Model Boat Turning Dynamics

[Bill Gibson]

My 20 Hydro(Intrepid) is without a doubt the finest turning model boat ive ever had...and i still remember Marty saying it could be made even BETTER! I greatly value the help and input Marty gave me with that and other boats of mine...his information is priceless to those who are willing to listen or read!

 

[Marty Davis]

Yes,

It is all of those components. COG, Wheelbase, Engine Placement, Top of Sponson Angle, Attack Angle, Strut Angle (by the way, I forgot that component in the Tech Article). I will need to add that.

If you take the informtion that I gave you and apply it, you will gain a huge amount in handling. Some built-in components you obviously can't change. You can, however look at what is happening and when you build a new boat, apply those things that you have observed into the new boat. This information will just make it easier to analyze.

Have you seen boats that when they hit a wave or rougher race water, the front end comes up? That is the extreme and NOT what you want, to be able to handle the rougher water.

I would like a boat that boat that does not come up at the front. Given that parameters such as "wheelbase", engine placement and so on are largely fixed by the designer it seems this may be difficult to achieve. Some boats have engine placement more rearward in relation to the sponson riding surfaces, can this sort of balance be achieved with these designs?

Ian:

Maybe it is time to design your own boat....

Using the information that I presented, you can come close. If you would like, I will post some percentages that I use for Engine and CG location so that you know the basics.

You need to consider the wheelbase as the distance from the sponson running points (rear of the sponsons) to the center of the prop. Then, everything is calculated from there.

 

[Marty Davis]

Yes,

It is all of those components. COG, Wheelbase, Engine Placement, Top of Sponson Angle, Attack Angle, Strut Angle (by the way, I forgot that component in the Tech Article). I will need to add that.

If you take the informtion that I gave you and apply it, you will gain a huge amount in handling. Some built-in components you obviously can't change. You can, however look at what is happening and when you build a new boat, apply those things that you have observed into the new boat. This information will just make it easier to analyze.

Have you seen boats that when they hit a wave or rougher race water, the front end comes up? That is the extreme and NOT what you want, to be able to handle the rougher water.

I would like a boat that boat that does not come up at the front. Given that parameters such as "wheelbase", engine placement and so on are largely fixed by the designer it seems this may be difficult to achieve. Some boats have engine placement more rearward in relation to the sponson riding surfaces, can this sort of balance be achieved with these designs?

I am not sure that these characteristics can be achieved with this type boat. Might be, but I don't have experience with the rearward cg type boats (Eagle, JAE). They obviously would take much different sponson configurations (width and attack angle). As I noted, Stu has a lot narrower running surface that takes a little more attack angle to get it correct. I did run one of his 67 Crapshooters for a season and it was very good. It was the small 67 boat and didn't handle rough water like the newer stretched version.

I think that you can get a super running boat with almost any of the currently available boats, but each will take a little different set of initial settings. You will have to tune the boat very carefully and keep records of your changes to achieve the best result.

 

[Charles Perdue]

After you get the balance, you will have to work on the aerodynamics to get the boat to FLY level. Aerodynamics not only effect the drag but have a much greater effect on the stability of the boat. The nose of the hull and sponsons has a large effect on the ability of the boat to stay level.

When building the hull, round the outside corners of the nose with about a 3/8 radius and blend that into the top to bottom radius. What this does is divert part of the air that normally would either have to go above the boat or below it, around the sides of the boat, so it has a more forgiving neutral attitude.

On the front of the sponsons add a small wedge shaped peice underneath the front tip and blend it in with a top to bottom radius. What this does is divert part of the air to go over the top that would be going underneath the sponson causing unnessary lift by moving the center of the nose of the sponson down and also doing away with the sharp nose which is either lifting or pushing down on the sponson. The sharp nose has no "Sweet Spot" where the aero is neutral. The larger the radius, the better. Make the wedges so they can be attached temporarily (double sided tape works good) so that the aero can be "Tuned" to what the boat likes.

Charles

 

[Ian Inverarity]

Ian:

Maybe it is time to design your own boat....

Using the information that I presented, you can come close. If you would like, I will post some percentages that I use for Engine and CG location so that you know the basics.

You need to consider the wheelbase as the distance from the sponson running points (rear of the sponsons) to the center of the prop. Then, everything is calculated from there.

Marty, designing my own 21 hydro is part of my plans, but I am a slow builder so it is a long process, at this stage I am considering my own front sponson design.

In terms of the "wheelbase" measurements, on my JAE21 I have photos that show the rear sponson seems to mostly stay on the water, and in my case the prop is a way back from the end of the rear sponson so there is quite a difference in wheelbase between measuring to the rear sponson and prop.

 

[Ian Inverarity]

After you get the balance, you will have to work on the aerodynamics to get the boat to FLY level. Aerodynamics not only effect the drag but have a much greater effect on the stability of the boat. The nose of the hull and sponsons has a large effect on the ability of the boat to stay level.

When building the hull, round the outside corners of the nose with about a 3/8 radius and blend that into the top to bottom radius. What this does is divert part of the air that normally would either have to go above the boat or below it, around the sides of the boat, so it has a more forgiving neutral attitude.

On the front of the sponsons add a small wedge shaped peice underneath the front tip and blend it in with a top to bottom radius. What this does is divert part of the air to go over the top that would be going underneath the sponson causing unnessary lift by moving the center of the nose of the sponson down and also doing away with the sharp nose which is either lifting or pushing down on the sponson. The sharp nose has no "Sweet Spot" where the aero is neutral. The larger the radius, the better. Make the wedges so they can be attached temporarily (double sided tape works good) so that the aero can be "Tuned" to what the boat likes.

Charles

Charles, interesting stuff. Do you have pics of the wedges you describe?

The hydro that I had that did fly level was an A/S Cobra, these do not have rounded tips at the front of the sponsons and tub. I understand that aircraft wing profiles do not have pointed LE's so that the airflow does not separate as easily at different angles of attack. But I would have thought that for a hydro we would WANT separation so that the lift isn't so great when the hull pitches up?

 

[Marty Davis]

Ian:

Maybe it is time to design your own boat....

Using the information that I presented, you can come close. If you would like, I will post some percentages that I use for Engine and CG location so that you know the basics.

You need to consider the wheelbase as the distance from the sponson running points (rear of the sponsons) to the center of the prop. Then, everything is calculated from there.

Marty, designing my own 21 hydro is part of my plans, but I am a slow builder so it is a long process, at this stage I am considering my own front sponson design.

In terms of the "wheelbase" measurements, on my JAE21 I have photos that show the rear sponson seems to mostly stay on the water, and in my case the prop is a way back from the end of the rear sponson so there is quite a difference in wheelbase between measuring to the rear sponson and prop.

The front sponson design is something that people get caught up with. Why not keep it simple and just make a sponson that is absolutely as simple and light as possible? No need to try to re-invent the wheel. Angles are MUCH more important than sponson shape. Cosmetics are importent to some people so whatever is your choice.

The prop way back is just the same as extending the boat, except that the rear sponson (if any) don't work as well with less leverage in relation to the prop position.

KEEP IT SIMPLE and refine, THEN add the cosmetic touches.

 

[Marty Davis]

After you get the balance, you will have to work on the aerodynamics to get the boat to FLY level. Aerodynamics not only effect the drag but have a much greater effect on the stability of the boat. The nose of the hull and sponsons has a large effect on the ability of the boat to stay level.

When building the hull, round the outside corners of the nose with about a 3/8 radius and blend that into the top to bottom radius. What this does is divert part of the air that normally would either have to go above the boat or below it, around the sides of the boat, so it has a more forgiving neutral attitude.

On the front of the sponsons add a small wedge shaped peice underneath the front tip and blend it in with a top to bottom radius. What this does is divert part of the air to go over the top that would be going underneath the sponson causing unnessary lift by moving the center of the nose of the sponson down and also doing away with the sharp nose which is either lifting or pushing down on the sponson. The sharp nose has no "Sweet Spot" where the aero is neutral. The larger the radius, the better. Make the wedges so they can be attached temporarily (double sided tape works good) so that the aero can be "Tuned" to what the boat likes.

Charles

Charles:

I use a little different approach. I add a wedge angle to the top of the sponsons so that this area tends to keep the front end down solid. The sharp front tips are EXACTLY as you say and will blow the boat off if it is loose at all. The best way would be a sponson that is only as long as the front sponson tube with a huge rounded front. Just doesn't look very nice. I decided to use the sharper front and use the top to keep it on the water.

 

[Ian Inverarity]

The front sponson design is something that people get caught up with. Why not keep it simple and just make a sponson that is absolutely as simple and light as possible? No need to try to re-invent the wheel. Angles are MUCH more important than sponson shape. Cosmetics are importent to some people so whatever is your choice.

The prop way back is just the same as extending the boat, except that the rear sponson (if any) don't work as well with less leverage in relation to the prop position.

KEEP IT SIMPLE and refine, THEN add the cosmetic touches.

Marty, changing the riding surface angles/widths is more what I am looking at rather than cosmetics, I don't intend my design to look a lot different. Weight and stiffness of turn fin mounting is also a consideration.

 

[Charles Perdue]

Ian, there is a lot that goes into make the boat stable in flight, the rounded nose and sponsons definately make them more forgiving with attitude changes

I will post some photos tonight.

When you design and build your own first boat, do not worry about paint or making it pretty. Make it so that everything that you possibly can make, is adjustable. Read an memorize all of Martys Tech Articles. Study other boats, how they are made and what you like and dislike about the way that they run, handle and why. Spend a year or so trying different combinations, you will be surprised how many changes that you can make, so that you can learn for yourself what different changes have what effects on the boat. WRITE IT DOWN. Then build your second boat with what you have learned. Every boat that you build after that will still have minor changes as you progressively lean more and more. For me, that is the fun part.

Charles

 

[Lennard Cuenco]

Marty et al...

hope you dont mind me reviving an old post...this one has stuck in my brain..cant let it go

noticed that heavier boats winning heats from lane 4 on out (never getting closer than approx 20'-30' from the bouys) by carrying a ton of speed thru the turns negating inside lane advantage, ie boats in lane 1 were scrubbing so much speed staying on the inside lanes, they were being passed on the outside (1/8 scale, not riggers..ie - slowing a heavy/underpowered boat is WAY more costly than slowing a light/overpowered boat)...the introduction of rudder input seems very gentle, designed to not upset the forward momentum of the boat...very different looking dynamic than a boat doing snap turns (in the end, i guess its all about lap time)

do you have thoughts on setup of boats that stay outside, maintaining inertia thru the turns rather than snap turning? as i write this, im thinking there is a weight limitation? for a boat to do snap turns?....over a max weight, it makes more sense to make smoother turns?

 

[Marty Davis]

Marty et al...

hope you dont mind me reviving an old post...this one has stuck in my brain..cant let it go

noticed that heavier boats winning heats from lane 4 on out (never getting closer than approx 20'-30' from the bouys) by carrying a ton of speed thru the turns negating inside lane advantage, ie boats in lane 1 were scrubbing so much speed staying on the inside lanes, they were being passed on the outside (1/8 scale, not riggers..ie - slowing a heavy/underpowered boat is WAY more costly than slowing a light/overpowered boat)...the introduction of rudder input seems very gentle, designed to not upset the forward momentum of the boat...very different looking dynamic than a boat doing snap turns (in the end, i guess its all about lap time)

I don't agree with your analogy at all. IF you have the turn fin and the rest of the dynamics correct, the boat will pivot on the fin and you will see almost ZERO water off of the fin. The rudder input it only the amount to make the diameter of the turn - no more. There have been people that have set records having a boat that runs outside the buoys some and running real fast. Both ways will work, but I can tell you that running on the buoys is by far the best way to win heats. That is ONLY if the boat is turning superbly as noted in the tech note.

do you have thoughts on setup of boats that stay outside, maintaining inertia thru the turns rather than snap turning? as i write this, im thinking there is a weight limitation? for a boat to do snap turns?....over a max weight, it makes more sense to make smoother turns?

As for setting up a boat to run outside, that is really not in my vocabulary Weight is super important to all aspects of an outrigger, but mostly in the areas of launch and acceleration.

 

[Peter Gise]

I have wanted to write a Technical Paper about Turning Dynamics of Hydros for a long time and finally got a little time to complete the project. I have posted the new Technical Paper at http://rcboat.com/past.htm It will be the top article.

I think that a lot of the items in the paper will apply to almost any of our classes.

Comments welcome.....

Article is gone?

 

[Marty Davis]

I have wanted to write a Technical Paper about Turning Dynamics of Hydros for a long time and finally got a little time to complete the project. I have posted the new Technical Paper at http://rcboat.com/past.htm It will be the top article.

I think that a lot of the items in the paper will apply to almost any of our classes.

Comments welcome.....

Article is gone?

Here is a link that works. http://engine-analysis-software.com/turning_dynamics.html

 

[Bob Morton]

Great information. Thanx Marty

NAILS.

 

[dr gary turner]

Marty, you mentioned Ed Lacky and working with turn fins. I remember when he came to the lake/reservor with a box of fins of all different shapes angle wide and narrow and etc. I remember seeing my first bent turn fin. It was the Beards, Ben and Bud.

We had a time trial at Huntsville and Mark Bullard was working at the 60 oval with a Pinkerd hydro. He had a "straight" turn fin. I took that thing and put a 17 degree bend at about the water line and he went out and broke the record.

As you stated the fin comes after the other refinements. Just "musing" Gary

 

[dwilfong]

On the subject of turn fins I have found a interesting setup that works for me.

Testing with my SG and SGX 90 boat I have found some different results than what most would consider.

I have made a new fin same shape and depth but wider out of .050 titanium I got of Ebay cheep.

This fin is like a piece of spring steel in the way it bends as the turning load is applied.

I put a backer on this fin as that is what I was to understand was needed to make it stiff as it should be.

Well this made the boat very hard to drive. the boat reacted to every thing and had a mind of its own. real twitchy.

I have found that with a fin that bends as the load is applied will be more forgiving and stabilize the boat under changing load conditions.

The titanium will act as a shock absorber as it will spring back to shape.

I know this is not what is the main stream thought on the fin but it works for me.

 

[shoboat]

David,

I think you may be on to something?

How is your rudder?

Thanks,

Mark

 

[dwilfong]

David,

I think you may be on to something?

How is your rudder?

Thanks,

Mark

That 1.25 rudder from B&B works killer on my SGX set at the same length as the fin with a 3 deg back cut on the bottom..

Didn't mention that don't want to upset the apple barrel to much.

 

[shoboat]

David,

The apple blossoms are just in full bloom these last two weeks up here.

Thanks For Sharing,

Others,

Don't forget the rudder and turnfin should work together. Both are very

Important in the turning of any boat.

Mark