The vacuum pumps that home builders use show an amazing variety and range of capability. Everything from aquarium pumps to vacuum cleaners. A professional outfit at retail prices can easily cost $1000 US. That’s a steep price to pay for an amateur builder. So somewhere between a $1000 pump and your home vacuum cleaner there must be a rig to get the job done without bankrupting the builder.
The two important things are that a pump be reliable (if it quits during the cure you are screwed) and it has to have enough capacity to tolerate minor leaks in the bag or mold. Theoretically, if a bag is totally sealed, a very small pump is all you would need. Real life is very different. Even if you do a complete dress rehearsal with the bag and pump before committing to wetting out the reinforcements, and the bag proves to be tight, there are still things that happen during the actual lay up. For instance, a drop of wet resin where the bag seals to the mold will prevent the sealant tape from sticking and will cause a leak. There may be a minor leak in the pleats and folds of the bag that is nearly impossible to find. For these reasons, some significant pump capacity is needed – the more the better.
I’m sure there are a lot of opinions on this but to vacuum bag something the size of a Tornado hull side; I would recommend a pump of a least 4 cubic feet of air per minute. This equates to around 1/3 horse power which is a little bit more than the average HYVAC air conditioning service pump. There are oil filled pumps that can develop more vacuum than dry rotary vane pumps but they are messy and require a lot of maintenance. I have acquired several pumps over the years, a couple of Gast 1/3 horsepower rotary vane units, a one horse power (14 cubic feet per minute) Gast vane pump, and a two horse power oil filled vane pump (16 cubic feet per minute). Although most pumps can produce 27 to 29 inches of vacuum, 20 inches will produce good quality parts. If you can’t get above 15, the quality of the part will definitely start to suffer.
Finally, this brings us to the issue where to buy a pump. I swear by Ebay. If you search under ‘vacuum pump’ there will be a hundred or more hits. The 1/3 horse power Gast units are common in computer rooms and machine shops. A unit in excellent condition can be had for around $100. They retail for around $400. My 14 cubic foot pump cost $180 on Ebay and was brand new surplus. Retails for about $800. So, the deals are out there if you look around a little.

Thanks for the Tip!

When I set up my vacuum pump (home made) I also bought a vacuum generator as well as a backup. In case your main pump fails you should have a backup "plumbed" and ready. http://www.vacmotionorders.com/vcv10hs-basic-venturi-vacuum-generat.html

While I have never had a problem, it's nice to know that I have a spare in the shop.

When I first started looking for my main pump last year I went to a dairy farmer friend that was just in the process of selling their farm.
I asked if they wanted to sell one of the pumps they use to milk the cows ( a roots vane type) but another farmer had taken them earlier in the week

Yes, it's great to have a spare set up. I think builders that take on big vacuum bag projects are really compulsive gamblers. Half way into the layup with a thousand bucks of carbon and a bucket of resin with the clock ticking you start to wonder 'what the hell am I doing'! But if the result is a quality part that you know is better than 90% of what professional builders make - it's worth the stress.

I'll have to say I don't feel the pressure. The epoxy I use will sit in the pot or roller tray (10 oz. at a time) for one hour before I start to worry about it. It takes me about 90 minutes of "wet work" to layup a side of an A Cat hull. Most of my time is spent in the prep. work. And checking the bag before I turn up the heat. Once every thing is good it goes to 130/140 and six hours later I turn every thing off and walk away for 36 hour, then pop out the part. I used to use West Systems and always had to change rollers and brushes and mixing pots because the resin would start to pop off to quickly. But back then Gougeon only had two flavors "fast" or "slow".

We probably do a lot of the same things ahead of time to minimize "open time" during layup. The reason I was feeling some pressure was that my layup was a 34 foot tiamaran half hull. There were 4 layers of reinforcement - Kevlar, carbon fabric, carbon uni, and glass. That's about 6X the size of A-class. I used a Polyamide curing agent with a 90 minute pot life at 65 deg F. But I was usually working in 80 to 90 deg F. The part took about 6 liters of resin so I would measure out shead of time three batches of epoxy/hardner and mix them as needed. Once the epoxy is out of the pot an into the fabric it takes a lot longer to begin cure. Do you have any time saving ideas to help Mike with his 20 foot trimaran layup?

Since I am using a mold there is probably not a lot of crossover.
If he does use the "band of epoxy where the seal tape" (another thread) he should cover everything from that band outward with blue painters tape. I put down my "tacky tape" and then cover it as well as most of the flange before I start the wet work. Then after everything is in the mold (glass, core, peel ply, release film, and breather) I pull up the blue tape and every thing is "dry" and clean.

I do the same thing to protect tacky tape during layup. I also cut all pieces of glass and carbon out to size first. Check them for fit on the mold then number all from forward to aft in the upper aft corner. They are stacked on one end of the mold in the reverse order of the finished part. That way it's very hard to get mixed up and place something backwards or upside down.
My last boat was male mold using a caul plate to mold the outside skin. The caul plate is a flat plate of 3mm PVC sheet with a glossy finish. It's outside edges match the male mold edges when vacuumed against the hull side. Treated with mold release before layup.

do some of you use an vacuum resevoir? [reverse of an compressor tank]

this guy did it [btw used an old fridge compressor]

see link [use bablefish translator or something- org. text is in dutch]

http://www.xs4all.nl/~ebb/kiteboard/html/handleiding/hl_24bou.html?page=eee

Yes, my pump has a four gallon reservoir and then across the shed from where the compressor and vacuum pump are I have a six foot section of sched. 40 pipe setup as a reservoir and it has three connectors on it. One fore a vacuum gage, and the other two connect to vacuum line to go to my bag.

Lesburn and Kevin,

you are having a very informative thread here. Even if not too many post, please do go on, it is all valuable information.

I do not use a resvoir but aways install a resin trap to protect the pump. Can be made from PVC pipe and fittings. I have found it very useful to install a plastic ball valve in the vacuum line just before entry into the bag. That way you can control how fast a vacuum is drawn and stop the process if needed. For instance, if the bag is drawing down and you notice a piece of foam core or fabric is a little bit out of position, you can shut the valve and reposition it. Once a hard vacuum is drawn it is impossible to adjust anything. And, without a valve, you would have to go secure the pump and disconnect supply piping.

try putting the tacky tape on the bag first. place it one or two mm back from the edge. still use the painter's tape on the part, but the tape on the bag will cut down on the time the epoxy is 'out' before the bag is down. i have used both methods, the tape on the part and filling in folds and the tape on the bag. i did the first for about 20 years. after having used the second, i realize i wasted 20 years.

Putting the tape on the bag instead of the mold is a great idea. I'll try this next time. How do you handle multiple vacuum ports for large parts? If there is more than say ten feet between the port and the far end of the bag, the vacuum at the far end can be too weak. I usually put a 1/2 inch PVC pipe inside the bag with 1/4 inch holes drilled every six inches or so. I seal the pipe in a pleat in the bagging film at the edge of the mold. To prevent any distortion of the part, I tape strips of 1/8 inch plywood about 2 inches wide along the length of the pipe. The pipe with the plywood taped to the bottom is outside the peel ply but inside the breather layer. Seems to work pretty well on hull sides and spar layups.

If you already have even small aircompressor you can build a very cheap vacuumsextaktor using this:
https://www.airtechintl.com/Airtechstore/product.asp?Dept_ID=7&ProductID=20
If somebody is interesting I can upload pics of my setup.

Of course there are sombody who are interested
(at least I am)

Here you are, the total price about 100€

What is that? The venturi device, two gauges and a pipe as reservoar? What is the reservoar made of? Where did you get the tubing? Lots of questions

Quite intriguing!

Looks like a bit of pvc pipe that is connected to your WC [Wash it 1st] and a pair of end caps.

That's exactly what it is, to primitive for a lot of freaks...
Here i action making a ply/glass spar http://www.gust.ax/gallery/sydvast_2/08-02-28/
Ply/carbon http://www.gust.ax/gallery/mast/08-04-24/
And the next will be Foam/carbon