View Full Version : Looking for help with cross-bag design for integrated craft?

27-Jun-12, 07:17 PM
It’s time for a complete re-haul of my integrated racing craft,and I would like to incorporate a new cross-bag under the hull. The craft if 12 feet long and due to it’s four-cylinder four-stroke engine relatively tail heavy.

At what length would one recommend to place the bag, and what diameter does the feed hole/pipe need to be?

Any good technical advice is greatly appreciated, Thanks, Raoul.

Keith Lovell
27-Jun-12, 09:21 PM

What make of hull is it? Pics perhaps
We have a Vortex Storm mk 6 with a cross bag.


Keith @ Bearwood.

28-Jun-12, 04:24 PM
a general concensus is to find the balance point of the craft with you in it and position the bag forward of that point

just put a steel tube or piece of 2x2 wood under the craft and find at what point it balances

T shaped bags with a bag to the front skirt are popular, i once tried a Y shaped bag which improved stability greatly but made the craft harder to turn, i tend not to use them as it's all a bit too much fuss keeping them maintained/repaired

feed holes don't need to be large my friend uses a 1/2 feed pipe, another technique is to connect both ends of the bag to a segment

more questions than answers i'm afraid


1-Jul-12, 08:40 AM
...., another technique is to connect both ends of the bag to a segment

more questions than answers i'm afraid


Thanks for you reply Tony.

Is this technique of not having a separate feed and connecting both ends of the another whole below the segment recommended for an integrated craft? I also try to use my craft for hight speed cruising, and I think I'd like to keep pressure on the bag when I lower the rpm?

The bootom line, I'd like to get more comfortable at higher speeds. What shape bag is suggested for that?

Thanks again, Raoul.

1-Jul-12, 09:03 AM
tests i've done show that bags have equal pressure to the plenum pressure which is higher than skirt pressure, because the bag doesn't rely on a flow of air it can have a relativley small hole to feed it, i have fed bags with a single 1/2" tube, one thing i noticed was that as i stopped the craft it took longer to touch the floor - the pic is Ricky Gooosey with a Y bag fed through a tube into the plenum, recently Ricky just ran the front tube and he said it worked

if you are looking for more lift at lower revs then bags are not going necessarily going to help you, consider more blades, you'll get lift earlier but may lose top speed - the pic also shows that even though the side skirts have lost all pressure cos the craft is on its side the bags remain inflated


Keith Oakley
1-Jul-12, 03:18 PM
Hi Raoul - is this the beast you're upgrading? Variable splitter plate to increase lift at low revs is the best answer for low speed cruising but I suspect you're more into high speed cruising!


5-Jul-12, 04:58 PM
Hi Raoul - ... I suspect you're more into high speed cruising!


U are correct sir. OK, it's defiantly gone be one bag across the forward-middle Bag, with the main purpose is to help with slowing down from high speeds, and to make the craft more stable & comfortable w/o plow-in when I reduce throttle.

The main question remains, should I have feeds from the plenum chamber on both sides, or I think what might be better is a separate feed from the back on one side?

What are the pros and cons between these two methods of feeding the bag?


5-Jul-12, 09:37 PM
Hi Raoul,
On my Rocket I tried feeding the cross bag from the normal skirt holes on both sides but didn't like the results (not firm enough unless I was at full throttle), so I changed it to feed from a tube located behind the fan in the splitter up to one dedicated 2" hole into the bag (with no exit hole). The result was much better because the bag was pressurised quite hard. I probably should put some small drain holes in it. The front would never drop even in tail winds. You can see the cross/front bag on my Raptor when we race in Sept. It's a little different than what I did on the Rocket.