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Aug 31, 2008, 07:01 PM
RELAX. You'll live longer
785boats's Avatar
Great job cyberhoops.
Personally I think the speed was fine bearing in mind the vintage that the model comes from & the style of it.
Being a lover of speed myself I think that using the same geometry but building a frame & superstructure more reminiscent of some thing out of Star Wars would look better going faster. Perhaps powered by one of these.
Just a thought.
Best regards.
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Sep 03, 2008, 02:52 PM
3 Blades to the Wind
Shaun Hendricks's Avatar
You'd need to watch getting too much weight too high up using that pod, but it is nice looking! Too bad it only fits a 380 motor.

Yeah, I figured 3-blades would work much better behind that cabin. They also provide more static thrust than 2-blade props do. Since the water resistance is so high, you aren't going to hit the ceiling speed of the prop anyway so you won't see any performance drawback from the 3 blades versus 2.
Sep 03, 2008, 08:46 PM

Good job cyberhoops.

Its funny how two persons in different parts of the world can have similar ideas, one year a go I decide construct two boats, one what I lost 20 years ago (a sterling puddle jumper) and other what I see on Popular Science when I be kid, the project its un finish but I want share this two pictures.
Sep 04, 2008, 10:25 AM
3 Blades to the Wind
Shaun Hendricks's Avatar
Very nice! 785boats pod would look right at home atop that canopy, maybe lowered just a smidge.
Sep 05, 2008, 12:56 AM
Registered User
highace1's Avatar
Have you been able to do any more testing of the new motor combo? wouldnt mind seeing some more video. Looks great on the water!

This thread has sparked my interest in this model, im playing around with some ideas for a new streamlined cab.

Sep 05, 2008, 03:16 AM
Registered User
I have been following this one with a lot of interset as I am a big fan of old technology / design and the ability to make it work well. A long thesis there that I can elaborate on if you want.
This is a very neat and simple design that has potential possibly beyond the original designers ideas given the powerplants available today as opposed to when it was designed.
I will continue to follow this thread as I am keen to move into foils soon as a new area of design and development for me.
I love speed: hydros, 'riggers, tunnels, canards.....I have all of those.

I want to work with foils and learn.
I also am working on a set of plans for a tug.
Sep 11, 2008, 11:36 AM
Go Fast, Turn Left
cyberhoops's Avatar
Thread OP
I added one last test run movie to the video post, which is a bit longer run with the new motor and prop combination. I found that at full throttle the motor is pulling just over 12 amps, so it peaks out at somewhere between 125 and 140 watts.

I have tweaked the "lift" in the rear tabs somewhat, but over 3/4 throttle the model still has the "stuffing" problem where it noses in. You can see that happen a couple times in the video. I still plan to tweak a bit more to see if I can fix that problem. It might be the due to the motor thrust angle lifting the boat too high, the up angle on the rear tabs, the angle of attach on the front foils... who knows.

But, for the most part, it works pretty nicely I think. I didn't really know what to think when building this model and the overall result certainly achieved and exceeded my expectations.

I do like the cabin design posted above by mroca. That kind of shape will make the prop far more effective than mine. I would be fun to see some other builds of the hydrofoil up and running.

Thanks for all the feedback and comments... this was fun.

Sep 11, 2008, 01:06 PM
Registered User

This is entirely speculation on my part, but since it seems as if it hasn't really be addressed, I'll throw in my $0.02 ... I'm wondering if your "stuffing" problem is a foil ventilation issue. The other idea I had was that it could be CG related, but from the looks of the videos that aspect seems to be okay, actually (the boat looks like it sits fine - front to back - in the water and while foil-borne).

What I'm getting at is that the boat seems to suddenly lose lift on the front foils, for some reason. It seems logical that if they are ventilating at full throttle, their effectiveness would be reduced to the point of no longer lifting the boat.

Perhaps another way to look at it is that as the speed increases, the foils come up high enough in the water that they eventually get to a point where the flow around them starts pull/mix air in over the lifting surfaces ... and since they're designed to work in water, not air, they no longer lift as well; causing the boat to "stuff" or "nose dive".

That's just my quick guess, not really based on much other than me watching your videos and thinking about it for about 30 seconds or so (as in no serious analysis more along the lines of I wonder what could cause that? *pause* Well heres one idea ) ... anyone wanting to chime in, please feel free to point out where/if I've got it wrong.

Sep 11, 2008, 03:17 PM
3 Blades to the Wind
Shaun Hendricks's Avatar
All foil designs have a maximum possible speed before they ventilate/cavitate.

If tweaking the foil settings doesn't elminate the 'stuff' situation, then in all likelyhood you have reached max speed for the hydrofoil design.

There was some video running around of a Naval Hydro doing the same thing. I think it was the High Point or Plainview. Imagine being ON the hydrofoil and having that happen...

I'll provide this link again for those interested:

International Hydrofoil Society

Lots of data and examples of every kind of foil there.

Interestingly enough, it would look like the platform would be good for experimenting with different kinds of foils.

I'd kind of like to build one BACKWARDS so it is in canard configuration and the prop hangs off the back. It is a nice, nifty design that PS has here. I still like to watch it run.

Any thoughts as to try the air rudder? That might help with low speed maneuverability.
Sep 12, 2008, 11:02 AM
Go Fast, Turn Left
cyberhoops's Avatar
Thread OP
I think the ventilate/cavitate answer might just be correct. I did a few more runs with less lift on the back foil and while the stable top speed has increased a bit it will still nose dive.

Looking in slow motion at some of the video it seems that at higher speeds it is riding very high on the front foils, then if the boat turns slightly you get a bit of a rolling action and the boat leans slightly away from the turn. That brings the foil on the inside of the turn up even higher leading to the loss of lift. You then get the nose dive. It digs into the water about 1/2 way up the cockpit on the nose. I can't imagine being on board a hydrofoil and having that happen... dang - that would throw things around a bit.

I can now get it to run at full throttle for a bit, but at that point the slightest twitch for direction change and it will "stuff". At 3/4 throttle it has become quite stable.

I think this could make a nice little test platform. Making a design were the front foil is hinged so that the angle of attack could be adjusted might be a place to start. I could also see trying various foils including some with a bit of "washout" as you get to the tips where the angle of attack is less toward the bottom of the foil. The possibilities are endless. (Where is my computerized ride control when I need it? Maybe a servo to adjust the attack angle on the front foil based on a percentage mix of the throttle channel?)

Sep 12, 2008, 01:35 PM
Registered User
charlie eaton's Avatar
Have you given thought to the angle of thrust on the motor? If it is parallel to the line of travel or if it is tilted downward at the rear(upthrust) it may lift the tail causing nose over. Try a bit of downthrust(lifting the rear of motor) or put a spoiler (like the race cars) to hold the tail down at speed.
May 30, 2009, 05:01 PM
Registered User
Wow! Where do I begin?

First, I am in awe and amazement! I do so love this thing!

I too am a plane man but before I became a plane man I saw this article long ago and tried building one for a science fair project but my building skills were almost non existent and a radio was only a dream, plus there were many other over whelming things at that time.
From time to time I would look at the design and wonder how in the world it could be run by R/C.

As for speed it was never designed to go fast as it was to be run free or on a tether.

I have some suggestions for improvement.

The biggest thing you have to overcome to make it go faster is less drag. I know, there ain't much hanging in the water once up on foil but the foils are too thick. If made from thin hardwood or better yet thinner metal and filed to airfoil shape you can reduce drag more than you may realize.

Get rid of the rudder! I know, sounds stupid at first but before I read the article I was looking at the pictures and got caught up with them. I saw what looked to me like a post in the "rudder" fin I thought that was how you were going to turn the craft. In fact you could easily do it that way. I feel sure the steering ability would be more responsive even when 'down'

Another real problem it the high thrust line. As proven it is not hard to get it up and into the air, even at relatively low speeds but once up, you have raised the thrust line even higher.
Consider this: Most of the drag, in flight is at waters surface and below. The high thrust line plays heck with this!

Maybe a cabin less tall (also contributing to overall less drag) with, say a couple of EDF units, one each side of the cabin and mounted as low as reasonably possible could help.

I wonder if and how a prop drive might somehow be added below the water's surface...

On the other hand I this project was a complete success on every level and with hardly any real changes brought a dream of mine to life!
Mar 31, 2012, 06:55 AM
Registered User
wparsons's Avatar
Originally Posted by cyberhoops
Let me think about this:

I think this method breaks down. If we test the approach above using a 45 degree angle would be 45/16 over 3 9/16. That would be 2.8 inches over 3.56 inches. However, 45 degress should have equal lengths and be 3.56 inches over 3.56 inches.


Andy - All that being said you are still correct. The offset is wrong and the angle is too big.


I used the theory where the opposite side of an angle is the radius * sin(x). For a shallow angle like that my 1 inch wedge has a radius that is still really close to 1" - so

sin(3 degrees) = 0.052
sin(4 degrees) = 0.069

Since 1/8" = 0.0625 - it is actually much closer to 4 degrees but in a decent range.

WRONG - 1/8 = 0.125. 1/16 = 0.0625. Using 1/8 gives an angle between 7 and 8 degrees - not 4.

Meanwhile - I didn't realize the 049 had that much kick. I think I will adjust my motor up slightly. I have no idea what kind of speed to expect from this hydrofoil. I think I will be surprised if it hits 10 mph, but you never know.
Way late to this party... but I thought I would add 2c about the math in case anyone else is looking at this thread too.

For the 45* angle, you were looking at the hypotenuse over one of the equal length sides, not comparing the two equal length sides. In that case, the height should be the radius divided by sqroot of 2.

For a radius of 3.56", the height would be 2.517" for 45*. I'm not awake enough yet to see where the other math is breaking down, but 45* angles are easy to work with.
Apr 04, 2012, 06:28 PM
Registered User
Awesome project and thank you for the mathematics behind it as well!
Apr 04, 2012, 09:27 PM
Mariner BC Canada
BluenosePacific's Avatar
Viva la hydrofoil!!

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