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[firstperkasa]

Hi.
I'm a returning modeller that has been out of the hobby for over 10 years. I have run an IC boat in the past and have previously made a 37" wooden hulled perkasa that got wrecked before completion.

I'm about to start a build on a precedent 49" perkasa with a fibreglass hull.
I'd like to keep things very simple and run this on a single motor.
I'm not looking for blistering speed just scale and possibly a little more but looking to keep things at a sensible cost as everything will have to be bought new for this boat.


so can anyone recommend a single motor brushed or brushless with an esc ?
I'd also like to know for that motor/esc combination, what propshaft and prop sizes I would need.
I'll also need to know what batteries and chargers i'll need to drive it.
My knowledge of brushless technology and LiPo batteries is zero save for info recently given to me.




Many thanks Rob

[lakesidebob]

Hi Firstperk,have the same boat myself,plus the smaller one as well,i like them so much,MTB with attitude! My 49" runs on a single MFA Torpedo 850 Brushed Motor with a 50mm racing propeller,power is supplied by a 12v 9ah SLA Battery using a 50 amp Tornado ESC.[Howes].It runs superbly and planes too,great sounding as well,can get 1 and1/2hr duration [not full speed of course]...The boat appears heavy with that battery,when you lift it,but as long as your build is light you should have no problems.Chargers available too [H's]. All the best.Lakeside Bob.

[firstperkasa]

thanks for the reply bob. I'm much more comfortable with a brushed motor as I understand the technology.
I'll definitely go for the torpedo 850.
Batteries I may try to improve on to get the weight down.
What was the exact spec of the prop and what size and diameter shaft are you using ?
also have you bought or made the rear propshaft support thingy ? not sure what its called but I need to get the correct size to ensure the right shaft angle.
atb Rob

[firstperkasa]

I do have plans to keep the weight down by replacing as much as i can of the superstructure with plastikard and also maybe simplifying it too. I've seen an earlier model of MTB with a much smaller simpler superstructure.
Thats ok with me as I'm not looking for a spot on scale model just a working boat.
cheers

[radiojoe]

If you go for lighter batteries you will only end up using lead ballast to get her down on the water line the 49" is a big hull if it was me I'd use the same battery as Bob did far better to have "useful ballast" and a longer run time as a bonus.

[lakesidebob]

Hi Perk,i will check and measure the prop shaft length and prop [it is 50mm racing] as soon as I get the model out as it's stored underneath my workbench,tomozz, abw.The only drawback with these big boats,storage and transportation....but brill.on the water!There isn't really all that much difference using plasticard imo as opposed to the ply supplied,my superstructure is very light.If you use Nim Batt cells as opposed to the SLA,for weight loss,you may not get the duration.I am a relative new boater,two years,others may offer different advice but I know mine works fine.Bob.

[firstperkasa]

Thanks both Joe  and Bob.
The SLA would be great as they're cheap and I already have a good charger.


Bob - if you could give me as many measurements as poss I'd appreciate it. Position of battery etc.


I'm trying to source build instructions too at the minute. I have the complete kit but all the paperwork is missing


itching to get started on this. As soon as I have the info I'll get ordering parts.
thanks Rob

[firstperkasa]

bob where did you get that 12v 9ah battery ? checked my local supplier and all they do is a 7ah.
thanks

[Stavros]

Message sent ref motor


Dave

[malcolmfrary]

I have never understood the perceived need to ballast a performance planing hull down to "waterline".  Adjusting for static trim, OK, but boats like this have a very variable waterline anyway depending on whether they have a full load of fuel/stores and are setting out or whether they are returning empty. 
SLA batteries, by their nature, are incapable of supplying the current required to give the performance required for more than a few minutes without giving themselves a disappointingly short life.  Other battery types, while costing more at the outset, will be more reliable over time.  If the hull rides high in the water and really needs to sit lower in the water, it needs more battery for increased endurance, not more dead weight.

[inertia]

Just for the hell of it, here's some fag-packet maths:

A 12v 9AH SLA will deliver maybe 4.5A, so power input to motor will be 12 x 4.5 = 54Watts (that's assuming 100% efficiency from the speed controller).

An MFA 850 brushed motor, to be generous, is 60% efficient, so power to prop will be 54 x 0.6W= 32.4Watts.

Original 49" Perkasa powered by HP 61F R/C 10cc glowplug motor, which put out 1BHP = 720 Watts.

That's 22 times as much power as the 850, and considerably lighter...

I can only conclude that the Perkasa with a MFA 850 and SLA battery will have all the speed and menace of a canal narrow-boat, but without the charm of the kettles and roses.
It is science but it's nowt to do with rockets.

As someone on here was wont to say, suit yourself.

[Dave Cook]

Hi Perk , As DM and Malc say a SLA bat will not deliver the amps ( i know that from my own big Perkasa ) go with nimhs you wont be disapointed.
Dave 

[derekwarner]

 ......DM...as you well know...I have near ZERO understanding of all things electrical  .......however clearly remember that1 hp = 745.699872 W so....

1. have British watts been down rated to 0.966 of the actual 
2. was Mr Watt not a Brit to start with?
3. is this ~~ 3% down rating all to do with Chinese amps being the new universal UOM?

Derek

[radiojoe]

Rob, if you ask questions you will always get varying answers we all have our own way of doing things, if as you say you're looking for scale speed (and this will also vary from one modeller to another) then go with Lakesidebob, he's happy with his setup so it may well be right for you, IMO some guys have these MTB's running much too fast but that's up to them, yes they were fast boats but off shore grand prix racers they were not, just go with what you are happy with but above all have fun with it.

[inertia]

......DM...as you well know...I have near ZERO understanding of all things electrical  .......however clearly remember that1 hp = 745.699872 W so....

1. have British watts been down rated to 0.966 of the actual 
2. was Mr Watt not a Brit to start with?
3. is this ~~ 3% down rating all to do with Chinese amps being the new universal UOM?

Derek

Yeah, Derek - Wattever..... 

[firstperkasa]

just to add to this, I've spoken to anglia model centre at length today. they set their perkasas up pretty much as bob suggested with maybe a few differences with propshaft length and prop spec. for the minute this is the way I'll be going. 
as much as anything this is an experiment in build and the usage will be minimum.

Thanks for all the input, Rob

[malcolmfrary]

Taking the information here - https://sites.google.com/site/kdperkasa/Home/the-real-boat - and using the normal standard for giving a model scale speed (sq root scale), I would say that it will need to go about 11.5mph to act like the real thing.  A 50Kt boat is a fast item. Letting google do the converting -  http://www.disastercenter.com/convert.htm -
But you makes your choice and you pays your money.  Usually in that order.

[Jonty]

 I bought a broadly similar Fairmile C with this motor and a 12v SLA. The battery was in the only place it would fit, in front of the motor under the wheelhouse. It behaved like a dynamic dive submarine. The only way I could get the CoG far enough back was to use two 7.2v packs wired in series and fitted well towards the stern either side of the propshaft.

[canabus]

Hi
Go brushless and Lipo.

A 3648-1450kv 1600watts, 100amp ESC on 3s or 4S 5800mah Lipo battery and 3blade 37mm prop or 45mm 2 blade(or use IC  prop). It will probably faster than the old boat and quieter!!!!