Hi Doug thanks for clearing that up I bought the standard type couplings and thought I must have got that wrong as well
regards fred

Hi Roy, to dispel any possible confusion; by 'double jointed' I mean the true Cardan link. Which is in 3 parts connected by 2 UJs. invented by an italian mathematician and engineer several centuries ago to transfer rotation from one axial shaft line / level to a different (within reason) line without using gears.
They inevitably create more friction by applying more side pressure to both input and output shafts, thus more wear on the motor output bearing and the tube input bearing 🤔 Plus friction and rattling/grinding generated in two 'out of line' UJs.
If, as they should be, your motor and prop shaft are in (near) perfect alignment you don't need a Cardan!
My maxim is 'less flexibility - more alignment' 😉
I like the coupling in the pic; called Stegkupplung in German, don't know the English for that😉 They are flexible but not floppy and totally silent👍
I once made the mistake (laziness🤔) of using a Graupner Cardan to couple up a geared Decaperm to the shaft in my Sea Scout. Never ever again 😡 After the first trials I took it apart and modded the motor mount properly. The original motor (perfectly fitted by my Dad) was a Taycol Target - the one I have recently modded for controllable forward and reverse. That will now be used in an ancient Billing Fish cutter inherited from an Aunt.
Cheers Doug 😎

Doug,
Regarding your comment, "Avoid using double jointed couplings, they waste power and create wear and noise". Can you please explain this in more detail? I have heard this comment before, but my experience has been quite the opposite. No noise, no wear (yet) and no noticeable loss of power, presumably evidenced by the couplings getting hot.
Roy

Hi Doug 3.45 am to hot to sleep as you said I have the motors so will give it a go only half way through the build but will let you know when it takes off if you see a bright light in the sky it may be the boat
regards fred

Hi Dave, Yep, agree, to a certain extent. I have also had surprising results with 385 / 400 sizes; for instance with my 110cm heavy and cramped submarine. On the surface it outruns most boy racers 😁
Not exactly scale but all good fun. A little down angle on the forward planes and it throws up a beautiful handlebar moustache of water 😊
Not too much angle at speed or it sticks it's tail in the air 😲
No, the main question was that Fred already has the 700s so I simply suggested a decent match, 5mm shaft and so on.
If he wants to spend on more motors fine. the the 3mm shaft would surely save weight, mostly through the smaller tube. Don't know the rest of the detail of the boat; beam, draft, safe waterline etc, but I would have thought a 3 footer would have a reasonable carrying capacity, like my 3 and 4 foot warships. Most of those run on multiple Speed 600s. And there I have the usual warship high length to beam ratio and associated stability problems!
Like I said; he pays his money and takes his choice. Personally I would give it a whirl with the 700s since they are in the box! Maybe though with a 3 or 4mm shaft and appropriate coupling so he can adapt later if he wants to. Would still think a pair of 35 props would do the trick.
Whatever, have fun Fred. Look forward to the Sea trials Report!
(Wrote, read and commented enough of 'em in my old job!)
Cheers Doug 😎
Oh no! Yet another thunderstorm just started, my terrace is already swamped 🤔

Hi Doug
I agree a 600 is probably too heavy. Looking on Cornwall Model Boats site the motors sold for the kit (currently unavailable) look more like 400 size.
I have used JP Power 480 motors in small craft and they are very punchy, light 92g and will work very well on 7.2v with a shaft diam 2.3mm.
I agree the Raboesch prop shafts are good but to keep the weight down I would be looking at 3 or 4mm. Prop Shop have three bladed racing props specifically for this motor.
Dave

Hi, The 600s strike me as very high revving for direct drives in such a twin set up.
The the alternative would be the #1793 600 Speed. Remember Fred it has only a 3.17mm output shaft. Weight difference is not dramatically less, 15g. (250-235)
Power is only 2/3 (60W) so it will probably 'potter'. Current draw at max . eff. is 1A less (>10%). So I don't see that the 700 needs a significantly bigger / heavier battery - don't have to put the pedal to the metal ALL the time 😉Anyway; Fred already has the 700s, 6316s! 👍
Whatever you do Fred enjoy it not regret it. 😉
Cheers Doug 😎

Thanks Dave taken onboard

regards fred

Hi Fred
For a 3' boat the 700 size motors and required batteries will make for a very heavy model. The table Doug has provided indicates a speed 600 size motor would be more suited and would certainly be about half the weight. I have two 700 motors in a 4' hull for a RN Exploit model which does require the weight and can carry the large capacity batteries. Each motor can and does draw about 15-18 amps.
I do not know your model but if it is anything like the Rivas then weight and internal space will be at a premium.

Here's the whole Graupner Speed spec. table so you can see the other parameters and headers.
Cheers Doug 😎

Any time👍
If you're not making a racer out of her but a Gent's Runabout 2x35mm 3 blade will probably do the job nicely at 7.2V. The sintered bearings will probably also do the trick, keep 'em lubricated! Happy sailing 😎
Shaft alignment will be important, as always! Avoid using double jointed couplings, they waste power and create wear and noise 😡 Minimum flexibility and maximum alignment 👍

Hi again Doug batt 7.2 and the motor 6316
i will take your advice and go with the Raboesch again thanks for taking the time to help a amature

kind regards fred

Hi Fred, I take it you have the 91W version (#6316) see pic for specs.
It has a 42mm dia. case and a 5mm shaft.
So I would try 2x 35 to 40mm max. props.
Shafts: 5mm, if you can, find one with needle roller bearings, try Raboesch (the most expensive of course! But very good and robust.) if you are going for 'all out speed'! Zoom zoom 😉
http://raboeschmodels.com/index.php/en/
If not ball bearings, last resort sintered phosphor bronze bushes.
Fit an oiler if the tube doesn't come with one.
Will you use a 7.2, 9.6 or 12V battery (or a LiPo somewhere in between!)?
Good luck and happy sailing, Doug 😎

Hi again Doug I meant to say the boat weight is approx 4 or 5 pounds

Hi Doug the motors (2no) are graupner 700 turbo the boat is about 3ft long not sure of the weight at a guess when finished 4\5 pounds ?
kind regards fred

Hi Fred,
How long is a piece of string??
A bit more info is necessary! 🤔
What type motors?
How big 'n 'eavy is the boat?
Cheers Doug 😎