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thataway
Joined: 02 Nov 2003
Posts: 21390
City/Region: Pensacola
State or Province: FL
C-Dory Year: 2007
C-Dory Model: 25 Cruiser
Vessel Name: thataway
Photos: Thataway
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 Posted: Fri Feb 14, 2025 10:57 am Post subject: |
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| Donald Tyson wrote: | | https://www.nrs.com/learn/inflatable-boat-use-care-instructions?srsltid=AfmBOoqRtOcP-AVN-XnBOBJftLS8PTVb52IMiu2UkRqXeLqBCE-bbUTv |
Why would you carry an inflated dinghy on the roof of C Dory while it is being towed?
Of course the altitude has a major effect on pressure in a "capsule". One of my friends carried his inflatable on the cabin top inflated on his almost 400 mile trip back to Pensacola. He was surprised to see that the bottom had been entirely blown out of the inflatable. This was a high quality inflatable which was almost new!
Interesting that some folks have found a problem when traveling in a day trip on the water. Air will expand 1/480 part per 1 * F temp rise.
_________________
Bob Austin
Thataway
Thataway (Ex Seaweed) 2007 25 C Dory May 2018 to Oct. 2021
Thisaway 2006 22' CDory November 2011 to May 2018
Caracal 18 140 Suzuki 2007 to present
Thataway TomCat 255 150 Suzukis June 2006 thru August 2011
C Pelican; 1992, 22 Cruiser, 2002 thru 2006
Frequent Sea; 2003 C D 25, 2007 thru 2009
KA6PKB
Home port: Pensacola FL |
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ssobol
Joined: 27 Oct 2012
Posts: 3567
City/Region: SW Michigan
State or Province: MI
C-Dory Year: 2008
C-Dory Model: 22 Cruiser
Vessel Name: SoBELLE
Photos: SoBelle
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| Posted: Fri Feb 14, 2025 11:24 am Post subject: |
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| Donald Tyson wrote: | | Is altitude an issue for these boats. Does the pressure change enough btwn Donner Pass and Sacramento warrant deflating them before descending. Somewhere in the middle here my the suction cups of my EZPass often let go. And tires change pressure. Doe this threaten an inflatable. |
More important to let out a little air before ascending. Going down isn't a problem. |
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ssobol
Joined: 27 Oct 2012
Posts: 3567
City/Region: SW Michigan
State or Province: MI
C-Dory Year: 2008
C-Dory Model: 22 Cruiser
Vessel Name: SoBELLE
Photos: SoBelle
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| Posted: Fri Feb 14, 2025 11:33 am Post subject: |
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| thataway wrote: | ...
Interesting that some folks have found a problem when traveling in a day trip on the water. Air will expand 1/480 part per 1 * F temp rise. |
Particularly in RIBs, the air pressure in the tubes is relatively low, ~1.5 psi, and the normal pressure range is about .5 psi. The tubes are also high volume which means a little change in temp has a bigger effect. Add in darker tubes (orange, gray, or black) and that more of the tube is not in contact with the water, and you can have noticable changes in tube temp during a day. Larger, fancier RIBs have automatic pressure systems that vent or inflate the tubes automatically to keep the pressure in the proper range.
A SIB that is kept in the water will have less temp and pressure swings than one that is not. In Colby's example, rowing ashore, pulling the SIB out, and letting it sit for awhile will result in more pressure change than if it had stayed in the water.
When I used to have a larger RIB that I towed around, I had to keep an eye on it to manage the pressure. Sometimes I had to stop on the side of the road to let pressure out or to pump it up (floppy tubes when trailering can result in damage unless they are tied up). I got pretty good a judging the tube pressure via looking in the rearview mirror. The pressure was more stable once it was in the water. |
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Donald Tyson
Joined: 24 Jul 2023
Posts: 465
Photos: Thistle
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| Posted: Fri Feb 14, 2025 12:22 pm Post subject: |
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Remember, I'm the new Guy. I just learned why not to leave it up on top. Not that that was my goal but now I know for sure. NRS supposedly makes a gauge but I've not found it.
| thataway wrote: | | Donald Tyson wrote: | | https://www.nrs.com/learn/inflatable-boat-use-care-instructions?srsltid=AfmBOoqRtOcP-AVN-XnBOBJftLS8PTVb52IMiu2UkRqXeLqBCE-bbUTv |
Why would you carry an inflated dinghy on the roof of C Dory while it is being towed?
Of course the altitude has a major effect on pressure in a "capsule". One of my friends carried his inflatable on the cabin top inflated on his almost 400 mile trip back to Pensacola. He was surprised to see that the bottom had been entirely blown out of the inflatable. This was a high quality inflatable which was almost new!
Interesting that some folks have found a problem when traveling in a day trip on the water. Air will expand 1/480 part per 1 * F temp rise. |
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Donald Tyson
Joined: 24 Jul 2023
Posts: 465
Photos: Thistle
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| Posted: Fri Feb 14, 2025 12:24 pm Post subject: |
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Pretty sure you meant the other way around. Thoughts?
| ssobol wrote: | | Donald Tyson wrote: | | Is altitude an issue for these boats. Does the pressure change enough btwn Donner Pass and Sacramento warrant deflating them before descending. Somewhere in the middle here my the suction cups of my EZPass often let go. And tires change pressure. Doe this threaten an inflatable. |
More important to let out a little air before ascending. Going down isn't a problem. |
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ssobol
Joined: 27 Oct 2012
Posts: 3567
City/Region: SW Michigan
State or Province: MI
C-Dory Year: 2008
C-Dory Model: 22 Cruiser
Vessel Name: SoBELLE
Photos: SoBelle
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| Posted: Fri Feb 14, 2025 3:30 pm Post subject: |
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| Donald Tyson wrote: | Pretty sure you meant the other way around. Thoughts?
| ssobol wrote: | | Donald Tyson wrote: | | Is altitude an issue for these boats. Does the pressure change enough btwn Donner Pass and Sacramento warrant deflating them before descending. Somewhere in the middle here my the suction cups of my EZPass often let go. And tires change pressure. Doe this threaten an inflatable. |
More important to let out a little air before ascending. Going down isn't a problem. |
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External air pressure increases as you descend. This increases pressure on the outside of the tube, decreasing the pressure difference between the pressure inside the tube and the outside.
As you ascend, the outside air pressure decreases, so the pressure differential increases.
RIB and SIB tubes are designed for a certain pressure difference between the inside and outside air pressures. When you inflate a tube you are increasing the pressure X amount above ambient pressure (in the case of a RIB about 1.5 psi, a SIB is about 3 psi).
If you inflate a tube and then increase its altitude (or reduce the ambient air pressure), it will eventually burst. This can happen with any pressurized object if the pressure differential exceeds its strength.
If you pump up an inflatable at a high altitude to normal working pressure (i.e. some amount above ambient) and then take it to a lower altitude, the pressure differential will decrease and the tubes will get soft.
While SIBs and RIBs seem like they are very rigid when inflated, it doesn't take very much pressure above ambient to accomplish this. This is a benefit of large volume. Smaller things like inflatable SUPs and kayaks need a higher pressure because the volume is a lot less. I have an inflatable kayak that needs 15 psi (above ambient) to be inflated properly. But the acceptable pressure range is also greater. |
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