Scratch built ribs and outboards

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A kit of parts to build version 2 of the VC jet

 

Nice kit! Ready for an e-shop !

 

New crew
Always on the lookout for new crew members for my Ribs, skis and kneeboard. Got a bargain with Luke Skywalker, normally about twenty quid, got him for a fiver including another figure, Rey. Been wanting a male figure for a while, too many girls up to now anybody want a light saber

 

If you use Luke or Rey, you need to change your outboard into this brand:

 

quite pleased with the finish, just waiting for motor coupling now.

 

Here she is Yamahas top of the range 105 hp at the pump, driven by a 150hp engine.

Tomorrow is test day, video will follow, hope its good.

 

Quiet running, fast acceleration and a good top speed, result. Shore shot video soon.

 

Looks very good indeed,and for sure it moves fast to.

 

Indeed she runs very well!
No prop-walk, no parts sticking underneath the hull!

I love the slow-motion footage with sound.
It sounds like a real outboard then, amazing!

The sound is great anyway, since you have no noisy gears or propellers.

You told me she did 155A on 6S in this video.
That is way too much to run flat-out for longer than a few seconds.

Your model is something in between 6 and 7 kilos, which is even for a 80cm hull a lot!
Then again this is a detailed scale model.

Very clearly visible in the slow-motion part is the exit water-jet-stream velocity.
It is much faster than the speed of the boat itself.

A jet-boat is based on sir Isaac Newton's 3rd law of physics:

action = - reaction

So if you shoot a certain mass to one side from a boat, the hull itself will move to the exact opposite of that direction.
The mass and speed defines the amount of energy.
The lighter the boat, the faster the speed, or the more mass and/or velocity of the water-stream, the higher the speed of the boat.

Visible is that the jet-stream has a very fast exit speed.
On my boat which is 5 times as lightweight as the thrust being generated, the hull speeds comes close to the jet-stream velocity.
This tells you something about how hard the jet-drive has to work to propel the vessel.

155A means over 3kW of power!
That is a lot!

 

That high amp draw was at 21mph, just before we started filming, she really skimmed across the water. So when we filmed I kept the speed down a bit, my motor is only rated to 125 amps, didn't want to damage it.
The new impeller, on the right from your recommendation should bring the amps down to about 100 A.
I definitely need to get my Neptune rib finished as it will be lighter and therefore faster. Now this outboard is up and running I can spend some time boat building.

 

Great result! I enjoy watching your videos, it looks very scale.

 

Amazing! I estimated the speed on the video as being the 21mph (155A) run!
Do you have any idea what the speed and current was in the video?

If so, you can use it to calculate the possible pitch decrease.

Given a fixed motor type and cell combination, you have basically two ways of controlling the current usage:

- Impeller pitch. Higher pitch results in more pressure inside the bowl/volute at the price of higher current
- exit stream hole diameter. A smaller hole means less pressure release from the bowl, results in a faster water-jet at the cost of higher current

So by changing one of those features at the time, you can play with the maximum current and speed.

 

I can tell you exactly what readings were. 83 amps, 15mph, 1700 watts. I judged it was about 7/8 throttle, so in that last bit the amps go sky high, which is what you said it would do. Without thinking I have fitted a slightly smaller exit as per the prototype, I might have to go back to the original size.
Dirk Jan, thank you you are very kind, wonder what this outboard would do on a light weight F1 hull !

 

Although the graph rpm versus power is parabolic, going from 7/8 throttle to WOT should not result in such a big leap: from 1700W and 15mph to 3000W and 21mph!

For this we need to know the rpm, which we do not have.

Does your Esc have an option for linear versus exponentially throttle curve? Maybe it is not set to linear, although I highly doubt that (the Swordfish Esc. did have this option)

Another option could be that the impeller blades start to flex, which would result in a lot of amps.
Example: my very first test with the Recoil with a Nylon impeller showed 45kmh and 160A (!) of current use, and a high rpm value.
With the identical impeller made out of metal 45kmh at 105A. A huge decrease! Also less rpm at WOT, but it does match the same top speed.

If the throttle curve was lineair, 1/8th less pitch might be a good start.
Then you can keep the current narrower exit nozzle!

 

Had a great afternoon, boating and kneeboarding in beautiful surroundings. We are in the Yorkshire dales again in the same place as last year. The lake was flat calm, i should have had a film for u tube, but the camera let me down. It caused some interest with people getting phones out to film it, even noticed a drone flying over the boat, wish i had that film.
On my way down to the lake I saw a man sat on a stile with a golden eagle on his arm! He said he'd been hunting with it, you don't see that everyday.

 

Managed to sort the camera out today. I also managed to see a tawny owl in the woods next to the lake.