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Thread: Shellback Dinghy - 100lbs of buoyancy bags enough?

  1. #1
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    Default Shellback Dinghy - 100lbs of buoyancy bags enough?

    Hi all,

    I have read every post here about dinghy flotation/buoyancy and still am not sure how much to put in and where to put it.

    I have a Shellback sailing dinghy. It's about 11 feet and around 100-120 lbs. I have seen the pictures of one flooded and floating well below the water, a possible death sentence in our cold waters of the PNW.

    I am going to add buoyancy bags. I would like to keep the bow free of bags (so I can stow other stuff there), so intended to put a bag under the main seat and rear.

    But for the space I have that only gives me about 100 lbs of buoyancy. Is that going to be enough to get back in and bail it out? Keep in mind this has a center board that is open in the boat.

    Am I going to have to put some in the bow also?

    Thanks for your help.

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    Default Re: Shellback Dinghy - 100lbs of buoyancy bags enough?

    Try it and see.

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    Default Re: Shellback Dinghy - 100lbs of buoyancy bags enough?

    Quote Originally Posted by Tom Christie View Post
    Try it and see.
    An excellent idea. But I am having to order in bags the right size and for some reason they are being hard to find. So I would like to get it as correct as possible at first attempt. But I will do capsize tests once I get them installed.

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    Default Re: Shellback Dinghy - 100lbs of buoyancy bags enough?

    I should say 2nd attempt. I already bought some optimist bags that were too big.

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    Default Re: Shellback Dinghy - 100lbs of buoyancy bags enough?

    Pictures would help ( good luck with that one... do a search for how-to threads ).

    You're going to get about 60 pounds of buoyancy for every cubic foot of air in the bags.

    What made the Optimist bags too big, they didn't fit, over floated the boat...?
    Last edited by Tom Christie; 02-04-2016 at 04:07 PM.

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    Default Re: Shellback Dinghy - 100lbs of buoyancy bags enough?

    The opti bags are 39 x 18. That worked OK, but not great under the center seat, but it bulged out a bunch. And under the back seat it was really too big of a bag. I could partly inflate it, but it was not a very good looking option.

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    Default Re: Shellback Dinghy - 100lbs of buoyancy bags enough?

    There are lots of discussions about this issue here, but most of the dinghies using flotation bags have them running fore and aft along the sides, under or alongside the seats. You also may need to put a triangular bag in the bow, even if that in inconvenient for gear storage.

    Here's the set of Holt Opti bags being fitted to my Cosine Wherry for an open ocean row, not fully strapped down in position yet. Source = https://www.facebook.com/david.luckh...9763645&type=3



    Your biggest issue will be that uncapped / open CB case. Any way to cap it? Otherwise you absolutely must have enough flotation to keep the slot above water - even in rough seas. The online plans show a daggerboard, not a centerboard, which is a bit easier to cap -- some sailors have made a top piece that can be bungeed over the DB slot, or the DB top seals well enough if secured by bungees or lines.

    The reason why testing was suggested is that actually bailing it out while swamped may be the only accurate method to determine what is needed and where. All else is theory, which may not be the best thing to trust your life to.

    Personally I rarely sail very far from the shore in small open boats, and when I do I'm sailing in company most of the time. If I swamp the boat I'll haul it to shore and either dump it out or bail and pump there. In many cases if it is windy and rough enough to knock your boat down, it is rough enough to keep you from bailing it effectively. But these methods rely on a lot of "maybes", so having sufficient flotation for self-rescue is a wonderful thing!
    Last edited by Thorne; 02-04-2016 at 04:13 PM.
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    Default Re: Shellback Dinghy - 100lbs of buoyancy bags enough?

    Thanks. Yes, it's a daggerboard. I assume if I swamp the boat the board will be down because I will have been sailing. That should keep some water out. But I do have a nice cap for the slot as well.

    My primary concerns running the bags for and aft on this little boat is that when swamped the bags themselves will get in the way of getting water out (sloshing it out before trying to get in).

    But now you have me thinking about running them for and aft farther forward. Maybe I will keep these opti bags after all.


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    Default Re: Shellback Dinghy - 100lbs of buoyancy bags enough?

    There are some threads here showing some very clever tiedown loops made from nylon straps, and epoxied to the bottom and sides of the hull interior. You can also try the hard nylon strap loops, but they have to be screwed to a piece of wood that is glued to the hull, and can snag gear and trip crew. These tiedown loops must be well attached, as the flotation will be putting a lot of pressure on them via the bag straps when swamped.

    http://forum.woodenboat.com/showthread.php?191073-Eye-strap-pad-eyes-ideas-please


    Last edited by Thorne; 02-04-2016 at 05:16 PM.
    "The enemies of reason have a certain blind look."
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    Default

    Quote Originally Posted by Yeadon View Post
    Isn't there be some sort of actual methodology to figuring this out? Some sort of mathy framework that takes into consideration the overall displacement of the boat, and how much water the bags would need to displace within the interior volume of the boat if/when the boat swamped? I'm also a fan "just try it and see what happens." No shame there, of course.
    Just the sort of topic for two or three boaty friends to debate while trying to make a six pack disappear. To break the stalemate, said vessel is dragged bodily to the water accompanied by oaths of, " I'll show you!" and, "You'll see!" 😀

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    Default Re: Shellback Dinghy - 100lbs of buoyancy bags enough?

    Quote Originally Posted by Yeadon View Post
    Isn't there be some sort of actual methodology to figuring this out? Some sort of mathy framework that takes into consideration the overall displacement of the boat, and how much water the bags would need to displace within the interior volume of the boat if/when the boat swamped? I'm also a fan "just try it and see what happens." No shame there, of course.
    And where's the fun in that, eh? Mathy Methodologies be damned, I say! I'm with Breakaway, get happy and pull the boat over a few times to see what happens.

    If ya really wanna get all mathy on us, check out the gazillion threads here on the topic. A different theory for every day of the month, at least.
    "The enemies of reason have a certain blind look."
    Doctor Jacquin to Lieutenant D'Hubert, in Ridley Scott's first major film _The Duellists_.

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    Default Re: Shellback Dinghy - 100lbs of buoyancy bags enough?

    Any other Shellback owners have experience here? Andrew?
    If one advances confidently in the direction of his dreams, and endeavors to live the life which he has imagined, he will meet with a success unexpected in common hours.
    -Henry David Thoreau-

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    Default Re: Shellback Dinghy - 100lbs of buoyancy bags enough?

    I am now thinking of running two bags down low, for and aft. Something like the Holt 60x10 or Crewsaver 58x9. That will give me close to 150 pounds of buoyancy on each side, a total of nearly 300 lbs.

    WestMarine may be able to order the Holt's for me at a good price. But I am concerned about their longevity since they don't have a nylon cover like the Crewsaver line. I also like the looks of the Yellow Crewsaver bags better. Anyone have experience with the durability of either brand?

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    Default Re: Shellback Dinghy - 100lbs of buoyancy bags enough?

    My Holts are made from a very strong nylon material. I wouldn't dance on them with hobnails, but they seem pretty strong and are a standard part of racing Optis.

    What Yeadon is suggesting above is that the placement of flotation may determine your ability to right the boat if turned upside down, and/or your ability to re-board if you got thrown out or had to exit for another reason. So depending on the level of self-rescue you're looking for, you may need to have flotation in the ends of the boat rather than the sides. This is where testing really helps, as it will make very clear what you need, and where.
    Last edited by Thorne; 02-05-2016 at 04:55 PM.
    "The enemies of reason have a certain blind look."
    Doctor Jacquin to Lieutenant D'Hubert, in Ridley Scott's first major film _The Duellists_.

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    Default Re: Shellback Dinghy - 100lbs of buoyancy bags enough?

    It seems the Holt Opti bags are nylon covered. But the blue Holt bags you see sold at West Marine and other places are made from a "tough and thoroughly tested heavy duty PVC", and they have no nylon cover. One sales person that sells both said the Crewsaver nylon bags seems more durable. I wish I could compare both in person.

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    Default Re: Shellback Dinghy - 100lbs of buoyancy bags enough?

    Quote Originally Posted by Yeadon View Post
    Just for fun, test whether it's easier to recover your capsized dinghy by A) putting the bags on the side, or B) placing one in the bow and one in the stern.

    Personally, I believe fore and aft is better.
    It's a good idea to test. And I am aware of the benefits of both for and aft and side bags.

    But the difference in the shapes and size of bags for those two different applications is an issue when you are trying to buy the correct thing first. For side tanks I will need long and small diameter tubes. For bow and stern tanks completely different sized bags. There is not a lot of room under my rear seat for a very big bag, and I desire to keep the bow clear. So sidebags it is.

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    Default Re: Shellback Dinghy - 100lbs of buoyancy bags enough?

    My old Lowell dory-skiff was similar to a Shellback in dimensions, though obviously heavier. When capsized on a lake in Connecticut October gusts (no flotation) it could be righted but it kept rolling over again (doing 360s -- aluminum mast with loose foam particles inside, an old O'Day mast cut down). When I finally got bow to wind I could carefully keep it upright but even the waves on a lake (whipped up by a constant high wind, definitely small craft advisory weather), filled it again easily. It was more of a horrible situation than it sounds -- just 50 yards off a shore of empty summer houses, I was turning literally blue trying to get the boat back up for 45 minutes, so a a little like summer PNW sailing I suppose. Thinking back on that, I really could have used float bags up high to stabilize boat to catch breath, and raise a bit for bailing. At the least, to stabilize the boat to sail downwind -- I did end up sailing it back to the ramp downwind, loaded with water in a balancing act. -- Wade

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    Default Re: Shellback Dinghy - 100lbs of buoyancy bags enough?

    Wade - Thanks for your story. I completely understand, and that is what I am trying to avoid.

    BTW, I went to West Marine to inspect some of the blue Holt bags they had. I was not impressed at all with the quality for the price you pay (although West Marine's price is much better then another popular seller in the states). Unless that material is much tougher then I suspect, I will not be buying them. They reminded me of kids blowup balls or cheap inflatable dinghies. Which is odd, because I have seen them praised on this forum. I will be going with the Crewsaver brand with their nylon fabric. I prefer the yellow color anyway. Crewsaver bags are really hard to get in the U.S. though. I can only find one store that lists them at very high prices. For 3 bags they want $220. I can order them from the UK, including air shipping for $160.

    So what is my plan now?
    I started with the idea of 100 lbs buoyancy, based primarily on what would fit under my middle and back seats.
    Then, considering how much buoyancy is used in the optimist dinghies, and rethinking about placement, I decided to go with two long tubes on each side with 300 lbs total buoyancy.
    After more thinking I have moderated a little. I am going to go with two side bags, slightly forward of the center, each worth about 80 lbs of buoyancy. And one under the rear seat. The max size I can fit there gives about 39 lbs of buoyancy. So I will have a total of 200 lbs, spread out in a tripod shape for stability.

    Once I get it installed I will share pictures and provide video capsize tests so that more people can benefit from my experience. There are a lot of these Shellback dinghies out there.

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    Default Re: Shellback Dinghy - 100lbs of buoyancy bags enough?

    GP,

    You seem pretty invested in this project.
    May I suggest you make your own bags out of
    Hypalon ( a trademark for chlorosulfonated polyethylene (CSPE) synthetic rubber (CSM) noted for its resistance to chemicals, temperature extremes, and ultraviolet light. It was a product of DuPont Performance Elastomers, a subsidiary of DuPont).
    Cheaper no doubt, waaaayyyy better (the best) and exactly the diameters and size you want.
    N/C

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    Default Re: Shellback Dinghy - 100lbs of buoyancy bags enough?

    PS What about foam tubes along the outside gunwales like the West Marine dingy?
    They come in 8 and 10' lengths... Walker Bay.
    Don't buy WM though, $1000 kit!

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    Default Re: Shellback Dinghy - 100lbs of buoyancy bags enough?

    Quote Originally Posted by Tom Christie View Post
    GP,

    You seem pretty invested in this project.
    May I suggest you make your own bags out of
    Hypalon ( a trademark for chlorosulfonated polyethylene (CSPE) synthetic rubber (CSM) noted for its resistance to chemicals, temperature extremes, and ultraviolet light. It was a product of DuPont Performance Elastomers, a subsidiary of DuPont).
    Cheaper no doubt, waaaayyyy better (the best) and exactly the diameters and size you want.
    N/C
    Thanks for the suggestion Tom. I considered making my own. But I am just too busy to add that much effort into this project. I have a sickness about making everything myself I have to fight!

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    Default Re: Shellback Dinghy - 100lbs of buoyancy bags enough?

    Quote Originally Posted by Tom Christie View Post
    PS What about foam tubes along the outside gunwales like the West Marine dingy?
    They come in 8 and 10' lengths... Walker Bay.
    Don't buy WM though, $1000 kit!
    Thanks Tom. I am by no means an expert on this subject, but the tubes up high on the outside strike me as a poor place for reserve buoyancy. It may help prevent a knock down. But if you do capsize, there is a higher chance of the boat going fully turtle I would think, and it would be much more difficult to get it get upright. And once you do, the buoyancy is so high that it won't help much with keeping water below the dagger board. But mostly, I think it would be pretty ugly.

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    Default Re: Shellback Dinghy - 100lbs of buoyancy bags enough?

    How can you resist then?
    They'd be exactly the shape you need.
    Who else could do this, ahhhh, then get them made $$$
    Your call...

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    Default Re: Shellback Dinghy - 100lbs of buoyancy bags enough?

    Quote Originally Posted by GPBoatHead View Post
    I am now thinking of running two bags down low, for and aft. Something like the Holt 60x10 or Crewsaver 58x9. That will give me close to 150 pounds of buoyancy on each side, a total of nearly 300 lbs.

    WestMarine may be able to order the Holt's for me at a good price. But I am concerned about their longevity since they don't have a nylon cover like the Crewsaver line. I also like the looks of the Yellow Crewsaver bags better. Anyone have experience with the durability of either brand?
    Yes I've been a program director at a sailing school with dozens of Opti's. The Holt Opti bag is by far the better product. I can't remember trying the Crewsaver ones.
    Last edited by Hwyl; 02-06-2016 at 07:25 PM.

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    Default Re: Shellback Dinghy - 100lbs of buoyancy bags enough?

    Would it be possible to capsize test with a combo of cheap blowup toys tied into the ends /cheap pool noodles tied together on the gunwales? That might give you a better idea how much/where you need the floatation before you spend the big ones on good quality bags. You might end up finding that "keeping your bow free" means that you only go out in nice weather...

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    Default Re: Shellback Dinghy - 100lbs of buoyancy bags enough?

    Quote Originally Posted by BOI View Post
    Would it be possible to capsize test with a combo of cheap blowup toys tied into the ends /cheap pool noodles tied together on the gunwales? That might give you a better idea how much/where you need the floatation before you spend the big ones on good quality bags. You might end up finding that "keeping your bow free" means that you only go out in nice weather...
    It's not a bad idea. Attaching them is a slight issue, but could probably be worked out.

    One idea that I found buried in the forums, is to use Aere Beach Rollers that are sold by Duckworks, strapped down low.

    http://www.duckworksbbs.com/gear/beachroller/index.htm

    They are long enough to give good buoyancy for and aft, providing something over 150lbs for each side, and they can also serve double duty as beach rollers and as fenders. These are heavier, but really tough, which I like the idea of. They seem worth the money. I think I will bite the bullet and try them.

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    Default Re: Shellback Dinghy - 100lbs of buoyancy bags enough?

    In addition, you can glue 2" pink foam to the underside of the seats. That's what I did.

    Dave

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    Default Re: Shellback Dinghy - 100lbs of buoyancy bags enough?

    Quote Originally Posted by Dave Hadfield View Post
    In addition, you can glue 2" pink foam to the underside of the seats. That's what I did.

    Dave
    --- Yes, the cool thing about the closed-cell insulation foams is that you can experiment and also shape -- with a little effort -- for the spaces they are to fit. You can test them by just lashing them in, and save any gluing-in until you are certain about what you want. You can also epoxy on a layer of glass cloth to encapsulate them.
    -- Wade

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    Default Re: Shellback Dinghy - 100lbs of buoyancy bags enough?

    Quote Originally Posted by GPBoatHead View Post
    It's not a bad idea. Attaching them is a slight issue, but could probably be worked out.

    One idea that I found buried in the forums, is to use Aere Beach Rollers that are sold by Duckworks, strapped down low.

    http://www.duckworksbbs.com/gear/beachroller/index.htm

    They are long enough to give good buoyancy for and aft, providing something over 150lbs for each side, and they can also serve double duty as beach rollers and as fenders. These are heavier, but really tough, which I like the idea of. They seem worth the money. I think I will bite the bullet and try them.
    Just picked up a couple of the Are rollers to replace some of my HA bags which some seam issues. One of the challenges on the SB is fastening them down. You might look at the big fabric D rings structures on offer at places like NRS for rafts. I have also had good luck saturating a bit of webbing for a loop in glue and then holding it in place with a weight. Good news with modern glued structures is that they are nice and smooth inside. also the bad news when you need to fasten stuff down.
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    Default Re: Shellback Dinghy - 100lbs of buoyancy bags enough?

    Do people ever use fenders for this? http://www.discountmarinesupplies.co...vs-xoCdbTw_wcB

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    Default Re: Shellback Dinghy - 100lbs of buoyancy bags enough?

    So here are some pictures of the Beach Rollers installed in the Shellback dinghy. They actually work great. I have not done a capsize test yet. But they make a comfortable back cushion when sitting on the bottom sailing. My attachment points seem to be working OK also. Although I will have to rebond a couple of them. Overall I am happy with it.



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    Default Re: Shellback Dinghy - 100lbs of buoyancy bags enough?

    Quote Originally Posted by GPBoatHead View Post
    So here are some pictures of the Beach Rollers installed in the Shellback dinghy. They actually work great. I have not done a capsize test yet. But they make a comfortable back cushion when sitting on the bottom sailing. My attachment points seem to be working OK also. Although I will have to rebond a couple of them. Overall I am happy with it.


    Have you done a capsize test yet? Any further developments?

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    Default Re: Shellback Dinghy - 100lbs of buoyancy bags enough?

    Quote Originally Posted by tink View Post
    Have you done a capsize test yet? Any further developments?
    That was over two years ago...

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    Default Re: Shellback Dinghy - 100lbs of buoyancy bags enough?

    Quote Originally Posted by Tom Christie View Post
    That was over two years ago...
    Yes I’m aware I am re igniting in old thread but it is a good one but doesn’t reach a conclusion, it is like a good book with the last chapter missing.

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    Default Re: Shellback Dinghy - 100lbs of buoyancy bags enough?

    Quote Originally Posted by GPBoatHead View Post
    It's not a bad idea. Attaching them is a slight issue, but could probably be worked out.

    One idea that I found buried in the forums, is to use Aere Beach Rollers that are sold by Duckworks, strapped down low.

    http://www.duckworksbbs.com/gear/beachroller/index.htm

    They are long enough to give good buoyancy for and aft, providing something over 150lbs for each side, and they can also serve double duty as beach rollers and as fenders. These are heavier, but really tough, which I like the idea of. They seem worth the money. I think I will bite the bullet and try them.
    A cheap and easy way to test the volume needed is to bag up a bunch of 2 liter bottles in a trash bag. Try tying it (them) in different locations 'til you have the combination of flooded floatation/balance you're after. If you're worried about splashback from the daggerboard trunk, fit a foam gasket on the board and secure it with a bungie...the bungie is a good idea regardless.

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