This design came about from the query of Jack Z, a member of the Hobie e-mail discussion group.
I have not built this tramp and it will require each builder to ensure measurements and details are correct for each boat. The net is not designed for heavy loads or adult passengers. Heavy loads, such as teens or adults, will upset the balance of the boat anyway. It is designed for carrying lightweight loads such as camping equipment, a picnic hamper or Flash the wonder dog. Equipment will need to be secured against movement. Children ALWAYS need PFD Type 1 vests and instruction about where not to sit or move in certain conditions.
This is the final general distribution version, you will have to do some fine tuning to the design for yourself. The instructions are very full, so it is a long page. However, there are no pictures. Perhaps one day I will get some, but my H14t is very susceptible to fore/aft balance and I do not have a great need to make this tramp yet.
The design differs between Hobies which have tramp frame pylons (14
& 16), and those which run the tramp directly into the deck (17 &
up). On several occasions I give alternative suggestions for the different
design of the boats.
HULL LOADS AND STRESSES (For the technically minded - I used to be a structural engineer)
The hulls are designed to carry heavy rigging loads and have shown that
they can take anything that most people throw at them. Because the forestay
attaches to the tang at the tip of the hull the forestay loads exert the
maximum leverage possible on the hulls. With full mast bend under heavy
main sheet loads this amounts to a lot of bending moment which is resisted
quite well by the hull.
You shouldn't be too concerned that the net will add a great deal of stress to the hulls. The loads of the net will be distributed along the length of the hull from pylon to tang and will not contribute anywhere near as much stress as rigging loads. The mathematics for the net loads are not quite as simple as we might hope as the loading of the net will be directed fore/aft as well as sideways. It is not as simple as calculating point loads on a wire, more like distributed loads on a bouncy floor in all directions.
The hulls are monocoque construction members, an engineering term equating the hollow shape to the exo-skeleton of a beetle. This means that they are extremely rigid and able to absorb stresses in many directions. So while the forestay rigging pulls the hulls inwards and upwards, and the net loads will pull the hulls inwards and downwards, the hulls can take multi-directional loads without over-stressing. The inwards pull is more significant than the downwards loading.
Remember that when carrying things anywhere on the boat the whole thing is fully supported by the water. Rigging loads try to bend the hull upwards, but net loads will push the nose into the water which is supporting the fore-hull for its full length. (That is why the boat is floating anyway) You can easily test this support by having somebody stand on a front hull. Try it, just walk out to the bow, even bounce a bit, and see if your boat breaks in half. Remember that this is only a test of hull rigidity. Hobie hulls do not like people constantly treading on the flat surface in front of the front pylons as the glass can delaminate over time with this treament.
The upwards loads from the rigging are highly stressful because they form an enclosed stress system from tang to masthead to mainsheet to lower block and back through the hull to the tang. (Sidestays also take part) It is because it is such an enclosed system that enough force can be applied to bend the mast. Imagine standing behind the boat and pulling back on the mast head, you will not bend the mast you will only tilt the boat. Similarly, when you load up the front net you don't apply much stress to the boat, you only tilt the bow into the water.
Taking the need for some sideways hull support into consideration, the
net has a front member which acts as a bow spreader to limit the amount
the bows can move inwards when under load.
HOW TO ATTACH THE FRAME AND NET
Jack's original thought was to drill the inner lip of the hulls to lace the net in place. A safer way is to drill the outer lip and take the lacing over the hull. This loads the deck lip for the distance between the holes, whereas drilling the inner lip stresses only local points at each hole. Hoever, it is not a good idea to damage the place where the hull is joined together. The bigger boats have more overhang on the lip for drilling, but they also carry greater loads.
The following design has built in sideways support and no drilling of
the lip. The net can be easily fitted and dismounted. The net consists
of a tube frame which is attached to the bow tang and pylon on each side.
The frame takes the load without having to drill the fibreglass.
THE TOURING NET FRAME
The net has three frame members, one on each side and one at the front. Each member is an aluminium tube, probably slightly more than 1" diameter.
Shop fitting clothing racks are often made of tube sufficiently strong
to take the loads. If you can hang carefully from the middle of the tube
of correct length when it is supported from each end, and it doesn't permamently
bend, then it is strong enough for child net loads. Ask a friendly shop
fitter for aluminium or stainless steel discards, (not chrome plated steel).
FRONT FRAME ATTACHMENT - STYLE A - SOCKETS ON TANGS. (Original suggestion)
The side members are located fore and aft at the tang point and the front pylon. A socket (a slightly larger tube about 3" long, front end closed) is permanently secured against the tang fitting. The side member is slid into this when the net is fitted.
The front member fits between the two side members almost at the tang
point. The side members have a T piece which points inward near the tang
and the front member locates into these T's. This front member takes any
side loads off the hulls. To assemble, just fit the front member into the
side T's and slide both side members into their sockets. Tie the aft end
of the side rails to the pylon. Don't permanently fit the front rail into
these T joints. When the net is removed just take the front rail out of
the T joints and the whole thing will fold easily.
FRONT FRAME ATTACHMENT - STYLE B - SHACKLES TO TANGS (Easier than sockets.)
This is the preferred method if your boat does not have pylons.
There are no sockets to be made up or fitted but the net frame remains generally the same. A plain stainless steel saddle is pop-rivetted to the front of each side member. This is connected to the shackle holding the forestay to the tang. A similar saddle is rivetted to each end of the front member and is located at the same point.
You might prefer to have a shackle at each tang dedicated to the net.
This means that assembling the net does not interfere with your normal
REAR FRAME ATTACHMENT - STYLE 1 - FOR BOATS WITH PYLONS
The aft end of the side member rests against the outside face of the front pylon and is located with something as simple as shock cord through a hole in the tube and then wrapped around the pylon.
A plastic buffer, about two inches by four inches, rests between frame
and pylon to protect against abrasion. Cut the buffer from a polyproplylene
kitchen cutting board, the white plastic variety. Screw or rivet the buffer
to the net side frame member.
REAR FRAME ATTACHMENT - STYLE 2 - FOR BOATS WITHOUT PYLONS
This requires two sockets made to be from strong fabric and fitted to the underside of the trampoline. The socket slides into the slot which holds the tramp in the side frame. The tramp fabric locks the socket into place and it is left on the boat when the net is not fitted. The sockets fit into the side frame but very close to the front tramp cross member.
Cut a piece of strong canvas fabric five inches by six inches. Fold and sew a half inch hem along one long side. This becomes the front of the socket. Fold it in half to four and a half by three inches. Sew across the remaing three inch end. Roll the long side open edges over a couple of times and sew them into a bulky hem. This bulky hem slides into the tramp slot with the socket opening pointing forward. The bulk stops the socket from pulling out of the slot.
The net side member slides into this fabric socket and clips to the tang with a shackle. It won't move fore/aft or sideways beyond the slack in the shackle.
You might want to reinforce the front edge of the socket before construction with some webbing tape. These measurements are estimates, you will need to measure up for your own application. Ensure that the socket is strongly made.
The side member might also have to be bent slightly to fit underneath
the front cross member and into the socket. This is another thing for you
to decide on site.
THE NET DESIGN
The net itself is made from mesh, such as tramp fabric, with minimal stretch. I suggested knitted (not woven) shade cloth in an earlier mailing. Whatever you use, check for its ability to take the loads. Because the net does not have to be as tight as a tramp, make it with just sufficient slack to assemble or dismount the whole thing. However, too slack and everything will roll into the middle. Experiment with tension. (See note about tensioning the net at the end of this document)
If you use non-mesh fabric it will collect water unless it has drain holes.
Cut the net fabric to match the toe-in/toe-out angle of the side rails and with enough seam allowance to double sew a three to five inch wide pocket down each side. Sew a similar pocket into the front edge. The side and front rails fit into these pockets, make them roomy enough. Cut and hem a quadrant cutout at each front corner so you have room to fit the side rails to the tangs.
The rear edge of the net is folded over and double sewn with a 2 inch
hem and some support fabric such as seat belt webbing. Grommets are fitted
to the rear edge with spacing to match another set of grommets in a piece
which is permantly attached to the boat.
ATTACHING THE REAR EDGE OF THE NET TO THE TRAMP FRAME
The rear edge of the net is attached to the tramp slot in the front member. The design is different for different boats. Your boat has the tramp fitted to the front cross member in a slot which faces either fore (H18) or aft (H14/16). If Hobie has designed other boats with top/bottom or elsewhere for the tramp slots, you will have to adjust this design principle to suit.
Make another piece like the rear tramp lacing piece (a long strip with a rope sewn into one edge and many grommets on the other edge). This fitting piece must be as long as the tramp slot and wide enough to suit, but not so wide that it gets in the way when not in use. You will only need a thin edging rope as the existing tramp edging rope will jam the new piece inside. Three sixteenths should perform well enough. You might even just roll the edge a coupld of times and sew the bulk as in the socket design above.
This strip can be made of strong canvas with a webbing reinforcement to take the grommets. It does not have to be made of mesh.
Loosen the tramp so this new piece slides into the slot alongside the tramp, then re-tighten the tramp.
If you have a forward facing slot this piece will slide in on top of the tramp fabric and point forward to give you the lacing holes for the front touring net. Make sure that you clear any fittings below the mast. You might have to make up a two piece strip.
If you have an aft facing tramp slot this piece slides underneath the tramp fabric, and turns under the frame to face forward. This means that the strip must be made so the grommets come sufficiently to the front. Measure your boat before making it up. You might also need to clear the dolphin striker. If so, make the strip with a slot in the centre for this purpose, although you will probably not have to make a two piece strip.
The grommets in this strip must match the spacing of the grommets in the rear edge of the net. Because you will not want more than a couple of inches clear air space between these two, the grommets must be fairly close together, probably not more than two/three inches spacing, four at maximum. This means more grommets but a lighter rope and smaller grommets can then be used. Space the grommets to fall midway in the spaces between the other set, don't identically match the grommet positions. Check your tramp to get an idea of the zigzag lacing pattern.
When you lace the two together, take the rope around the side rail to
stop any tendency of the net to pull the grommet strip inwards at the ends.
PROTECTING THE HULL SURFACE FROM SCUFFING
Because the side rails will rest on the foredeck, add some closed cell
foam to prevent scuffing of the gel coat in contact places. Plumbers sell
large bore neoprene tube which can be slit and opened out (expensive),
or builders supply stores sell four inch wide rubber strip normally used
to separate concrete footpath sections (cheap). Anything like this can
be used to protect the deck as long as it is strong enough not to get holes
in it when squeezed. Glue it to the mesh to hold it in place.
SOME EXTRA THOUGHTS
When cutting holes in mesh or synthetic fabric for grommets, use an electric soldering iron (at least fifty watt) to seal the edges. You will not need to cut them first this way. Don't burn a hole which is too large or too small for the grommet, make sure you have a comfortable push fit.
There is a lot of industrial products out there which most people never hear about. The humble velcro is available in many forms and grades. I have even heard of stainless steel velcro for attaching industrial motors. One thought I had was to glue some heavy duty velcro along the top of the foredeck and attach the sides of the net to that. No side frames needed. A front frame member can easily be shackled to the tangs as above.
The no-pylon boats will have an air gap between side rail and deck. An extra flap of mesh that reaches out the the deck, and fastened with velcro might be all that is required if you want to cover that gap.
You might like to attach a rope or shock cord to each rear corner of the net. This can be used to tie to the pylon or suitable fitting to take side loads off the grommet strip on the rear edge of the net.
The Hobie Sport Cruiser (picture in 1995-96 Hobie catalogue) seems to have its net laced to a permanent cross member attached just behind the tangs. This means modifying the above design but somebody might prefer that on their boat.
If your net gets too slack sew a line of two inch webbing six inches in from one side. Fit grommets along this strip every four inches. Lace some shock cord through the grommets and around the side rail to tension the net. You might also prefer to lace the rear edge of the net to the grommet strip with shock cord to give constant net tension.
If you end up with a large air gap between the net and the boat you can easily make up a flap which slides into the lacing so you don't lose your Coke cans.
If your side rails pull in too much when loaded add a grommet to the net at the centre of each rail and just inside the rail position. Make up a solid plastic or metal clip to fit the outer lip of the deck, and loop some shock cord through the grommet (around the side rail) and to the clip. This will be more noticeable on the longer boats and the extra support might even be needed as standard on the H18 and over.
I thought of designing clips for the whole side support, but they presented difficulties of making, fitting, damaging the glass, holding in place when capsized, bruising your kids' shins, catching toe nails, and many such problems. This one clip doesn't present such a lot of problems.
With a bit of on-site imagination this design will give you a touring net which can be easily fitted or taken off. It has no permanent joints so is easy to fold up and roll for storage. It means minimum alteration to the boat. It can be built at home, although a tent maker might be needed to handle the sewing.