Model Boat Mayhem
Technical, Techniques, Hints, and Tips => The "Black Arts!" ( Electrics & Electronics ) => Topic started by: cdsc123 on May 16, 2007, 02:41:08 pm
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Gents I've been asked to help out with this query;
Hello All
Just been given a model Brave Class? Circa 52" long with fibreglass hull and single electric motor. Plans that came with it say "Modav PRECEDENT .... KD Perkasa Class, designed B Ayres Jan1976" Could that be the age of the model?
The model is not very detailed but better stay that way, cos of kids ages. Now for the important questions where I need a bit (lot) of guidance really:
1. Could it be based on a real boat given that it is marked P47 and had some big missiles on the stern?
2. It has s a single 6v 9000rpm electric motor, will that be sufficiently powerful?
3. Any good (and cheap) books/guides out there as to how to set these things up including RC etc...
Any thoughts most welcome.
Thanks
Any help with questions 2 and 3 please?
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Is the 6volt 9000 rpm motor a Jonson with a fan inside the casing?
Bob
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The PRECEDENT .... KD Perkasa Class, is made by Metcalfs but is only 49" long.
Richard ;)
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I beleive, and stand to be corrected, that the Perkasa was a development from the 'Brave' class gas MTB's. I think she was built for the Malaysian Navy, possibly in the 50's or early 60's.
Ian
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Thanks for the response you guys, I have redirected the owner to this site as ours isn't so good with technical modelling questions
http://www.bmpt.org.uk/forum/forum_posts.asp?TID=340&PN=1&TPN=1
Regards, Christian
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To get real performance from this large model I would suggest 2 x Graupner 700BB Turbo Motors.
These are fairly power hungry, but will get this large model planing well.
Bob
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Thanks for the response you guys, I have redirected the owner to this site as ours isn't so good with technical modelling questions
http://www.bmpt.org.uk/forum/forum_posts.asp?TID=340&PN=1&TPN=1
Regards, Christian
Be fair Christian,
You hadn't given us folks 6 hours to respond before you redirected.
There has been a lot of proplulsion advice on PT boats on this site in the past.
Try a search of this site as well.
I don't think 1976 is necessarily the age of the model, it is the year the design was produced.
Good book guide is https://sslrelay.com/s84068217.oneandoneshop.co.uk/sess/utn;jsessionid=15464bb50324a0d/shopdata/index.shopscript
Hugh Brights book 'The Complete Marine Radio Control Manual' 12.95 pounds
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TT he means he has redirected the owner to this site [Mayhem] as his site[http://www.bmpt.org.uk/forum/forum_posts.asp?TID=340&PN=1&TPN=1] is not so good with technical modelling questions.
Richard ;)
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Intriquing ???
I sold on my Perkasa many years ago, to a shop in Preston, it had a 6v motor and I made a missile launcher for the stern deck using missiles from an aircraft kit. Apart from the length that sounds like it. That motor might be an MFA Marlin as they powered up those kits nicely, I ran mine on two 7.2v racing packs connected in parallel for a greater run time.
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Any chance of some photos Christian ?
Richard ;)
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Hi Richard
I'm trying to encourage the owner to join in on Mayhem himself, in the meantime I'll work on it via the bmpt page.
Rgds, Christian.
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Hello All
Christian (big thank you) has pointed me in your direction. What an excellent site!!
Firstly I'm not really a modeller or boat expert so excuse me for any faux pas. However as an engineer I'm not too bad at putting things together and making them work. What would really help me is a good interior shot or two of some boat electrical intallations to give me a pattern to follow.
Anyway, about the boat:
Actual hull length 47.75" plus a 2" platform at the stern. (Previous measurement taken with 6 year old holding the other end of the tape measure.)
It has 3 turbine outlets at the stern so I guess it could be based on a Perkasa. The hull and superstructure are spot on, its just the armament that confuses me.
The motor is made in Germany, has Marx moulded into the base plate and a paper label on top with the following info: Decaperm-5pol;6V=leer60A9000u/min;load7A7000u/min;350cmp;20W;90degC;.5ohm; Jmax 1h 7A 5' 10A
I've now got a 6V 4ah sealed battery to go with the motor, but rather think I'd be better off with two in parallel. Perhaps one mounted each side to balance the boat. (A local model shop (in big village just north of Oxford) seemed happy enough to take over £100 from me in gear to get the boat working but disinterested in offering advice to the point of rudeness!!)
Sorry no photo available but the boat is cream (or v.v.pale grey) with a green deck. A cluster of four side by side rockets on a plinth at the stern with metal mesh around it. On the foredeck there is a gun with a curved shield around the breech and loader's seat.
It would be great to find the guy who built it, its nice to know the history of things.
Thanking you all in anticipation.
Cheers
Dave
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DMSmith
While the deccaperm is a good reliable motor, but overpriced, at the gearbox outlet it only runs at about 2750 rpm. You would need a huge prop to make her go well.
Sorry to say that this motor isn't really suitable for a fast patrol boat.
Bob
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I would think he would probably need larger batteries too Bob.
Richard ;)
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Hi,
I scratch built a 36" Brave Borderer a few years ago. I put in 3 electric motors, port and starboard for cruising, with the centre motor kicking in at full throttle at full speed only. The ESC (electronic speed controller) drives the port and starboard only.
This set up gives a reasonable scale speed - see pic below. As a note of interest, the borderer was the first fully armed and loaded British warship to exceed 50 knots! (needed 3 gas turbines to do it!
Hope this is of interest to you
Ian
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dunno how i did that twice! lol!
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Hello,
If you live near Oxford have you tried Howes? They always seem to offer good prices and have been around a while.
Clive :)
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dunno how i did that twice! lol!
Glad you said that, I nearly poured the last glass down the sink ???
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There are wiring diagrams on the Mayhem site accessable from the home page.
They are for twin engines but can be used for one.
http://www.modelboatmayhem.co.uk/Common/Electrics/Images/setupsA4JPG.jpg
Richard ;)
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Thanks chaps for the pointers.
Went back to Howes today and bought a second battery and bigger (55mm) prop. This time had a different assistant so got more useful help!
On a budget so will have to keep motor for now.
Will spend a few days putting it all together and will see how she goes at our local Marina. I'll just have to avoid the full scale boats..
If performance is pitiful I'll try a three motor set up as that is what a brave and perkasa would have had for real. I think I can get away with leaving the centre shaft in place as is, although I note that the shaft is at a 14 degree angle from the flat aft hull section and may be steeper than original?
Cheers to all.
Dave
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DMSmith,
There isnt any right or wrong way to power a model.
There are some more tried and tested ways though.
Motors are very rarely produced for model boats as the market is too small, so a motor labeled as a 550 can vary in speed and current consumption.
Here is a pic of my old "Perkassa-Type" patrol boat. She was 36 inches long but very heavy.
I found a 9.6 volt 550 motor in an old battery drill and used this on 9.6 volts.
As you can see she handled quite well.
Bob
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Thanks Bob and forum members.
Spent pretty much all day refitting driveline and electrics and just road tested her in the bath (3") to spare. I reckon she'll go alright, hell of a force against the hand and nearly chucks the water right out over the side, jacuzzi style. Toyed with the idea of putting bubble bath in!!
Made little wooden boxes for the batteries and araldited them to the bottom, eitherside amidships. At rest waterline is spot on. Made a plinth for the rudder servo c. 5" ahead of the rudderstock. In the end the layout was partly dictated by the length of the wires supplied with the motor controller and the receiver.So with a bit of luck I'm there! :D
Only remaining problem is slight water leak between shaft and elderly (polyfiller type?) sealant. Any good products out there I could paint on to seal and stabilize this sealant?
Thanks
Dave
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Dig out and make a small box or use blu tak and pour in epoxy resin.
Will only take a few minutes
Bob
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Will do thanks.
In the interim I painted over the porous sealant inside with epoxy and this has done the trick well enough to allow a "sea trial" at the marina. ;D
She went well, not planing, however she was moving swiftly and the bow wave looked good and to scale. She ran for over 1 1/2 hours, mostly full throttle, and only slowed down a bit towards the end. Motor was warm at finish too but no smells so I assume OK ??
I think I've got room for a 60mm prop. Are there any single engines out there that would spin that fast enough to lift her fully onto the plane?
I rather think I'm getting the bug!! :o
Cheers
Dave
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Selection of a prop for maximum performance for a given motor/hull/battery combination is a "black art". Conventional wisdom is that often, if the motor has more revs to give, a larger prop will not allow the motor to spin fast enough to increase speed, and may well cause the whole thing to slow down while using more current and overheating the motor. It is possible that you may need a two blade prop to allow an easy increase in RPM, and eventually, speed. Counterintuitive, but...........
Hopefully, someone with more experience in making heavy boats go fast will give some accurate advice.
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If the motor is giving that duration (but on what size battery) and is only warm, you have 2 options.
To try a larger prop but make sure you keep monitoring temp, or fit a higher performance motor.
I assume you dont want to go to 2 shafts?
Bob
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Bob & Malcolm
Thanks.
I'm using 2x 6V 4A jelly cells in parallel, and yes I now think I'll stick with one motor if I can.
You mention the Decaperm o/put via g/box is c 2750rpm. That makes sense looking at the prop free running speed. However I'm even more confused when I look in Howes catalogue and see motors like Graupner 3308.700 turbo 4.8-14.4v 15,000 rpm. This appears to be a direct drive motor. Should I use a gearbox? What voltage should I run it at?
Other motors are also listed e.g MFA torpedo 800 12V 5167rpm & MFA torpedo 850 12V 9778rpm
Having purchased a 15A speed controller I guess next step would be 2 more batteries for series parallel set up 12V 8A to keep current down. Now which motor to go for? ???
As before, any ideas welcome
Cheers
Dave
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The best way to increase performance with a particular motor is to reduce weight. It might well be that using just one 6V 4A battery will increase perfomance and still give an acceptable run time. Only problem then is how to mount the battery and still leave the boat balanced.
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DMSmith,
if you go for a motor with much higher performance on direst drive you will have to reduse the prop size.
The faster motors will struggle to turn a large prop and pull horrible amounts of current.
These motors need to be able to rev, so you must remove the load by reducing the prop size.
It might be worth asking some of the fast electric boys how they would go about it.
I have 2 x Graupner speed 600's on an epicyclic gearbox.
These motors go like the clappers, but pull over 6 amps in air withput a shaft.
One of these would suit, but you would need a boot full of batteries!!!
I haven't found a use for these yet!!
Bob
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Thanks chaps for the advice again. I'm pretty much getting the picture. Its very much a question of boat mass (displacement) v's thrust. Small prop big rpm, large prop lower rpm and finally current required dictating battery size bringing me back to weight again. I'm struggling to remember my o'level physics from the 70's but its coming back slowly.
The Decaperm with its gearing is able to turn the 55mm prop OK but is pretty much on its limit. It does appear to give what looks like a fair scale speed. Although reading up as best I can about this motor makes me realise it is better suited to a model tug boat or similar.
I'm a bit worried about the thought of 5,000 or 10,000 rpm through the current (old) shaft and the ability of my children to handle what could become a real missile. So I'll leave as is for this summer and we can all get some fun out of it until motor or shaft gives up.
On the subject of scale speed what guidelines do you follow? My guess is that a real Perkasa is c.100' so 4' = 1:25 scale so 50mph top speed would translate to 2 mph. Obviously that would not look at all right and the boat could not plane at that speed. My guess is she can reach 5mph at present although not timed. Is that a fair enough speed or should I aim higher?
Thanks for your time and trouble
Dave
ps I'm trying to talk my friend, who gave me the boat, into letting me have a go at getting his 30" Fairey Huntsman going using what I have learned from you.
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Dave,
As a rule of thumb, if she looks right she IS right.
I can give you the formula, but why bother. As I have said in previous threads I ALWAYS overpower my models as the wind and waves aren't scale.
As to fast patrol boats MY preference is for 2 shafts, not for power, but if it is planing with over 1/2 the length out of the water, 2 props "digging" in the water makes the hull that little bit more stable.
If you are happy with the deccaperm stay with it, but these motors reach £25 plus on eBay which is just about enough to get 2 x Motors and shafts and fast props.
But you might have to upgrade speed controller.
Bob
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I agree with the looks right / is right approach.
Its v hard to scale speed, cos water and waves (ripples) dont scale.
Ian
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Speed does scale, thats why test tanks work. To get the scale speed, divide the speed of the original by the square root of the scale - in this case, sq rt 25 = 5, so 50/5 = 10 mph. The waves produced will look right, the only thing that doesn't scale is the creamyness of the wash, as the bubbles produced are full size. This is a function of surface tension, and we are stuck with it.
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Thats me told! ;)
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Thanks chaps, I'll be taking my stopwatch to the Marina next time, this is facsinating stuff. If she doesn't make 8mph at least, then piggy gets the hammer and it'l be new engine time. However the beasts below look a little pricey.
http://www.pandanmodelboats.co.uk/acatalog/Our_Online_Catalogue_Graupner___Robbe_and_Geared_Motors___Drive_Systems_250.html#a581
Cheers again
Dave
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If you have access to a hand held GPS... you could send the boat out with it on board. Mine gives max MPH.
Adds a bit of weight though.