Novice Modeller

[Robert MillsParticipant @robertmills13867]

 

So I'm a novice modeller, although I have spent my life messing about with full sized boats!

So wanting to set my self a challenge I've decided to design my own hull from scratch and then go from there to come up with the plans to build it

Attached below are some initial design images taken from Sketchup, for scale the hull is approximately 52cm long

Please feel free to comment with ideas and suggestions and when I get around to uploading stl / sketchup files where I shall post links if people want to have a closer look around the model

Edited By Robert Mills on 15/04/2016 10:08:42

[Robert MillsParticipant @robertmills13867]

[Dodgy Geezer 1Participant @dodgygeezer1]

The kink in the keel looks odd – is this because your drawing package doesn't do curves?

The prop and the rudder both look very large – together with the narrow hull and the semi-circular cross-section I think you might have some torque issues? And I expect that at speed the water may climb up the sides unless you add spray rails. The big prop and general layout of the boat look as if it's a planing boat, but the hull form looks like a displacement one…

But of course any comment depends crucially on what the hull is for. Is it a racing boat, a sports boat, planing or displacement? What sort of power plant will you use? What material are you expecting to work in? It would help us a lot more if we knew what you wanted the finished boat to be able to do.

Before designing a model boat (or, indeed, doing anything!) it helps to have a look at what other people have done before you. There are a lot of boat plans available on the net that you could look at to give you an idea about constructional techniques. At 20 inches the boat would be quite small – you might be working in Balsa? I run a small website with simple plans for novice builders to introduce them to constructional techniques at http://eezebilt.tk and there may well be simple kits which can be bought from your local model shop…

[Robert MillsParticipant @robertmills13867]

Hi Dodgy Geezer,

Thanks for comments, I definitely think that having a go at one or two of your plans to practise construction techniques!

The basis of the hull design is a semi-displacement one, so something a more displacement centred hull but with concessions to make it plane. Hence going for a, probably, oversized propeller. Certainly a smaller one would allow for the prop shaft to sit closer to the hull and have a smaller rudder. Spray rails are certainly something to be added as I agree water is likely to creep up the sides!

For construction I was looking at balsa frames with a ply keel and then planked with balsa finished in either epoxy or polyester resin (probably with some kind of glass fabric to strengthen the hull further.

For the power plant I was considering a brushless motor as I have several on hand from various bit and pieces.

Thanks for you comments so far and I look forward to picking up all sorts of little insights as my project progresses!

[Dodgy Geezer 1Participant @dodgygeezer1]

Your hull design is rather reminiscent of the German 'Schnellboot' of WW2. The differences are that they had a flatter underwater profile at the stern, giving hydrodynamic lift to offset the tendency of a displacement hull to squat down in the water at speed, and more of a flared bow to give lift and throw water aside in rough conditions… see **LINK**

[Paul TParticipant @pault84577]

Hello Robert

Welcome to our happy little sanatorium.

I like your design and apart from the oversized propeller and rudder I think it has the makings of a decent little boat. I can certainly see a few different influences in your work, have you sailed dinghies such as Enterprise or GP14.

The severe change of keel angle is reminiscent of some pre WW2 designs and will act as an excellent 'step' for the boat to ride on whilst at full speed and the multiple chines are good for fast turns and will make for a lively handling.

I would construct this model out of 1/4 Ply frames with a 1.5 LitePly skin as the hull will need a little extra strength to survive high speed manoeuvring.

Looking forward to seeing this boat in the water.

Good luck

Paul

[Dodgy Geezer 1Participant @dodgygeezer1]

The severe change of keel angle is reminiscent of some pre WW2 designs and will act as an excellent 'step' for the boat to ride on whilst at full speed…

Hmm… wouldn't a working step be further back? This looks more like a continuation of the bow curve – though much deeper. Steps work by dropping the pressure directly behind them and allowing the hull to plane on a water/air mix – there is no room there for an attached air bubble behind the 'step'. Steps are notoriously difficult to get right, only work well for one speed anyway, and I'd be more worried about porpoising with that keel shape – it's equivalent to having a lot of rocker.

So it might be more manoeuvrable – that's the sort of thing they design for the slaloming kayaks they call 'Banana Boats'. However, there the deep point which the kayak revolves around is directly below the G of G – we don't know where that is on this boat, and the big rudder and wide prop-strut both mitigate against rapid movement….

[Paul TParticipant @pault84577]

Hmm… wouldn't a working step be further back?

No not necessarily it all depends upon the boats centre of mass.

This looks more like a continuation of the bow curve – though much deeper

Which is exactly how the first hydroplanes were conceived, Roberts design has a good strong bow profile which will enable a rapid rise to the plane without expending to much power.

Steps work by dropping the pressure directly behind them and allowing the hull to plane on a water/air mix

Not really as a step can also be a tight change in angle without the usual upstand, true the air/water mix isn't as good but it still provides a hard point upon which the hull can ride.

Steps are notoriously difficult to get right, only work well for one speed anyway.

Steps are difficult to get right and this one look perfect Robert is to be commended especially as this is his first design.

I'd be more worried about porpoising with that keel shape – it's equivalent to having a lot of rocker

There is a significant chance of porpoising but only when the power is reduced after a high speed run, if Robert allows himself plenty of distance to slow the boat in he shouldn't have much trouble, its all part of the lively handling that will be associated with this design.

Judging from his drawing skills and obvious knowledge of small boat construction I don't think it will take him long to sort out the Centre of Mass and where all of the hull sweet spots are. Apart from the propeller and rudder size the design is excellent and I am looking forward to seeing how Robert draws the internal framing and locates the heavy equipment such as motor and battery.

Paul

[Dodgy Geezer 1Participant @dodgygeezer1]

Ah, Paul, you are confusing me again! You agree 'that's not really a step' and then you say "Steps are difficult to get right and this one look perfect".

I can't see it working as a step – I anticipate that the angle of attack will be too shallow. It's much more likely to act as a pivot-point. I'm not actually sure that a tight change in angle is intended there anyway – it may be an artifact of the drawing package he's using.

But more importantly, as I said earlier, we need to know what the aim of the boat is, and have some idea about the mass distribution before we can say whether this boat is well suited to its task or not, let alone whether it's 'perfect'. Assuming that the motor is well forward (look at that prop-shaft length!) he has lots of room to move the batteries, so pitch issues can be addressed to some extent. But I would still anticipate quite a lot of heel in a turn with that semi-circular cross section. The rudder could usefully be halved in size, and I expect some resonance issues with 8" or so of unsupported prop-shaft.

 

Edited By Dodgy Geezer on 15/04/2016 14:14:27

[Paul TParticipant @pault84577]

Ah, DG, No I didn't say it wasn't a step I actually said that "a step can also be a tight change in angle without the usual upstand" I would have thought that you, more than most on this forum, would appreciate the problems associated with miss-quoting.

Now my comment "Steps are difficult to get right and this one look perfect" isn't so difficult to understand.

As for everything else all I can say is that this is Roberts first attempt at designing and there are bound to be teething problems. I think it is to Roberts credit that he has brought his design here and asked for our opinion, I don't believe that 'slings and arrows' of critique is the best way forward in encouraging this unique design. Yes some details might seem incorrect but these are easily refined as the project progresses.

Paul

[Robert MillsParticipant @robertmills13867]

Paul, you are indeed correct in guessing I'm a dinghy sailor, and have raced both Enterprises and GP14s!

I have to admit the apparently highly contentious 'keel issue' was a happy accident as I was drawing the keel line and liked it so it stayed, certainly the design will be sensitive to weight distribution but most fast hulls are. I agree it is somewhat unusual but then so is the design below which has won the microtransat series several times.

With a smaller prop and a steeper shaft angle it would be possible to mount the motor slightly further back to reduce prop shaft length (the two thick black lines by the bow mark the bulkhead where the motor is mounted) and the the batteries could go forward of the motor right into the bows.

[Colin BishopModerator @colinbishop34627]

I think Robert has enough advice to be getting along with at the moment without the need to overdo things by delving into the more abtruse elements of design just yet.

His design shows a number of characteristics which will obviously influence performance depending on what he is hoping to achieve. Model powerboat design is something of a science with a solid history of what works and what doesn't.

Robert, you have obviously given a lot of thought to designing your hull and it would be helpful to us if you could explain the various features and why you have incorporated them. I assume they have been influenced by your experience with full size boats. They may or may not work well in model form depending on how the model will be used.

As far as constructional techniques are concerned, I suspect you would find the EZE Build models a bit too simple and unsophisticated. If you are intending to build in wood construction then one of the classic designs by Vic Smeed where the plan incorporates the constructional instructions would be a good place to start as just about all Vic's designs worked well and he produced a wide range of them, many of which were very straightforward to make. I should think that there is something not too dissimilar to your design which you could look at.. Also Dave Wooley's Navaho design resembles yours in side view although I think the hull is considerably flatter: http://www.myhobbystore.co.uk/product/16807/navaho-mm1274

Colin

[Paul TParticipant @pault84577]

Hi Robert

I used to sail dinghies back in the 1970s and enjoyed the club Enterprise in heavy weather but my own boat as a mahogany Fireball which didn't like to much chop but was like a greyhound on flat water.

You could mount the motor at the lowest point in the hull and have a shallower shaft angle with a smaller prop nearer to the transom and hold the shaft in place with some A frame brackets. The batteries could then be positioned either side of the motor which will concentrate the mass on the area directly above your 'step'

Paul

[Robert MillsParticipant @robertmills13867]

Hi Colin,

The two biggest decisions were at the bow where the very vertical upper section was intended to give better performance at low speeds when the boat is in displacement mode, the low section was designed to be much flatter to enable the boat to get up onto the plane when desired. It is best perhaps though of as having two water lines, one which runs from approximately half way up the lower vertical horizontally to the transom. The second runs from the curved section in the bows back to the transom when the boat is on the plane. Hopefully at this point the boat would be on the on the lower 4 chines only (two either side of the keel).

[Colin BishopModerator @colinbishop34627]

Most of the development work on the characteristics of model boat planing hulls was done 40 or so years ago and all the possibilities explored pretty thoroughly. Apart from the simple quest for speed, a lot of the work involved handling characteristics so if you are not following an existing design then you will probably just have to build the model and see what happens.

In which case it would be a good idea to make allowance for modifications such as changing motor, prop and rudder; making provision for fitting spray rails in different positions, fitting adjustable trim tabs etc. Dave Wooley, who used to be very active in this area is an occasional visitor to this forum and might have some ideas for you.

Colin

[Dodgy Geezer 1Participant @dodgygeezer1]

Posted by Paul T on 15/04/2016 14:54:51:

Ah, DG, No I didn't say it wasn't a step I actually said that "a step can also be a tight change in angle without the usual upstand" I would have thought that you, more than most on this forum, would appreciate the problems associated with miss-quoting.

Now my comment "Steps are difficult to get right and this one look perfect" isn't so difficult to understand.

As for everything else all I can say is that this is Roberts first attempt at designing and there are bound to be teething problems. I think it is to Roberts credit that he has brought his design here and asked for our opinion, I don't believe that 'slings and arrows' of critique is the best way forward in encouraging this unique design. Yes some details might seem incorrect but these are easily refined as the project progresses.

Paul

 

I see I have misread 'not really as a step' to mean that the feature would not really work as a step, which you did not mean. My apologies – I must blame pressure of time with many things to do by week end… but now I can respond…

My concern about your use of the word 'perfect' was driven by the belief that we didn't have enough information at this stage to know what the hull was intended to do.

However, I still consider that, if the boat were made as specified, the hull would rise under power, with the hull rising rapidly due to the upswept bow profile, which you noted, and then the stern would squat, since it is quite rounded and has little hydrodynamic lift, tilting the boat quite strongly upwards, so that it is running with the waterline parallel to the rear part of the keel. Under such circumstances the discontinuity would not act as a step at all.

It might be persuaded to act as a step if the boat ran with the deck parallel to the waterline. For this to happen the stern would be skimming the surface of the water and support for the rear of the boat would be provided by the upthrust from the propeller. I anticipate that to be an unstable condition – hence my concern about porpoising. And the CG would be critical – I'm not sure that batteries forward of the motor would end up with a balanced boat (unless there was a lot of lead in the back!), Incidentally, where is the CG expected to be? That will help us a lot in determining how the step might work.

I estimate the prop at 1.5" (38-40mm)? This is a huge prop for a 20" boat – and dropping to about 2" below the hull. Props around 1"-1.25" are more common, and produce less torque, which I have indicated earlier might be a problem. If the hull is intended to work with a step I would love to see the expected waterline position at speed – the expected attitude of the boat in the water will give us a much better idea of the expected dynamic forces.

I am not sure I need to 'encourage' this design – RM has obviously well committed, has put thought and work into it, and we all hope it will proceed to a satisfactory conclusion. However, I can recall Blish pointing out in his critiques of young science fiction writers that it was a false charity to hide your concerns out of sympathy for a beginner – people will never overcome issues unless they can see where these are, and I presume that RM has put his design up here to make use of our opinions?

I reiterate that it would be useful for RM to indicate what his design aims were – it is hard to provide advice if there is no specific goal in mind. Is the intended construction going to be multi-chine with formers and sheet skinning? If so, I might pick 1/8" hard balsa or lite-ply for the formers – 1/8" or 3/16" for the keel and 1 – 1.5mm for the skin. Strength is important, but so is 'adding lightness' and I think strength is better acquired by clever design.

Edited By Dodgy Geezer on 15/04/2016 19:09:36

[Robert MillsParticipant @robertmills13867]

Thanks for all your input so far, I’m by no means committed to the design at this stage, this design was the product of a couple of evenings working in Sketchup. So I’m open to lots of suggestions as to how to improve it. This design was mostly what looked reasonably fast and pleasing to the eye, I know what tends to work on full size boats but at this scale there are differences.

Edited By Robert Mills on 15/04/2016 18:47:36

[David WooleyParticipant @davidwooley82563]

I have been reading all what has been said and perhaps I can offer some thoughts on the subject of performance hulls in model form . Colin mentioned that I was involved in high speed work some years ago and at that time experimentation was par for the course excuse the pun . I had a book published way back in 1980 under the title Radio Control Fast Electric Power Boats and was involved in three quite separate spheres of experimentation . I'll explain. The first was to build very high speed steering boats for the international competition F3 the purpose of this type of performance hull was to get a model to stick lick glue to the water and turn on a sixpence all flat out . This required a short but proportionally wide ratio hull with moderate V forward to a shallow V aft then increase the spray rail to form a moderate tunnel effect so the boat had a controllable slide but in its own length and accelerate out of the turn with out turning over . The second was to design a very high speed design for what was called F1 that also required some of the attributes of the F3 boats but as these machines where being designed to travel from one point on a triangular course to another then turn 360 degrees on the bottom of the course to repeat the course in the opposite direction with straight line speeds touching 50 MPH . This was seat on the pants stuff and to increase the lift and reduce the wetted area beneath the hull rails were added and the angle of each rail was critical to stability . Yet the standard theory of the C of G should be as low as possible and slightly ahead of the centre of lift when planning .stability becomes critical when the moment between C of G and the centre line in excessive . . . The third type of design was for multi boat racing FSR here the boats required a hull that gave both speed and the best possible stability .Attributes from the first type could to some degree be joined with those of the design built for speed as the requirement was to be able to sustain a relatively long period on the water with other boats .here the V form increased at the bow and at the transom with more pronounced spray rails this design survived better than the design which provided less wetted area. I have only scratched the surface of high performance hulls for models, there are of course other factors, power out put, shape and size of propeller is critical as is the shaft angle and its position .also the use of wedges beneath the hull at the transom or controllable trim tabs even dagger plates .These can be all factored in but essentially it is the hull design and what you wish to achieve with that design . Lastly I would like to mention what was the old records days in which boats traveled from point to point over a measured distance . here the design was more fluid as the direction was linear , no abrupt turns . As such American designs that were shaped like delta wings reduced V forward and a flat transom these could reach speeds of 100 mph in a straight line in perfect conditions .these used a step design in the hull with the shaft and prop as a out rigger aft of the transom . Lastly water conditions are also a major factor , for speed only flat calm would suffice . I spend many years racing in all three and didn't do too badly but that's another episode. To sum up the chosen design reflected its requirements all the other factors were down to experimentation trial and a few errors but in 1960s -70s we just happened to be at the cutting edge of model power boat design .

[Tony HadleyParticipant @tonyhadley]

Robert,

If the magazine item (with construction notes for a ply hull) and plans for this old design would be of value with your project, please send a message.

Tony

[Colin BishopModerator @colinbishop34627]

Thanks for the information Dave, it doesn't get much more authoritative than that. There was a huge amount of development effort in those days, for those who don't know it, Dave had a monthly column in the magazine for a number of years on fast electrics and won many awards although scale warships are more his line these days.

Colin

[Malcolm FraryParticipant @malcolmfrary95515]

A bigger prop does not make a boat go faster – a faster spinning smaller one does. The proportion of size of the rudder and prop would be great on a tug. On a narrow hull, with a big prop, one of Newtons Laws comes into play – the prop will do its best to stay put while spinning the hull.

[Colin BishopModerator @colinbishop34627]

Yes, getting the most efficient combination of prop diameter, pitch, number of blades and speed to suit a particular hull has always been something of a black art requiring trial and error. So best to allow for the maximum likely diameter of prop consistent with keeping the driveline as horizontal as possible and take things from there.

Rudder application immediately introduces a degree of drag so the rudder should be a small as possible and moved as little as possible to achieve the desired degree of manoeuvrability. A rudder on a powerboat situated in the propwash needs less blade area than one for a sailing boat for an equivalent turning circle.

Colin

[Dodgy Geezer 1Participant @dodgygeezer1]

David Wooley's comments remind me that the heyday of model boat hull experimentation was in the period we would think of as 'Vintage' today, and a good example of what was produced can be found on a recent posting in the Vintage forum here: **LINK** The wide ratio hull, deep Vee flattening towards the stern and extensive railing that DW talks about are all quite evident in the hull pictures.

As an example of what this kind of hull could do back then, this video provides a good illustration: **LINK**

Colin made two points which I agree strongly with – that handling is an important feature (you can see the wide, flat stern and dart-shaped planform providing stability in the turns in the video above), and that EeZeBilt constructional techniques may not be optimal for this class of vessel!

[Colin BishopModerator @colinbishop34627]

The advances in Dave Wooley's hull development are just as valid today as they were back in the day. All that has really changed in the interim is more efficient motors and batteries. In a sense the hull design technology was essentially 'mature' back in the late 70s and has not appreciably improved since then.

Colin

[Robert MillsParticipant @robertmills13867]

Thanks for all your input, I think it is time for me to back to the drawing board and redesign the the stern section of the hull to be flatter along with a smaller rudder and prop.

Thank you David for your pointers, I've managed to find a copy of your book for sale in the UK for a reasonable price so I think I'll get that for a bit of light reading!

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