The Vacuum Bagging Techniques Engineering Essay

Published: Last Edited:

This essay has been submitted by a student. This is not an example of the work written by our professional essay writers.

Start by laying down the plastic over the area that the hull will be vacuum bagged to. Lay the Styrofoam on top of the plastic. Mix epoxy completely for 2 - 3 minutes and spread onto both Styrofoam surfaces that are being joined. Spread the epoxy on with a paint roller like a thick coat of paint. Recoat lf the epoxy soaks into the Styrofoam. Use long screws or nails to hold the Styrofoam in place while the epoxy cures. Next, lay the 2nd piece over the entire hull. Lay down blankets to cover the entire hull. Next, lay the vacuum hose onto the hull as in the drawing below Cover all of the exposed hose with another blanket. If the optional scale will be used, put the scale close to the center of the hull. The scale should read between 50 -150 pounds when the vacuum is turned on. This setup should put about 1lb./sq. in. or I44-lbs/sq. foot on the hull, The larger piece of plastic can now be placed over the hull. Tape the plastic down around the perimeter, 6'' away from the hull. This will leave slack In the plastic until the vacuum is turned on. Attach the vacuum to the hose with adapters or duct tape. Turn the vacuum on. Within 30 seconds, the plastic should be pressed firmly up against the hull. Check for leaks in the vacuum bags and seal with tape. Pull on the plastic in a few different areas. It should be difficult to pull. If it isn't, there are leaks in the bag. The leaks need to be found and taped. The vacuum should have some air moving over the motor. It may be necessary to position o fan next to the motor on the vacuum to cool the motor. Watch the setup for at least 1 hour before leaving. The vacuum should run for 4 - 12 hours until the epoxy in the cup is cured.

The enclosed vacuum bag is the same except that there is no level surface to vacuum to, so the bag goes around the entire hull. The plastic can either be one large piece folded in half, or 2 pieces. The bag on the top and bottom are taped together at the seams. The east piece of plastic is the larger one. Next, lay down the small piece of plastic, then the hull. After the hull is glued, lay another small piece of plastic over it. Put on the blanket, then the hose and then another blanket. Finally, cover the hull with the other large piece of plastic or fold the larger piece over the hull. Tape the seams and turn the vacuum on and check for leaks.

Carefully remove the vacuum bag and save it for the next job. Mark a center line on the top of the hull and measure left & right of the line to get the collect hull side shape at several positions as shown on the plans (for pointed carft). Bend or hold curved a long thin piece of wood or metal intersecting previously marked points. This piece of wood will give the finished shape of the carft. Mark and cut this cube carefully with a haywire or saw. Be sure the cut is 90 degrees or perpendicular to the hull's top surface. A gravity hot wire may be used for this cut (place a weight on the end of a wire). Sand the edges so the skirt attach strips fit with no gaps between the foam and wood. Glue the outer skirt attach strips to the foam hull. Hold in place with 4 ½" screws until the glue is dry.

Now shape the hull top by removing form at the topsides to for a smooth curved surface for bonding the 1/8" plywood to the top foam and to the top of the skirt attach steps. Fiberglass from the top plywood around the attach strip onto the bottom. Use the vacuum bag to attach plywood onto the deck.

Using the vacuum bag again, glue the bottom foam and plywood which supports the landing skids and (he inside skill attach strips on to the hull. Use 4 ½'' screws to hold the foam from moving while the vacuum bag is put ill place. The inside skid attach strips are glued and stapled to the 1/8" plywood after the vacuum bag is removed. The aft skirt attach strip glued into a slot cut in the foam hull.


Place the carft on a low bench and scow the four comers inside and outside just. Then adjust the skirt to the position with the least amount of uniting and even hanging all amend. Install the skirt so that the contact line is the same height all the way around the hull. This is impartial in achieving good performance. What happens to the rest of the skirt is not important. Use _x#6 round Phillips sheet metal shows every four to six inches. Run the lib engine to see if any further adjustments are necessary whale the crate is on the bench.

It is best to check the skid adjustment with the craft hovering on a hard level surface such as a cement poor. Put extra weight on the front of the crate to trim and level the carft when on cushion. Inspect the skirt for winkles and high or low areas, especially at the comers. Adjust by moving material at the skid attach strip or by moving the tack lane. In extreme cases, you may have to redo a comer to take up more material. But the extra 5 to 15 mph incense in speed is worth the effort.

The aft part of the skirt is likely to get the most wear and tear. Extra care should be taken when attaching the skin especially across the as inside attach strip. The skirt-attach strip should be checked after high-speed operation on rough water. Rough water imposes high pulling forces on the skin and skirt-attach strips. Loose attach strips or screws can cause see large tears in the skin by failing at high speed and permitting the skin to stop water. Properly placed skirt-attach strips, good diving techniques, and a properly balanced carft will prevent a lot of skirt wear.

The best way to attach a sell cogently the just time is to mark perpendicular chalk lanes on the skill even they fed on each side. Then mark the correspondent zincs on the hull perpendicular to the outer skirt-attach strap or tangent line at that point, marking the outer and inner skin attach amp. Attach the inside skin at these points and adjust the excess material evenly between these points. It is easiest to do this whale the carft is indexed, before building the top stricture.

The skirt should be checked and repaired if necessary before each outing. One loose screw can cause others to come loose until the skirt no longer functions properly. A small tear can quickly get large enough to impair performance if it is not repaired.

When a hovercraft is shut off on the water the bag skid will slowly fill with water because the skirt tends to sink. It would be very helpful if the skirt could float or if there was a good way to attach some flotation to the skirt. Most methods we have mud seem to be more trouble than they are worth. Therefore, a good skirt drainage system is necessary to drain the skin after the carft has been shut down on the water.

Two large slits 3 to 4 inches long starting just behind and above the contact line in the aft skirt should drain any skin in 15 to 30 seconds with the lift system running at power and the thrust system at 1/4 to l/3 power. On single engine craft, use just enough power for a stable hover. Up of mere system running at ICR causes excessive spray and propeller blade erosion as well as more noise and possible belt slippage. Note that slipping belts will wear aluminum sheaves very rapidly.

In snowy terrain, most of the snow drawn into the skid system will be removed if a 3 to 4 inch section of the corner is left open, just above the ground contact point Choose the rear comer closest to the fan and in the direction of rotation of the fan as it passes over the skirt air-feed duct.

If the front skirt tucks back easily when the craft is at cruise speed, check for loss of skirt pressure at the tack strips. Sometimes you may have to use fiberglass to seal and help hold these tack strips to the hull. Also check for holes or tears in the skirt. The skid air-feed opening should be about 3 to 4 inches, depending on the size of the crate, at the widest point to ensure that enough air is fed to the skid. This skid air-feed duct should expend up close to the lift fan to keep the skirt at a higher pressure than the air cushion chamber. Fan tip clearances should be as small as possible, especially over the skirt air-feed duct. The lift fan should be placed as high as possible In the duct to prevent water damage during a plow in condition, but at least one inch below the bottom of the lie duct inlet lip radius. Lift ducts and thrust ducts alike must be guarded with wire mesh. A handle should also be installed on the front of the craft to simplify relocation chores. The lift duct should never be used as a handle for relocating a hovercraft.

Hovercraft with rear-mounted lib fans are generally snore difficult to get user planning speed because the skid does not recover from tucking as quickly as it would with a front mounted Ilt1 fan. If the craft won't plane out at full power, ease off the throttle to let the skid recover from the bow wave (about 1 to 2 seconds) and then quickly apply full power again. Planning is more difficult in shallow water (6 to 30 inches), but easiest in vet shallow water (up to 6 inches). To make planning easier, reduce lift power so the craft is just barely on cushion. If the air pressure in the skid is low, the bow wave will pass under the skirt easier. If the craft won't plane as it should. Inspect the skirt and skirt-attach strips for damage or loose shows. Also check for water-in-the skid or in the hull. Check for tears or drain holes that may be scooping water. A small flap over the drain hole may help prevent scooping. Check the height adjustment of the skirt. If some areas of the skirt are too low, excessive drag will result. The craft could also be out of balance (driver and passengers may have to shaft their weight). The wrong lift fan will also patent planning. Do not use two bladed propeller or air conditioning or automotive fans as lie fans. Other causes of poor performance are: poor running engines, incorrect engines, incorrect thrust prop or drive belt ratios, lift fan or thrust prop blade-erosion which causes poor lift and thrust, slipping belt due to impair tension, snow or water spray and want sheaves.

Excessive spray is undesirable for all the aforementioned demons, usually the result of the skid being out of adjustment craft out of trim (too much weight as) or overweight and/or too much lift power.

The two primal disadvantages of bag skirts are the need for good damage and the bounce that occurs over smooth surfaces. To alleviate the drainage problem: be certain that there are no openings that can scoop water, especially near the back of the skirt. Be sure that the skirt is tingly attached to the skirt-attach strips with no openings for water to enter. Be sure that the skirt drain holes are large enough and high enough to drain the skirt should it fill of water. When a skirt is full of water, it deforms as thrust is applied. Therefore, with a lot of thrust, drain holes might become scoping holes instead. If you intend to use your craft for fishing, or leave it parked on water for long periods of tame, then you may consider a very large drain slit with a nylon lace so it can be loosened just bet-ore starting the engine(s) and then pulled tight once the water is amines. Make the lacing line long enough to reach the operator's seat.

The skin bounce problem is most seven on ice or smooth pavement, so most builders who operate very little on these surfaces need not to be concerned about bounce. . But reduced lift power means more skin wear on ice and smooth surfaces and more likely to plow at higher speed smooth water.

There are many methods to control skirt bounce. One is to add weight (preferably weight that floats on water) to a line half way up from the contact line to the top outer side of the skirt. We prefer using four layers of skirt material measuring 5, 4, 3 and 2 inches wide sewn to the entire length of the skirt. Floatation is achieved by trapped air. This method also helps control spray.

Another method incorporates the use of an anti- bounce web sewn to the bottom of the skirt and tacked to the skirt-attach strip. This web should be installed along the entire perimeter of the skirt and have several holes to permit water and airflow. Unfortunately, the web is usually difficult to adjust and hard to keep sewn to the skirt. Another method of controlling bounce is to simply use heavier skirt material. However, this will cause more drag on rough water and on land. This is why we prefer the first method if skirt bounce must be mitigated.

Skirt Repair

Most skirt damage occurs in the rear and could be avoided by trimming the craft correctly, driving carefully, avoiding snags and sharp objects, and avoiding backwards motion when departing from a parked position on a hill. Furthermore, do not let the skirt slide under the skid when landing or departing.

The vinyl-coated nylon adheres better than other skirt materials, allowing the corners to b glued without sewing. Patches can also be glued without sewing. Tears can be repaired with inside/outside patches or by "whip stitching" the tear with 1.8 inch stitch spacing. For increased wear resistance, coat the exposed thread with vinyl or urethane glue. The inside/outside patch is applied by gluing the patch on the inside in the forward direction and on the outside in the aft direction.