Flat bottom jet-boat (almost) - RC Groups
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Oct 31, 2017, 05:47 AM
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unusual_rc's Avatar
Idea

Flat bottom jet-boat (almost)


We had some discussion in the 3D printed jet-drive topic about if a flat hull jet-boat will work.
The real scale drive manufacturers and hull-designers all say you should not.

But than I came into these boats!
It has a real shallow dead-rise of only a few degrees. Check out the details on their website.

4K footage:
SJX Aerial Action-4k video (1 min 51 sec)


Here is a commercial (so beware of 100% pure sales-talk! ) video showing how nice these boats drift through the bends, like an airboat!
SJX Jet Boat Performance Video (6 min 16 sec)


Really nice looking hulls if you asked me.
I am considering this hull to use for the 3D printed jet-drive project.

Here is another, more action video. Yet it does not show the drifting that good as the other (cheesy) video above:
SJX Jet Boats Unleashed "You gotta see this!" Vol. 1 (4 min 29 sec)
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Oct 31, 2017, 02:33 PM
Just Plane Nutts
AirDOGGe's Avatar
Yeah, really flat hulls for minimum draft in shallow gravel rivers, but not fully flat like many airboats hulls. I measure approx. 8 degrees per side...If I measured from a head-on shot, it would probably come up as 6 or 7 deg.





OBSERVATION:
It appears that the hull has a recessed center section to help preventing the intake from "vacuuming" up gravel. I guess that would work if the hull was still submerged enough to keep the water intake underwater. Perhaps having the recessed section open like that all the way to the bow helps channel water into it and to the recessed intake proper. That may be something to experiment with on future projects with boats that will run in shallows.

Last edited by AirDOGGe; Oct 31, 2017 at 10:47 PM.
Oct 31, 2017, 02:49 PM
Registered User
Yep, it seems pretty common to have a fairly flat vee in the world of jet ski engine powered mini creek boats as well:





Also, subscribe to this guy on YouTube. He puts out amazing vids, and is a mini jet boat guru.

Jet boating in Westland , New Zealand (6 min 59 sec)
Nov 01, 2017, 06:13 PM
Registered User
dunk911's Avatar
Are you planning on making a flatish bottom jet boat?

I was going to for the first boat I made but my mate (very accomplished sailor) talked me out of it. Still wish i had because i think the displacement would have let it sit higher in the water and let it go faster.

I'm building a large boat on a steeper v at present and previously made a hull from fibreglass that had a much shallower v, very much like the one in this video just so I can get a better understanding of how it effects the boat. I'm not one for learning from reading, I like to see the difference:-D
Nov 01, 2017, 07:20 PM
Registered User
I would love to, but my experience building hulls is zero. If I were to try, I think I would buy sheets of ABS and plastic weld them together with an old soldering iron or something along those lines. For my use, the hull needs to be tough to withstand bouncing off rocks and such. As mentioned in another thread, I think a scaled-up version of the NQD hull would be a good place to start design-wise. I had mine apart last night to remove some goose fluff wrapped around the impeller from my last run and was having a closer look at the hull. I think the key to the way it handles is the low deadrise in the keel, and a bit of hook at the end of the hull presumably to stop porpoising and keep the bow down when on plane without the use of trim tabs. My SicRC 540 hull on the other hand has a more pronounced keel deadrise, no hook, a deeper vee, and a wide, deep-stepped keel section. This results in laser-sharp turning ability, but I get occasional porpoising, and the boat gets pushed off-course easily when it encounters currents going in different directions in the creek.

Can you laser cut sheets of ABS instead of ply with the setup you have?
Nov 01, 2017, 07:47 PM
Registered User
Firehawk....take a look at Propless RC....he has a couple of small hull designs...440mm x 190mm.
he has a couple of videos on them...ABS moulded hull...I believe very NQD ish but better.
He does make his own drive but I think he would sell a hull without a drive fitted if you have your own drive.
I have one here but just not got around to completing it...there was some issues with the vesconite bush in the drive overheating...he now has a slightly different drive but I am thinking of getting a bare hull with the cab...this is supposed to help self right without any fancy flood chambers or weighted couplers.
I have offered up a KMB 28 to the hull and it should go...transom is large enough and at 90 degrees....maybe a slight bit of work around the triangle at the front of the drive.
I like the KMB as the motor goes right on the drive...
An alternative is the drive I have mentioned that will be released soon.....I will be chasing that up this week from the designer.
Nov 02, 2017, 02:32 AM
Registered User
dunk911's Avatar
Quote:
Originally Posted by Firehawk989
I would love to, but my experience building hulls is zero. If I were to try, I think I would buy sheets of ABS and plastic weld them together with an old soldering iron or something along those lines. For my use, the hull needs to be tough to withstand bouncing off rocks and such. As mentioned in another thread, I think a scaled-up version of the NQD hull would be a good place to start design-wise. I had mine apart last night to remove some goose fluff wrapped around the impeller from my last run and was having a closer look at the hull. I think the key to the way it handles is the low deadrise in the keel, and a bit of hook at the end of the hull presumably to stop porpoising and keep the bow down when on plane without the use of trim tabs. My SicRC 540 hull on the other hand has a more pronounced keel deadrise, no hook, a deeper vee, and a wide, deep-stepped keel section. This results in laser-sharp turning ability, but I get occasional porpoising, and the boat gets pushed off-course easily when it encounters currents going in different directions in the creek.

Can you laser cut sheets of ABS instead of ply with the setup you have?
Ahh I don't have a laser cutter, just a knife and a ruler lol

Most lasers that do ply can to certain types of plastic too and there are lots of companies around that can do it for not that much. :-)

I got quoted 60 for a 700mm hull in 2 and 5mm ply a month or so ago.
Nov 02, 2017, 03:04 AM
Registered User
unusual_rc's Avatar
Quote:
Originally Posted by Firehawk989
I would love to, but my experience building hulls is zero. If I were to try, I think I would buy sheets of ABS and plastic weld them together with an old soldering iron or something along those lines.
Can you laser cut sheets of ABS instead of ply with the setup you have?
You can "weld" ABS plastic with acetone. But be careful with that stuff!
CA glue, epoxy and many other glues will give good results too.

I am considering this jet-boat, but I planned to build a NZ tour-boat initially, where I already started making the drawings for the scale version.
But still this hull is tempting. I like the looks of the boat!
Nov 02, 2017, 12:30 PM
Registered User
All good info there, thanks guys.

Oyley, I'd be looking for something in the 500-600mm range ideally.

Dunk, good to know that laser cut plastic is doable and relatively cheap.

Unusual, thanks for the info on acetone, I didn't know that. I have access to more acetone than you can shake a stick at here at work, so I should do some experimenting. I work with it on a regular basis, so am aware of the hazards.

NZ tour boat sounds like a fun hull to build as well! Post a thread when you get started on it.
Nov 02, 2017, 01:52 PM
Registered User
KMB Jetsprint Slash maybe? 535mm long and a beam of 220mm 119 euros or 155 euros with their 28mm drive.
I'm looking right at one on my floor right now...
Nov 02, 2017, 02:27 PM
Registered User
Youngsta's Avatar
Fire
you can mix acetone with ABS flakes to make a paste,
it's quick grabbing and a good strong ridged bond
it's also good for beefing up joints too,
hope everybody is good,
Mark
Nov 02, 2017, 04:13 PM
Registered User
That Mark is good info,I never knew that.
When is your Pariah V3 available to buy?
Nov 02, 2017, 05:33 PM
Registered User
The SicRC hull I have is an ABS copy of the KMB Splash hull. That hull was designed in the era of brushed motors and NiCd/NiMH batteries, and so only works well with a ton of weight in it; see the notes I made about it in my earlier post. In short the keel is too deep and the bow vee section is too steep/sharp. It corners really well at speed, but that's not what I'm after.
Nov 02, 2017, 05:35 PM
Registered User
Thanks for the additional info there Mark. Will come in handy if I decide to take the plunge and build a hull.
Nov 02, 2017, 05:54 PM
Registered User
The KMB hulls do sit high without weight...I bought a used one some time ago and the guy had made 'wells' for the batteries and he had lead sheet pieces in the bottom of each well so it was balanced ok but it did make it a bit slow off the mark.
It did corner well but was not quick to corner..just stable.
Being a glass hull it would not be too hard to alter..like flatten it out a little.
You could literally slice off 450mm of the central part of the hull base,55mm wide and I think it would be pretty close....I have the dremel in one eye and the hull in the other !!
I will make a template up and offer it to the hull and see if it looks right...height wise it would be about 15mm lost and that loses almost all of the 'V' still enough height for the pump.
The 'V' would be at the bow for around 80mm or so...


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