It depends what diameter and length your 800kv motor is (ie 2836, 3548, 4240, in-runner or out-runner etc) and the min and max voltage ie 2cell, 3cell 4cell etc - (8v 12v 16v - for practical purposes) If you want to run a bigger prop use a lower voltage and a smaller prop, a higher voltage. Both in runners and out runners will get hot if you run them slow, and with both in boats (as DG mentioned previously) water cooling is really advisable. In-runners have cooling jackets and fans available and out-runners have water cooled front plates available.
The larger the diameter and length of the motor generally the more torque is available (longer motor=more windings, longer rotor, more torque) bit like a 4cyl and 6cyl petrol motor. Your choices with electrics are endless. You should only need a 30A ESC (I use the Hobby King 30A car ESCs (these have a cooling fan output if you find you need one) in my HSL (2x 2845 2000kv brushless in-runners,- 1 ESC per motor on 2cell 2200mah LiPos ) and they don't even get warm (30mm 3 blade props with a high pitch - around 40mm I think - Chinese from AliExpress and very good AND cheap.
Choice of prop size and pitch is also a matter of experimentation. No good running a large diam (say 50x20) prop at 16000rpm when you could run a 30x40 prop and get more thrust, (if you get my drift) better to run a 50x40 at 5000 for eg. Hope there is something of use here. Have posted some eg s of cooling types (silver one is o/runner front plate cooler) and the 30A ESC mentioned.

Hi George,
Yes. I ran mine on 12V without problems but not all day long at full bore!💥
Remember, even if the batt attached to the ESC is 12V you are not always applying the full 12 to the motor. (Or are you?😉 A 'pedal to metal' kinda guy?)
Even at full bore the supply voltage to the motor is pulsed by the ESC so the average V applied is less than 12V.
Just in case start with a 7.2 / 7.4V accu (NiMh 6 cell / LiPo 2S).
You should be able to go to 9.6V (8 cell NiMh) without problems. Check the motor temp with your finger tips on testing. If you can keep your digits on the motor without screaming you're OK😊 As said by others as well; I'd take a long hard look at the prop and shaft alignment as well. Dodgy's tip with the spring balance pull testing is good too. I've been doing that using a redundant fish scale. Gives a good (if relative) indication of push and pull.
I also use a little Wattmeter as well. Useful for initial shaft alignment. Twiddle things around until the motor runs sweetly and the Wattmeter shows the minimum current.
Before each test use a little accu capacity meter to make sure the accus are up to scratch and the playing field even for each test. Pics show the ones I use. Available all over the shop, e.g. FleaBay😉, for peanuts. Black one is checking an NiMh, white one a LiPo.
Happy tugging. Doug 😎

Cheers Doug thanks so you think even if the motors state 6v they should run be able to run 12v then if needed

Bollard pull now there is something I never thought of I will give it a try thanks again for all
the help and advice I will still measure the boat and props for my own peace of mind I will post a couple of pictures up aswell 👍

Hi Georgeo,
I can confirm that the Decaperm is a torquey low revving motor and has a 1:2.75 one stage gearing. It's a low speed plodder with plenty of torque rated for props up to 50mm.
Look for a 50mm prop with a fairly coarse pitch.
You can run it at up to 12V with no great problems as long as motor and shaft are well aligned and lubricated to minimise friction and power loss.
I had one in my Sea Scout which produced very sedate performance. The Sea Scout now has a 1000kV brushless.
Two of these Decaperms should give a good amount of 'tug' to overcome considerable wind.
I'm wondering if the superstructure of your ship is offering too much 'attack area' to the wind!
I also attach a table of typical brushed motors if you are considering alternatives.
Look for one with low RPM off load. They are the more 'torky' ones😉
Bon chance mon ami🤞
Cheers, Doug 😎
PS: I think you meant Smit Zweden
http://www.shipspotting.com/gallery/photo.php?lid=1588898

So we have no precise way to measure increased power... It might be an idea to do a bollard pull with a spring balance before you start doing any alterations to obtain a base figure. I don't suppose what you were noticing was just battery run-down, was it?

A spring balance and a watt-meter should give you many happy hours of making slight adjustments and measuring them...

Basically I was towing over the weekend and the slightest bit of wind and I couldn’t pull the tow back the tug didn’t have the power to bring it back inline

These are the cheap ways of increasing power. You might also check that the transmission of power from motor to prop is running easily and with adequate lubrication, so that no power is lost in this way.

Why do you want to increase pulling power? Are you interested in competitions, or do you have a load that you can't pull fast enough? How would you measure any increased power?

Dodgygeezer thank you very much for all your help I will measure the boat prop and boat when I get to n snd defo will checking how many blades it has 👍

The Decaperm 6v geared is 1:2.75. And very high torque, so it ought to swing a pretty big prop. Measure the prop diameter when you get home as well, also the number of blades.

Have you considered experimenting with different propellers? That can radically change a boat's performance with little effort. Props can be expensive, but you can buy a few cheap ones, or borrow some from a club colleague for experiment. Either increase the diameter or the pitch (or both!) if possible, or go for a 4-bladed one rather than 3-bladed....

I don't know about a Kv/torque relationship - the key distinction is inrunner (high speed/low torque) vs outrunner (high torque/lower speed). Although that is a very general statement, and almost certainly untrue for some motors. KV is a measure of how many revs per volt you can expect. So an 800KV would give you 4800 revs at 6v - not a lot, though that depends on the prop, of course. I normally go for around 1000Kv, and run at 7.2v.

Voltage is important for a brushed motor - the brush gear is designed for a particular voltage and will wear badly with increased volts. Brushless do not have this problem, and can run on a wide variety of voltages. Running at 24v is quite possible...

The ESC you want for a brushless is simply one that can handle the maximum current your motor will draw - which you can either look up or measure with a Watt Meter. The key concern I usually have with brushless motors is thermal. They are normally designed for aircraft use, where they will get lots of air cooling, and we are putting them in sealed enclosures. Unless you want to water cool, I would suggest running a brushless well down below maximum power, to keep the temperature down. So overspec it, or run with a lower than maximum voltage...

Yeah I understand I will measure the boat when I get home and yes the tug is twin screw the 2 motors are decaperm geared motors not sure of the gear ratio as I understand the lower the kv of a brushless motor the higher the torque so hoping someone might be able to help me work out what size speed controller would be required to run a 800kv brushless motor

Not a lot. I couldn't easily find any data on the full sized boat on the web, so I don't know how long a 1/32 model of it is. Perhaps it's 36"?

You have 2 motors - does that mean two screws?

A '6v motor' is not much of a description - do you know the make? You might simply be able to run them on 7.2v or more to increase the power. I doubt whether changing the ESC will be necessary, though of course we will need to know what that is to estimate whether it can take more power....

Thank you for replying basically I have a Smith zweden which is running on 2 6v motors which have not got enough pulling power. I think the scale is 1/32 scale does this info help?

Without much idea of the tug size and load we are rather working in the dark. Here, for instance, is a 12" tug being made...