Twin Cat Strut assembly on a budget -> help please ;)

[norbique]

I'm building a 1.1m cat from scratch and as I progressed, I reached the building phase where I need to have the strut assembly to make certain mods in the inside of the hull and so on.

I was looking for struts of many kinds, (on ebay, on swissmodelpowerboatteam, etti, hydroandmarine, astecmodels, etc, etc) and I'm still on the same opinion, I can't afford to pay €150 for a set of Arneson Drives. I can't, so I have to start thinking about how to make these drives on my own. I can use the lathe and the milling machine in the local club. Although I don't have too much experience with the mill, I think I can master the knowledge to produce a strut. If nothing else, this project will help me become more experienced.

I'd like to ask those who had done this before, to help me with their knowledge.
Some technical drawings would be nice too. :) Just to help me understand how these drives really work and look like from the inside. Do they have bushings on the prop side and bearing on the flex side? How is the whole thing sealed at the hull, yet still possible to move up and down? All your thoughts are welcome.

Thank you!
Norbert

[norbique]

Just a picture of the cat.

[Flying Scotsman]

First off cudos to your scratch build cat with a stepped design. If you are short of cash, I would suggest a single drive, what motors, power, speed and run time are you considering?

Douggie

[norbique]

Hey Douggie!

Thanks for the reply!
My biggest problem is I want the cat to look more or less scale... That said I am thinking in a twin setup. This is the problem. I'm aware of the fact that twins cost roughly twice as much a single drive setup does. Still I want to make a twin cat, (never had one before).
I was searching the Internet for various motors, ESC, and since I can't afford a NEU a Plett, or a Lehner, I gotta stick with the "low-end" chinese motors. I was thinking of the KB-45L perhaps the 10 turn version which is 1800KV, and power it with a 6S Lipo.
I have a small 3S A123 pack that I built and use it in my Hydromite and I love it. So at the beginning I was thinking of getting A123 cells for the big cat as well, but my maths quickly showed me I'd need roughly $500 for the 2x 8S2P A123's. Which is a LOT of money. So I think I'll have to stay with thecheaper "fiery" Lipos.

I downloaded the FE calc, and the modded database, plus added my own stuff. I'm playing with it, but it has no feature for a twin setup.
There are other motor alternatives as well, the Feigao 580XL8 or there are the beautiful 6pole 2240 Tenshock motors @ €150each.

My goal was to reach 100km/h, but I would settle with anything above 80. Run time is not really important.

FEcalc shows me around 1500W input needed for that speed, don't really know what a twin setup should say power wise.

Norbert

[Flying Scotsman]

Ok, so two motors each on 6S power and what ESC's? This is getting expensive

Douggie

[norbique]

I was thinking of the Turnigy's. Either the 80A plush ($55) or the 100A ($65). I'd modify them and add water cooling as the first step, then some extra capacitors. And try to prop it so to stay within the limits of the ESC. FECalc shows results around 70A.

[astecme]

Norbique, what about using 4mm or 3/16" stinger units - e.g. HOR/TFL or ETTI (4mm only).

Allan

[G Doggett]

If you can settle for a top speed of 'only' 90 kph why don't you use the set up I have in my twin Jolly.
2 Ammo 2300 motors at $67,99 each, and 2 X-Power 100 ESCs at 74.99 each, all from OSE.
I power this with 2 4S 5000mah lipos and the average current draw is 50 - 60 amps per motor.
I have scratch built my own drives from stainless steel tubing, but you could maybe use a pair of SuperVee units.
Graham.

[norbique]

Hi Graham,
I found the picture showing your SS tube drives, it looks really nice but can you tell us more of the building process? Did you make bronze bushings on both ends? What diameter flex shaft did you use and where did you get the counter rotating flex shaft from? How did you seal the part where the shaft enters the hull?

I am thinking of putting SS bearings on the motor side. Maybe bushings on the prop side. I was thinking of getting two pieces of brass and drop it in the lathe to make the right shape.

I am thinking of getting lower KV motors and use higher voltage. 6S perhaps.
The Ammos look really nice, and those 100A ESCs would be Ok if they could handle 6S lipos. On the other hand I'd have to pay $45 only for shipping these.

Allan, do those HOR/TFL or ETTI flex shafts come in counter rotating versions as well?!

Thanks!
Norbert

[G Doggett]

Hi Norbert
My drives are K and S stainless tubing with mounting brackets welded ( easyflow) on the top. If you have found the pictures this should make sense.
The mounting brackets take the thrust so the tubes are just forced into the back of the sponsons through rubber sleeves that allow a bit of up and down adjustment.
I used r/h and l/h wound flex cables from OSE with 3/16 stub shafts and r/h and l/h Octura X442 props also from OSE.
The 'bearings' are simply brass tubing with lube slots cut in them running inside another brass tube that in turn is pressed into the stainless tube. Does that make sense ?
Graham.

[norbique]

Yes it makes sense Graham. You you basically have a brass sleeve running full length of the SS tube acting as the bushing right? Interesting approach! Do you have Teflon tubing inside the brass where the flex cable runs?

How long should the strut be for this 110cm hull? Or how do I calculate it's length?

[astecme]

Allan, do those HOR/TFL or ETTI flex shafts come in counter rotating versions as well?!

They can. I do not list the LH lay for ETTI but do have LH lay 130 cable in stock. That is probably a bit light for you.

The TFL/HOR ones can be fitted with both Lays. - I do not stock the reverse lay but HOR are selling a pair of cables on Ebay. Johnny is very good and will I am sure supply a handed pair.

[G Doggett]

Yes it makes sense Graham. You you basically have a brass sleeve running full length of the SS tube acting as the bushing right? Interesting approach! Do you have Teflon tubing inside the brass where the flex cable runs?

How long should the strut be for this 110cm hull? Or how do I calculate it's length?

Yep, you got it. The stainless tube is the same diameter as the drive dogs, and is sleeved down with the brass tube that is fixed inside.
The brass slotted 'bushing' is only the length of the 3/16 stub shaft and can float or spin much the same as it does in a Speedmaster stinger.
The teflon tube is pushed into the inboard end of the tube until it is just short of the stub shaft and bushing.
I think from memory in my case the teflon was a bit of a sloppy fit in the tube so I sleeved it up with some heatshrink to be a nice push in fit.
Works great.

The rule of thumb for strut length is 10% of hull length to front face of prop. So for you 110cm hull the strut should be about 100mm.
Graham.

[sailr]

You can run inside just the brass tube and forget the teflon unless you need it for ease of strut adjustment. Most of the 'go fast' boys are running without teflon with great success. Less drag.

[norbique]

Thanks M. Sailr, I've heard this somewhere before and was about to ask anyway. So just run the flex shaft through the brass tube only, right?! When doing so should I pump the brass full with grease?, oil? what kind? Or just put some grease on the flex prior to sliding it in the brass tube?

Thanks Graham, I'm asking all these questions because I've never had a factory made strut or stinger in my hands. So I can't really imagine how they work exactly. I only made my own strut for the Hydromite, but that's a very simple unit, brass shaft running in brass sleeve. I grease it every now and then, and it holds up pretty well.

Another question: maybe I'm over "refining" (read: complicating) the strut and I was thinking of adding the SS bearing on the motor side of the strut, to support the prop shaft where it mates with the flex. Is it worth the trouble?

Also how do I know the diameter of the drive dog I'm going to use? I'm thinking of using 4mm prop shaft, so tell me what outside diameter drive dog goes there?