C.G. measurement

[sport hydro]

I am currently rebuilding a 1974 PAY-N-PAK wood hull. I am to the point that its primered and in first sanding. With everything mounted in the hull, I measured the C.G. at 19 1/2" foward from the transom. The overall length is 43 1/2". Is this a good C.G. or should I try to get it more foward? There was no feul in the tanks.

 

[P THOMAS]

80-90% of the afterplane should be close, depends on where you can put the motor, fuel, and radio in a scale. The after plane is the lenght from the transom to the rear of the front sponson pad. Dont be afraid to add some extra weight to get it to balance the boats have alot of lift. You may have to see how it runs the first time to decide on that.

 

[sport hydro]

Right now the motor is mounted with the starter belt groove on the flywheel in line with the trailing edge of the sponsons. I'll have to measure the transom to sponson length to see if I'm in the 80-90% range.

Thanks.

 

[FloridaScaleBoater]

Sport Hydro;

A good rule of thumb is to get it to balance at a point in the middle of the turn fin, if that makes sence??

 

[sport hydro]

I'll try that method as soon as I get my Dick Tyndal turn fin on saturday. I did measure the sponson to transom length and right now its in the 86% range. The fin should make it even better. I finally posted some pic's on this site. You can see how everything is fitting in the hull.

 

[Hydro Junkie]

That boat looks like it's from a kit. I'm curious as to who makes it. Another thing you might want to look at is what your prop weight is. If it's more than 1.75 pounds, you're too tail heavy, regardless of where the CG is located. I would have put the engine just behind the sponsons and the radio box in front of the engine, keeping all weight out of the rear of the boat, but that's just my opinion

 

[sport hydro]

The person I got the boat from said it was an original Dumas kit. A X457 prop better not weigh over 1 pound. That would be an afull lot. I can't put the radio in the front because the front cowl is glued in perminantly.

 

[Hydro Junkie]

I had the feeling it was a Dumas boat.

I think you mis-understood what I was talking about when I referred to prop weight. Prop weight isn't how much the prop weighs, but is how much weight the prop has to hold up when the boat's running. You can check this by balancing the boat on the sponson transoms and putting a scale under the prop.

 

[Dasboatman]

Hydro Junkie,

You said: "Another thing you might want to look at is what your prop weight is. If it's more than 1.75 pounds, you're too tail heavy, regardless of where the CG is located."

This statement doesn't make any sense. If the boat is a pound nose heavy using the proper center of gravity - - the prop weight is directly affected.

A prop weight of 1.75 pounds on a Dumas Pak - - - that puppy would fly like a bird !!!

Sport Hydro,

The figures Phil gave you are good, and the "middle of turn fin" - Rick gave you is also a good place to start. It sounds to me that you are already in the ball park right now. Run Fast and turn Right !!!! CHEERS !!! Bob Johnson

 

[Hydro Junkie]

Actually, it makes perfect sense. Think about it, if you balance a airplane on the plan's CG and the CG isn't right on the plan(which does happen, like on the Laddie Mukalasko designed Arrow seaplane), your plane will be very unstable and probably unflyable. On a scale type hydro, you have to look at how much weight the prop and/or air pressure will need to lift to keep the rear of the boat from dragging. A figure of approximately 1.5 pounds is a good starting point, since if you get much lighter on the prop, it won't get a good bite and heavier, you drag the transom. It's also easier to adjust the prop weight than trying to balance a boat on the CG. You also have to look at what modifications were done to the boat when it was built. Were the sponsons made longer or shorter than the plans, or was the afterplane made shorter to make the boat corner quicker? These changes will move the CG away from where the plans say it should be, so why worry about it? When I look at a set of hydro plans, I just ignore the CG because when I set the prop weight, the boat is balanced where it needs to be anyway

 

[sport hydro]

Hydro Junkie;

Sorry for misunderstanding you post. I will try your method and see where it comes in at.

 

[FloridaScaleBoater]

because when I set the prop weight, the boat is balanced where it needs to be anyway

Hydro Junkie;

Your comments about prop weight have been toying with my brain since I read this the other night!

So I just came in from the garage, after measuring prop weight on 3 scale hydros I that are set up, good running boats, this is what I came up with:

Boat #1 is an epoxy glass U-4 hull weighs in total weight, somwhere aroung 16 pounds, with the boat set up on wood blocks placed under the aft. inner point of ride plate (ass end of sponsons) and aft tip of the strut touching an oz scale, I come up with 19 oz. 1.25 pounds, yes? If this thing had any more tail weight, it would be up and over, flyin like a bird as "dasboatman" stated.

Boat #2 is a W.O.F. U-4 total weight 14-15 pounds, tail weight 9oz.---------1.75 pounds???? can you say tweet tweet!!!!

As a statement of reference, this hull, engine combination was run at the NAMBA NATs this past summer in sport-X Running against a boat with a MAC 84, and hangin right with it, till the CMB 67 we were running, sucked something through the intake, and destroying the piston and sleeve.

Basically what I'm saying, not a bad running boat!

Boat #3 another W.O.F. U-4 total weight 12 pounds, tail weight????? 6oz.

So My conclusion is, prop weight is going to be a ratio of the total boat weight, if you have a boat that weighs, in theory 25 pounds 1.75 pounds prop weight would probably not be enough.

If you are lucky enough to have a real light scale, 8-12 pounds, a prop weight of 1.75 pounds, you would never keep it on the water.

As with boat #3 at the end of a heat, were I have run hard, and burned most of my fuel, the last couple of laps can become a tip-toe afair.

Your statement "since if you get much lighter on the prop, it won't get a good bite" NOT A CHANCE MY FRIEND, OUR PROPS EAT AAAALLLLL DAY LONG!!!!!!!

So go ahead and run your 1.75 prop weight, and get good at feathering that throttle.

Just my opinion!!! and I'm stickin with it

Rick