Hydroplane C/G Starting Point

[Terry Heddin]

Need help to find correct C/G for this hydroplane? Hopefully I have included enough info in my crude sketch.

 

[Dave Gilles]

at the turn fin

 

[Mark Anderson]

Something tht big, 2 inches behind the back of the sponsons. biased to the left.

 

[FloridaScaleBoater]

Balance point should be mid-way through the middle of the turn fin

 

[Bill Schaare]

Guys, asking this question as I'm new to rc hydroplanes. From the answers I read above, I'm to assume the balance point is set (meaning interior gear moved forward/boat back) to make boat balance where the turn fin is set. From everything I've read and seen in set up videos, the turn fin is placed based on the balance point and not vise versa as noted above. Am I missing something? Appreciate any clarification applicable. THX-Bill

 

[Mark Anderson]

The C.G. needs to be where it needs to be, based on lift/weight/speed. The turn fins relationship to the C.G. changes the boats steering response. Without any knowledge of the boat in question, all anyone can do is make an educated guess of a starting point that will need to be tuned.

 

[peterjuo]

Hi mate
I am like you new to hydroplane
And what I have done with my boat is carefully pick the boat up between my fore tinger tips at the point just behind the sponson transom. And adjusting back an fourth as necessary until you pick the boat evenly at that point that will be the balance point an c g . Not exact but a good starting point.
Ps
With the boat empty no running gear . This point will change when adding motors ect.

 

[FloridaScaleBoater]

Hi mate
I am like you new to hydroplane
And what I have done with my boat is carefully pick the boat up between my fore tinger tips at the point just behind the sponson transom. And adjusting back an fourth as necessary until you pick the boat evenly at that point that will be the balance point an c g . Not exact but a good starting point.
Ps
With the boat empty no running gear . This point will change when adding motors ect.

determining the CG of an empty hull accomplishes nothing.
mount all of your hardware. put everything in the radio box basically where you want everything (battery included)
Place your fuel tank where you want it (1/2 full of fuel) mount your engine in the mount and move the movable things (engine/mount and radio box around till it balances at the half way point vertically of the turn fin.
Once you do this, you will be very dang close!

 

[Hydro Junkie]

And that's where the fun starts, where to put the motor, radio gear and such. If you're doing a 1/8th scale boat, the turn fin is generally 1.5 to 2" behind the sponson. Sport 20s and 40s are closer, gas scales similar to 1/8th scale or slightly further back. With that said, I have to agree with Rick. An empty hull won't tell you anything. If the boat will have a tail or rear wing, that needs to be installed before trying to check the balance since it's weight at one end of the boat that may need to be offset by weight in the front. The rudder and strut and should be installed as well.

 

[Mark Anderson]

Build the boat with everything where it makes sense. Engine at the back of the sponson step. I always put the radio box in the front. After it is all built and painted. Tape a dixie cup to the tip of the left sponson. Fill the cup with lead shot until the boat balances 2" behind the step. Now very near the front of the left sponson, on the bottom. Drill a 1/4 inch hole. Mix enough slow setting epoxy in the lead shot to coat the shot, but not a bunch of extra epoxy. Roll up a piece of paper to make a funnel, pour the shot/epoxy mixture in the hole. Tape the hole up and stand the boat on the sponson tips until the epoxy sets. The epoxy will seal the hole and require just a bit of touch up paint. That should get you in the ball park and bias the left side. The C.G. may need to be adjusted during testing.

 

[FloridaScaleBoater]

And that's where the fun starts, where to put the motor, radio gear and such. If you're doing a 1/8th scale boat, the turn fin is generally 1.5 to 2" behind the sponson. Sport 20s and 40s are closer, gas scales similar to 1/8th scale or slightly further back. With that said, I have to agree with Rick. An empty hull won't tell you anything. If the boat will have a tail or rear wing, that needs to be installed before trying to check the balance since it's weight at one end of the boat that may need to be offset by weight in the front. The rudder and strut and should be installed as well.

Strut & rudder are what was meant by hardware.
correct on the wings, forgot about those parts, I try to stay away from those boats... Too much "stuff" to overcome.

 

[Hydro Junkie]

I was agreeing with you and clarifying for the new builders. If we were dealing with those that have built boats in the past, I wouldn't have gone there, other than tails and wings. You have to remember, however, hardware can refer to any metal parts on the boat. That includes tail mounts, tail bracing rods and mounting points, tuned pipe mounts, fittings for running the exhaust through the transom, cowl mounts and retainers, etc. Obviously, I'm including parts for scale boats in that list but, you have to remember, they all add weight

 

[Jim Walker]

And that's where the fun starts, where to put the motor, radio gear and such. If you're doing a 1/8th scale boat, the turn fin is generally 1.5 to 2" behind the sponson. Sport 20s and 40s are closer, gas scales similar to 1/8th scale or slightly further back. With that said, I have to agree with Rick. An empty hull won't tell you anything. If the boat will have a tail or rear wing, that needs to be installed before trying to check the balance since it's weight at one end of the boat that may need to be offset by weight in the front. The rudder and strut and should be installed as well.

The “generally 1.5”-2” is that the center of the turn fin or the leading edge?

 

[Hydro Junkie]

That depends on who you're talking to. In the case of my 1/8th scale 2000 Elam Plus, it's to the center of the fin. To the furthest forward point of the fin, it's about an inch to the sponson transom.

 

[peterjuo]

Hi every body
I thankyou all for your kind wisdom I for one am very much appreciate all your advice
Thankyou this is where i am at with my build

 

[Hydro Junkie]

Looks like one of the boats Scott Meyers builds. Would I be correct in assuming that it's a kit build?

 

[peterjuo]

Yes it is
I am very great full of his series of videos that has inspired me to have ago with a lot of patience and very limited time this is how far I have progressed in 6 -7 months work takes up all of my time.

 

[Hydro Junkie]

Trust me, I get it. When I'm working, I spend anywhere from 9 to 17 hours a day, not counting commuting time to and from work, on the job. Unfortunately, I haven't been able to work on my boats for six weeks due to health issues that I'm presently recovering from. Not going to go into it here but I've (finally)recovered enough, at this point, to start rehab and to work on the boats again.

 

[peterjuo]

God willing your recovery will be speedy and you will be able able
to get back to building racing all that fun stuff.

 

[Hydro Junkie]

My doctor has already cleared me to go back to work in early March so I figure I can go back to work on the boats, that is as long as I stay within the physical limitations they have me under. Yes, it tends to suck that I'm limited but it does beat the alternative