weight.

I haven't been drysuit diving yet but I hear that one uses the suit as a BCD and thus the wing is inflated even less.

What do I have wrong here?

 

[docmartin]

i have been thinking about this a lot, too and here is what i came up with. i am sure someone will point out if there is a flaw in the logic.

you may be underestimating the wetsuit buoyancy. in your initial post you say that wearing your 50F wetsuit you would carry 9# (bp)+7# (p-weight)+6-8# ditchable. all of this weighs 22-24#. you also may carry a light and certainly a reg. that's another couple of pounds so you are at 24-26#. the full tank (AL80) will be about 4# negative at the beginning of the dive. therefore, you're actually carrying 28-30# when you start.

When you subtract the 5# of weight you are presumably carrying to compensate for the swing weight between a full an empty tank you are left with 23-25# of weight. if you are properly weighted this means that this is the amount of weight you need to be neutral at the beginning of the dive i.e. it will make you descend on exhale. The only reason you need all this weight is to overcome the inherent buyancy of your exposure protection. in other words your wet suit is 23-25# buoyant (in actuality it is a little more since virtually all humans are somewhat negative).

when you dive deep at the beginning of your dive the wet suit will be compressed and lose much of its buoyancy. the question is obviously how much because that is the lift you will need to stay neutral at depth. i have read varying estimates. however, i have seen 20# mentioned several times for a depth around 130fsw. this would basically mean an 80% reduction in wetsuit buoyancy compared to the surface. at first glance that seems a bit much but again i don't have better data. if you buy into this you would need to have at least 20# of lift to compensate for the loss in wetsuit buoyancy. furthermore, at the beginning of the dive your tank is still full so you are negative by the additional 5# of swing weight you are carrying. the halcyon 27 supposedly is more like 24# so you would be one pound short of the 25# of lift you need.

btw, for a drysuit 22# buoyancy is fairly common. in this case you want to have enough lift to compensate for a catastrophic flooding i.e. you need at least 22# capacity plus swing weight in your wing.

all of this assumes no pony which is a requirement on virtually all boats here in new jersey and possibly around boston. if you wanted to add the potential swing weight of a pony the 27# would be cutting it very close.

these are my rather lengthy 2 cents. maybe i have made a mistake in the logic. if so i am sure someone will jump in and correct me.

 

[JimC]

I dive a dry suit with a 27# Halcyon wing without problems.

SS Plate (5#)
Homeade P-Weight (8#)
Belt (10#)

The wing floats it all with an AL80, without dificulty. Changeing to a steel tank would just mean droping weight off the belt.

 

[docmartin]

the only way you could possibly encounter problems with a drysuit would be if it flooded totally.

 

[lairdb]

DiverAnderson once bubbled...
Maybe I'm thinking of buoyancy wrong. I thought that if properly weighted, at the beginning of dive I would be a little heavier than neutral in order to account for the fact that the tank is lighter at the end of the dive and you want to be neutral at the end of the dive. Then the wing is only inflated to account for the wetsuit compression which is significant but not even half of 27lbs. {...}
What do I have wrong here?

I think there's a lot of implicit assumptions in your calcs. (They may be valid, but I'd rather they were explicit.) (Fair disclaimer; I've been refiguring this stuff for myself lately, so this also serves as a "what-have-I-missed" post.)

For each gear set, you need to calculate for three situations: start of dive at surface (I want to descend), end of dive at last stop (I want to be neutral at my stop), and start of dive at depth (I want to be neutral at depth).

They're in that order rather than chronological order because the first two will give you answers in pounds-of-lead, the larger of which becomes an input to the third, whose answer will be in pounds-of-lift.

The factors are {diver at nominal average lung volume}, {gear}, {wetsuit}, {tank}, {weight}, {lift}.

Suppose you're +2 buoyant by yourself.
Your fins, reg, knife, mask (in place), light, etc. are -2.
Wetsuit: +24 at surface, +4 at maximum depth.*
Tank: -2 full, +4 empty.*

*the suit-at-depth and the tank-at-empty are opportunities for conservativism; you could conservatively assume the suit compressed to zero at depth; I used the numbers that seem most common in discussions here.

Sit.1: At start, at the surface, you'll be +2-2+24-2=+22; you need to be >22 pounds heavier to sink. Some is available via deep exhalation, some by swim-down, most by lead (or other weight, e.g. BP.)

Sit.2: At the finish, at the safety stop, you'll be +2-2+24+4=+28. You need to be 28 pounds heavier to stay at your stop. (There's another conservative assumption here; that the wetsuit will be fully uncompressed at the stop. I prefer making that assumption, since it means by implication that I can become neutral, i.e. be completely controlled, anywhere between the stop and the surface.)

Sit.3: At depth, at the beginning (heaviest tank), you'll be +2-2+4-2+{weight}. From situation 2, we got +28, so we'll add -28 of lead for +2-2+4-2-28=-26. You need 26 pounds of lift to be neutral at depth with the full tank and the weight. If you assumed full suit compression, you'd need 30.

(Yes, there are some terms that are identical in all three situations and could be netted out or canceled out, but I find it more useful to reman conscious of them and clearer to explain when they're left in. Also, I think you can ignore situation 1 in practice, since unless your wetsuit compresses non-linearly situation 2 will always be higher.)

You may want to consider a fourth&fifth situation: you want to be able to doff my gear at the surface or toss it in and don it. Same theories, only you have to run a calc of you, your you-attached gear, and your wetsuit (and your weight if a weightbelt) and a separate calc of your tank, harness, etc. (and weight if BP, STA, integrated, etc.)

Now you do the same calcs for the warm water gear set to get those answers. I haven't run the numbers for the case you propose, but at a glance and depending on some numbers that are personal to you (your inherent buoyancy, your wetsuit's lift) it looks like the 27 pound wing isn't enough lift even with an AL80 for cold water, and it's more than enough for warm.

Mistakes, folks?

--Laird