grael

Max,
I used ordinary balsa wood glue on this one, because it's more sandable than superglue. Superglue's faster, but I use it more if I'm bonding ply in a hurry. The balsa glue sets moderately hard in half an hour or less with ventilation and moderate warmth. So long as you keep putting in more tacks, and give it a rest somewhere hot after, then it's easy.

The finish... I brushed polyurethane resin on and between the cracks, surface tension held it there, and it locked the balsawood planks together where I hadn't bothered gluing. (more photos going on this message shortly.) It's ready to sand in less than half an hour, which is much better than the sandable primer, as you can lay it up thick too.

I sliced the middle of the build deck out this evening, so that I could reinforce the balsa planks from inside, I mixed up about 40mL of poylurethane resin, poured it into the front of the boat, then let it flow from the front to each cavity back in turn, then sloshed it around untill it was sealing all the balsa from the inside, as some of the planks were pretty thin, and wouldn't sand quite right. (flexing when sanded) I threw some woven glass fibre scraps over that, and now it's really solid. If it survives demoulding, I may even use the hull model for an operational boat.

I've cut into the stern and underside to make my shroud now, but it's pretty minimal, it's only recessed about 20mm in from the stern, I figure that I should be able to fix a steering nozzle pivot point under the shrouded cover, that's where the backwash usually leaks out. My existing outlet-stators can be modified to fit, but I'll probably do a new one to suit the small shroud space.

Anyway, pictures tell it best...
I've got what I hope is a final coat of polyurethane resin on, hardening at the moment. Then I need to sand with 400 grit, then 800, then apply planing strakes and polish. I'm planning to save myself a lot of heartache this time by coating it with silicone resin, then a layer of fibreglass, so that I can mould without release agent, the silicon will just peel off. Without the strakes, it would run very wet, and I want this boat to run very fast, so I'll be putting probably jut two strakes on, but it should also cruise at next to no power off the plane due to it's length to beam ratio and the rear cutaways.

sundogz

This stage of your boat (smooth bottom) reminds me of an idea I had for the circuit racers. A rudder midships - almost an extension of the keel - say, about 15mm long by 6mm tall (tapered up the front) operated by the steering servo. It would turn the opposite direction of the steering nozzle. Should turn on a dime and give you change back. Remember you heard it here first! Sdg

chf

grael

I think it would roll the boat ?
The steering nozzle is there to point the boat in the correct direction, if you try to steer with anything under the centre of gravity, it will just roll the boat off the steering[ornament].

However, I might be completely wrong, and would love to see a video of your boat performing with your invention...

TwinJetMax

I agree with Graham there. There shouldn't be any pivot points under the hull.

Grael: I really like your idea of strengthening the hull from the inside, so I copied it.
My hull wasn't strong enough to hold the filler together white it dried.
Here are some pics of my hull. I obviously don't have quite workmanship skills of Graham, but I make up for it with design skills.

grael

That's looking good Max.
I noticed that you didn't have any bulkheads in the back/middle section. I think I tried that a long time ago, but I found that it makes a sort of over-compansating warp and a hollow in the place with the biggest gap.

I've been making a deck today. My superglue doesn't seem to be sticking anything except for my fingers today, and so I switched to urethane resin, and then found that my hatch position was about 1-2mm further to one side than I wanted.[] You have to look at it reasonably closely to tell, but I'm going to use it anyway.

I've traced another deck outline, so I can always do another deck if I like the performance on the water and want to spruce it up a bit.

Next tonight, is sand the deck, polish the deck and hull, and then pour a coat of silicone over the top. I have a theory that if I put some muslin or dynel over the silicon just as it's getting tacky, then I can fibreglass over that, and have a non-stick self releasing, but otherwise rigid, mould.[8D]

TwinJetMax

I am way too lazy to put bulkheads in that section. It seemed to work out fine this time. The glass will strengthan it up somewhat.
The first phot looks like it has a warp but it is just poor sanding.
Where do you get the silicon from? I like the idea of non-stick.

Anyway, here are some pics of my carbon cloth. It is uni-directional but I think with two layers (perpendicular) it will be extremely light and strong.

grael

Topmark distribute the silicon, but Modelcraftandhobbies retail it in Wellington. I think there is a place out in LH that sells Dow Corning silicon resins too.


Be warned, it's quite expensive!

"Pinkysil" will go off in about 10 minutes, and be almost finished solidity in about half an hour. I had some of a cheaper version (tin cure Ultrasil) left, and thought I'd give it a try. If I'd used 3 x the quantity, I'd be expecting sucess, but as it is, I'm not sure. I laid up cheesecloth over the top for tear resistance, and was hoping the fibres would stick out for a layer of epoxy over the top to grab, but i'm not so sure it will work how I've planned, as I tried to do Deck and Hull with my mixture. I'm not going to risk epoxying tonight, as I don't want to risk a bad reaction between the silicon and the epoxy.

Regarding your carbon fibre, how were you going to lay it up ?
The tensile strength comes into effect on the side of your finished material that has to stretch during a bend or impact, the outside of the bend. The compression resistance is needed on the inside of the bend, but the cured resin will have compression resistance anyway. If it were me, I'd run crosswise on the outside of the hull, and lengthways on the inside. However, it can still shatter with a decent impact, and I would even use 3 or 4 layers if I wanted a hull that was strong as well as light.

One layer of thick chopped strand, or 2 or three of light grade woven glass fibre work pretty well in my opinion.

The other thing that will increase the hull's stiffness, is your design... if you have complex surfaces, steps and/or planing strakes, all these help to stiffen the hull. This is most important on the planing surfaces, where you can get traveling wave effects in the hull if it's not at least moderately stiff.

chf

HI guys.

No idea why my last post did not show. Anyway, I'm new to the world of boat building and "Jets" are what I am interested in. This all is great, but does blow me out the water a bit.

Some help would be appreciated with a few things:
I have a load of coroplast 5mm. Could I use this for a jet boat?
I am having trouble finding plans on the net, what should I aim for? or is there a good program to use?
Is it possible to build a jet or what else could I use to force water?

Hope you all are doing well.
Regards

TwinJetMax

I think that you will encounter issues with that. It doesn't bend well and can easily get filled with water and hence be heavier to get on the plane.

No body really builds these boats from plans because there is nothing suitable around. The boat that I a building now (at the top of the page) was designed on google Sketchup. It is really small but if you want plans I can send you some and you can blow them up. Google sketchup is a free program an is really easy to use, s you could design your own boat on it. The trick is to start with frames and a deck and then join the dots. The picture of my design is on the last page.

chf

That would be great thanks. I am downloading Sketchup now. I will see what I can do on it.

I will go for marine sealed ply then. Was looking at ways of sealing the coroplast but not much sticks to it besids hot glue.

Thanks for your help. I will keep you updated with what I'm up too.

regards
Chf

grael

Max is right, but you could try the coroplast anyway, here's how I'd do it:

Cut panels out for all the faces of your boat.
Tape them together.. masking tape or any other cheap thin tape.
Spray lightly with 3m adhesive (or similar)
Lay fibreglass cloth over the adhesive.
Brush a thin coat of epoxy resin into the fibreglass.
Sealing all the coroplast cells could be quite a mission though.
Alternatively, you could do the above process in reverse, applying the fibreglass to the inside, and remove the coroplast when you're done...

Your other question, is it possible to build a jet:
Yes.
It's expensive though, you would ideally need silicone moulding rubber, access to a lathe (or mounted drill), and some considerable modeling practice, or at least patience ! I've made various moulds myself over the years, but I'm still learning ways to improve technique.

The stator vanes need to be well fixed in the outlet, and to get enough strength, you would need to solder, braze, or cast them in one piece like I do. On a small boat, I'd go for 3 to 5 stator vanes, and a cut down 2-blade prop for the impeller. If you carve your inlet out of a piece of wax with a shaft centered in the straight section, then you could fibreglass directly around it to your boat hull.

TwinJetMax

I am on study leave now, so I have time to work on my boat a bit more.
Me no need very bit much study anywhose.

The epoxy resin seeping through the cracks in the boat served as a nice filler. So today I was able to sand the boat down to 1200g and apply the first coat of release wax.
I will hopefully apply another 6 coats, but I am down at the library studying for most of today.
It should be ready soon though.

grael

Sounds like your boat will be much shinier than my one [:@]

I demoulded my hull and deck from my silicon/epoxy sandwich today, and it went pretty much as expected- some minor and slightly less minor voids on the silicon. I mixed up the dregs of my pinkysil silicon, and filled those, but it's not very thixotropic, it was running slowly until it started to gel, and the transition from gel time to tear time was only a couple of minutes[:-]

There's a two part silicon putty that's really expensive, but that would otherwise have been perfect for the job.

I'll trim the globby bits later, and hopefully cast a hull and deck later tonight.

grael

Some more pictures, I've moulded two boats off my silicon/epoxy-glass mould already
I'm trying to get the performance of my old 750mm custom hull, and that's what the new one's designed for- It's shown in Carbon, glass fibre, and the original in Balsa.
The light blue deck and hull mould has silicon rubber inside glassfibre-epoxy, so I don't need release agent[8D]


Second newest 250mm long boat with 4200kv outrunner, and 11.1 V LiPo, but mounted too far forwards:
http://www.rcuvideos.com/video/Custom-jet-boat-testing
(I've run this in slow motion, and it's definitely porposing at speed, too much weight up front. I'll redo it at some stage possibly.. maybe with a long inrunner and saddlepack.

Old 750mm long boat with .40 ASP:
http://www.rcuvideos.com/video/JB-avi

TwinJetMax

I am struggling to get my plug to come out of the mold. It has 5 coats of wax and two of PVA and it is stuck.[]
Any hints?

grael

Max, is it just PVA glue, or PVA mould release (green) ?

All I can suggest, is to either give the resin more time to set (less sticky)

Good luck !

TwinJetMax

PVA mold release. I am not that thick.

I just cut the boat into tiny wee pieces and levered them out with a screwdriver. No plug left now.

grael

Got any pix of your nice new mould then Max ?

This is my glass boat, after a lot of modifying of an outlet and intake from my smallest moulds. I used the blue hobbyking props in the end, because I ran out of the r2hobbies ones. It seems to pump a lot of air... [8D]

I don't like the length of the inrunners.[X(] every time I tried layouts with an inrunner, the batteries looked too far forwards. So, I have settled on my original 2500kv outrunner with the short bell, and I'm confident it will still go well. the torque reaction seems minimal, and I don't think it will be drawing more than 20 amps.

TwinJetMax

I certainly do.

The first two pics are of it last night, and the third id this morning.

The mold had heaps of imperfections in it so I decided to experiment with wet sanding while it didn't matter. I nearly got the mold back up to gloss after sanding it down to 180g. It didn't take too long either.
I figured a mold with holes through it would not have the best non stick qualities, so I put another coat of resin on the inside and let is seep through, while also filling some of the bigger imperfections.
It just need to cure a bit more and then I will sand it down, before wet sanding it back up. It is at a gloss state right now but does not reflect light smoothly, and I am going to be picky this time and do it properly.

Anyway, your boat is looking good. Better than your last one. It will be interesting to see how it goes.

Max

Pics

grael

It looks like a boat Max...

It will be interesting to see what it looks like after some paint. I would concentrate on the boat now, to see if it does as you want. Got a deck planned ?

I'm off to sort my steering, and hopefully try running my boat today.

TwinJetMax

Yep I will take a copy today. I just wet sanded it back up to a semi gloss. Should I try easy cast and fibreglass first? I can get a boat quickly in that case.
Epoxy takes quite a bit longer.
No dec planned as of yet.

I will go and wax the mold up now.

grael

Up to you on the easycast....
I've found it doesn't stick very well to itself though.

Easycast doesn't give you much time to set it up either. It's OK with chopped strand and resin infusion if you time everything just right, and have a good runner for supplying the resin all around your mould.

grael

Ran my boat a couple of times today. The water was rough again, boat handled the conditions fine, but I have a leak that is very hard to find. I mislaid my normal small impeller, and had to cut down a prop with higher pitch, it proved much more prone to cavitation, the roots left after turning down are very high pitch. Also, I need to improve the steering nozzle a lot, it wouldn't turn very well. Might be because of being a long skinny boat, no flat, but I still have work to do on the outlet and nozzle.

The boat's not getting up to full speed on rough water, because it doesn't reprime very well on the super high pitched impeller.

The 2500kv outrunner motor is working fine, marginal torque effect from the very short rotor.

I have a suspicion that the water is getting in while turning via the steering rod tube, but it's hard to be certain. whatever the reason, I run in salt water, so it's not good for the electronics.

TwinJetMax

I just discovered that I had another bottle of epoxy. Which is really good because I skimped heaps when I did my first layer of carbon fiber.
Now I have added another layer of carbon the back half, where the jet and motor will go, and given it lateral strength with a layer of glass fiber.

Also, I have restarted work on my Jet sprint track. So now I will have somewhere to test my boat... in my back yard.
Jealous much Graham
Should be able to pull my boat tomorrow because I have no exams. yay.

Can't wait to see the result. My order from hobbyking is down the road at the Courier Post Centre, so I will be able to pick that up.
I have an outlet and stator so I just need to make an intake and I will be just about done.