Re-Visit Sponson Offset

[Grimracer]

Ninja sposons are built to 7deg and run around 5.5.. i have designed and run lower as well. 5.5 for around a 4deg running AOA. However if you run this type of AOA i have found you have to add toein to the chine.

Not that this info does anybody any good just laying it out there.

Grim

 

[John Finch]

Hey guys! Interesting reading. My experience with sponson spacing is as follows................ First lets assume you have a boat with both sonsons equal distance from the tub. The engine is in the center of the tub and the weight on each sponson is distributed the same. That's the benchmark. If you move the left sponson closer to the tub, the result is that more weight is now on the left sponson, so it digs harder in the water, thus making the boat veer to the left. But, that puts the right sponson further out from the tub than the left sponson which allows it to have more leverage in making the boat veer to the right. So, this senario can work for a particular boat. On the other hand, extending the left sponson on a benchmark boat will give more leverage in making the boat veer left, while more weight is distributed to the right sponson making the boat want to veer right. So we have opposing forces, and the one with the most strength wins. Now.............here is where the other variables come into play. Sponson angle of attack and sponson width. If you extend the left sponson from the benchmark boat to give more leverage to pull the boat to the left and give the right sponson more lift than the left sponson, you can get equal pressure on the water from both sponsons but still have more leverage on the left sponson. What I'm getting to is a boat where both sponsons are barely touching the water yet the leverage of the left sponson being way out to the left causes the boat to veer left and compensate for prop walk. Make sense? Most people set up their riggers...left sponson more negative than the right, which plays out well with the left sponson extended.

 

[Marty Davis]

Hey guys! Interesting reading. My experience with sponson spacing is as follows................ First lets assume you have a boat with both sonsons equal distance from the tub. The engine is in the center of the tub and the weight on each sponson is distributed the same. That's the benchmark. If you move the left sponson closer to the tub, the result is that more weight is now on the left sponson, so it digs harder in the water, thus making the boat veer to the left. But, that puts the right sponson further out from the tub than the left sponson which allows it to have more leverage in making the boat veer to the right. So, this senario can work for a particular boat. On the other hand, extending the left sponson on a benchmark boat will give more leverage in making the boat veer left, while more weight is distributed to the right sponson making the boat want to veer right. So we have opposing forces, and the one with the most strength wins. Now.............here is where the other variables come into play. Sponson angle of attack and sponson width. If you extend the left sponson from the benchmark boat to give more leverage to pull the boat to the left and the right sponson has more lift than the left sponson, you have less drag on the right sponson as it has more lift but still have more leverage on the left sponson. What I'm getting to is a boat where both sponsons are barely touching the water yet the leverage of the left sponson being way out to the left causes the boat to veer left and compensate for prop walk. Make sense? Most people set up their riggers...left sponson more negative than the right, which plays out well with the left sponson extended.

John:

OK, I agree with every one of your statements about the effect of each change in spacing. I also agree with your statement on attack angle.

Now, what do you think the amount of influence the change in spacing makes. Do you think that the added weight of the sponson moved in or the space of the sponson moved out is a greater influence?

How much spacing change do you think it takes to make a big difference? Obviously related to total weight of the boat and contact patch of the sponson to the water.

What do you run on a given heat race boat to make the boat totally neutral? The way that I like my boats to run. I guess the ultimate test of neutral is a rudder with ZERO left correction in it. VERY difficult to get on the larger high torque prop boats.

Probably too many variables for us to EVER arrive at a consensus...

 

[John Finch]

Hey guys! Interesting reading. My experience with sponson spacing is as follows................ First lets assume you have a boat with both sonsons equal distance from the tub. The engine is in the center of the tub and the weight on each sponson is distributed the same. That's the benchmark. If you move the left sponson closer to the tub, the result is that more weight is now on the left sponson, so it digs harder in the water, thus making the boat veer to the left. But, that puts the right sponson further out from the tub than the left sponson which allows it to have more leverage in making the boat veer to the right. So, this senario can work for a particular boat. On the other hand, extending the left sponson on a benchmark boat will give more leverage in making the boat veer left, while more weight is distributed to the right sponson making the boat want to veer right. So we have opposing forces, and the one with the most strength wins. Now.............here is where the other variables come into play. Sponson angle of attack and sponson width. If you extend the left sponson from the benchmark boat to give more leverage to pull the boat to the left and the right sponson has more lift than the left sponson, you have less drag on the right sponson as it has more lift but still have more leverage on the left sponson. What I'm getting to is a boat where both sponsons are barely touching the water yet the leverage of the left sponson being way out to the left causes the boat to veer left and compensate for prop walk. Make sense? Most people set up their riggers...left sponson more negative than the right, which plays out well with the left sponson extended.

John:

OK, I agree with every one of your statements about the effect of each change in spacing. I also agree with your statement on attack angle.

Now, what do you think the amount of influence the change in spacing makes. Do you think that the added weight of the sponson moved in or the space of the sponson moved out is a greater influence?

How much spacing change do you think it takes to make a big difference? Obviously related to total weight of the boat and contact patch of the sponson to the water.

What do you run on a given heat race boat to make the boat totally neutral? The way that I like my boats to run. I guess the ultimate test of neutral is a rudder with ZERO left correction in it. VERY difficult to get on the larger high torque prop boats.

Probably too many variables for us to EVER arrive at a consensus...

I wish I knew exactly what forces made the biggest difference, but I do not. I have often thought that the sheer aerodynamic drag of the sponson could be a big factor, especially after running oval time trials. When my oval boat reaches full speed the sponsons are riding on air and absolutely no water leaves the trailing edges of the front sponsons when traveling the straightaway. With only 3/8th of an inch extension on the left sponson it make a big difference in how the boat tracks. With that specific scenario I feel more like it is just aerodynamic drag further on the left side of the boat that makes the boat track better. When heat racing the sponsons are always touching the water, so I feel that aerodynamic and the water drag work together, but I have no idea how much each has on the tracking of the boat. I don't think the weight has as much effect as the aerodynamic and hydrodynamic drag at the speeds we are running. I only extend the left sponson about a 1/4 inch to 1/2 inch on most of my setups. I run 67 size and gas riggers. What I am experiencing might not apply to the 20 riggers. I run wider sponsons than most people because I like to float the sponsons on top of the water. It gets me around the corners quicker too.

As far as rudder setup.........I always alway always always set my boats, right or wrong, so the right side of the rudder is parrallel to the tub. The left side is at the angle of the blade. That gives me the best controlled speed with no hunting. If the boat veers into the course the rudder needs to be deeper. If it veers to the left, the rudder can be shorter. That's my golden rule. It works for me.

 

[Mark Grannis]

Hey guys! Interesting reading. My experience with sponson spacing is as follows................ First lets assume you have a boat with both sonsons equal distance from the tub. The engine is in the center of the tub and the weight on each sponson is distributed the same. That's the benchmark. If you move the left sponson closer to the tub, the result is that more weight is now on the left sponson, so it digs harder in the water, thus making the boat veer to the left. But, that puts the right sponson further out from the tub than the left sponson which allows it to have more leverage in making the boat veer to the right. So, this senario can work for a particular boat. On the other hand, extending the left sponson on a benchmark boat will give more leverage in making the boat veer left, while more weight is distributed to the right sponson making the boat want to veer right. So we have opposing forces, and the one with the most strength wins. Now.............here is where the other variables come into play. Sponson angle of attack and sponson width. If you extend the left sponson from the benchmark boat to give more leverage to pull the boat to the left and the right sponson has more lift than the left sponson, you have less drag on the right sponson as it has more lift but still have more leverage on the left sponson. What I'm getting to is a boat where both sponsons are barely touching the water yet the leverage of the left sponson being way out to the left causes the boat to veer left and compensate for prop walk. Make sense? Most people set up their riggers...left sponson more negative than the right, which plays out well with the left sponson extended.

John:

OK, I agree with every one of your statements about the effect of each change in spacing. I also agree with your statement on attack angle.

Now, what do you think the amount of influence the change in spacing makes. Do you think that the added weight of the sponson moved in or the space of the sponson moved out is a greater influence?

How much spacing change do you think it takes to make a big difference? Obviously related to total weight of the boat and contact patch of the sponson to the water.

What do you run on a given heat race boat to make the boat totally neutral? The way that I like my boats to run. I guess the ultimate test of neutral is a rudder with ZERO left correction in it. VERY difficult to get on the larger high torque prop boats.

Probably too many variables for us to EVER arrive at a consensus...

I wish I knew exactly what forces made the biggest difference, but I do not. I have often thought that the sheer aerodynamic drag of the sponson could be a big factor, especially after running oval time trials. When my oval boat reaches full speed the sponsons are riding on air and absolutely no water leaves the trailing edges of the front sponsons when traveling the straightaway. With only 3/8th of an inch extension on the left sponson it make a big difference in how the boat tracks. With that specific scenario I feel more like it is just aerodynamic drag further on the left side of the boat that makes the boat track better. When heat racing the sponsons are always touching the water, so I feel that aerodynamic and the water drag work together, but I have no idea how much each has on the tracking of the boat. I don't think the weight has as much effect as the aerodynamic and hydrodynamic drag at the speeds we are running. I only extend the left sponson about a 1/4 inch to 1/2 inch on most of my setups. I run 67 size and gas riggers. What I am experiencing might not apply to the 20 riggers. I run wider sponsons than most people because I like to float the sponsons on top of the water. It gets me around the corners quicker too.

As far as rudder setup.........I always alway always always set my boats, right or wrong, so the right side of the rudder is parrallel to the tub. The left side is at the angle of the blade. That gives me the best controlled speed with no hunting. If the boat veers into the course the rudder needs to be deeper. If it veers to the left, the rudder can be shorter. That's my golden rule. It works for me. During record trials if I can set the rudder this way for the straights and tweek to the right so the left side of the rudder is parallel to the tub in the turns..........it's the fastest way around the course no matter how wide the turn.

Cool stuff! John & Marty, I'm guessing you're keeping the turn fin constant with this information? Can you add the turn fin to this equation.

The reason I'm asking, I've recently been making a few differnet fins and changing the way we've been sharpening them for decades. What I'm finding out is unexpected. The length of the "chord" has interesting handling diffenreces.

John, I like your rudder blade setup idea. I've always tried to run dead straight. That's a new trick for this old dog!

MG

 

[Rokieboater]

Hey guys! Interesting reading. My experience with sponson spacing is as follows................ First lets assume you have a boat with both sonsons equal distance from the tub. The engine is in the center of the tub and the weight on each sponson is distributed the same. That's the benchmark. If you move the left sponson closer to the tub, the result is that more weight is now on the left sponson, so it digs harder in the water, thus making the boat veer to the left. But, that puts the right sponson further out from the tub than the left sponson which allows it to have more leverage in making the boat veer to the right. So, this senario can work for a particular boat. On the other hand, extending the left sponson on a benchmark boat will give more leverage in making the boat veer left, while more weight is distributed to the right sponson making the boat want to veer right. So we have opposing forces, and the one with the most strength wins. Now.............here is where the other variables come into play. Sponson angle of attack and sponson width. If you extend the left sponson from the benchmark boat to give more leverage to pull the boat to the left and the right sponson has more lift than the left sponson, you have less drag on the right sponson as it has more lift but still have more leverage on the left sponson. What I'm getting to is a boat where both sponsons are barely touching the water yet the leverage of the left sponson being way out to the left causes the boat to veer left and compensate for prop walk. Make sense? Most people set up their riggers...left sponson more negative than the right, which plays out well with the left sponson extended.

John:

OK, I agree with every one of your statements about the effect of each change in spacing. I also agree with your statement on attack angle.

Now, what do you think the amount of influence the change in spacing makes. Do you think that the added weight of the sponson moved in or the space of the sponson moved out is a greater influence?

How much spacing change do you think it takes to make a big difference? Obviously related to total weight of the boat and contact patch of the sponson to the water.

What do you run on a given heat race boat to make the boat totally neutral? The way that I like my boats to run. I guess the ultimate test of neutral is a rudder with ZERO left correction in it. VERY difficult to get on the larger high torque prop boats.

Probably too many variables for us to EVER arrive at a consensus...

I wish I knew exactly what forces made the biggest difference, but I do not. I have often thought that the sheer aerodynamic drag of the sponson could be a big factor, especially after running oval time trials. When my oval boat reaches full speed the sponsons are riding on air and absolutely no water leaves the trailing edges of the front sponsons when traveling the straightaway. With only 3/8th of an inch extension on the left sponson it make a big difference in how the boat tracks. With that specific scenario I feel more like it is just aerodynamic drag further on the left side of the boat that makes the boat track better. When heat racing the sponsons are always touching the water, so I feel that aerodynamic and the water drag work together, but I have no idea how much each has on the tracking of the boat. I don't think the weight has as much effect as the aerodynamic and hydrodynamic drag at the speeds we are running. I only extend the left sponson about a 1/4 inch to 1/2 inch on most of my setups. I run 67 size and gas riggers. What I am experiencing might not apply to the 20 riggers. I run wider sponsons than most people because I like to float the sponsons on top of the water. It gets me around the corners quicker too.

As far as rudder setup.........I always alway always always set my boats, right or wrong, so the right side of the rudder is parrallel to the tub. The left side is at the angle of the blade. That gives me the best controlled speed with no hunting. If the boat veers into the course the rudder needs to be deeper. If it veers to the left, the rudder can be shorter. That's my golden rule. It works for me. During record trials if I can set the rudder this way for the straights and tweek to the right so the left side of the rudder is parallel to the tub in the turns..........it's the fastest way around the course no matter how wide the turn.

Cool stuff! John & Marty, I'm guessing you're keeping the turn fin constant with this information? Can you add the turn fin to this equation.

The reason I'm asking, I've recently been making a few differnet fins and changing the way we've been sharpening them for decades. What I'm finding out is unexpected. The length of the "chord" has interesting handling diffenreces.

John, I like your rudder blade setup idea. I've always tried to run dead straight. That's a new trick for this old dog!

MG

I am lost on the rudder comment. Can someone elaborate or a pic or drawing? Awesome topic.

Thanks,

Robert

 

[John Finch]

Robert. Take a yard stick and place the edge on the right side of the rudder with the yard stick pointing towards the bow. The yardstick should be parallel to the tub. Like setting a turn fin on a mono to make sure the fin is parallel with the keel.

Mark.....What interesting things have you found out about the cord of the turn fin.

 

[Mark Grannis]

Robert. Take a yard stick and place the edge on the right side of the rudder with the yard stick pointing towards the bow. The yardstick should be parallel to the tub. Like setting a turn fin on a mono to make sure the fin is parallel with the keel.
Mark.....What interesting things have you found out about the cord of the turn fin.

Well, the pull was just the reverse of what I thought was going to happen. The fin with the shortest cord pulled the least and displaced more water spray. Without the use of a radar gun, the boat looked slower. The fin with the longest cord pulled harder to the right, seemed faster all around the course, and had much less water thrown high in the turns. I don't like having to hold left steering or use left trim down the straights. If left like this, I would have to run a constant arc in the straights. Forget about a left turn.... violent tumble.

I thought the longer cord would have more of a surface area acting as a lever to push left. I think the result was by increasing the surface area of the sharpened edge the fin caused increased drag on that side thus the more right pull. I purposely exaggerated how far back I sharpened this last fin just to learn.

That's what led to my question about the fin being a factor. I do run an offset to the left. I tow in the left sponson slightly more than the tow in on the right sponson. I have not run with both sponsos straight or with tow out.

With all that being said, do I have too many vectors fighting each other reducing my top speed, or am I just manipulating a poor setup to get through the turns faster? Just locking for the pudding before the proof.

I also made a radical fin for a gas hydro boater. He only changed the fins and it was much faster in top speed, but the fin didn't last but a few laps. Back to the band saw.

MG

 

[Stuart Barr]

One thing I that noticed many years ago when we ran in really rough water was where to put the weight. The turn fin side acted like a clamp to the water. Weight was added to the left side and the boat was able to drive through it. Think about "offsets" to the left with this in mind.

 

[John Finch]

One thing I that noticed many years ago when we ran in really rough water was where to put the weight. The turn fin side acted like a clamp to the water. Weight was added to the left side and the boat was able to drive through it. Think about "offsets" to the left with this in mind.

Yup! More weight further out on the fulcrum. More leverage using less weight. Oh Stu...Don F. just built two of your boats and he just can't stop talking about the quality. I was in his shop several times during the build and got to say..........I was impressed also. You got it goin on!!!

 

[Rokieboater]

Robert. Take a yard stick and place the edge on the right side of the rudder with the yard stick pointing towards the bow. The yardstick should be parallel to the tub. Like setting a turn fin on a mono to make sure the fin is parallel with the keel.

Then you adjust the trim with the length of the rudder??????

Robert

 

[John Finch]

Robert. Take a yard stick and place the edge on the right side of the rudder with the yard stick pointing towards the bow. The yardstick should be parallel to the tub. Like setting a turn fin on a mono to make sure the fin is parallel with the keel.

The rudder is only one way to trim the hull. I may or may not shim the strut to be angled to the left or I may offset it to the right or change strut angle to trim the hull. The rudder length is just one component. I think a little tweek here and there is better than using one single item to trim a boat. There are too many variables to say we should trim a boat a certain way and have it meet the needs of every boat, but the rudder explaination I used works well towards that end.