Carbon Propshaft for electric inboard

[feunatz]

Hi,

some time ago i've thrad regarding hull selection for a electric/hybrid boat project.
http://www.boatdesign.net/forums/boat-design/planing-electric-dinghy-56770.html

Now I'm a few steps further and have locked in some components:

Hull: Bateau RB14
http://bateau.com/studyplans/RB14_study.php?prod=RB14
Battery: up to 4x 20S(150Cells per pack) 18650 LiIon(1.6-6.5KWh)
Motor: Chinese Watercooled Outrunner (25 or 35KW peak) - limited to 15KW
Generator: 2KW honda

Now for the propshaft I'd like to use a 25mm solid extruded carbon shaft for wight savings vs steel shaft. Run in a carbon tube as shroud. Thrust bearing will be above WL. I'll need to use a thrust bearing, motorbearing can't handle the axial forces.

The rough sketch shows a solid shaft/motor coupling. I'll most likely have to use a flex(CV or dualchain) for easier motor lineup.

Sketchs shows the shaft shrouded all the way. great for overall rigidity but drag would be horrible?

-Any recommendations on Props? Solid Hub Props maybe 10x11. I'm thinking of something like the dinghy 30hp racers use. Overproping won't be a big problem on a electric motor.

-Prop to shaft mounting?
Tapering an keying the shaft will most likely not work(key tearout)
Gluing a steel taper/spline could work
Gluing the Prop to the shaft will work but make Propchanges expensive

 

[feunatz]

Maybe something like this as Prop?:
http://www.ebay.de/itm/8-X-12-Outbo...495352?hash=item5b2d90e878:g:4p0AAOSwUEVYEStB

16mm ID could be enlarged to 20mm or more. Pin and shoulder in front and a glued in M10 thread to fix the prop.

 

[gonzo]

Have you calculated if there are any real weight savings between your setup and a standard steel shaft? You can use a 3/4" shaft for the power you are specifying. Unless all other components, including the hull, are extremely high-tech and low weight, savings on the shaft will be minimal; particularly compared to the cost. The design you show if for a moderately heavy boat, designed for an outboard and not an inboard. The weight estimate is not realistic. It says it uses 6 gallons of epoxy, which add to 60 lb. The fiberglass will weigh about the same, totaling 120 lb. There is no way you can get it at 160 lb, even 200 lb is not going to happen.

 

[feunatz]

2m - 25mm Carbon is 1.4Kg / ~160$
2m - 20mm Stainless is 5Kg / ~230$

Saving maybe 7lb
Cost about the same since going steel would also enable me to use more std parts...

 

[feunatz]

Updated Overall Weight Calculations:

Empty Hull inkl Driveshaft / rudder / some mods: 120kg/265lb
Hull+Motor+2Packs+Gen: 175kg/385lb

Adding 2people gets me right to the designed max. so maybe some more weight saving needed?

 

[Irie]

What is the continuous duty rating for your selected motor? Are you fully committed to that drive line? Maybe stick with steel for the prop shaft and try to save weight any/everywhere else?

I've tested a 9.25x10 and a 10.5x12 prop on my boat. The 10.5x12 is over propped and only suitable for short speed runs (25mph) in cooler weather. It also doesn't perform as well below 3/4 throttle. The 9.25x10 is only a few mph slower (22mph) but performs much better throughout the throttle range. For continuous duty with the 9.25x10 I can go 12mph with the drive submerged or 16mph with the prop at the surface.

 

[gonzo]

You could save some money and probably weight by getting an outboard with a bad motor and replacing it with the electric one you have.

 

[Stumble]

I don't think a pulltruded rod will work, the fiber orientation is all wrong, you would need a filament wound rod, and i am not sure if a solid rod that size is readily available. If it is the cost would be substantial.

Carbon here really probably won't save all that much weight and would be incredibly expensive. Unless the entire boat is carbon/nomex already I doubt this is the best place to save weight.

 

[feunatz]

Sorry no continuous duty rating for the cheap china motors.

The smaller 120100 is speced at max 300A. I've seen successful tests up to 250A. So 200A cont with heavy cooling seems somewhat reasonable ~ 14KW cont

I'm not fully locked in on the drive line. Maybe just no outboard hanging off the back. Sorry gonzo, but i really appreciate your inputs straight to solution attitude!

I'd like to use the outboardless transom for a ladder. But maybe i really should give sourface drive another look...

 

[feunatz]

Stumble you right. On the other hand the rod is very resistant against deflection(vibration?).

I found no specs for torsional strength of pulltruded rods, probably for a good reason. Just one paper mentioning that they can survive 45° rotation/m with litle to no damage. I'm trying to get my hands on some 25mm stock to test, but I'm confident it will handle the required 50-75nm

 

[gonzo]

Have you calculated the CG for your proposed setup, compared to the design outboard installation?

 

[Joakim]

I don't know what you are looking for, but the arrangement shown doesn't seem efficient at all. That kind of boat + shaft with a tube is not going to work well with limited power and energy.

You can make a suitable boat very light. E.g. the racing version of TG-Special build form carbon is about 35 kg without engine, but with steering, seat etc. Even polyester resin + glass version will be less than 60 kg. LOA 3.8 m. The racing version reached 40+ knots with standard Mercury 20 from 70's and 52 knots with 80's Yamaha 25 modified to output 36 hv.

In the 80's I had some plans to put a motorcycle engine to a similar boat. Then I would have used a short propeller shaft going through the transom and a surface piecsing propeller. With that you don't need the shaft under the hull (a huge drag for such a light boat), CG is in a much better place and the thrust line is much more suitable for a planing boat.

So something like this: http://www.boatdesign.net/forums/bo...neffective-pwc-converted-propeller-21381.html

The propeller you linked is a cleaver. It works very poorly deep under water. You need to have it surface piercing.

 

[feunatz]

Have you calculated the CG for your proposed setup, compared to the design outboard installation?

I have not. My proposed drivetrain is comparable in weight with an 25hp 4stroke OB. Also I can't find any mentions of CG in the RB12 plans.
CG will be substantially lower vs OB but no idea by how much.

 

[Joakim]

I have not. My proposed drivetrain is comparable in weight with an 25hp 4stroke OB. Also I can't find any mentions of CG in the RB12 plans.
CG will be substantially lower vs OB but no idea by how much.

The longitudinal CG is much more important. OB is behind the transom and your arrangement shows the motor very far forward of transom. Then you have the propeller shaft at a clear angle downwards. All this will lead to very low trim (bow down) and long waterline while planing. Which means a lot of drag at higher speeds (20+ knots).

E.g. for a 200 kg boat (including driver) my Savitsky program shows 9.3 kW propeller shaft power at 25 knots in optimal setup. Then trim is about 3.5 degrees and CG 0.85 m forward from transom.

Now if you take 50 kg OB away from 0.2 m behind the transom and then put 50 kg 2 m forward of te transom, your new CG is 1.4 m from transom. That will reduce trim to 2.3 degrees and increase power needed to 12.9 kW. With propeller shaft below the hull you have no way to adjust the trim of the boat.

So you may waste 3+ kW power at 25 knots. Addiotionally you will loose about 2 kW due to drag of a 30 mm propeller shaft tube.

 

[feunatz]

Ofc longitidinal CG. It seems I'm a bit too much racecar guy...

At this point i do want to thank you all for your valuable input and throw my driveline idea overboard.

Switch to surface drive.
All heavy components will be placed as close to the transom as possible. CG will be forwards compared to OB but the surface drive should give me some trim capabilities to compensate.

How much steering angle do i need for reasonable maneuverability?