balancing point

[Dale]

How about some tips on c/g ,or balanceing of a tunnel.Ihave 3 htb 360s with 7.5 K&Bs with erwin pipes,all are alittle different.My boat that runs the best is at 10" from the transum .MY question is should i add weight to the frount or back to make this the same on my other boats ??? How important is the c/g to the handeling and ride,is this maybe the cause of my boat having a little porpusing :- Thanks Dale ,by the way i am a member of IW i still cant log in. ???

 

[Jerry Dunlap]

Dale,

I've never added weight to the back of a tunnel boat to balance it. This is to say it couldn't be done, just that I've never done it.

C/G is an important factor in how a boat will perform. Even if you have three of the same hulls/motors the balance of each hull is apt to be different.

Generally speaking, a model tunnel boat balances at about 1/3 the distance from the transom to the sponson tips. So, 1/3 of 36" is 12". I know on my HTB 360/CMB Purple Head 45 I'm running the C/G close to 12" from the back of the boat. I had to add about 6 oz. of lead to get that C/G location.

Lots of things go into maxing out a tunnel boats performance: props, prop depth, thrust angle, and C/G. The only way to find out what's best is to run the boat and try different combinations.

Jerry D.

 

[[email protected]]

Dale,

It used to be rule of thumb that a tunnels CG was about at 27% of the boat. To achieve that measure the entire boat and then measure 27% of that distance from the transom forward. For example, for a 36 inch long boat when you multiply by .27 you get 9.72. To determine the CG just measure 9.72 inches from the transom forward. Also once you do it and you set the boat on a dowl. I'd reccomend you fill the fuel tank half full. That's about reallistic of milling and racing weight. That's optional but worth doing.

The engine and radio box are a big factor. Here is one reason. If you put the CG to far forward you will cause the boat to dig and spin out in corners and you'll have all sorts of problems.

Just my two cents. I'm sure the real experts will give you much better advice!

Also I'd e-mail the moderator, have them remove you as a member and then reapply. Something went wrong when you signed up. Did yyou delete your "cookies" from your computer? If you did that's why you can't log in.

 

[Dale]

Thanks for the info, iv tryed everything with this boat > still slaps at hight speed ??? Im hopeing the guys i meet in kalamazoo will help me find out my problem. Ill check on my login info with the medeator thanks. Also my 360 htb c/g is 10-1/8 " from the back, the nose of the cowl is alittle heavy to me on this boat, also iv even added 4 ounces to the back of this boat to get it to balance. I set up my sons boat the same way and his runs great,my next step is to move the radio box around and see what that dose. could it maybe be the prop causeing the boat to hop or slap runs real bad with a 1450 and ok with a x450 both run great on my sons boat :- could it be the hull what can i check on that. any ideas will help Thanks Dale

 

[[email protected]]

Sounds more like the way you have the motor set. Maybe it's to high?? And the angle is wrong. I heard a guy tell me he had an HTB that hopped really bad. Guess how it stopped doing it? One day the motor mount came loose, just a little and it caused the motor to tilt slightly to the right. He didn't know why the boat ran so good and stopped hopping until someone said "man why's your motor crooked?" When he fixed it the boat hopped so he retilted the boat and it ran fine. Don't know why?? But hey, worth a try.

JD,

ever have this happen?? How about you Mr. Garcia?

 

[waterbug]

Go to HTB Boats web site and the set up specs are right there in black and white !!!!!! Sounds like you need to lower the engine to me .

 

[Jack Garcia]

Guess I'm a couple days late but to answer Rcboats question, I suppose it happened to me at some time or other, but I agree with both you and Dunny (JD). 30% is a good starting point. It could end up at 27% . It just depends on all the combos that Dunny mentioned. What we're trying to say is that there is no cut and dried CG point.

Dale, by porpoising do you mean it hops like a bunny, or does it raise the nose and slap down? AKA womp womping. You know, womp womp womp womp. ;D

 

[topfuel443]

I had a 20 rigger that I could not get to do anything more than mill speed (40 mph maybe) after trying every thing known to man, almost ready to give up with it (new boat that I scratch built) I ran it one more time, and after about two laps it was like it changed into second gear and picked up what seemed like 10,000 rpm, running about 74 mph. on radar. got it back to the shore and realized the strut bracket came loose and it found the sweet spot all on its own. It was such a bizarre angleand height, I couldnt believe it, so I tightend it down a threw it back out, to this day it still has the same crazy angle and height but runs like a scolded ape with no bad habits whatsoever. I get alot of odd looks when people look at the strut, but it works for this boat........ weird ~ James

point being......every boat has its own setup that works

 

[Jimboat]

The reason that a tunnel boat can still misbehave even after you've "balanced" it, is that you can only do a 'static' balance on the table. A tunnel hull in flight is, however, a critical combination of hydrodynamic forces (sponson or running surfaces lift/drag on water) and aerodynamic forces (tunnel and air lift/drags). The dynamic balance of an operating tunnel hull changes at every speed throughout the operating range. The location of the 'center of lift' (also refered to as center of dynamic balance) can change as much as 80% typically. It is quite possible to analyze and predict the changes of dynamic balance in operating tunnel boat - it isn't trivial - but it can be fun!

/Jimboat

 

[dale]

To answer some of your questions it goes wop wop wop wop on the water not like a bunny they wop way to fast.Ill try some motot tilt and see what that dose side to side, iv tried up and down no change there, another thing if i run with a little turning it runs perfict if i run a straight line it hops or slaps for some fo you :- Keep the ideas comeing one day ill get it running good.

 

[Jerry Dunlap]

Re: Accidentally Finding "The Sweet Spot"

Last August I was running my JD WOF 30/CMB 21 Green Head in practice water before a day of racing. The motor came loose and the handling got all goofy. When I saw the motor had come loose, I just retightened the bolts and went out again. It was running the best ever. When I got the boat back to the beach and check the setting, seems like I'd upped the prop depth and it certainly helped.

But, as has been mentioned, there's many variables that come into play.

JD

 

[Trickey]

Dale

I am running a 36" tunnel with a 11cc K&B inboard and I am haveing similar problems with poupresing and have also found that a touch of rudder stops it!! Im new to this sport so dont know much about the setting up side of things but could it have something to do with prop torque??

Trickey

 

[Jerry Dunlap]

Tricky,

Prop torque on a 11cc OB can be an issue. I've run Octura X450 and X452 and Prather 235 and 240 on 11 cc K&B motors.

Generally speaking, the 11cc size motors need to be run lower on the transom than 3.5s or 7.5s.

Jerry D.