Modifying a Nacra 5.8

Hi All,
I have just purchased an old cheap Nacra 5.8 catamaran. The hulls are structurally ok, just a bit neglected (mouldy). The rig, sails, beams, deck gear and trailer are all in good condition.
As I am already going to be doing work on the hulls (minor repairs and a full respray) so I would like to lengthen the bows a bit (add 0.4m to make 6.2m LOA), so as to qualify the boat for local trailerable multihull races that usually exclude beach cats. I also plan on adding wing seats as trapezing is also banned.
I have experience with timber/epoxy work and also poly/vinylester mould construction, but I haven't ever lengthened a catamarans bows before I figure I should use foam to shape the new bow and sheath with epoxy/glass.

Im just not sure what type of foam to use or how much glass would be enough (I thought maybe 400 to 600gsm?)
Any help would be appreciated.

Here is a pic of a similar catamaran with lengthened bows that Im trying to replicate.
Cheers, Dave

 

The laminate schedule depends on which resin you'll use. The foam should be no less than 5 pound (2.2 kg) density, preferably heavier, especially if you sail in rough conditions. A polyester laminate would be a few alternating courses of mat and cloth or roving, finished with some mat, so you have something to fair on. An epoxy laminate could be a couple layers of 1208 to gain bulk quickly, though the lighter and stronger arrangement would be a straight biax schedule with a 1 ounce (34 gsm) mat on top. Lastly, you'll likely want to stiffen the centerline and bow of this nose piece with additional fabric. If you use good quality, higher density foam, you can go fairly thin on the schedule, but if you use a modest density core, the skin has to be thicker.

 

Indeed, the foam should serve only to shape the new bow. The foam can not be accepted as a resistance element, thus the final thickness of the skin is independent of the weight of the foam. Should you get the new bow a thickness equal, or very similar, to that in the keel of the boat.
Moreover, what PAR proposes seems very correct, except that I do not think a centerpiece of reinforcement is needed.

 

My thoughts on the bow reinforcement are for abrasion resistance and modest impact, as it is a beach cat.

 

Yes, that is what is called a "protective Bow", which is achieved by increasing the thickness of the laminate, never putting an inner reinforcement.

 

If you just extend out the bow it will be out of the water and not be of any value to your waterline length. One way to integrate the new bow would be to spring a batten from the centre of the hull and extend it forward to the current waterline/bow height to give you the new outline. Then you can fair it into the existing hull.

I know it's more work but you could add a small sugar scoop to the rear of the boat. I could see how you could make a nice stern extension with a cutout for the rudder and it would make an easy boarding step too.

Just a few random thoughts, good luck with your project

 

The bows on the Narca 5.8 will be "engaged" most of the time, even with a 15"-16" extension. Even with the slight rocker in the bows, just extending them forward, along the same curve will be beneficial as well as conform to the rules. I'll be interested in reports on balance and rig tune with this modification.

 

keep it light

I used to repair beach cats of all brands, and except for the bows and keels, they were usually built of two layers of fairly thin glass on the outside, 1/4-3/8" foam, and one layer of glass on the inside. Plenty strong, but not very puncture resistant. I can't see why an extension needs to be any heavier, even if it broke off, the boat would still float. The 5.8 is a bit of a tank already, it certainly doesn't need much more weight. I suspect the extended bows will help some in heavy conditions, but they will make a wet boat even wetter- bring a snorkel
B

 

Thanks for the advice everyone.
I will outline my current plan, please agree or shoot down in flames my proposal
I intend to cut a profile of the shape of the new bow out of 4 mm Okoume ply and glue it to the centreline of the bow. Then I will build up either side of the ply with enough foam to fair back into/onto the hull sides. Then I will sheath with upto 600gsm (too much? or not enough?) and do final fairing. I will be using epoxy throughout. I have attached a picture of a mock up of the shape of the new bow.
Please correct any of my above plan or suggest a better way.
Would a high density polyurethane foam sheet be OK to use (something similar to these http://www.afpt.com.au/afpt_polyurethane.html ?

Thanks again for the help.

 

Hi,
for foam I would use styrofoam (xps?) this is a insulation foam with a density of around 35 - 40kg/m³. This is the same foam ( a little stiffer for the boat) the european surfers use (30kg/m³) for there home made windsurf boards. It soakes no water if you damage your laminate.

A woven 560 - 600gr/m² glass fabric with the fibres aligned in 0/90° and the second layer diagonally at 45/45° will do the job perfectly.

Best Regards, Michel

 

Hi Dave - Don't get too carried away with the reverse bow. They are a bit of a fashion at the moment and probably will go back to vertical soon. The dreadnaught bow is exceptionally wet and if done you will need to put on a spray rail. I follow the 5.8's around the race course doing rescue and have to right them occasionally. The more bow volume the better!! Originally the reverse bow comes from making the fwd stations an upside down pear shape. Which means as the hull goes forward you run out of geometry at the top first. This was done so that when a hull dives the usual flat deck does not drive the bow in due to hydrodynamic effects. It then became a fashion, so as you have done the designers leave a small vertical at waterline but add the reverse for fashion at the top. The reverse looks aggressive and does its job in terms of asthetics but doesn't function well .(IMHO) The other side of bow design is that a modern bow is designed to go over the top of the water not through the water. Your extension gives you the opportunity to create this. So don't try and get the knuckle in the water, keep it high. Even monos are doing this now and AC have been doing it for decades. Look up Davidson bow. Cheers Peter S

35kg PU is perfect and 600gsm is probably too heavy, 450 would be fine. Hobies and small cats use 250gsm for their hulls so 600gsm is hugh!