Wetside of Tunnels...

[Kris Flynn]

Hey

I noticed a while back that he full szed tunnels have a little 'Step' around 1/4 to a 1/3 of the length of the hull from the back on the ride pads...it looks so small and pointless, yet it is on most of them (in the UIM championship anyway) http://www.f1boat.com/02/waterford/waterfo...yaction_02.html

and i wanted to know why most guys didnt put them into their tunnel design (Villain? Dunlaps?) although Jerry seems to have put some on his new Rinker style 21 stock tunnel...

anyway anyone who has experimented with them of have an opionion, it would be good if you could give me (us) your thoughts...

Kris

 

[Jerry Dunlap]

Kris,

The built up section running the length of the tunnel floor on the F1 in the photo is intended to stabalize the boat. You will notice there is a slight vee, or diehederal, in the raised section. Since the center section acts like an airfoil, or wing, this dihederal is supposed to provide stability like the dihederal on an airplane.

I've seen many full size F1s blow off the water, so I'm not sure such an addition to the tunnel floor of a model would make all that much of a difference. It's much easier to build a WOF hull with a flat tunnel floor. However, someone building a plug for a fiberglass hull could incorporate this design feature into the mold. There are several fiberglass model tunnel boats that do have dihederal in the tunnel floor.

JD

 

[hg-md]

There are other theorys to stepping a hull also. The step in the hull also draws in air and aererates the water that you are running on. This will lessen drag by reducing the surface tension of the water in that area. This is done on allot of offshore boats in particular, including cats.

 

[Propjockey]

Old Prather Lap Cat

 

[Propjockey]

Better view

 

[Kris Flynn]

well that about sums it up Jerry i think i understand the point of it now...but i wont test it on my 21" electric tunnel as i dont think it would make much of a difference at the speeds i will be going at.

hg-md thats what i was my understanding...because the cats are the same sort of hull shape (having a tunnel and two sponsons) but with the CoG being much further forward than the OB's, like i said im surprised i dont see them on the RC Tunnels more often

you see PJ i didnt even realise that old hull had steps

ok so my question now is...why arnt the steps bigger than they are? as i said they are just so small and in significant...

Kris

 

[Grimracer]

Kris

This is what I see when i look at tunnel hull design regarding steps. Steps (like the prather hull) and pockets (like the picture of the one to one hull) are two different animals.

Steps are just that. Like the steps in a house. The ride height of the hull changes as the steps go rearward. "Step down, step down and so on" do you get that?

Pockets (there may be other names for this and steps could be one) just break surface tension. Maybe add down force with little or no drag. (not sold on that one however)

Now steps and pockets do the same thing (kind of) its just that steps can make the boat act weird in the corners, Loss of bite.

Does anybody really know what the heck those pockets are called? They might be called steps but this makes no sense to me.. O well steps it is..

Grim

 

[Jerry Dunlap]

Grim,

In full scale cats and vee hulls, the term "ventilated bottom" is utalized in descibing the notches/and/or stepped running surface/s. Since the idea of interrupting the running surface with notches/steps is to move air across the surface, this seems like a good description.

JD

 

[osracer]

Hi, I'm not an expert on models or the real thing but I do have a lot of fun with the models. I have also sat in many different kinds of racing equipment over a long period of time. When you start comparing a real tunnel to a model you have to take into cosideration that the real deal is ultra high tech and can be trimmed for corners, chutes, upwind, downwind etc etc. Our models always run in a compromise. The need to corner, go fast down the straights, handle the wind and rough water all with no adjustments once we throw them in. My thought is you could build an exact replica of the best real tunnel in the world and run it against a Villian, Dunlap, Hopper etc. and it would probably get it's doors blown off as these hulls are all designed to compromise the situations they face. Just my two cents. Bill

 

[Jimboat]

On Step design...

Steps in planing surfaces is a controversial subject. It's history goes back a long way, but the difficulty of designing a proper step makes it a very 'design-dependent' feature, and not one that all boats will benefit from.

The concept of steps in the hull running surface was originally proposed by Reverend Ramus in 1872. He proposed both a single step with tandem planing surfaces, and a combination of 3 pontoons with one forward and 2 aft. There were published drawings for small stepped hulls with hard chines as early as 1906, and W. Fauber obtained the first US patent for hulls with multiple steps in 1908. There is quite a history of step design... the Solair set a record of 46 mph with 70 hp with a 12 stepped hull in 1910... and the record was upped each year to 1929, when the Estelle IV set the record at 105 mph with 2000 hp on a 35 ft. hull. It was an interesting era, more outlined in the History of Powerboat Design book.

Stepped hulls dominated race boat design until about 1938 when Adolph Apel patented the 3-point hydroplane configuration. Even though 3point hulls were very successful in small limited class racing, stepped hulls were still running competitively in Unlimited class racing until 1949. In 1950, Slo-Mo-Shun demonstrated 'prop riding' and boosted the world speed record significantlly.

There are a number of reasons why stepped hulls did not continue their popularity for pleasure boats...complexity of design, cost of development, they were banned from Gold Cup racing from 1920 through 1931, and there were many, many huge war-surplus aircraft engines available after WW1 at low prices, so it was easier to buy a big engine for a standard mono-hull, than to develop an efficient stepped hull. Quite a history...

The most significant issue with step design was, and still is, the very difficult engineering challenge of properly locating an efficient step on the hull. The length of planing surface behind the step (ie: the location of the step) and depth of the step have a huge impact on the performance of the setup. To design the step improperly can actually decrease performance. The issue of multiple steps makes the challenge even more tricky.

During the early development of stepped hulls, the reason that performance varied so drastically, was just exactly this issue...the designers didn't always achieve the best engineering solution to the placement and depth of the step.

This is still the case today...performance of stepped hulls can widely vary. So it's difficult to assume that any boat with steps, will necessarily be any better than one without.

I recently wrote a series of articles on Factors that influence performance in high performance powerboats and tunnel boats. You can download it for free in ....TBPNews, my newsletter for high-performance and tunnel boat enthusiasts....check out "Rocket Science! (Part 2, Planing surfaces)" (Series of articles on high performance powerboat design, and technical opportunities for performance improvements by design).

 

[Kris Flynn]

ok now i see the difference Mike, i was thinking of the 'Pockets' but called then steps...

i think i will leave then off for now, just because i cant be bothered doing any testeing for a bit to see if there is any performance enhancement

 

[Grimracer]

Jimboat

Cool stuff... stay with us we can all use your help.

Hea guys,, cool stuff here,, happy reading..

Thanks Jimboat

Grimracer

 

[lohring]

One thing that most model tunnels have that is missing from the picture are "stumble blocks". These are the surfaces on either side of the tunnel at the front that keep the bow up in the turn.

I just came back from the District 19 Shootout where Jeff Michaud was running a new cat design that featured a stepped forward sponson to eliminate the need for "stumble blocks". The idea was first run by Jim and Jimmy Johnson on their tunnels. From the way the cat cornered, the idea looks very promising. The Johnson’s tunnels only have the front lower angle area and a continuous 15 degree bottom on the rest of the sponson.

(img)http://www.intlwaters.com/yabbse/attachments/Insane cat 3.jpeg(/img)

 

[lohring]

I don't know what happened to the picture. The server won't accept another post because the "file exists". You can also find pictures of Jeff's cat at http://www.hometown.aol.com/nambagas/page1.html

Lohring Miller

 

[lohring]

You can allso see the idea on the tunnel with its sponson tips up in the middle of the pictures on http://www.f1boat.com/02/waterford/waterfo...yaction_02.html . I think the advantage of a clear tunnel area with the stepped sponson to limit the "set" in the turn should improve speeds.

Lohring Miller

 

[Jimboat]

lohring...I may be able to help clarify the sponson design features of question, but I need to better understand the 'terminology' you're using for the features you're talking about. What exactly are you refering to when you reference the 'stumble blocks'?

Also, lohring, what are you referring to in the reference to the 'idea on the tunnel with its sponson tips up in the middle of the pictures on http://www.f1boat.com/02/waterford/waterfo...ction_02.html.'?

 

[Kris Flynn]

These are where 'Stumple blocks are positioned and they help reduced hooking in the trurns...they lift the bow in the turn

 

[lohring]

The idea of stumble blocks is to limit the set of the boat in the turn. When the bow comes down, the stumble blocks hit and stop the movement. They also disrupt airflow in the tunnel. The alternative design uses a low angle area at the front of the sponson to do the same thing. When the bow comes down the flat area hits the water and stops the movement. I think this may be a better design.

Lohring Miller

 

[Jimboat]

Right... these "stumble blocks" are also refered to as "skis" sometimes. Having them located inside the tunnel like this is a somewhat inefficient design, as you pointed out, lohring. Controlling the amount of "set", or the dynamic stability of the hull in rapid deceleration and turning, is better done through an active sheer and chine design, and/or varying leading plane deadrise.

The aerodynamic lift gained from the 'aerofoil' made up of the tunnel roof and deck surfaces, is most efficient when it sees an undisturbed air flow. Particularly during rapid decelleration and/or turning, the dynamic mix of aerodynamic lift & drag with hydrodynamic lift & drag is changing very rapidly and very dramatically. The location of the "dynamic center" of the hull will, under such conditions, move fore/aft by as much as 80% of boat length. Consequently, it is somewhat dangerous to add disturbed airflow, and a "quick fix" hydrodynamic "stop" (like the "skis" to the transition.

There are, however, many ways that designers use to "skin the cat". I don't subscribe to all of them. And I try to prove that the overall impact of any, will be 'positive' under all circumstances, before I actually build it into a real boat!

 

[lohring]

jimboat

One of my observations is that our tunnels are very prone to blow overs immediately after exiting the turn if you don't straighted out smoothly and get on the throttle slowly. Could this be aerodynamic (with reference to your aerodynamic center comments) or is it the result of too fast accelleration?

Lohring Miller