infatable pressure loss

I have a 12ft inflatable with a detachable transom,the tubes are continuous all the way round the boat.I have fitted a ply floor for rigidity and comfort.
I noticed after half an hour in the water the tubes seemed to deflate,and the outboard started to push itself under the stern.I believe this is the water temperature cooling down the air in the tubes.What I do now is take a hand pump with me,and re-inflate the tubes after half an hour or so.Has anybody else had a similar problem? Initially the tubes were inflated to the manufacturers spec.

 

Might hold true if there was a significant difference between air and water temperature, otherwise you are looking for a leakage somewhere. If the inflation holds up out of the water, your theory might be right, but I have never heard of such an effect. Water here probably too warm!

 

Inlatable boat

When the boat is stored inflated on dry land it holds its pressure indefinately,
When the evening cools down and into the morning until the sun comes up it deflates,then regains its original pressure.
I suppose the difference in the climate,like you say,is the reason.
I have swum in lakes and lochs here in the summer,and the temperature is very low.When snorkeling the temp. changes after about 4ft to a couple of degrees above freezing.It causes quite a lot of fatalities here in the uk to unsuspecting swimmers.When I was learning to dive one of the tests was to snorkel down to a diver at 20ft 3 times,it was almost impossible without a wetsuit.

 

thats what happened with mine. in the cool night air it would look half flat and during the day it would go tight again.

 

This is a very common problem with small inflatables and non-elastic tube materials like Hypalon. In cold air or water they go limp, on a summer day in the Meds they blow off air through a safety valve or rip a seam if there is no valve.

One solution would be a compensating cylinder with a spring loaded bellows, but to be effective it must have approx. 10% of the tube volume, which makes it quite bulky.

Another possibility would be to pour a small quantity of a hydrocarbon liquid with a boiling point near 5 degrees C. in the tube. Such liquids are used in spray cans for contacts or video head cleaning.

 

That's why we call them "deflatables." What you described is a common occurrence with every deflatable that I have seen or owned. You just have to pump them up every morning or anytime there is a decrease in temperature in the air in the tubes. I always used to take the pump with me because without fail every time you got back to the dink after an evening out it would be deflated. Also damp from dew. Don't forget to keep a towel on board.

 

Yah sure. Temperature changes . A fully inflated boat will blow off air thru its valves in hot sun. Hence, hard during the day , soft in the morning.

Also look for leaks. A typical one is the valve itself. Give it a blast of silicone spray when you inflate. The "O" ring stays plyable and seats better.

 

limits

I have re-inflated whilst it is in its deflated period,but I am not sure whether this idea takes it beyond the makers inflation limits.They dont specify a time to inflate,or limits.Mine is marked with a 6" measurement,and they supplied a tape to check the correct distance for inflation.Even when I use the above method it doesn't exceed the limits,but the chambers are rock hard.
I suppose this is one of the joys of inflatables.

 

yes they are horrible things but very convenient when you can put them in the boot of the car.

 

Is it made of Hypalon or PVC? With Hypalon, you should inflate it until it's pretty hard and the entire boat is stiff. Don't worry about over-inflation, you won't be able to get enough pressure to do that with any foot pump or inflatable electric pump that I know of. Most can't even come close.

I worry a little bit more about PVC if it's a really hot day, but not much, I've never seen one actually blow a seam unless the seam was abraded badly or about to blow anyway.

BTW I used to own an inflatable boat business for years and we did a lot of repairs.

Frankly, overnight when it gets cold, most inflatables deflate a little. Just pump it up in the morning and go on your business.

 

Many inflatables are fitted with pressure relief valves to prevent overpressure in the tropics. Give them a clean if you suspect leaks.

http://www.youtube.com/watch?v=AIB0eq60kFo

 

A detachable transom?

 

Hi,
The inflatable I have has a continuous inflatable tube,with no gap for a transom,[Quite common on less expensive inflatables]It has attachment points for a stainless tubed frame that fixes to it.On the frame there is a wooden transom to enable a motor to be attached.I have two short shaft seagull engines,a 40 minus and a 40 plus,both these engines work quite well on it.Of course the choice of engine is your perogitive.Bearing in mind the max.size recommended by the boat manufacturer.It is possible to go slightly over their engine size,bearing in mind the extra thrust on the boat.
I use the boat in a cold climate where the water is always cool,so I find that the boat loses pressure when it first goes into the water because of the temperature change,I take a pump along in the boat to bring the pressure back up.If I dont do this the prop ends up taking up the lack of rigidity and ends up under the stern about a foot,which of course changes the angle of the motor and makes it totally inefficient.I might add that seagull engines are not easy to set up because they are dependant on the correct depth of the cavitation plate because they depend on the back pressure on the two stroke engines,if they are too deep they will not start or run correctly.
Hope this helps.

 

This is just one of the reasons I gave up on RIBs years ago.

It was cheaper to buy a 3 passenger 215 hp Seadoo than to buy a good quality 3 passenger RIB with an outboard.

 

"it is you- know that your were doing it wrong.... "

http://www.youtube.com/watch?v=6rb13ksYO0s