Reply
 
Thread Tools Search this Thread
 
Old 30 October 2006, 15:51   #11
RIBnet admin team
 
Poly's Avatar
 
Country: UK - Scotland
Boat name: imposter
Make: FunYak
Length: 3m +
Engine: 2 stroke YAM 20 HP
MMSI: 235089819
Join Date: Sep 2005
Posts: 10,113
Quote:
Originally Posted by BogMonster View Post
Really? I'd have thought it was a lot more than that, purely going on "feel" (I have no pressure gauge). The tops of the tubes have black wear patches on them and a couple of weeks ago I saw them go from slightly sagging one day (to the extent of there being wrinkles in the tubes) and then zinging tight the next day in about 18 deg C and direct sunshine. I let some air out of them because I was worried they were too hard! But maybe it was only a few psi then, how do you work out the pressure change with temperature?
its either Boyle's law or Charles' law I can never remember which is which. But anyway :


Assuming the total amount of gas stays the same, and the volume is fixed (its not - but near enough - and the volume is getting bigger - so will relieve the pressure).
pressure at high temp = pressure at low temp x (high temp / low temp).
TEMPERATURES MUST BE ABSOLUTE - i.e. deg C + 273.
__________________
Poly is offline   Reply With Quote
Old 30 October 2006, 16:20   #12
Member
 
Country: Other
Town: Stanley, Falkland Is
Boat name: Seawolf
Make: Osprey Vipermax 5.8
Length: 5m +
Engine: Etec 150
Join Date: Mar 2006
Posts: 3,688
OK, so a 20 degree increase in temp is not going to make a very big difference in pressure. Interesting, thanks

On the theme of inner tubes, are there any other manufacturers apart from Gemini? I am told somebody here got a set of really heavy duty ones that needed to be inserted by cutting the end off the cone and have lasted for years, but I haven't caught up with him yet. Sounds like a different product to this but Google was mostly silent on the subject. Must be UK based, not worth the hassle of trying to get stuff shipped here from the States etc.
__________________

__________________
BogMonster is offline   Reply With Quote
Old 31 October 2006, 05:19   #13
Member
 
Pete7's Avatar
 
Country: UK - England
Town: Gosport
Boat name: April Lass
Make: Moody 31
Length: 9m +
Join Date: Aug 2001
Posts: 4,850
Quote:
Originally Posted by BogMonster View Post
OK, so a 20 degree increase in temp is not going to make a very big difference in pressure. Interesting, thanks
I beg to differ. On a hot summers day with dark coloured tubes they could be pumped up hard, but by dusk they will be soggy just down to changes air temperature during the day.

An idea - If its just the cones that are gone and the rest of the tubes are fine you could try solid end cones out of GRP, been done before. I think some of Dave Pictons early ribs had these.

Pete
__________________
.
Ribnet is best viewed on a computer of some sort
Pete7 is offline   Reply With Quote
Old 31 October 2006, 06:17   #14
Member
 
Country: Other
Town: Stanley, Falkland Is
Boat name: Seawolf
Make: Osprey Vipermax 5.8
Length: 5m +
Engine: Etec 150
Join Date: Mar 2006
Posts: 3,688
Quote:
Originally Posted by Pete7 View Post
An idea - If its just the cones that are gone and the rest of the tubes are fine you could try solid end cones out of GRP, been done before. I think some of Dave Pictons early ribs had these
Where would those be available from? Just thinking about it I guess they might be a lot easier to fit than floppy ones!

Forgot to take photos of it yesterday, will do it at lunchtime if I remember, and post tonight...
__________________
BogMonster is offline   Reply With Quote
Old 31 October 2006, 06:32   #15
Member
 
Pete7's Avatar
 
Country: UK - England
Town: Gosport
Boat name: April Lass
Make: Moody 31
Length: 9m +
Join Date: Aug 2001
Posts: 4,850
Quote:
Originally Posted by BogMonster View Post
Where would those be available from?
your going to have to make a plug and mould two (if the wifes pans set isn't close or you can't find a paint / chemical drum the right diameter, or try e mailing Dave Picton to see what diameter theres were.

Quote:
Originally Posted by BogMonster View Post
Just thinking about it I guess they might be a lot easier to fit than floppy ones!
Agreed the thought of trying to glue a floppy end cone into a deflated tube thats covered in contact glue, hmm
__________________
.
Ribnet is best viewed on a computer of some sort
Pete7 is offline   Reply With Quote
Old 31 October 2006, 07:30   #16
Member
 
Country: UK - Scotland
Make: HumberOceanOffshore
Length: 8m +
Engine: Volvo KAD300/DPX
Join Date: Oct 2002
Posts: 4,962
Quote:
Originally Posted by Polwart View Post
I don't think you would need to deflate the tubes all the time. Even if you inflate them to 5 psi at 0 deg C and it heats up to 60 deg C without finding anywhere to expand to (probably less of an issue for an inner tube than an actual tube - which is fixed rigidly to the hull) the pressure will only be 6.2 psi.
Neil . Assuming an 18" dia tube compartment 48" long, I make that about 1.44 tons of increased load for that tube compartment to contain.
__________________
JW.
jwalker is offline   Reply With Quote
Old 31 October 2006, 11:30   #17
Member
 
Country: Other
Town: Stanley, Falkland Is
Boat name: Seawolf
Make: Osprey Vipermax 5.8
Length: 5m +
Engine: Etec 150
Join Date: Mar 2006
Posts: 3,688
Quote:
Originally Posted by Pete7 View Post
Agreed the thought of trying to glue a floppy end cone into a deflated tube thats covered in contact glue, hmm
The thought of ending my days starving to death while irrevocably bonded to the punctured wreckage of an inflatable boat had crossed my mind
__________________
BogMonster is offline   Reply With Quote
Old 02 November 2006, 09:07   #18
Member
 
Country: UK - England
Town: Dartmouth
Boat name: TIDEL III
Make: AVON SEARIDER
Join Date: Jun 2003
Posts: 823
stephen you could try covering a circle of ply and sticking that in the end of the tube .it does work i have seen it on some of the p&o ferries rescue boats
__________________
paul tilley is offline   Reply With Quote
Old 02 November 2006, 10:53   #19
RIBnet admin team
 
Poly's Avatar
 
Country: UK - Scotland
Boat name: imposter
Make: FunYak
Length: 3m +
Engine: 2 stroke YAM 20 HP
MMSI: 235089819
Join Date: Sep 2005
Posts: 10,113
Quote:
Originally Posted by jwalker View Post
Neil . Assuming an 18" dia tube compartment 48" long, I make that about 1.44 tons of increased load for that tube compartment to contain.
Jeff, I haven't checked your calculation - but that is certainly one way of looking at it. I don't think you would inflate to 5psi normally anyway - more like 2-3 so the increase is also proportionately smaller. My point was that if "nominal" pressure was x, then ~20% over doesn't seem that high. (and that is in the extreeme case of filling at freezing and it reaching 60 deg C).

Quote:
Originally Posted by pete7
I beg to differ. On a hot summers day with dark coloured tubes they could be pumped up hard, but by dusk they will be soggy just down to changes air temperature during the day.
I think this is a slightly different phenomena. When the tube is inflated to normal working pressure it has essentially reached its maximum volume (in this case the inner tube has reached the volume determined by the hypalon outer) and therefore any heating either has to "stretch" the fabric - or increase the pressure. However, if a tube is firm when hot and then cooled down, the volume of the tube can contract as well as reduce in pressure.
__________________
Poly is offline   Reply With Quote
Old 02 November 2006, 12:10   #20
Member
 
Pete7's Avatar
 
Country: UK - England
Town: Gosport
Boat name: April Lass
Make: Moody 31
Length: 9m +
Join Date: Aug 2001
Posts: 4,850
Quote:
Originally Posted by Polwart View Post
I think this is a slightly different phenomena. When the tube is inflated to normal working pressure it has essentially reached its maximum volume (in this case the inner tube has reached the volume determined by the hypalon outer) and therefore any heating either has to "stretch" the fabric - or increase the pressure. However, if a tube is firm when hot and then cooled down, the volume of the tube can contract as well as reduce in pressure.
Hmm, I think its the same thing and agree with what you saying. So at 30.c on a hot summers day the tubes are hard and at 10.c that night they are soggy - so a huge difference in tube pressure with a change of 20.c in temperature. Pump them up at night time and leave the boat cover on during a hot summers day and you could end up with a burst seem, probably along the weakest link.

Pete
__________________

__________________
.
Ribnet is best viewed on a computer of some sort
Pete7 is offline   Reply With Quote
Reply

Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off





Copyright 2002- Social Knowledge, LLC All Rights Reserved.

All times are GMT -5. The time now is 13:07.


Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.