New concept in foil

Linton Jenkins and aeronautical engineering guru Jonathan Howes explain their new Tomahawk

Friday March 13th 2009, Author: James Boyd, Location: United Kingdom
One of the stars of the RYA Dinghy Show last weekend was certainly the new Tomahawk foiler displayed on Ovington Boats’ stand. It transpires that this is the latest development of the fine vessel Full Force’s Linton Jenkins was trialling in a clandestine way down in Weymouth last summer.

According to Jenkins the concept of the boat is as an ‘every man’s foiler’. Originally he was contacted by Mark Wagner, a businessman, who by coincidence had built his own foiler during the 1970s. Wagner wanted to buy a Moth, but Jenkins convinced him he needed something purpose-built.

The new boat was designed by uber-dinghy designer Phil Morrison and according to Jenkins the brief was “that it is supposed to be a really easy boat to rig - Mark wanted it as easy as a Laser, quickly rig it put it in the water and sail it. We are a little way away from that, but that is the general gist of it. And make what you want of it. You don’t have to be a good sailor to sail this boat. You have to be a good sailor to sail it fast, but that is it.”

Like the RS600FF, it is designed for those who are larger than spindly Moth sailors - around 75+kg, also for those who are not as agile. It is also the first boat of its type designed specifically for foiling.

Significantly it was originally conceived with a conventional foil set-up similar to the Moth or RS600FF. However in parallel to this project, Wagner had also been approached by one the UK’s top aeronautical designer, Jonathan Howes to help fund development work on a speed sailing project called Monofoil. Someway into the development of the new boat Wagner asked Howes if he could reconfigure his unique ‘surface running, ventilated hydrofoil’ he had been developing for the Monofoil, to be used on his boat.

More on the science of the Tomahawk foil later on, but the result of Howes’ latest developments was the establishment of the company holding the patents on this foil: Tomahawk Foils, owned three ways between Jenkins, Howes and Wagner. The foil has also been fitted to a sailboard, which the keener-eyed will have noticed on the foiler display in Palm Court.

Compared to a Moth, the Towahawk dinghy is a little longer – 14ft – while the production version is likely to be slightly narrower, around 8ft compared to the present 9. There is substantially more volume in the hull compared to a Moth, making it a more stable in light winds, but this makes the whole deal a lot heavier than a Moth – while these now have an all-up weight of less than 10kg, the Tomahawk at present weighs in at 55kg.

Significantly different from a Moth, the boat also has two sails. This rig is something both Morrison and Jenkins were keen on as it makes it more user friendly and Jenkins maintains that there are many users who find it easier to trim a boat from its headsail. It also lowers the centre of effort of the sail plan - a bonus for foilers as it reduces pitching, thereby stabilising the foils.

Also there is also no wand control mechanism – the Tomahawk foil is surface-sensing and therefore has less moving parts. At the Dinghy Show the boat was fitted with an RS600FF rudder with the usual motorcycle-style twist grip to alter the pitch of the rudder foil. However once the Tomahawk is into production, Jenkins reckons that this will be a fixed arrangement too.

Ovington Boats were bought into the equation at a fairly late stage to handle the marketing and production. According to Jenkins it is likely that the boat will be available in three versions – without a foil, with a foil and also a version for the more performance-orientated, with a trapeze. Despite its mass production, it will be fully built in carbon fibre. The plan is for it to go into production from the end of May with the first boats available in June-July time. But the good news is the price - all up for around £8,500, around about 60% of a fully tricked up Moth.

The Tomahawk foil

Jonathan Howes has a formidable background as an aeronautical engineer, an industry he has worked in since the early 1980s, including six years working on supersonic wind tunnel testing, and the bulk of his career spend at the Civil Aviation Authority, ultimately ending up as head of Loads and Dynamics, where he carried out the European structure certifications on the Airbus A380 and A390 and A321. He currently runs his own consultancy. Based in Sussex he is also an avid sailor, originally in dinghies and more recently in what he calls “5 tonnes of rotting rainforest on Chichester Harbour”.

Howes describes the intricacies of the Tomahawk foil: “The first thing you notice about it is that it has got a concave camber on the lower surface and it has steps on the trailing edge of the upper surface. When this foil rises fully to the surface, it generates all its lift on the lower face only, which means that that face is optimised to produce an even load distribution, so that the surface works evenly across the chord.

“Before it can get to the surface it has to work below the surface, because it has to climb up. Now a foil that is designed to create all its lift on one face will create less lift than a conventional hydrofoil section, because a normal hydrofoil generates most of its lift from suction on the upper face and a small amount from pressure on the lower face. This one generates no lift at all from the upper face once it is up and fully running. So to get the thing out of the water you need to make the upper surface work and the way we do that is by having a series of facets from about 50% chord on the upper surface. What happens is that as the foil accelerates when it is completely submerged is the pressure on the aft-most facet on the upper face sees a significantly lower pressure than the next forward facet and lower pressure than the facet forward of that. So the first place that air goes is down to the last facet. So as you accelerate, the pressure drops, and when the pressure has dropped sufficiently, air is sucked down to the last part of the upper surface and the flow then runs across the upper surface round the first corner to the second corner where it separates cleanly and forms a big bubble and the bubble follows the foil around and the bubble is actually effectively past the foil itself in that condition.

“As you go a little bit faster, the pressure drops in the next facet forwards and eventually air will suck in and ventilate that facet as well. Then the flow is leaving the upper surface of the foil at about 50% chord.

“Now as each of those changes occurs, the net deflection of flow by the foil is reduced. So it starts out with the flow fully attached and all the facets are wet and you have the maximum possible flow deflection and this is directly equivalent to an aircraft wing with the flaps fully down. It is performing exactly the same function – it is increasing the angle by which you deflect the flow. As the first facet gets ventilated, that angle gets reduced slightly so the lift reduces a little bit, but you are going faster which means that there is more lift available. In fact overall lift tends to climb, but you get these steps and as you go faster the foil reconfigures itself for a higher speed.”

So in short aside from the facets that work at low speed to get the boat and foil airborne, its upper surface is designed to produce a zero pressure differential with all the work being done on its underside.

Howes continues: “As you accelerate, you don’t need all that lift, the lift must be reduced or the drag will become excessive. So as you go faster it automatically adjusts for the higher speed by reducing the lift, by letting the air into a slightly more forward location on the upper surface. When you are going full speed the entire upper surface either matches the natural shape of the bubble or it is the bubble itself. So when these foils are going absolutely flat out, what you’ve got is this huge bubble following it in the wake. The way you design these foils is that you actually treat the bubble as part of the foil itself. So you design the free surface as well as the foil surface.”

This results in a very even ride compared to a Moth set-up. “When you are going absolutely flat out because it will never cavitate, it remains consistent. It not the most efficient, but what it is is a foil that will give you absolutely even performance across a huge speed range. The boat will lift out extremely early and the boat will remain handle-able, consistent and smooth right up to the point where you run out of nerve! Which was the point of Monofoil. With Monofoil we were designing a 100 knot foil.”

The Monofoil speed sailer is worthy of a separate article, but is unique in sailing along, or perhaps more accurately flying along, on with just a single point in contact with the water. As Howes states: “What wipes out most high speed sailing boats is that they generally have two or more points of contact with the water. If you have a rudder and a foil in the water, sooner or later one will be in a position where it fails to make water contact and most boats I’ve seen wipe out when that happens or have slight control issues. So I wanted a foil which would be completely consistent across the speed range and a boat which would use only one point of water contract, hence Monofoil.”

With just a single point of contact and effectively nothing in the water, Monofoil is more aircraft than sail boat. It is a two tack boat. All the R&D – the six models built and the virtual modelling too – has provided encouraging results, however at present the project is on hold while Howes awaits more backing. Video of the Monofoil model sailing can be viewed on YouTube.

“I think it was a bit too radical for the sailing world unfortunately. We tried to cast around for sponsorship but obviously you can’t get it now. I had a few abusive emails about the videos being fake which made me laugh a bit,” says Howes.

According to Howes, ventilated foils such as the Tomahawk have no place in the aerospace world – they are only effective in two media such as water and air. “They are a sub-set of cavitated foils, only instead of using water vapour in the cavity you let air in, which means it happens much earlier and it is much easier to control. Obvious the advantage is that running on the surface you don’t need a wand or anything like that, the surface is where the surface is.” Ie the foil itself senses where the water is and reacts accordingly.

So what happens upwind when the foil is inclined? “You get some of the lift reacting against leeway. There is a small fin underneath the main foil on the Tomahawk which provides a little bit of high speed centreboard area. It doesn’t need to be as big as a conventional centreboard, because it only comes into play when you are sailing fairly quickly. What I am waiting for is when we get some really good heavy bodies, who are capable sailors on that boat, because when you are a beam reach to a broad reach this thing doesn’t have a top speed limit, it is only limited by nerve and aerodynamic drag in the end. So although it will slower than a Moth probably upwind, and slower than a Moth in moderate conditions, my suspicion is that under the right conditions it will probably keep going when a Moth won’t and in those conditions it should be frighteningly quick.”

While the foil, and the boat it attaches to, have been in development, so Linton Jenkins has been the principle test pilot. He explains that the Tomahawk foil works in entirely the opposite way to a conventional Moth foils in that it is good for air to feed into the foil.

Rather than the quietly fizzing sound a fast-moving Moth makes, the Tomahawk tends to slap along the surface more, rather a pebble. “It is a bit noisier,” describes Jenkins. “But it is smoother, because whatever system you have on the Moth or the 600, you have the wand and all the time that is altering the height. So this is a lot lot smoother and it is just simpler.”

Howes provides the physics: “It is obviously noisy when it is coming up because it is sucking air down, through ventilation. Once it is on the surface you will see some commotion and spray, because it works at the surface. With a conventional foil subsurface, it is pulling a pair of vortices along with it which you can’t see on a conventional foil because they are under water. It has all the same junction grad problems that we’ve got where you attach it to the vertical strut, which again you can’t see those on a conventional foil because it is underwater. The fact that you can’t see it doesn’t mean it is not there.”

The Tomahawk requires a slightly different technique to sail it compared to a Moth, but there are some aspects which are the same, such a canting the boat to weather when sailing upwind. Jenkins explains: “Basically the leeward side of the foil can pop out of the water and it makes the boat go quicker because you lose foil surface area. So the more you have it canted to weather the faster it goes, that’s upwind. Downwind the foil comes to the surface, stays on the surface and then there is in theory no top end speed.”

Howes adds his ha’penth. “We found that once we introduced the sweep back that controls the ventilation, then sticking the tip out of the water wasn’t a problem, because the sweep back works to retard the ventilation and it seemed to come out nice and gently. There are another couple of iterations before we are completely happy with where it is. But it is moving in the right direction. What is obvious is that it is quite easy to sail, which is what we were aiming for.”

However at the end of the day the Tomahawk foil is at times not as efficient as the conventional style foils one might find on a Moth. Howes reverts to the physics: “When you are talking aerofoils and hydrofoils you are normally talking about the ‘glide ratio; of the section, that is not the same thing as the glide ratio of the complete foil – it is a two dimensional thing. Now the glide ratio of this foil section in two dimensions is about 20:1, whereas a conventional foil it could be 100 or 120:1, so obviously it is significantly worse. However when you come into three dimensional flow, which is what you get with a foil with wingtips - in other words a finite foil - things get much more equal. So with a conventional hydrofoil you may get a glide ratio under perfect conditions of about 15 or 18:1, while with this one we are down to about 10:1. And that is under perfect conditions. As you go faster and cavitation starts to fight the conventional foil, that is when we will overtake a conventional foil and get better. So for easy sailing with no mechanism but a bit more drag, we are good. For absolute balls out speed, we will also be good. In the middle, when you are looking for performance at low speed a normal foil would actually be quicker, but it is horses for courses.”

As has been mentioned the ultimate speed of the foil is potentially much much higher than the conventional Moth foil set-up since there is a lack of cavitation. As Howes says: “Cavitation is the result of pressure dropping in a flow below the vapour pressure of the water. So what happen is that the water boils locally, it is vapourising like kettles up a mountain. So you get this cavity of vapour forming, but if you give it an air path, the same cavity is populated with atmospheric air and that is what we are doing. Plus we are designing this foil from the outset to go along with that cavity present. Everything else is done with positive pressure and not negative pressure, so all the lift depends on positive pressure. If you have got overpressure you can’t cavitate, because there is no suction. You can only get cavitation when there is suction.

“We are not the first - other people have messed around with ventilated foils and cavitation-avoiding foils, but changing the lift with speed with these facetted steps, that we think is new. And what it gives you a very very low lift off speed. One early version Linton reckoned it lifted out at about four knots, which was too low. So you want to go a little bit fastest than that - with the steps it comes out a little bit faster, probably about six knots.”

With the present concept boat Jenkins says they need around 9 knots to get airborne, although with the production boat being lighter they are aiming for this to come down to 8 knots for a 80kg crew.

According to Jenkins it pitches very much less than a Moth. This came through the boat being longer and having greater separation between the foils, but also due to the nature of the foils and its lack of wand. Howes expands on this: “We don’t have the surf-following wand. If you are trying to sense the surface and track it you have a mechanism and a linkage and a response rate in there, so things can get out of sequence. But if it is surface following, all is does is if a steep wave comes along, it slices through the top of it, takes a bit of air with it and pops out the other side. It doesn’t care about waves, it just gets on with it. There are no control issues, it’ll just skip through it.”

It will be interesting to see whether the new Tomahawk lives up to expectations. While the foil may not be suitable for ultimate performance boats like the Moth, it is certain to have other applications and Howes and the team are already investigating the possibilities for powerboats for example.

More photos on the following pages.

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