Is prop too big, too small, or wrong pitch?

[deshojo]

We tried a new prop on our home made boat today, and the results seemed odd.
It's a 32" medium-V mono hull, with a 35/60 2300kv inrunner, with 2 x 2250mah 4s 20-30c lipo's.

From a stand-still it seems to churn the water, revving quite highly but not moving as fast as one would expect, but then it suddenly lifts up, gets on the plane and absolutely flies. There seems to be no gradual acceleration, as we've had with previous props we tried, but the top speed is mental!

This is an Aluminium (or should I say Aluminum) 1818 Mocom prop. Previous props we've used were cheap crappy plastic ones, just to get an idea of the right size, but when we uprated the batteries the boat would tear the blades off them in the first 30ft of accelerating.

We're not sure if we need a bigger or smaller diameter, or deeper pitch - this prop is only about a 1.1x pitch, being 47mm with 52mm pitch.
I know we should really be using surface props, but the shaft is an M5 thread (the boat was never meant to go this fast, but you know how it is ) so it would mean a complete new drive system, as no surface props seem to come with M5 screw fittings.

Any suggestions welcome (preferably not two word ones ending in "off" though).

Cheers,

Matt

PS. In case it helps, here's a link to a short youtube video of the boat when we were using a single lipo.

[domwilson]

Welcome to the forum.
We would be more than happy to help you.
But first:
Do you have pics of the boat? The setup? The hardware?
Any specifics on the hardware and some pics would allow us to better assist you.

[deshojo]

Thanks Dom,

I don't have any still pics right now, but I can take some and post them tomorrow (it's 10pm here now and the boat is at my friend's place 7 miles away).
The best I can do now is a link to our first video with a brushed motor from a 12v drill. You can pause it at the beginning and get a rough idea of the set-up.

First build with brushed motor video.

Thanks,
Matt

[domwilson]

Can't really see the hardware. Is that a solid shaft or a flex? Tomorrow will be fine.

[domwilson]

Will also need to see the inside with the batteries placed at their "running locations" too.

[deshojo]

It is a solid shaft - a brass outer tube with M5 threaded end for the prop.
I'll be sure to get a good detailed set of pics, with everything fitted as it would be in use, and maybe try and post a video of it running with the new prop too, so you can see exactly what it's like.

[deshojo]

Good evening,

Here is a link to some close-up images of the boat, hopefully they will show what you need to see.
Photos

I have also uploaded a video of a bit of today's run, so you can see and hear what the boat is doing with this prop. The file is 6.3mb, so any readers not contributing, please don't use up all my bandwidth by downloading it unnecessarily - thanks.
EDIT: Video now removed.

Cheers,

Matt

[domwilson]

Do you have pics of the whole boat? Possibly top, front, sides and back. One thing I see is that you may have too much negative prop angle. But that would be hard to determine from the pics you have provided. Do you have a smaller prop around 40mm with a 1.4 pitch? I am leaning towards your current setup is cavitating. I sure would like to hear others thoughts on this one.

[tiqueman]

Im going to have to say from the looks of it, it does seem there is a bit too much negative prop angle. But as Dom mentioned, its a bit dificult to determine in the picture. That may be a key role in getting this to perform the way it should... and I would think a smaller prop would help as well.

I had a 32" cat on a 2045KV on 4S and it screamed... got hot.. really hot, but it screamed on a Octura X442. Im guessing you are running the 4S in Parallel, so your at 4500mah?

Also, looking at the vid again, Im going to say your strut angle is in fact quite negative. There is harldy any roost.

Kind of a silly question, but what does it sound like at top speed. Is it really winding out or does it sound loaded up? Hows your temperatures after a run?

[ED66677]

IMO 36x60 2300KV under 4S is not appropriate for a 32" mono, this motor must correspond to a Feigao 540 L size, witch is a bit small, a XL would be better in any case 2300KV under 4S is way too much, 1900-2000 is the maximum and 4S is not even the best.

[deshojo]

Hi Guys,

Thanks for your comments, they are much appreciated.

I've added a couple of pics of the whole boat side-on, which may help.

The negative prop angle was one of our concerns.
When we ran it the other day with the trim tabs up, the boat was literally running on the prop alone, bouncing up and down with the entire hull a couple of inches above the surface of the water momentarily. Lowering the trim tabs and adding a turn fin brought it down and stabilised it tremendously.

We have tried smaller props, but a 40mm just frothed the water and didn't push the boat at all. Maybe pitch is a factor here? Could it have been too little? (It was not specified, but probably about 1.2). Is cavitation caused by insufficient pitch?

The 4s Lipo's are in parallel at 4500mah, yes. You can just about hear the motor on the short video clip, I wouldn't say it sounds really loaded, but it's not screaming at full revs either.

With regard to the size of the motor, we are on a fairly limited budget, and this one was the biggest we could get cheap at £16 (about US$26). The shop we use does have a 36/74 9xl 1900kv, and a 36/74 12xl 1400kv. Which of those do you think would be the most suitable if we were to change?

Also, does anyone know who offers a decent range of props with an M5 thread? It seems there's either Graupner props for M4, or Octura etc. for 3/16th drive dog shafts. This prop is really a sub-surface prop, and really intended for gas/nitro engines.

Thanks,
Matt

[domwilson]

That's a roboesch straight shaft? Is there anyway you can convert to a flex shaft with a strut? This will give you a wider range of adjustments and options to get this hull running correctly. You can probably make your own strut to save cash or check the swap shop. As for the motor choice that depends on a few things, like desired speed and runtime. I think the problems that you are experiencing are related to prop depth and possibly angle. I understand that you are probably new at this. So the best advice I can give is before you spend any cash, research alot, ask questions and look for deals within your budget and ask more questions. For the most part, everyone here is very helpful and will try to assist you as much as possible.

[deshojo]

Thanks Dom. Yes it is a Roboesch straight shaft, it was fitted in the hull when we got it, along with the rudder and prop, all for £5 on a well known auction site :-). And yes, this is a new thing for us as we only got the boat on a whim 2 months ago. We didn't appreciate at the time what a complex science it is, but it's great fun learning.

I think you are right about the need to change to a flexi-shaft. It's something we've considered, and I think now the project has reached the stage where it is going to be necessary in order to advance any further.
My partner in crime is a mechanic/engineer, and I'm sure he'll be able to make a strut once he's studied their design.

Thanks for your help, I'll be back with more questions soon enough!

Cheers,
Matt

[domwilson]

We'll be here to answer any questions along the way.

[ED66677]

If you want to stay with 4S, the 9XL is the best, it'll spin a big prop, at least 45mm

40mm as you tried isn't big enough, one thing you have to understand is that for a given hull size, there is something like four or five prop size that would do it, I'm not talking about pitch or brand, just diameter, on my 29" DF copy the smallest I use is 40mm, 42mm is a good average and 43,5mm is hotter, according to this on a 32" a 42 or 43.5mm prop is certainly the minimum, below this minimum size the prop slippage become too important and the prop is not able to transfert the amount of power that this boat requires to move fast. The last point is that for this given hull size if you want to spin at least a 42mm prop under 4S at 30-32000rpm it is clear that a 35/60 motor can't do it for long!