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Author Topic: Vosper Perkasa 1/32 Mediocre Patrol Boat (Hooben/Dragonland kit)  (Read 4057 times)

BobbyShaftoe

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Vosper Perkasa 1/32 Mediocre Patrol Boat (Hooben/Dragonland kit)
« on: February 29, 2012, 01:13:21 PM »

I've never done marine RC before although I've been doing most other forms of RC (i.e. land and air) for about twenty years.  I've not done 'proper' electric before, I've played almost exclusively with IC.  I have a long history of micro-electronics so the electronics side isn't a problem.  I figured I'd start with a relatively cheap kit as if I then screw it up I haven't lost much.  I've also little interest in accurate scale representation although I'd like it to look reasonable when complete.

Because of my motivations for building a boat I figured I'd go adventurous.  I've built a triple-screw setup with three ESCs, differential drive and separate rudder control.

I'm still only part way through the build, I've done a few bathtub tests and sealed holes where I found them and today took it out for its maiden open-water voyage.

I've discovered something distressing...

I don't believe my battery setup can dump enough current into the stock (i.e. cheap) 550 can motors to drive the three-bladed semi-scale brass props.  With one fully charged 7.2v pack I get about 2 seconds semi-planing before it slowly crawls back to me.  With three 7.2v packs in parallel (one per ESC) it lasts marginally longer (maybe 10 seconds) with the bow coming out of the water but still it's all but dead after 30 seconds on the water.

I think it's probably the batteries, I don't know the current demands of stock 550 motors but I suspect they're pretty high.  If it's 10A a pop then 30A is a lot for a 7.2v NiMH to provide.  I also don't know how good condition these packs are in, they've been in the garage for a few years and despite being high tech at the time (3600mAh) I doubt they're as capacious as they used to be.

Other options that come to mind are overheating somewhere, maybe in the wiring.  I don't think that's happening but it seems plausible.

So, onto the questions, if you have the time and inclination to answer them...

1) Are 3 x cheap 550 can motors capable of getting the boat up onto the plane?  Their performance in the bathtub is impressive, giving huge swells at either end of the bath.  If not what's the most cost effective way of getting this result?  I don't want to spend the cash on a racing setup, just a modicum of performance to earn the title 'Fast' Patrol Boat in preference to 'Mediocre' Patrol Boat.

2) What battery setup do people use for high-current demands?  I don't want to go LiPo unless I really have to due to the high costs of every aspect of this power setup.

3) Do you have any other ideas of what could be causing these symptoms?

Thank you all in advance :)
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rsm

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Re: Vosper Perkasa 1/32 Mediocre Patrol Boat (Hooben/Dragonland kit)
« Reply #1 on: February 29, 2012, 04:16:55 PM »

I think your battery packs may be knackered. Check out this build log; http://www.vosper-perkasa.com/blog/ . This guy built the same kit as you and his setup seems to work pretty well.
Regards.
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BobbyShaftoe

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Re: Vosper Perkasa 1/32 Mediocre Patrol Boat (Hooben/Dragonland kit)
« Reply #2 on: February 29, 2012, 04:44:41 PM »

I've just found out that NiMH batteries die if they're stored discharged.  I didn't know this.  I knew it was true of lead acid batteries, didn't know about NiMH.  Chances of my NiMH packs being in good nick after 5 years in the garage seem pretty low now.

Thanks for your reply, prompted by this I've ordered a new battery pack which should be here in the next day or two and after a decent charge we'll see if it solves the problem.

Long term I'm going to need to figure out how to stop my battery packs dying when they're stored for long periods.
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triumphjon

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Re: Vosper Perkasa 1/32 Mediocre Patrol Boat (Hooben/Dragonland kit)
« Reply #3 on: February 29, 2012, 07:46:37 PM »

as you have stated your model has three independant motors / escs have you tried using one battery per esc ? 
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BobbyShaftoe

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Re: Vosper Perkasa 1/32 Mediocre Patrol Boat (Hooben/Dragonland kit)
« Reply #4 on: February 29, 2012, 07:56:08 PM »

as you have stated your model has three independant motors / escs have you tried using one battery per esc ? 

Yes :D

I don't believe my battery setup can dump enough current into the stock (i.e. cheap) 550 can motors to drive the three-bladed semi-scale brass props.  With one fully charged 7.2v pack I get about 2 seconds semi-planing before it slowly crawls back to me.  With three 7.2v packs in parallel (one per ESC) it lasts marginally longer (maybe 10 seconds) with the bow coming out of the water but still it's all but dead after 30 seconds on the water.

It definitely improves things massively, over 3 x improvement in longevity, which indicates that the power delivery is the issue.  Still not good enough, hence the 'dead battery' conclusion.
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triumphjon

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Re: Vosper Perkasa 1/32 Mediocre Patrol Boat (Hooben/Dragonland kit)
« Reply #5 on: February 29, 2012, 10:19:02 PM »

will your speed controllers handle more volts ?  i know you have already said you dont want to upgrade to lipo type batteries , but they offer some advantages , the capacity of discharge is much greater , they take up far less space inside your model and are over half the weight of a standard nimh pack .
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BobbyShaftoe

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Re: Vosper Perkasa 1/32 Mediocre Patrol Boat (Hooben/Dragonland kit)
« Reply #6 on: March 04, 2012, 02:56:37 PM »

Ok, I bit the bullet and bought a Vapextech 4000Mah 7.4v (2S) Lipo since the JST-XH charging connector is the same as the Lipos I use in my Honey Bee helicopter.  The reason for the Vapextech one specifically was that it's cost effective (25.25 shipped) and comes with multi-leads so I can use it with Deans and Tamiya connector.

It fixed the issue.  The boat still isn't fully planing but it's getting the bow out of the water and it doesn't die like it did at the beginning of the thread.  The motors sound like they're revving quite high and I may look to see if I can find some semi-scale 3-blade brass steeper pitched props.  The battery pack is good for 90A and the ESCs are good for 25A which means that I can push the motors pretty hard before worrying about either of them.

Now I need to learn that trying the boat on RAF Greenham Common is a mistake because of all the foliage.  I got one of the three props tangled and stalled the motor.  It doesn't appear to have caused problems so I'll strip and rebuild the shaft and try it in the Basingstoke boating lake next week.

Thanks to all for your help.
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