The recommended air pressure vairies with make, model and manufactor, but most are surprisingly low. For example my AIRE Couger tops off at 2.5 pounds per square inch, some floors have a PRV valve rated at 1.5 psi.
I purchased a special low pressure air gauge along with my first boat. This was very helpful in the begining. After a while I could judge the pressure pretty accurately by pressing a knuckle into the inflated boat.
Keep in mind that air pressure is a functin of temperature, the air pressure in the boat changes a lot when it's moved from the hot beach to the cold water or visa-versa. Be careful of boats parked on hot sand I have seen them rupture when warmed up in the sun!
I never knew a low pressure gauge existed till my last trip. I have a 19 foot JPW cat that is from the late 20th century. It's old. Only has two patches but they are each four feet long. I asked the guy with the gauge the same question.... how much is too much? I HAD been running 3/4 to 1 1/4 PSI utilizing the time honored thump test.. He assured me that 2 1/2 pounds was the upper limit. Thus, I inflated to 1 1/2 PSI. One of the fun things about a gauge is you can see the effects of parking in the sun. The sunniest most beached chamber of the boat would go to almost 3, the shadiest might sink to under 1. Having all been equal at sun up inflation of 1 1/2. Be careful.
OK, not suggesting you do this, but I have a 15.5' PVC glued seam Star. Star recommends a max of 3.0 PSI. I have owned the boat for 9 years now and guided fishing from it for about 6 of those. I use this boat a LOT. I always inflate it to 4.8 psi in the shade because it almost always drops .5-1 PSI when it hits the water. At 3 PSI it is a bitch to row but at 4+PSI it gets a lot easier. I have had it on the trailer in the sun and have confirmed it getting as high as 6.5 PSI in the sun. I have never had a problem with blowing a seam or any valves etc. I know with smaller boats made of hypalon and thinner PVC you have to be much more careful. I have a pressure gauge and I always use it when inflating. I also use an air compressor to inflate. I know I'm breaking a lot of rules here but I hate a mushy raft. I'd be interested to hear where others keep their boats inflated to. No, I'm not a patsy for the raft repair industry either.
Word of caution. You can blow seams at pressures well below the recommended limits if the glue gets hot enough.
Once I foolishly blew up my boat on a section of fresh asphalt. Not noticing the surface temperature of the jet black pavement was waytoo hot for my floor. Blew every seam in the floor before I could even feel resistance in the pump.
As noted, the amount of pressure to explode a seam varies so much. Construction, temperature, material, age, exposure, care, the list goes on and on. If the boat is drum tight and left in the sun it can certainly exceed its limits, whatever they may be.
2.5-3.5 pounds is usually a good number for most standard sized rafts to run at. The amount of pressure needed to make a tube hard is related to its diameter. The bigger the tube, the less pressure it needs to be hard. A good way to see this is on a cat tube. Next time you see a cat on the water, if you dont own one, push on the tube close to the tip and then push on it where the tube is fat. You will find that the tube will be much harder in the middle where the tube is big and very squeezable at the ends even though the whole tube is under the same pressure. On the water, i always use the palm test that I have used since i was a kid. I want the boat to give a little when i push my palm into it at the fat part of the tube or i dont feel comfortable. I am certain that todays boats and materials will take much higher pressure than the boats I grew up with, but i'm not ready to let go of the palm test. Pumping a boat to 4.8 lbs would scare the hell out of me if it was my boat. Obviously, it has worked fine for Carroth but i certainly wouldn't recommend it. I did look at a Saturn i had in the shop today and it said max pressure 5.8 lbs. Keep in mind I am doing seam work on that saturn. I pump my boats up according to the palm test in the morning and rarely bleed air out during the day. I have a pressure gauge at the shop but it rarely gets used. Bottom line is, if the thing is drum tight in the morning and it sun bakes all day, especially on land, don't be surprised if you burst a seam. If you pump it drum tight in Denver and drive over Vail pass, don't be surprised if you burst a seam. I think that most rafts are very forgiving but common sense goes a long way. If you want to run it tighter than the palm test, run it just where it wont take a palm anymore, stop pumping and you should be fine. I have sat on rafts that just feel hard on my ass. If a rubber raft feels like you are sitting on a board, it might be a little tight. By all means, test until your hearts content, pump away until you are blue in the face and then click on my signature below to set up an appt.
My 30 yo hyside (Hypalon) gets pumped to about 2psi. With it's age I'm scared to pump it up anymore than that. It makes it give just a little bit for a hypalon boat...and leaves a little room for the sun to heat it up.
My Revolution (Urethane) also gets about 2psi but it is ROCK HARD at that pressure. I like it but it took getting a pressure gauge to realize that I wasn't pumping it up all the way when I first got the boat. At 1psi it feels as squishy as the Hyside does at 2.5psi.
My Sea Eagle ducky (PVC) is about 4psi I think and that makes it super stiff (not like revo tho). I believe it gets about 7 psi in the drop stich floor.
My cheap chinese bucket boat (pvc) gets enough to float-probably 1.5-2psi, but it has a slow leak I just can't fix (or don't care to put the effort into fixing)
Eitherway I am super anal about bleeding air off the tubes when it gets hot and topping them off when it cools along the way.
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