"suggested 600 motors"
Hmm! That depends on WHICH Speed 600 was suggested!
There are several versions, from 7.2 to 12V and off load RPM from 14300 to 21500😮 So which spec to use as a basis for choice of a brushless???
My choice would be a pair of 750 to 1000kV brushless.
Prop size significantly affects current drain and speed.
Often a bigger prop simply sucks more current with no performance gain.
Maybe the contrary in fact.
Size of brushless motor:-
They are quoted as xxyy, in my Sea Scout 2830 (Outrunner by 'Propdrive' HobbyKing). That means 28mm diameter and 30mm stator length.
The larger the diameter or the longer the stator size the greater the torque but usually lower kV. (Revs per Volt OFF LOAD)
So for tug pulling perhaps a big fat, long outrunner with kV 250 or so.
For max speed a short, slim motor for hi revs for the competition racers.
I reckon for your Lifeboat a decent compromise between start torque (to overcome the boats inertia) and a good speed would be a pair of something similar to the 2830 1000kV I used in my Sea Scout.
I only had one in a 28", 2kg boat, your boat is bigger 'n 'eavier so two should do the job nicely😊
Also recommend a pair of the Quicrun ESCs to drive them. Type Hobbywing Quicrun 16BL30 or current equivalent. They're good for 30A but I don't think the motors I suggest will ever draw more than 10A in your boat, WITH THE RIGHT PROP SIZE! Prob. 35 - 40mm MAX.
Get the Programming card to go with it as that easily enables you to switch off the irritating Brake function and to set the Start to Soft, which eliminates the problems Rowen had with 'jump starts' and slow speed manoeuvres.
I run my Sea Scout on a 4Ah 3S (11.1V Nom) battery which gives me well over an hour runtime and great performance. See my Boat Harbour, Blog and Video Gallery for trials videos!
If you can fit one similar battery per motor I think you'll be a very happy sailor😊😊 You can always throttle back!😮⚡😭
Enough ramblin'😮 Hope this helps more than confuses 😉
Pics show my Sea Scout 'Engine Compartment' and differences between old motor and prop and the new😉
Bon chance mon ami👍
Cheers, Doug 😎