If we can believe the specification this ESC can withstand a continuous current of 280A. I smell Chinese amps again! Even if we divide that by ten it should be meaty enough for your 850. MFA rate these motors at 11A but I've seen them blow 20A ESCs without so much as a "by your leave". I would follow Malcolm's advice about fitting a progressively heavier fuse until you find one that doesn't blow. I'd start with 15A, and would recommend auto blade fuses rather than those spring-loaded cartridge types (which have poor contacts).
Dave M

To put 280A into perspective

My 5 litre diesel 4×4, which weighs 3 tons , has a maximum current draw of 120A when cold starting. To provide this level of power the car is fitted with 2 huge batteries and the cables to the starter motor are 20mm in diameter.

To understand how powerful 120A is: 

If the starter motor turns for more that 30 seconds it automatically shuts down to prevent it from burning out.

Edited By Paul T on 03/02/2013 06:48:22

Good Morning Alun

I also have to admit that the dark art of electronics is something of a mystery to me as well and that I am far more comfortable with IC.

I can say that after bitter experience with a Chinese ESC I always buy British manufactured equipment which always works and as they say 'does exactly what it says on the tin'

For expert advice on anything electronic I consult my good friend Dave Milbourn (resident of this forum) who is the dogs dodahs when it come to anything more complicated than changing a fuse.

If you talk to Dave dont forget to ask him how the new workshop is getting on.

Paul

Alan, when your boat is in the water is it lying evenly balanced fore and aft ? Those 12 volt 7ah SLAs are quite heavy (about 2.5 kg) and you might get improved performance by using similar capacity NiMH battery. I've got a Nautical Marine HMS Biter model which is smaller than the Sea Queen. It does get "up on the plane" but I know from measurements that each motor (twin shafts) is consuming about 10 Amps at 7.2 volts which is approximately 70 watts. Assuming an efficiency of 70% then actual power into each propshaft is about 50 watts – which is 2 x 50 = 100 watts total.

According to MFA's own data the MFA 850 motor will deliver 80 watts output with a 12 volt supply with rpm = 8300 at an efficiency of 62% (which implies that it will dissipate about 50 watts heat !!). Given that my boat is smaller than yours maybe you need a motor capable of giving more than 80 watts output to get it planing. Perhaps Larry could say whether his boat planes and what motor and battery he is using.

Regards.

Ian

**LINK**

…. and here is another link where someone is asking what motors and batteries should be fitted into a Sea Queen. If you really do want fast performance then it is feasible but comes at a price – the run time is going to be short. One other point – what is the scale of the Sea Queen and what is the top speed of the "real" full size boat ? Many people run their model boats in excess of scale full speed.