i was thinking

[Marty Davis]

Marty,You are from the North. Ever go down a hill on a sled in the snow and need to make a quick turn. What did you do? You stuck out your right leg and dragged it in the snow to turn right and stuck out and dragged your left leg to turn left. Same thing. You put the left sponson further from the hull and the boat wants to turn left. Longer leverage arm. I tested this at record trials about 6 years ago because the straightaway boat was turning to the right as they all tend to do. I extended the left sponson at different lengths until the boat tracked straight. During one test with the sponson a bit over an inch further on the left side, the boat actually veered to the left while tracking down the straightaway. Since there are many ways to compensate for propwalk I use a little bit here and a little bit there rather than using all one method. I remember back in the 60s when people used to add weight to the left sponson to counter act the torque roll. Making the right sponson further from the tub does the same thing like you said. It is a double edge sword. You help the sponson from digging into the water hard by extending the sponson out, but you also extend the leverage arm that makes the boat turn right. This is not theory. I have tested it over and over. Maybe that's why your flat right sponson works well for you. It helps keep the sponson from digging into the water.

John:

I like your analogy....

OK, if you drag your foot out on the left side of the sled, you turn right. I agree, BUT you are sticking your foot out behind the cg at the rear of the sled. What happens if you stick your foot out on the left side of the sled in front of the cg? (besides ripping your leg off) It turns right. The key is that you stick your foot out toward the front of the sled. (Sponsons are well in front of the cg)

AN easy test since the front sponson tubes can be easily reversed and the offset will be automatically placed on the opposite side.

Marty Davis

 

[John Finch]

You said it backwards, but that's ok. Depends which end of the sled you are on. Facing down hill or up hill. Lets look at it this way........ A boat is riding on three points. The prop and two sponsons. It's running light on all three.........the left sponson hits a plastic bag blowing accross the pond.

The boat will continue to move forward, but the drag on the left sponson will slow that side of the boat and the prop will push the right side of the boat more forward than the left. The boat turns left. The thing we all seem to forget is that the boat is flying on top of the water and it only takes a crosswind to deflect the boat from side to side. Even with a turn fin and rudder, because of the speed the model is traveling. The faster it goes, the easier it is to pull, drag, blow or push the boat off track. That's why at record trials it takes less action to cause a reaction. That leads us to people trying different things only to get different results. It seems like every time a boat's speed increases the effects of the rudder location etc. affect the boat differently. Example...........I will use a mono, because this is seen very easily. At racing speeds the rudder on the right side of the transom works great with the average racing hull. The boat torque rolls to the right. When you build a super light hull for record trials, The boat is super light on the water and all of a sudden the boat leans to the left instead of to the right. The rudder actually lifts the right side of the boat when you get the boat super light. Completely different results at different speeds with different boats.

Don Ferrette had a twin that pulled to the right terribly. He had a dual pickup rudder that was very wide and mounted on the right side of the transom. He went to a narrower rudder and the problem went away. He was dragging his foot too hard on the right side of the sled. (analogy).

 

[Scott Norris]

Apparently you guys have designed a way to have alot of adjustability in your front sponsons, or, you have boxes full of em, or, you have a sxitload of holes drilled in your sponson tubes.

 

[John Finch]

Apparently you guys have designed a way to have alot of adjustability in your front sponsons, or, you have boxes full of em, or, you have a sxitload of holes drilled in your sponson tubes.

All of the above, Scott. I have an attic full of sponsons.

 

[Marty Davis]

I am not opposed to re-visiting anything. There are so many variables in our hobby that we need to make it easy to see the WHY. In order to make this easy to see, you need to remove the turn fin and then offset to the right and then to the left. It should be easy to see the effect.

The plastic bag is a very good analogy.

I will re-visit this next time that I go to the pond to test and report on my findings.

Marty Davis

 

[John Finch]

I am not opposed to re-visiting anything. There are so many variables in our hobby that we need to make it easy to see the WHY. In order to make this easy to see, you need to remove the turn fin and then offset to the right and then to the left. It should be easy to see the effect.

The plastic bag is a very good analogy.

I will re-visit this next time that I go to the pond to test and report on my findings.

Marty Davis

Sounds great Marty. let us know what you find. I will get back on some of my testing this weekend too with the rear sponsons.

 

[Kelly Miller]

Here is one for you.....if the turn fin is bent a bit so that the trailing edge is closer to the center section than the front......will the boat track to the left or the right......will the fin try and run straight through the water thus twisting the boat counter clockwise.....or will it just create drag and pull the right front sponson causing it to pull to the right.....

Kelly

 

[Kelly Miller]

John you touched on this a bit......but the question is that with prop walk and keeping the boat light on the water can we actually reach a point that the boat begins to move laterally to the left yet appear to be pointed straight? hope these are not stupid questions.

Thanks

Kelly

 

[Marty Davis]

Here is one for you.....if the turn fin is bent a bit so that the trailing edge is closer to the center section than the front......will the boat track to the left or the right......will the fin try and run straight through the water thus twisting the boat counter clockwise.....or will it just create drag and pull the right front sponson causing it to pull to the right.....


Kelly

It will turn to the right...

Marty Davis

 

[John Finch]

John you touched on this a bit......but the question is that with prop walk and keeping the boat light on the water can we actually reach a point that the boat begins to move laterally to the left yet appear to be pointed straight? hope these are not stupid questions.
Thanks

Kelly

Yes, that is exactly what happens. And like Marty said, the bent fin makes the boat turn right. Think of the fin like the front wheels of a car. Wherever you point it, that's where it leads the boat.

 

[Ian Inverarity]

John you touched on this a bit......but the question is that with prop walk and keeping the boat light on the water can we actually reach a point that the boat begins to move laterally to the left yet appear to be pointed straight? hope these are not stupid questions.
Thanks

Kelly

Yes, that is exactly what happens. And like Marty said, the bent fin makes the boat turn right. Think of the fin like the front wheels of a car. Wherever you point it, that's where it leads the boat.

How about turning the turn fin to turn the boat?

Ian.

 

[John Finch]

John you touched on this a bit......but the question is that with prop walk and keeping the boat light on the water can we actually reach a point that the boat begins to move laterally to the left yet appear to be pointed straight? hope these are not stupid questions.
Thanks

Kelly

Yes, that is exactly what happens. And like Marty said, the bent fin makes the boat turn right. Think of the fin like the front wheels of a car. Wherever you point it, that's where it leads the boat.

How about turning the turn fin to turn the boat?

Ian.

You change the angle of the fin in relation to the DIRECTION OF TRAVEL OF THE MODEL when you turn the rudder. So, that's really what we are doing when we turn the boat. You can look at the fin as a pivot or a steering device. Depend on your perspective. Earlier I talked about the fin being bent and how it turns the boat like the front wheels of a car. Look at it as a car where the rear wheels do the steering. The fin is the front wheel and the rudder is the rear wheel. Amazing how things can be seen in different perspectives!

 

[Ian Inverarity]

As I understand it the turn fin generates most (but not all) of the centripetal force to hold the boat in the turn, and this is done by the direction of travel of the boat relative to the turn fin and rest of the hull, the slip angle. On the earlier design riggers I had, like a A/S Cobra 90, the turn fin was not very large by modern standards and the slip angle could be seen in corners, that is the boat had a high slip angle to generate the cornering force from the fin and hull surfaces. My early 45 SG had a turn fin of similar area on a boat that was about half the weight, and seemed to run much lower slip angles. The higher slip angle designs are pushing the hull sideways to a greater extent thru the corners so are less efficient in the turns and lose more speed. So what my comment was about is if the turn fin was made to have less area and is turned at the same time as the rudder (but in the other direction) then the cornering force could be achieved potentially with little or no slip angle on the hull itself. And maybe the boat could lose even less speed in corners. :huh:

Ian.

 

[Ray Sametz]

Ian, I'm open to new ideas. What your saying is run a rod from the fin to a servo in the tub. Mixing in some rudder to it. Then it would pull the fin closer to the tub in the turn. Interesting. Ray B) B) The old sundowner O/B hydros had a adjustment rod on the turn fin. I believe full size scale boats have hydrolic rams to move the fin in the turns.

 

[Kelly Miller]

My question earlier was prompted by he thought of what happens to the CG of the boat at speed.....and is it possible for the turn fin to act as a transom rudder....instead of a front wheel. I explain to people all the time as I deliver a new boat (full size) to customers that in steering a boat we are moving the back of the boat side to side to push the front. the turn fin on static bench is a bit forward of the CG....and I am just asking because I don't know...is there such a thing as the CG pushing foward, from propeller lift, thus putting the turn fin more on top of the CG......( I can see people looking at their computer now like the RCA dog saying "is this guy stupid or what". reason why I say this is double checking my hydro because of a severe crab down the straights.....i would say 20 degrees crab yet going straight. changing things one at a time I found the turn fin trailing edge away from the tub and felt as though I saw a significant difference making sure I had the fin parallel to the tub. I looked at it as a drag issue....ie paper bag....but began to wonder about the "rudder" possibility vs a "front wheel" issue. not 2 ways about it we are pushing to friction points up front and if one side has more drag that the other we will begin to pivot on that point. I have dropped the left front attack angle to compenstate for turn fin drag....but I think I like the thought of increasing the leverage of drag from the front left by placing the sponson farther away.

If the running CG moved far enough foward toward the front edge of the turn fin, and the fin was large enough ie roadrunner is it possible for it to become a rudder thus moving the back of the boat instead of the front?

I will be experimenting.

 

[Kelly Miller]

I think too much.............If the turnfin was pointed to the left..trailing edge away from the tub....this would push the front to the left..thus act like a wheel and allow the prop to walk freely to the left and create a massive crab angle. As I thought about it ....which we can already tell is a dnagerous thing for me to do. the boat was drifting rather hard to the outside.....like a lateral movement to the outside but at a crab of about 20 degrees pointed to the inside. don't mean to ramble but I answered my own question.

Kelly

 

[John Finch]

Ok, back to the car analogy.........No matter where you put the front wheels, the car turns because of the relationship between the front and rear wheels. The cg (engine) can be anywhere. Mid engine, front engine, rear engine. The cg has nothing to do with the two devices that steer the car until you try to negotiate a turn.

The car with the rear engine will tend to spin out. The car with the forward engine will plow. The mid engine car will corner really well. The boat steers by the turn fin and rudder. It takes both, unless you are running slow and the sponsons are into the water and they become the wheels. We used to run our turn fins on the tub at one time. Anyone remember that? Sometimes halfway between the engine and transom. They steer the boat with the fin behind the CG. NOW..............how well does the boat handle in the turns. Pretty poor with the fin behind the CG. The thing about the fin is that it is like a front wheel on a car until you want to throw a boat into a turn at 75 mph and have the boat stay in the turn. Then, the boat is hanging onto the fin to control centrifugal force. Another totally different world. Here is a simple thought.........hold your boat by the turn fin. The left sponson falls down and the right is up. If you move the fin forward on the boat........the transom falls low because there is more weight aft of the fin. If you move the fin rearward on the boat.........the front of the boat falls and the transom comes up. Now, picture the boat ripping a turn on a teather tied between the fin and a pole in the middle of the pond. No rudder on the boat. If the fin is forward the centrifugal force is going to swing out the transom. If the fin is too far back the front of the boat wants to move away from the fin. If the fin is on the cg the boat follows a nice track around the pole as weight is distributed evenly front to back. Now, add a rudder. It helps to keep the transom from swinging away from the fin, so, you can move the CG more forward since you have the rudder to help keep the rear of the boat from sliding out. That's why the turn fin works better in front of the cg. That's why if you have a boat that spins out ........a longer rudder can cure that problem. Just trying to put things in simple terms that might make sense. I think a movable turn fin would be too sensitive for our boats, but I have not tried it. Has anyone here tried it?

Also a thing to remember is that the rudder is sometimes just keeping the prop from pulling the transom around and sometimes it takes a longer rudder to keep a boat tracking straight. I'm starting to ramble too. I have a race to get ready for.

 

[Don Ferrette]

Don Ferrette had a twin that pulled to the right terribly. He had a dual pickup rudder that was very wide and mounted on the right side of the transom. He went to a narrower rudder and the problem went away. He was dragging his foot too hard on the right side of the sled. (analogy).

The boat John is referring to was my twin 80 boat & actually it wasn't a pulling problem per say but what happened on hard acceleration. Poor Ray Sametz found out when I was about to pass him on the outside at a race 2 seasons back. We both got to buoy #1 about the same time & I just momentarily chopped the throttle to plant the boat & when I hit the trigger the boat did an instant right hand turn into Ray (I still feel terrible about that). Once I got the boat fixed the next time at the practice pond I found that I could duplicate this condition at will. I was running the Speedmaster dual pickup rudder blade which is .250" thick at the trailing edge. After a lengthy phone conversation with Andy one afternoon I went back to the shop & milled off equal amounts from each side until the rudder was at .190" (that was a pain in the ...) & the next time out the "right turn Clyde" syndrome was gone with no other changes to the boat. There is no one magic bullet here but a correct combination of all the pieces to make that killer heat race boat.

 

[Kelly Miller]

I understand......I get on some crazy rambles sometimes.

Kelly

 

[Ian Inverarity]

Ian, I'm open to new ideas. What your saying is run a rod from the fin to a servo in the tub. Mixing in some rudder to it. Then it would pull the fin closer to the tub in the turn. Interesting. Ray B) B) The old sundowner O/B hydros had a adjustment rod on the turn fin. I believe full size scale boats have hydrolic rams to move the fin in the turns.

Ray,

I was just considering the theoretical situation. Practically it would take a bit of thought. The forces on a turn fin are greater than on the rudder, I would say the pivot would have to be behind the leading edge to reduce the load somewhat. The mounting of the fin and linkage system would have to be very stiff, and servo very strong. I am not sure if the rudder would still need to move, but will still be needed to oppose prop walk at the least. The turn fin, if narrower than normal turn fins, would move more than the normal slip angle of a boat to generate the same cornering force. Maybe the rudder would still be needed to move to control the attitude of the hull in turns.

Ian