Redoing carbon electric outboard

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allweather

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After gaining some much needed experience last year and busy with other projects over the winter I now want to redo my diy electric outboard construction. This time in carbon to save some weight and get it stronger at the same time, but mostly to get some adjustments in(needs to be 20mm smaller to pass the backstay adjuster effortlessly) and insert some proper slide bearings.

To that end I want to replace the 30mm/M30 stainless shaft that comes originally with a 19/12mm Carbon shaft(don't want it thicker than that) and would like to hear if anyone has advice how to best get an M30 thread on that shaft.

Ideally I'd love to get it in G10, but don't even know where to find something like that in Europe, so my next step would have been to have a stainless piece machined that I then can glue onto the carbon shaft. Which isn't great, but okay.

Ideas?

Picture is how it is right now, I want to have the next version to be slimmer and smaller overall so that the round/turning part can use 60mm slide bearings that I have found relatively cheaply and readily available.

2021_04_08_IMG_3454.JPG

 
Zonker

Zonker

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Google "Pultruded Rod" - but pultruded rod is NOT what you want for torsional loads like a drive shaft. You want a filament wound shaft with fibers at +/- 45 degrees.

How much torque are you putting through it? A 30mm steel shaft is big enough for 20 kW or more. A small outboard of say 3 HP will have a 3/8" diameter stainless shaft.

Bonding metal bits to carbon to transmit torque isn't trivial. I've never done it and I do know the carbon driveshaft people have had a bit of a learning curve getting it to work. They don't talk about it much. Wouldn't surprise me if they glued AND shrink fit the metal bits.

 
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allweather

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Oh, I'm sorry for my in retrospect very unclear question. Can only partly blame my grasp on the language...

When I say shaft, I mean the straight pipe that connects the pod/motor with the outrigger.(aka, what a long shaft on a petrol outboard is)
That kind of load should be addressed with normal, straight uni directional fiber, correct?

I am reasonably secure in choice of tubing there(considering I currently am using uni directional glas fiber...), what I am not sure is how to best enlarge the diameter for the larger threads used to connect that tube to the pod.

So, barely any torsional loads at all and pretty much all transverse loading if that makes sense?

As for the thrust that then acts on the tube, it is nominally 60kg thrust produced from a 1,2kW motor. So nothing much.

 
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Zonker

Zonker

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Now I understand. You really don't want a round tube. Round tubes in flow will flutter. A round structural tube with a fairing like you seem to have is fine.

Could you use carbon or fiberglass tape to attach the tube to the pod or is the pod metal or plastic?

Otherwise I'd get a machine shop to get a long M30 bolt. Cut off the head. Drill inside for your 12 or 19mm shaft. Clean all the cutting fluid out of the hole (acetone/brake cleaner etc). Glue the shaft inside the stub of the bolt. I'd go for 3-4x shaft diameter for hole depth.

image.png

 
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allweather

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Thanks for the additional advice on what to ask for at a machine shop! In particular the part about 3-4x diameter is great to know for discussing with the machinist.

The pod casing itself is metal, I suspect aluminium but don't know for sure.(no manual for that, only know that the original shaft was proper stainless steel) So I don't see myself trying to fiberglass anything there, screwing seems more convenient too...

It is basically a trolling motor like all the others(there are threaded M30 fiberglass shafts around, but as mentioned, I want to go a bit slimmer)
I agree with you on the fairing, the current plan is to go for a 20x40 oval. Which while not ideal at all as far as drag goes, is the compromise I need since this part telescopes into the upper section which in turn needs to fit through slide bearings.

Way more complicated than a basic fairing and small bearings to turn the motor, but the only way I was going to fit through the hatch and still reach deep enough into the water to be of some use even in a bit of waves. Sure, won't do me a lot of good in more than a meter or so in wave height, but then that is always like that with outboards. As is it is already at least as deep if not moreso than the long shaft Suzuki.

But I am digressing. Thanks for the advice on how to go about it again! Now I just need to find a machine shop with some time and patience ^_^

 
Zonker

Zonker

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The first one (lots more 0 degree uni).

The second is just braided +/- 60. Easy to wind. Not as stiff.

 
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allweather

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Basically that, in uni, if not for overall cost and shipping getting rather prohibitive...
I did some napkin calcs that come out quite a bit cheaper, especially when I buy leftover fabrics from outlets which is more than enough for my purpose. Well, my time not included, but that I can live with...

Right now I am testing(once I'm home again anyway) if the bearings I picked up work as I envisioned, then gather the other materials...
Though I am playing with the idea of using uni directional glass in most areas. For all that carbon fiber is miracle stuff, as I understand it at the lower end strength to weight ratio the difference isn't that big, but carbon still vastly outperforms in terms of stiffness no matter the grade.

But since I don't necessarily care too much about the straight parts bending, I need to consider if it's worth the cost for me there. Different for the "outrigger" that takes a lot of torque, biaxial carbon seems the most useful there.

 
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