phickey06

I know there's some pretty smart cookies on this site so i'm going to ask my main burning question of you all.[8D] I have studied and hung around hydroplanes my whole life and I still havent gotten the definitive answer to this.[] When you examine modern limited hydros about 70% have the rudders on one side and about 30% the opposite. If the theory or school of hard knocks knowledge is in,why is this so? What is the theory and why does'nt it apply to everyone?

TERBObob

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Very simple answer - on the right because that is the direction they turn when racing . Whilst on the otherside , a lot of groups over-seas turn the opposite direction when racing

Now , if you want turning to be almost equal , then place it in the middle behind the strut/prop

( oh yeah .... sorry , but I'm not one of the smater ones here )

phickey06

Thanks for the repl terobob but look at pictures of modern inboard hydroplanes at various meets in one season all turning left to see what I mean.

TERBObob

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Sorry , thought you were referring to RC . Something to realize .... RC and real , can NOT be compared . BIG major differences .

Hydro Junkie

I think it depends on who followed the 12 degree rule and who didn't. In the early days of hydroplaning, it was felt that the rudder needed to be placed at the position on the transom that was 12 degrees in from the skidfin. This placed the rudder on the left side of the transom. Some designers even followed this into the picklefork era. On the modern turbines, it has become accepted that the rudder is on the right side and the skidfin is on the left. The theory behind it is it makes the boat able to turn sharper to the left. True or false, who knows. What is known is the turbines are pulling u-turns to the left as fast as 200 MPH

phickey06

Hi Hydrojunkie. If the turn fin of a hydroplane can be considered to be like the hinge on a door,about the boat rotates,and the rudder the door handle where the force is applied to cause the rotation ,many aspects must be considered,IMHO. In the fifties most hydroplanes didn't really have turn fins,just small anti skid fins on each sponson and the rudders were almost universally centered. When the boats grew real turn fins the rudders started to be placed almost universally on the left[left turning boats].The turn fins got biggerand the rudders started to shift to the right[unlimiteds] with some boats having brackets on both sides[if they werent sure].Now they have settled [unlimiteds] on right placement,but this is not true on limited boats ,check it out! I would think limited hydros are most like these models and in response to Turbobob models are still real aren't they just small? Is this the 12 degrees you refer to?

Hydro Junkie

Actually, the boats had a 4 to 6 inch deep skid fin on the inboard side of the left sponson till the crews and designers found that having it behind the sponson worked better. As for a centered rudder, I've never seen one. Every boat I've been around(25 or more from the Slo-Mo-Shun IV and V to the turbines) have been offset to one side or the other by varying degrees depending on transom width.
As for the limiteds, it could be as simple as which side of the hull the steering cables run down. Something else to look at is when the various limited hulls were built, as practices differred from year to year.
The diagram you posted is exactly what I was refering to for the 12 degree rule.

phickey06

Anyone else have an opinion on the forces at work here?

Seabiscuit

As my physics teacher used to say:
"At that speed [200mph], you're not driving your vehicle, you're aiming it!"

This is an interesting topic.
While I know nothing of boat racing or why one would put the rudder in a certain position I can postulate one theory.
When turning (one way or the other) one would want the rudder on the side which is in the most water.
It would seem that when in a turn the side opposite the turn tends to lift and if your rudder is on that side, you will have less control.
For this reason cars that run on circular tracks use different sized tires on the inside and outside.
Similarly, boats that run in offshore races with left and right turns, have their rudders centered.

SB...

cyclops2

" Cutting edge ", in anything, is a good example of your EGO overpowering your brains ability to protect you.

You know you are going to get creamed. It is always just a matter of when and how bad.

People who disagree with that logic, have passed the too large of a ego test.

biteme

For whatever reason, planned or not, I think it comes down to leverage. With a turn fin on the left, and the rudder on the left you get a certain amount of leverage (to turn the boat). Increasing the distance between the fin and the rudder will increase leverage, The turn fin could be moved forward, but I don't know how this would affect the boat. The rudder could be moved rearward, but what to attach it to?* By moving the rudder to the other side of the boat, the distance between is now the hypotenuse of the triangle (the longer side). This would seem to be a good way to increase the leverage, without changing the hulls' overall charicteristics.
does this make any sense? Sean


*I have a couple airboats(pontoon/catamaran type), with air rudders mounted far beyond the transom. This was my attempt to increase leverage as pontoons are hard to turn(twice as many hulls). Works OK.
Sean

phickey06

Thanks for all the responses so far.I'm not just idely curious about this I really need to understand . I have several different feelers out unproductive so far but plan on expanding my inquire to obtain the answer or lack there of.[] I am also reading a book that is somewhat over my head[I suppose thats the essense of learning] Linear Vector Equations,Products of Vectors,Vector Coordinates.Maybe something will come of this.Dogmoat I think as you go from position A in the diagram to D you decrease leverage at the hinge point[turn fin] but increase swing motion?

Hydro Junkie

Dogmoat, I see a few problems with your theory on moving the rudder aft and the skid fin forward:
On the rudder, the further back you move it, the larger the boat becomes. This makes the boat's turning radius increase. This is why many experienced R/C scale hydro racers take advantage of the 10% rules. By making the sponsons longer, the boat becomes shorter, thus turning sharper. This is due to the skid fin being moved closer to the rudder.
As for the skid fin being moved forward, there are several problems here. Where do you mount it? If it's mounted on the inside of the sponson, it can't hold the sponson down due to the water kicked up against the bottom of the boat. This has the effect of lifting the skid fin out of the water, making it vertually useless. It also can't really be put on the outside of the sponson for the same reason. Mounting it in an outrigger position is asking for hull damage, along with making the hull longer

LtDoc

Hydro Junkie,
Sometimes it takes me longer than others to understand what I read. It's a 'glitch' in my thinking, usually, so this isn't a critisism, I just want to understand what you meant by, "By making the sponsons longer, the boat becomes shorter, thus turning sharper.". It seems like a contradiction in terms, and I know I'm missing something here. Would you help me clear up my 'glitch'?
- 'Doc

(I know it's gonna be a simple idea, and being 'simple' myself it's embarrasing to miss it.)

Hydro Junkie

Actually, you're not missing anything. What many of the experienced racers do is to take advantage of the 10% rule. To explain it, let me give you the APBA and NAMBA rules on size for a 1/8th scale hydro:
Scale length + 1"
Scale width + 10%
Scale tunnel width + 5 or 10%(depends on club and santioning body)
Scale afterplane length + 10%
In this case, we are dealing with afterplane length. This is the distance from the sponson transom to the back of the boat. I'll use the 8806 hulled Miss Madison since I'm very familiar with it. The afterplane length on this boat is 18 feet. This works out to 27 inches for accurate scale. When the 10% rule is added to the mix, the afterplane can be +2.7 inches from scale length, AS LONG AS THE BOAT REMAINS WITHIN THE LENGTH LIMITS. By legally adding an additional 2.5 inches to the back of the sponson, the skid fin is also moved back 2.5". This makes the boat HANDLE like a shorter boat by bringing the forward water contact point(the rear of the sponsons) and skid fin closer to the prop and rudder, even though the boat hasn't actually changed in length. This is just one of the little tricks that can put you in the winners circle

phickey06

Me againTo whom it may concern,does this make sense?[sm=confused.gif]

biteme

Hydro J,
Not theories, more thought experiments. a big 'what if we could..' The mods cannot be made for the very reasons you pointed out (I tried to say this as well, must've messed it up). Let me try to say it another way...Without changing the boats' dimentions, how can leverage be increased? Use the boats longest linear dimention - the diagonal. That way you get the leverage of a longer boat, in a shorter hull. Does that make more sense?
Sean

phickey06

OK . I have just heard from some people at Roostertails.Net and their explanation is that having the rudder on the right {left turning boat} holds the right side of the boat up in the turn ,the water pressure on the rudder does this resulting in flatter turns. I guess this makes sense to me and may be the best answer so far .