MHZ Alligator 1250mm 49" twin outboard build

[Rich]

Hi, I just finished this build and had a question on how to stop it from rocking side to side when at full throttle. My setup as follows:

Twin 4092 730 kv on 6s 8000mah each side
ZTW 300 amp each side
COG at 28%
x457 rotating inwards

I moved the trim on the outboards down and it helped the problem but didn't solve it. Any suggestions? Here is the video link

https://www.youtube.com/watch?v=6bo8gaM73P0

[2turbofords]

Looks like the nose of the boat is riding pretty high in the water under throttle, try a little weight in the nose and give it a test? Man that cat with the twin outboards is so amazing, and the sound is like no boat I've heard before. Those big boy builds you have are sick!

[Rich]

Looks like the nose of the boat is riding pretty high in the water under throttle, try a little weight in the nose and give it a test? Man that cat with the twin outboards is so amazing, and the sound is like no boat I've heard before. Those big boy builds you have are sick!

Thanks! I might just add two more batteries and make it 6s2p on each for the added weight. I am thinking outboards need more weight in the middle- rear of the boat to keep the hull in the water. The cat you are talking about doesn't have the outboards on it anymore. I blew one with the power I ran in it. This is the other project that inherited the outboards. Now the cat has twin inboards, I just hit 60mph on that one today.

[RaceMechaniX]

Try moving the batteries forward to shift the CG forward. If the batteries are fixed add some lead weights in pool noodles and stuff them in the front.

[Brushman]

hello,

I would try a little change in prop angle, more negative... Also, if you were obliged to put your batteries at the front to get a proper CoG, you will get porpoising. try to add batteries isntead by keeping the same CoG, make sure your batteries are sitting as low as possible in the V.

Vincent

[Rich]

Try moving the batteries forward to shift the CG forward. If the batteries are fixed add some lead weights in pool noodles and stuff them in the front.

hello,

I would try a little change in prop angle, more negative... Also, if you were obliged to put your batteries at the front to get a proper CoG, you will get porpoising. try to add batteries isntead by keeping the same CoG, make sure your batteries are sitting as low as possible in the V.

Vincent

Guys, is it possible that the transom is getting to much lift and causing the rear of the hull to be out of the water? If that's the case shouldn't I put the cg more aft and put the trim down? Am I thinking about this correctly?

[Rich]

So I messed around with a few ideas. 1st I put on 50mm x 1.4 pitch props and ran it. It still rocked side to side but not as easily as the x457s. Next I put the trim down all the way and got about the same results. Last I changed the rotation of my props to spin outward and the problem got noticeably better. I also noticed that I could here one or both of the props coming out of the water from time to time. It seems that the transom is balancing on the two outboards and by changing to props out the distance between the balance point widened causing it to be more stable. I feel like I need more weight in the back or I should move the outboards up so the hull is in the water more. Is there any truth to my thoughts or am I just thinking into it too much?

[Brushman]

Hello,

any change? Did you move the outboards up?
I have a few ideas : do you have strong servos and linkage to the outboards? An oscillation could occur...
also you have 4092 motors at high height, it can cause this balancing effect. On 6s , I have to say that your motors are not efficient on 6S, It would be better to use lighter outrunners, like : https://www.xnovamotors.com/xnova-4025-560kv/, two 130Hv seaking esc's and 12 S with 52-54mm props, you would have a good efficiency and a "scale" speed of 50mph.

Vincent

[Rich]

Hello,

any change? Did you move the outboards up?
I have a few ideas : do you have strong servos and linkage to the outboards? An oscillation could occur...
also you have 4092 motors at high height, it can cause this balancing effect. On 6s , I have to say that your motors are not efficient on 6S, It would be better to use lighter outrunners, like : https://www.xnovamotors.com/xnova-4025-560kv/, two 130Hv seaking esc's and 12 S with 52-54mm props, you would have a good efficiency and a "scale" speed of 50mph.

Vincent

The outboards can't handle more than 20k rpm. I haven't tried anything since my last post. I should be able to try something this week

[Brushman]

hello,

these outboard are good for 30K rpm:https://www.mhz-powerboats.com/shop/...number=1729-4M

[Rich]

hello,

these outboard are good for 30K rpm:https://www.mhz-powerboats.com/shop/...number=1729-4M

I don't mean to argue but there's been a few threads on Facebook where the owner of the company says 20,000 RPM is Max I think there's a typo. Also always ose also says 20,000 https://www.offshoreelectrics.com/pr...?prod=bel-2061

[Brushman]

okay,

this is your boat, I hope you will find solutions!

[Rich]

Ok so I have tried just about everything. Moved the outboards up, in and out. Changed the prop angle and moved the lipos around. I even put trim tabs with no avail. Running 4 lipos stops the rocking but makes it heavy in turns and a it's slow. I currently have the outboards as close to the transoms as I can with the nose cone 1/2 inch up from where the hull is at the spread of the outboards. I put 2 lipos in the very front battery trays and it ran fine without any rocking and it was pretty fast. It seems the props aren't getting quite enough bite though. I put the next set of lipos in the same spot and it rocked again! I had one last thought, the water pickup is mounted in the center most lowest point on the vee. I brought it in and I removed the pickup and the rocking went away again! I'm thinking I need to move the pickup or get rid of it all together since the escs were cool to the touch. I'm going to lower the outboards back down to get more bite out of the props and keep the water cooling off to see of the rocking presists.20210815_155802.jpg

[LUKEYxxLUKE]

Is it rocking side to side or is it porpoising? It looks like porpoising in the video, if it is then that?s a CoG issue. If it?s dancing then you could be getting too much lift and the strakes on the hull aren?t stabilizing it. You could also adjust you trim tabs down some so it pushes the bow down.

[Rich]

Is it rocking side to side or is it porpoising? It looks like porpoising in the video, if it is then that?s a CoG issue. If it?s dancing then you could be getting too much lift and the strakes on the hull aren?t stabilizing it. You could also adjust you trim tabs down some so it pushes the bow down.

It's hard to tell in the video but it is rocking side to side. I put the cog forward more to stop the porpoising, I think the water pickup was causing it to rock side to side, I must have hit it out of alignment when I swapped out the batteries.