Hull Balance

[twissted]

You shouldnt lose any speed in fact you might gain some,it wont make her run wet so long as cog is not too forward but having it a bit further than it is now should allow the boat to run more level and stable and gain some speed as well.Id be looking at having the cog at around 34 maybe even as much as 36%.
Mart
I got this reply from another web form and was wondering if cog means center of the front to back of the hull? this was sent in regaurds to my green Zonda that is running with the TP4050 3Y motors and at the time X945/2 props.

I don't want to move my Lipos and so I was thinnking of moving the esc up another inch or as much as possible and then lower the props a little to get a better plane at speed.
What are your thoughts. I will be using 1448/2 CNC props soon and will see if it changes how the hull rides. and also top speed.

[kevinpratt823]

CoG is Center of Gravity. It is the point along the hull that the boat balances at. Typically you measure from tip of sponson, to transom, but some people with cats that have an overhanging transom may be measuring from the back of the sponson. The 36% would be 36% of the over all length, measured from the rear, then make some small marks at that measurement. Now shift stuff around a bit so the boat balances at that point. You can do it roughly just by picking it up with a finger on each side at that mark, or placing a dowel or other roller under the boat like a see-saw. Example, if your boat was 45", you would go
45" x .36 = 16.2" forward from the transom for a balance point.

[twissted]

So of the 3 spots where would I messure from? 41'' hull same as the fightercat Fantazim so I would be looking at 14 3/4 inches? or is it different depending on the type of hull?

[kevinpratt823]

Most likely it would be the top right mark, but you may want to check with the guy that told you 36% and see where he's measuring from. I've never had the overhang, so I just measure from the transom. It's really just a reference point to start with anyway. You can match his setup suggestions and try it, but I think it's more about knowing where your CoG is now, and playing around with shifting it forward and back. Maybe somebody who has more experience running cats with the overhang will chime in and tell you everybody measures from the transom, but in the back of my mind for some reason I'm thinking some people go by the back of the sponson. The videos of your boat didn't seem to be that far off, so I'd find where it balances now and mark/measure that, then divide that number by the overall length of the hull from the transom and it will give you that percentage from that measuring point......

[twissted]

Thank you I will work on it. I feel the same way I will move the esc's sideways and up by the carbon front bulkhead and that shoud get me there. I feel I'm very close.

[kevinpratt823]

Thank you I will work on it. I feel the same way I will move the esc's sideways and up by the carbon front bulkhead and that shoud get me there. I feel I'm very close.

In the future, when you build, it helps to get an idea of what type of CoG you're looking for. Then after hardware and everything else possible is in, place your batts and motor/esc's in and shift them around so you can get a good idea on where to place them, leaving room to shift the batts to adjust the CoG to fine tune it after it's built.
Good luck.

[Fella1340]

I always thought the cog was measured from the rear edge of the sponson, or the top mark in your photo. That way it's balanced off the ride (wetted) surface surface of the boat. The overhang varies greatly from cat to cat and measuring from any other place seems rather random. I don't understand how it could be done any other way. I could be wrong but that's what I do.

[kevinpratt823]

I always thought the cog was measured from the rear edge of the sponson, or the top mark in your photo. That way it's balanced off the ride (wetted) surface surface of the boat. The overhang varies greatly from cat to cat and measuring from any other place seems rather random. I don't understand how it could be done any other way. I could be wrong but that's what I do.

You may very well be right, it's been a while since I had my DF Explorer,(I forgot, that did have the overhang), but that may be why it's in the back of my head like that. Personally, I only really do the measurement once or so to get the initial marks in the ballpark, then adjust as needed using those marks as a reference, but with boats I'm just sport running for fun. I'm sure others are much more meticulous.

[twissted]

The problem with the hull didn't start untill I added horse power! That is why I know I'm very close. I would think that the wet area would always be the starting point as well. This is my secound year with rc boats and so with higher speeds I will need to pickup on more. I will be moving the esc's and then taking messurements and see what the center point really is.

As some of you know this was a ATR boat called a Zonda. I have made it my own with changing almost everything. And that is the reason the center point was never considered till the power steped up. I knew that I was going to go for more speed and that is the reason for the changes.
That and I change everything I own at some point!

[keithbradley]

Nevermind. I think I'm going to stay out of this one.
Best of luck with your boat.

[twissted]

No, it is very smooth. Just high speeds. Here is the video that has the boat in the air. The last flip is when the prop breaks.

[twissted]

I hope this does the trick. It's close the way it was.
When the new cnc 1448/2 props from OSE get here I will test again. I ordered last thursday so they should be here soon.

[785boats]

I always thought the cog was measured from the rear edge of the sponson, or the top mark in your photo. That way it's balanced off the ride (wetted) surface surface of the boat. The overhang varies greatly from cat to cat and measuring from any other place seems rather random. I don't understand how it could be done any other way. I could be wrong but that's what I do.

This is correct. COG is measured from the rear of the sponsons. Not the overhang.

[Fella1340]

Where did you end up cog wise with the move of the esc's? You must have been almost dead on before the move. Or did they get you where you need to be? Just curious as most cats have the motors mounted to the very rear with batteries in front. I suppose the weights of the motors to battery position could net a similar result for cog. Hope it runs good for you!

[rickwess]

This is correct. COG is measured from the rear of the sponsons. Not the overhang.

Makes perfect sense since the CoG would be based on the wetted surface.