I intend to lenghten the hulls of an old SC20. Basically I only want to extend the bow by changing the shape to a more modern straight bow but keep the original hulls and structure like it is. So the principle would be glueing foam onto the existing structure, shaping it and cover with carbon fibres than. I think of an overall hull lenght extension of 1-2 feet with most volume added to the lower part of the currently eliptical shape as it is intended to end up with a straight bow. As the sctructural integrity of the hulls is not touched, this bow extension would only have to withstands the forces from the water/waves.

I wonder if anyone has done this before and is willing to share his knowledge which would be highly appreciated!

Kind regards

Dirk

Cool bit of Photoshopping Dirk!

Sorry I can't offer any advice with your project though - other than to ask: Why? <img src="http://www.catsailor.com/forums/images/graemlins/smile.gif" alt="" />

May be this link will help:

http://a-class.org.nz/album/tech%20&%20detail%20pix/changing%20stuff/index.html

@John:
two reasons

1. functional
the SC20 has a nosediving tendency which is reflected in the fact that the boat nowadays only exist in a 2 feet longer version (ARC22). I think I don't have to raise the benefits of more volume in the bow, specially when its increasing the waterline length... as intended. curently the boat also generates quite some spray, something I relate to that the lines have quite some radius in the bow section, which also would become smoother if extended.

2. appearance
lenghtening the hull will stretch it lines and make it's appearance significant more elegant (personal view). and an eliptical bow, well that's really somthing from the past...

Fit T foil rudders - Far cheaper and simpler !

Erez, thanks for the link, but this guy cut away, not added! :-) hope so he added volume by opening the underside, otherwise he ended up with a real wave piercer... ;-)

simon, surely would help nosediving, but you have to inforce the transom, change the ruddercastings (how about kick-up?) AND still have to build new rudders. AND you still have an old fashion eliptical bow...

ORMA 60 style curved daggerboards would be cool so, just guess they would need to be positioned further forward than the current ones.

You might also consider actually shaping the foam extention so that it is closer to a wave piercing design than the current shape of the bow. As to how to approach the project, you might try contacting Ben Hall at Hall spars and have him send you the information of his renovation of a Waterat A-Class cat. The project was dubbed the Flyingrat. You should be able to attempt this extention by roughing up the gelcoat/paint in the area that you are attaching the foam to, getting the shape of the foam so that it fits over the end of the existing hulls. Epoxy glass to the inside to stiffen and strengthen the foam. Use a structural filler (West System has a few good fillers for this) and then fit it on. Glass the outside of the foam, use carbon fiber toe at the connection point if you feel that the hinge point needs greater stiffening than glass will give you. I would look at approaching the actual connecting joint as a scarf joint. This would allow you to increase the surface area of the epoxy joint. Anyway, good luck with the project. You can find Ben Hall's email address on the usaca web site. I will try to attach the Flyingrat file to another posting.

FLYING RAT PROJECT

The 2002 World Championships provided a great benchmark for comparing boats, spars, sails and of course sailing technique. Once again the Aussies showed us the way. Glenn Ashby's performance was nothing less than remarkable. Glenn excelled throughout the varied conditions (5-20 kt.).

The weapon of choice for the 5 of the top 6 finishers was the Flyer. Many of the top sailors in the class have gravitated to this design due to its success in the previous three World Championships. Other competitive designs sailing were the Melvin Waterat, the Boyer Mk 5, the Marstrom and the Boyer Mk 4.

I had been tempted to go with the flow and sail a Flyer at the Worlds but I had a lot of time in my Waterat and it was setup exactly to suit my style of sailing. My speed at the worlds was quite good, especially in the 5-10 knot range and at the top end in 17-20 knot range.

After analyzing the results and on the water observations, I determined that if I could improve my existing boat just a little bit I'd be very close to Flyer performance in most conditions. The big question was how to improve this boat.

One of the most noticeable differences in the Waterat and the Flyer is the height of the bows. The lower windage bows of the Flyer are definitely a plus. The other noticeable difference between the Flyer and the Waterat was the under water shape of the bows from the main beam forward. The Flyer here is basically "U" shaped while the Waterat is "V" shaped. The Flyer shape allows for easier turns and less pitching.

I decided I liked the back half of my Waterat but wanted to change the bows to be more "Flyeresque". My first thoughts were to modify the existing bows by a series of pie cuts, tucks, adding etc. and then fairing the re-shaped bows. While talking with Andrew Gaynor (USA 133), he jokingly suggested to just cut the bows off and put on new ones kinds like "Prada" was doing on a regular basis in Auckland. I thought about it for a while and decided that this was "not a bad idea". The next step was to bounce the idea off Pete Melvin. Pete said he would take a look at the existing design to see if it made sense. Pete ran a bunch of numbers and proposed a new bow design that started 3" in front of the main beam. He did ask me "Why not do the whole boat?” The problem was I did not have time to make a whole new boat. I wanted to sail in the Tradewinds and A Cat "Olympic" Key Largo Regattas. It was already mid October and I needed the boat on the trailer ready to head South just after New Years.

The challenge was to make a set of female molds that would produce a set of parts (bows) that would exactly match the existing hulls just in front of the main beam. This is where computers, DFX files, and CNC cutters make a job like this possible. Pete sent the lines files to Henry Elliot (C Class "Cogito" builder) who fired up his CNC router and cut the female molds. To reduce time and cost Henry chose dock Styrofoam as the mold material. After the shapes were cut, Henry covered the Styrofoam with three layers of 4 oz. Fiberglass cloth. We then sprayed the molds with Duratec and wet sanded them to 400 grit. We then Freekoted and waxed the molds. Instead of gel coat we sprayed Awlgrip 545 epoxy primer in the molds and let it dry overnight. Prior to lamination we scotchbrited the 545 and wiped it clean. The laminate used (starting from the outside skin) was 4 oz. glass cloth, 6 oz. carbon cloth, ¼" Nomex honeycomb and 6 oz. carbon cloth on the inside. We chose Gougeon Proset (laminating) epoxy resin. The entire laminate and core was put in the molds in one shot and then vacuum bagged. In the molding process we created a centerline flange in each half to have a good gluing surface. Once the bow halves were molded, we glued the two halves together to make a complete bow section. The excess glue was cleaned up on the seam and then we were ready to attach the bow to the old half of the hull. We put a 4" wide rebate (about .030 thick) in the area where the new bows joined the old hull. The core in the original hull is 3/8" Nomex honeycomb. The joining method ended up very simple. We dug out the core of the old hulls to a depth of about 1 ¼" but were careful to leave the inner and outer skins in tact. Since the new bows had the rebate and a ¼" core, the bows were inserted into the 1 ¼" deep gap with lots of thickened epoxy adhesive.

We set up a rigid fixture to hold to two halves at the hulls in exact alignment (a lot going on here) while they were being bonded together.

Once the bows were glued on, we taped the seam with a 4" band of carbon around the entire joint and then faired the area.

The next step was to "beam-up" the hulls. I had decided to get rid of the previous bolt on method (OK but it did leak a bit and would require a frequent tightness check). I made the decision to glue the beams in place and over wrap the beam/ hull joint. This creates an incredibly stiff connection. The bad news was the boat would no longer be able to winter in my basement (door width only 7"). To improve water flow from the bow the over the beam we fashioned a "ramp" to fair the hull/beam joint. This ramp was foam covered with one layer of carbon cloth.

The final modification was install "rails" to the hull to make the boat max width for trapezing. Again we used shaped foam covered with one layer of carbon cloth.

The next step was sending the boat off to the paint shop. We used white Awl grip 545 Epoxy primer and wet sanded the hulls to 400 (leaving a semi gloss finish).

After paint I installed a new bright blue Kinder tramp and reinstalled all the hardware. Wow the boat looked brand new!

It was now time to head off to Key Largo to try out the "Flying Rat" for the first time. I was able to tune up with Mark Sellnau the day before the Tradewinds. Mark has a Waterat virtually identical to my boat (before the modification). In the lighter wind range our speed was very similar both upwind and down...So far so good...at least it wasn't slower. As the wind increased to the 14 knot I had a slight speed advantage... maybe due to less weather hull windage?? The other thing that was quite noticeable was the ease of turning the boat. This was most evident when making the turn at the bottom mark.

Now it was race time at the Tradewinds. Weather was nasty...cold... with a frigid 18-20 breeze kicking up a nice shop. These were great conditions to see if this modification was worth it. Good news...Three firsts... I am happy, the boat preformed well on all points of sail. Lower windage, easier turning, and less pitching all made small improvements around the course.

@windswept

thanks a lot for the detailed tips and posts!
although I do not plan such an extensive (and structural) modification than the flying rat is about, it indeed consist of some useful hints!

I thought about the flyer waverpiercing bow, but it's there already: I just should mount the hulls opposite up! on a more serious note, the SC is quite a typical V shape and while extension seems makeable, to add that much volume like nowadays designs would surely mean to cut the lower part of the existing hull open to widen it, something I really don't want to do. I would have to extend it to an amount people might call it team phillips afterwards ;-)talking about this 'wavepiercing' hype, I am pretty sure the SC 20 is much more wavepiercing (and less pitching) than my Flyer A-cat ever was... the Flyer shape has much more lift generated planning tendencies, that's where it's major advantage becomes evident.

your comment on the scarf joint is interesting! what exactly does it mean/look like?

do you think stock foam(closed cells) like used for underground building insulation (mostly grey, green or blue) would be suitable? I have seen the ship-models used for tanktesting also uses this (while not encountering any side forces so).

In the next links you will fined useful information

http://www.boatdesign.net/articles/foam-core/
http://www.diabgroup.com
http://www.gurit.com/index_en.html
http://www.alcanairex.com/alcan/acsites.nsf/pages_acai_en/index.htm

I'm not sure anybody else has said this, but wouldn't it be easier to simply get a 22? It's essentially a 20 TR, built very nice with 2 more feet.... <img src="http://www.catsailor.com/forums/images/graemlins/wink.gif" alt="" />

dear erez, thanks al lot for your links! specially the first one on boatdesign seems a very valuable source! I am aware that foam cores for marine sandwichconstruction differ from the building material foams used mostly for thermal insulation reasons. but having to deal with the limitations of the Chinese market and the not to strong forces to expect in that area, would this kind of foam still work?

keith,

if you know someone willing to change boats and ship it here, very happy to 'upgrade' indeed...;-) for the moment I have to scope with what I have... and honestly also the bow of the ARC 22 (while more straight than the one of the SC20) is also not the most beautiful one...

Dirk

you will probably find that your resin will melt the cheap foam, things get messy and a bit out of shape then! test a chunk of foam first.

paul

Colored polystyrene closed-cell foam will dissolve in polystyrene resin, styrene, gasoline, acetone and a million other solvents. Polyurethane foam will not.
More than this gets more involved.
PS: If you ever get to Florida, stay away from Bill Roberts who might want to punch you in the eye. (Thats a joke- he wouldn't really)

paul & dacarls, thanks again for the input!

regarding the designer of the boat, I think he designed and build a wonderful boat. this boat is from 1980 and at that time it surely must have been outstanding in terms of how much detail thoughts went into the design. its one of the few beachcats you really can see that many issues were thought about deeply and answered in a very clever way, something you still lack nowadays with a lot of modern or so called high-tech designs. I am pretty sure if the designer wouldn't have developed this boat back in the late 70s and would start nowadays at least the bow section would look different. In that time it was designed, curved bows were quite standard on tornado, dart, hobie etc. and you can also not blame a producer to keep his line. look at the hobie tiger in comparison to the nacras. nacra meanwhile builded 4(?) F18 designs while hobie just stick with their initial one and still be able to pull it off in the worlds.

when th A-Flyer came up, I remember a lot of people saying how ugly it looks, that the boat is upside down, etc... I simply loved the look! So really hope nobody wants to punch me in the eyes just because my eyes don't like to see too many eliptical bows nowadays any more... ;-) I am just trying to see two less in 2007!

And Florida? Well, I really try to avoid too beautiful places in fear I don't would like to leave ... <img src="http://www.catsailor.com/forums/images/graemlins/grin.gif" alt="" />

Actually they are only on the second hull shape with the infusion.

Dirk what's better the "Spitfire" or "Folke Wulf" ? <img src="http://www.catsailor.com/forums/images/graemlins/grin.gif" alt="" />

I make it 3 F18s Jake;
Inter 18
F18
Infusion