Hi Kjeld, sorry, missed your message somehow. Have you been to the big pond at Gulf Harbour? the GH r/c yacht club sails there and it's a good pond with a small jetty (Regency park Drive lake). The pond where I've been running the airboat is where the Golf club used to be at Red Beach/whangaparaoa (now a subdivision). It is more suitable for electric as it is a catchment pond with reeds planted all round. There is one access which might be suitable to launch a yacht but the mud is pretty nasty and deep and could be a bit dangerous going past your ankles (nearly lost a jandal going in shin deep over reeds trying to get the airboat back)😁. Let me know if you want to have a look at it, and perhaps we can meet up and I'll show you where the best launching spot is, (99% is not easily accessible.) 02102591684.

John. (I'm just along from the fire station at Hilltop)

You can buy a universal chain and pulley set, from Maplins, if you do a site search for "geared motors, you will see both some reasonably priced geared motors, but also a chain and toothed pulley set, so no risk of a rubber drive belt slipping as it ages and stretches.

OR, so model boat shop sites do list "motor gearboxes" which may fit your motor.......

Thanks guys,I too have been looking at the aircraft belt drives but don't know anything about them. I'm thinking this Johnson motor is of the numbered motors family I see referred to everywhere so perhaps it would bolt up. I'll try to add some pix of the boat's progress soon. GJ

HI John
Welcome to the site.
Gregg's advice is spot on. 😊 At about 6' your model will look superb on the water but it needs to move at a scale speed. Good though the Decaperm is you already have a 550 motor and I have used this type of motor with a belt drive reduction unit that the model flyers used in their electric flight models. 😀 Unfortunately I don't know of any suppliers in the UK but possibly in the States you will find one. if you have any flyer friends or local clubs they may still have some discarded units, as they now prefer brushless motors such reducers are not required. 😉 Another alternative is to make your own from small timing chain belts and geared pulleys. if you are not too concerned about a belt slipping and you have access to a lathe make your own pulleys and use a Hoover type belt. The flat version works best.
As to speed, this will be in the hundreds of rpm and will depend on the pitch of your blades, the weight and shape of your model and depth below water. if the draught is not great there will be a tendency for the prop to cavitate so slow revs are essential when starting.
I have nearly completed a 1:96 model of the RMS Olympic and that is 8' long. I have three high tork direct drive motors (777) driving similar size props. Ballast is 56Kg so your model weight will be similar. if the 550 proves insufficient you can always upgrade to a 700 or higher. 😀
Why not start a build blog on this site and share your experience with others.
Good luck

To be honest, to drive a prop in excess of 35mm on such a scale model, you do really need a geared motor. 3:1 is good. 12v battery on a 540 motor, would seriously give you too many rev's and the ships speed would be too excessive.
Plus due to the sheer size of the prop, the motor would take a high current to drive it, and either burn out, or the battery would last less than 5 minutes before full discharge.
I have a Sun tug 1/24th Scale with a 60 mm prop and that uses a decaperm 3:1 ratio geared 6v motor, it gives me superb slow speed torque, allows the battery to last all day without recharge and still has plenty of power left to get me out of trouble if need arises, Plus in your case, it would allow you to ballast the ship to give better waterline appearance and still not overload the motor.

its far better to "over engineer" the drivetrain, so it lasts, what you dont want to have to do it replace the motor after a couple of sailings and then find the motor mount needs changing too, and you cant fully get to the mount to do so.