Thanks Guys I will try that, Spence

Spence,
Do as Razor1955 suggests. it really works and I do this on all my builds.
Len

When faced with a similar problem I have taken off the coupling, found the witness mark on the shaft and then ground a small flat on the shaft with a Dremel. Reassemble and "feel" for the grub screw engaging the flat on the shaft. A dab of Loctite on the grub screw doesn't do any harm.

One thing I have noticed with my prop. shaft assembly is the grub screws don't hold very well. I managed, when fiddling about, I could pull the coupling off with the grub screw tightened up. I am always careful when tightening not to tighten too much as I once stripped the threads on an alloy coupling. Anyone else seen this problem? Cheers Spence.

The shaft can be replaced, sometimes with great difficulty. I had to do that with my Wiesel schnellboot. I built it with three shafts (per Graupner's instructions) and then changed that to the correct four shafts. That required a 5 inch by 6 inch rectangular hole in the bottom.

Lew
Florida ⛱️, USA 🇺🇸

Correction (edit): The cutout was a cross shape. Added photos in the order of the repair.

Must say the prop. tube is glued in, can't replace, Spence.

Just tried my set-up again today without the prop. shaft connected and everything worked well, motor remained cool throughout, no problems. Will now try with everything connected up. I've noticed where the prop. shaft comes out of the tube at motor end, it is at a slight lateral angle although it turns smoothly. Correspondingly the motor will have to be too. I haven't strained it in any way, I'm guessing it has been supplied to me like this, bit strange. So far so good. Thanks Lew and Chris, you were right, cheers Guys, Spence

Btw Lew thanks for the illustrations, Spence.

Lew, I was going to use the wooden coupling as a temporary measure to get every thing aligned then swap over to the flexible one as permanent. Sorry to have misled you there 🤣. I've just ordered a solid metal one. Spence.

dog bones are my go-to joint. If not use 2 u joints as Lew points out. Len

Spence, That's a new one! I never heard of anyone making a model boat coupling out of wood. For the coupling's small size and speed you most likely need something stronger, balanced, and precise.

I checked on ebay.co.uk and did see some Dumas couplings there. There ae some hobby shops in the UK that carry Dumas model boats and accessories, try checking with them. (Photo #1)

Alignment issues - difference between one and teo u-joints. (Photo #2)

Another alternative is getting two u-joints with a short shaft between them. (Photo #3)

About photo #3, that is from the drive set I ripped out of the Krick FLB2 that someone gave me (someday I hope to get back to it). I added the short shaft and new couplings. Photo #4 is an exaggerated misalignment but serves as a guide as to why two u-joints are a vast improvement over a single u-joint. (Also, I did not realize I had any MFA Torpedo 500 motors. Ha!)

Lew
Florida 😎, USA ❤️

I tried making a solid coupling out of wood but with the motor being 3.2mm and the prop shaft being 4.0mm couldn't get the hole straight. Don't think the Dumas dog bones are available here in the UK. They have a solid metal coupling to get things aligned where I shop, but it is too long for my design. I was under the impression, rightly or wrongly, that you could have a slight mis-alignment with a flexible coupling. Maybe you could advise, or tell me what else I may use. Spence.

This is why I pretty much exclusively use Dumas dog bones. I swear for them, never at them! Even three in my 47" heavy PT61.

Lew
Florida ⛱️, USA 🇺🇸

If the overheating issues started with using a single UJ that points to an alignment issue with the motor and propshaft.

I use mainly Microflex couplings and you need to ensure that alignment is accurate - to do this I use a solid coupling of the same length and then replace it with the Microflex coupling. If you use a motor mount that is screwed or bolted into place you will need to shim with washers to maintain the alignment.

Hi Lew, it isn't at the moment, had to take everything apart after the UJ came adrift and the motor overheated. Will post photo when everything assembled again, but I'm going to be using a Microflex coupling. The issues with the motor overheating began when I was using a single UJ. Hope this clarifies. Maybe I'll wait till everything is put together again before posting further. Thanks for your trouble thus far. Spence.

Also... how is the motor connected to the shaft? PHOTO please.

Lew
Florida 😎, USA❤️

MFA 500 Torpedo motor specs: Voltage: 6-12v, 13,350 rpm at 12v, Current draw under load: 1.4A, 5 pole, Length: 50mm, Diameter: 35.7mm, Shaft - 3.2mm.

Do you have the ability to measure some voltage and/or current at the motor wires? That is a common setup that I use often (Even the Hobbywing 60 brushed ESC). Different motor brand, but about the same size and I use larger props with no issues.

Lew
Florida ❤️, USA ❤️

Have you tried running the motor without he propeller to see if it gest hot?

The boat is a 1.5 Kg catamaran and the prop. is 25mm, 3 blade, sorry don't know the pitch. Meant to say my prop "shaft" is the same angle as motor about 15 degrees. Thanks for your patience Guys, Spence.

I agree with Doug. Still some missing information. For all we know you could be turning a 4 inch prop on a 60 inch, 100 pound tug. I'm sure that is not the case, but I hope you see the dilemma in trying to answer this question.

Lew
Florida ⛱️, USA 🇺🇸

All very well Spence but the vital information is missing☹️
What size prop (pitch as well if you know it)?
Rather than 'treat the symptoms'; i.e. by fiddling with heatsinks, I prefer to find the cause or causes and 'cure' the problem. As Lew and ToraDog have indicated.
Cheers, Doug😎

Thanks for the replies, will photograph my set up. The motor is a MFA Tornado 500 and has about a 15 degree tilt downwards and the prop. is the same angle towards stern. It is a 4mm greased shaft through a 4" tube. At the time I was using a Hobbywing 60a ESC.

In the past when tuning motors on RC power boats we used an adaptor on the end of a hose pipe to connect a silicone tube to the water inlet and another tube to take away the used water. This worked well for outdoor bench running.

First off, a photo or two usually helps in answering questions like this.

First I would look at possible reasons for overheating. Is the motor being driven too hard? (Big prop, free/stiff running shaft, u-joint binding...) Is this a high speed boat? Is the ESC overheating? Does it overheat fast or gradually? Give us more info.

Water cooling isn't too difficult and probably the most efficient.

Lew
Florida ⛱️, USA 🇺🇸

I fitted a couple of small PC fans on mine🤔

First off, I would not expect to run motors very long when bench testing. Besides, they would not be under load. They should not get very hot.
A simple coil for a water jacket with a pick up aft of the prop should provide more than enough cooling.
Then the is the question of whether or not you may be overloading the the motor with too much pitch and or diameter of prop. Not knowing the application, I can not answer that.