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Able Seaman
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PostSubject: Jet drive boats   Jet drive boats EmptySun Mar 15, 2020 10:22 am

Is there anyone with experience of jet drives im doing (stalled) on a twin jet drive set up using 2 kehrer 28mm drives with reverse buckets. There is the R/C group's jet drive forum but I'm more minded to ask on here first.
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PostSubject: Re: Jet drive boats   Jet drive boats EmptySun Mar 15, 2020 12:56 pm

Depends what your issue is, 

jet drives are very simple tech I’m familiar with a few of the original graphner drives, what’s your issue?

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PostSubject: Re: Jet drive boats   Jet drive boats EmptySun Mar 15, 2020 3:37 pm

Generally it's choice of motors esc etc  I'm going to go brushless which is a whole new world to me so as long as there is some help once I really start this I'll be posting some questions.
There's advice to be had in R/C group's but there's also a lot of contradictory posts and some quite argumentative posts so I'd much rather not to there. pale
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PostSubject: Re: Jet drive boats   Jet drive boats EmptySun Mar 15, 2020 4:14 pm

Jet drives work best on brushless inrunners for their high rpm which you need for the thrust.

An out runner is generally used for high torque applications ( high Pitch air craft props or for boats which require torque on the prop over rpm.

Most general purpose rc boats you can run on an out runner brushless without issue or water cooling, But for high speed applications where rpm is a must then inrunners are the way to go, for jet drives specifically a brushed motor 400/600 sizes run at 16-18000 rpm which is why they were good for jet drives in the old days ( and all we had) today with modern brushless a outrunner can produce a ton of torque but at low rpm which is unsuited to jet drive applications, hence an inrunner is needed.... but .......

Because an Inrunner is the best option you have to take Into consideration the inrunners needs with water cooling both the esc and motor ( cooling jacket) most decent jet drive units have a water cooling input but cheap ones may not. Take this into consideration if you have no cooling input ( and fitting an input on a jet driven boat is a pig) the. You may have to opt for outrunners at the cost of rpm.

This can ofc be offset with the right size battery, using the standard kv calculation...

Motor KV X input voltage= no load rpm.

In your case since they are 28mm units, 16-18,000 rpm is what your aiming for as drive input to the prop, so a brushless motor inrunner or out runner would be required to have....

1500kv rating run at 11.1v (3s)lipo =16.650 rpm to the prop. This setup can be achieved with an outrunner setup so no water cooling needed. But it’s easier to achieve it with a inrunner since they are rated up to 4050+kv in standard 28mm can sizes.


On a side note.... inrunner setups are expensive over out runners, and this is increased also with inrunner esc costs as inrunners tend to run much higher amp ratings, you maybe hard pressed to find water cooled escs suited for 28mm inrunners....

Outrunners given your 28mm unit size maybe the best solution not only for the fact they will work but rpm and torque are achievable easily with the outrunner at reduced cost, if you were running a much larger unit then inrunners would be your only way.

My advice,address your budget, look at what’s required to running specific types of brushless motors ( water cooling etc) and decide what rpm you should be running at, then the choice is made clear your only option will be which units to buy and subsequence costs attached.

A known fact is that outrunners are cheaper and easier to use ( 28mm can motors are drone types so easily available cheap and honestly they don’t go wrong in boats)

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PostSubject: Re: Jet drive boats   Jet drive boats EmptyMon Mar 16, 2020 2:43 pm

All looks like great info from my experience. What do you think Swab? 
I have two boats both using out runners but a big issue currently is obtaining parts sourced from China
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PostSubject: Re: Jet drive boats   Jet drive boats EmptyMon Mar 16, 2020 4:19 pm

This alternative that is preferred by a fellow builder uses 3000kv inrunners powered by 7.2V NiMh sticks.
As Roadrunner said 18,000 rpm is optimal and is achievable with those. 
The pricing on these is very competitive so worth doing a good search but I advise buying from one that says dispatched from  UK

https://www.ebay.co.uk/itm/13T-3000KV-Brushless-Motor-60A-ESC-Program-Card-Combo-For-1-10-RC-Car-Truck/352994278501?hash=item5230193c65ⓂmqX9ABnpV_7uQ9-W256w2jQ

Worth noting is that PROP SHOP do a 3 blade bronze impeller conversion for the Kehrer units
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PostSubject: Re: Jet drive boats   Jet drive boats EmptyMon Mar 16, 2020 9:59 pm

https://hobbyking.com/en_us/watercooled-brushless-inrunner-3660sl-3180kv.html
I've seen this recommended for a twin Kehrer set up and I do trust hobbyking at least so far.
I'm not bad at building and I'm competent at fitting it all in but this is a step up for me so thanks for the advice when I start it again I'll be posting more.
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PostSubject: Re: Jet drive boats   Jet drive boats EmptyMon Mar 16, 2020 11:22 pm

Quite a beast. I use Hobbyking often myself.
I don't use water-cooling anymore. On my first build I did through the ESC but realised it wasn't necessary.
The out runners I adopted are a little over 50 amp rated but they are aerodrives so the rating is based on driving an aircraft prop.
In practice I fitted 25 amp fuses and never managed to blow one with 28mm impellers. 
I use the Turnigy Marine ESC's but don't bother with the pipes.

I look forwards to you revealing your build when you're ready for more Smile


EDIT
I use 4s LiPo 5200 x2
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PostSubject: Re: Jet drive boats   Jet drive boats EmptyTue Mar 17, 2020 12:39 am

It’s I don’t recommend motors In general that I haven’t used, hobby king is a good source for a lot of rc tech but the reason there “competitive” cough cheap... a lot is the cheap chinese knockoffs usually poor constructed esc’s or motors with bad thin windings and bronze bushings over bearings ( and with brushless bearings are a must)

The only couple of brands I personally recommend are overlander and leopard motors as they check all the quality boxes but they do have a price tag attached, most overlander motors 1000kv motors are from £20+ when you hit 1500kv they jump to £35 and the prices go up from there...

Leopard motors are in the high end sports range which I used to use when I raced mono 1, specifically now thy a 4050kv inrunner setup, motors start at £50 and at the time when I raced, a 4050 was closer to £200, I think three vastly cheaper now though!

I will say this when it comes to electrics always buy the best you can afford, I’ve tried cheaper options in the past but they have let me down on a more regular basis, this was more so when I started mono 1 racing and was needing 300 amp escs, frequently blowing them was a common occurrence in the sport, but coughing up 100’s on an esc isn’t a practical option, that said I’ve also used cheaper gear in non racing boats and regretted it as they never last more than 2 seasons, now all my boats are fitted with top quality so no matter how long it’s been when I go to a boat, loan a boat to my old man for a season on the Cornwall tours they just work.

Guess it’s personal preference in my case, but in yours not knowing that brand of jet drive I can’t vouch for it’s quality long term over what I know a grahpner jet drive ( we do see a lot of Chinese knockoffs now) so overpowering the drive could cause you a lot of issues, especially damaging the prop as replacing them can be difficult.

Anyway just my 2 cents on brushless might help you decide.


On a side note I looked at that motor you linked, you say your jet drives are 28mm, the motors linked are 36mm...They may be to big to actually fit to the unit....??? 

28mm are 400 brushes motor sizes
36mm are 540/600 brushes motor sizes

Just be sure to check motor diameter that will fit your jet drives, turn around on delivery and returns from hobby king is slow.

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Last edited by Roadrunner on Tue Mar 17, 2020 12:59 am; edited 1 time in total
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PostSubject: Re: Jet drive boats   Jet drive boats EmptyTue Mar 17, 2020 12:54 am

I bought Kehrer for my present build but before I started I bought these from Hong Kong and sold the KMB drives unused.

The drive in my hand is what I ordered initially but it shocked me that it was nothing like a 28mm drive as advertised. 
Weeks of communication with OceanWorks Victor and bit by bit he created those on my model to my requests. These are continually being improved. I did the prototype instal blog on another forum.

The jet was increased in size to match other 28mm makes
The reverse buckets were re modelled and reflect back at a better angle to the transom
The Transom plates were made square
We made trim tabs and intake grills that Kehrer won't even consider.
New upgraded shaft tubes are currently being tested with ceramic bearings as I type 

Jet drive boats I-LkzQjtt-X3

Jet drive boats I-4wRjD7F-X3
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PostSubject: Re: Jet drive boats   Jet drive boats EmptyTue Mar 17, 2020 1:06 am

https://kehrer-modellbau.de/en/KMB-JET28-Rev.-3/RC-marine-jet-propulsion-drives/KMB-JET28-Rev.-3-including-Riva-Calzoni-like-steering-and-thrust-reversal

If these are the right ones, then I think you have the ability to fit the 36mm sized motors, personally if these are also the right ones I would look at 2200-3600kv on 2or 3s they are quality machines going of the specs nd can take a lot more power. This would be a beast running twins.

Anyway let us know

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PostSubject: Re: Jet drive boats   Jet drive boats EmptyTue Mar 17, 2020 1:06 am

Video of mine 
https://youtu.be/Kjj7nuEvKwE


The very first water trial. Many changes since but not had a re run yet 
https://youtu.be/FtUBJRC0G50
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PostSubject: Re: Jet drive boats   Jet drive boats EmptyTue Mar 17, 2020 3:12 pm

https://kehrer-modellbau.de/en/KMB-JET28-Rev.-3/RC-marine-jet-propulsion-drives/KMB-JET28-Rev.-3-including-KaMeWa-like-steering-and-thrust-reversal
These are mine I got that motor recommendation from the R/C group forum who seem a bit speed obsessed. I just want a good turn of speed and reliability.
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PostSubject: Re: Jet drive boats   Jet drive boats EmptyTue Mar 17, 2020 6:34 pm

I can’t fault the recommended motor you had off the other forum, looks to be fine for the job, but I can’t recommend it’s quality as never used them before.

Jet drives are ment to go fast, so even if you built it over powered and it’s too much to handle just don’t throttle up to the max!

Let us know how you get on!

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PostSubject: Re: Jet drive boats   Jet drive boats EmptyTue Mar 17, 2020 9:46 pm

By the way your installation is absolutely spot on i think I'm in the right place to get this job done.
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PostSubject: Re: Jet drive boats   Jet drive boats EmptyWed Mar 18, 2020 8:01 pm

Thank you. I have hundreds of pictures available on request of both Speedline and OceanWorks 28mm

OceanWorks also sell the intake grills as accessories and they should fit over KMB jets comfortably, trim tabs too I think but I doubt they would work with the KMB buckets. 
I think the OceanWorks transom jet and bucket assembly can be fitted with a KMB drive
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PostSubject: Re: Jet drive boats   Jet drive boats EmptySun Mar 29, 2020 9:19 pm

I've ordered two brushless motor mounts from Kehrer today i will make this my next build along side a really ambitious (for me build).
On another point but still sport boat related i watched the latest 'grand tour' called 'seamen'  and despite the vile graphics it's plastered in i absolutely love Richard Hammonds Scarab is there anything available as I think it's a beautiful shaped boat and I've got a real taste for fast pointy things.
Is this normal planning 3 or four builds in front?
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PostSubject: Re: Jet drive boats   Jet drive boats EmptySun Mar 29, 2020 10:23 pm

The boat Hammond uses is a planing hull, but it’s not a jet driven boat, it’s a Twin or triple outrigger depending on the model ( rudder and prop as one unit similar to that of an outboard, currently known as Z drives) powered of direct inboard motors. The hull itself is relatively straight forward in terms of design, a common place planing hull in the model speed boat community, even fast electics opt for this type of shallow v type hull.

Blistering fast on brushless!

I thought I would have liked Mays wooden river boat going off the adverts when it was seen, but overall I have to say clarksons PRB was by far the most impressive with its twin jet drives. Just a shame about the damage he caused to it... sigh...

Jet drive boats 066ced10

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PostSubject: Re: Jet drive boats   Jet drive boats EmptySun Mar 29, 2020 10:31 pm

And yes you can buy that boat as a model...


https://www.gianhien.net/model/woodpro/all/scarab_31.htm

There is also a “cheaper” Dumas kit called the scarab of similar hull design but uses a single sub prop ( normal arrangement) and rudder as the drive since model z drives are a more recent creation in the model boat world, graphner did the only originals of the z drives before going bust but now they are back I think they maybe back In Production, that said there are several z drive types available mostly for the model boat F1 systems ( more powerful  than the outboards now, and there is a nitro engine ( can’t remember its name of the top of my head built as a dedicated z drive system.

Just googled z drives there are many versions and sizes now in Cornwall models boats website, prices from £15 to £150 depending on size....
 It wouldnt be to hard to pick up a standard fast electric boat hull of similar style and add the overhang and two of the smaller drives of the back, just leaving you with a custom cabin to suit as a freelance build.

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PostSubject: Re: Jet drive boats   Jet drive boats EmptySun Mar 29, 2020 10:41 pm

I've found the ocean works website and there are a few items i want for my boat, I'll stay with the twin Kehrer units as they fit my boat and it's shape. I've also looked at leopard and overlander motors that's a decision yet to be made.
The r/c jet boat board gives quite a few ideas on water cooling from the jet stream which is relatively easy to plumb in which gives me a bit of choice of motors. I'm really tempted in making this a beast Bournville is not a big pool but I'd like to go to give it a run on bigger venues. 
Mine is a shallow vee do you think trim tabs a useful item to add to it?
I want a scarab now!  Very Happy
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PostSubject: Re: Jet drive boats   Jet drive boats EmptySun Mar 29, 2020 10:45 pm

Z drives

https://www.bauer-modelle.com/epages/Bauer_Uwe46269592.sf/en_GB/?ObjectPath=/Shops/Bauer_Uwe46269592/Categories/"Schiffsantriebe%2C%20Spezial-%20und%20Wellenantriebe"/"Z-Antrieb%20für%20Motorboote"#
Not cheap but im told the quality is excellent.
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