Rocking the boat
by John Fildes 28 Sep 2020 15:59 BST
Rockatross © John Fildes
For those of you reading this from outside the UK and thinking 'what the hell is a Merlin Rocket?' and 'why the hell is anyone building a clinker boat in this day and age?', here is a bit of background; The Merlin started life on the Thames at the end of the second world war. It was designed as a dry-to-sail boat with a lot of deck (this is pre self-bailers and transom flaps) in contrast to the International 14s at the time which had no deck and the crew bailed out all the way around the course.
A couple of years later, along came another class up in the north; the Rocket, designed by Dick Whyche, which had a similar set of lines but a lot less deck than the Merlin. The Royal Yachting Association, concerned there was too much choice, amalgamated the two together and we had the Merlin Rocket class. The class today is deemed a restricted development class but runs open class rules, and although not fully understood by most I fear, basically the rules are such that if it's not written, it's not implied.
While boats up until twenty years ago were predominantly wooden, today the majority are foam sandwich hull (no carbon allowed in the hull) but everything else is carbon sandwich. Throughout the years there have been many great designers; Jack Holt, Dick Whyche, Ian Proctor, Phil Morrison and Hugh Wellborn to name a few, as well as many great sailors in the class and today it's a bit like a champion of champions class in the UK. The clinker design was chosen in the day as it was cheap to build (sadly that's not the case in glass fibre these days) and over the last twenty years the class has in effect moved more and more towards being one design, firstly because the major builder produces a good reliable product which has minor updates each year to update the boat a little, as well as I would guess that people have become more risk adverse, and as the costs have risen they have been more keen to make the right decision, so this leads me on to where we are now.
The standard hull we have today is all based around a Canterbury Tales design which first arrived in 1987 from the pen of Ian Holt and was a little fettled by the great Jon Turner (formerly of Rowsell and Morrison). The boat was designed to plane earlier in marginal conditions than the Morrison boats of the time and it has been mightily successful. But back then the boats were wooden with alloy rigs and dacron sails and were sailed off a PY 1028 handicap. In contrast, the modern boats sail with PY 985 and the boats have carbon masts, bigger spinnakers and poles, as well as a raking system we call a 'one string' which in effect joins the shrouds, lowers and forestay together and allows you to pull one string to change the rake of the rig while keeping everything at equal tension. So, in short, we now have a class a bit like the TP52s of dinghy racing in the UK which means costs are relatively high by dinghy standards with a new boat and all the gear around 25k on the hitch.
So this brings me on to my mad idea one year ago.
With all of what I have written in mind, I felt that logic said we must now be able to build a faster hull shape which is still as good all round as a Canterbury Tales because every aspect in boat building has moved on in the last 32 years, so I set out to find a designer thinking they would do all the hard work with a bit of influence from myself and I would get someone to build her. I was going to find out that would be a long way from reality.
After spending a bit of time talking to a very well regarded Moth designer as well as a regular contributor to this publication, it was suggested I have a chat with Jo Richards who proved to be somewhat busy, but eventually we met up and hatched a plan that he would draw the planks, decide the rig and board position, and the rest would be my problem.
A few words from Jo Richards, "John Fildes approached me to design a new Merlin Rocket. At that point I had very little time so said I would draw the hull lines and position the mast, but after that it was up to John. The result is a slightly different solution to the same Merlin / Twelve problem - i.e. it is beneficial to have the hull as shallow as possible at mid length to push the 'rise-of-floor' measurement up, and as a consequence reduce the waterline beam. The downside of this is that the displacement still has to go somewhere!
"I am a great believer in not pushing the water any further out (or down) than is necessary and then trying to put it back where you found it! As a result Rockatross is a low rocker boat and absolutely minimum rise-of-floor, however the crew should be prepared to move right back to the transom if required, to keep the bow out, hence the almost full width stern.
"The plank run on a Merlin is a compromise in different conditions. I am a believer in keeping some width on the garboard and twisting this up to the stem. This has the effect of twisting some of the water down under the boat rather than pushing it out to the sides. It is less distance for the water to travel and provides 'cheap' lift. There is a temptation to push buoyancy into the ends, I have tried to avoid raising the prismatic coefficient too high to keep light air performance.
"Every 'quick' boat that I have sailed has been dead straight in the last 5-600 mm. Any curvature here sucks! Rockatross is slightly reduced in freeboard to try to keep the aerodynamic drag down and the maximum beam is carried as far into the crews area as possible to maximise righting moment. The layout and construction is all the work of John Fildes so I will leave him to describe that."
God knows if Jo thought I was a dreamer but he was very enthusiastic. A few weeks later I went to take a look at the hull lines on his computer and I was fortunate enough to have a top Merlin sailor along with me who gave their thoughts, and a few days after that I received an email with the plank lines, and this project was now very real. I tried to find a few people to work on drawing my internal ideas and for a number of reasons it just didn't work. I'm a firm believer that what is meant to be will be, and as it turned out it was a blessing in disguise as it allowed me much more time to formulate my ideas while the builder put the CNC plug together and made the split mould.
Although being very dyslexic I have always enjoyed reading books in adult life and trying to learn from the past as well as modern concepts. The builder Simon Hipkin has a real passion for dinghy history and when I first met him he talked about Manfred Curry and his book 'Aerodynamics of sail' from the 1930s, as well as his amazing boat Aero II. So I rushed out and brought a nice original copy. While technology has moved on, and most will dismiss his crude wind tunnel tests which are nearly 100 years old now, the philosophy remains the same in my mind; How can you bend the wind and water to make the biggest advantage and then put it back down while paying the least penalty in drag?
The other book I read straight after was Adrian Newey's 'How to build a race car' and again thanks to one of Seahorse magazine's regular contributors for the recommendation. What I learnt most from the book was that Adrian had done many things which had not worked, and while we only tend to remember the things that have worked, he says you must not just discard an idea until you understand why it did not work - that way you have gained some knowledge, and more importantly, just because something is not working in a particular application today, does not mean it won't work when used in some variant in the future. If I had a pound for every time I have been told 'so-and-so tried that in such-a-such year and it didn't work then, so it will never work' I would be very rich (on second thoughts, maybe not, I have a boat problem) but what they can never tell you is why it didn't work or why it wouldn't work today - they may be right but they will never know.
A couple of years ago I bought an old wooden Merlin which apparently never went very well with anyone who had owned her previously and I turned it into a working experiment much to the fleet's amusement. We won't get into the subject of sheerlines in this article but it formed a lot of useful ideas in my brain as to what I wanted and what I thought would work in a new boat, so I will try to break it down and explain my rationale.
Hull
I had looked at a number of areas in the rules that were, shall we say, open to interpretation. One was to stack the planks around the rise of floor measurement to, in effect, create a chine. Phil Morrison had done this near the bow in his first China Doll National 12 and was possible as there was no minimum plank exposure, and if challenged, I planned to show the planks were different widths inside the boat.
Secondly, the planks can be between 6mm and 12mm but it's not written very well and I thought about doing the bottom four planks in 6mm and the top four in 12mm which would in effect push in the rise of floor by 6mm each side and the bigger planks would act as spray rails shedding water where you did not want it. In the end, I decided this was going to be a step too far for the class (needless to say, the class are at present changing the rules to ban this) so in the end, the hull is quite conservative (Jo's words not mine).
We went for minimum wetted area in the bilge keels and I chose to have the smallest plank radius to help lock in the water below the water line and shed it above. While this may make the water more disturbed on the plank edges when moving around, I feel the water is more locked in and she planes a bit earlier, which in turn reduces the overall wetted area at that point - most things in life require a trade of one thing to make a gain somewhere else. I also worked hard to remove the disturbance around the slot gasket by making an inset in the tooling to take a stainless plate with the gasket stuck to it which seems to have given us a much cleaner flow of water around the box.
I guess I should point out that I was given the choice of a normal bow and a raked back one which pulled in the planks at the bow, and as you can see I went for option two; the benefits of no gunwhales and reduced windage, along with helping the jib work better were always going to be a no-brainer for me. We spent a long time finishing the CNC cut plug kit, and the boat (I would say) is crisper that the competition but it's only my opinion.
Internal layout
This took a great deal of debate with myself as to what was 'what I wanted', 'what would sell' and 'what could I get away with'. The one comment that I heard a lot, was that people look at a Merlin and think 'that looks scary' and 'what the hell does all that string do' so my first mission was how do I make a boat look less daunting but have all the same control options as the other modern boats, and that was going to be a difficult call to get right; time will be my judge I guess. I had thought that the inside of most Merlins don't look too different from a wooden 1980s boat, but just more and more stuff has been retro-fitted. I wanted revolution! At the moment there are two other main builders in Merlins; one design has a central thwart, and one has a similar central line console to what we have ended up with.
We started out with a "rockapole" for the mainsheet that went up and down and was canted side to side each tack, but we have now moved to an adjustable height hoop with a traveller, that works like a pair of crutches and this offsets the constraints in height of a normal hoop (where the boom is wanting to hit it when the mast is fully raked). We have a central console that looks very similar to the Winder but is actually lower, and narrower to allow more freedom of movement around the boat. There is also the fact that that all of the controls except the pole launchers are there in the middle. Many other boats have a fair amount of controls at the front as well as the middle, but I found that as you start to move back in the boat they become inaccessible to the crew while the helm is now too far back for the centre ones but they are now perfectly placed for the crew.
We made subtle changes to the angles of the cut-outs which house the cleats to improve the workability of them and we were able to spread out the control tails much better by having a carlin that in effect allows you to operate all systems in a tactile manner while keeping your head out of the boat. The boats are not allowed to self-drain and our rules state that a rear tank can come only 1100mm forward of the transom.
Most boats have had two rear air bags up until a couple of years ago but Jon Turner introduced a new style low tank that went from side deck to side deck and was about 60mm high in the middle which was great for stiffening up the rear end of the boat and removed a lot of volume which meant the boat would drain much quicker after a capsize. We had sailed his boat a year before and it was great in lots of ways, but I didn't like the fact you had to step up on to the tank and then over the box to tack, so I decided to do something a bit different.
The Winder boats had fitted bags until a few years ago and have replaced them with a carbon moulding that replicated them, but made the boat much stiffer. The problem for me is that the older boats got narrower at the back end and the tanks restrict the amount you can hike with the combined effect being that they push you to rotate and sit on the transom. So my design brief to myself was to remove as much volume as possible, allow fast draining, get maximum righting moment and combine all this with comfort and control.
I had started this in my last boat after the transom fell off and it looked like a good opportunity to try something different but it was not quite right. Anyway we have ended up with a big tank in the centre of the boat and shallow 'cheese wedge' tanks on each side which join into the carlin. This removes the maximum amount of water when righted but allows the side to sink a little in a capsize to help you get on the centreboard. The space under the big tank is not used as the tiller is over it, so it's good utilisation and the sides of the central tank give you great control as they are like giant kick-offs and the slim sides enable you to sit nearly 400mm further out than the Winder giving massive benefits in righting. I also chose not to fill in any of the back end. We have to maintain a minimum of 40mm between the hull skin and the inside back there and we are pretty close to that where the cheese wedge side tanks meet the side of the central tank. The result is something I hope Manfred Curry would approve of as the wind and the water is able to leave the boat with much more ease than the other models.
The large kick-offs help to give good control in the boat and some of this up to 110mm from the transom is also tank along with the case sides adding to the overall buoyancy. Coming back to Jon Turner's floor tank and stepping up and down, I decided to use a fixed floorboard from 1100mm forward up to the back of the 'Napoleon's hat' (where the central moulding raises up) giving a much cleaner working area and stable platform for the crew downwind.
To enable the boat to drain we built a pair of tunnels through the rear tanks to the transom flaps which allow the water to leave the centre of the boat and put two cockpit drains at the back of the helm floor into the tunnels to drain that area. I've also managed to run the spinnaker pump system in the tunnels with great success. We made every effort to make the tunnels as small as possible to maintain maximum volume in the tank, but they needed to drain the centre section of the boat fast, so at the back end we opened them up in size to relive pressure, and in hindsight we could make that area smaller in the next boat.
Sharing is caring
I somehow ended up with two very pivotal boats in my life that I have learnt a lot from; first the great Firebird catamaran from the pen of Martin Smith, and second, Albatross speedboats designed by Archie Peace.
Both of these men by chance worked at the Bristol aircraft company (is it Airbus today?) as stress engineers - Martin on concord, and Archie on WW2 planes before being sent to Grumman's in the US. Both boats are worlds apart but share the same principles of removing weight and creating rigidity by sharing loads between components when joined together. If I can remember correctly, the Firebird's main bulkhead which carries the beam and other hull weighed 4lbs and had 104 different patches in it. This took the British aerospace computer some days to work out in 1985. The result was a boat that in effect killed the Micro multi hull class.
The Albatross is all aluminium and riveted like aircraft, and the top end ones have Coventry Climax light alloy engine with over 110HP in a boat that weighs 280kg all up - the underlying fact is you could look at aspects of both boats in component form and think they are not strong enough, but put all the bits together in the right way and they form a formidable structure.
I have tried very hard to use this philosophy in the Rockatross so there are five bulkheads crossing the boat and then longitudinally the kick-offs link the front bulkhead / tank to the rear tank sides. The centreboard box top in the driver's area joins down into the cockpit floor and out to the kick-offs, and the bulkhead inside the rear tank continues the box to the transom. In the crew area, the box capping ties into the front tank which is not the case in some of the other Merlin's and by having carlins, we link all of this to the gunwhales. The result is a boat that is in a different galaxy in stiffness, but the next step is to think about how this is affecting our stock mast and foils.
Other Merlin crews have retro-fitted more and more bits, and one of the latest fashions is for what can only be described as 'picnic benches' where they mount the jib and kite turning blocks and cleats. The crew sits to leeward downwind with their back facing outward to be able to look at the kite, so these picnic benches were never going to make them feel very comfortable. The jib sheeting has become more and more complicated and finite in the boats with the move towards the jib tack going forward and the re-proportion of sail area from main to jib in the last few years.
The effect has been a need to have the jib sheeting further back in the boat and the problem with this has been that the crew has been left with less and less working area and tends to get involved in the sheet when sitting to leeward and while this is not so important downwind the gybe is now more complicated as its harder to release the jib before the gybe for the crew as they are likely standing or sitting on some part of it. Where some are running a three-track setup there is now the added problem of the controls for them making it more cluttered again.
As such I have gone radical and moved the jib sheeting out of the cockpit altogether freeing up space and cleaning the boat up, but I will come back to that in the foredeck description. The rest of the crew area features a centreboard box which is lower by 30mm to give more space and a slight reduction in the top width but there is a fair amount of carbon in this to keep it stiff. The board features a Magnum PI style 308GT gearbox which allows you to lift the board back or forward to change the pivot point and move the centre of effort.
The front of the case looks like a wishbone and that gives much more room for all the control lines to be rigged through. This moulding also supports the mast supports. I am not a great fan of using tubes like most boat builds as it has such little bearing area as well as causing a road block directly under the mast, so we have used cedar that has been bound with carbon sock and this forms a very light, strong structure and we moulded in to recesses to the tank top and case top to accept the ends of these. Under the mast is a piece of 8mm Tufnell encased in carbon which replaces the Corecell foam used every where else. We have notched the posts to support the underside of this Tufnell as well as support the side load and all of this allows us much better support and much more working space.
The front tank rises as it goes forward ensuring any water that comes into the boat through the spinnaker chute falls straight back into the boat and out of the self-bailers. Jib and kite controls are hidden in the carlins and make for an even cleaner look and feel. The large kickoffs continue up to the bulkhead helping the crew cross the boat. The pole launcher lines exit from the back of the foredeck with the cleats hidden behind.
Deck
I spent a long time deciding on the deck and what to do with it and as we made the side decks first I guess I should start there and work forward. Jo had suggested having carlins that came down into the boat to make her easier to slide in and out of, and I guess I never realised at the time how that was going to define my thinking. With so many controls in such a small space generally the boats struggle to have good leads from the cleats and organise the take ups.
Some of the controls need to be continuous but also be on elastic take-ups and it was always going to be a challenge to make it all work well. I decided to put two access hatches on each side in the side decks; one above the central controls and one near the back. We used the new Allen through-deck bushes which screw together from both sides making far less friction as the rope is pulled either way. This allowed me to space out the controls nicely under the side deck.
We have some turning blocks on some of the lines and some just go straight to the take ups. There is a sliding carbon card that slots in the back access hatches with all the take-ups tied to and that lifts out if you need to do maintenance to any of them. The doors are just bungeed shut, and by popping a ball off a loop inside the boat you can open them up and do what you need to do at any point. It has allowed us to group fourteen controls in a very small working area.
The shroud placement into the deck was a major gamble that has somehow come off. I knew that at that point I was going to need some control lines to pass the shrouds and lowers without getting involved with each other. I was also going to need the turning blocks for the jib and spinnaker in there as well. We did mock-up a fair bit of it and worked hard to get the jib pull angle across the boat at that point, and the kite pull angle to be more directed towards the middle of the boat.
It's worth considering that all of the controls need to be operated well with the crew inboard as well as hiking and that results in a very big variable of what is possible. It's easy to make them work for one position but not the other so I am ecstatic that she has turned out so well and with such functionality. So, we have ended up in this area with the shrouds and lowers running under everything, the control lines passing over the top very close, and then the sheets over the top of the lot; quite a feat.
It's also worth mentioning that the gunwhale is not U-shaped underneath like most dinghies, but has a flat underside and then comes up. This is part of the hull mould and the side deck areas in effect creates more space under the deck for control lines and a box girder structure giving great strength, the deck is then bonded down on to the outside flange.
At the shrouds the boat is 180mm wider than most other Merlins, allowing for more righting moment and also helping us run less rig tension. As we go forward to the bow, the gunwhale disappears. The shape of the side decks was given much thought and I struggled to find someone to back up my thoughts. Our last boat had side decks which sloped outwards and down and I'm guessing that was to help you hike hard. But I found myself sailing around with the boat heeled to leeward.
My question to myself was based on a line from the film Rush when Nicky Lauder says to a girlfriend, "I am paid for my ass and its ability to feel." So was my ass telling my subconscious that it wanted to sit on a level surface and not one that makes you want to fall backwards?
With that in mind Simon Hipkin and I worked hard on finding a nice shaped inner carlin and gunwhale angle to make hiking as comfortable as possible and I chose to start the side deck at the shroud with the outside dropped which has the benefit of positioning the crew a little bit lower than the helm when hiked giving the helm more visibility over the crew.
The helm's normal position is flat and as we go backwards the outside of the deck lifts raising you up as you are more likely to be perching than full hiking most of the time back there.
The foredeck was difficult as it had to blend into the side decks nicely and while the hull was still in the mould this was hard to visualise. I remember a Clarkson quote about a Renault Laguna estate, were he said it was a shame the chap that designed the estate bit had not been told what car it was going on as it looked very out of place. With this in mind we did not start the foredeck until late in the build.
I spent a good deal of time looking at TP52 kite drops and what made a slick kite hoist/drop, and my conclusion was chute radius or consistence of radius. I worked hard to make a fair radius that was not just at the back of the chute but also on the sides and front and the result is a greatly reduced friction on the drop making it faster. Behind the chute the deck starts to rise.
We have our biggest area of jib where it has no overlap with the mainsail, and we are not allowed to fill in the area between the kicker and the mast to solve this. I chose a twofold approach to deal with this. The deck rises more than 100mm higher than the other models and is conical in shape which helps to push the wind that hits the deck up at about 20 degrees towards the mainsail.
In time I will look at the merits of reconfiguring the rig to drop the boom 100mm and this would further reduce the area that is not benefiting from overlapping the mainsail. I also did the unusual thing of dropping the mast into a pit. This has several benefits; a reduction in windage but mainly we are able to have the mast heel at almost the same height as the shrouds so that the geometry of the rig lengths has less variable between upright and fully raked.
The foredeck is much longer where it would normally be cut out to allow the jib sheets to pass. I have decided to put in cross tracks under the deck so the jib sheets go through the deck and then lead forward to a turning block under the foredeck before heading back to the shrouds and out to the cleat and across the boat. There are a number of reasons for what I have done. It keeps a very clean layout in the boat and also controls the jib much better as the car and clew are much closer together, this stops sheet stretch in the gust which will put depth in your jib at the point in time you do not want it.
The slots in the deck rake forward, as when you rake the mast you naturally move the car out, so the idea is the camber is matched to the rake effect on the clew. Although the foredeck looks long, it is in fact only 20mm longer in the centre behind the built in Tacktick bracket. I spent a lot of time working out if the infill around the jib would impede the crews position in the boat but figured that it was a dead space that could not be used so we have ended up with a really nice end-plate effect on the jib that also helps direct the wind up on to the main, minimising the confusion as best we can in the non-overlap areas, and also a lot more structural stiffness between the shrouds and the mast.
We are able to change the angle of the sheet using the jib cunningham and halyard by running a halyard inside the jib luff which raises or lowers the jib a very small amount and these controls are led back to the middle of the boat.
The rig and foils we started with are straight from our last boat which are Jon Turner J and S Pro carbon rig and foils. I chose to keep these to start with to give us a datum point. We have since moved to a new mast and boom, and we have a fixed forestay and the rake control comes through the bow. This is for a number of reasons; firstly it has given us more rake range and secondly it has removed compression from the rig which is something that will deaden the benefits of a high modulus mast.
The foils are still a work in progress. We are now making a new centreboard and are just about to start on a new rudder stock. The rudder blade has been delayed due to a rule clarification dispute that will hopefully be resolved shortly and allow us to move ahead with the last bit of the jigsaw.
The construction is Corecell foam throughout, glass biaxial hull, and carbon everything else. We used Sicomons epoxy and a Scott Bader spray gel coat. We have ended up with a super stiff, light boat that drains fast and tries to manage the wind disturbance easily.
To Summarise
I guess you will be asking now that all of that's well and good but is she fast? And I would say most definitely yes. It's still a learning curve and we haven't helped ourselves by biting off so much at once with so many ideas, but we are on top of all of the initial teething problems with control systems and now ahead of the game in that respect. It also does not help that we keep putting non Merlin sailors in the boat to drive with no practice.
It's not been an easy project by any means, but it has been fun and rewarding. If you want to find out more please have a look at our Rockatross Facebook page.