big problem with cavitation

[bmd8991@aol.com]

Friends
I have a wooden tunnel hull. Was made for nitro engine but I added electric motor. It is an outboard that I bought from OSE.

When punching it, it goes and stops, goes and stops. I tried a larger prop, but still does same thing. Think it is cavitation.

What can you tell me about this issue? What should I change or improve? Does the weight of the boat affect it? How high or low should the outboard motor be set? I have a video but it is too large to upload here.

Thanks
Brian

[boredom.is.me]

"Goes and stops" isn't really much to go on. Is the motor cutting out? Is the boat bobbing up and down? Is it creating a lot of foam, but going no where? Upload the video to youtube and paste the link here.

Any details on the boat?
- length
- beam
- prop specs
- motor kv
- voltage

[Fluid]

The photo really tells us nothing useful. Take a few pics of the boat’s transom from the side and rear. In addition to the requests above:
- where is the CG?
- with the hull on a flat surface and the OB overhanging the edge, where is the centerline of the prop relative to the surface?

[LibertyMKiii]

If you can describe the "stop" a little more.
Is it still running but the power is cut? Many times I see that the ESC hits amperage or temperature limits and cuts power.

Pulling the specs from the red outboard on OSE's sale site. "Scale Sevan Outboard"

"
Recommended setup:
These work well on 30" (760mm) to 35" (900mm) length boat in the 3-5 pounds total weight range. Keep in mind there are many options, each boat will be a bit different. Below is a general setup that will cover many boats. Any questions about your particular project, please email us before the order.

Requires:
(1) Raider 90 esc
(1) flh-d3548-1500kv motor with a 3s lipo 5000mah 30c pack(4s maximum)
(1) cnc-3814250
"

What motor did you put in there because they recommend a 3s and you are running a 2s I see...

[bmd8991@aol.com]

Thanks everyone for the replies.
Here is my youtube video so you can see and hear the problem- defilippo video 2 boat 110421 - YouTube
Also see my pics of the bottom and where the center of the prop meets the bottom of boat.
You can see how it stops and goes with the throttle at full.
My setup is as follows-
motor 3650kv
2s
ESC 90a
Transom is a wood block
length- 28 inch- all wood
Prop- Diameter 40mm(1.58"), Pitch 1.4x40=56mm or 2.20"

I think it is cavitation but do not know how to fix it- is the OB too high or too low on the transom?
How do I change ESC so it does not cut out if too hot?
Boat is tail heavy but I can put weight in the cockpit. I know how to balance an RC plane but how do I find the CG on a boat like this?
I appreciate any input on this problem.

Thanks

[jkflow]

What ESC model do you have? Make and model.

FYI, Video is private and can't be viewed.

CG is 30% from transom (total length). At least it's a good starting point.

[bmd8991@aol.com]

ESC is Raider 90a
try video now- https://youtu.be/HPn5ZaUDnr0
I will measure for CG. What method do you use to balance it?

[boredom.is.me]

I'm not familiar with the outboard, but it's gear driven. Why is there a space between the drive dog and the foot?

How heavy is the boat?

It's really hard to tell, but it also looks that you outboard has a negative pitch.

Did you check your motor coupler?

[Fluid]

…- where is the CG?
- with the hull on a flat surface and the OB overhanging the edge, where is the centerline of the prop relative to the surface?

We can’t help you without more information. We need to know the position of the prop relative to the bottom of the sponsons. Your photos do not show the sponson transoms so are of no help. Also, the prop centerline should be parallel or slightly up at the back end compared to the bottom of the sponsons when the boat is on a flat surface.

The CG is the position where the boat - with everything installed exactly like it will be when running - balances. Measure the distance from the back of the sponsons (not the extreme back of the boat) to that balance point, then measure the length of the boat from sponson transoms to the bow. Divide the first number by the second, that’s the position if the CG. The CG can be changed by either moving the battery or adding weights.

I suspect that the problem is either the prop position or the prop type, but need more information to know for certain.


BTW, the setup in post #4 is too mild to work well the OP’s boat. It might work in a mono, but his boat is not a mono. The OP’s motor/voltage should work with the correct setup.


.

[bmd8991@aol.com]

Fluid
I added weight to cockpit and came up with CG point. When picking it up with two fingers, it seems more balanced.
See description of prop in previous post. Is that prop correct?
What is OP? What do you mean by mono? Is my setup not correct for a tunnel hull?

Boredom
Please see pics. You can see the prop is even with sponsons.
There are spacers in between drive dog because of the thickness of prop and to allow the nut to screw tightly.
I checked that the drive dog and coupler are not slipping.

Jkflow
I reposted the video- can you see it now?
Now that I balanced it better, I will take it out tomorrow to see the effect. Hope it will stop or improve the original problem.

Thanks for your continued suggestions

[jkflow]

Yes, video is up now. Nothing wrong with your electronics. Your issue is below the waterline. I let others chime in with more expertise.

[Fluid]

What is the CG as I described the measurement? That has still not been answered.

OP means the original poster - you.

The prop is some type of plastic, the design is as important as the dimensions. Brand?

Mono is a type of hull, yours is a tunnel hull. The motor/voltage should work for that hull, but check for heat.

Based on what I can see (it is difficult to help when questions go unanswered) the prop probably needs to be at least 1/8” deeper and angled up at the aft end a few degrees when the sponsons are flat on a table. Even then the prop design may not be suitable, an Octura x440 may work better, but try the new prop depth/angle first with CG closer to 28%.




.

[Alex.v.Schie]

Try to lower the outboard.
Get writ of the trimtabs ,no need on a f1 tunnel.
And maybe an other bigger prop.
What motor your running ,how big is the boat what prop?how heavy is the boat what battery are you running.
Lots of questions I now.

Verstuurd vanaf mijn SM-A405FN met Tapatalk

[bmd8991@aol.com]

Fluid
Another poster suggested 30% for CG. So I measured about 8 inches from transom to center (boat is 27 inch long). How many inches if 28%?
The prop is from OSE- Graupner K Series 40mm
I will try angling OB up a little bit (instead of down that I had originally)

Alex
See my original post for answers.
What will lowering the OB do to the flow of water?

Thanks

[Fluid]

1) You mentioned two lengths in your posts. Is the boat 28? total length, 27? long, or 27? from the back of the sponsons to the bows? If 27? from sponson transoms then the CG should be about 7-1/2? ahead of the transoms.


2) The prop is probably big enough, but you could try a K42. That might be too much for your setup but is a cheap trial. A better prop design is the Octura x440. https://www.offshoreelectrics.com/pr...od=SB-oct-x440


3) Lowering the prop will keep more of the prop in the water, perhaps reducing cavitation. Try that before buying a new prop.


4) One thing not mentioned, you can try tossing the boat onto the water with the throttle pinned. This can be tricky since the boat has to land flat on the water with forward movement. Sometimes this will help a cranky boat get on plane, but if it comes off plane out on the pond then you?ll have to motor it back in slowly.





.

[bmd8991@aol.com]

Fluid
Boat is 28 total. CG then is about 7.8. Added 2 oz weights in cockpit.
Before writing to you, I already brought up the OB 1/4 inch on the transom because of the problem.
What do you mean by pinning the prop? When on the water, how do I get it on plane?
I am very new to boats and you have been great. I hope rain will stop today so I can go try the changes.
Thanks

[jkflow]

You throw it into the water with forward momentum. Might get it over the critical issue that you are experiencing.

Lower your prop first and see if that helps.

Launching:
https://www.youtube.com/watch?v=5iMJz9ZXTf0

[dasboata]

those shafts are squared ends ,,to me it's a mechanical problem like a slipping shaft , take apart the lower unit !!

[NativePaul]

2s and 90A isn't a lot of power for a 28" boat, especially an ex nitro conversion which typically will be heavier than an electric boat.

Also with 3650Kv on 2s with a 40mm prop, I doubt you are using the full potential of your setup. While it is impossible to say for sure without any temperature data or datalogs, I suspect that you may have some headroom to prop up with. Next time you run measure the temps and if they are significantly under 140 battery, 160esc, 180motor, I would buy a 42 and a 45 mm prop, try the 42,take temps again and if still cool enough try the 45.

[JestDanny]

I think you are way under powered, power to weight ratio, just not enough power, under powered over proped? Starting points, 28 inch hull 30% CoG = 8.4 inches from stearn. Prop ride height, bottom prop shaft = bottom of sponson + 1% to 2% negitive trim angle. Try smaller prop unload engine more 38mm x 1.4

[JestDanny]

What motor is in that OB? need specs. 36x50 mm or 3650 kv?? Recomended motor out runner 35 x 48 mm, 1500 kv, 2S with X440 prop = 14 mph on 3S = 21 mph ???

[bmd8991@aol.com]

Jkflow
Never knew it mattered how you put it in water. Been putting it in motor first. What do you mean lower the prop? Do you mean the entire outboard or angle the prop upward?
Appreciate the video!!

Dasboata
What do you mean by shafts are squared ends?
The OB cannot be disassembled. But the shaft and drive dog are secure.

NativePaul
I will begin a datalog. I have a 40mm prop now. I will also try the 42mm and 45mm props once they come.
OSE said my 3650kv should only use a 2s.
The battery and wires do get hot. What would cause the heat?

JestDanny
OSE said my Leopard 3650kv should only use a 2s.
Where do you recommend buying a 1500kv?
See above for my props

Thanks

[boredom.is.me]

I dont think your 3650kv on 2s is ideal, but changing it wont fix the issue here. Keep what you have and focus on the boat itself. Just as Paul said, you probably arent even using what it had to offer.

[JestDanny]

Leo in-runner 28x45mm 3650kv motor is that correct ? Have you tried running with the CoG at 30% from rear yet, once set leve it there, any improve mints. next set prop ride height to a base line setting, test it an make changes from there. one at a time an test it. I like starting with small prop then seeing how big I can get away with.

[NativePaul]

Other than 2s you haven't said what the battery is, but if it is getting hot it means that I was wrong and you don't have leeway to prop up with your current setup.