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[Dave_S.]

I have just acquired a second-hand 46" Crash Tender in reasonable condition. It is currently fitted with a tired looking MFA 850 motor driving a single shaft. My only previous experience of this size of model was a Veron Vosper Fast Patrol Boat of (I think) 52" length which had an MFA Barracuda driven by a pair of 12v lead acid gel cells in series. It performed reasonably well, but looked like the full size would have done at less than half throttle. It was also very heavy to lift out of the water!


That was years ago, before the model miracles of Brushless motors and LiPo batteries. I am looking at a 380 kv outrunner which I will drive with a pair of 3 cell LiPos in series, which should give a maximum prop RPM of just under 8,000. I don't want to drive it around at full throttle all the time, prefer a more prototypical cruising speed, with an occasional burst of power for fun.


I'll be using a 90 amp ESC and 6000 mAh packs. The motor is supposed to be good for up to 90 amps, but doubt that I'll need to prop it that much - that'd be approx 2.5 hp. I seem to remember these going well on Merco 60s back in the day, probably about .5 hp - but it's always better to have more than you need than to be pushing everything to the max, I reckon.


Does that sound like a set up that will perform as I want it to? Max speed would need to get the first third of the hull out of the water, as the full size would have done on its way to an emergency.

[red181]

my opinion, the kv is too low


I had the 4 foot aerokits fireboat, a pair of 800kv ourunners,direct drive,  with 4 cell lipo per motor (twin screw) 25 mph on gps, and planed as it should. You will need good c rating batts, and good mah, and expect about 15 mins running

[Dave_S.]

Thanks for the info, I am a bit in the dark here. Most of my modelling experience has been with aircraft, and most of that was with IC power. We would usually prop engines for general sports use for around 10k RPM, so was using the same assumption.


My Crash Tender is already built with a single prop, converting to twins screw, though desirable, would be too much work. I was considering using a lower kv and more cells to get the power without excessive current drain. It wouldn't be on full throttle much of the time - just when it needed to get a move on.


I'll have a re-think - thanks again for the suggestions.

[Leaky Bottom]

What's the biggest size of prop you can physically fit to the boat as that may be the factor that determines the kv of the motor you require, a small prop with a low kv motor such as the 370 you suggest will not give the performance you want

[Dave_S.]

Good point - I'll measure that and get back here with the result.


My aim with the lower kv motor is to use more cells in order to get a decent amount of power without excess current draw, plus the memory of my Veron Vosper's performance on the Barracuda and 24 volts of gel cells.


I've plenty of time to gather information and ideas before committing my credit card - both of the second hand boats I bought 2 or 3 years ago required re-motoring, it's not something I want to do too often with any model, so would rather get it right first time. Too big a motor can easily be run on fewer cells or a smaller prop, too small means more surgery to the model.

[Dave_S.]

The prop fitted to the model when I bought it is a 50x two bladed, there is sufficient clearance to go to 55mm, but that's the lot. I am not sure what voltage the builder ran the MFA 850 on, I have yet to untangle the battery to ESC cables, but it seems that he used 4 packs. The MFA 850 is a 12 volt motor according to orders still advertising it, and can be run at up to 11 amps.


If I use a 500 kv motor and 6s LiPo pack, the theoretical RPM would be approx 11,000. I suspect in the real world it would be between 9 & 10k on the water on the rare occasions I used full throttle.


Looking at old ads for Aerokits boats, the 46" Crash Tender was sold as being suitable for 2.5-5.00 cc engines, whereas the Sea Queen, same length but 1 1/2" more beam was advertised as being for up to .60 glow - though I saw plenty back in the day with larger engines, including petrol of at least 20cc.

[madwelshman]

These are some that I have saved as reference for some of my boats.
Sea Queen and they were very pleased with:
Surpass Hobby C4260, 600kv, 125amp esc, 4s, 55x prop.


One that someone had in a 47" Perkasa:
Hobbyking T600 (kv unknown), 180amp esc, 6s, prop unknown.


42" Veron Huntsman:
SSS 5964, 800kv, Alien 200 esc, 4s, prop unknown.


Will

[Dave_S.]

Thanks, that helps me narrow down the choices. I'm not in a great rush to get it on the water, next spring will be fine, plenty of time to weigh up the options.

[Dave_S.]

I settled on a 500kv motor running on six cells which should provide sufficient urge.


I ordered the motor and 90A ESC from Hobby King in Hong Kong on Friday, and it was in my hand this morning! I paid a few quid extra for DHL Express as in the past I've had to wait 6-8 weeks. The extra payment is definitely worthwhile!


Here is the new motor in my grubby mitt with the boat behind. I hope to start on the removal of the MFA 850 and installation of its brushless replacement as soon as I have cleared enough space for a 4ft boat.

https://flic.kr/p/2mBYe99

[canabus]

Hi Dave


I have a 450kv on 6S Lipo batteries( 2 x 3S 4000mah) in my boat and it goes very well !!!


I had to place the batteries each side of the rudder to get it to sit up on the plan right.


Running a 2 blade 52mm prop.


Canabus

[Dave_S.]

Thanks, Canabus, that's reassuring as some people seemed to think the kv would be too low. I think a lower KV and more volts is better - also means I can get away with a lower current to prolong time on the water and reduce heat build up - wouldn't want my Fireboat to catch fire!


The ESC is water cooled, so should avoid scorching the plywood.

[canabus]

Hi Dave


No watercooling and the ESC is a car one with a fan on top.


Canabus

[Dave_S.]

Do you know what the max current used is? On a 6S pack, 35 amps should mean over one HP - probably more than the average .45 two stroke that was common in the late 60s, early 70s.