Center of Gravity on Tunnels

[HTV Boats]

Been doing some set up and talking with friends with similar boats and everyone says just set your CG here. Many threads here and a lot of opinions given. My problem is two indetical boats may be using different engines that are not the same weight and pipes that are different. Add a lower that places the prop further back or space an engine with various mounts and those two hulls are way different. So is CG just a guideline or a mythical pivot point that is the fulcrum to turn with per individual style of hull? Is there some standard for transom vs rear edge of sponson which again with mount and lower is different?

I have always went for the lightest set up and if weight is needed you put it where it affects handling. Far forward to keep the nose down or midship for hooking. Keeping the fuel tank over the balance point makes sense as that takes the variable out as you use up fuel. Or do you put it forward and have control the first 3 laps and gain speed as the front lightens.

I am just looking for opinions and ideas here. Nothing written in stone. The best answers are probably from the guys with the quickest oval times.

 

[topfuel443]

Good post!

Heres my take:

Say you have two identical boats, doesnt matter what kind, but one runs a stock OS in the Sport class, and the other a full blown mod, with tuned pipe/lawless...blah blah blah. I set the balance point (not CG) pretty close to the same. Not exact, but close. I run my tunnels slightly loose, not flighty but not glued to the water. Obviously the one with the motor hangin further off the boat and more weight, you will have to add more weight to the front to acheive this "common" balance point. The sport boat, you wont have to add as much (I actually dont add any weight to my sport boats as I can get them where I want by moving radio equipment)

I would say I run my sport boat at 31% from the transom and maybe 33% from the transom on the mod. 2% difference = not much in actual demensions. But, with the slightly under powered (when compared to a Mod) sport boat, I want it a little lighter in the front to get the hull to ride more loose, and get a little more speed out of it. The Mod, typically having more power, needs the opposite to keep it on the water. Out of all of the aspects of running tunnels, be it engine tuning, prop/pipe selection, I feel this is the most "thin line" of all adjustments, followed closely by engine angle and height.

This rough 33% mark, give or take, is what most manufacturers recommend. Most of the ride pads on tunnels are the rear 33% (or 1/3) of the sponson. and the rocker is usually right at this measurment.

My fuel tank is usually on the back portion of the balance point, but the balance gets set with an empty tank. I dont think it really matter to much, as long as you are consistant with this method.

My opinions, take them for what they are worth.

~James

PS, I always set my boats up so that the lower unit pivot pin is in line with the rear edge of the sponson ride pad, regardless of transom inset or mount spacing.

 

[HTV Boats]

Good post!
Heres my take:

Say you have two identical boats, doesnt matter what kind, but one runs a stock OS in the Sport class, and the other a full blown mod, with tuned pipe/lawless...blah blah blah. I set the balance point (not CG) pretty close to the same. Not exact, but close. I run my tunnels slightly loose, not flighty but not glued to the water. Obviously the one with the motor hangin further off the boat and more weight, you will have to add more weight to the front to acheive this "common" balance point. The sport boat, you wont have to add as much (I actually dont add any weight to my sport boats as I can get them where I want by moving radio equipment)

I would say I run my sport boat at 31% from the transom and maybe 33% from the transom on the mod. 2% difference = not much in actual demensions. But, with the slightly under powered (when compared to a Mod) sport boat, I want it a little lighter in the front to get the hull to ride more loose, and get a little more speed out of it. The Mod, typically having more power, needs the opposite to keep it on the water. Out of all of the aspects of running tunnels, be it engine tuning, prop/pipe selection, I feel this is the most "thin line" of all adjustments, followed closely by engine angle and height.

This rough 33% mark, give or take, is what most manufacturers recommend. Most of the ride pads on tunnels are the rear 33% (or 1/3) of the sponson. and the rocker is usually right at this measurment.

My fuel tank is usually on the back portion of the balance point, but the balance gets set with an empty tank. I dont think it really matter to much, as long as you are consistant with this method.

My opinions, take them for what they are worth.

~James

PS, I always set my boats up so that the lower unit pivot pin is in line with the rear edge of the sponson ride pad, regardless of transom inset or mount spacing.

I agree with the pivot point at the rear of the sponson and thanks for rest of the information in your reply. Any thoughts on say a 3.5 lower stock K&B, Lawless or 7.5 K&B adapted to 3.5. All have different heights and prop locations. Any pro's or cons?

 

[topfuel443]

Ive never had any reason to run a 7.5 lower on a 3.5 engine. Seen plenty of guys do it succesfully. Maybe ive been lucky, but i havent had any problems. Most guys run them due to the stroger cable. I would not want the extra weight honestly. A good balanced prop, free drive line, and a boat that doesnt porpoise, shafts shouldnt be a propblem. Im sure there are some who would argue the other way, but like I said, until I start breaking shafts im sticking with the normal 3.5 lower.

~James