No need to be gutted, these motors are fine if treated just right
.. Firstly for direct drive you will need 20amp Vipers not 15s ( past experience with two single shaft 37" Perkasas. One had a 15 amp Viper, one had a 20 amp Viper, both had these Johnson motors on direct drive, 35mm 2 blade 'S' props and 7.2v 3600 mAh NiMhs. The 15 amp equipped model was just that bit slower until we swapped over speed controls. Both achieved run times of about 14 minutes, not bad for pre-brushless times. Both models got up on the plane..both models were the glass fibre variants so slightly lighter than the wooden ones.).
Second, as stated above, 6v is just that bit low for a supply voltage. Under full load the speed controls are tripping out as your available voltage is dropping below about the 5v mark. That or your Rx is dropping out and loosing bind temporarily ( had that last week with a little RNLI Lifeboat, those fifty quid things. I stuck two ex-coaxial heli motors in it for a set up on 4.8v..NOT the best idea. Ran like a mad thing and kept dropping out and re-binding after a few seconds, too many amps by a long way! Now happily equipped with a pair of MFA 280/1 motors, sweet as a nut..).
These motors have had a bad press, not least because of some of the less scrupulous people suggesting their suitability for 6v to 12v operation, my advice, don't exceed 7.2v on direct drive, after that slap them on some sort of reduction drive gearbox. I have run them up to 14.4v on Robbe 3.3:1 reduction drives with 42.5mm 2 blade props, pulling about 6.5 amps in the water. i am also using them on 5.5:1 reduction boxes in a 68" landing craft that I am slowly building, that is running on a pair of 6v 10amp gel batteries, one per motor.
lots of motors can cause problems. Only a few weeks ago, at a model show, I saw a scale 'grey funnel line' battleship with four Graupner 600 12v motors, a bit extreme for a five foot boat I thought. However the local expert had recommended them as he had used the same set up in his much faster boat and all was well.."anyway.." said he, "you can always just not use all your throttle.."
Good advice..better advice would be to put the right motors in, but I digress..to quote a great man...
Suit yourself