Rudder design for solar boat

[lost at sea]

I was researching rudder design options when I had stumbled upon this forum site. I am currently a mechanical engineering student who is doing a senior design project on the rudder design and steering mechanics of our team's solar boat that competes in June.

While the first semester I spent designing what I thought was an acceptable rudder profile and geometry, my advisor seems to think intuitively that my design is flawed. I chose to use a NACA 0012, quater elipse profile since, through research, I concluded that the lift to drag ratio would be maximized at this thickness. I then proceeded to test various spans and depths using the Visualfoil software. My final geometries were chosen to be a span of 7 inches and a depth of 11 inches. The profile is to be constructed using the balanced 80/20 technique. This was the most efficient performing profile I could design that could be actuated using the power from the gearmotor produced by the steer-by-wire design that an electrical engineering student placed in the boat last year.

I plan to use a set of dual rudders mounted directly behind the slipstream of the dual prop configuration. I also have to mount it at the transome of the boat due to the placement of the twin screws in relation to the stern of the boat (limited space caused by other boat circuitry).

My question is: Is there a better profile and geometry I should be using? My feelings about last year's problems with manueverability are that they are likely to be caused more by the type of hull and speeds reached, than the rudder design itself (last year's design was larger, but of the same profile). I was told to reduce the drag of the design since the rudders were in stall most of the time and created a drain on the instrumentation batteries for our race. I'm to the point of maximum frustration, and any help provided would be most graciously accepted.

[SailDesign]

What sort of course do these boats have to go round? Is it a maneouvering trial or a speed race?
In other words, are you trying to minimise drag completely, or improve efficiency of the rudders while keeping drag "a bit lower"?

Steve

[ClarkT]

Hi lost,

Good for you for doing the race. It is a blast, and some of the boats are pretty cool.

Steve,
The race as I remember it is a 2 hour endurance (carrying solar cells) and a 500m sprint (removing solar cells and using batterys only.

The NACA0012 does not sound like a problem, I wonder where your advisor is coming from?

But why are you using two rudders? Given the speed you are going to be doing in the endurance part of the race, I can't see the need. Some boats in the past have simply used a trim tab to offset the side forces from the prop, then heeled the boat to steer.

If the two rudders are for the sprint part of the race...fugetaboutit! UNO has that division wrapped up.

Good luck! Let us know how it goes.

[lost at sea]

The sprint race is like a quarter mile drag on water (though only 300 meters long). Steering is not of utmost importance expect for correcting the path. Therefore, myself and the software engineer will allow for limits to be placed in the program to allow for a maximum steering angle of 0 to 5 degrees (we need to test it on the water later on to ensure if this is sufficient).

The main area of concerns were in the slalom and endurance races. The slalom tests the manueverability of the boat where the course includes a corkscrew turn. The speeds target speed for this year's boat is approximately 16 knots.

As for the endurance race as described by Clark T, we hope to acheive a target speed of 8.25 knots for the duration of the two hour race (it's a lofty goal I know Clark). The advisor's thoughts are that he was never happy with manueverabilty of the boat during this race. My thoughts were that by using dual rudders mounted behind the screws I could take advantage the extra force created by the prop wash. I have also devised a way to use a toe-in technique for the rudders to hopefully add greater turning force to increase the performance.

We plan to use the same hull from last year's team. This being a semi-displacement hull with a shallow V and a varying deadrise. The boat has a length of 17.5 feet with a maximum beam of 2'10". The theory for this hull is that it'll be able to cut through the water at slower speeds (endurance race)while the varying deadrise permits it to plane at higher speeds (sprint race). Is using two rudders being overkill if they thought last year's boat wasn't turning fast enough? Could trim tabs be a good suggestion to utilize the prop forces? And most importantly, is my design justified as I had researched and concluded?

If there is a better way to do this, please inform me.

Thank you all,

Erick

[SailDesign]

Lost at sea says:
"I have also devised a way to use a toe-in technique for the rudders to hopefully add greater turning force to increase the performance."

You have a two-part problem here. For max turning force (like the Ackerman effect on car steering), you want to aim both rudders at the bow, roughly. This also works well for twin-rudder sailing boats.
For minimum drag, this is not good, however, and you need to set both rudders as follows. Loosen the steering gear so that you can adjust it for toe-in. Get the boat up to speed and let the rudders do what they want - then tighten them. They will have weather-cocked to the lowest-drag position.
If you can "remember" toe-in setting, you could use both in their right place. Otherwise I would set for min. drag and forget them.
Steve

[lost at sea]

Steve,

I was planning on using a four bar linkage with adjustable arms to test and set the offset utilizing the Ackerman angle from each rudder arm to the center of floatation. Hopefully through testing we can find the best fit for each event.

Do you think though that using a quater elipse NACA 0012 is a good choice given the speeds we will be operating under, or would you think a 0015 or 0010 would be better choices? Maybe I should not be using the 00 series altogether, or just use a different profile than a quater elipse?

Erick

[SailDesign]

Erick,
The 0012 is probably fine. The drag difference between that and the 0010 is probably negligible (except in the lab) and the 12 will stall later, which would be handy for the slalom section. I have seen 0020 used on faster boats (counter-intuitive, but logical in the end). As far as the quarter-ellipse goes, it should be good, too.
Other sections to look at would be 63-012 or 63A-012, but with the cusps removed (just fill in the "dent")
Steve

[tspeer]

You might want to look at http://www.tspeer.com/Planforms/Planar.htm for alternative planforms. It depends on how much influence you think the surface and hull have on your rudder. Rectangular planforms are not all that bad. If you want to reduce induced drag, make it deeper.

I'd advise against the NACA 6-series sections. And filling in the "dents" gets rid of the most beneficial aspects of these sections while retaining the worst behavior (leading edge stall at low Reynolds numbers). The best section is one that you design yourself using XFOIL to meet your particular requirements.

[SailDesign]

tspeer says:
"I'd advise against the NACA 6-series sections. And filling in the "dents" gets rid of the most beneficial aspects of these sections while retaining the worst behavior (leading edge stall at low Reynolds numbers). "

We've used those sections for years, very successfully, on larger boats. On a dinghy, I wouldn't touch them with a bargepole, but on something where the helm is unlikely to be shoved hard-over quickly, I still think they're a good bet. YMMV (and obviously has )

Using XFOIL to design foils, or even Panda, is dangerous if you have no experience. I've seen foils that looked great in the puter, but were so intolerant of cross-flow or small speed variations that they were useless as anything on a boat. Unless there were NO waves .

Steve

[ClarkT]

As I understand the way the rules are interprited, any fixed gear must remain fixed, but if it 'kick's up' or might be able to, it may be removed for other portions of the race.

If the rules are still read this way, you could have a bow and stern rudder. The stern rudder could be your primary, and the bow rudder could overhang the bow just for the manuberability portion. If it kicks up (like a sunfish rudder) then it is legal to remove it for the sprint and endurance. For the slalom, it should turn on a dime.