Center of Gravity for a 1/8th Hydro

[Doby]

As the title says, any specifics???

[JimClark]

probably 1 to 1.5 inches behind the aft end of the sponson.

Jim

[G Doggett]

The rule of thumb for c/g on all hydros (from full size unlimited to micro models) is 10-12 % of the afterplane length as a starting point.
Most 1/8 scale hydros measure around 26 inches from the rear of the front sponsons to the propeller so c/g is usually 2.5 - 3 inches behind the sponsons.
Roundnose style might need to be slightly less than this because of the airlift under the nose at high speed, and extreme picklefork style might wind up with slightly more than 3 inches to help put some weight on the prop.
You didn't really expect to get a simple answer did you ?
Graham.

[JimClark]

Except if you are running Lipos I would think the CG is closer to the aft of the sponson's

[eric-paul]

Except if you are running Lipos I would think the CG is closer to the aft of the sponson's

Good point!

[Blueprint]

Hi Doby
I have experimented with the CG in several positions on my 1/8th electric. Using my nitro experience and with these boats, keeping the motor and everything as far forward as possible then adding weight up front and getting the CG about 2 to 2½” behind the sponsons (about 2LBs on the strut) the boat would still flip over backwards and sometimes doing a full loop. The flips were then cured by putting more angle on the strut.
When I started doing the Electric 1/8th the opportunity to get the CG as close to the sponsons was possible by juggling the motor and batteries (NiMh) around. The first attempt at launching by a standing start was a failure as the boat wanted to become a submarine. (I was trying to avoid a throw launch) A few more attempts with moving the batteries back I finally achieved a standing start.
The CG ended up being 1½” behind the sponsons and the strut was set parallel. I think it needed more angle on the strut. This was a T4 Budweiser with the vents, check the video.

http://www.youtube.com/watch?v=U32pikSS-yc

I then converted to Lipos my old nitro T2 Budweiser (No Vents) and experimented with the CG and found again 1½” behind the sponsons to be the best for launching and have been adding more angle to the strut to stop the boat blowing off as can be seen in these two videos. Funny thing about these blow –offs, both were down wind, and I was easy on the throttle into the wind but gunned it downwind and it still took off.

http://www.youtube.com/watch?v=7NZ6nEWwyBk

http://www.youtube.com/watch?v=HR0RVhK-xO0

You will also notice in the videos that when the boats reached the apex of the flip it didn’t go over backwards but went forward instead showing you that the CG is quite away forward.
So Doby the answer to your question is to get the CG as far forward as possible for an easy launch the do your fine tuning on the prop angle. This is based on boat speed of 70-80mph, probably not so critical on slower speeds.

Hope this helps

Merv

[Ub Hauled]

Merv, I just wanna say that your boats are awesome! I love the way you cut into the sponsons to place the cells... great idea. A few questions, are the cuts in the sponsons reinforced? And also, what is your setup? Voltage, motor, prop, are you using 1/4 flex?

Great videos, thanks for sharing.

[Doby]

Blueprint;
Thanks for the information. 2 of my 4 LiPos will be in the cockpit and 2 will be under the cowling. There are pics in my build thread. With that configuration the CG is about 2 inches from the back of the sponsons so it sounds about right. By the way, I saw the first vid you posted above quite sometimw ago and love the way it sits in the water during the high speed pass at the beginning. Not much in the water at all, looks about perfect!

[calcagno45]

In watching the video's linked, I noticed that upon first throttle, the nose seems to take on some water, almost dive. Is that normal? Just trying to understand these hulls.

[G Doggett]

Merv is 'Mr 1/8 Scale Hydro' in this part of the world, so listen to his advice as I do.
His is the only electric 1/8 running in NZ at present but some others are bound to follow soon as the technology evolves and the price of components gets more affordable.
I have a Circus Circus ( modified Dumas kit ) that I plan to power with 8 - 10S as my next project.
Graham.

[Doby]

In watching the video's linked, I noticed that upon first throttle, the nose seems to take on some water, almost dive. Is that normal? Just trying to understand these hulls.

Hydros are like that, they run wet untill up on plane.

[calcagno45]

Thanks for clearing that up. I looked into a Hydro and that would have freaked me out on the initial run, not knowing any beter.

[Blueprint]

UB Hauled, my hulls are fibreglass and as the electric boats are water tight I filled up the sponsons and afterplane with a 2 part closed cell expanding foam. This makes the hull ridged and super strong and makes the cutting into sponson deck easy. I do make a frame up for the battery hatch that has the same contours as the deck. Also the cuts on the deck are strategically placed so I didn’t cut through the Budweiser Decals.
Motor is a Plettenberg 370MB/50/A1S, ESC Schulze 40.160WK, flex cable is ¼ inch and 36s2p 3300 GPs on the first video. 2nd & 3rd video are 12s2p 3700 Polyquest. The props for coursing are X460 and straight a ways an ABC H30. I am now going to try 10s2p Lipos as I would like to travel to the USA in the future and bring both my nitro and electric 1/8ths over and possible race at the MC and maybe a race or two at URC.
By the way I have been watching the DMWB thread on the red forum and waiting to see what develops from that.

Doby, I also been watching you build thread with interest as well and hope all goes well with your project.

Graham, thanks for your compliments and hope to see your CC out soon, heard rumours that there could be another two elec 1/8ths for our Club on the way as well.

[Ub Hauled]

Merv, thanks mate, pne other thing id you don't mind, what is the RTR weight of your lipo 8th scale?
It's great to know that you have been looking at the SSC boats...
I have been trying to figure out what setups will work good on 8s, so far I have been more impressed with 10s (of course). It's pretty clear to me that it will be a NEU powerplant, now I have to decide (along with da fellas), on the ESC and cells and how we are going to make an 8th scale fast enough on 8s to be interesting to the curious eye, ideas are always welcome BTW.

[Blueprint]

Just weighed the boat with my setup 12s2p and it is 19LB exactly.
Don't think weight is a big issue these days.
Hope this helps.