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[swiftdoc]

Kiyo, what material did you use to get the bellow watertight at the transom? Is that black silicone? I am asking as I still have to do that on my build. Yours is looking good

[Backerther]

Arno to be contacted later
so sorry

[Backerther]

Hi Arno,
I'm awfully sorry to have kept you waiting so long due to my autumn trip to Okinawa located about 1250 miles southwest
 of Tokyo where I live. It was about 30 degrees C still now in the region and nice place to stay for autumn vacation.

By the way, the material I am using for watertight purpose around the steering rod is truly black silicone rubber originally intended for dust covers for shock absorbers of my 1/8 RC Jeep.
 I have diverted the item to the watertight part this time.
But as you may know, similar item for the watertight purpose of the RC boats is available easily in the internet dealers.
　　

[swiftdoc]

Hi Kiyo, many thanks for your reply. I think we have quite the same approach. I will use black silicone in order to fill the gap between transom and bellow.


A trip to sunny Osaka, that really sounds promising, a nice vacation in order to gain new energy for your build

[Backerther]

Yes, indeed. but I spent the vacation in Okinawa (southern islands of Japan) and not in Osaka...Arno.!


By the way, I made an intensive calking procedure all around the jet drive thrusters to prevent the water ingress as seen below
and finally set the radio gears in the hull only for testing the jet drive function and cooling water sucking function as well as water ingress. Especially, last two functions are fatally important for a jet thrusting fast boat like this MTB to be sailed on the lake.

Pic1 is before calking.
Pic7 is what I have used for calking which is one of major manufacturers of Japan.
Last pic is the location of Okinawa where is very far away from Tokyo as if it was a foreign country in its distance.

[swiftdoc]

Many thanks for your reply, Kiyo! And please excuse me for not writing Okinawa, I sent the post after a hard day‘s work yesterday

[Backerther]

Oh, No, Arno. Never mind at all. !!!
Even I do not have much geographical knowledge of your country.....

from Kiyo in Okinawa, semitropical and heavenly islands of this world!

[swiftdoc]

Simply wonderful

[Backerther]

Thanks Arno for your always friendly comment.!!


I tested the jet pump units in the bathtub with these setup and a good impression for a fast boat fundamentally.
However I don't know and am not confident that this result should be really Ok for a fast runner to sail on our lake...
As the initial impression of the jet flow, it looked sufficiently OK! but...but...but....??? something a bit unsatisfactory to me when I imagine the boat is run as a fast boat on the lake......If this boat was run with a brushless motor, it could produce much more
powerful jet BLAST-like water flow...?? Well then, change the brushed motors to brushless motors for potentially advantageous power source for the jet boat.!
 
Pic1 shows Tamiya DMD by which I tried so-called tank type steering with the present brushed motors.
Worked well except on max. continuous speed run over around 1minute. Auto-cut function did a good job.!!

[swiftdoc]

I am using brushless outrunners, Kiyo. No need for water cooling in my case. After the bath tub test I changed from 2S to 3S Lipo batteries.
I have ordered super x from an ebay seller in Tokyo   I like that it is without solvent and will give it a try.

[Backerther]

I placed an order of inrunner brushless motors for my application to be water-cooled.
The motor is 4000KV and 50000RPM max with no-load so that water-cooling is absolutely indispensable.
Therefore, I think 2S lipo will do sufficiently for my jet propulsion MTB perhaps.
3S lipo will cause the MTB to fly a bit over water-surface just like a flying fish..???


"Cemedine Super X" is really nice for a water-tight application and I have been using this one for many years.
 
Pics below are the brief test with 2S lipo at 2/3 throttle. I was feared to increase more speed due to its jet flow...
Prior to this test, I used 3S lipo....Oh my Goodness! It's like really the jet "blast" against bathtub wall.
But the motor was kept cold throughout the test thanks to water-cooling.!  

This time I made water-cooling coils by myself to meet my requirement to get the motor cool intensively.
This is because the coil became longer(number of turns, around 7) around the motor can than those being sold in the market.

[Backerther]

While waiting for the brushless motors to arrive, I set a pair of bilge keels as shown below.
These bilge keels actually and prominently worked very well in my second PT-109 on the large lake at high speed quick turns.

[Backerther]

A practical RC boat does need a good serviceability and I cut the deck into two for maximum and full access into the hull
as seen below.
"No good serviceability, no good RC boat sailing" is my basis  to enjoy RC boating on the spot and in my work shop.!!
The front deck is supposed to fix to the hull, needless to say.

[Backerther]

I decided not to glue these three "hatches" to the deck for regular checking of steering servo and motor couplings before sailing
at the boating spot. The gun mount and engine ventilators ? are positioned conveniently for this RC jet model maintenance,
aren't they?

 

[Backerther]

The brushless motors arrived here at last and I made a couple of hand-wound water cooling coils to attach to the motors.
So far as I made the coils by myself, I made them symmetrical in their inlet and outlet directions each other unlike ready -made ones having been sold in the hobby market.   I wonder non-famous and cheap Chinese-made motors should really work
properly in the hull to sail on the lake for over 5 minutes at speed in each time of sailing...???

These two motors never work properly for over several minutes at MTB like speed without water cooling, and will surely be blown off unless properly water-cooled.
Overall length of each coil will reach about 21" which will be possibly enough to cool the brushless motor.!

[derekwarner]

Kiyo....


I think the mirror reverse inlet/outlet of your wound cooling coil will pay benefits in optimizing/minimizing cooling tube length/mass...[and that's how full size manufacturers would have constructed them] 


I understand your natural concern optimizing the heat transfer [cooling] especially considering the use of higher output brushless motors


When you think about the physical design, we have single point contact between the spiral OD of the individual spiral tube set and the motor outer shell.......all of the heat generated by the motor that does not touch this single spiral point in the tube coil, is wasted heating the air between the next coils in the tube set [in this case, air is a poor conductor and thermally inefficient]


A relatively inexpensive trial would be to use a thermally conductive paste spread fully into the coiled spiral airspace to the inner tube coil set, before you slide the tube set over the motor shell


These products are designed for the electronics industry to conduct heat from components to lower the operational temperature 


A trial using such a paste would only add maybe 10 to 15 grammes extra weight to the vessel displacement, and maybe $20 total for two motor coils


Let me know what you think


Derek
-----------
 
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PS....I would only use a Registered Named Heat Transfer Paste, and stay away for e-bay [Asian no name goop] 

[Backerther]

Derek,


Thank you for a kind and professional comment on heat transfer of the water -cooling coils I made.
Yes, indeed , what you have mentioned is surely true and reasonable for an effective cooling of the motor cans, as well as
 is ideal and better to do.  However, as you know a brass (tube) itself is also so good a material to conduct the heat and I made
the coils to get them to touch as much surface of the motor cans as possible to contact the water inside the coil tubes.
Even this level of craftmanship will I suppose do a good job from my experience of brushed motor boats in the past.
This style of cooling is not the best, but necessary and sufficient style for this PT-109(JP).
I have learned a new knowledge that you suggested to use thermally conductive paste.
I will keep it in mind   .!!!

The pick of steel tank (98 pounds) is equipped with a cooling water coils and a radiator to run in the summer.
All the drive system is fully covered with steel which becomes toooo hot in mid-summer. Only forced water cooling system
could save this tank in summer run.       Just an appendix!!!!


Thanks Derek.

[JimG]

Perhaps they are too small but did you not try to get inrunners with a waterjacket for cooling. I generally don't use inrunners as my main use for electrics is aircraft but I do have 2 fast electric models where the motors have waterjackets for cooling.

Jim

[Backerther]

Hi JimG,
Thank you for always constructive comment.!!
I selected my inrunner motors basedon the some factors.
They are the size of diameter, price, comparatively better heat dissipation and sufficient KV value for this size of boat.
Moreover, there is no need for me to choose so powerful brushless motors for my 29.5 " light weight pla-model boat.
Even my motor could produce about 0.55 Hp each and two are equipped in my boat.

The pics below are marine-application brushless motors with water-jackets in my fleet. But too powerful and no adequate(smaller) ones could be found in the market when studying what to choose.
Above all, inexpensive for the quality is main reason this time for this experimental pla-model boat.!!




 

[Backerther]

Well then, here is a post for today.
I could reach a final setup with the brushless motor basis at last. !!!!
Chinese-made brushless motors and ESCs as well as lipo in the US-made hull are largely expected to run powerfully by which the PT-109(JP) is able to sail like a modernised MTB with the jet propulsion...!!??

[dodgy geezer]

I wonder if steering might be an issue?
My PT boats are light and overpowered with a brushless  (which is a good idea because with not much load they run cool and don't have any cooling issues)
However, the hull form will cant almost 90 deg if you do a sharp turn at speed. I cut back the rudder to minimise this, and suffered some low speed lack of steering authority.  I'm not sure how a jet boat will behave over a wide speed range - the hull is not a racing flattie....

[Backerther]

Hi dodgy geezer,
I'm so glad to have your kind comment and information of your PT boats.
Steering angle of nozzles of my PT-109(JP) does not look sufficient at a glance, however a boat is inclined to turn easily and
sharply at speed. Moreover,I haven't normally tried to turn sharply in order to avoid an upset so far.
See the video below for reference.


In addition to the above, I set a pair of bilge keels to reduce risky cant, which I have felt is largely effective as in the video below.
Anyhow, everything will be made clear in the maiden voyage to be held within the month...
I wonder she could really run well enough as my first jet driven MTB....???


Lindberg 1/32 PT109 sailing test resulted in success in the third trial run.

[Backerther]

Jet thrusting power test was done in the bathtub for close to 10 minutes in 3 times of different RPMs of the brushless motors.
I have gotten the motors to run 2/3 throttle in the final test starting from the idling basis.
The most anxious was the water ingress and motor cooling in this bathtub test. But successfully done about the points.
With this test I have felt that this PT-109(JP) could be worth actual sailing test on our local lake.
Without the actual test run on the lake, I can do nothing any more to know if the boat could really run like a water jetted MTB
able to run on the lake.   To tell the truth, I am not confident that she could perform a nice job like MTB because of no experience to build or to modify at all so far.....  I wonder how she should behave in the lake test....??  Bow up?  speed?
nice and stable turn?   overheat by "continuous" high speed run..??  mechanical trouble leading to a sudden stop far away
from me??   No experience and few knowledge do do do get me fear the test on the lake....


Pics; Can you identify the difference of water force of drain between the two pics? (No2 and No4) ?
        Pic No1 shows 2/3 throttle.  More power? Oh no, I was scared with the jetted force even with this level at this distance.

[Backerther]

Primarily bath test was done successfully at the 2/3 throttle with which I was satisfied to some extent.
Well then, do I go to the painting work as I am eager for an initial trial run within this month to get her commissioned within the year since this is an autumn project..??   And it will be colder for the actual test...
For the above reason, I painted the hull and deck in a hurry.

[Backerther]

Fundamental painting was done all over the boat and she had better be weathered a bit.!!
Since she just does look like so-called a "pla-model" unless weathered.!!!???