Tunnel Hull Design

[RodneyPierce]

Alright guys, dead rise angle. What are its effects on a boat, and its handling characteristics? Would a boat with a steeper dead rise angle make a boat less prone to porpoising? How would it effect the overall ability of the boat? Please discuss.

 

[Baxter Walton]

A higher deadrise angle will give better performance in rough waters, but a lower deadrise angle is more efficient and thus can generate better acceleration and a faster top speed. As far as porpoising, it can be cause by a number of things. here is an artical that explains its causes and some solutions.

Porpoising is a common trait of performance hulls - power catamarans, vee hulls and vee-pad hulls in particular are susceptible. Each boat has it's own unique speed at which is can experience the transition through phases of planing performance. This transition speed is called the "hump zone".

 

All such hulls are planing on running surfaces and get their performance from a combination of hydrodynamic (water) and aerodynamic (air) Lift. The transition of the Vee-Pad hull to "running on the pad" can cause dynamic instability. Here, the hump zone represents the speed at which the amount of Lift from the highly efficient "pad" section of the hull (aftward, center located, flat planing surface) becomes significant compared to the Lift generated by the veed (full length, higher deadrise) portion of the hull.

Porpoising comes from a rapid change in the location of the center of Lift as the boat accelerates. The location of static weights is one way of dampening the rate of change of the CofL...so it's not always obvious whether to move weight fore or aft in order to cause the "dampening". The solution can be calculated, but we use software for that. It's not too difficult for you to find out through testing, whether moving weight fore or aft will help your particular problem.

 

Propeller selection can often change the dynamic balance of the hull/setup. For example, a change to a prop that provides more aft-Lift can alter the dynamic balance of the hull, and similarly change the speed and range of the "hump zone" – often eliminating porpoising. Weight distribution changes can also have a positive effect on "where" the "hump zone" will occur, as can change in power application. Changing "trim" angle while driving through the "hump zone" even if less efficient, will also provide a better experience, and when well controlled, can "close up" the range of "hump zone" substantially – eliminating porpoising.

 

[Slideblues]

I would REALLY like to hear Rod G's thoughts on this.

His Idea of doing a updated version of Tommy's turn fin boat sparked my interest.

Goes like stink down tha chutes,turns on a dime and leaves ya change......

Gene

 

[LarryConrad]

Rodney, If your tunnel is porpoising, Check to see that your running surface is flat. A small dip in the sponson bottom can act like a suction cup to the water surface, catching and releasing which will make the boat porpoise severely. Raising your prop shaft too high will also cause the boat to porpoise. Also some tunnels will porpoise at low speed and stop bobbing as the speed increases.

 

[lohring]

One of the big differences between the Lynx and the Leecraft XTR-21 is the deadrise angle. That makes the Lynx a better rough water boat. The water is always rough at 3.5 tunnel scale. My very limited experiment with high deadrise resulted in a boat that didn't turn well.

Lohring Miller

 

[RodneyPierce]

what would be considered a "high" dead rise angle?

 

[Grimracer]

above 15 deg

 

[Carl Van Houten]

When you go avove 15 degrees, things beging to get touchy. The Lynx is ~18 degs. Going through the turns with a 18 degree sponson that is wanting to pull the hull down and dig it into the infield will be your battle. Keep good records of all your changes and what they did or didn't do. Don't expect to get it right the first time.

-Carl,

 

[RodneyPierce]

Carl, can you enlighten us a little more? If not, i understand. Wouldnt flattening out the front of the sponson help with the digging in problem? As lets say the last 12 inches are 18 degrees, and flattens out rapidly from there, to almost dead flat in the front. Wouldnt that give you both benefits? Great rough water boat, yet when it lays down for the turn, it wouldnt still want to dig in?

 

[Grimracer]

Rodney,

That is how the Nemesis is designed,(new sponsons)... It has progressive deadrise.. from the flat section to the front. I think it end up around 5deg (or so) at the tip.

If you are going to run steep deadrise you need a deep tunnel roof.

For me I don’t care to much about racing in rough water.. they can stay in the trailer in those times.

Grim

 

[lohring]

I've tried sponson angles from 10 to 20 degrees and ended up around 15 degrees. That has been the standard for a very long time. Tunnel hulls ride on a combination of wing and planing lift. The cornering is mostly in planing mode and the straights are mostly in flying mode. I think I understand the flying mode, but cornering is where the sponson angles get important. Control of how much edge is immersed and the position of the center of lateral force compared to center of gravity determines cornering behavior.

I've seen several ways to control how much the sponson sets in a turn. The standard is the "stumble block", invented by Rod Geraghty, I believe. Another is a very flat forward sponson surface that I first saw on Jeff Michaud's early cat design. Most cats, however, don't have any special way to control sponson set. They do have deep tunnels compared to the outboards.

I don't think there is one right combination of all the factors. That's what makes tunnel design fun. Unfortunately, there are very few people working on new ideas. Nitro outboards are pretty much dead since K&B went out of business. Gas cats are a fast growing class, but almost everyone copies Aeromarine hulls. Carl is the last person to develop some really great hull designs. I loved the Villan/VS-1 and the Nemesis. I also saw some great ideas from Rod. Where is everyone else?

Below are some ideas to get everyone started.

Lohring Miller

David Preuse's XTR-21 turn fin

Jeff Michaud's cat sponson

Rod Geraghty's 3.5 tunnel

Notice the lack of non trips, the little wing, and the sponson fence. Think about what each of these things do.

 

[RodneyPierce]

Lohring, awesome information and pictures!!! Where is Rod at? I figure he would atleast sneak in and give us his input. That is an interesting tunnel design there. Ill have to think on that one for a little while.

 

[Slideblues]

I'd give the PS/NS-295 high marks for inovation.........

Sometimes its what ya dont see h34r:

Gene

 

[LarryConrad]

Gene,

Is that one of Mike L's (spooncrx) hulls?

 

[Slideblues]

Yea, ML Boatworks

 

[Carl Van Houten]

Rodney,

Lohring has nailed it. A tunnel is a very interesting model. The things that makes one model work will screw up another. It is a blend of a lot of things that all have to work together. I have been playing with the O/B tunnel concept for a long time and I am still learning. Actually when you get old, you have to relearn some things. LOL

Anyway, I have tried twisting sponson dead rise both ways. True, flater sponsons in the front will keep a boat from digging in but you can only go so far with this concept and you quickly lose the ability to turn thru lane one without sliding out, hooking or spinning out. Think also what the water is doing when you wet the sponson in a turn. If your dead rise is more in the rear than in the front....your lifting water. Be careful with this one because it taxes your cornering speed big time if you over do it. You will have fun for years playing with this type stuff if you are weird like me..

I have been talking to the FE O/B guys and even though they are doing well converting their nitro tunnels into FE boats, there is some room there for some improvements both in weight distribution as well as some performance changes. Buy the time I make my initial model prints, I will have thought about all of these things for weeks. Trying to decide how wide I want my sponsons & at what angle, tunnel depth, O.A.L. & width, tunnel width, and none of these things are based on looks. I rarely look at full size tunnel boats. Full size boats and what they are doing and why, is way off from what I am trying to do. When they start pushing their boats 600 mph, then their boats will begin to look like mine.

I have some basic stuff I like to build into all my models. Matching the top to the bottom is where I spend most of my time. Not to say that it is the only big design issue but once you get everything else worked out, this is what you will end up playing with along with a lot of prop work. Be ready to make changes. Keep good notes.

The FE O/B is a very dependable platform for boat design.

I didn't mean to write a book.

-Carl,

 

[RodneyPierce]

Carl, thats perfect! not a book! That is exactly what i was looking for. Thats why I started this thread. Thanks you, sir!

 

[Geraghty]

Rod will be chipping in for sure now........I hope.......

I've been waiting for a pic of that orange boat to show up. It is way cool. I "think" Tommy Lee built that boat. I met those two, Rod & Tommy at the '96 Internats. Pretty sure it was 96, in Ft. Wayne. I bought one of the 1st XTR's from Tommy & picked it up there. I also "think" that was about when the sport tunnel class came about & that was the boat Tommy had entered, man could he drive!! Seems like that thing was like 11"x24" or so, tiny little beautiful boat.

Do you have that boat now Lohring?

Want to sell it?

Am I even close Rod?????

Joe

Your close.....Tommy built the boat in 1995 and I have ressurected it from the dead 3 times from very severe damage.....

The boat currently owns the IMPBA "A" mod tunnel SAW record with David Hall at the wheel.....

The boat will run 50+ mph all day long with a Novarossi .12/gearbox combo machined up by David....

Some day we will go out and kill the 2 lap....that is on the bucket list of things to do in the future.....

The boat still looks the same as the photo Lohring posted.....

The boat will never be for sale......

 

[LarryConrad]

Rod,

The JAE design hydros have proven to be extremely popular and competitive designs. A tunnel bearing the JAE logo incorporating design features from your vast experience would be awesome.

 

[RonShaw]

Rod,

You want to tell us about the new gas tunnel you are working with JAE to design. The stage is yours................................