Scratch built ribs and outboards

Wow, that is indeed not bad at all for a scratch build axial jet-outboard based on the TFL 29mm 4 bladed impeller!
Great video, as always!

12mph/18kmh is not bad at all. It does look faster by the way in the video.
From what I hear the rpm is very high, which could mean it is less then 20% of the unloaded maximum rpm possible.
Did you measure the maximum current?
That could support my thought that it is running in the low region of the power potential of this motor.

As I said before, you could reduce the exit nozzle diameter.
Mind you: it is not the circumference which counts here, it is the surface area of the exit nozzle!
So a slight reduction of the diameter already has big influence on the surface area.
On my 45mm FJD I did .5mm diameter reduction at the time, which was noticeable in both current consumption and top speed.

I would love to hear a current number.
On how much cells was this? 3 or 4?

Edit: you kept this note away from us, the Dutch audience

 

Thanks Michiel, I thought you would be pleased. I have drawn the new swivel to raise the motor one at 10mm and one at 15mm

 

Yes, I am!
This is an excellent way of getting insight in what you are developing.
This will prevent damage to hardware for sure.

I am confident more speed can be achieved by setting the height of the outboard correctly.
The jet-nozzle should be in free air once the hull is on plane.

Clearly visible in the video is excessive spray coming somewhere from the base of the end-part (intake) of the outboard.
Possible due to the fact it is protrudes too far below the keel of the transom.

 

Up 15mm, but still too low.

 

Much improved by adding Michiel's ventilation plate, I was sceptical, but once again he was on the right track, I should have tried it sooner. Now she seems to skim the water line.

 

Since the video I have added the flat plate, hoping to guide water into the intake.

 

VC jet
She's ready to rock and roll, but will have to wait for jet fox to move over. She looks so sweet I am itching to do another one, but I need to see if it works first.

 

Jet Fox and VC jet
So good news and bad news, the batteries I took for the GPS weren't big enough, so no readings, sorted for next time.
Jet Fox, I did get one quick reading which was 9mph with the large plate, and 7mph with it removed. I think it's time to try something new like reducing the outlet, as the amp readings were 45 and 50, watts 900 to 1000.
Now some good news, I took the finished VC jet to show the lads, but with the disappointing results from jet fox, I set too and fitted it at the lake. We dipped it in and tried it, holding on, very promising. So then I found by moving the printed steering arm up I could connect it. So maiden voyage, it went really well, good speed, if I had to guess I would say 17mph.
It is very quiet too, we have made a video which needs sorting out. When I stripped it down to check the impeller it was fine, just slightly polished where it touches the wear ring in one spot.
You will see on the video water spraying out of the joint between the chamber halves, my glue couldn't cope with the enormous pressure. I will print a jig so I can superglue it together and it will be lined up correctly for the wear ring/impeller.
The 4082 motor in the VC has a max amp rating of 126amps

 

Indeed I have a gut feeling the Jet-Fox needs some optimization concerning the exit nozzle.
If your lipos can cope with 100A current, you can reduce the outflow.
Be aware that even a single millimeter reduction in diameter has a large influence on the speed and current use.
As an example I went with my 45mm drive from Ø31.5mm to Ø30.0mm.
It consumed 10A more and increased speed with 4 to 5kmh!

Remember my calculations are quite accurate and useful to determine the reduction of the diameter.
Maybe I already did (cannot remember, fiddle a lot with the sheets).
If not, I can help you out making an educated guess.

Is the reducer you use not the one from Bermy Lizard?
As I remember in that design the stator-housing does not reduce at all, or minor.
The compression/reduction was build inside the steering nozzle.
Interesting design, but as you found out with the VC-drive the water-pressure will find a way out!

Centrifugal pumps are used for high head applications in general.
Which means if a fluid needs to be pumped to a high level, those are the pumps you need.
In other words, the pressure is even more high than in an comparable axial impeller pump!

Try to make a design where the water pressure helps closing the gaps.
A Lip/Groove might be a solution. If you place the lip inside the two halves, water pressure might push it out into the groove.
Tolerances are important of course.

 

Based on the following input:
1500kV motor
6S
26000rpm (80% of unloaded rpm)

Impeller 4 bladed: Ø29mm
Hub diameter: Ø9mm
Impeller pitch: 19mm

If you make the exit nozzle internal Ø15mm you will consume 2kW of electrical power at full throttle, resulting in around 90A of current.
What is the current internal diameter of your used exit nozzle?

Since you do not have real-time telemetry available, I would be careful during the initial tests.
Might start with Ø16mm, which consumes almost 500W/20A less!

And remember if something goes wrong with your bevel gears, energy consumption can go up as well!

I use 8000mAh lipos for 110A currents continuously, which I have to preheat to 35°C in order to keep them in mint condition.
Almost all lipo brands exaggerate their maximum continues unloading value.
Mine are rated 20C.
Luckily somebody in the US tested my type of lipo, and calculated a more realistic value of 12 to 15C.

 

Jet Fox...nozzle dia 22mm, this has now been reduced to 20.5mm a 1.5mm reduction ready for next time.
Lipos are 5000mah 100c graphene
Things are going to get confusing, testing two similar looking jets, I have had to edit the video myself, Paul kept adding in views of jet fox to this VC jet video

 

Wow!
She's moving your big and relatively heavy RIB with ease. Any speeds recorded and current data? Purely judged from the video it already shows good scale like speed!


The last shot shows there is more to get out of this current set-up once you solved the leakage issue. Shows how much pressure is being build up by the Francis vane centrifugal impeller. It is a small impeller! Amazing!

Jet-Fox:
20mm diameter exit hole 30A/650W WOT of power needed, without losses of the bevel gear, according to my calculations.

 

Jet-Fox:
20mm diameter exit hole 30A/650W WOT
At 22mm I am using 60A /1300W, so now i have dropped to 20.5mm will be higher still. Real world is a lot different, is it the gearbox soaking up the power, leaving less for propulsion.

 

Bevel gears even in real outboards show a lot of loss due to friction. The big brands have reduced this number to 15%.

When I worked in an outboard shop many years ago, I learned each outboard had custom adjusted fitting of these gears. Notes with the values were carved in the inside of the two shelves forming the gearbox.

In smaller scale this will be more, however 50% sounds like a lot!
Do these gears still have to run in?

 

It could be weeks to fully run in. After today's run I need to get the outlet even smaller.

 

Considering you are not using telemetry your iterative approach of testing is the best. Your 3D cad and 3D printer just have to work harder

So in the end the current draw will be less and less until fully braked in?

 

Your C rating seems very low, this is the ability for current to flow from the battery as I understand. Which is why I went for 100c the highest I could get. How do you preheat your batteries? I imagine you have a 12v heated blanket to wrap them in. Robert on rcg thinks my jets are cavitating which would explain the loss of power at a certain point in the throttle use.
This forum is now refusing my photos

 

The C-rating the supplier (often not the manufacturer) puts on a lipo is almost in every case marketing BS!
There is no international standard for measuring the C-rating, or a law which says the information should be correct on the labels unfortunately.

That is why I try to focus on the batteries which are correctly tested. All show an exaggerated rating, and often a capacity as well. Not every 6Ah has the same capacity.

Have a look yourself!

https://www.rcgroups.com/forums/showthread.php?1767093-Battery-Load-Test-Comparisons

The photos, are you uploading them to this forum, or just adding a link to RCG for example?
There is a limit in total capacity you are allowed to use if you upload.

Commented on RCG about the cavi...ventilation

 

Might as well close this post, no more photos,, must have reached a limit.

 

Tried to look into the forum rules.
Unfortunately they are outdated. Not a single clue how much memory you are allowed to put in a single topic.
I know it is somehow limited, yet never encountered the physical limit!
For (paid) forum members this space is larger, like RCG has.