A topic to think about

[Carl Van Houten]

Does aerodynamics matter much on tunnelboat design? Here is your answer : ABSOLUTELY YES!

How can you know that what I am telling you is true? Here is your 3 part answer:

1 ) Old style boat designs (I don't have to name them because you already know witch ones they are) REQUIRE you to run at least some negative trim on the best of them and a lot of negative on the ones that are not so good in order to combat the aerodynamic fight going on under as well as over your boat.
2) Old style boat designs REQUIRE you to run your prop low in the water to help combat the aerodynamic fight going on under as well as over your boat.
3) Old style boat designs REQUIRE you to run tons of weight to help combat the aerodynamic fight going on under as well as over your boat.

The next time you go to a race look at the boats in the pit area. Notice who has their engines tucked under. Those same guys are trying to trim a boat that is aerodynamically out of balance. They are typically the same guys who have tons of 1/4 ounce wheel weights stuck all over their forks. Those guys are most likely going to be in the way when the race starts. When you see a tunnel boat that has his engine set high, no negative trim and no wheel weights on his forks...you had better get ready because that guy is fixing to smoke your A$$.

Your final race ready prop height, prop angle and the total amount of weight you were required to add to your boat in order to trim your boat correctly tells the real story. Plus the type of prop you can run on your boat tells how good of a job you did designing your boat.

Boats who have been designed to properly take advantage of aerodynamics run NO negative trim (NONE) plus the props on these boats run very high. My 3.5 mod for instance has the center of the shaft .312" (5/16") above the bottom of the sponson. Try that with an old style tunnel boat. The C/G of my 30" moidified 3.5 tunnel is 7.75 (7-3/4") again this C/G percentage will most likely not work well with a flat deck round track model. I can achieve this because my boat is designed to "Fly" correctly at this C/G and not only can it fly down the straights as it should (with hardly any added weight to hold it down) it also has the ability to set smoothly and carve out a perfect turn without slamming, hooking, flat spinning or rolling and with only 1-1/2 ounces added in front of the radio box (not on the tips of the sponsons).

All these traits are hard to achieve from a tunnel boat. The easiest thing to achieve from a tunnel boat is one that can run fast in a straight line because none of these other required round track traits mentioned come into play. How well your model uses the air flow on top of as well as below your boat will determine how much power you will have to rob from your engine using negative thrust and low prop settings to trim your boat correctly and if you don't take advantage of the air flow then the faster you go the more weight it takes to keep your tunnelboat/kite on the water. Forget what the full size boats look like because they have hydraulic trim to perform these functions and your RC tunnelboat don't.

When the first aerodynamically tuned tunnel boat hit the pond in 2003 there was no going back. Let's not go backwards. Experiment with your boats and find what works best for you.

 

[Edward Drock]

 

[Grimracer]

All that having never asked the question..

 

[Carl Van Houten]

Oh there is a reason for the post beside the fact that it's true. There are some that continue to tell people that aerodynamics have practically nothing to do with tunnel boats. I will believe that when they bolt a F-150 truck body on a INDY car frame and actually win a race with it...

 

[Jeff whiddon]

Good info Carl !

 

[Grimracer]

Oh there is a reason for the post beside the fact that it's true. There are some that continue to tell people that aerodynamics have practically nothing to do with tunnel boats. I will believe that when they bolt a F-150 truck body on a INDY car frame and actually win a race with it...

I must have missed that being posed...

Grim

 

[Terry Keeley]

My first attempt at designing my own hydros back in the 90's didn't work very well, they blew off at anything over 70 mph.

Tried to "fix" my .67 boat so I put 1 lb. of weight on the nose. Guess what, it still flew.

Aero has a lot force and effect on our toys boats.

 

[john v]

great post carl, i am a little slow but learning from you!

 

[LohringMiller]

While air drag is insignificant, managing air lift is the key to high speed performance. My favorite example is where we added 25 mph to a SAW sport hydro by using spoilers to kill lift. The boat ended up very stable at around 100 mph and still holds the NAMBA GX1 SAW record.

Thanks Carl. Getting tunnels to work is hard.

Lohring Miller

 

[Wasted wages]

I figured out early that added weight further away from the cg didn't do much other than to create porpoising (?) as the the front and back of the boat was now trying to do a balancing act around center mass..

I always try to move components around to achieve the CG with no added weight... compensate for fuel at a half tank and trim from there.

 

[HTV Boats]

Adding weight to the nose of a tunnel is a crutch and makes a teeter totter when you let off power and the front dives and hooks. Above 40mph the air flow becomes a factor and proper design starts to really show. That said if your happy with the handling of your hull small wings at different heights catch a lot of air and drive the front down at speed.
Mic

 

[Carl Van Houten]

I think everybody has stuck their hand out the car window at 55mph and noticed how much up or down force is generated just from changing the AOA of your hand. Divide the size of your hand into the area of even a small 3.5cc size tunnelboat. You will be shocked.

IN A NUTSHELL: Once a tunnelboat is on plane you have a huge surface hanging on the front side of three points. With the two sponson points being your fulcrum points and the prop being your third point. The lift of the prop and the lift generated by the design of your boat should be closely matched or you will forever have to compromise the fight between these two forces through your trim settings. Trim setting come at a high cost from the performance of your engines ability to push the boat forward. A great boat design will be one that is aerodynamically and hydrodynamically matched.

Think of it like a playground seesaw and how you have to use your legs to push your end off the ground when the skinny kid gets on and wants to ride. Its the same thing your engine is having to do when your boats aerodynamics are poorly designed.

 

[Julio Almazan]

Hi.
Carl,
Thank you for this reflection and teaching ... I think it is always good to remember some concepts ...

 

[Julio Almazan]

Hi:

Getting tunnels to work is hard.

... but it is very rewarding and exciting

 

[Marty Davis]

My first attempt at designing my own hydros back in the 90's didn't work very well, they blew off at anything over 70 mph.

Tried to "fix" my .67 boat so I put 1 lb. of weight on the nose. Guess what, it still flew.

Aero has a lot force and effect on our toys boats.

I bet that you had sponsons that had more length on top rather than the bottom. How I know, I did the same thing. And 70mph was the same for me.

 

[Terry Keeley]

I bet that you had sponsons that had more length on top rather than the bottom. How I know, I did the same thing. And 70mph was the same for me.

Think they were like the CR sponsons of the day, I made them big tho to handle rough water. That with the lift from the tub and there was no way to keep it from being an airplane!

 

[Kris Flynn]

So who is this person or people that have your knickers in a twist? ;-)

Of course aero matters, but I bet most gains have been due to hydrodynamic changes in the past 15 years.

 

[Carl Van Houten]

Hello Kris. The hydrodynamics you are probably referring to have been tested here years ago. Trimming to a heavily hooked bottom has it pros and cons. The Seaducer tunnel and the Trophy boat both played around with that type of bottom style back in the 90's and I even tried it as well but was not happy with the results. Their happy place was moderately chopped water (depending on the amount of hook). If the water was too smooth it struggled to keep up and too rough and it was highly likely to stuff before six laps were done. The hook robed so much power that you had to run small props and scream the blades off them to get it to perform with boats who were running straight pads. They required more fuel onboard to support the high RPM's and bearings had to be changed out more often. I personally don't care for that style of bottom. Everybody has their preference. I had rather trim against a straight pad and work with flying the boat. It is not easy to find that perfect trim but the sweet spot seems to be more forgiving over all as long as you have done your homework. Regardless and as you know tunnel boats are persnickety little models no matter what you do.

 

[dr gary turner]

AERO?
Top boat 104.8. Incredibly stable
Thanks Carl for the musings

The more the better since it results in faster and better handling boats

 

[Kris Flynn]

I do not mean hooks and such...just simply getting the hydrodynamic lift correct. I found I had too much when going for 80mph. All the aero mods and bandaids did nothing...a few small ride pad tweaks and WOW. Look at how the JAE design has changed riggers. Mainly due to how simple it is and what it going on underneath.

Do not get me wrong, aero is important, just expanding the subject a little.