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.

[RandyatBBY]

Looks like to me the boat is too far out of the water. Raise the motors till it settles.

[RaceMechaniX]

Have you tired any 3 blade props? An outboard guru was telling me the smoothest running happens with 3 blade props over 2 blades.

[Rich]

Looks like to me the boat is too far out of the water. Raise the motors till it settles.

Randy, if I go higher the props wont be in the water. I do agree that it was acting like I needed to raise them up. We will see what the new mount does.

Have you tired any 3 blade props? An outboard guru was telling me the smoothest running happens with 3 blade props over 2 blades.

No I have not tried 3 blades, what is the reason for them being better on an outboard? Any insight to this?

[Rich]

Well I tried the new mounts and it rocked pretty bad. These mounts were .5 inch down and about .375 back. I'm going to lift the motors from there and see where it stops rocking. On the other hand the boat planed out almost immediately and ran about 5 mph faster then before. Not to bad, the nose cone of the lower is below the hull so bringing it up .25 inch should put it just about the hull, we will see. I might try 50 and 52 mm props just to see if it helps with this configuration.

[Rich]

Well, I know its been a while since I posted this thread but I ended up finding you guys were right. The lipos needed to be way far forward to stop the "rocking" I was seeing. That being said I ended up making my own lowers in the hopes to run 12s reliably in an outboard here is a video on 6s:

https://www.youtube.com/watch?v=vPIUhatybe4

This is one of the first test. I ran the boat twice back to back at about 8 min per. I drained the oil (75W-140) I had in it and port side had water in it. I still need to figure that out. Let me know what you think. I did run these on 12s the time before this with a breather tube but all of the oil came out. Ill try to find the leak and fix that then try 12s without a breather tube.

[Rich]

Question for you guys. I was running the boat today and it ran perfectly. I am running 2 4092 700kv on 6s per side with x457 counter rotating (out) props. I want to see if I can gain more speed with the same 6s setup. I am currently getting 33-35 mph and would like to hit 45. I am thinking I should start with a larger prop but I can't seem to find any that are counter rotating besides the x465 (which I have on hand). I am not sure if the 4092s can handle that big of prop. I checked the temps on the motors today and they only hit 93F after a 8 min run pretty much full throttle the entire time. What do you guys think I can do to gain more speed with this setup?

[Bande1]

That's a super low RPM...stock zenoah type rpm. Consider 8s IMO...or even 10s. Under load you're probably turning 15-17k rpm. Go up in voltage, lower prop diameter, and raise pitch. I run 850kv on 12s on my 42" cat with 43mm props. It does very well like that.

This will open up way more props for you. I would use the prop calculator to find a prop for 8s or 10s that will achieve somewhere in the 50-55mph range. That's just me.

[Rich]

I was thinking about going to 8s. Since I'm using a gear drive I dont want to go past that. In short 8s might be all i need to do. Now i just have to find some 8s batteries.

[Bande1]

I was thinking about going to 8s. Since I'm using a gear drive I dont want to go past that. In short 8s might be all i need to do. Now i just have to find some 8s batteries.

I run these. priced pretty good. I pull 165A on them.

https://www.amazon.com/Socokin-Batte...zcF9hdGY&psc=1

[Rich]

I'm thinking 8s1p. It will be less weight and less wire. Thanks for the info though

[Bande1]

I cut my batt wires to 1" and add 8mm bullets. Then series connect with 1" of 10awg wire as a jumper. Keeps everything short and sweet. Just throwing it out there as a possibility.

[Rich]

That is an idea which I could always do. I do like the idea of having less weight though hmm... I think I'll balance and sharpen the x465s and do a test run and check motor temps. If that doesn't give better results then I'll save up for some 8s lipos

[Peter A]

Way back Tyler suggested 3 blade props. You also said there were a lack of L&R props. Have you bothered to check out what OSE and RC Boat Bitz stock? They both have two and three blade props in 50mm + sizes in both L&R.

[Rich]

Way back Tyler suggested 3 blade props. You also said there were a lack of L&R props. Have you bothered to check out what OSE and RC Boat Bitz stock? They both have two and three blade props in 50mm + sizes in both L&R.

Peter, I did not look at the RC Boat Bitz website because I didn't know it existed until now... What I meant was a lack of L&R props that were either bigger than the x457 that I am using or the same size just a different pitch. That being said will I gain speed going from the same size and pitch 2 blade to the same size and pitch 3 blade?

[srislash]

Peter, I did not look at the RC Boat Bitz website because I didn't know it existed until now... What I meant was a lack of L&R props that were either bigger than the x457 that I am using or the same size just a different pitch. That being said will I gain speed going from the same size and pitch 2 blade to the same size and pitch 3 blade?

not really. A little stronger out of corners but burn 1/3 more power is my experience. I may have a set of Old School Prop Shop props that I had Chris do up for me. I’ll look for them in the morning.

Shawn