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Jun 15, 2021, 01:55 PM
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Got the windscreen fitted today even though it was very hot in the workshop.
The side to side balance seems ok.
I agree the weight keeps it steady on the water and a realistic look.
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Jun 17, 2021, 12:01 PM
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Cushions done, ticked off the list.
In the film at 38secs I put full throttle in and it shoots off to the left, my camera man looses it. It's usually a left turn sweeping , not a 360. Puzzling. Bit awkward when your near the bank.
Jun 17, 2021, 12:33 PM
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Quote:
Originally Posted by returntohome
In the film at 38secs I put full throttle in and it shoots off to the left, my camera man looses it. It's usually a left turn sweeping , not a 360. Puzzling. Bit awkward when your near the bank.
I can imagine. I have the same with our Recoil 26 featuring an overpowered jet-drive (45mm impeller)

It steers very bad, as if the servo is malfunctioning. I even placed a GoPro inside the boat to see what's the servo is doing. That was not malfunctioning I found out.

For my boat it is a combination of torque caused by the jet-stream exiting the reducer. There is still an unwanted rotation visible and noticeable.

However all this would be no issue if the CoG would be better. Because of the limited buoyancy at the stern due to the large jet-intake and the heavy motor and batteries, the CoG is too much to the front. This seems to cause the uncontrolled turning at speed.
At first I thought it was always the same side, but later I found out it was in both directions.

Will do a new test on a pond soon with the CoG much more to the aft, while taping all seams of the access hatch. Otherwise I am afraid it would make too much water.
Something my footage inside the boat also revealed!
Jun 18, 2021, 12:35 AM
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Thinking outside the box, what if I put some hydrofoils on the front to keep the nose up they would help to lift excess weight at the front end. Thinking like two separate skis. A guy in Australia builds full size and they act like dampers stops the bow from slamming down over the waves. He makes them retractable either by hand or electric so the boat can be trailered.
Jun 18, 2021, 03:43 AM
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That would make your beautiful scale model not scale any-more.
Maybe you have still some options for the CoG?
That will certainly do the trick.

Another option could be to add a rudder to the steering nozzle: https://jetboatpilot.com/pages/jet-boat-steering

Or, add small fins at the back or on the bottom of the hull.
Something you see with large scale boats as well to enhance the handling.
Also the Thrasher RC mixed flow jet-boat has side fins, see their website.
Jun 19, 2021, 12:46 AM
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Thanks for keeping me on track. I could try removing the console and see if it makes it worse or better.
I am thinking of altering the shape of the hull under the jet nozzle to a v shape like my outboard version of the same boat. This would create a trim tab effect. Like fins on an arrow keeping it straight.
Last edited by returntohome; Jun 19, 2021 at 12:51 AM.
Jun 26, 2021, 12:45 PM
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Took the bull by the horns and removed some metal. See if it works tomorrow.
Jun 26, 2021, 01:40 PM
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Let us know!
I have similar issues with the 45mm FJD testbed. At first my CoG was way to far to the bow. It went nice in straight line, but then the end of the pond was there..it just did not want to turn.

Next I placed the CoG on the point where one would put it in the prop driven version. Much better and more predictable in taking corners, but going fast in a straight line...impossible! I am confident that is partly casued by the hull shape.

Your hull is much better suited for a jet-drive. This might work for you!
Jun 27, 2021, 01:59 PM
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So the v made no difference.....shame. Next I removed the console.....if anything it was worse ,. So next console back on and a spare battery stuck under the front seat. Ah improvement,. So it looks like I have gone too far with moving all the weight back. It also points to it being a c of g problem.
So I have two things I can try. Move the drive battery back to where I intended it to be under the deck. ,or move the light battery back under the front seat. Both are easy to do I can try it in the workshop and get some measurements.
Jun 27, 2021, 02:46 PM
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Good to hear you have made progress with the design!
A good start-off point is the CoG of the original boat.

In most cases the CoG lies roughly on the same position as for the propeller driven version.

It sounds your original CoG was in the ballpark after all. Then again it is always good to test the boat for the best CoG, concerning it is a one off scratch build scale model.
Jun 28, 2021, 03:34 PM
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So i put the drive battery back under the deck, this moved the c of g forward by 1" so i ve gone for this. Moving the little light battery moved it 1.5 " forward!
Will test on Wednesday
Jun 30, 2021, 07:48 AM
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Test


Testing today,. No different the problem persists. No matter what I tried . Then I think I had a light bulb moment, watching it run past me I could see a lot of water hitting the stern and more so to the right side which could push the boat to the left which is what is happening. So to cure it I could make a new nozzle bigger in diameter to let more water go backwards where it should. Question is how to make it.
Jun 30, 2021, 11:56 AM
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Our default FDJ nozzle has the same issue, although I have not heard that it did push a boat out of it's line of travel.

If you watch this video at 3:40 seconds, you see the back-wash clearly:
Water Jet Thruster For DIY RC Boat (4 min 56 sec)


I also have stability issues concerning traveling at speed in a straight line. I only see back-wash when I steer, not when the nozzle is in neutral position.

To end this back-wash, 3D printing could be a solution. But you need to design the replacement parts first.
Last edited by unusual_rc; Jun 30, 2021 at 02:10 PM.
Jun 30, 2021, 09:31 PM
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I have the same jet in my KMB Jetsprint 2 and as I don't want a backwash I changed to a KMB stator and nozzle ...
Here are also STL files for KMB style stat & nozz
Jul 01, 2021, 02:50 AM
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There you go!
Well done Dennis! 3D printing is a great addition to the hobby. I would not know how to create a drive without it!

You can either control the back-wash by stator-housing = steering nozzle interface design (just make sure it does not happen by design, working on that now), or what I noticed a small incline of the exit nozzle.
Since I added 5 incline instead of a tube perpendicular to the impeller axis as the jets water stream exit, it is practically absent while using the original steering nozzle of our FJD.


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