# Rc sailrocket

Discussion in 'Hydrodynamics and Aerodynamics' started by Glueandcoffee, Feb 9, 2021.

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### AlexanderSahlinJunior Member

I used the same equation for lift-force as you used in Post 4. Then I assumed that the force from the sail was as large as the force from the foil in water, to set up the equation for equilibrium. Then I divided both sides of that equation by the the speed ^2 multiplied by the density of water/2.
I came to the sail-area 11 sqm. by the hypothetical assumption that the CL of the foil was 0.6 and that CL of the sail was 1.0.
In the real world you have the sail-area you have made, and the CL:s will adjust themselves so you get equilibrium.
My assumed CL ≈ 1 for the sail is a rough estimate from my experience of model-airplane wings. It can be lower, it can be a little higher depending on the aoa, but it is hard to get it higher than CL=2 for a single-element airfoil, especially at model scale. I recall that we had CLmax slightly above 2 in a windtunnel-measurement on an early version of the wing-sail for my paravane-speedsailer, but that was a very extreme high-lift wing-section, that depended on laminar boundary layer in the front part. The final wing-design for the paravane-speedsailer was a more robust one with slightly lower CLmax, that worked also for fully turbulent boundary layer.
However, my conclusion is that you can start sailing with the foil and sail you have built, and after you have made it work, you can try to reduce the foil-area to reach higher speed. Very best luck!

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### GlueandcoffeeJunior Member

I know I'm getting ahead of myself but let's say what I have built is a v1. Proof of concept. If I was to develop a v2 to not only set a soft water record but get close to the physical limits of what's possible at similar scales, what would be the best place be to start and how would someone go about editing the v1 design to get there.

Very interested to see some pictures or videos of the paravane speed sailor in action and testing.

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### AlexanderSahlinJunior Member

Here is a link for our home-page for the paravane-speedsailer The Swedish Speed-Sailing Challenge https://www.trampofoil.com/speedsailing/index.html
You have the time-line there as well as video-clips and a schematic explanation of the configuration.
The reason that it is not updated is that we have not been sailing since 2006. If we had put in a similar effort as Sailrocket to find the optimum spot to sail and a similar amount of testing at that spot and maybe also the persistence to continue after some spectacular high-speed crashes, we would have been able to reach about the same speed as Sailrocket did.
But now Sailrocket has done 65 knots average (with a peak at 67 knots), so if any more speed-record attempts shall be meaningful, you have to design for 70+ knots.
Before we built the full-scale paravane speedsailer, I built a free-sailing model (sorry, I did not find the picture) 3 sqm. sail, I started with a 0.01 sqm paravane, that should have been the right size at speed, but could not make it start from rest. Then I tried a 0.035 sqm paravane, and after some initial struggling it took off at some 20+ knots (twice the windspeed 10 knots). I had no GPS or similar installed, but I determined the speed from the fact that it did not stop until it reached some 10 m up on the rocks on the other side of the bay.
If you can get reasonably flat water I think the Sailrocket configuration is maybe the best configuration for speed-records in model scale. I think the full scale Sailrocket had some 3 or 4 times the all-up weight as aerodynamic force in the sail. For a model Sailrocket you can have much more force/weight. When your model sails at such speed you will probably have to deal with stability issues. The first Sailrocket was e.g. aerodynamically unstable in pitch when it jumped out of water once at 50 knots, resulting in a looping. Sailrocket 2 has the main-beam behind the C.G. and is therefore stable in pitch, but I am not sure that SR2 is stable in yaw with fixed rudder. On such a big boat "only" sailing at 65 knots this is maybe not a problem, but it may be on a model, sailing much faster relative the size. But I have only followed Sailrocket as a competitor from the outside, Malcolm Barnsley, Paul Larsen and the Sailrocket team must know more.

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### GlueandcoffeeJunior Member

Oh yeah. I've seen your page before. Quite interesting.

As far as I can tell sr 2 is stable in yaw. If the sail is overpowered compaired to the foil it pushes forward more than the foil which in turn yaws it towards the true wind angle. With a small amount of leward rudder trim sr2 in gusts accelerates and bears away which momentarily increases load between foil and sail. Then gets back to equilibrium when it overtakes the gust so as load decreases and it points back closer to wind.
I'm not sure if this simplifies things or even if this explanation is correct but I imagine in consistent wind sr2 would find some stable heading in relation to the true wind angle. I don't think this concept changes much for different boat scales or wind speed.

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### AlexanderSahlinJunior Member

I am not sure I would consider what you describe as directional stability, but as the changes in aoa on the main-foil will be small, because it is somewhat oversize, I am quite sure you will be able to keep your model on the desired course by actively controlling the rudder. Obviously Paul Larsen managed to keep the full scale craft on the desired course, so my worries were probably unnecessary.
Is there any limits in size for a model sailboat challenging the 52 km/h speed-record? or is the only requirement that no human is on board? It would be fun to challenge with one of my paravanes connected to an ultralight model glider at one or a few m^2 (the so-called ultimate-sailing concept).

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### GlueandcoffeeJunior Member

As far as I can find on Google there is no official speed record for a remote control boat. Without anything being official there are currently no rules. 52kmph is just the fastest ice boat I could find and I was curious how fast I could make my boat go. I started building before I started looking for a contender so the boat initially had no target speed. Then I had a "why not" moment and decided to see about getting a record and found there was none.

I think it would be a fun project to set up a speed class of rc boats with a small set of rules as it seems like most other classes would be racing classes designed for all tacks and headings. If we did have a class of boats with only one design requirement to go fast how fast could they go.
I think a set of rules something like this would be a good place to start
10 foot or 3m box rule
4sqm max sail area
Minimum of 1 hull at rest
Start from rest.
Some standard of servos and battery.
100m 250m 500m and 1km records
I'm sure I could think of more.
Any suggestions?

The upcoming 80kt syroco is similar to that ultimate sailing concept.

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### AlexanderSahlinJunior Member

I think there should be as few rules as possible. The rules shall ensure that the model is propelled by the wind and that it is a model, RC-controlled or free-sailing. Your idea of sail-area limit is reasonable. A simple box-rule can be nice, but I think the rule shall be open also for kite-propelled boats. Why require a hull? is it to prevent someone competing with a helium-ballon drifting with the wind without any contact with the water? With such a configuration you can side-step such a rule by connecting a very small hull to the ballon towing that hull with a string. Anyway, such craft can never sail faster than the wind. Configurations where you cancel the heeling moment, like Sailrocket, can sail at more than twice the windspeed.
Having some limit on the size or power from servos is a good way to limit the size to model-scale. A maximum all-up weight is also a possibility.
When deciding the distance you have to consider that a pilot on a fixed spot with an RC-transmitter shall be able to control the craft with the sight of the model as only feed-back. Hence, I think 100 m is reasonable.

Hope the syroco-project build their machine. It will be interesting to see how fast they can go.

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### Will GilmoreSenior Member

Course speed vs momentary speed?
Maybe both.

Since the power is dependant upon the natural forces of wind, current and sea state, should a record be conditional upon those things?

-Will (Dragonfly)

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### GlueandcoffeeJunior Member

It prevents hurricane helium balloon towing and ensures the craft is heavier than air but lighter than water , and excludes land or ice boats and Ensures start from rest. I have no problems with kite power but props to anyone who can control a kite under 4sqm in record breaking conditions with model size servos and batteries without an onboard flight control. Autonomous sailing is not in the same category as remote control or free sailing.
But in saying that I realise I'm only setting rules for the perfect boat because I know it's out of my league.
If you can make an autonomous soft water wind craft that goes 65 knots, more power to you.

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### AlexanderSahlinJunior Member

I think we can learn a lot from the WSSRC here, they have been organising the 500 m speed-sailing world-record since 1972, and have considered a lot of different issues for the speed-record. A reason to average over a long enough distance e.g. is that some boats, like the present AC-75, can reach a substantially higher peak when they bear away round the windward mark than they can do on a straight line.
The WSSRC-records are set independently of the wind-speed, but it would also be nice to have a record for best boat-speed relative the wind.

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### AlexanderSahlinJunior Member

Here is a link to the speed-record for some wind-powered models
In dynamic soaring (a technique developed by the albatross long before anyone learnt to sail) you use the wind-gradient. and with a RC model glider the speed-record is now 476 knots. The models flying at such speed experience some 100 g when turning. At such speed I would probably use less than 4 sqm sail-area if using standard model servos.

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### GlueandcoffeeJunior Member

My point exactly why we require at least one hull and to start from rest. "This isn't a boat" would be a pretty good argument for a dynamic soarer. Also makes me reluctant to veer away from a box rule as it's quite easy to put the gps on the kite or glider instead of the hull to squeeze a few extra knots out.

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### AlexanderSahlinJunior Member

You will handle the problem of unsteady kites by averaging over a reasonably long course. I suggested 100 m, but the RC-helmsman can certainly handle 200 m as well. I posted the link to dynamic soaring just as some inspiration. I don't plan to use it for the model sailing-speed record. The starting from rest rule and requirement of pure wind-propulsion will stop that. But I consider a kite tethered to a paravane as sailing on water, like the kitesurfer, that held the WSSRC speed-record before Sailrocket 2 did. So there is no need to state that the craft must have at least one hull.

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### GlueandcoffeeJunior Member

That is true. I suppose the hull or paravane would almost certainly exceed the speed of the kite while bearing away along the course rather than the other way around with the kite starting farther back at the start of the course and farther ahead at the end.

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### GlueandcoffeeJunior Member

Just an update. Looks like the weather forecast is finally right for a run or 2 on Thursday maybe. 15kn from the west with high tide. Going to do some finishing touches on version 1.
I got a delivery of carbon fiber and resin from easy composites. The next step is to see if v1 works, make some small changes to the setup to see what works best , then design and build a v2 and work from there.
The plan for the fuselage of v2 is to carve out a mold from insulation foam , cover the form in 3 separate sections running lengthwise (bottom, top left, top right). Once the epoxy is hardened remove sections from the foam mold. Glue the sections back together and then laminate the hollow fuselage with a braided carbon fiber sleeve . Then wrap that like a COPV with 25mm unidirectional carbon spread tow tape and a final layer of epoxy or polyester resin to seal it.
I can see this being quite the project and cant wait to see it sail.

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