Bluebird K4

[HowardParticipant @howard79726]

I intend taking a break from IOM radio sailing and building an RC model of Sir Malcolm and later Donald Campbell's Bluebird K4 in her pre-war aero-engined form. I have enough info to work up drawings for the build and the natural choice for motor type might be brushless electric. Naturally the boat's body or 'fuselage' is quite slim and the motor would operate in a more confined space than the MTB hulls I have previously built, so would the motor need water cooling and if so how is this best done and has anyone published info on the subject? Or maybe a decent brushed motor would generate a better scale (1:10 or 1:12) speed. I would appreciate any comments or information before I blow the dust off my CAD skills!

[HowardParticipant @howard79726]

Brushless cooling?

[ashley needhamParticipant @ashleyneedham69188]

Hi Howard.

As far as I know the only way to cool out runners is to have a water cooled mount, as the can rotates obviously there is not much else you can do.

However, inrunners can have a water jacket, and these are quite common, so just select a motor and jacket together. You would need something with quite a high Kv….2600 or so and perhaps a flexidrive. Is the intention to go very fast or just achieve “the look”? The goal on my K7 was To go as slow as possible whilst getting that surface skimminglook, our pond not being over large for supersonic speeds .

Having just had a quick surf….I saw a good snap showing the prop, a funny squared off surface piercing type which according to the blurb said was geared up to produce14000 rpm. ?? Sounds a bit high but what do I know!

Ashley

[Chris EParticipant @chrise]

I agree with Ashley about outrunners  needing the mount cooling as this is the bit attached to the core of the motor and the coils..

What often isn't recognised that it is the centre coils & core of an outrunner that get hot and that the bell can still be cool whilst the coils melt with overheating. This can happen very quickly.

You often see people testing how warm their outrunner motor is by touching the bell. In reality this tells them very little.

Edited By Chris E on 03/12/2020 09:12:37

[HowardParticipant @howard79726]

Thank you both. I guess my main concern is where best to site the ‘scoop’ or pickup for water, and is it the case that the water finds its way to the cooling jacket (via piping obviously) propelled purely by the force from the propellor’s backwash and/or the boat’s forward speed? To answer your question Ashley, I’m going more for the ‘look’ of the boat plus a bit of speed obviously, as I’ve always considered the original a rather elegant design and, although she only ever captured the record once, this was done quickly and by a creditable margin with minimal ‘tweaking’.

[HowardParticipant @howard79726]

PS. ‘Supersonic’ speeds are not that welcome at our pond (my Higgins PT boat sometimes provokes the cry ‘hooligan!) but I think this model might attract interest.

[Chris EParticipant @chrise]

In the old glow motor days I always put the inlet pipe in the propwash behind the prop blades to get as much water forced up the pipe as possible.

The outlet pipe came out of the side of the model well above the waterline so that I could see that the cooling pipe hadn't become blocked.

[ashley needhamParticipant @ashleyneedham69188]

If You are not aiming for max performance, then an out runner should be ok as long as there is some ventilation…ie…air through the boat. If the motor is not being overloaded it should run without “forced” cooling.

How big will the boat be? A lot of people simply look at the size of the motor and prop and go for large motors, but the old rule of thumb (prop no larger than an diameter) does not apply for brushless units.

if we were talking about 500-600 mm and of light-construction, I would not hesitate to put a 28mm motor in, and if using a surface piercing prop it might take a 50mm unit no probs. Likely you would not need such a large prop. I will illustrate that statement below, and it is a 50mm prop! Ashley

[HowardParticipant @howard79726]

Thanks Ashley and Chris,

At present I'm planning an overall length of 830 mm at 1:10 scale. Also considering 1:10 which comes out at 690mm.

If I could mount the motor behind the cockpit as the original I'd have no problem getting air to it but I'd have to emulate Campbells 'V drive' that directed the drive shaft forwards to a gearbox in the nose and then back to the propellor at the rear. Practically I think I'll be mounting the motor at the boat's front and I'll have to think of a clever (invisible) air intake where there was none on the prototype.

But hell, problem solving is what makes this hobby interesting!

Ashley, your reference on the propellor seems to reflectt the fact that the boat was converted to a 'prop rider' configuration by Donald Campbell making the propellor the third planing point à la Slo Mo Shun. I figure you might need a healthy RPM to enable that!

Thank you both for your interest. Howard

[HowardParticipant @howard79726]

I suppose a rear-mounted motor's drive could go forwards via a solid shaft to spur gears then back via a flexible drive. Could be noisy and power-consuming though. H.

[Paul TParticipant @pault84577]

Howard

Mount the motor with the shaft facing forward and use a belt / pulley to connect with the propshaft.

The cooling systems that you are discussing will only work when the boat is in forward motion and won't prevent a heat spike when the boat is stationary after a high speed run.

I would suggest an independent forced air cooling system such as high capacity computer CPU fans as the air intake can be adapted to suit the boats existing air intake design.

Paul

[ashley needhamParticipant @ashleyneedham69188]

At 830mm you may need a 35mm outrunner. As I suspect the rear cover had some louvre vents, that would be plenty ventilation. No problem with brushless to get lots of revs……

Both my K7 and Cobb were designed to allow the battery to go from as far forward as possible, to as far back, as you never really know where the c of g needs to be (unless you are very clever). I would steer clear of any sort of gearing if possible. A flexishaft would allow a front motor and a nice flat drive underneath.

At the pond we have come to the conclusion that 2800mAhr 3s Lipo batteries are the ideal compromise between run time and weight. BUT the difference in overall weight between my K7 and a 2200mA 2s, or the 2800mA 3s is quite something, and although you get more oomph, in the K7 the extra weight was too great a penalty…so I changed the fan motor to a higher revving one and it is fine on 2s.

ALSO something to consider is sealing against water entry at you access point (cabin etc). Almost impossible on K7 and Cobb due to the big intakes, but more feasible on a conventional type. The cobb is very wet at low speeds.

Ashley

[HowardParticipant @howard79726]

Again thank you to all who have responded to this thread. I must confess to not having used a brushless setup yet and I am little daunted over choosing motor type and size, matching it to an ESC, cooling etc., but I have saved articles by Messrs Milbourne, Parker and Rees and I will re-read them! My next step is the drawing board (iMac), so it'll all go quiet for a bit. Thanks again, Howard

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