Tunnel Hull Design

[Slideblues]

Larry and Bill thanks for the reply's

Its always been a battle for me the speed vs cornering equation.

Would be nice to have tha boat still in one piece after the turn :lol:

Gene

 

[David Moyle]

During anyones testing what did you find to be the hardest to achieve ?

Turning in without spinning out or stuffing, Or speed without blowing off.........

Gene

Hi Gene,

The hardest to achieve that I've found is transitioning from high speed down the straight (high, dry and floating on air) to initiating the turn. Without backing off, all the steering servo achieves is to crab the boat and keep going in basically the same direction. Backing off and settling the boat, through a speed reduction, then gets the boat into cornering mode where it's not too bad, but far from perfect, at getting around the turn.

I'll be experimenting more with hinge pin angle, some sort of turn fin arrangement, and maybe even an air rudder if I get around to it.

The attached pic kind of shows why it doesn't react too quickly to steering input at full throttle.

 

[Slideblues]

Hi David,

Yes the transition has been a booger for me,Thanks for the tip!

This is my first OS so I do have ability for pin adjustment now.

Never messed with that before,

Gene

Just saw the pic, Yea thats aired out I see what ya mean

Cool shot!

 

[David Moyle]

Yep, I agree Rodney, that's the only way I'm gunna be able to convince myself.

At the moment, I'm experimenting with different props, so I'll try it once I think I've got the prop with the best speed & handling combination..................... this could take a while :unsure:

James,

I've used and tested pin angle on every O/B tunnel I've ever built with an OS or TT lower. Some benefit, some don't. From what I've seen of your tunnel - I think it would benefit.

Things that work for me.

1. Pick a prop and CG % and stick with it when experimenting specifically on pin angle.

2. If you go changing or tweaking the prop whist trying to determine the effect of changing the pin angle, it introduces another variable. Lift characteristics alter the effect.

3. Reset the angle behind the pin after making a change to pin angle or the boat will run too light or too wet in the straights depending on which way you are going with the shims.

My current sport tunnel runs a shim between the transom and the bottom engine mount screws. I have tested i with and without, & it keeps the nose from digging in too hard on our 5 buoy turns. Certain prop characteristics exagerate the phenomenon more than others. When I switched props a while back, I had to go thru pin angle again as the new prop had to run with more negative in the straights to prevent it being too loose. More negative behind the pin usually needs more shimming to keep the nose up in the turn.

Great info Tim... thanks! We'll certainly be doing a lot more experimenting with pin angle and hopefully find something that gives good results.

 

[RodneyPierce]

David, man that thing looks cool aired out like that!!!!

 

[RonShaw]

I Shimmed the bottom of the mount to give the hinge pin a top forward angle on one of my XTR-21 Leecrafts, that I had an OS lower on. It help a bit with the some of the cornering issues I had with that boat. Jerry Wyss turned me on to this trick back when I was running the XTR-21.

Larry,

This is EXACTLY why I believe the K&B lower works so much better on the LeeCraft than the OS. The angle adjustment is in front of the hinge pin!!!!

 

[RonShaw]

I have found that if you leave the "Rod Blocks" off (That's what I like to call them to make sure that he "Rod" is always credited to have stuumbled onto them) Just kidding Rod! I love them!

But all of my boats have them and here is why: It is a variable compromise that takes out a lot of negative and inherit problems you will incounter out when you design or "try to design" something that wants to dart infield at the slightest turn of the wheel. Two things a O/B tunnel loves to do is TURN and BLOW OFF!

As you know, I have designed all my boats using Rod Blocks. I design mine to be progressive with the intent of only using what I need. Why slap it all down at one time and pickup all that drag if you don't need to???? Remember "Drag is the devil".

Now, I will go ahead and tell you guys who are thinking about designing a tunnel, and this is a fact, you can not fix a ill turning boat with Rod Blocks alone UNLESS it is already close to turning good to start with. I design mine to "feather" my turns ONLY. They can't do but so much because of the ratio of surface area combined with their angle of attack weighed against the length of the sponson. The sponson will win that battle if it is far off.

Can you build a boat that will go thru the turns without Rod Blocks? Yes. But there must be a reason why there is not one O/B tunnel that turns good without Rod Blocks after all these years. If it was that easy, someone would already have one. If there is, I havn't seen it.

I like to choose my battles when I have a choice. In this case, don't fight it, use them and use your forehead for head butting the really big problems you will be faced with like how to keep a really fast tunnel on the water for six laps or how to design a boat that will not require 10 to 16 ounces of lead to finish a race and not stuff at the end of the straits, tail walk, barrel roll, hook or flat spin in the turns. These are your first big issues.

I want to see people get onfire for tunnel design. It is not a "plug & go" task. I average one fair design about every 5 to 6 years and I have done nothing but O/B tunnel design for 30+ years. I guess what I want to say to you guys is find a test pond and test your boat until you dry all the water up!

I am signing off this thread because I don't want to step on any toes here. I am finishing up my last model now. It was fun. I learned a lot. Drake will build my boats until he gets too old like me to build them. I hope I said something that helps.

-Carl,

Carl,

I thank you for the nice words......

As you well know in the last 30 years Tommy Lee and I had been all over stumble blocks from every angle imaginable.........

Boats with blocks were "always" faster around the race course than boats without....That told us something......

We proceeded to find out why this was true.......

Tommy and I also noticed that boats without blocks tended to do something stupid [spin out] when you least expected it.....

99% of the tunnel records in IMPBA and NAMBA are all held with boats with blocks......that should also tell us something.....

Once again thanks........

But...the best part of your kind words is it gives me the perfect opportunity to get off this keyboard merry go round on this subject......

Have you noticed that it is very seldom that model boating national champions or record holders and icons seldom if ever contribute to a subect such as tunnel boat design.....

Virtually every technical subject raised on keyboard forums eventually digresses into a pissen match.......there is always somebody that tells you that you are wrong but they "never"show you how you are wrong with scientific evidence or good emperical testing and they never tell you how to do it better......

It is always he said or she said ....or I read ......but never "I know because I have tested this and ,,,,yada yayda yada "...............

There is a ton of model boating experience out there that will never contribute to this or any other forum because they know they will eventually end up trying to defend a post they know to be true because they personally have been there and done that.....What people fail to realize is all these champions,builders and record holding

experience has spent many years doing it wrong and that is how they learned to separate the flies from the manure......They are not any smarter than anyone else just more persistent in search of success.....

There is no magic bullet to tunnel boat design........there are no speed secrets.....there is only experience and how to apply.it....................

I extended an invitation to talk to anybody,anytime,one on one about tunnel boat design.....That invitation still stands...

I will be more than happy to share anything I know to be true because I have been there with either good science or good emperical testing.....

Rod Geraghty

Boy,

Both of you guys are "spot on" with your comments. Things that SHOULD work on paper don't always transfer to the pond, that's for sure. You too have emphasized to everyone following this thread that once again, nothing beats many hours on the pond experimenting. Rod and Carl, I would like to say thank you for the time you spent on this thread trying to pass so many of the "tricks of the trade" you two have tested and been very successful with over the years. I can only apologize for many of the boaters on IW that make the same mistakes my salespeople make everyday, they are talking when they should be listening and keeping their sidebar comments to themselves. When guys such as yourselves are giving us just a little of what you have learned from many years in this hobby, we all need to listen and ask questions, not try to discredit your words. Actions have always spoken louder than words and many boaters on this thread are getting "wordy"!!!!.....hahaha. I would appreciate you two continuing to share your knowledge with those of us that appreciate it. This is EXACTLY why you would never hear Tommy Lee in a group talking about tunnels. He was a radical kind of tunnel racer that was always the gentleman and put his mouth where it counted, on the race course. He too is a member of your team of boaters that have made tunnel history and what it is today. So thanks.......... If it wasn't for your team, (too many boaters to even try to name in fear I will miss someone), we would all still be running Prather LapCats!!!!!!!........hahahahahahahahahahahahaha.......................

Ron

 

[LarryConrad]

Dave, Awesome shot of your tunnel all aired out! My imagination can almost hear the CD on the PA in the back ground right after the shutter click...Dead boat on the front straight, lane one between the entrance buoy and buoy one. Ron, If you shim your K&B to incline the hinge pin, how do you trim the prop shaft back to a neutral or negative angle? I ran a K&B lower for nearly 2 full race seasons on my Leecraft before switching to the OS lower, The compound mount is a great tuning aid, but the real superiority of the OS lower is the prop shaft that isn't worn out and wobbly after a handful of runs. I was able to maintain a higher entrance speed into the turns, with less spin outs, with the top of the pin slightly angled toward the bow.On my boat it was definitely a win win situation changing my lower unit over to an OS.Your mileage may vary. I had a long standing love hate relationship with the XTR-21 before I switched to racing hulls I could finish nearly all my heats with...Loved the acceleration down the straights, hated the way the boat behaved while turning.

Larry

 

[RonShaw]

Larry,

I ran every LeeCraft ever made, and like Rod and Carl point out above, there may not be any factual reason why, but this has been my experience. I believe the fact that you are able to trim the lower in front of the hinge pin on a K&B, instead of behind the hinge pin like the OS, probably eliminates the need for both. I have never had any issues with turning a tunnel that had slight negative angle from the transom back, including the hinge pin. Another hint, personally I have always had better results on tunnels with the front of the hinge pin, even or behind the trailing edge of the sponsons. Good luck...........

 

[Grimracer]

Ron,

We call that under-steer (in front of the sponson backs) and over-steer (behind)

That’s different then what we are talking about here.

All good however.

Grim

 

[LarryConrad]

Ron, If you shim the top or bottom of an engine mount like used for a K&B or Lawless lower to angle the hinge pin, you angle the entire lower unit. If you do this to a OS or a Thunder Tiger it is possible to angle the pin and set the prop shaft back to neutral or negative, and induces a trim setting change when you turn the engine, which changes the ride attitude of the boat.

Larry

 

[Tim_Duggan]

Larry,

I ran every LeeCraft ever made, and like Rod and Carl point out above, there may not be any factual reason why, but this has been my experience. I believe the fact that you are able to trim the lower in front of the hinge pin on a K&B, instead of behind the hinge pin like the OS, probably eliminates the need for both. I have never had any issues with turning a tunnel that had slight negative angle from the transom back, including the hinge pin. Another hint, personally I have always had better results on tunnels with the front of the hinge pin, even or behind the trailing edge of the sponsons. Good luck...........

Something else to consider in the K&B 3.5 to OS comparison is the distance from pivot pin to prop face - the K&B 3.5 is shorter than a Lawless or an OS. While the distance of prop from transom can be altered by a longer mount - its also moving the pivot point further away from the trailing edges of the running surface.

 

[Carl Van Houten]

I like the top of the pivot pin to lean sightly forward and I sometimes will lean the flywheel starboard in big heats to take out the lag (the time it takes to properly set the sponson) or if I am running someone that can put some heat on me, I can do both and pull some weight out to get the full effect out of my set up. I guess it depends on what effect you are needing to get you through the transition and into full set.

You want a sponson that has a forgiving or "wide sweet spot" getting into the turns. You never want a sponson design that is too slow to set in rough water or one that grabs, pulls in or hooks on smooth water.

With a forgiving sponson design, you can use these things to feather your setup before you have to dink around with the more agressive thrust angle, prop height setup changes to get the results you want.

-Carl

 

[kesso2118]

Yep, I agree Rodney, that's the only way I'm gunna be able to convince myself.

At the moment, I'm experimenting with different props, so I'll try it once I think I've got the prop with the best speed & handling combination..................... this could take a while :unsure:

James,

I've used and tested pin angle on every O/B tunnel I've ever built with an OS or TT lower. Some benefit, some don't. From what I've seen of your tunnel - I think it would benefit.

Things that work for me.

1. Pick a prop and CG % and stick with it when experimenting specifically on pin angle.

2. If you go changing or tweaking the prop whist trying to determine the effect of changing the pin angle, it introduces another variable. Lift characteristics alter the effect.

3. Reset the angle behind the pin after making a change to pin angle or the boat will run too light or too wet in the straights depending on which way you are going with the shims.

My current sport tunnel runs a shim between the transom and the bottom engine mount screws. I have tested i with and without, & it keeps the nose from digging in too hard on our 5 buoy turns. Certain prop characteristics exagerate the phenomenon more than others. When I switched props a while back, I had to go thru pin angle again as the new prop had to run with more negative in the straights to prevent it being too loose. More negative behind the pin usually needs more shimming to keep the nose up in the turn.

Thanks Tim,

As usual, excellent advice and explanation

I didn't see your post yesterday because I was at the Puddle testing props. I had a great day, with good conditions over a variety of smooth and wind affected water.

Four hours and 3 litres of fuel later, I came away with best results from a couple of mod X640 props as well as the Grimracer 40x52/3. My mod GR40x53 props went pretty good as well. The stock GR 40/53 was still showing understeer tendencies and the T-101 was fast but scary in the turns

The X438 backcut and stock X637 gave lots of revs but were down on boat speed and the stock GR 42x55 was a bit too big for the set up I was running.

The 5 port Nova with Cooper Pipe and OS Lower is providing a good test bed.

Regards,

James

 

[shoboat]

James,

Try the x-640 or the x-642 reduced.

Some are using the x-442 B/C with great results on a Nova.

Thanks For Reading,

Mark Sholund

 

[Jimboat]

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.

A shallow deadrise (less than 8 degrees) will provide the most efficient lift. Deeper deadrise angles (like 12 to 20 degrees) will make a much cleaner presentation to the water surface in rougher water conditions, exhibiting a much more comfortable ride, but less efficient lift. Again, a design tradeoff.

Also the deadrise of planing surfaces can affect the potential for porpoising. A higher deadrise hull design (less efficient Lift generated) will allow the hull to operate at higher trim angles before porpoising is experienced.

What else can we do? - Other methods of diminishing the susceptibility to Porpoising for this design are –

· Move (longitudinal) dynamic CofG forward,

· using wider running surfaces (sponsons, center pod, vee surface, vee-pad)

· change motor height

· add transom wedges to allow for more negative trim travel

· Change to a different propeller with more aft-lift.

· Adding afterplane tabs or wedge to reduce the required operating trim angle.

In general, to fix a porpoising problem, reduce the running trim angle at the problem speed.

Here is an article on "Porpoising"

 

[Carl Van Houten]

A boat that has problems with porpoise, can have many problems or just a couple of problems. A poorly designed boat does not have a good balance of aerodynamic forces of lift and drag acting against the hydrodynamic forces of lift and drag.

If you are trying to learn about design, start with 14 to 15 degrees dead rise and work with the deck and tunnel until you get a boat that works. You can build a fair boat with these angles. Then build one that is a little bit different and see what changed. Fix that and go to the next step. You will start to see what works and what don't after a few models.

A sponson with a low dead rise is fast but it won't turn well without a great white turn fin. Some of the first O/B tunnel 2- lap oval records were broken by putting a turn finn on a tunnelboat. I don't like turn fins.

A good way to learn some cool stuff is to GO TO SPEED TRIALS!!!!! See what it takes to get a 3.5 O/B tunnel turn a 26 second 2 lap time and see how it is done through tweeking props and changing setups. Ask Bill Britton. He went and got bit HARD by the speed bug. What you will learn from the 2-lap oval times will help you with your race setup.

-Carl,

 

[kesso2118]

James,

Try the x-640 or the x-642 reduced.

Some are using the x-442 B/C with great results on a Nova.

Thanks For Reading,

Mark Sholund

Mark,

Thanks for your input.

Would you please advise why you are suggesting the X640 and/or cut down X642 or the back cut X442, is there a characterisic of these props that would make them perform differently to what I described with the stock GR 40x53, or are you suggesting them because you feel the Nova can handle more pitch than what I have used??

Regards,

James

 

[shoboat]

James,

The Nova runs very well with a X-442 Lynx cut propeller.

The 40 x 53 is not enough propeller to take advantage of

the Novas powerband. Just our experience.

Keep Testing,

Mark Sholund

 

[Slideblues]

Carl and others,

I read that the OS lower works best with the Lynx.

Transom mount to drive dog mine measures bout 6 inches.

K&B shorter,Lawless I think is a smidge longer.

Question is do you feel there is a sweet spot on this measurement?

Gene