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May 16, 2019, 02:03 PM
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Build Log

Rebuilding a 410mm jet sprint boat with FJD 35mm jet drive


I’m rebuilding an old jet boat that I have put over 300 hours bashing about in the rocky rapids next to my house. The boat is very beat up and needs a new hull and jet drive.

The old boat is a 410mm SicRC jet sprint with a KMB jet drive. Steve Dales (Ondabeach) designed and printed a three dimensional thrust nozzle for me about five years ago after a little arm twisting. I believe this was the first 3D nozzle to get used in a jet boat of any size. There is now one being developed for a full size jet boat but I don’t think it is in production yet.

At first a 3D nozzle was hard to learn to steer for me. After 20 one-hour sessions it started to get more intuitive. This was the control I had hoped for in the three dimensional world of whitewater. I now have hundreds of hours on the water with this nozzle. I would like a 3D nozzle for the FJD 35mm jet that I will make for the boat. To accomplish this I needed to learn enough CAD to make the nozzle and get the jet drive to fit this small boat and my piloting needs.

The 3D nozzle will get some improvements that I have been planning for a long time. Dunk911 sent me a 3D nozzle for my 45mm jet drive that he designed. It works very well so I am borrowing parts of his and Steve’s design and adding my water hook concept. This hook works a little like a reverse bucket on a jet or like a Pelton wheel to change the direction of water with a sphere. This acts like a steering nozzle that can turn 90 degrees but without the complexity. A boat with this nozzle should do some interesting tricks. Check back to see a video of this boat in action.
Last edited by Kayaker; May 16, 2019 at 02:34 PM.
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May 16, 2019, 02:06 PM
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I will reuse the Leopard 4074, 1800kv motor that I ran on 4s with a Turnigy Marine 120 amp ESC. The four batteries in this boat are 2s, 2200mAh that I connect in series and then in parallel. The boat barely floats with all the weight, water line is just above the deck/hull seam. I like a heavy boat because it sticks to the water better but this is a little too much.

Here is an old video of the boat before the rebuild. I think the new jet will be a big improvement.

Jetkayaker with a three dimensional thrust jet nozzle on a SicRC boat (12 min 2 sec)
Last edited by Kayaker; May 16, 2019 at 02:30 PM.
May 16, 2019, 02:47 PM
Just Plane Nutts
AirDOGGe's Avatar
Too bad we can't coat the hulls with the same stuff truck tires are made of. Rocks are hard, but even harder when you run into one!

Enjoy the build. Pics, please!
May 16, 2019, 04:04 PM
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The bow bumper on the old boat is truck tire rubber. I see that Shapeways is introducing a printable rubber. I知 thinking of making a bow bumper with it for this new build especially if I can figure out a way of getting more bow rise. This hull is very pointy.

This log will have lots of photos, I promise.
May 16, 2019, 10:02 PM
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KiwiKrawler's Avatar
Ha Kayaker you read my mind. I was going to do a build thread on my SicRC 410 jetsprint...but you致e bet me to it. So is that hull you have one you bought prior to Steve pulling the plug?

Looking forward to your build.

🚤🌊🌊🌊
May 17, 2019, 02:41 PM
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I値l show you mine if you show me yours.

When I got the first hull from Steve I also got a spare, I knew this day would come.

It will be interesting to see the difference between the 27mm KMB jet drive and the FJD 35mm jet, almost everything remains the same in this boat, even the servos are the same.

One of the things I like about this little boat is my mistakes are smaller and less expensive. On the other hand I知 having a hard time fitting everything I want to use in such a small boat. Will it even float?
May 17, 2019, 06:03 PM
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KiwiKrawler's Avatar
Quote:
Originally Posted by Kayaker
I値l show you mine if you show me yours.

When I got the first hull from Steve I also got a spare, I knew this day would come.

It will be interesting to see the difference between the 27mm KMB jet drive and the FJD 35mm jet, almost everything remains the same in this boat, even the servos are the same.

One of the things I like about this little boat is my mistakes are smaller and less expensive. On the other hand I知 having a hard time fitting everything I want to use in such a small boat. Will it even float?
No no no would rather keep the spotlight on your project. 🤤

I致e already posted a few pics in another thread of my orange & black SicRC boat with comparisons to the NQD hull. Shame I got mine with the holes already cut so I知 limited on the jet I am able to install. It値l have to be the KMB or the TFL unfortunately.

So I知 interested to know why you run four batteries. As you mentioned how low it sits in the water? Wouldn稚 two with higher amperage be sufficient...take up less space and be lighter. You can still move them round to find that all important centre of gravity.
May 18, 2019, 03:15 PM
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My spare hull was cut to fit a KMB. The FJD 35mm jet is shorter so I patched the intake hole with stainless steel and Shoe Goo.

One of the problems with using a big jet drive in a small boat is that it needs a bigger motor. A big motor needs a bigger ESC that is hungry for amps from the batteries. I try to reduce the resistance to moving amps so that electrical parts don’t waste energy overheating. After a hard run I quickly open my boat and feel for heat in the motor, three wire connectors to the ESC, ESC, cap bank, servos, wires and plugs to the batteries and the batteries. I now have them all well matched with the batteries being my weakest component, producing the most heat.

The way I have the batteries wired in series and parallel I’m getting 4400mAh’s at 4s (14.8 volts). With a 65c discharge rating this should be plenty of mAh’s with new batteries. I boat in the rapids next to my house 3 days a week or more. My batteries aren’t new for long. I sometimes only run 2200mAh batt’s to run a light boat for jumping but I risk damaging a cell and this is how I mostly kill batteries.

That covers why I’m using 4400mAh batt’s. The reason I use three or four batteries to get the weight in the right places is a much longer explanation that I hope to answer but that will come later.
May 20, 2019, 02:49 PM
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An amazing jet drive


I致e been waiting for a long time for a jet drive to come along that can do everything very well with few short comings. I believe it痴 finely here with the FJD. I知 in the process of building two boats with these jet drives. One of them is using the 45mm FJD jet drive in a 28 inch hull. It is slow going because I知 designing the hull and deck plus learning to make plugs and fiberglass molds. For testing I thought I should get this 410mm (16 inch) boat with the 35mm jet drive going first. This hull is understood by most people so they can understand the performance increases they see when I post videos of this boat and jet drive.

The reason I知 so excited about the FJD is it痴 designed like a professional bit of equipment not a toy. With this jet drive I can take my boats performance to the next level. Down in the canyon I boat in and with the number of hours I boat I need the reliability this jet drive is capable of. The strength of Shapeways Versatile Plastic can take rock hits so they printed all the plastic parts. The impeller is printed in Shapeways steel and the bearings are Boca ceramic including the motor and thrust bearings.

I melted brass inserts into the housing for the stator screws. The stator is using an 前 ring for a seal. This makes it very easy to change impellers or stator sizes for different types of water I boat in.
May 21, 2019, 12:06 AM
Just Plane Nutts
AirDOGGe's Avatar
35mm is a huge drive for a Tear Into-size hull.

Looking forward to seeing this rocket take flight.
May 21, 2019, 12:41 PM
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I think a 25mm jet would be better in this boat because it could be a much lighter build than the 35mm. I felt the need to go bigger than the KMB I had in the old boat but probably the 25mm FJD would outperform the KMB anyway. A lighter boat would add to that performance.

This boat is so close to not floating that I知 adding a bilge pump. This system is similar to what I知 using in my Thrasher. This takes suction from just ahead of the impeller and uses a gas pick-up clunk from a weed eater. There is a 3 gallon an hour dripper in the suction line so when it痴 sucking air the jet doesn稚 loose prime. I知 hoping someday to have the CAD skills to design a bilge system that works like AirDOGGe痴 at the end of the steering nozzle.
May 21, 2019, 01:56 PM
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KiwiKrawler's Avatar
I’ve got a 25mm drive on the slow boat from China which I intend to drop into a Tear so will be interesting to compare notes.

On the siphoning of the bilge issue. I run a second outlet on the stator that provides plenty of suction without compromising prime...as it’s on the positive pressure(thrust) side of the impeller. And she’s a thirsty little beast!


NQD bilge pump (0 min 39 sec)
Last edited by KiwiKrawler; May 21, 2019 at 01:59 PM. Reason: .
May 21, 2019, 03:11 PM
Just Plane Nutts
AirDOGGe's Avatar
Quote:
Originally Posted by Kayaker
... I’m hoping someday to have the CAD skills to design a bilge system that works like AirDOGGe’s at the end of the steering nozzle.
"Sundogz" you mean perhaps? I have made no water removal systems for my boats (not needed).
Jun 05, 2019, 11:42 AM
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a true compression nozzle


I have now made a nozzle that acts like a 90 degree nozzle by changing the direction the water travels with a sphere inside the nozzle to redirect the thrust. I wanted this to be a true compression nozzle, not a deflection style nozzle that can’t hold water pressure. I think this gives a better handling boat, especially at top speed, and it has no water blow-by when turning sharp. Unlike other jet drives the water compression is done at the nozzle not the stator with my design.

Shapeways, Versatile Plastic prints accurately enough to get all the parts to fit tight for the rotating water compression seal. The stator nozzle I’m using is the biggest one for the FJD, 35mm jet drive at 26mm. I made the steering nozzle opening start at 21mm and it gets bigger if I sand a little off of the flat end. A smaller opening gives a faster boat; a bigger opening gives better handling characteristics.

Here’s some video of learning to steer this boat in the first couple of sessions on the water. Notice the “traction” the FJD 35mm jet drive has. This is like putting a four wheel drive on a sports car. In aerated water it continues to have good traction and rarely looses prime. This jet drive is proving to be everything I want in a jet drive for river use.

PM me if you would like the STL files for the nozzle parts to try or just to look at. This is my first CAD part, so just about anyone with CAD experience could improve on it.

One of the tricks in this video I’m calling the “Ester Williams”, can you spot this trick?

3D steering on 35mm jet drive (6 min 24 sec)
Last edited by Kayaker; Jun 05, 2019 at 11:47 AM.
Jun 05, 2019, 09:03 PM
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KiwiKrawler's Avatar
Ha great minds think alike Kayaker. I have this old bath with a tap that has a ball and a 360 degree directional nozzle on its outlet. I have spent many an hour in this bath looking at this tap thinking if I could reproduce this in a jet drive it would be the perfect proportional steering nozzle. And...you have gone and done it! Brilliant!!
And once again....wicked video. Is the Ester Williams the bum down bow up 360痴?

I for one would love a look at your STI files if you are keen to share. Will be in touch...


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