Hydroplane C/G Starting Point

[peterjuo]

Well the alternative. I hope it doesn't come to that .we'll we don't have to use heavy cranes so on the bright side there is nothing to heavy to worry about. Working on our boats .a stray razer blade or hobby knife may do a bit of damage but hopefully not to serious.

 

[Jim Walker]

Does anyone use a formula for CG such as 88-90% of after plane length? Seems like most go straight to the 2” behind the sponson transoms for a starting point.

 

[David Ashcroft]

2-3inch from the face of the sponson to the transom has always been the general measurement.
Just test until you find what works .

 

[Hydro Junkie]

Jim, that distance is the go-to because it's where most boats start to run well. What you will find is that, unless you have a boat that is very "tail heavy", most boats won't need much, if any, ballast in the nose to get it to run decently at that balance point. If you go much further aft, the boat will want to blow over where, conversely, much further forward will cause the boat to pound the sponsons or plow through the water instead of riding on top of it. If you use that measurement to start, test it and then adjust as needed, tuning the boat will be easier.

 

[Jim Walker]

Jim, that distance is the go-to because it's where most boats start to run well. What you will find is that, unless you have a boat that is very "tail heavy", most boats won't need much, if any, ballast in the nose to get it to run decently at that balance point. If you go much further aft, the boat will want to blow over where, conversely, much further forward will cause the boat to pound the sponsons or plow through the water instead of riding on top of it. If you use that measurement to start, test it and then adjust as needed, tuning the boat will be easier.

I understand. Really good explanation. I’m building a T-5 hull. I’ve got everything built, sealed (interior), and installed except I still need to glue the decks on. I want to get the ballast in before I deck it.

 

[Bill Eilers]

Yes. first it's setting the cg to your hull type. Will be a percentage of your after plane.( mono, rigger, round nose or pickle fork hyd.) All use a different cg. According to the amount of fwd lifting surface. Once that is established. Your turn fin is set. Hydros.. trailing edge of fin at water line will be fwd of cg. Note.. as u get farther fwd of cg it will turn quicker. (Not always good to have a hull turn to quickly) its best to set proper cg. Other than just keep pushing fin farther fwd. Because the cg isn't set properly

 

[Bill Eilers]

Yes. first it's setting the cg to your hull type. Will be a percentage of your after plane.( mono, rigger, round nose or pickle fork hyd.) All use a different cg. According to the amount of fwd lifting surface. Once that is established. Your turn fin is set. Hydros.. trailing edge of fin at water line will be fwd of cg. Note.. as u get farther fwd of cg it will turn quicker. (Not always good to have a hull turn to quickly) its best to set proper cg. Other than just keep pushing fin farther fwd. Because the cg isn't set properly

John Finch, Andy Brown, Youngblood and others that understand hydros are not boats. In reality there aircraft that must run light but not blow off at speed. That damn fin is not desirable. But is needed to control amount of slide in the turn.

 

[Bill Eilers]

John Finch, Andy Brown, Youngblood and others that understand hydros are not boats. In reality there aircraft that must run light but not blow off at speed. That **** fin is not desirable. But is needed to control amount of slide in the turn.

One more IMPORTANT.note. servos that don't have the nessary torque. Will make you think the fin is to far back. If your cg. Is set to be as fast into the wind as it is Running with the wind. Fin set fwd of that just enough to make a tight one if u need to. A really good gage on your setup is how many milaamps your bat needs to achieve full charge after a full two days of heat racing. I know mine are there or getting there when a ballance charge is a total of an Amp or less.(D, E, and F Hyd)

 

[Bill Eilers]

One more IMPORTANT.note. servos that don't have the nessary torque. Will make you think the fin is to far back. If your cg. Is set to be as fast into the wind as it is Running with the wind. Fin set fwd of that just enough to make a tight one if u need to. A really good gage on your setup is how many milaamps your bat needs to achieve full charge after a full two days of heat racing. I know mine are there or getting there when a ballance charge is a total of an Amp or less.(D, E, and F Hyd)

Fin to far back. High servo load to make it turn. Fin is a fulcrum for rudder to act on. Fin surface area is dependent on the hull weight. Rember. Fin length does affect right sponson getting suked down. Even if it's aligned

 

[Hydro Junkie]

Jim, you don't want to add ballast to the boat before you run it. If the boat doesn't need it and you added it, then you would have to tear the deck off and replace it to get that ballast out. Something else you need to remember is that you can lift the back of the boat by using a lifting prop or using a downward thrust angle in the strut. Adding ballast to add ballast isn't the way to go

 

[David Ashcroft]

Just throw it in the water and run it ..lol

 

[Hydro Junkie]

Starting at half throttle and increase slowly from there

 

[Jim Walker]

Jim, that distance is the go-to because it's where most boats start to run well. What you will find is that, unless you have a boat that is very "tail heavy", most boats won't need much, if any, ballast in the nose to get it to run decently at that balance point. If you go much further aft, the boat will want to blow over where, conversely, much further forward will cause the boat to pound the sponsons or plow through the water instead of riding on top of it. If you use that measurement to start, test it and then adjust as needed, tuning the boat will be easier.

I understand. Really good explanation. I’m building a T-5 hull. I’ve got everything built, sealed (interior) and installed except I still need to glue the decks on. I want to get the balance point close as possible.

Jim, you don't want to add ballast to the boat before you run it. If the boat doesn't need it and you added it, then you would have to tear the deck off and replace it to get that ballast out. Something else you need to remember is that you can lift the back of the boat by using a lifting prop or using a downward thrust angle in the strut. Adding ballast to add ballast isn't the way to go

Jim, you don't want to add ballast to the boat before you run it. If the boat doesn't need it and you added it, then you would have to tear the deck off and replace it to get that ballast out. Something else you need to remember is that you can lift the back of the boat by using a lifting prop or using a downward thrust angle in the strut. Adding ballast to add ballast isn't the way to go

The CG of the boat setting like this is 4” behind the sponson transom. Everything is in the boat. Decks are just setting there.

 

[Jim Walker]

I understand. Really good explanation. I’m building a T-5 hull. I’ve got everything built, sealed (interior) and installed except I still need to glue the decks on. I want to get the balance point close as possible.


The CG of the boat setting like this is 4” behind the sponson transom. Everything is in the boat. Decks are just setting there.

Great advice by the way, thank you for that.

 

[Hydro Junkie]

You still have to seal the bottom of the deck and epoxy the deck down and that adds weight. You still have to fill and seal the outside of the hull, after installing the decks, and that adds weight. You have to prime and paint the hull, add decals and clearcoat over all of that, again adding weight. Since the sponson normally have a drain hole through the transom, you can put a rolled paper tube into the sponson through that or cut a hole through the sponson inside with a doubler to epoxy a plug to and add a mixture of epoxy and lead shot into the front of the sponsons, one at a time, propping up the hull so the location you want the weight to be is the lowest point. Now the question is how do you know how much weight to add? By test running the boat and adding stick on weights to the deck in the area you want the weight to be in under the deck. Find out how much weight you had to add and mix up a batch of slow set epoxy with lead shot added to it to equal the stick on weight used in testing.

 

[Jim Walker]

You still have to seal the bottom of the deck and epoxy the deck down and that adds weight. You still have to fill and seal the outside of the hull, after installing the decks, and that adds weight. You have to prime and paint the hull, add decals and clearcoat over all of that, again adding weight. Since the sponson normally have a drain hole through the transom, you can put a rolled paper tube into the sponson through that or cut a hole through the sponson inside with a doubler to epoxy a plug to and add a mixture of epoxy and lead shot into the front of the sponsons, one at a time, propping up the hull so the location you want the weight to be is the lowest point. Now the question is how do you know how much weight to add? By test running the boat and adding stick on weights to the deck in the area you want the weight to be in under the deck. Find out how much weight you had to add and mix up a batch of slow set epoxy with lead shot added to it to equal the stick on weight used in testing.

My thoughts were to add some of the ballast to move the CG forward to about 3” and finish the boat and test and balance it from there. You are right, I have no idea what and where my weight will be or the exact CG the boat wants.
Thanks for taking time to teach me, obviously my first build so this type of help is huge!

 

[Bill Eilers]

Moving eng, fule. And or raido gear is much more preferable than adding weight. Try to look it like this.. Any weight added is detrimental.

 

[peterjuo]

It's a fine line between pleasure an pain

 

[Jim Walker]

Moving eng, fule. And or raido gear is much more preferable than adding weight. Try to look it like this.. Any weight added is detrimental.

I understand.

 

[Hydro Junkie]

Here's some pictures that show the location of the radio box, engine, fuel tanks and turn fin on my 2000 Elam Plus.
I put more information in a private message but these pictures are in both