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Is it already Winter up there? :ernaehrung004: Didn't want this to fall to far down.

Autumn has arrived in Finland, next week temps drop below water freezing point. Boat is out of water and ready for winter, antifreeze in and so on. Soon I move it in home garage but it need to clean first. Now are little break from Fountain, sailing trip that was succeed, maybe paramotor flying and all other fun activities. I build one BMW six cylinder rally car race engine recently for my friend.

https://cimg3.ibsrv.net/gimg/www.off...f2f3dd3a90.jpg

Sailing with 47 foot boat in big seas was so fun that I think rent boat in next year too, autumn trip to deep blue waters continue Finland short boating period nicely. There was near ten foot waves in Greece when we were there.

But back to Fountain, I ask my friend's friend to calculate my drive shaft critical speeds, he is professional strength calculator.
Steel, CrMo and aluminium drive shafts will not work, how ever they are made, lenght is too long. Critical speed will be about 3500rpm that is way too low. Carbon shafts will work because strenght/density ratio is better than CrMo and aluminium. Two piece drive shafts get rpm up but CV joint angles will get worse, not really option. Old carbon tubes was just wrong made, too big resin/fiber ratio and fiber orientation was wrong too. Factory fabricated tubes are too expencive to me exept some nice topic reader want be sponsor and buy them. I have to made tubes myself, different method than earlier. I have to talk local composite company if they have a big enough oven and could they get right style carbon fiber for me.
I have think to make them from biaxial and unidirection fibers so tubes get more bend strenght that influence critical speed. Clothes have to assembly dry, vacuum bag and inject epoxy or make it from prepreg stuff. There is few options but this time I will test tubes before put complete shafts together. Earlier I didn't understand critical speed things but now I can calculate it myself and things are more clear to me. It's not easy learn design skills in all fields, but every other day I learn something new.

Made your own high pressure autoclave, for shaft it is wery simply and cheap.Take a seamless steel pipe with a large enough diameter to fit your shaft inside. I’d suggest at least DN300.

Close one end with at least 10 mm steel plate.
On the other end, weld a DN300 PN16 flange.
Use another blind flange DN300, PN16 as a cover.
Weld on a fitting for connecting compressed air.
Attach a fabric heater with a built-in thermostat to the steel pipe, set to 80 °C (or 120 °C if you’re using a prepreg that requires it).
Wrap it with 5 cm of mineral wool and cover it with aluminum foil or a similar material.
Wind the prepreg fabric onto the mold so that the fibers are at a 45° angle.
Tightly wrap it with peel-ply fabric, cut into strips so you can wrap it like bandaging a hand.
Add some felt over the peel-ply layer. No need to overcomplicate it — you can use felt typically used for pipe insulation in central heating systems.
Put everything into a vacuum bag made from high-temperature vacuum bagging foil, sealing the edges with high-temp Tacky Tape for vacuum bags.
Insert a DN3–5 mm plastic tube, and place a bit of felt at the tube’s inlet. Tube for air in the aquarium it is ok...

Once everything is ready, evacuate the air with a vacuum pump from a bag and then pull the tube out. The bag must remain sealed and under vacuum. If not, find the leak and seal it with Tacky Tape and repit.

When vakuum stay in the bagg insert the whole assembly into the steel pipe, ensuring the “shaft” doesn’t touch the pipe walls (use supports underneath).
Turn on the heater and pressurize the steel pipe with 7–8 bar of compressed air. Leave it for at least a day, beter two, then demold.
Make sure to release the pressure before opening the flange..

This method allows you to quite easily produce a carbon laminate of F1 or aerospace quality.
Do not just apply resin manually to the fabric — the result will not be comparable to that achieved with prepreg. It’s possible, but it requires a lot of experience with resin control and the correct resin system.
Vacuum infusion is fine for simpler laminates, but in your case I really don’t see any other solution than using prepreg. Take UD (unidirectional) fabric and wind it alternately in one direction and then in the opposite direction. Don’t use thick fabrics — go for a maximum of 1000 g and apply more layers instead. A laminate made from UD fabric is stronger if it’s made from several thinner layers, because the stresses are distributed more evenly and it won’t delaminate as quickly as thick layers do.

For prepreg, vacuum film, and Tacky Tape, you might want to contact R&G. They’re not cheap, but their products are excellent.

I doubt that local fiberglass company will have knowledge to made this carbon pipes. If they do not use high pressure autoclave just go away! Believe me, it will not work!

For more infos you could contact me on pm.

Propulsion:
The Viking 90 uses shaft drive propulsion with powerful engines like the twin MTU 16V 2000 M96L, which produce 2,635 HP each, and that boat weighs 192 thousand pounds, if those shafts can stand up to the engine torque and wave action-

Why not use an inboard shaft bought to the diameter needed to handle twist and load and to the lenght needed to machine the ends to accept whatever coupler to fit transmission and drive. Interested to know why or why not

awesome project.

do not use carbon fiber shafts on this boat its not safe theres plenty of videos of them shattering on race cars you need big heavy steel dont risk it you have done an awesome job many of us hard core boat geeks are following this build thank you

[size=13px]A carbon shaft made from prepreg in a high-pressure autoclave can withstand loads and is lighter than a steel one. This process provides predictable material properties and allows for precise strength calculations. Passenger and especially military aircraft have many load-bearing components made from carbon laminate, and there are no issues with strength for example.[/size]
[size=13px]With an appropriate safety factor, it’s not a problem — but I would still give serious consideration to using steel shafts.[/size]

Thanks for help plavutka! Method you recommended is just like my plan. Maybe I try vacuum infusion with surplus biax glasfiber clothes first to learn right way to work. I think it’s not so hard to get resin flow even if there is like ten points where resin come in. I can shut them individually.

I will use non-crimp biax so fibers are as straight as possible 400g or 600g cloth, not heavier. Tube wall will be increased from 4mm to 6mm but torque strenght is not problem, bending is and that can be made better with some unidirectional layers longitudinally.

prepreg will be best option, there are out of autoclave products in market so it can be made just under vacuum but I have to figure out how much strenght penalty it can make. Then oven can be made from thin wall steel air pipes. It’s better cure tubes vertically to avoid any bend.

please understand that drive shaft torque transfer is not problem, rpm is. You can’t compare max 2000rpm yacht shafts to 4600rpm Fountain prop shafts. 120mm diameter is max there can be used so 2350mm long tube can not spin more than 3500rpm if it made from crmo or aluminium. If you don’t beliewe please learn something about term of ”critical speed”

For infusion with epoxy you need a stronger vacuum pump and it’s not as easy as with vinylester. In more than 90% of so-called “epoxy” laminates, vinylester is actually used.
You must be very careful choosing a resin that is thin enough for infusion while still having the required physical properties.
Vacuuming the tubes isn’t simple either. In your case you must not use spray adhesive when laying up the laminate, because the adhesive weakens the laminate.
Whatever you make will contain more resin than a part made with prepreg and a pressure autoclave. In your case the torsion produces significant shear stresses, which are the cause of delamination between laminate plies. That’s why I advised you to use more plies of thinner fabric. Even with infusion there is too much resin in between and it will delaminate earlier than a part made in a pressure autoclave.
Do a test: laminate two plies of, for example, 250–300 g UD fabric placed at ±45° and after curing try to pull them apart. Very likely you will quickly split the laminate into two plies. That is exactly what torsion does. The fibers in tension pull on you, and the fibers loaded in compression lift out of the laminate. I think the propeller generates sufficiently dynamic loads, sometimes even pulsating, so that such a laminate will delaminate.
Prepreg with vacuum infusion is a better option. Because the prepreg laminate is tacky it’s easier to lay up, but the vacuum is too low and there remains too much void/space between plies for it to be homogeneous — at least in your case. It will be stronger than ordinary infusion, but you still won’t reach the designed laminate strengths. Fatigue is the main problem.
Heating the laminate is also problematic. It’s hard to guarantee an even temperature and you must maintain pressure the whole time, because I doubt you can seal the bag well enough that it will hold vacuum all day while you heat it.
Make a sample laminate as in the infusion test (two plies) and try to delaminate it. It will be harder, but you’ll still manage.
Prepreg with vacuum infusion plus a pressure autoclave gives you a homogeneous laminate that you cannot delaminate in a simple pull test.

The best indicator of what I mean is laminate thickness. Check the thickness of your test laminates and you’ll see how much thinner the autoclave laminate is.

Believe what I’ve written — I have many years’ experience with laminates.
For business we use a lot of hand-laminated parts and parts made by vacuum infusion. As a hobby I have a lot of airplane model-building experience, where in a pressure autoclave I make stabs and bayonets for wings for F3F and DS models; those parts are designed for at least 50 g. That means an 18 mm high, 1600 mm long spar root must carry 750 N for a 3 kg model and it weight it is less than 150 g for spar!. Although it’s a toy, that is real high tech — and it’s the same kind of technology you need if you want to make a carbon shaft.

On R&G’s website you have all the required strength data for prepreg, so your friend can do the strength calculation, and because of material fatigue I warmly recommend you reinforce the laminate at the flange joint and taper it slowly so the loads are distributed smoothly. Use as high a safety factor as you can while keeping the mass of the shaft acceptable.

The price of these shafts will not be small, because they require a lot of material, but with the right laminate plan and a pressure autoclave they will hold. Although you’re young I’ll tell you time goes fast. In your project you’ve used a lot of innovation, and you’ll certainly have enough problems without trying to prove to yourself that infusion is the right way for this shaft. It would be a waste of money and time to destroy two or three more seasons — each trial will cost you at least €1,000 only in materials.

And somewhere in the middle of the shaft, isn’t it possible to install a single bearing to halve the effective length? Such a support doesn’t need to be very strong, it just has to prevent the start of oscillation. I would make a slender spar into which I would install three small bearings to keep the shaft stable. That would be easier and cheaper to make with a CrMo tube than to bother with carbon. And a CrMo tube with a slightly thicker wall can be machined so that it is thickest at the flange, then tapered elliptically toward the middle.

P.S.
How are the shafts from the front engine to the drive arranged in a staggered configuration by other speed boats, like probably original by yours Fountain?

Here is the formula

https://cimg1.ibsrv.net/gimg/www.off...17c2ef34b8.jpg
Drive shaft is 90.5" long, max OD that can be used is 4.724"
If 0.1967" wall aluminium tube is used, critical speed will be 3715rpm, safety factor 0.75 rpm will be only 2790rpm

same dimensions but steel tube, critical speed is 3730rpm, safety factor 0.75 rpm will be 2800rpm

https://cimg5.ibsrv.net/gimg/www.off...1409a34b55.png
https://cimg8.ibsrv.net/gimg/www.off...8b68f03ec3.png
modulus of carbon fiber is 6-77 MPsi depends what grade fiber are used
Density is 0.05834 lb/in3

with carbon fiber critical speed can be calculated by this formula.
Same dimension tube will have over 5000rpm, safety factor 0.75 rpm is over 3750
I can shorten tubes 6" with aluminium spacer between CV joint and split box flange. Then critical speed go up to 5500rpm so smaller safety factor this should work. Fountain prop shafts spin 4600rpm max, it's same if BBC turn 5700 with 1.24:1 gear ratio

Somewhere around the middle of this shaft, make a support.
Since the initial oscillation forces are very small, you can use three small bearings, with the upper one being pressed on the shaft by a slightly stronger coil spring, so that there are absolutely no vibrations.
You definitely have space for such a line support, and this will be much cheaper and simpler than a carbon laminate.

When calculating the laminate, you must consider the strength properties of the laminate as a whole, not the fibers. With manual lamination or infusion, you need to make a test laminate and test it, whereas with prepreg, everything is already known; you only need to account for the lamination method.
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