Another Hare-Brained Idea

[Dr. Jet]

I was deep into a YouTube rabbit hole when I came across this:

https://www.youtube.com/watch?v=RcCcb2d-z-M

Now that looks like a hoot! I researched jet pumps and they are available from small to HUGE with several sizes in between. The "small" (15mm impeller) pump would make a good micro......

This got me to thinking...

Years ago, I built several slope gliders out of EPP Foam. You could "paint" the shaped EPP foam with thinned Goop, and could even laminate Kevlar fabric into the Goop covering. I've flown my Kevlar/Goop protected Moth glider full speed into a concrete guard rail and just threw it off the slope to fly again. It is, in fact, bullet proof.

So here's what I'm thinking: Has anybody built a jet boat out of EPP? Anybody have some ideas? Suggestions?

[J Mack]

[QUOTE=Dr. Jet;758555 Anybody have some ideas? Suggestions?[/QUOTE]

No but you had me at "It looks like a hoot"
I cant wait to see what you come up with!

[Dr. Jet]

I obviously need to finish a few projects that were put aside recently to work on some guitars. I have two more guitars in the building queue, and I can get back to finishing the Atlas and the Miss Madison at that time. While I'm finishing those, I can work up a design for a bulletproof jet boat.

Some preliminary criteria:

Simple to build.

Off-the-shelf parts (more on this later)

Self-righting. Duh

Easy to seal hatch.

Protected jet nozzle.

[properchopper]

The boat in the vid (https://www.youtube.com/watch?v=RcCcb2d-z-M) has some major cajones. I swear that I could make out a hungry bear on the shore scratching his head....

[Dr. Jet]

The boat in the vid has some major cajones..........

That is the Thrasher V3 from Streamline boats. https://order.streamlinerc.com/collections. A bit pricey at around $1K RTR, but plenty of cajones. I'm thinking smaller, cheaper, and build it yourself.

[Dr. Jet]

I thought about this as a starting point: https://www.hydromarine.de/Hulls/Mic...ion::1982.html because the hull shape could easily be adopted to a jet. But even made of Kevlar/epoxy it would eventually shatter into a billion pieces whilst bouncing off of a multitude of rocks. The "glue" in the composite structure needs to be flexible; epoxy isn't. Goop on the other hand, is!

[NativePaul]

I think you may be overestimating the forces involved and underestimating epoxy.

A clubmate has a brushless NQD Tear Into (worth looking up, it it your size and scale, available for about ?40, it is toy grade but brushless conversions are common and I suspect you have the parts needed on hand already), it is a dirt cheap plastic hull that you would want to change out in the longterm or at least get rid of the screws but it has taken years of abuse with just scratches to show for it. It might be worth buying for the pump, and give you something to run in the intrim while you work on other projects, and help you narrow down further what you want form the hull that you make yourself.

Speeds are lower with jets even than with submerged drives never mind surface drive, and I think you would have a lot of trouble getting to 30mph in a mini jetboat, and weighing only about a pound there is not that much energy in a collision, especially with glancing blows against slippery wet rocks little of that energy will be transferred to the hull.

While epoxy is less flexible than Kevlar it is still reasonably flexible, a well laid up epoxy Kevlar won't shatter into a million pieces (or any pieces) its failure mode is for the epoxy to crack reducing the hulls stiffness, it locally turns into a kind of stiff kevlar bag, any brittleness is either caused by excessive amounts of epoxy in the layup, use of vinylester resin instead of epoxy, or commonly by use of gelcoat (which is the spawn of Satan). Often on RTRs you find the trifecta, with excess vinylester resin used due to a wet layup and thick gelcoat used to ensure a good finish, the gellcoat has great compressive strength but almost no tensile strength so when you flex it away from the gelcoat, the gelcoat cracks, and when you flex it towards the gelcoat it gives added leverage to break the fibres in the layup.

My Mini Hydro is a vacuum molded kevlar item mostly using 92gsm, with doubled up areas for local reinforcement (no gelcoat), it has hit numerous bouys, and boats at race speed over the years and I have splintered countless solid carbon booms in it but there is not a single crack in the hull. Even though there are no flat areas on it and it is all double curvatures, I can bend the flattest part of the hatch into a circle with about a 1" diameter without cracking the epoxy and it springs back to shape just as it was.

I am very intrigued by the kevlar goop over foam you have used though. What do you thin it with that doesn't eat the foam? Can you use any foam or does it have to be a special sort? Does it add much stiffness, or is it just abrasion and tearing resistance for the foam?

I have done epoxy kevlar over foam, then melted the foam out for lost foam moulding, but found the wave to be too prominent for hydrodynamics on the bottom and for tape to stick well on the top, so resorted to a thin layer of sacrificial glass over the kevlar so I could sand a flat surface onto it, and then realised the kevlar wasnt worth the hassle and switched to plain glass with local reinforcements for lost foam moulding. What sort of surface finish can you get with kevlar goop?

[Dr. Jet]

You may be technically correct that epoxy is somewhat flexible. I use it on the wings of my glider and they flex a LOT. I may have Oliver build a Kevlar Firefly, just to test out powerplants and setups. I'm also aware of the NQD boats and the hop-ups being done to them. Very cool indeed.

I'm well aware of speeds and the limitations of jet pumps; the boat just has to be faster than the river current, it doesn't need to set speed records.

The one thing I've seen is that all encounters with immovable objects are not necessarily glancing blows. The T-bone is common; the worst situation is when it launches straight up and lands on the transom (think rockets that land on their tails). That's why the boat in the video above has a nozzle protector on the bottom of the hull.

I use expanded polypropylene foam (EPP) and thin the Goop with Toluene (hard to get in Kalifornia). EPP is highly flexible and nearly indestructible. You need that special "cheese grater" sandpaper screen used for drywall to sand it. The Goop adds some stiffness, depending on the thickness applied (weight). A very smooth finish can be obtained, but it will never rival well-built wood or gel-coated fiberglass. Kevlar, being a flexible cloth only adds abrasion resistance.