It seems my dear friend Mr Baker stumbled across this thread, and in particular, the discussion about spray rail termination.
Of course, his chance find of this thread also means Mr Ring might wake up one morning to find a Horses head on his pillow, for trying to tear a loyal customer away from the centre of the known world of powerboats (Sittingbourne). A 'just' wake up call I believe.
Anyway, he sent me these words to show how he's come to do the rails the way he did on the various Phantom designs.
Justification for Termination of Spray Rails forward of Transom
The following comments explain some of the rationale behind the treatment, by Phantom, of where the spray rails should finish on approach to the transom. No attempt is made here to give reasons for the way Phantom adopted particular treatment of rails on approach to the bow or to the number, position and detail of rails.
There is no wish to infer that the final design actually approached the optimum but in the event the ideas adopted proved reasonably successful.
Some rudimentary ‘Tank Tests’ were performed on a scale model of the proposed ‘16’ design in an attempt to establish the optimum configuration. These Tank Test are described in a separate article.
Results of tests suggested that the lowest and the next pair of rails should terminate early, with the lowest most forw’d, the next pair finishing later. Seen in plan this produces a clean, V-shaped running surface with its apex forward.
This conclusion seems logical if one assumes that the primary purpose of the rails is to deflect the spray root from the hull after it has made its contribution to lift. Indeed, it could be argued that it is preferable that to minimise drag in this running area there should be as little disturbance as possible. If part of the rail spends the majority of its ‘working life’ totally immersed it is not functioning as a spray rail.
Obviously the actual point at which a rail should terminate is only ‘correct’ for a particular set of circumstances, speed, weight, trim angle, etc etc. With a competition hull this would presumably be to maximise the speed in race conditions. With, say, a planing cruiser perhaps it would be sensible to ‘tune’ the length of rail aft so as to minimise the drag in the cruising-speed range that is provided stability was still acceptable at higher speeds
Thus in the situation under discussion the aim was to try to establish the ‘best’ position with the V hull trimmed for its maximum speed but running in a stable condition.
One problem was that due to the limitations of the test tank etc it involved many assumptions and much guess-work. It in no way attempted to address the situations which occur when running ’full size’ in the open sea with the boat free to move in three dimensions and under human control.
This point about the open sea is made because in some situations results from tests in effectively flat calm conditions can not necessarily be indicative of what happens out there ‘in the real world’. For example. Most of full size testing for Phantom was done on a relatively narrow section of the river Swale . Extensive testing was carried out to find the quickest set-up including a Merc. prop that suited the boat. Fortunately, however, a ’purpose built’ propeller was also eventually included in the trails. This was significantly slower in the river than the Merc prop but readings taken in the open sea proved the opposite. With the boat leaving the water but prop still partly immersed the drive persisted and the boat proved faster.
Perhaps there is a parallel with the spray rails and results obtained for the calm cannot be assumed to hold in rougher water. Say the rails run right aft. Then if a boat is frequently flying, or even only lifting significantly, is the overall drag less than in similar sea conditions but shortened rails? Phantom did not carry out any testing which might have offered a definitive answer.
The un-cluttered running surface became a feature of the Phantom 16,18,23 ( early version ), 21 ( early versions ), 25 and the 28 ( to a lesser degree ). None of the designs following the 16 were subjected to sea-trails specifically to optimise the rail cut-off points.Subsequent to development work on the 20, latest versions of the 21 and 23 featured an area of reduced deadrise or ‘pad’ which was an obvious departure from the ‘clean running surface’ principle. This ‘pad’ was formed between the lowest pair of spray rails and was carried through to the transom.
The outer edges of this area were treated differently on different models.
With the 21 this work involved running the ‘old’ hull form at its max achievable speed in mirror calm conditions with a 200 Merc It was observed that at max the bows would tend to ‘wander’ slightly.
This could be eliminated by continuous steering correction to ‘balance’ the hull but this scrubbed off some of the speed. ( Obviously ). Modifications were made to this actual hull by introducing the ‘pad’. The object being to increase the ‘top end’ speed but without compromising the turning ability or any other aspect of stability.
The result was a hull which ran more quickly and was dead steady at max speed.
In retrospect it was acknowledged that although the development work produced a ‘better’ hull, strictly speaking the test program ought to have included extending the rails by small amounts on the ‘old’ hull. It was therefore never discovered whether this would have produced a similar result to that achieved with the pad.
Earlier, reference was made to the ‘primary function’ of the rails. In addition the rails themselves contribute to lift, drag and stability ( both directional and roll ). It would be sensible not to take any one aspect of design in isolation.
Steve makes reference to a 'test tank'. This contraption of 40 years ago may be of interest to some, as it was to say the least, unusual. He describes it with the following words:
Background
Tank Tests
I had, for some time, been interested in boats generally and in particular the relatively fast small boats used for water sports in general. I dabbled with some ‘home-made’ designs of mine and also spent many happy hours in, or skiing behind, one of the late Bill Shakespear’s 15ft sports-boats. A few races in my father’s Avenger 21 and I became interested in the possibility of competing more seriously in Class III Offshore. Being unable to find a hull which appealed to me I eventually became determined to build a boat that could be suitable.
G.R.P. was chosen as the construction material.
I don’t think that I can justify my initial choice of hull form by logic, but the constant ’V’ form somehow seemed as good as any. I decided on a 16ft hull having a 22.5° deadrise aft and with 3 spray rails per side. I thought this would be about right to accept an 80hp Merc so as to run in IIIB.
Study of similar hull forms indicated that often the part of the hull aft which, to put it simply, spends most time in the water, was devoid of rails. I decided to try to establish the ‘cut-off’ points for the rails by means of some sort of tank tests.
Now for the joke bit. It seemed totally impractical for me, having no funds to spare, to consider constructing a conventional test tank in which the water was stationary and the model moved, so I thought I’d try moving the water past a fixed model. First thoughts were to use an old Coventry Climax/water pump discharging through a channel but this couldn’t produce anywhere near the flow required. ( Some simple sums would have told me that had I done them beforehand. ) I then considered running water form a ’water tower’ downwards then through a horizontal channel, but couldn’t see any practical way of controlling the flow rate or indeed building an elevated reservoir with sufficient capacity. In the end I chose to make a test facility which might produce results but at the same time would almost certainly expose me to ridicule. But hopefully not by those who were fans of Emett.
The Big Wheel
Unfortunately no details were permanently recorded of dimensions etc of this project or of the actual figures obtained. The following description is from memory and I’m afraid that forty-odd years have rather blurred the picture.
A large ( 8ft ? ) diameter ply disc was cut out and a rim and return attached to form an open trough right round the perimeter. The idea being that with the wheel mounted vertically and spun, the trough could be filled with water. I think this trough was about 8ins wide and 4-6ins deep.
An old Ford was acquired and the back end stripped off leaving the front reasonably intact forward of the dash. The transmission and rear axle were left sitting in the chassis but I vaguely recall that an additional gear-box might have been interposed to give a better control of the speed of the wheel. This heap of junk was mounted several feet off the ground and the ‘wheel’ bolted on where one of the rear road wheels would have been. At the other end of the axle the brakes were adjusted to lock that drum.
It was quite exciting ( for a ‘child at heart’ ) to start up for the first time and run the wheel at sufficient speed so that the trough could be filled with water.I changed the speed to observe the result. Noticeable was the fact that the effect of gravity was greater than I had anticipated. The water levels in the trough varied noticeably from top to bottom.
An appraisal of the centrifugal force produced ( if you believe this exists! ) by the wheel at various rpm would have given me some idea of the forces involved compared to gravity. Or indeed I could have applied some common sense.
At the outset I had realised that any results that I could obtain were only going to be a ‘first approximation’ so to speak. They would not be definitive and thus useful only for comparison purposes. I therefore decided that it was not sensible to try and take this particular gravity effect into account.
Test Model
Based on my initial parameters I drew out the hull form for the actual boat and decided on a scale that would produce a sensibly sized model. I guesstimated what speed might be achieved with the ‘actual’ and then calculated the rpm required to represent this speed for the model. ( Please give at least twelve month’s notice if you wish me to repeat this sum ) With the wheel running at these rpms I marked the max water level in the trough and then made a plot of the profile of the water in the area where the model was going to sit. I re-drew the hull form at the dimensions for the model but this time I curved all of the fore/aft lines to echo the water profile. I sensed that this procedure might also be unacceptable to the establishment. I then made my model which clearly presented an alternative meaning to the term ’banana boat’. Some critical running surfaces were made from ’Perspex’. ( The stuff seems to shrink when you heat it up to bend it ). The bottom of the hull was marked with calibrations so that one could hopefully see what was happening underneath. Some balsa-wood spray rails were made up and glued to the bottom to give some sort of starting point.
A mechanism was made out of ‘Meccano’ that would suspend the model in the trough whilst allowing freedom of pitch. The mechanism incorporated a crude balance so that the drag of the model could be determined - again not necessarily the actual numbers, but a realistic figure which could be used for comparison purposes.
Estimating the all-up weight of the full size boat allowed me to calculate a reasonable value for that of the model and thus the ballast that would be required.
Testing.
Quite a number of ‘runs’ were carried out to try and determine the optimum cut off point for the rails aft. It was possible, ( or did I imagine it ) to see through the Perspex where the spray root formed.
This was repeated at various speeds and with the c.g. of the model pretty well at its aft-most position but not so far aft that it started to become unstable in pitch. New rails were fitted as seemed necessary to minimise the drag at the maximum speed.
Some other subtle changes were made to the model to try to reduce drag. One which was incorporated in the 16 and several subsequent designs was to ‘round-off’ the keel as it approached the transom. This had the added bonus of spreading the load between the trailer rollers and the boat. It did, however, make the plugwork slightly more difficult and I guess present a problem when manufacturing a hull from ply.
Quite interesting, I'm sure you'll agree.
Linear Mode
