Construction of a PVC (Divinycell) cored boat
The following post is a short explanation and description of the construction of a 100% PVC cored boat, with Fiberglass, Kevlar, Epoxy and Carbon Fiber as the shell material. Its purpose is to show those interested the various steps of the construction of a boat using this method and materials. (Please forgive my English).
Composite material boats are either built with or without a "sandwiched" core.
Boats built with a core, being just the hull, the top sides, deck or interior or other structures can have, nowadays, a wide variety of materials available in the market, that range from Balsa Wood to modern lightweight rot free materials such as PVC, which is nothing more than a high rigidity micro balloon foam. (Almost like very hard Styrofoam, with tiny little air spheres).
The PVC comes in plates inside cardboard boxes for material protection during shipping and is quite expensive.
Construction of a larger boat in the range of the 42 feet LWL will require approximately 645 Sq Feet for the hull, 320 Sq Feet for the deck and roughly 355Sq Feet for the internal furniture, since the boat in question has all the internal furniture cored for weight control and increased structural rigidity.
However, furniture corners and main beams are solid wood for panel stiffness, and the core is first covered with hi strength marine plywood, then covered with a 2,5 mm Cherry wood veneer for aesthetics purposes.
One of the core brands used today in high performance light weight racers or race cruisers is Divinycell.
Divinycell comes in large variety of shapes, densities and purposes, whose choice and specifications are based on the objective and purpose of the application.
Some core has compression strength, some has directional or multi-directional rigidity, some is just for cosmetic purpose applications with minimal structural benefits. Meaning that for each application on the boat, there is a specific type of core to be used, with a specific thickness and directional rigidity.
However, correct application and choice of a core is very important, and is probably one of the very first steps in engineering a new boat, not only for the building purposes, but as well as for the safety of the vessel and ultimately its sea going performance.
A good guide towards the procedures, the choice, application and calculation of Divinycell can be found here.
Obviously a core used for the hull differs from that one used on a deck and from that used inside or for combings.
There are therefore 2 ways of building a boat with a core. With a mould or with a Positive plug.
With a mould:
The procedure for building a cored boat inside a mould, is practically the same as building with solid laminate, except that there will be a time when after the gel coat and outer boat skin material, a layer of core will be applied, and resin impregnation assured by the application of a vacuum, then the application of the inner boat skin.
With a positive plug.
A “male” frame is constructed, normally in wood, where the core is stitched (later the stitches are cut) to the frame, to bend it to follow the contours and desired shape, then, after the shape is obtained, the outer shell is covered with resin and skin material, that will later be sanded down several times and painted with Awlgrip or other paint.
Once the outer shell resin cures, the plug is turned upside down, the stitches are cut and the wood removed. At this stage, the outer skin will maintain the shape of the piece being built, and the inside is now covered with the inner skin laminate.
For increased stiffness in dedicated points, stiffeners can be added, inside before the inner skin is applied, and those are either made in carbon fiber, Kevlar, wood or metal.
At this stage, the backing plates are also applied to increase structural rigidity of the areas where dedicated deck hardware’s bolts will need a solid base to be tightened.
The boat you will see bellow had the hull built using a mould, and the deck, cockpit and interior using a positive plug.
For this boat, it was decided to build the hull on a mould for several reasons.
1) Since the shape of the bow and stern on this boat is somewhat different, building inside a mould would be better to guarantee zero hull deformation while the deck and interior was being built. (the boat was removed from the mould with the interior and deck all finished.).
2) Since the curing of Kevlar, CF and glass are different as far as times and procedures, it was better to maintain the boat in a mould to allow proper curing without pulling
3) The skin would have a better finish in Gel coat for increased rigidity
4) The transition form the Kevlar bellow water section to the above water sections built with a different lamination would also be smoother
5) No need to risk turning the hull upside down for internal lamination
6) Easier to install the vacuum system.
Construction of the hull:
The mould was made first and glassed. The bow section of the mould was reinforced athe the bow to prevent mould deformation in a high structural area.
Above the mould can be visible, it was made using wood planks, on a structural bed, then glassed inside.
Gel coat was applied, with brush, not air gun. This procedure was very important to assure no bubbles in the outer skin of the boat. Time consuming obviously.
The the Divinycell PVC core was applied and impregnation assured with vaccum. The underwater hull thickness on this boat is larger than the sides. In the photo bellow this can be seen.
The whole underwater is kevlar and carbon fiber, the inner skin is a mixture of all where applicable.
The entire central section of the boat, from the bow to the stern was not cored, and that can be clearly visible. The whole central section is highly reinforced to prevent cracking and increase stiffness.
The boats floor beams (in this boat are part of the hull structure to increase stiffness and reduce flex, were also made and glassed at this stage. This boat uses the space created by these beams for water tanks, Stainless diesel tank and keel. The fresh water tanks have a series of valves for ballast, and can, if desired be filled with salt water.
Then, once the hull was finished, the deck positive plug was built over the hull.
At this stage, this frame allowed us to see the final shape of the boat, and several modifications were made until the lines merged in a smooth manner to please the eye. Its at this stage that one can change anything. after glassing...well...too late. This boat had 3 cabins and 3 sterns made before the final ones were glassed. Think of it like the "rib cage" of the boat. That wood later was removed as explained above.
The brown board above the deck is the Divinycell plate.
On the deck, a pred-cut PVC was used to allow better shaping and better impregnation.
Then, once the shape was satisfactory, the whole deck was covered with PVC and the corners glassed to help maintain shape.
Once the corners and edges were glassed, the whole deck was also glassed. On key points, such as chainplates and deck joints, Kevlar was used.
Once the whole deck was glassed, the wood inside was removed and the deck lifted off the boat.
It was laid on the side, up side-down and the inside glassed. Where the deck gear goes the PVC was removed and either wod or metal plates were added to provide a good base fot the bolts and screws. This being a reace boat and therefore subjected to more stress than a normal cruiser, the stiffening has to be doubled of the regular boats.
At this stage the deck stiffners also in foam were applied.
The deck was sanded and painted some 10 times before final painting was done.
The hull was painted inside with a speciall paint for the water tanks, and white elswhere to allow detection of leaks and to brighten the bildges.
Above the aread where the speed and depth probes go, so they can be removed with the boat in the ater withour risk of sinking. The upper walls are higher than the water line.
The above photo clearly shows the shroud bases that in this boat go all the way to the bottom of the hull, spreading chainplate loading all along the hull. Each side of the boat has 3 beams that run vertically to the keel, so the pull caused by the mast is even, allowing for sailing with higher winds and loading.
Once the deck was completed, the inside planning started. The location of the bulkheads was decided at this stage, as well as the inside alyout. much easier to do without the deck above.
Above a view of the keel box structure, before the CF reinforcment H frame was installed.
The inside was first made with pressed wood so the owner could have a good feeling of the inside.
Inside all the heads were also made out of PVC and were finished outside then slid into the hull.
The inside was finished in the nice wood....
The engine was installed, in this case a 40HP Yanmar withan SD50 racing saildrive and Gori folding prop.
Above, the deck was almost finished then installed on the boat for the last time, while the furniture was completed inside. Note that throu all this time the boat is still inside the mould, which prevents deformation.
The keel box was finished and reinforeced accordingly.
The deck is applied and attached to the hull, in this boat with composite materials, no bolts or screws, ans the deck has a male edgle all around that enters 2 inches inside the hull sides. Then the edges are glaees winside and outside.
Then, the boat is finally de-moulded.
Once outside the boat is finished. In this case it was an absolute requirment that the decks and cockpit be in teak, so 15mm thick teak wood was applied by glueing and screwing to outer deck shell with epoxy filling.
The deck fittings are installed
The keel installed
mast in and tested
Et voila....a boat made in a few minutes...;)
Thank you, and should you have any questions, please do ask.
(Later I may edit this post.)
What a wonderful thread, Thank you.
Thanks Alex! If I was going to build a boat, it would be difficult not to build another one like yours.
Great thread......pictures are worth a thousand words!
Are the teak decks fabricated from teak sheet goods (like plywood) with the individual board shapes and king plank routed in? If so is that the normal method of installation on modern boats?
Great job Giu! A lot of well thought out description and explanation.
One of the edges has a male prtotusion, while the other has a female entrance, so that when the planks are all installed there is a small groove covered with sikaflex caulking.
The planks are first laid with out glue, only screws. Then each screw hole is epoxied and redrilled for the screw. Then its glued and screwed down onto the deck.
The screw head is then covered with a small plug epoxied in place.
I can't even begin to imagine the craftmanship and patience required to do that.
Most of the process was described in your blog - I admired it then as much as I admire it now.
Superb hull shape, best materials and precise craftsmanship - no wonder she is such a beauty and such a performer.
Just wonder: your water tanks, which can be filled with sea water - do they significantly contribute to stability and/or heel or are they too small to make a real difference - water ballast tanks on dedicated racers are usually much larger and further on the sides of a boat (with large pumps to move watter left or right). Of course you could not sacrifice the living spaces for that - so do you use them as ballast tanks ?
What you do is fill the tank half level, (that's the weight we calculated to help reduce heel), then transfer from port to stbd as needed. I wish we could have higher tanks, but.....you're right..the space. Still since the beam is so wide the bottom tanks do a pretty good job. Note that the boat was built to be sailed hard without ballast, the ballast helps in certain conditions. The day we made the 15 knots on that video I had the balast on...and it helped. Last week returning home they were also ballasted.
At the stern, I have a hole, that dumps all the water out of the tanks with another pump. It empties the whole boat in 4 minutes.:p :p
I am talking with the builder to be able to not only transfer form port to Stbd, but also forward and aft. Which will help downwind if the water goes aft, and forward for beating.
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