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Old Nov 30, 2007, 03:53 AM
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Miami crashboat hull

I wanted to build something quickly in a planing boat so I used the Miami crash boat plans that are elsewhere on the site. Im putting in a brushless motor. The setup I've come up with is 11.1V 1300mah Li-Po, KA450H Pulso motor, 3200kV, Maxx gearbox 3:1 or 2.7: or 2.3:1, std, straight shaft stuffing tube, 40 amp reversing brushless controller 35mm or 30mm graupner carbon filled prop. While I haven't actually run the boat yet I have put into the wading pool and promptly covered myself in water .
Seeing as the last fast boat for me was a whipper snipper mono bout 1.5 meters long. I'm just curious what some of you fast electric guys think of this setup what I should watch out for etc.
Regards
Nick
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Old Dec 01, 2007, 05:07 AM
Boaters are nice people.
Sneek, Netherlands.
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Hi Nick,

Set-up sounds good to me.
As it has submerged drive, gearing down the 35000rpm+ is a good idea, just keep in mind you might have (prop)torque issues due to the power available, go easy on the throttle for the maiden run.

Can you switch between gear-ratio when all is build in?

I'm asking, because if you can, it's very easy to balance the propsize (read prop torque) versus the rpm.
I've had a submerged prop driven boat running on its side due to (prop)torque (too large), very scary on a testrun without the hatch in place...

Got some pictures?

Regards, Jan.
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Old Dec 01, 2007, 05:57 PM
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Hi pompebled,
It shouldn't be to hard to change the ratios, I've made it so the whole motor gearbox is removable in one piece, the gearbox only has 3 ratios available and currently have the largest reduction in place.
Ill post some pictures when I get the boat tidied up, balsa is far to easy to damage/dent
Regards
Nick
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Old Mar 08, 2008, 03:32 AM
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First run, Whoops!!!!!!!

I ran the crashboat hull today, holey cow it was way too fast, It was planing on the last quarter of the hull, and at times porpoised wildly. I didn't notice any propwalk as thought.

First I tried some recycled laptop 18650S li-ion cells 1350 mAh in a 3S-2P configuration, but the boat kept cutting out from the LVC in the speed control (these batts will be relegated to sail boat rx batteries I think) a FE boater let me borrow a 5000mAh 11.1 li-po, I couldn't get it in the centre but ran the boat anyway (from my previous post I cooked the li-po I had, first I left it connected to the ESC over a couple of days then I didn't balance charge it and it went all puffy on me, a real lack of experience on my part).

Subjectively it looked to be going at least 30 kph but was only running for less than a minute when the boat flipped, turning around hard I think the battery already off centre shifted outboard. It was lucky the whole thing is made of balsa because the hatch wasn't sealed and only a push fit into the opening, It remained afloat upside down until the rescue boat could retrieve it. After drying the receiver (ESC waterproof) when I returned home all up and running again. From what I've seen I think I will put high capacity Nimh battery's in it as weight is not as critical as I thought. Eight cells (or ten ) , four on each side of the keel.
Regards
Nick
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Old Mar 08, 2008, 06:00 AM
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Pictures

Pictures
of
boat
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Old Mar 11, 2008, 02:42 PM
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Nice boat Nick!

As you mentioned, having bit of ballast in the form of NiMH C-cells isn't bad with that kind of power on board.
Don't overdo it though, you'll ruin the swift running appeareance of the hull.

Regards, Jan.
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Old Mar 16, 2008, 02:33 AM
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Ran the crash boat hull this morning with a gps, 23 kph max speed, It appears my estimate was 30% out . Using 14.4 v nicad out of a 3 yo dodgy GMC battery drill pack. I did get 10 3200 mAh Nimh batteries for it to make up.
It porpoises badly once over about half throttle, could the prop be too big? its marked X35 a graupner maybe? or would trim tabs help?
I think I should cut the rudder down significantly as well, maybe so the bottom of the rudder is level with the prop hub instead of bottom of the prop?? would that be about right, it has a nasty habit of digging in a chine and flipping.
I need to get the hatch sealing right as well, electrical tape on painted balsa is not watertight. I water cooled the ESC and after the first run the motor has minimal heat in the can so it appears the amps are not too high. One day I will treat myself to a data logger or e-meter.
regards
Nick
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Old Mar 16, 2008, 12:43 PM
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Hi Nick,

One of the disadvantages of having a submerged drive, is the need for a superb under water hull.
The slightest rocker in the keelline will cause porpoising once you get over a certain speed, as you'll have noticed.

The Eco class raceboats with their submerged props can run up to 50-60 kph without the slightest sign of porpoising, so it's all in the design and set-up.
Our mildly powered monohulls run 35-40Kph, on a 600 motor and 7 NiMH sub-C, but they have a surfacepiercing drive.

If your hull has a rocker, forget about 'ludicrous speed', you can tame it a bit with trimtabs, but at the cost of some topspeed.

A large/deep rudder can certainly cause the bow to dig in, check the angle of the ruddershaft; when viewed from the side, if the ruddershaft (below the hull !) is tilted towards the bow, it will lift the bow in a turn, tilted towards the transom it will push the bow down in a turn.

Also, the edges of the hull need to be sharp, preferably with strakes running from the bow about 1/3 of the hull. to deflect the water away from the hull, without these deflectors, the water will creep up the hull, causing it to dig in. (and flip the boat is you're running fast enough...)

Regards, Jan.
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Old Mar 17, 2008, 02:34 AM
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Hi Jan,
Thanks for your advice, I wasn't ever expecting surface piercing speed with this set-up, actually I was surprised with how fast it appears to go, hence the over estimation of speed in my previous post . Looking at the rocker the keel is straight to where the prop shaft exits, it has a slight bend up towards the waterline after this point. So I guess that answers that question. the rudder is parallel to this angle so the top of the rudder post is closer to the bow than the bottom of it. Strakes along the chine are specified in the plan, I never put them on , About time I did I guess.

At this stage I think I'm going to finish it off as a nice scale boat, replace the motor with a brushed 400 or something and put the brush-less into something else. Like a Micro Wallypower.
Regards
Nick
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Old Apr 13, 2008, 12:31 AM
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I ran the hull today with some new cells, I have dropped down to 10 cells 3000 mAH NiMH. The boat is much more manageable, still a little skittish with a fresh charge, it settles down after a minute or two. I get a full 10 minutes runtime at nearly all full throttle before the speed sags off and I can run for another 5 or so minutes until the LVC cuts at 0.7 V, I should set it a little higher to around 0.9 v I've been told. After the first run the pack voltage was 10.5 V when I first put it on the charger, put 2250 mAH into pack and the second time it was 12.2 volts and is charging at the moment.
Regards
Nick
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Old May 18, 2008, 12:31 AM
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I Ran the Crash-boat today with some slight mods, I fitted the spray rails. Its a much better boat now, it still porpoises but when turning it doesn't dig the bow in and I couldn't flip it at all. It was a very windy day and the chop was quite large. I had a ball jumping waves with it. The batteries must be breaking in as it would porpoise at full throttle for several minutes.

Regards
Nick
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Old May 18, 2008, 08:00 AM
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Nice!

Could you manage some running pics?

(I just went out for a spin with my boat, got to the water and noticed my transmitter had been left switched on since yesterday morning..., it was the shortest outing ever.)

Regards, Jan.
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Old May 19, 2008, 12:29 AM
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Hi pompebled,
Someone was taking photos, they may appear in the Tygum thread, he was having trouble keeping it in frame.
Regards
Nick
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Old May 19, 2008, 03:44 AM
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A couple of runing photos are on the Logan City Marine Modellers thread, while they dont show her at full speed it certainly shows how windy it was on Sunday.

A query came up, as I'm running a gearbox and it is rather noisy , is a 1000 kv direct drive outrunner going to be able to spin the same size prop as my current 3200 kv motor and 3:1 gearbox assuming same stator size? I.e. does the low Kv motor in direct drive have the same torque as the geared high Kv motor?
Regards
Nick
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Old May 19, 2008, 08:03 AM
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Sneek, Netherlands.
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Most likely yes, BUT, see if you can find out the torque of that 1000KV outrunner, ideally it should be three times the one you're running now.

I doubt if that will be the case, so either go for the next size outrunner with the torque required, or go for a smaller prop and a higher KV motor to equal things out.

Regards, Jan.
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