KeilKraft Mermaid

[carl brothertonParticipant @carlbrotherton75833]

Eze-Build

[carl brothertonParticipant @carlbrotherton75833]

I know nothing about model boats.

Although pre teens I built a KK Mermaid. which went remarkably well, other prior to finishing, that is unsealed/painted, I ran it in the bath. At that point the sheeted hull warped.

During lock down for a number of reasons I decided to build another, and yes a lot of water has flowed under the bridge in-between. That is about 62 years of it. In that time a lot has changed.

I now have a hull, the top slightly modified, just to make it less fussy and the hull detail has been amended to avoid the the warping (hopefully).

I now need to fit it out. I do not know if a 2mm or 4 mm shaft assembly is best suited to such a small model. Plus I intend at the moment to fit a 60w input motor on a 2s Lipo.(probably about 300-600ma) the Kv is approx 1200. In short I do not know what a good combination will be to provide a decent turn of speed, with a reasonable motor run.

I have sealed the model with Water Based Varnish, about 4 coats.

Advice is needed intially on the prop and motor arrangements.

[ashley needhamParticipant @ashleyneedham69188]

Carl Have a look at the excellent “eezebilt.tk” site. A veritable mine of information.

The suggestion seems to be a 2mm shaft. Do we assume your boat is 14” long?A brushed speed 280 motor and 25mm prop would likely be more than enough for your boat unless you already have a motor as per pictures?

Ashley

[Chris EParticipant @chrise]

You might care to have a look at this video.The model is powered bu a 180 motor!

Weight is a critical factor with a model like this.

KK Mermaid

[Ray Wood 3Participant @raywood3]

Hi Carl,

The interesting part of the youtube video is the boat is fitted with spray rails along the chine line which helps lift the bow out of the water, these little boats were never envisaged to actually get up and plane with tiny motors and Ever ready torch batteries. Ashley is spot on with the motor size I always use 4mm shafts even on the little boats.

Regards Ray

[ashley needhamParticipant @ashleyneedham69188]

Cor! Very small “in the flesh” as it were.

yes a 180 would seem to be adequate. J.Perkins do some aircraft style brushed esc that weigh 5g including wires!

..GWS -ics100

Ashley

Edited By ashley needham on 22/01/2021 09:32:42

[Charles OatesParticipant @charlesoates31738]

Someone educate me please. I thought I had a reasonable grasp of the common brushed motors, but the model in the video has me wondering. I haven't seen a 180 motor for a long time, but I don't remember them as anything special, so I thought he meant a 280. The problem is that he's using 7.4 volts, and the only 280s I've used were either 6 volts max or 24 volts max. I presume there are variants I've never seen, or have I got something wrong?

Charles

[ashley needhamParticipant @ashleyneedham69188]

 Not looked up a 180, but all my 280s run on 7.2v and get a good thrashing. Not had one burn or worn out yet!

No doubt running a 280 for days on end at 7.2v will give it a slightly shorter life than using 6v but considering the briefness of model boating?

Ah the wonders of opening another iPad window whilst replying to a post… a well known component shop does a round 140 motor takes up to 9v andseveralothers.

Ashley

Edited By ashley needham on 22/01/2021 11:15:39

[Chris EParticipant @chrise]

This is what I don't like about brushed motors. The specs are generally so imprecise. I note that Component Shop list 3 different spec 280's & I am sure that there are many more out there.

I agree that the motor in the video looks like a 280 but what spec? I note that Ray's Ferret is about the same size & also has a 280. Perhaps Ray will be kind enough to tell us which spec?

[Ray Wood 3Participant @raywood3]

Hi Chris,

I get the feeling we have had this debate before on here many times ? the only specs I know about or the ones I wear the Ferret MTB had a Robbie Slow Fly 280 it was one of 4nr I bought for an aeroplane I never built 20 years ago. I ran the little boat on a 2 cell lipo giving 7.4volts.

I was frightened by the photo of the brushless motor and surface piercing/ flexi drive set up in in Carl's opening post !!

Regards Ray

[Chris EParticipant @chrise]

Posted by Ray Wood 2 on 22/01/2021 14:58:41:

Hi Chris,

I get the feeling we have had this debate before on here many times ? the only specs I know about or the ones I wear the Ferret MTB had a Robbie Slow Fly 280 it was one of 4nr I bought for an aeroplane I never built 20 years ago. I ran the little boat on a 2 cell lipo giving 7.4volts.

I was frightened by the photo of the brushless motor and surface piercing/ flexi drive set up in in Carl's opening post !!

Regards Ray

Yes it is one of by frustrations in life!

The brushless motor frightened me as well. It looked much much larger/ than any I would want to put in a model aircraft as a 280 replacement.

Keep Well

Chris

 

 

Edited By Chris E on 22/01/2021 16:04:36

[carl brothertonParticipant @carlbrotherton75833]

Ray

No need to be overly frightened, those bits were bought with a view of powering a model of John Cobbs "Crusader"

A model still waiting completion after years of sitting about. The bits I bought are not suitable, as now I needed and need help in sorting out the running hardware. It took a while for me to realise that the rear sponsons were there to support the heavy back end due to the jet engine, therefore the CG should be well aft.

In the case of the Mermaid, I realised early on the CG needs to be towards the stern. I was also confident that the model would float.

To all, I have now read the KK eezebilt site material.

I am encouraged that without prompting I have but in a lot of Stringers? to support the sheeting. I have also tissued the hull, to improve the surface finish, and to increase the strength. I have also varnished all the interior (before closing the various chambers) and exterior with several coats of varnish.

Where I am totally without knowledge is the hardware. It seems that either a 2 or 4mm shaft will do. I am perhaps mostly concerned (now) about frictional losses than strength. I have a 4mm prop shaft (to long for this application) which in principal could be better in that the shaft is bushed each end, the middle therefore not in contact with the outer tube. I suspect that it is much heavier than a 2mm shaft.

The video has also been useful, in that I was intending to put in a higher wattage motor than one of the ones I have to hand. Th video convinced me that the model probably could not hydrodynamically cope with so much thrust.

Now I will look at the motors and ESCs I have to hand.

I am also considering the rudder assembly, in that I intend to use a outrigger assembly Not pretty, although functional, not requiring the hull to be pierced. It is also ugly in that the control arm will be visible.

I am considering offsetting the rudder, out of the direct prop wash, so that I can remove the prop shaft for drying and lubrication, without removing the rudder. Would odd setting the rudder cause the model to become sensitive to which direction it is turning?

All the time I am worried about weight, the hull is light, I would like to keep the finished model light.

The spray rails are interesting, are they helpful? is it as the model accelerates, turns? Perhaps rather bizarrely, I had considered increasing the beam by attaching a sort of side pod either side, to hopefully increase the stability at speed. I then thought, it would no longer be anything like a KKT Mermaid, more a hybrid, that you might as well started with, but for me experimentation is what I like doing. I this case I I am less clear what I am trying to achieve.

[Dave Cooper 6Participant @davecooper6]

Hi Carl re: the Mermaid. I had been tempted by this one as a young lad. What put me off was the small size. I didn't have the skills to scale up but, if I had, would probably have gone to twice-size.

The one thing I would say about small boats is the way they 'shrink' out on the water (visually not physically). I think I could cope with a small tug, but not something approaching Mach 1.

My RAF launch (16&quot is OK on small water but I wouldn't go to a big lake with it. It has a 2mm shaft on a 380 brushed motor – that's probably pushing it, but, it will plane on about 3/4 throttle with assistance from a small step under the hull.

Does 2" make that much difference (no rude comments please) ? Try it and see…

Have fun,

Dave C

[ashley needhamParticipant @ashleyneedham69188]

Spray rails are usually a must, the stop water creeping up the bow.

Like the Crusader, an unusual beast and something that not many people would tackle. Does yours float bow or stern heavy? Please let us know how you get on, or feel free to ask questions,

Ashley

[Dave Cooper 6Participant @davecooper6]

Ashley

As a matter of interest, how big should the spray rails be ? Say, as a percentage of beam, or, perhaps just a nominal width /depth…

I was thinking of maybe 1/16" square for my little RAF launch.

Regards,

Dave

[ashley needhamParticipant @ashleyneedham69188]

Dave. If In doubt, overdo the size. If they are too small then they would be ineffective….I would go with as large as looks ok, Obviously with a pukka to-scale model you would have some sort of guide to start with I suppose, but 3mm square strikes me as a good starting point? It also depends on where the rails are..real life fastish environmental agency type boats I saw in London had no more than a small sort of shelf at the front just above the static water line to start the water rolling off the bow.

several fast boats here at the pond transformed by the use of spray rails, and some of our specials have had them added afterwards to help with water management, the Walrus being among them.

Ashley

[Dave Cooper 6Participant @davecooper6]

Thanks Ashley – I'll try 3mm as a 'starter for ten'. Can also add more later if needed…

Sorry to have hi-jacked the thread Carl.

Dave

[carl brothertonParticipant @carlbrotherton75833]

Dave, you asked my next question.

I have spent the last week waiting for a very small order from Hobby to arrive. Unlike many companies these days they do not inform you that an item has been dispatched. Waiting after 3-4 days started a small amount of anxiety. Now I know what to expect, also a plus their prices are very good, with a good description, which is helpful when you are a newbie.

I have started the process of installing the prop shaft and motor mounting. Again not being aware of the usual methods used with model boats, I made up my own method, based on what I have seen in Heavy Engineering. The beam of a alignment telescope , has been substituted by the prop shaft, in its tube, for intermediate targets. The Bulkhead marked where the shaft struck, the motor mount marked from the shaft. A piece of close fit tube, fitted to the plate at 90 degrees as the alignment lense.

I am know making up a mounting system for the plate.

The rudder purchased is a supposably Micro rudder, a lot bigger and chunkier than I anticipated, even from the dimensions supplied.

I have purchased a spring coupling. My preferred double UJ taking to much space and pretty chunkier. I was concerned, would this a pretty good UJ be upto transmitting the power. Given that the motor on a 2s Lipo only gives about 80w in. it should be ok. That is except that the grub screws are the tiniest I have ever seen. I may be looking for a solution to this issue,. Any Ideas?

My intention was and perhaps is to mount the rudder as an outrigger. I have realised that I have already cocked up, in that I simply intended driving the rudder externally by a simple push rod or short snake. I am no longer as certain, having forgot that this required it to go through the removable top. I ham more interested in function than elegant appearance.

It will take me a few days to sort out the motor mounting.

As for the Crusader, I just sits on the water, slightly nose down. Indicates all the weight has to be at the back. If needs be the front planing wedge may need increasing in width for buoyancy. If I get the Mermaid sorted, I will return to the Crusader. In both cases I intend initially to run them the once, to prove i can do it. The Crusader essentially being a straight line model, dor something like 100 yards. I have bought a load of stuff for the Crusader, non of which is probably any use. I intended a flexi shaft drive with a 2 blade surface breaking prop. I have toyed with a DF (Ducted Fan) although I do not believe it can be made to work with it. Air inlet/weight issues. Paradoxly I think a Blue bird type could be made to work.

[ashley needhamParticipant @ashleyneedham69188]

Carl. Discard the spring coupling. It is for low power motors only.

Could try the normal brass inserts but use silicone tube instead of a u.j. I bought some thick wall tube ages ago and it is a good tight fit on the splines and can be cut longer or shorter as required.

Crusader with an edf?? No problem (hopefully with more testing this summer)…

Bluebird with edf?? Easy, done and dusted…

Ashley

[Chris EParticipant @chrise]

That looks like an very large rudder for what is going to be a fairly fast model.

[ashley needhamParticipant @ashleyneedham69188]

Ah, on reflection you won’t develop much power using the standard prop despite having 80w in the motor, so the spring may be ok, but I am sure there are better small couplings out there.

Ashley

[Malcolm FraryParticipant @malcolmfrary95515]

That is one big, heavy rudder for a very small boat. There are many similar size RTR boats on the market, and often, spares are readily available, because exciting toys get bits broken off.

Googling for the Feilun FT009 spares (because I can remember that one) shows things like rudder assembly and drive parts listed.

Putting a large motor in a small hull and driving it with a large prop gives a problem that Newton predicted. The reaction of the prop being turned in the water results in the prop trying to spin the boat.

[carl brothertonParticipant @carlbrotherton75833]

I have bought a dumb bell type Constant Velocity type UJ listed as a Feilun ft009. Even if it doe not fit with the existing drive train, it will come in handy for another model.

The rudder assembly appears to be similar to one that I currently have.

If necessary I could narrow the chord, to produce an effective area similar to the Feilun rudder.

The Feilun appears to be about 60mm longer than the Mermaid. A rough approx. from the little data that is available from adverts. It uses a 1500 mah or 0.15 ahr Lipo. the run time is approx 5 min the volts quoted at 7v. Using W=AV, I am guessing that it pulls about 200w. I can only pull about 80w 0n 7v or maybe 120w 0n a 3s.

Perhaps the most disturbing the beam on the Feilun appears to be 112mm the Mermaid is 90mm max, at the stern it is only 70mm. Possibly about 50mm narrower. I could always build on sponsons if needed.

I cannot see how to get the area of the inlet big enough on the Crusader, to allow the fan to draw enough air without major losses. Perhaps Bluebird having already successfully done as a DF is perhaps not such a surprise. The inlet already as a scale item is pretty large, a little bit od stretching here and there is possible, an additional cheat inlet, then Robert was my uncle. Also the boat had a generous are at the back. GRRRR, perhaps a me too

[Malcolm FraryParticipant @malcolmfrary95515]

On a small hull, the weight difference between the plastic rudder hanging off the transom and the weight of the brass lump plus whatever is used to mount it will be considerable..

A jet engine will take air in, then make a huge amount of gas o squirt out the back by burning fuel, giving it lots of drive. A ducted fan, however, only accelerates whatever air it can suck in, so getting a lot of air out the back needs the opportunity to suck a lot in at the front. If Crusader had been running a ducted fan, it would have needed huge nostrils as well. I suspect that DF Bluebirds also tend to have oversize inlets, and the smaller the scale, probably the more extreme the exaggeration.

[carl brothertonParticipant @carlbrotherton75833]

Yep, there is quite a difference in the weight in the rudder assemblies. I will probably use the plastic version.

Now for another bit I am less than certain, the prop shaft installation.

How close should the prop shaft come to the motor shaft?

How much should the prop shaft tube come into the hull interior?

With respect to the propeller, how many diameters or part diameters is ideal for the prop be distanced from the hull underside?

I also have issues with fixing the prop shaft outer. Just seal the ends, or perhaps fix the whole of the outer in Epoxy?

I have also been thinking about lubrication and cleaning, I thought I could drill a small hole at the upper end of the tube, so that I could use WD40 for both after running cleaning and pre run lube.?

[Author]

Posts