i was thinking

[Ian Inverarity]

Sometimes a picture is worth 1000 words so I did a diagram of the forces acting upon a conventional rigger in a corner:

https://www.intlwaters.com/gallery/displayimage...=241&pos=11

This is all just my understanding of course! From the diagram, as the turn fin is moved forward from the CG the moment created by the turn fin centripetal force is greater, which make the force required by the rudder to turn the boat less. I think I may have drawn the rudder force in the wrong direction. Moving the turn fin further from the tub will increase the turning moment on the hull in turns. While the slip angle probably reduces the effectiveness of the prop, the prop thrust has a component that will oppose the centrifugal force.

My thoughts about turning the turn fin would change the diagram so that the hull has little or no slip angle in a turn and the turn fin force is generated by angling the turn fin into the corner. By reducing the slip angle the centripetal force generated by the hull would be reduced, but so would the drag, and the prop would not be acting at such an angle. Just a thought.... B)

Ian.

 

[Marty Davis]

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.

John:

VERY good analogy on a lot of items in your discussion....

I like to think of the way that a boat turns in a little different analogy. You could consider the turn fin a post that goes down into the water and the boat pivots about the post. In fact some of the older full sizeed race boats actually dropped a pipe into the water to turn on and then lifted it for the SAW.

The important thing for my analogy to be as efficient as possible is that the turn fin must be right on the "Dynamic CG" for it to pivot about the turn fin. When you find this spot, the boat will turn with only the amount of rudder throw which corresponds to the circumference of the turn you are making. It does not have to over turn (MUCH DRAG), just pivot the rear of the boat about the turn fin pivot point.

Where is the "Dynamic CG". I have not been able to quantify the spot exactly but can come close by using the "Static CG" as the starting point. I do know that the Dynamic CG is in front of the Static CG by a little. The center of pressure of the turn fin will be on that Dynamic CG. There is a little bit of a compromise since the tank being full, empty or someplace in between affects the Static AND Dynamic CG.

Marty Davis

 

[Joe_Knesek]

Marty touched on a good point. The size of the turn fin has a direct relationship to the size of the rudder needed. You can find a lot of speed by balanceing them out.

 

[John Finch]

Marty,

Just came back from the Delmarva race. I said I would report back on my testing. I ran an OUTLAW rigger this weekend that performs better than any rigger I have ever driven in all my 39 years of racing. This is the setup..........Engine in center of tub.( MAC .67 engine). Both sponsons same distance from tub. Left sponson 5 1/2 on the 3/8th block. Right sponson 4 1/2 . The rudder is on the right side of the transom. The right side of the rudder blade is parallel to the tub. DIEHEDRAL is about 3 degrees on both sponsons! The rears at 1 to 2 degrees and even with the bottom of the strut. So, the only measurement that was not neutral on the sponsons was the left sponson being more negative than the right. That setup is perfect for that boat. It runs in the 80's with no hop. no darting, and very linear turning. A dream to drive.

Looks like it is accellerating around buoy two and three. No modifications needed or wanted. Joe did a great job of engineering and building the boat.

The rear sponson testing ( on another boat) proved out what we were talking about last week. If the sponsons extend to the prop, the boat is more stabile than ending the rears at the transom. Also, if they are on a wide track, they make the boat more stabile. If you make them too thick, the boat can tend to run wet at the front sponsons at slow speeds.

Also....... The flat front right sponson ( no diehedral) with a tunnel in it worked fine too.

The bottom line from the testing is that boats can be very different and still work very well.

 

[Terry Keeley]

Marty,Just came back from the Delmarva race. I said I would report back on my testing. I ran an OUTLAW rigger this weekend that performs better than any rigger I have ever driven in all my 39 years of racing. This is the setup..........Engine in center of tub.( MAC .67 engine). Both sponsons same distance from tub. Left sponson 5 1/2 on the 3/8th block. Right sponson 4 1/2 . The rudder is on the right side of the transom. The right side of the rudder blade is parallel to the tub. DIEHEDRAL is about 3 degrees on both sponsons! The rears at 1 to 2 degrees and even with the bottom of the strut. So, the only measurement that was not neutral on the sponsons was the left sponson being more negative than the right. That setup is perfect for that boat. It runs in the 80's with no hop. no darting, and very linear turning. A dream to drive.

Looks like it is accellerating around buoy two and three. No modifications needed or wanted. Joe did a great job of engineering and building the boat.

The rear sponson testing ( on another boat) proved out what we were talking about last week. If the sponsons extend to the prop, the boat is more stabile than ending the rears at the transom. Also, if they are on a wide track, they make the boat more stabile. If you make them too thick, the boat can tend to run wet at the front sponsons at slow speeds.

Also....... The flat front right sponson ( no diehedral) with a tunnel in it worked fine too.

The bottom line from the testing is that boats can be very different and still work very well.

Interesting John, got a picture?

 

[Marty Davis]

Marty,Just came back from the Delmarva race. I said I would report back on my testing. I ran an OUTLAW rigger this weekend that performs better than any rigger I have ever driven in all my 39 years of racing. This is the setup..........Engine in center of tub.( MAC .67 engine). Both sponsons same distance from tub. Left sponson 5 1/2 on the 3/8th block. Right sponson 4 1/2 . The rudder is on the right side of the transom. The right side of the rudder blade is parallel to the tub. DIEHEDRAL is about 3 degrees on both sponsons! The rears at 1 to 2 degrees and even with the bottom of the strut. So, the only measurement that was not neutral on the sponsons was the left sponson being more negative than the right. That setup is perfect for that boat. It runs in the 80's with no hop. no darting, and very linear turning. A dream to drive.

Looks like it is accellerating around buoy two and three. No modifications needed or wanted. Joe did a great job of engineering and building the boat.

The rear sponson testing ( on another boat) proved out what we were talking about last week. If the sponsons extend to the prop, the boat is more stabile than ending the rears at the transom. Also, if they are on a wide track, they make the boat more stabile. If you make them too thick, the boat can tend to run wet at the front sponsons at slow speeds.

Also....... The flat front right sponson ( no diehedral) with a tunnel in it worked fine too.

The bottom line from the testing is that boats can be very different and still work very well.

John:

INTERESTING....

Were the front sponsons wider than normal? Width?

Can you tell what the attack angles on the fronts are in degrees?

Ther setup is almost exactly what I run, except for the dihedral and sponson spacing. It makes for a VERY stable platform with the rears in a supporting position at the bottom of the strut. Is the strut without any angle? What style prop were you using?

Sounds like the sponson spacing was not of importance to you on this boat, since there was no offset???

Marty Davis

 

[John Finch]

Both boats used more negative on the left sponson to keep the boat tracking straight. Both boats are set for a slight neg angle when on the table. The amount of neg is determined like so...............I first determine how much of the hub will be out of the water at full speed. I then build a block that I slip under the strut when on the setup table to keep then strut at that height off the table. then I set the strut to be flat on the block to keep a perfectly flat angle on the prop when at full speed with only 2/3rds of the blade in the water (give or take). The size of the block depends on the depth the prop rides in the water. That process works for me. I have also found that running the strut flat on the table gives a positive angle on the strut at full planning speed. This positive angle actually deters propwalk because the blade on the right side of the hub then has more pitch than the blade on the left in relation to the motion of the hull. I don't know the angles of the sponsons because i have not measured them since setup has been completed. It's just easier to use the blocks on the table for quick setup. I use the digital level on my sgx sponsons because they are flat on top and easy to get a quick reading.

 

[John Finch]

Marty,

The sponsons are normal width on both boats. 2 to 2 1/4 inches. The extended boom was not needed on either boat. Easier to just put the left sponson negative. On my oval boat I use the left sponson offset to free up the sponson more.

 

[dschigoda]

On the subject of Hydroplane design -

I am re-designing/tweaking a 1/8 scale picklefork, from the '80's. I have heard that the "afterplane- length (APL) to tunnel-width ratio" should be around 2.0 to 1. Is this a good place to start? If the ratio is less than 2.0 - would it make the boat more stable in the turns and less stable in the straights? If it is greater than 2.0 - would it make the boat more difficult to turn quickly and efficiently? If the sponson "ride pads" are spaced further apart than the tunnel width - would that dimension be more relevant to the ratio than tunnel-width?

Thank you for any help anyone can provide. These hydroplane design issues are really interesting!

Dave

 

[John Finch]

2.0 to one is a good number. The hull design will dictate the sponson width. for example, I have a roundnose that is 23 inches on the APL. 10 inches between the sponsons. If You were to change those numbers, the boat would not be to scale. With 1/8 scale you need to make the original design work for you.

 

[dschigoda]

2.0 to one is a good number. The hull design will dictate the sponson width. for example, I have a roundnose that is 23 inches on the APL. 10 inches between the sponsons. If You were to change those numbers, the boat would not be to scale. With 1/8 scale you need to make the original design work for you.

True, but... NAMBA does allow a +/- 10% tolerance on tunnel width, APL, and boat width. IMPBA allows a +/- 10% tolerance on boat width and +/- 5% of the boat length.

Plus or minus 10% on the APL of my boat translates to 21.60" to 26.40", for a difference of 4.8" !

Dave

 

[John Finch]

Sounds like you know what you are doing then, so it's small mods you are asking about. The wider stance on the sponsons would be more stabile, but the air trap gets wider too, so you catch more air. I have a hull 11 wide on the inside of the fronts and 25 AFT that works really well on a picklefork design. I had to add to the depth of the sponsons for it to ride better at slow speed. I would stick to the 2 to 1 ratio and add two or three inches to the AFT.

 

[joefroooo]

Sounds like you know what you are doing then, so it's small mods you are asking about. The wider stance on the sponsons would be more stabile, but the air trap gets wider too, so you catch more air. I have a hull 11 wide on the inside of the fronts and 25 AFT that works really well on a picklefork design. I had to add to the depth of the sponsons for it to ride better at slow speed. I would stick to the 2 to 1 ratio and add two or three inches to the AFT.

John,

Im glad the outlaw is working well for you. Accelerating thru the turns is all in the set up of the hull, turn fin placement and prop. By changing or off setting the boom tube length like you have done looks like also helps in the handeling. This has been a very informitive thread and will help the future hull builders. Keep it going. Thanks

Joe Moceri

 

[Ray Sametz]

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.

Don, The boats been pulled from the rafters and being restored. Hope to race this season. With these colors I might catch a break jumping the start. h34r: :lol: :lol: Ray

 

[John Finch]

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.

Don, The boats been pulled from the rafters and being restored. Hope to race this season. With these colors I might catch a break jumping the start. h34r: :lol: :lol: Ray

Ray,

Thanks for bringing out the Mongoose at the last race. it was interesting to see the left sponson way out from the tub. The engine offset in the tub to the right too. I did not know that boat had those design features till I saw yours.

 

[Ray Sametz]

I do have a 5 year old 20 mono designed by some guy, I forget he's name. I just can't bring myself to spend 700.00 for a 20 hydro 80% built. Last race 20 hydro 3 boats. 40 hydro 4 boats, two were sport 40. 20 mono 2 boats, class canceled F hydro, John you know that deal. I'm building an 1/8 scale for 08. No matter what I run, I refuse not too have fun a the races I attend. Hanging out with a great bunch of guys. Ray B) B)

 

[John Finch]

Ray,

I raced my sport gas boat yesterday against Don McGurn and had a ball. I hate the smell of gas, but that's where the competition is in our district right now. I have to go with what's giving me the best bang for my buck and the most fun, so next race I will probably be running gas again. It's all model boating. Gas or nitro. That's whats great about our hobby. You can race what you want when you want.

Have fun this weekend. I will be at the muscle car power fest in Dinwiddie, Va.

Catch you at the August race in Chesapeake.

 

[Marty Davis]

I have a question.............what are the advantages of running the rear sponsons past the rear of the transom?Any tips or experiences there?

John:

I was re-reading this discussion and found this one that was not really discussed enough.

The reason that I believe that the rear sponsons should extend beyond the transom is that you can then run slightly higher attack angle on the rear sponsons. Why, because the lever arm is longer and it will not make the rear end have the "high frequency hop".

Just 2 or 3 tenths of a degree will start this hop, so lengthening the lever arm allows you to add a couple tenths.

I think that it makes a difference on the .21 and .45 boats and less as the size increases. A change in rear sponson length on the smaller boats is a greater percentage increase in track because of the shorter track and the amount you can lengthen the rear sponsons behind the transom.

The is a little bit of a logistics problem if you lengthen too much. The rudder won't have space to turn (will hit the rears). I saw a boat that Ralph Almirola made for a guy here that had barely enough space to turn left (don't need much left anyway).

Marty Davis

 

[Marty Davis]

bump

 

[Scott Norris]

I think John is at a race this weekend Marty. Bump it again on Monday.