KeilKraft Mermaid

[ashley needhamParticipant @ashleyneedham69188]

I believe the KK rudders were made from brass wire and thin sheet soldered together, not very heavy. A decent enough rudder could be made using 3mm brass rod etc e5c.

Shaft tube inner should be as per photo. Outer end needs to be where the prop is supposed to be if that’s not an obvious thing to say. A bit of give and take is reqd re motor location and shaft length is needed!

Whereas there may be an optimum distance viz prop and hull, with something like this it really won’t make much difference practically speaking…not toooo close of course, wouldn’t like to give an actual measurement…but it is also a function of how steep or shallow you want the shaft angle to be.

The shafts on these radio active setups are not rustproof in my experience and my advice would be to grease the shaft before insertion, and clean/regressed every now and then. Wd40 too thin as a lubricant. The shaft bearings are not too tight and let in water also in my experience but I would not pack the tube with grease. Best water lubricated as nylon bearings.

Shaft angle and shaft lubrication both contentious issues on this forum as far as agreeing on what is best. For my money considering the run time of these things….

I can’t see it is possible to make a decent to-scale edf Cobb or Bluebird without massively enlarging both the the inlets and outlets, so a looks-something-like appearance I reckon is all you could manage, if a decent scale model is required then a prop driven jobbie would be needed, or a jet unit.

Ashley

[Chris EParticipant @chrise]

These shafts did not come with a propeller lock nut. I would strongly advise that if yours doesn't have a lock nut then you should fit one.

If necessary, because the shaft is too long, you can knock out one of the nylon shaft bearings and then shorten the tube, shorten the shaft & replace the bearing.

That motor looks enormous in the model. Is it a 1200kv CF2822?

[ashley needhamParticipant @ashleyneedham69188]

The issue there is that although the shafts are billed as 2mm, I think I measured mine at 1.8mm….because….I bought some 2mm stainless bike spokesthinking I would replace the original shafts but they would not fit..So you may be lucky to find a locknut to fit. Perhaps a spot of superglue.

As Chris says they are very easy to shorten.

For the money these shafts are great value when you need the lightness, but they do take maintenance in the long term. All our landing craft were so equipped but 4 years down the line I have changed them to slimline 4mm. BUT weight is not an issue there !

Ashley

[Ray Wood 3Participant @raywood3]

Hi Carl,

I'm afraid the 2mm shaft and scale type 3 blade prop are not up to the power your trying to put through it !!

4mm shaft X30 prop decent coupling and cut those rudders down 50% or it will only act as a brake or flip the boat over.

You will definitely need sponson's poor little boat

Regards Ray

[Chris EParticipant @chrise]

Ray

I am not looking to confuse the issue further but why have you recommended a X30mm prop when you similar sized Ferret MTB looked to run so well on 25mm which had a much lower pitch?

A CF22 isn't all that loaded with a 30mm prop so should achieve say 75%+ of its 1200kv x 7.4v which is around 7000rpm. I don't think that your 280 with a 30X prop (or even on a 25mm lower pitch 4Bl brass prop) would got anywhere near that.

Overall I suspect that this model is overpowered and probably heavy. Not a good combination for a hull that was designed for a small motor and a bell battery.

I agree about reducing the rudder area but I would add that it should be by reducing the height rather than the width.

Chris

Edited By Chris E on 02/02/2021 11:36:28

[redpmgParticipant @redpmg]

Hi Carl – suggest you have another look at the Eezebilt site – there are some leads to double sized versions – and if you check Dodgys own 50+ versions you will find that he also uses brushless motors – plus descriptions of props & rudders etc – that should give you enough information to finish the boat.

Rudders are easy enough to make your own – I use a bit of 1.2 mm brass plate with a 3mm brazing rod encased in a close fitting brass tube(well greased before use) – with a filed out brass nut soldered on the bottom- use a washer between the rudder and the soldered nut – same at the top. The blade is fitted by sawing a hole down the centre of the rod and then soldering the blade into that – as a belt & braces mod I use two small brass pins through the shaft/blade as well before soldering .The rod should be +- 1/3rd of the way along the blade to create a balanced rudder. – for the tiller arm you can use the steering arm meant for RC planes (1/8" is close enough to 3mm as they seem to come in inch sizes) Make sure the rudder is well in the stream of the prop and not too far away from it and use minimum movement on the rudder until you are used to the boat. You may find you have to reduce the size of the blade but follow the pattern of the one you bought.- that is a good shape . By the way I always use a low melt "silver" solder from the US called stay-brite with a pencil torch – still available and makes a far better/longer lasting rudder unit than the plastic type you bought

Gearge Turner used to use the Graupner 2mm Shaft units with 7.2v Speed 400 motors and 28mm 2 blade props for his fast boats – worked very well (there is a difference between the 6and 7.2v versions) . Not sure you need a 4mm shaft at all if its not too long – very long one could cause whip at high speed. .

[Chris EParticipant @chrise]

Posted by redpmg on 02/02/2021 12:47:11

……………………………George Turner used to use the Graupner 2mm Shaft units with 7.2v Speed 400 motors and 28mm 2 blade props for his fast boats – worked very well (there is a difference between the 6and 7.2v versions) . Not sure you need a 4mm shaft at all if its not too long – very long one could cause whip at high speed. .

A 7.2v Graupner Speed 400 had a kv of around 2300. Given the greater willingness of brushed motors to slow down as the load increases this is probably not a bad comparison with a 1200kv CF1200 although the Speed 400 might well end up turning the prop a little faster.

Was George Turner putting this combination in a 14in vintage model?

[Ray Wood 3Participant @raywood3]

Hi All,

The only reason behind using a 4mm SS shaft is they generally have bronze or brass bushes, Carl's looks like plastic bearings ? and the little prop it comes with is ment for something more sedate.

Chris the only reason I used a brass prop on Ferret was it was in the spares box (£12.00), the X props are as cheap as chips in comparison

Regards Ray

[carl brothertonParticipant @carlbrotherton75833]

Well, there is a lot of food for thought in the various posts. Particularly when this recipient knows so little about boats.

I Have been looking at the Feilun, initially principally for spares. Doing a rough (W=Av) calc on running time, Lipo size and volts I have come to the conclusion that it pulls circa 200w. Even if optimistic I expect that my circa 80w in, is pretty low.

I have re-read the Eze-Bilt site, and am not sure what is thought to be relevant that I should look at?

I can see that some fitted a 400 sized brushed motor. My experience suggests that 480 (which I have and had a few) are good for only 80w and then not for long, as brush/commutator limitations soon contribute to a short useful life.

Any way, onto what I have got at present.

With all the major bits, stuck in at present it weighs in at 202g. There will be an extra 9g servo -Rx a bit of paint slapped on say circa 250g finished. Is that heavy?

I have now started on my own rudder.

It weighs a bit more than I would like for the size. I know it is not balanced, but it is made from copper (a old 1d). I am thinking of narrowing the chord (width) by say a third. Does that seem a good thing to do. I also intend to continue filling it down in thickness, to loose a few grams more. It is tedious, as it took some time to get rid of all of the raised bits.

I have bought a Feilun UJ, which is a good piece of kit (IMO) although not immediately suitable for the kit I have. Dam cheap as well at £2.50 (delivered). Unfortunately, being a dork I do not know what some bits are for. I get the two shaft couplings, plus the bomb bell, but what are all the other bits for?

I can imagine that I am already upsetting the KK purists, as I can see that I am drifting away from the purity of the original model. There is a major plus side, I am learning a lot, I am starting to think again, plus a degree of experimenting is what makes hobbies fascinating.

Something one of you guys had me waking up thing about "Dimensionless Relationships". I thought of Froude Numbers and how it relates to boats. Thinking why is the Mermaid long and narrow, when I am intuitively thinking mush wider at the stern would be better. Then the penny dropped, ger clunk, it would go a little faster in a straight line, than my wider boat, which would perhaps be more stable turning.

Now what is relevant, that you could and hopefully will help me with. I have been considering a little skeg/keel/bagger board, forward of the propeller, with a view in keeping it running straighter. Rather than just relying on the rudder and prop thrust interaction.

All of which reminds me of trying to establish where the longitudinal CG is. Hopefully again your knowledge and experience will educate me to how it works, or if it is relevant.

I know I have slowed up, partly because I am uncertain what to do for the best.

[ashley needhamParticipant @ashleyneedham69188]

Carl. 30W would be more appropriate for this model, and as several people have said, the 2mm shaft you have would not develop any more even with a million horsepower behind it!

Not a good idea to have a dagger board in front of the prop as this would likely just fight with the rudder. I would mount your new rudder blade lengthways and not upright as you have it in the picture.

rails running fore-aft underneath would help straight line stability at speed. However, with such a light boat, going fast will be fraught with danger….

Ashley

[carl brothertonParticipant @carlbrotherton75833]

I am not sure how fast, is fast? From what is claimed, the Fielun manages 20 mph full throttle on 200w, it seems a inch or two longer, much wider (from memory). On 80w, I would be considering something in the ball park of less than 10 mph. I now it is a guess, would that be fast,?

Not having built a model boat I have no idea as to what is fast.

I am assuming that the propeller is the issue? What is thought to happen, cavitation, limiting performance?

I was thinking that a narrower chord rudder will reduce the torque required from the Servo.

Some one has suggested the model will be heavy, is 250g heavy or light for the size the model is?

I am now thinking about varnishing/sealing the housing for the prop shaft, then just sealing the top and bottom of the outer prop tube, to make it easier to replace it with a 4mm shaft, if necessary, is the concept viable?

What do people think will happen with the current prop shaft, is it wear of the plastic bearings? The shaft hitting a critical speed (whirling), a torsion issue.

It seems a number of lessons could be learnt with the modele

[ashley needhamParticipant @ashleyneedham69188]

Fast is relative to the boat in question. kK boats will be nippy but not fast compared to a nitro offshore powerboat model. Unless you get an onboard gps tracker it is difficult to say exactly how fast something is.

Theprop on these cheap shafts has very little pitch (twist) on it and this limits the amount of thrust it can generate.

I use micro servos on rudders twice the size you are contemplating. Why would you want to reduce the servo torque?

250g is light for this size of boat.

Glue the propshaft in with epoxy. Line the shaft up, secure in place with a few drops of superglu then finish with epoxy. Everybody has their own method.

The bearings are likely to out last you for the use they get, With my previously recommended regime of regular disassembly and grease smearing. The shaft should be stiff enough to counter any whip issues.

Ashley

[carl brothertonParticipant @carlbrotherton75833]

Ashley

I have been taken back some 50 years. I was new to a design office, not long qualified.

The Chief Engineer was doing a round of one of the projects in the department. He had stopped at one of the boards, talking to both the section leader and the Draftsman. I was thankful that it was not our project, as these encounters were often traumatic to both the section leader and specifically the draftsman.

Suddenly there was a vocal explosion. What do you call that! It is a shaft came the reply, The Chief Engineer had not even allowed the final word of the reply to be said, "A shaft, I have seen knitting needles that are stronger". He then continued, "I feel like a ruptured duck on crutches feels". After a brief pause, "are there no engineers here".

Next thing, a waste paper bin is kicked, going flying down the office. Abruptly, he suddenly storms of down the office back to his liar. A refrain could be just faintly heard, "get it sorted".

On that note I have varnished the hole which will contain the prop shaft. I will then cut the outer and shaft to lengths. Just Epoxying at the top and bottom. If necessary I will swap out, hopefully with minimum hull damage for a bigger prop shaft assembly. I will also start work again on the rudder assembly.

Please recognise this is only the second boat I have ever built and that was 63 years ago.

 

 

 

Edited By carl brotherton on 06/02/2021 16:30:36

[ashley needhamParticipant @ashleyneedham69188]

Carl. I am simply answering all the questions you posed, nothing more. I am sorry if it came across as lecturing.

There can be a tendency to over-think these things. As long as the bits are secure, painted over and (for the motor/shaft) not tight, then things will be ok.

I would not worry about stability, power, endurance or anything else just yet. Fit the bits you have and chuck it in the water. As has been said, there are no right ways of doing some things, we all have our own methods.

After a while you get the feel for these things and just do what you know will work.

Ashley ps…..bet you cant guess where the propeller is on this one!!

[Chris EParticipant @chrise]

Ashley

That is a nice model & much more convincing than the Alligators that are proliferating everywhere.

 

 

Carl

Nobody is wanting to "lecture" to you. We all have our experience and are offering it to help you. It is entirely up to you what you do with it.

We would like to think that we are helping make your model a success.

 

Edited By Chris E on 07/02/2021 09:44:28

[carl brothertonParticipant @carlbrotherton75833]

Chris

I do not think or suspect that anybody is lecturing me etc.

There is a problem that those who know, assume quite often that others have a common understanding of what is being said/written. In my case the nuances are often missed.

There is also a problem in that most things are not absolute, that what is correct/acceptable/doable in some circumstances, yet change the condition, no longer the same.

Experience is often what builds intuition. In my case I lack experience of model boats. My intuition comes from an engineering background. Much of engineering is built on observation, then asking why, why does that work or not. An example would be hardening steel. Why has that happened, I can see that it has, yet not always, hmm, why. On that basis I ask those who have experience, I am not challenging if and when I ask why. It is about building some sort of understanding.

[Chris EParticipant @chrise]

That is one of the problems of a forum. If you are speaking to a beginner you can read from their body language and voice whether they are understanding what you say. With a forum their is no such interaction.

Having said that there are times when you think that you have explained something face to face and it appears that someone has understood it – but then they turn up with what they have done putting it into practice…………………

Then there are times when they don't need something explaining but you don't know that and finally..

Very very occasionally you get it just right.

 

Edited By Chris E on 07/02/2021 13:03:23

[carl brothertonParticipant @carlbrotherton75833]

I think there is one other aspect to misunderstandings, is that there is more frequently than we would like difficulty in providing mood into Emails.

Anyway here is a picture of a model I built some 63 years ago, as 10 year old. it probably took another 5 years to completion. Then 63 years to slowly disintegrate.

If you wonder why it took so long, it was filing and sanding the bows in particular to any shape at all. All I had was a Gillette razor blade, a tiny piece of sand paper, an a half round file (probably second cut and worn out).

Both of my boats were terribly underpowered on there original motors an block Ever Ready batteries. In that the wind had more effect than either the power or rudder. My experiences (with models) there is no substitute for power.

Edited By carl brotherton on 07/02/2021 13:32:56

[Chris EParticipant @chrise]

That is a really nice neat small coaster. For a 10 year old with a razor blade she is quite amazing and we are seeing it around 60 years later. Judging by its location I am guessing that she is quite small.

Are 10 year olds allowed, by most parents, to get near razor blades now?

I am afraid that electric and underpowered were comfortable bed fellows until quite recently.

[carl brothertonParticipant @carlbrotherton75833]

Chris

For a 10 year old it would have been quite a triumph.

The reality it was an illustration of ambition over ability. The ten year old struggled to just stick the hull together. The adhesive was something called Aerolite (from memory), a powder mixed with water, put on one face, a acid of some sort painted on the other. this user ended up with a rash on their hand from possibly the acid. Unfortunately the double edged razor blade was not up to the job, neither was my abilities.

The model ended up in Limbo, nothing happening with it. The removal of the vast amounts of balsa from the bows, the principal obstacle, due To my limited abilities and knowledge. It languished for about 4 years.

It was when I was approaching 15 years old. I somehow found the determination. Plus a book on how to build Model Boats.

It was mostly a 14-15 year oi-ld that built it. It is a Caribbean Coaster by Veron,@ about 30" length,

My mother was rather indulgent. I also have a Graupner Adler Von Lubeck, again my abilities being at odds with my abilities. I was then 15 when again my mother indulged me. I now feel guilty, that it cost so mush and I did not have the conscious to think, or ask, can you afford it.

[Tim CooperParticipant @timcooper90034]

I have the sister model to that Veron Coaster. Built when I was about 13. So it's about 58 years old. No radio control to start with but now has 2 channel set. Probably been repainted 3 times, and a coat of tissue and dope about 20 years ago to stop leaks.

Tim

[Tim CooperParticipant @timcooper90034]

Should have said that mine is the President trawler.

Tim

[carl brothertonParticipant @carlbrotherton75833]

Well Tim, yours looks  far better than my model.

I have toyed with the idea of refurbishing my model. Rather alarmingly I have considered in inserting another section, as Camell Lairds have now doe on a few occasions. Thankfully I have managed to restrain myself.

I can see that there are similarities in detail. The same handrails, ventilators, even the Lifeboats and davits seem remarkably similar.

One aspect that has disturbed me is that my lower cabin area, is curved. I know that Engineers and Draftsman love straight lines, simply cut plates etc. As fancy shapes cost money, particularly if there is no benefit. Again I have toyed with redesigning the superstructure at the rear. Also, removing the hatches (having lost one, some how) to be piled high with ISO Freight Containers.

I should do something as most of the Joints have fallen apart. I guess I used Balsa Cement.

Plus a decent motor and RC.

Edited By carl brotherton on 08/02/2021 16:46:39

[Tim CooperParticipant @timcooper90034]

I will get the name right eventually. Trident Trawler.

Mine has had several overhauls apart from the hull sealing. New rudder when changing to RC. The superstructure has come apart a couple of times and the hatch had to be changed to get either the battery or the RC gear in. It has lived in its own wooden box since being built.

Tim

[carl brothertonParticipant @carlbrotherton75833]

The first stage of the model is coming to an end. The next stage is seems that it could take longer than I would like. This is getting it to work and understanding what is acceptable, how to rectify the issues and for me is seeing the way forward with regard to completing Crusader , as a planning, straight line model.

I am already concerned that the coupling is just not up to the job, specifically in transmitting the drive given the pathetic sized set screws.

The screws themselves are tiny, as a consequence the screwdrivers need to tighten them are equally small.

I am thinking I may need to permanently fasten the coupling in place using Outfight. Is there a better way? Also I mam not convinced that the spring is strong enough to transmit anything above a nominal amount of power.

I am now awaiting a couple of lipos to power the model,,as can be seen there is not an awful amount of spare space

I am not enamoured with my top, which is intended to functional, rather than elegant. It seems I have sussed in avoiding any elegance.

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