i was thinking

[John Finch]

Due to this thread I have been inspired to test some new sponson designs at the Delmarva race this coming weekend. The right sponson is flat on the water for about an inch and a quarter at the deepest point closest to the tub. Then there is a tunnel for the next 3/8th, and then there is a 3 degree outside edge riding surface 1/8th off the table and 1/2 inch wide. Just playing. The left sponson has two riding surfaces both two degree deadrise. Each takes up half the riding surface. One is an inch further forward than the other and both riding surfaces cause a bit of an air tunnel in the middle of the sponson. This was already tried last race. The boat liked it. The right sponson is the new experiment. Got different rear sponsons to try also. lOVE TO EXPERIMENT!

 

[John Finch]

Oh, by the way, to not confuse anyone, when I say my boat is set up a certain way and then later in the thread I say it is set up differently................It's because I change the darn things weekly, and I have 10 boats. So nothing is standard. when I was testing the tub curves I had 3 identical boats running at the same time and was modifying all three of them one way or another. One might be an outlaw, one might be an sgx, one might be a scratch built, one might be a hybrid. What works on one might not work on another. Just wanted to get that out there.

 

[Joe Warren]

Oh, by the way, to not confuse anyone, when I say my boat is set up a certain way and then later in the thread I say it is set up differently................It's because I change the darn things weekly, and I have 10 boats. So nothing is standard. when I was testing the tub curves I had 3 identical boats running at the same time and was modifying all three of them one way or another. One might be an outlaw, one might be an sgx, one might be a scratch built, one might be a hybrid. What works on one might not work on another. Just wanted to get that out there.

John Are you rigger tubs running a center located engine? & are these tunnel effect sponsons set w/same off set? or do you have the RF closer to the tub than the left.

 

[John Finch]

Oh, by the way, to not confuse anyone, when I say my boat is set up a certain way and then later in the thread I say it is set up differently................It's because I change the darn things weekly, and I have 10 boats. So nothing is standard. when I was testing the tub curves I had 3 identical boats running at the same time and was modifying all three of them one way or another. One might be an outlaw, one might be an sgx, one might be a scratch built, one might be a hybrid. What works on one might not work on another. Just wanted to get that out there.

John Are you rigger tubs running a center located engine? & are these tunnel effect sponsons set w/same off set? or do you have the RF closer to the tub than the left.

Center loaded with left sponson anywhere between same distance from tub as right sponson to 1/4 inch further that right sponson. Usually, most boats, 1/8th inch is all it takes with 2 to 3 tenths of a degree more negative angle of attack than the right sponson.

 

[Bob Morton]

Wow John,

This is great stuff. Change makes for inprovments B) . Soooooo much to do so little time hu

See ya at a race

Bob

 

[John Finch]

Bob,

I like your TWIN roundnoses. I'm racing scale this year. Come on down to one of our races and bring a scale.

John

 

[Lay26]

wow this thread took on a life of it's own. Now take a look at the reason for this question that i asked. Thank you all for your informative post. consider these sponson pics in my gallery

 

[Bob Morton]

Bob, I like your TWIN roundnoses. I'm racing scale this year. Come on down to one of our races and bring a scale.

John

Ok thanx . I be around. You will be hard to beat in scale. Hummm any class for that matter

Keep the good stuff coming

 

[John Finch]

Bob, I like your TWIN roundnoses. I'm racing scale this year. Come on down to one of our races and bring a scale.

John

Ok thanx . I be around. You will be hard to beat in scale. Hummm any class for that matter

Keep the good stuff coming

Hey, you won the big twin shootout last year. You ain't no slouch.

 

[Bob Morton]

Bob, I like your TWIN roundnoses. I'm racing scale this year. Come on down to one of our races and bring a scale.

John

Ok thanx . I be around. You will be hard to beat in scale. Hummm any class for that matter

Keep the good stuff coming

Hey, you won the big twin shootout last year. You ain't no slouch.

 

[Joe Warren]

Oh, by the way, to not confuse anyone, when I say my boat is set up a certain way and then later in the thread I say it is set up differently................It's because I change the darn things weekly, and I have 10 boats. So nothing is standard. when I was testing the tub curves I had 3 identical boats running at the same time and was modifying all three of them one way or another. One might be an outlaw, one might be an sgx, one might be a scratch built, one might be a hybrid. What works on one might not work on another. Just wanted to get that out there.

John Are you rigger tubs running a center located engine? & are these tunnel effect sponsons set w/same off set? or do you have the RF closer to the tub than the left.

Center loaded with left sponson anywhere between same distance from tub as right sponson to 1/4 inch further that right sponson. Usually, most boats, 1/8th inch is all it takes with 2 to 3 tenths of a degree more negative angle of attack than the right sponson.

John I too have moved both sponsons to improve corning 40 boats maybe 1/4 diff, 60-90 stuff 1/2 to 3/4 & twin as much as 1 inch off set on the sponsons. many rigger tuners believe if you move the leading edge of the turn fin towards the front of the sponson it turns best. But they are moving the leading edge & the trailing edge & the same time by moving the entire turn fin. You would be amazed if you leave the trailing edge where you know it should be & move the leading edge towards the front of the sponson by building the wider turn fin. it dont take much. This is all considering you have a Boat that is set up on a base line set up & already running well. Good luck on you set up, Most are still running Don`s ole theory`s from the days passed. But they all will perform better with the correct prop & pipe & set up fine tuned. Joe W Memphis Tn.

 

[Tom w]

.

 

[Tom v.d Brink]

as a 67 does not have 3 times more power then a 21 scaling rules should applied

so you have to scale the engine power as scalepower=originalpower*scalefactor^3

as a .21 has something around 2.2bhp and a .67 has something around 5bhp

(bhp values taken from CMDi website as bottom given power for each engine)

this wouldmake the .21 as a function of the .67 scale as

2.2=5*scalefactor^3

scalefactor=(2.2/5)^1/3=0.7606

or a 1:1.315

I checked it on sg lengths as given by CMdi

given lengths: 21=30" and 67=39"

scale here would be 39/30=1.3

pretty close i'd say

 

[Joe Warren]

Somewhat OT but how does all of this relate to the scalability of riggers? When scaling from say a 21-size boat to a 67-size do you keep everything proportionate or are there some hard fast rules that you follow? Do you keep your tubs the same width? Do you keep your ride pads the same width? Obviously a 67 is not 3 times longer than a 21 but is the volume (l x w x h) proportional? Or is it proportional based on horsepower? Or is it based on RTR weight? Or is it based on potential speed?
Just curious what you gentlemen think and/or have experienced.

Tom W

Tom Many of todays Tubs are very narrow & only allow for center engine location, Rather you move the RF sponson IN a little more towards the tub of the boat or you move the LF out. The boat see the offset as RF came closer to the tub than LF. Putting move turn fin weight in the boat, Many Name Brand Riggers work well with 1 inch off set. it helps when you have to turn left while on the power & it corners well being it puts turn fin weight in the boat. Dick Tyndall builds some nice turn fins, I believe the Stone age of the turn fin is over. You can buy a nice turn fin that works very well if the guys know what type boat you are putting it onto. Roadrunner S/G & crapshooter all supply turn fins with their size kits that are proven Race winners & they work best on that paticular boat as well. Most supply you with a set up sheet that if followed closely will allow you to build a very competive boat for you class of racing. I have witness a paticular Roadrunner 20 radared many time run 82 83 mph on a stalker gun 7 years ago & it only came from many hrs of tuning. Even today you dont see it @ the races 70mph in 20 hydro can Win anywhere against Anybody.

 

[John Finch]

To scale up a boat by a specific number to enlarge a model might make sense in reproducing the boat in a larger scale, but I believe that the increased torque from the engine and size of the propeller for example create the need for characteristic changes in the model. A good example is that you can not run a tub but so close to the water. Doesn't matter what size boat it is, you need a certain clearance. Big engine torque with big props on big boats provide different challenges than the 21 size boats with small props turning high rpm. Just my observation.

 

[John Finch]

wow this thread took on a life of it's own. Now take a look at the reason for this question that i asked. Thank you all for your informative post. consider these sponson pics in my gallery

I see that the sponsons pictures need to be rotated. They are laying on their sides when viewed.

 

[Marty Davis]

John:

What is your reason for offsetting the sponsons to the left side (wider spacing on the left side)?

Why don't you offset the sponsons to the right side?

Seems that we have a difference of opinion here....

The reason that I offset the sponsons to the right:

The prmary reason for a boat torqeing into the course (trying to turn right and having to add left rudder trim) is the prop. The prop entering the water tries to twist the boat clockwise looking from the rear. This adds weight and wets the right front sponson, making it have more drag. If you want to offset this twisting effect, you lengthen the lever arm (widen the right sponson spacing). The larger the boat, the more the prop tries to twist the boat, and the more you have to offset the sponson.

Thoughts....

Marty Davis

 

[Marty Davis]

as a 67 does not have 3 times more power then a 21 scaling rules should applied so you have to scale the engine power as scalepower=originalpower*scalefactor^3

as a .21 has something around 2.2bhp and a .67 has something around 5bhp

(bhp values taken from CMDi website as bottom given power for each engine)

this wouldmake the .21 as a function of the .67 scale as

2.2=5*scalefactor^3

scalefactor=(2.2/5)^1/3=0.7606

or a 1:1.315

I checked it on sg lengths as given by CMdi

given lengths: 21=30" and 67=39"

scale here would be 39/30=1.3

pretty close i'd say

Tom:

Not sure that I agree with you on this....

I think the power is irrelevant. For length, what counts is the speed of the boat versus the size of the waves it has to traverse, not the power output. Weight of the boat is also a primary input item.

Thoughts....

Marty Davis

 

[John Finch]

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.

 

[Tom v.d Brink]

I think the power is irrelevant. For length, what counts is the speed of the boat versus the size of the waves it has to traverse, not the power output. Weight of the boat is also a primary input item.

power is not irrelevant to length if all factors are scaled(power, weight, length and speed)

but on the other hand yes to accuratly scale a boat you need to scale the weight as well. then after you scaled the weight you need to scale the power to push it so in the end it all comes back to the scalefactor TomW was looking for.

this of course only works if you perfectly want to scale something down. this would ofcourse also affect the speed result the boat would achieve to keep the model in perfect scale. any deviation from this(i.e lighter, faster more power) and you lost your perfect scale and entered a new area and would need to adjust the design to compensate. however often the numbers add up pretty nicely the mistake people often make is scaling directly by engine size.