Multihull Structure Thoughts

[oldmulti]

A topic that frustrates me. Underwing clearance. A good cruising design has good clearance, modern production cruising boats often don’t. Yet they sell why? Because we have two different markets happening. The “Roomarans” that “cruise” 100 miles in very well chosen weather and those who are speed demons. The roomarans are chosen on accommodation space, features such as roller furling, electric winches, bar fridges, a loo per double bunk etc with sailing capability second. Pounding to windward is not often tolerated by this group especially on forward of the mast berths. But you say many Prout cats have gone around the world and they have low bridgedecks that pound. True, people can adapt to being in a prison cell as well, but it is not a preferred choice. Those who have experienced high bridgedeck clearance as a way of life understand there are better ways to live life.

Another frightening truth is the average wind speed globally is about 12 knots. There are many sailors who have not experienced 30 knot plus wind speeds whilst going upwind. They may be 2000 miles from their home port but are parked in a marina until the storm passes. The speed demons fall into 2 categories. Around the buoys with some coastal racing who choose their weather EG Farrier tri’s that are capable of a lot but choose when and where they go and the serious racing guys who do serious offshore racing. The serious offshore racers do not have low wing decks, if they do, they often have broken boats and I include ORMA 60 foot tris in that statement.

Some advertising blurb says “As a rule of thumb, we believe that clearance of between 5 and 6% of the LOA of the catamaran is good.” Nice statement, the only problem is, is it a narrow cat with fat hulls or a wide cat with slim hulls. A Lagoon 42 has 22 inches of underwing clearance loaded for cruising and has 8:1 hulls. A Catana 381 has 31 inches of clearance and 11:1 hulls.

One Lagoon 440 owner has 26 inches of clearance which he says works really well with the big round hulls. The nacelle at the front works well to split any waves that can potentially hit the underwing. He nearly bought a Leopard 47 that had a bridge deck clearance from his memory was 14 inches and they had narrow hull shapes that needed according to him 30 inches of clearance.

The Catana 381 mentioned above can point to windward at 35 deg in 25/30 knots of wind. Other cat designers that are looked on favorably here have for example. Oram 44C is 44 x 22 foot and has 28 inches of clearance at lowest point, fully loaded for live aboard cruising. A Grainger Ocean 470 48 x 26 foot has 36 inches of clearance. She was lightly built and a good fast sea boat. A Schionning (26ftx49ft) Wilderness 1500 has a clearance of 33 inches. Fusions Catamarans are 40 x 22 foot and have 31 inch underwing clearance.

Translation It is not just length that defines bridgedeck clearance its total beam and hull shape. The more clearance the better. BUT if you do own a low wingdeck cat and have a structurally sound boat (that will remain as built) and can put up with the noise and discomfort good luck to you. It is there are better boats out there that can really sail well upwind in strong conditions without doing damage to the boat or you. PS I enjoyed one night racing across Bass Straight close hauled, fine reaching in 45 to 50 knots of apparent wind. Boat speed up to 12 knots and diving into the back of every 20 or so waves. Fun. Yes I love big bridgedeck cats capable of sailing anywhere at average speeds of 8 to 12 knots.

The jpegs start with a Prout Elite 37 study plan versus the reality of the boat in the water for cruising. Next some big French cats and some samples of OK boats.

 

[oldmulti]

I read this comment on another site. “Infusions dirty little secret is that after the resin front passes the vacuum in the bag behind the front drops to zero. That's right...the clamping force disappears. To combat this the double bag technique has been developed, capable of creating 70% fiber/resin ratios and parts half the weight.” Double-bag infusion: 70% fiber volume? http://www.pfonline.com/articles/double-bag-infusion-70-fiber-volume

Please read the response to this article before accepting this article as fact. There is one very big caveat. Double bagging solves problems

A typical double-bag infusion for a 20-ft boat hull starts at the keel and flows outward, using vertical feed lines to achieve wet-out to the top edges. “Often you end up with a resin-rich laminate lower in the boat and a drier laminate as you move up the hull sides,” says Emanis. Similarly, there is a change from vacuum pressure to hydrostatic pressure on the fibers as the resin front moves forward. He illustrates, “If you take a flat piece of plate glass with fiber under vacuum on top of it, and you measure the thickness of the fiberglass before and after the resin flow front, you will see that it is thicker behind because there is no vacuum pressure left there to hold it down.” Emanis asserts that even with full vacuum, the pressure difference in the bag could be 15 in. Hg over 4 ft/1.2m. In other words, the pressure in the bag drops even when the vacuum gauge at the pump reads 29 to 30 in. Hg. Emanis believes double bagging overcomes this because there are no effects from fluid dynamics in the outer bag; it acts purely to achieve compaction, pushing the inner bag down. The outer bag enables a positive pressure not possible in the inner bag and, Emanis contends, is also the cheapest insurance against a blown bag and laminate inconsistency. Likewise, changes in ambient temperature and moisture and any differences between resin and tool temperatures will change the resin flow profile, making it impossible to replicate results consistently. Stored core and reinforcement materials must be protected against ambient moisture absorption and/or dried before infusion to avoid outgassing during infusion and cure, which causes voids. Even light can elevate temperature, if only a little, causing resin to react more quickly. According to Emanis, “If you test panels … in a very well lit lab and then move to a dimly lit open shop to run the full infusion, you won’t get the results you tested for.”

The response to above came in with one very big caveat. “I am not an engineer but have been doing carbon epoxy infusions commercially for over 10 years and try to keep up with the technical side of the process. The vacuum preassure does not go to zero at the resin front in an infusion... Given you are doing a properly set up infusion over a standard laminate. What you will see is that before you open up the feed lines the entire laminate is at max pump vacuum. When you open up the feed line the dry side of the resin front is still at your full vacuum while there is a vacuum gradient in the infused laminate going from Atmospheric pressure in the open resin cup to close to full vaccum at the resin front. Once the part is infused and you clamp the feed line the entire part will equalize back to your max pump vacuum as long as the resin does not gell before this happens. If your resin gels before the part can equalize you can measure your laminate thickness and see that the part is slightly thicker near the feed pot compared to the vacuum lines...this is not good. A double bag infusion can help to equalize the part sooner. But, if you run your single bag infusion properly, allowing the part to equalize, there is no difference between the double bag infusion and a standard infusion.”

Also the following comment from the same source to a question. “The bag pressure wants to equalize and the entire bag will closely match your inlet vacuum pressure pretty quickly once the resin feedline is clamped. You can watch this happen if you set up some vacuum gauges on your bag. The laminate at that point is richer at the feed line and leaner at the vacuum line and thus it is important that the resin does not gel at this point so that it can still flow out to give you a uniform laminate. That can take 15 to 30 minutes depending on the laminate and resin. If you have a leak on the other side of your resin front or the resin starts to gel then you have some other problems.”

Translation. Single bag resin infusion will work very well as long as the resin infusion is completed at least 15 minutes before any resin starts to go off (or gell). The total vacuum bag needs to equalize the pressure all over the glass resin panel surface to get maximum compaction of the glass and resin. Hopefully you will achieve the 66% glass to 34% resin ratio everyone wants. The jpeg is of the Double Bag process.

 

[oldmulti]

From small scraps of paper come interesting things. Cheers was the first Atlantic proa created by Dick Newick. The 40 x 16.7 foot long 3000 lbs Cheers came 3 rd in 1968 OSTAR against behind 2 sponsored 55 foot plus monohulls. Cheers was a gem of a boat but had accommodation for 1 person as long as they only wanted to lay down and eat dry rations. Dick Newick in 1980 designed a racer cruiser version of an Atlantic proa. The 60 x 24 foot proa had a 48 foot waterline length and weighs 6000 lbs. Its two 44 foot West free standing wing masts form a schooner rig that carries 860 square foot of sail in 2 mains. The main hull has 16:1 length to beam ratio, the float has a 24:1 length to beam ratio. This boat under its standard rig would be fast, add a genoa or the 160 square foot jib and it would be very fast under offshore conditions. The lines are like all Newick multi’s, designed for speed in all conditions and should provide a relatively smooth ride. The proa has internal room to stand up, sleep 3 people, sit at a table and cook at a galley.

Now the words on the diagram probably will not be readable so I will write them here. The cross arms are constructed from two 9.5 mm plywood bulkheads with the ply scarphed so the grain is at 45 degrees. On the top and bottom is 4 square inches of fir on each bulkhead. The fore and aft edges of the cross arm has 2.5 inch fir. The skin of the Cross arm is the same layup as the hulls with the grain at +/- 30 degrees from the axis. (Bias 66% across the boat 33% fore aft)

The hull layup is 3 plies of 6 mm juniper laid up on a cylindrical mold covered with 4 oz polypropylene in West epoxy. The mould is based on a 6 foot radius male mold that can either be 21 or 31 foot long. The panels are scarphed over 8 inches and backed by 10 inch butt blocks. Frames are every 4 foot. Sheers are 2 by 19 x 52 mm fir, Keels is 3 layers of 19 mm mahogany, stems are 10 layers of 19 mm mahogany, decks are 12 mm ply except for the sponson and cabin side which is 9 mm ply 25 mm VERTACELL and 4.8 mm ply inside. Decks are sheathed with 10 oz (330 gsm) cloth and epoxy. This boat would make one very fast minimalist cruiser that is capable of recovering from a 90 degree capsize. Not many modern multi’s can do that.

 

[oldmulti]

This post was made on a Sailing Anarchy blog about composites from a guy whose job it is to make resin infused carbon composites for over 10 years. It supports an earlier post from Rob Denny about resin infusion of carbon fiber. Resin infusion is not hard but takes practice, test panels and real evaluation of finished parts to make perfect.

“As to the sandwich panel properties of an infused panel compared to a wet bagged panel.... There are some differences. A proper infusion does flood all the open cells in the foam core. Because of this, the infused foam core panel can weigh more than the wet bagged panel. Diab changed there H80 core some years back to reduce the cell size in their cores which have helped narrow this weight difference that is due to resin uptake of an infused core. You will also find, to some surprise, that in very thin foam core laminates (<1/4") using H100 core can make a lighter panel compared to H80 core because the cells in the H100 core are smaller and the core absorbs less resin... So you should have a superior laminate to core bond in the infused panel. But another difference in the infused panel compared to wet bagged panel is that the laminate thickness in the infused panel will be thinner compared to the wet bagged one... With the actual laminate having a higher fiber to resin ratio.

Thinner laminate thickness will adversely affect the panel stiffness and thus you will need to add some laminate or core thickness if you are trying to achieve a target panel stiffness. So if you infused 18oz of carbon on either side of 1/2" foam and then compared the properties to a wet bagged panel you might be surprised to find the infused one is slightly heavier and not as stiff. If that lack of stiffness is causing the side of the boat to oil can and constantly flex...it makes sense that this will eventually break down the laminate to core bond... Which is the achilles heel of sandwich construction. So, I guess this is a very long winded way to say that the infused hull sides are likely delaminating because they need a thicker core and/ or thicker laminate. There are so many variables that if you really are serious about weight and panel properties you should make many test panels.

Delamination of infused carbon and foam panels can occur because of the following. Carbon fabric by itself infuses very slowly because the fibers are 1/10 the size of fiberglass and under vacuum can form a very dense, hard to infuse laminate. So, a very common way to infuse a light weight carbon panel is to use flow media, like green flow, onto the surface of the laminate. Resin flows very quickly through the green flow. There is a huge difference between the flow rate of carbon and flow media. The resin likes to take the path of least resistance and thus will tend to flow extremely quickly in the flow media covering the surface of the laminate while the laminate is barely absorbing resin in the z direction... Toward the mold surface. If this happens too quickly, the resin front will race past the dry carbon leaving behind voids and dry fabric. If you are using close to full vacuum the carbon will eventually wet out but is resin starved and full of voids. This is very easy to do with carbon infusions and will lead to some very crappy parts with poor laminate to core bonding. If you are shooting for a clear carbon finish you will see tons of pin holes and voids on the mold side of the part.

The ideal infusion rate for carbon is around 1/2" to 1" per minute (that’s in my shop with our processes and resin system). We achieve that by metering the flow of resin from the resin pump with a gate valve/clamp. We also use elevated temperatures of the part and resin, along with micromesh interlaminar flow media to improve the infusion rate of carbon and solve these lag issues.”

The jpegs are of a Harryproa that is about to be vacuum infused and are from a web site that describes a mold and "intelligent infusion" http://harryproa.com/?p=1845

 

[oldmulti]

Back to Basics is a Newick tri that has many interesting features that were not widely available to a home builder when it was designed in 1996. The tri is 26 x 18.5 foot and can fold to 8.5 foot by swing arms. The displacement is 2800 lbs and has a tow weight of under 2000 lbs. The length to beam on the main hull is 10:1. Now the fun begins You can choose a sloop rig or a lug rig of 287 square foot of sail area. The sloop rig has a large mainsail and small fractional jib. The lug is just one big 4 sided sail. Lugs were use by British fishermen who sailed 80 foot long mono’s with 2000 square foot lugs with 2 people. Lugs are not easy to tack but are very powerful on reaching and running courses. They go to windward well if tuned and sailed correctly but will not get down to 35 degrees apparent. Dick Newick claims the bioat will sail at between 60 to 80% of wind speed under either rig. The advantage of a lug is the short (21 foot) free standing mast and its ability to quickly reduce sail in any wind direction Dick Newick specified lug sails on several other of his designs including a 48 footer island trading tri. Only one Back to Basics was built but others were interested in the design.

As Newick said he sacrificed sleek looks for full headroom between the 54 inch wide aft double bunk and forward loo. There is a small galley space and the cockpit can act as a dinette in good weather or at anchor.

The construction of the prototype was 9 mm WRC with 10 oz (330 gsm) cloth on both sides using West epoxy. The floats are the same but have a unidirectional carbon fibre strip on the top and bottom of the floats to strengthen them longitudinally. The Aka’s (cross arms) are douglas fir with uni directional carbon fibre and sheathed in glass fibre. The top and bottom planks are 1900 mm long x 125 mm wide and 19 mm deep. These planks have uni directional carbon fire on either face. There are 125 x 19 mm diagonals to support the triangular shaped cross arms. The cross arms are attached by pins 25 x 400 mm Stainless Steel through the crossbeam and attaching to the main hull. At the float end a 25 x 300 mm pint goes through the cross arm and is attached to a float bulkhead. This boat was conceived as a fast cheap performance cruiser for those who had reasonable ambitions but not deep pockets or they live miles from the nearest water. The jpeg is shown in 2 forms to try and get a better size.

 

[oldmulti]

The following is a bit of history. The Clipper 14 hydrofoil stabilised trimaran won a design competition in a 1970 Trimaraner magazine design contest. Dick Dodd went on to detail the design and subsequently wrote a book called “Hydrofoils and high speed sailing craft” by Dick Dodds. At least 2 of the 14 foot boats were built but there are no reports of the boats sailing capability. The 14.75 x 8 foot tri weighed 200 lbs and carried 102 square foot of sail. The second jpeg is a study plan of the structure of the tri which as 9 mm 4 lbs/cubic foot polyurethane sides and bottom covered with light (guess 200 gsm) glass cloth on either face. The bulkheads were of the same construction. The main hull stem was 12 mm ply. The deck and centre board case is ply. The foils are T shaped. Subsequently Dodd designed a cruiser option tri named Mallard 25. The tri was 25 x 16 foot displacing 2600 lbs and carried 310 square foot of sail. The tri was foil stabilised with variable incidence T foils.

Both these designs were around a “golden age” of initial sailing hydrofoil development. The first really viable well funded development was Monitor. A 25 foot hydrofoil developed for the Navy that topped 40 knots in 1957 and it only cost $20,000 of 1957 money. You could had purchased several houses for $20,000 in 1957. Monitor has aluminium ladder foils on a “monohull”. The first known hydrofoiling Moth was developed in 1970 in Queensland Australia and was racing in 1972. If the wind was above 10 knots it was OK but could not really sail to windward and below 10 knots a Sabot could beat it.

But the one of the first “all round” amateur built foiler was done by Don Nigg in 1967-68 in the US. His first foiler in 1964 had 3 small floats and a plywood panel structure in between. It foiled but was fragile. His next design was Flying Fish a 16 foot long x 20 foot beam tricycle that weighed with crew 480 lbs. The main hull was a sealed 6 mm ply hull shaped tube to carry the torque loads and the cross beam had a vertical web inside with a 3 mm ply tubular skin to carry the torque loads again to keep the wooden hydrofoils in alignment. The foils had a total surface area of 15 square foot with the 2 rear foils taking the majority of the load, the front foil was the steering foil. The foil sections were 7 percent Ogival sections. When sailing at full speed only about 5 square feet of foil area is in the water. This boat could sail on any point of sail taking off in 5 to 7 knots of wind and achieving 20 to 30 knots after a few years of development. Flying Fish could only sail in winds below about 18 knots due to its structure. The last jpeg is Flying Fish. The PDF should be read if you have a real interest in the topic, its 9 meg. and gives a lot of theory and history from people who have really done it. Yes we have come a long way but again with the early Moth foilers and Don Niggs boats they did not require variable incidence control to sail them.

 

[oldmulti]

The 23 x 17 foot Marples Cyclone tri folds to 8 foot. The tri weighs 800 lbs and displaces 1600 lbs. It carries a Hobie 18 rig with 240 square foot of sail area. It has 8 foot long cockpit seats to sleep on. Has a small storage cuddy forward. The boat draws 1ft centreboard and rudder up. Plans show a lifting rudder as well as CB in cockpit.

The construction is constant chamber with the master mold for the hull panels is 4.5 x 14 foot. The longitudinal camber is a constant radius of a circle, so you can line the finished panels up together on it. There is a crosswise camber that changes in radius across the mold. Once you have a pattern for the WRC that fits together, the same piece can be constantly cut out, so there is no spiling, saving time. Cyclone main hull is built with 3 veneers of 3mm wrc (one was built with 5 layers of 1.5 mm wrc) and floats are built with 3 veneers of 1.5 mm WRC. Decks are 9 mm plywood.

Once the panels are made there only needs two internal frames in the hulls due to the stiffness of the hull construction. The plans are fairly comprehensive. The dimensions are very accurate producing fair hulls and a completed boat that floats on its lines.

The Hobie 18 sail rig is cheap and available. Two reef points are added to the mainsail. The mast and boom can be modified to allow roller furling/reefing. The folding unfolding and launching-retrieving process can take two people about two hours. One owner reported since launching he had sailed 13 times covering around 130 nautical miles with a max speed of 12.8 knots in less than 20 knot winds. Another owner has reported a maximum speed of 16 knots. The owners are very happy with the all-round performance of there boats.

 

[oldmulti]

The second Marples tri we will talk about is the CC 26. The tri is 26 x 18 foot that weighs 2000 lbs and displaces 2800 lbs with 308 square foot of sail in main and foretriangle or a max of 367 square foot on a 30 foot mast. It is trailable folding to 8 foot using a simple float fold under approach. It takes about 2 hours to fold and unfold. Marples has designed the Sea Clipper 24 hard chine plywood tri with swing arms. Also he designed the DC 3, a 26 foot centre cockpit constant camber build, swing arm tri with more accommodation than the CC 26. Each tri has its advantages and complexity of build. But all sail well, are structurally strong if built to plan and are trailable.

The CC 26 main hull has a deck beam of 5.5 foot with 4.7 foot headroom. The accommodation is a simple forward “double” berth (if your thin or in love), a small galley and some seating. The designer claims “it is a fast coastal cruising sloop trimaran that is a great weekender for a couple. In a pinch, there is room for two children as well. This boat folds for highway trailering. It has a four-person daysailing capacity. This boat is a lot of fun to sail as it easily cruises at 8-10 knots, with bursts of speed to 15. The construction is particularly streamlined with the Constant Camber panels, minimizing the number of parts and simplifying assembly.” Marples is an honest designer.

The structure is constant camber with all hulls 3 layers of 3 mm WRC. The decks are either 9 mm ply or 3 layers of 3 mm ply if it is a curved surface. 6 oz glass cloth cover. Gunnel strips and laminated roof beams. Centre board case 12 mm ply. Mast bulkhead and transom 12 mm ply. The cross arms are ply timber with a metal strut acting as a water stay and a folding arm. The jpegs have a very early proposal of the CC 26, a few actual boats and shows some of the initial panel builds for a CC 26.

 

[oldmulti]

The following beach cat was popular in Australia and had a global following and formed the basis of some fiberglass cats of similar proportions. The original version was designed in the late 60’s but an upgraded version appeared in the 80’s with a bigger rig and kites. These boats were home built mostly out of 4 mm tortured plywood. The aluminium main beam 91 x 84 mm and has a dolphin striker. The main and rear cross beams are permanently attached to the hulls being glassed in. The diagrams give an idea of the deck structure etc. These are good fast boats that gave a Tornado a hard time and could sail in 25 knot winds with good sailors.

 

[oldmulti]

This is Nic Bailey and Cornish Crabbers Coolcat 26. It is 26.2 x 15 foot production cat that weighed 2275 lbs and displaced 3000lbs. The racing version carbon fibre single spreader mast 40 foot mast carried 420 square foot in the fully battened square topped main and fore triangle The Coolcat 26 was Built by Cornish Crabbers (yes the people who produce open and cabin clinker glass mono’s) in 2005. It was designed by Nic Bailey. Cornish Crabbers wanted to diversified their markets and only built a few of these cats before they discovered the labour and cost of a multihull were more than expected. But the boat was a very good design with its short high wing deck and deep dagger boards which allowed excellent upwind performance.

The CoolCat 26' is definitely a ship for people who like to sail fast. The Coolcat 26' was conceived and constructed to fulfil the needs of speed and stability of multihull sailing with the comforts of double berths. All accommodation was in the main cabin on the wing deck and had sitting headroom. The boat was available in Cruising and Racing versions the Coolcat 26' is moulded in a single upper and lower units joined at the gunnel line for more strength. In tests done by Ross Hobson the cat was doing 16-20 knots and he stated the Coolcat 26' is a very capable all-round cat. Ross who slept a night on board said it was a comfortable boat as well. The construction was foam glass all the way through with PVC foams and polyester resins. The rig was attached with synthetic ropes etc. This was a fast cat with some accommodation. The jpegs give the idea.

 

[oldmulti]

After the Coolcat 26 Nic Bailey did some design of a similar nature but they were not popular. They were conceptually very fast designs but had the usual problem, they either did not have enough accommodation space for full cruising or they were not perceived as “hard core” racing enough. One of these designs was the Feral 32. The Feral was 32 x 18.25 foot displacing about 5000 lbs and carrying a 45 foot wing mast with a 380 square foot main and a 180 square foot fore triangle. The hulls length to beam is 13.8:1. The dagger boards are deep. The bridge deck shape is very aerodynamic. This cat has all the characteristics of a very fast boat that has 2 double berths some seating and a small galley. There is only sitting headroom in the main cabin. The accommodation is all in the main cabin as in the Coolcat 26.

The construction was proposed to be full foam glass with rig options ranging from aluminium to carbon fibre masts with stainless steel or synthetic rigging. Study the hull shape, this was a good shape before the move to vertical then lower buoyancy reverse bows on higher performance cats. This would have been a very good racer or fast weekender for those who needed the accommodation basics but were not into full cruising.

 

[oldmulti]

First a small announcement. From today there will only be one formal post per day not 2, life is starting to get in the way. After the recent fast racer cruiser cats, I will do a few cruisers that need a very generous rating to race. One of these cats was designed by Derek Kelsall and started out with the name of Belerion 24. The Belerion 24 is 24 x 14 foot displaces 4400 lbs and carries 295 square foot of sail area on a fractional rig. The sail area could be increased to 500 square foot with genoa’s etc. The hulls length to beam ratio is 7.8:1. The hulls have low aspect ratio keels. The designs hull shapes are optimised for carrying loads, minimising pitching and good handling by centralising weight around the pitch centre and minimising rocker. There were options of a central rudder or spade rudders under the hulls. The performance predicted by Kelsall was a realistic maximum speed of 8 - 9 knots and do not expect sparkling windward performance. This is for a person interested in cruising the performance would be adequate.

It went into production and changed name to the Solaris Sunbeam 24 and had a raised roof option over part of them bridge deck to give full headroom. Now the main feature of this boat is its accommodation. Two double (really large singles or very friendly doubles) berths in separate cabins, separate loo and shower, separate 7 foot galley and internal seating for 5 at a permanent table that could be lowered to form another double berth. There are not many 24 foot boats of any type that could genuinely accommodate 6 people for a week. The double berth cabins are in the idea position for sleeping at sea with your head near the pitch centre of the boat.

The construction of this boat was foam glass as far as I know. Kelsall has done later designs that have similar characteristics for home builders but that is for another day.

 

[oldmulti]

The second sample of Kelsall pure cruise cats. The following designs were initially developed for the home builder. The KC 21 was not as popular as the KC 27 of which many were built. Kelsall still has the KC 27 plans listed on his web site for sale in full ply, strip plank cedar round bilge and ply or foam glass. The jpegs are of a ply build.

The KC 21 is 21.2 x 13.2 foot with 260 square foot of sail in a main and foretriangle. It has 7.75:1 length to beam on the hulls. Its main cabin has 4.8 foot of headroom although there is full headroom in the hulls and galley. This would be a cruiser that could carry a small family for a week away. The KC 27 is in a different league. The KC 27 is 26.4 x 15.35 foot displacing 7000 lbs and carrying a 250 square foot main and a 135 square foot genoa. The length to beam on the hulls is 6.7:1. The design has fixed low aspect ratio keels and can either have a central rudder or spades under the hulls. There are 2 double berths aft, a 7 foot galley, separate shower head and a dinette in the saloon. This boat is a sensible cruiser that would have no problems doing a 4 week vacation up a coast or if the weather was well chosen short ocean hops.

The Tonga 25 (the close relation to the KC 27) hull plan gives an idea of the hull specs for the plywood version. The jpeg of kit panel construction gives an understanding of the simplified approach for construction and the round bilge option. The jpegs are of a plywood home-built KC 27.

 

[redreuben]

I love those older Kelsall Cats.
I have these pics variously labelled as 25 or 27 on my hard drive, I think they are all the 27, some ply some glass foam regrettably I don't think the ply plans are available any more and even if they were Kelsall's prices are astronomical.
Enjoy,

 

[oldmulti]

Redrueben. I like these boats as well and I saw the Marine Timbers being built by Barry Marmion (who also built Little America Cup winning C Class cats in 1985). This boat could sail very well in light to medium conditions beating a motely group of 30 foot boats but not able to keep up with Bucc 24’ or boats above 30 foot unless they were Pivers in racing conditions. Marine Timbers cat was a Tonga Tini 25.15 x 16.9 foot boat that weighed 1800 lbs and carried 320 square foot of sail. A later version became the Kelsall Tini and was extended to 27 foot with the same beam and sail area but the displacement was 4100 lbs. Marine Timbers cat was sold in Victoria for $5000 to I think a WA person.

Now remember I said Marine Timbers cat weighed 1800 lbs. The builder was very good and had access to many sheets of Gaboon ply which he choose the lightest sheets and he applied C class building standard quality. Everything was as light as possible and only minimum size fillets etc were applied. This boat had 6 mm ply hull sides, cabin sides and decks. The hull bottoms were round bilge 3 layers of 3 mm ply double diagonal. The lower hull stringers were 25 x 19 mm at 225 mm centre lines. The upper hull 6 mm panel had 2 stringers 37 x 25 mm. The sheer clamps were 2 layers of 25 x 75 mm. The deck stringers were 30 x 25 mm at 230 mm centre lines. The main beam bulkhead(s) are 9 mm ply either side of the beam landing point with the beam resting on a 15 mm plywood shelf. The main beam and other 7 mm hull bulkheads were framed with 80 x 19 mm timber. The stems were 80 x 60 mm. The keel lines were 25 x 200 mm on the flat. The wing deck was 12 mm with 80 x 19 mm stringers on edge underneath. The 6 mm ply flat surface internal furniture acted as structural reinforcements. The main beam was 500 x 200 mm box with 12 mm ply top bottom and sides. There was some timber reinforcement inside. The forebeam was a mast section 100 x 75 mm. The rear beam was a 275 x 190 mm box section. There was a central nacelle that carried the rudder, dagger board, outboard and anchors etc.

The 32 foot mast was 160 x 120 had a set of diamonds. The forestay and caps were 4.8 mm stainless steel 1x19 wire. The central steering system and dagger board worked well as long as they were clean and well shaped. Now a warning, the specs are from the Marine Timbers cat built Tonga Tini 25 which was lightly built, so it may not reflect exactly the plan build. There was no structural problems with that boat. The accommodation was surprisingly comfortable for its size but it only had sitting head room. This was a nice boat capable of coastal cruising. People have crossed oceans in Kelsall Tini 27’s.