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[Leaky Bottom]

I've got a 29" Riva sports boat with two speed 400 size motors, I used it for the first time yesterday and whilst the performance was what I was expecting after a couple of minutes running the motors got so hot they almost seized up then when cooled down again they ran again.
What size props would be recommended for these motors running on a 11.1v lipo.
The motors are MFA 457-RE385 6-15v

[inertia]

No more than a 2-blade 25mm diameter one. That said, you might be better off swapping the 385s for 380s, which are much faster motors (x2), and running them on 2S. You'll get change from a tenner for a pair of these, and they have the same mounting dimensions and shaft size as the 385.   http://www.componentshop.co.uk/380-dc-motor.html
DM

[bj]

....with two speed 400 size motors, ..... and whilst the performance was what I was expecting after a couple of minutes running the motors got so hot they almost seized up then when cooled down again they ran again.
What size props would be recommended for these motors running on a 11.1v lipo.
The motors are MFA 457-RE385 6-15v

Am I understanding you correctly ........ Are you running brushed motors on a LiPo battery? If so who's ESC are you using?
 I thought that only MicroGyro ESC's were able to be set to use LiPo and brushed motors, maybe I am wrong here.
Anyway, for what it is worth see http://www.modelboatmayhem.co.uk/forum/index.php/topic,37609.msg377048.html#msg377048 relating to BRUSHED motors

[malcolmfrary]

LiPos are just a source of volts and amps.  Motors do not care where they come from, neither do ESCs, just as long as enough volts at enough Amps are available.  LiPos, if the right voltage can be hit using them, can, for a given size, provide a lot more Amps. 
The only thing to beware of is how delicate LiPos are regarding over discharge.  They do need at least a warning circuit adding to monitor their state of charge.  Some ESCs contain a circuit to do this and restrict performance as a clue to end the session, some need an extra card adding to give an audible warning.

[Leaky Bottom]

Am I understanding you correctly ........ Are you running brushed motors on a LiPo battery? If so who's ESC are you using?
 I thought that only MicroGyro ESC's were able to be set to use LiPo and brushed motors, maybe I am wrong here.
Anyway, for what it is worth see http://www.modelboatmayhem.co.uk/forum/index.php/topic,37609.msg377048.html#msg377048 relating to BRUSHED motors

Yes, using lipo's and brushed motors with a jeti jes 350 esc

Use a fully charged battery at first then run it for a few minutes, recharge it and see how many milliamps go back in, do this a second and third time increasing the run time each time, this will then give you a indication of how much run time you can get from the battery without over discharging it

[inertia]

It would be a lot less effort to stick one of these onto the balance lead of the pack http://www.componentshop.co.uk/led-battery-status-indicator-alarm-for-1s-to-3s-3-7-11-1v.html

The Microgyros FR40 for brushed motors has a low-voltage cut-off to prevent LiPo packs from discharging too low. The MTroniks Tio Viper 15A ESC is described as "LiPo safe" and has a selectable battery type (NiMH or LiPo) BUT I can't find anything either on their website text or in the manual for the unit which says that it will automatically slow or shut down the motor if the pack voltage drops below a safe level. Strange, that. I don't know of any other ESCs for brushed motors that have this facility although it's pretty much standard on brushless ESCs.
 
  I understand that there will be a new ACTion brushed motor ESC with a list of features as long as your arm (including BEC...) and that automatic low-voltage cut-off is just one of them. No further details are available i.e. I haven't seen one yet! 

DM

[jaymac]

The MTroniks Tio Viper 15A ESC is described as "LiPo safe" and has a selectable battery type (NiMH or LiPo) BUT I can't find anything either on their website text or in the manual for the unit which says that it will automatically slow or shut down the motor if the pack voltage drops below a safe level. -
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I emailed  them to query  that and never even got a reply
Jay