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Old 25-06-2009, 17:54   #1
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Lightbulb Lifting One Hull for Maintenance

We don't have a big tidal range in Tampa Bay, Fl area so I can't dry out Cat Tales to do maintenance on sail drives and props. The saildrives/props need attention at least annually, but I'm trying to do haulouts every 2 or 3 years. I was thinking about lifting one hull out of the water at a time using lift bags. Has anyone experimented with this idea?

I found a company that specializes in lift bags for divers and salvage companies and they have bags with a 4400lb lift capacity with the right dimensions to snug up against my keels. I think I may be able to lift half my boat with 2 of these bags. If I tie the bags together (they have s/s tie rings) with heavy straps under the keel and secure them fore & aft with straps around the keel they shouldn't pop out, right? Is this a good idea, or great material for "Funny Home Videos?"

Another issue. Before inflation, the bags will extend up to waterline. As I inflate them, will I lose bouyancy as they start to rise out of the water? Will my 8800lb lift capacity be reduced to 4400lb when they are half out of the water? Isn't that what Archimedes would say? If I go for bigger bags, there will be more of the bags sticking out of the water so that might not work. Any ideas on best way to do this?

Greg, SV Cat Tales
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Old 25-06-2009, 18:16   #2
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It would seem to me that for the bags to work, you would need to rig a system where the bags stay under water to generate maximum lift. I suspect if the bags break the surface to a significant degree, you won't get enough lift from the bags to make it possible to do your intended jobs.

Working on the saildrives with the boat still in the water sounds like asking for trouble. If you open up the saildrives and any oil drains/drips into the water, the coast guard could hit you with big fines. Lots of people would be more than happy to report such incidents to the coast guard.

If you are just working on the props, you should be able to do that with scuba gear or a hooka set up, and no need for lift bags.

I'm not usually a naysayer, but in this case, I suspect that hauling the boat is the answer, especially if you are going to work on the saildrives.

I hope you can figure out a way to make it work.

Dave -Sailing Vessel Exit Only
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Old 25-06-2009, 21:30   #3
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When I first started looking at saildrives I thought "Cool, nice idea". But, man it seems like a too much maintenance.
That being said, I'm concerned about the stability of the cat once it's been raised on these balloons (lift bags) - I've used smaller bags.

Good Luck on your work, update us OK?
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Old 25-06-2009, 21:38   #4
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Yes, if they break the surface, they lose buoyancy. Use more bags to account for this. I sort of suspect that a Priv. 37 is more than 8800 lbs x 2 anyway
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Old 26-06-2009, 05:28   #5
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Thanks for the inputs. As Dave said, it would be easier to do mx on saildrives during haulout and I plan on hauling this winter. Unfortunately, the yards in Tampa area don't let you do your own work or rush you to complete work after they finish the bottom painting. I'm not a glutton for work, but I don't feel comfortable with letting someone with unknown skills taking my saildrives apart to reseal them. I also want to mechanically fasten the rubber fairing to the outside of the hull with strips of aluminum or plastic and I don't want screws penetrating the hull (glue alone hasn't worked for me.) Within the next couple years, I will need to replace the rubber grommets that seals out the ocean around the sail drives and that could be a time consuming job. I prefer taking my time to do these jobs carefully without the yard pressuring me to get back in the water. My plan would be to accomplish all the saildrive mx before haulout and then just "hand over" Cat Tales to the yard for bottom painting.

While working on the saildrives, I would position the dinghy under them as a work platform and to ensure that no oil would spill into the water. I know this sounds like a hair-brained scheme, but I'm fortunate to have my own dock with all my tools handy; a fridge with cold beer immediately available; and a garage with a nice workbench for working on stuff out of the sun and rain.

Evan & Dave, I think you are right about losing buoyancy when the bags break the surface. I found another source of lift bags that has long sausage shaped bags that measure 15ft long and only 29" diameter. They may be easier to hold under water than the large pillow-shaped bags.

I did find a yard in Key West that allows you to do your own work. Plan B is to haul out there and get all my sail drive work done. We are planning a trip to Bahamas next winter so Key West isn't too far out of the way.
Greg, SV Cat Tales
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Old 26-06-2009, 06:29   #6
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Cat Tales - how big is your back yard? ... Can you crane the boat out and park it there? Work at your leisure and crane it back in 6 weeks later when you've completed everything.
The light at the end of the tunnel are no longer the headlights of the oncoming train......yippee
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Old 26-06-2009, 06:35   #7
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OK so extrapolate this "weird" idea a bit further. Since you have your own dock and can take your time, why not build your own permanent or semi-permanent inflatable lift?

Build a cradle to contain your lift bag. The top can be flat with perhaps a few PVC guide poles to keep it in position beneath the hull before inflation. All you need on top of the cradle is a flat wood surface for the keel to rest upon. The key is to make the underwater part of the cradle big enough for the fully-inflated size of the lift bag so that the expansion does not stress your cradle from the side. Maybe a nylon strap or two to help keep the bag in position. Pad the keel resting surface with some old carpet. Maybe pad the whole thing to prevent chafing the bags.

Float the assembly into position under the hull you want to lift, then inflate the bags. This eliminates the problem of decreased lift as the bag comes out of the water. If you do one hull at a time you'll have to handle side forces of the bag within the cradle as the bag will try to roll out the side and pop to the surface. Doing both hulls level would be easier and allow you to use (build) a smaller / lighter cradle.

Just like those plastic docks with the inflatable tubes.... just not as refined. Not such a far-fetched idea....

And if you build this I'll be a customer of your 'mini-lift' yard in Apollo Beach
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Old 26-06-2009, 07:09   #8
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What is your draft and what is the clearance between your bridge deck and the water surface? If the clearance isn't that great you could back the yacht into fairly shallow water at some convenient location, like the northeast side of Emerson Point on the Manatee during Springs, inflate a couple of bags under the aft end of the bridge deck to raise the stern somewhat, which would allow you to pull the yacht in closer to the shore, and then, as the tide runs out, block the aft end of the hulls with cut-off lengths of discarded railroad ties to support the yacht until the next tide. The bow would likely settle on the bottom to lock the boat in place and you'd have proper support for the hulls aft. Absent that support, getting under the boat is a formula for disaster.

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Old 26-06-2009, 07:09   #9
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Greg, why don't you buy two of those air tubes and raise the whole boat. When it's shown how great it will work you can rent out the tubes to other cat folks and get your money back? I'll rent them for sure.
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Old 26-06-2009, 09:11   #10
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If you have only to get under the sterns, what about beaching her backwards? You probably wouldn't need much tidal range, or even a full tide cycle. Just enough to lift the aft ends of your keels a bit. With your bows downhill the sterns will lift higher as the water recedes.

Dave's concern about oil seems valid.
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Old 26-06-2009, 09:23   #11
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SVcattales, what kind of sail drives do you have. I have Yanmar SD40s. Yanmar sells an engine bed kit which forms the flange to which the saildrive bolts. The diameter of the flange is significantly wider than the hole in the bottom of the boat through which the foot of the saildrive passes. This forms a small shelf around the foot of the saildrive. Endeavour (my boat's an Endeavourcat 44) built a small fiberglass frame that screws into the bottom and holds the rubber boot in place. The screws do not penetrate any part of the hull proper, but only that shelf. Just to keep the water out of the layup, bedding compound is used between the boot and the hull. I've been using 3M 4200 but I'm sure other will work just as well. So far this seems to work pretty well. If your builder used the Yanmar engine bed, you might be able to use a similar solution. I replaced the boots last year my self. I also changed the diaphragm seals as Yanmar recommends every 2 years and mine were 7 years old. That was a total waste of money. The ones that came out looked just like the ones that went in and were just as flexible. I keep the old ones stored in a locker in case I ever need a spare.
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Old 26-06-2009, 10:32   #12
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As a diver, I am doubtful you can easily accomplish what you want with lift bags. The bag design takes advantage of breaking the surface of the water. You might be able to build a platform, enclose them in it and raise the entire thing.

By the time you are done, you may want to speak with Airdock instead. You are on the fringe of the limits and I have only see them used on a smaller power boat. That is not a personal endorsement as I have not personally used them, but the guy seemed happy with it behind his house.
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Old 28-06-2009, 13:56   #13
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Bag lifts are prone to surprises, and you don't want any surprises when you're under a boat. Bags burp. Try as you might to get the inflation even, one will have more lift, the boat will cock, and straps will slide off slick hulls, exacerbating the cock. By the time you've got the bags secured laterally as well as for and aft, you're exhausted and still have a day's work to do. The idea of floating an upside down airtight strongbox under the aft bridgedeck appeals to me because you can position it very precisely and secure it against moving in all directions. Then you can fill it first with inner-tubes and then with free air. I would prefer to lift both transoms so that the bows and the box form a widely spaced tripod. The box would probably have to be 7 feet tall, and long enough to span the bride deck. Maybe a half-size shipping container would work.

Nah. Go down to the Keys.
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Old 28-06-2009, 15:42   #14
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Thanks for all the inputs. You have given me lots to think about. Here is a link for lift bags . I've been looking at the "elongated pontoon" bags.

I'm concerned about keeping the bags from burping out, as some of you mentioned, especially since the dynamics will change as Cat Tales begins to heel over. Using two of the biggest bags (4400lbs ea) gives me just enough lift to raise half the boat, but I can't afford the bags breaking the surface and losing lift capacity. I liked Mark's idea, but I also have depth issues. Cat Tales draws 4' at the keels and the water depth is about 7.5' at high tide. I don't have a lot of room to have a cradle and the bags held captive underneath it. I also think the lift would me more stable if the bags were secured under the hull on both sides of the keel. I envision the tubes attached to each other with the keel keeping the inboard pontoon from escaping and also holding the outboard pontoon in place.

Kefaa, you made some excellent points. All the pictures of lift bags in use show them on the surface and that would be a more stable set-up. I have checked on air dock system, but its a lot more expensive (about $3,800), it is very heavy, and would be difficult to store when not in use.

Craig, please don't be disappointed if I don't size the lift to handle Silent Faith. To lift your Voyage 50, I'd need the "Goodyear Blimp lift bags".

My next task is to measure how far I need to lift the hull to clear the saildrive out of the water. Then I can figure out whether I can keep most of the tubes below the surface of the water.

Greg, SV Cat Tales
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