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How to power a 2m (6') Jet RHIB?

Old 08-26-2020, 05:08 AM
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MERV20
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Default How to power a 2m (6') Jet RHIB?

Hi Everyone,
Firstly let me say that I have read the 'Read First' section.....however it didn't really cover what I'm doing. I have built a 2m RHIB running a 75mm Jet drive. My issue is that every 'BIG' motor may be too small. I have had a look at the Aquastar 730KV, the Dynamite 6P 1000KV, Proteus x524 and they all are powerful motors, but I don't know if they are powerful enough and I don't really want to spend big $s only to find that it's sluggish.
One issue at a time though and my question is if anyone has a suggestion for a motor (and potentially ESC) large enough to power a 2m (6 foot) boat. Any suggestions would be appreciated, or even testimonials on the motors listed above,.
P.S. the Aquastar is meant to be 5280W, so if it's under that then it's not powerful enough.
Cheers, Merv.
Old 08-28-2020, 05:38 AM
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Got RPM
 
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Cool

We need more information to be able to give even SWAG advice:

- what speeds are you looking for?

- what run time are you looking for?

- what voltage are you planning on running?

- exactly what is the jet drive you will be running? (I assume is is a water jet and not an EDF)

- what rpm and wattage does the jet maker recommend?

- where will you be running the boat?

- how have you waterproofed the electrical area?

- how will you recover the model if it stalls in the middle of the pond?

- do you have photos of the model, inside and outside?

Your build is outside the experiences of 99.99% of the members here, but we can at least give you some suggestions once we better understand what you have and what you intend to do.




.
Old 08-29-2020, 09:04 PM
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Default Details of build

- Speeds - 20 - 30kts
- Run time - not an issue, as I will build the battery (18650s)
- Voltage - again not an issue, as above
- Weight - battery pack will influence, but may end up around 10Kgs or more

- Jet drive is not a particular brand. I downloaded a version from Thingyverse and adapted it in Fusion 360? (you are correct, it is a water jet and not an EDF)
- Rpm is yet to be determined, but I would suggest motor wattage above 5KV?
- Where will you be running the boat - Sheltered ocean
- How have you waterproofed the electrical area - I will be building a fully enclosed electrical bay. Considering a heat exchanger for cooling (don't want to run salt water through electronics) I was thinking maybe a modified PC liquid cooling system?
- How will you recover the model if it stalls in the middle of the ocean - boat
- Apparently I have to post 10 times before a can include a photo???


Old 08-29-2020, 09:13 PM
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MERV20
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Default Jet drive

I forgot to mention that I 3D printed the Jet Drive in ABS.
I have machined a drive shaft from SS stock and I'll be running SS bearings throughout.
Once paired with a motor I will do some trials to get a good idea of thrust.
I'm not 100% sure how to do this, but I was thinking of doing it next to a wharf with a set of fishing scales and rope connected to the boat.
If anyone has a better idea let me know and I will post the results here.
Old 08-30-2020, 07:47 AM
  #5  
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Cool

The questions I asked were not arbitrary. We need to know the pack capacity and voltage in order to make a motor recommendation. Electric tool batteries will likely not provide the power you need for decent performance in this big model due to excessive internal resistance - they are not designed for high amp draws and are not recommended.

Jet impeller rpm is vital to performance, too slow or too fast will not work. What does the jet unit designer recommend? Motor wattage depends on the jet unit design, too much will destroy the unit, too little won’t be fast. You are not providing enough information for us to help you...

Go to this forum and post ten times using just the word ‘test‘. Then you can post pics in your thread.
https://www.rcuniverse.com/forum/test-posting-forum-72/



.

Old 08-31-2020, 05:29 PM
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Default Details and photos

Hi,
Sorry if the details are a little thin, but this is all design as I go. I realise that makes it difficult to make suggestions, however I'm after your 'best guess' and I'm happy to work it out from there. I have included photos below.

'We need to know the pack capacity and voltage in order to make a motor recommendation', you're correct in saying that LiPos are able to produce more burst current, however a Li Ion is able to match any current requirements by putting more cells in parallel and spreading the load. Yes the physical size of the battery may be larger, however I am not short on space. Also, these are not 'Electric tool batteries', they are high capacity LG cells with a BMS. The reason for these over LiPo are to get long run times. By putting the cells in parallel the issue of internal resistance (which is limiting burst current) is overcome, and obviously I can put them in Series to meet any voltage requirements.

'Jet impeller rpm is vital to performance, too slow or too fast will not work', you are on the money and I completely understand how RPM can lead to under performance if not matched to the motor. To higher RPM will cause cavitation and lead to impeller damage and inefficiency, too little and the boat will be severely sluggish. What I have noticed though is that impeller diameter can give you a fair indication of required RPM and I am after a general figure so that I can match a Motor and Battery setup.

The diameter is 75mm and it is a four blade Impeller.

I can adjust the battery voltage to suit, but I need a general figure to build around. I will then find the sweet spot through testing (mentioned above) and I could then redesign the pack to suit the application so that I'm not working at the capacity of the motor or battery. At the end of the day any advise from those running Jet drives of all sizes is going to help. Based on RPM Vs impeller diameter, I can make a general hypothesis and then work around it. This is probably the most important piece of the puzzle.

If anyone out there has a Jet Drive, any advise is appreciated.


Interior of hull, pre-fibreglass.

Hull design, skirt will be held in recess.

Jet drive with ruler to give scale.

Last edited by MERV20; 08-31-2020 at 05:58 PM. Reason: Improve wording
Old 09-23-2020, 02:16 AM
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Default Updated photos

Hi Everyone,
The build is going slow due to work commitments and COVID, but it is getting there slowly. Currently I'm looking at the SSS 56104 as the proposed motor, but still up in the air about a suitable ESC.

If anyone out there has a Jet Drive of any size, any advise is appreciated. Mainly after an idea of impeller diameter vs RPM




Old 10-04-2020, 07:12 PM
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Default Jet Boat Power

Originally Posted by MERV20 View Post
Hi,
Sorry if the details are a little thin, but this is all design as I go. I realise that makes it difficult to make suggestions, however I'm after your 'best guess' and I'm happy to work it out from there. I have included photos below.

'We need to know the pack capacity and voltage in order to make a motor recommendation', you're correct in saying that LiPos are able to produce more burst current, however a Li Ion is able to match any current requirements by putting more cells in parallel and spreading the load. Yes the physical size of the battery may be larger, however I am not short on space. Also, these are not 'Electric tool batteries', they are high capacity LG cells with a BMS. The reason for these over LiPo are to get long run times. By putting the cells in parallel the issue of internal resistance (which is limiting burst current) is overcome, and obviously I can put them in Series to meet any voltage requirements.

'Jet impeller rpm is vital to performance, too slow or too fast will not work', you are on the money and I completely understand how RPM can lead to under performance if not matched to the motor. To higher RPM will cause cavitation and lead to impeller damage and inefficiency, too little and the boat will be severely sluggish. What I have noticed though is that impeller diameter can give you a fair indication of required RPM and I am after a general figure so that I can match a Motor and Battery setup.

The diameter is 75mm and it is a four blade Impeller.

I can adjust the battery voltage to suit, but I need a general figure to build around. I will then find the sweet spot through testing (mentioned above) and I could then redesign the pack to suit the application so that I'm not working at the capacity of the motor or battery. At the end of the day any advise from those running Jet drives of all sizes is going to help. Based on RPM Vs impeller diameter, I can make a general hypothesis and then work around it. This is probably the most important piece of the puzzle.

If anyone out there has a Jet Drive, any advise is appreciated.


Interior of hull, pre-fibreglass.

Hull design, skirt will be held in recess.

Jet drive with ruler to give scale.
I'm running a 28mm diameter drive in a 550mm hull with 4S and a 1500 Kv motor and it runs about 40-45 mph.
What sort of performance are you looking for?

Last edited by bigwaveohs; 10-04-2020 at 07:14 PM.
Old 10-05-2020, 11:14 AM
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Hi Bigwaveohs,
Yeah that is well beyond what I'm after. I'm hoping for about half of that. Thanks for the details though as it helps to build a picture of what I'm after. I will try and build a table so people new to this have a resource to refer to to get a general idea on what different setups offer. I know it won't guarantee anything but it might help as a guide. I need things like length, weight, hull design, esc, motor (Diameter, length, Kv) , drive diameter, battery set up, speed achieved. Thanks for being the first to reply. If you give me the details I've missed I'll put then in the table.
Old 10-05-2020, 11:26 AM
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