Jet boat design and build - RC Groups
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Feb 02, 2017, 03:26 PM
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Discussion

Jet boat design and build


Alright guys.

Been thinking a bit about doing another jet. Ideally a bit bigger than my first which was 475mm, I'm thinking from 600mm up to 1000mm length. But this time I'll spend more than 30!!!

Philosophy
I've had a couple of ideas, one has been to make the plywood hull a bit more resistant to being damaged by rocks. I definitely want to continue making my boats from predominantly wood and I want to have the option of running in flowing water so rivers and streams. I have done an experiment to validate my ideas. My idea is to coat the hull with a layer of epoxy then press a sheet of cloth (nylon) into the epoxy. Then skim over it again with another load of epoxy. I have done some test pieces, and it seems to make only a very small deference to the weight and the surface is much tougher and harder. I think I'll use this process for my next boat. Maybe you guys have some advice?

I have also been thinking about getting the plywood all laser cut to save time and improve accuracy. It will also mean that if anyone else wants a boat just like it; all they need to do is ask and I can get one done for them! Drawing up the plans and doing a full CAD job will actually be relatively straight forward as I am a design engineer by day so all I will need is to get hold of a home licence from work and get cracking. Once I am happy with the design I will do the .dxf files and get it cut. But this is quite a long way down the line and I need some help first, this is where you come in!

I have compiled a bit of a lessons learned list that I have condensed here, some feedback on your thoughts would be good here too please! Pictures below of my first boat for the un-initiated!

Lessons learned
Hull shape;
- keep some hull volume forward to prevent the bow from dipping. Needs more forward volume than on my boat.
- keep the "v" shallow, (shallower than mine at the back).
- shape has to be 100% convex or flat, this worked on mine.

Build up;
- interlocking pieces making a monocoque shell worked.
- modular installation of components kept my options open regarding where I locate stuff.
- Ply is good but surface is easily damaged if its to be used in a river etc. See above experiment.
- using long pieces of material that run all the length of the boat helped to stiffen it up and negated any possible weak spots or areas for water ingress on join lines.
- balsa wood is too soft for modular installation. Screws are damaging the pilot holes.

Practicality;
- leave as much room as possible inside hull to keep options open regarding component installation.
- waterproofing needs improvement. Ideas guys?
- keep RX, ESC etc. elevated or enclosed to protect from possible damage arising from water ingress.
- 10 screws to fit/ remove the rear panel is a pain in the obvious. Better method required!
- inlet really seems to want a grate unless the water is super clear.
- provision for ride plates and other ancillary fittings should be incorporated into the design.
- single drive trumps a twin.
- provision for a failsafe, GPS mount and camera mount favourable.
- no flat top so water can just run off instead of seeping around panel seals.
- varnish inside of boat too. This has paved dividends on my boat.
- properly planned cable harness, no rats nest.

So that's that. Hopefully you guys agree largely!

Can you think of anything that I have missed out?

Summary/ questions
- What do you think about the size I am looking at for the purpose? Too big/ not big enough?
- What about other ways of toughening up the wood or do you think a solution based on this is a good start?
- CNC laser cutting, good idea? I know that offshore cat further down the page looks good for it!
- Lessons learned, anything to add or comment on any of the points?

Looking forward to your input! :-D

Cheers, dunk
Last edited by Dunk_911; Feb 02, 2017 at 03:27 PM. Reason: title ammendment
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Feb 02, 2017, 09:06 PM
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helir22's Avatar
Hi Dunk, this sounds like a great project and job well done on your last one. What size or model jet drive are you considering? I have to build a jet boat hull for myself at around the 800mm range so I will be watching with interest. Best of luck to you, Joe
Feb 03, 2017, 02:09 AM
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dunk911's Avatar
Quote:
Originally Posted by helir22
Hi Dunk, this sounds like a great project and job well done on your last one. What size or model jet drive are you considering? I have to build a jet boat hull for myself at around the 800mm range so I will be watching with interest. Best of luck to you, Joe
Thus is one of the things I still have to consider, Joe. There is a 50mm unit from gravitex. But the best bet I think is to chose a jet unit and scale the boat to fit it Rather than working the other way around because there is relatively so little on the market.
Feb 03, 2017, 03:36 AM
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unusual_rc's Avatar
Looking forward to your build!
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Feb 03, 2017, 12:00 PM
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sundogz's Avatar
Me too! I have a 30mm drive from the 3D boat project that I won't need. Should be suitable for a 20 to 30" boat. I'll donate it to your project if you'll use it. It should be a good one and you'll have plenty of help from Joe and Brad (Firehawk) they both have similar ones they'll be putting in boats soon. Sdg
Feb 03, 2017, 01:53 PM
Registered User
Quote:
Originally Posted by sundogz
Me too! I have a 30mm drive from the 3D boat project that I won't need. Should be suitable for a 20 to 30" boat. I'll donate it to your project if you'll use it. It should be a good one and you'll have plenty of help from Joe and Brad (Firehawk) they both have similar ones they'll be putting in boats soon. Sdg
That's very kind of you sundogz! thanks!

I'm off out tonight with my girlfriend so no boat news to share! I do have some videos though from this afternoon when I went boating with a friend who helped a lot with the build. His boat is just a standard brushed job on 6 and 7s nimh packs. He made it years ago as a side project, his main thing is planes like me.

I'm more attracted to boats at the moment though because I get more pleasure from the designing, building and engineering. Its pretty hard to design a plane truly from scratch and incorporate something slightly experimental like a jet so the boats give me a more full experience. I want to do a car one day, now that will be a challenge!

I'll pop the videos on the main jet thread when they have uploaded :-)
Feb 03, 2017, 04:46 PM
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Kayaker's Avatar
The bigger a boat is the harder the river will smash it into the rocks. Make it as strong as you can but this adds weight and that makes the boat hit harder. Around and around I go.
Feb 04, 2017, 11:57 AM
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dunk911's Avatar
Quote:
Originally Posted by Kayaker
The bigger a boat is the harder the river will smash it into the rocks. Make it as strong as you can but this adds weight and that makes the boat hit harder. Around and around I go.
Yeah OK I understand. It makes perfect sense too. I'm not thinking mega rapids,vi don't like ruining stuff I've spent ages on but I'd like it to be able to bounce a bit!!!

Been on my bike today, spent basically all the ride checking out every river I rode past for potential! I'll start hand sketching at some point tonight. Should have some ideas to share soon.
Feb 04, 2017, 06:07 PM
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dunk911's Avatar

Preliminary sketches


Rough indication of the shape I'm thinking of here :-)

A 700-800
B 200-250
C c.70 depending on the size of the pump
D 3/4.C
E width of pump inlet + 3 either side
F curve of bow to start 1/2.A from transom. Will give me more forward volume. Previous boat was 2/7.A for comparison.
G sloped sides to prevent pooling of water
H running this line all the way to the bow. Will give me more forward volume

*dims in mm

Do you think the proportions are about right? Let me know if we need a better picture :-)

Cheers, dunk
Feb 06, 2017, 02:57 PM
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Kayaker's Avatar

Hull shapes and ride plates


I like your hull shape. The bottom is flat enough to give good primary stability and the sides are flared enough to give it good secondary stability. These both will help to keep it upright.

I like the amount of rocker you have (proportion of bow rise to flat planing bottom) . This will work well in small rapids and you will have lots of control but will be a little slower on the flats. This much rocker can have steering problems at top speed that can be fixed with ride plates and trim tabs. Longer , narrower hulls will give a little more top speed but sometimes can have it's own steering problems that can be hard to fix with trim tabs and ride plates. The ride plate that I use with “upside down fins” helps hulls with lots of rocker to go where you point it and also helps long hulls to go strait with out running off in unintended directions when it crosses eddy lines. These “fins” also control the slippery, sliding around that jet boats always have a problem with when planing. Have you ever tried to throw a dart or shoot an arrow with out feathers? I've been using aluminum door thresholds or door shoes (bottom) from the hardware store (about $15) to cut up for ride plates with “upside down fins”. I'm including a photo of a jet sprint 410mm hull with lots of rocker but very little bow rise. It's handling problems are well controlled with this plate.

On the hull with the red deck I pushed the V of the bow in about 2mm using a heat gun to get better directional control in turbulent, swirly water. Hard to see in the photo, it's the front two inches (50mm).

I sometimes tape some garbage foam to the bottom of an existing hull to test new and different hull shapes. This is very fast for developing new ideas and putting old ideas to rest before committing to a new shape.
Last edited by Kayaker; Feb 06, 2017 at 03:27 PM.
Feb 07, 2017, 02:05 AM
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dunk911's Avatar
OK, so these upside down fins are a bit like winglets on a jet plane? They stop the outward flow of air (water) over the edge of the wing (ride plate) , giving the boat better directional control?

I'll get some of that channel for door thresholds, it looks very useful stuff.

Thanks for your feedback on the hull shape, I know its not easy to get too much of the shape from two small views, i'll try to do a CAD this week which will makev it easier to see :-)
Feb 07, 2017, 11:41 AM
Just Plane Nutts
AirDOGGe's Avatar
Quote:
Originally Posted by dunk911
OK, so these upside down fins are a bit like winglets on a jet plane? They stop the outward flow of air (water) over the edge of the wing (ride plate) , giving the boat better directional control?

Actually, they act more like a horizontal stabilizer. The difference is that a stabilizer raises the front of a naturally nose-heavy aircraft to achieve aerodynamic stability, while on a jet boat the ride plate's job is pushing the bow down if it rises too high and keeping the hull running on a level plane.

Winglets capture the outward flow of air towards the wing tips and re-direct it rearwards, reducing drag by eliminating the vortexes that are normally produced and providing the effect of a larger wingspan, without needing the extra maneuvering space on the ground a larger wing would require. They aren't used for directional control except in cases where they are placed on canard-type aircraft with the main wing mounted in back , where the winglets serve as vertical fin stabilizers.
Feb 08, 2017, 05:09 PM
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dunk911's Avatar
Thanks, AD. Think I just got confused!

So I have been doing some more preliminary sketching. Let me know what you guys think please!

On my first boat I had parallel sides that mat at a point at the bow, curving from 40% of the way forward from the transom, kind of like A. Is there a benefit to be had from curving the sides in to the transom like B?

Similarly looking from the side, is there a benefit to having a curve lead into the transom as in view D or is a traditional 'flat bottom' like C best?

I know basically all the boats I see are fairly flat and square at the back, but I wondered if I could benefit from a curve into the transom to streamline things a bit? Has anyone got any experience with this!

Also I did a 3D view to communicate my idea more effectively. Sorry I'm no artist! I labeled it to help folks who struggle with my sketches. Please let me know what you think!!

Cheers, Dunk
Feb 08, 2017, 05:48 PM
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sundogz's Avatar
I don't think curving the sides in would be to any advantage, as there would be a wee bit more waterline that way which would result in a wee bit more drag. I have seen racing sailboats shaped that way, but I believe that has to do with the hull shape (rocker) when heeled over in turns. And you really don't want more rocker in the hull (D), I'd keep the back half flat. I don't understand your last drawing but maybe someone can explain....this is interesting!
Last edited by sundogz; Feb 08, 2017 at 06:13 PM. Reason: sp.
Feb 08, 2017, 06:07 PM
Steps? What steps?
zozer's Avatar
Straight back from the widest point to the transom, no taper! The issue is not more waterline - this is a planing hull after all - but keeping a wide enough bottom area to maintain lift. Do not add rocker to the bottom, this will slow the boat down and give lousy handling. Just look at successful small planing hulls - models or full scale. All are straight lines back. Don't try to re-invent the wheel here, the result will suck if performance is your goal.


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