Underwater prop
 

Looking for a bit of advise.
I have a light 40" plastic hull which is crying out to just see some water.
It has been drilled for an underwater prop.
What sort of rpm should i aim for?
obviously less than subsurface props.
I've lots of motors and escs and cells etc.
Aiming for about 30mph.
thanks.
Brod..

RE: Underwater prop
 

Heres a pic. next to my 29" delta type FE.
Brod.

RE: Underwater prop
 

60-80amp 1800-2000 rpm/v should get u in ball park, providin ur usin lipos to keep weight down. Ryan would have more accurate details.

RE: Underwater prop
 

Did you mean 2000kv/20,000 rpm. 2000 rpm would be really super slow.

RE: Underwater prop
 

your kidding right?

RE: Underwater prop
 

Thanks,looking for prop speeds to aim for,roughly.
I can then work out which motor.kv and cell count i can use..
Brod..

RE: Underwater prop
 

Hi Brodjack,

That hull looks a lot like the Graupner Giant Racer https://shop.graupner.de/webuerp/ser...2025.200&PIC=4 , now discontinued.

In order to get that hull up to speed, you'll better do some work on the hull; sharpen all training edges on the step and transom, stiffen the hull internally, as the Giant Racer hulls I have seen, were way to flexible to run reliable at the speed you're aiming for.

For a submerged drive, the 20.000rpm at the prop are a good starting point.

Motorwise you could look in the KB45 series from Hobbycity, read the customers comments to get an idea how these motor perform: http://www.hobbycity.com/hobbycity/s...hless_Inrunner

Regards, Jan.

RE: Underwater prop
 

Thanks Jan,i was hoping you'd reply.
Always good solid advise..
Reckon that big motor would push it well.
Brod..

RE: Underwater prop
 

You would be much better off closing the hole in the bottom, and using a descent hardware setup and a surface drive. It would be MUCH easier to get the speeds you are looking for that way.

RE: Underwater prop
 

That would be three holes to close, as the hull originally has three 540, each turning a prop, running on 8-10 Sub-C, if I'm not mistaken, with a mechanical two step switch...

Basically a surface piercing drive is faster, the work needed on the hull is about the same, so the choice is up to Brodjack.

Regards, Jan.

RE: Underwater prop
 

Yes,spot on Jan,3 holes,well a bit more than holes,sortof moulded plastic underside holes.
Thats why i thought i'd keep it simple with a submerged prop.
It oriiginaly,a few yrs back,had 3- 600 size? motors and a very poor electonic speed control which didnt last long.We ripped out the electrics,the middle motor too, and fitted 2 esc's which ran very hot with 10 cells.The boat actually ran quite well,looked good too.But 2 10 cell GP 3300's didnt last long maybe about 4-5 mins.
I thought a single large brushless with a decent maybe 6s lipo would be much lighter and therefore quicker.
I'll see what my LHS has in with couplings for a 6mm motor shaft.
Is there such a thing as submerged metal props?..
I'll probably send for that big motor that was highlighted.
This uk store dosnt stock the right KV for me.
http://www.giantcod.co.uk/inrunner-b...bf94ae0fdb7106

I'm trying to keep this cheap hull a cheap build;)

Brod..

RE: Underwater prop
 

Just found an old pic of it,remember my very first boat[X(] complete with a margarine tub for radio box!
The boat surprisingly stayed dry inside.
I think i can do better now:eek:
Brod..

RE: Underwater prop
 

Keep in mind these KB motors are rather amphungry, so a 'cheap' 6S Lipo is out of the question.
Also watercooling is a must, not a tiny aluminum coil around the can, but a fullsize coolingjacket, covering the entire can.

These can be bought, or home made, I have a 'tutorial' and some pics, showing how I go about it.

I wouldn't go for metal props as yet, find the right size and pitch by trying some Graupner K-series twobladed props, I know, these are officially surface piercing props, but I've been getting good results with them, as they are much stiffer than the plastic ones.
Start with something in the 37,5K, 40,5K size and work your way up, keeping a close eye on the components temperatures.

J.M. has a valid point in the surfacepiercing suggestion; as the hull is stepped, it will draw air under the boat.
As the suggested powerplant will allow the boat to run a lot faster than stock, the aircushion under the hull may become a problem when it reaches the prop and rudder.

If you use an K-series prop, the propulsion may not even suffer a lot, unless there's a massive amount of air drawn underneath, but the steering will.

Some time ago, I've been running a 30" stepped hull as a motor testbed, with a subsurface prop (Ø 40mm). On 12 cells and a 700 motor the speed was quite ludicrous, and the steps worked fine and left my rudder standing in a pocket of foam, behind the prop. My input from the transmitter was ignored completely as the boat raced towards a brick wall..., only a substantual reduction in speed, dropping the hull off the plane, gave me back steering control, just in time to avoid a collision.
Quite a heartstopping experience...

I expect your 'rather distinctive' step will have the same effect, so beware. (My testhull was polyester, quite solid, your ABS hull will disintegrate on impact...)

Making it a surface piercing drive would avoid such steering issues. (but would require another KV, aim for 30.000 rpm for such a drive)
This one would actually suit: Keda KB45-11L 1649kV 70A Brushless Inrunner, at 85% efficiency on 6S would be around 30.000 at the prop.

Regards, Jan.

RE: Underwater prop
 

Thanks again Jan,very good info.
Brod..

RE: Underwater prop
 

Jan, Are you suggesting a single L for this hull?

It'll barely move.

At LEAST an XL, preferably two.

If you want some speed out of this big hull with a surface setup, you will need one of these:
http://www.offshoreelectrics.com/pro...?prod=neu-1527
or these:
http://www.offshoreelectrics.com/pro...?prod=neu-2215

But I don't know how this hull would do with that much power.


The first step in making this hull go fast would be to take EVERYTHING out of it, including the "inner hull" and coating the bottom in a few layers of fiberglass on the inside. If you do this while you plug up the holes in the bottom, it can all be done in one simple step.

RE: Underwater prop
 

Hi J.M.,

Not knowing what budget Brod has in mind, I took the largest budget motorseries I know of, the link was only to point out the website.

If the set-up is kept light, these motor(s) will make it go at least twice as 'fast' as with the stock set-up.

I deliberately did not mention the Plettenberg/Lehner/Neu motors, as they are not only very good, but, in my opinion, too expensive for such a project.

Brodjack has his 'ballpark' figures, now it's up to him.

Regards, Jan.

RE: Underwater prop
 

Those are EXPENSIVE!
Look at these:
http://www.hobbycity.com/hobbycity/s...hless_Inrunner

RE: Underwater prop
 

Yes,they seem to get a good rating by users too.
Thats equivelant to about £20 in our money,almost free!!
I've never bought these cheap motors for boats,helis and planes before,sticking to mainly Megas,Neu's, Axis,and Kontronics,which are all running still like new.
This boat doesnt really deserve an expensive motor,it will probably only get one pack usage per visit,i mainly run gas stuff at the lake.
Trying to keep it cheap.

I have a spare 1/4" outdrive unit and flexshaft.But might be a bit big for the motors linked?
http://www.prestwich.ndirect.co.uk/wpesf2.jpg

Brod..

RE: Underwater prop
 

It is large...

Unless you decide to run twin 540XL's through a gear drive to one BIG prop that won't have to spin very fast to get that hull moving.

You might have a nice selection of props if you say you run gas...

Here's a good unit:
(in the middle of the page)
http://www.drcwebservices.com/ffe/hardware.htm

RE: Underwater prop
 

A KB45 L is a lot larger than a 540 XL.

Ryan

RE: Underwater prop
 

Diameter, yes.

A KB45XL is 77mm in length, a Feigao 540XL is 75mm in length.

No, they aren't nearly equal in power, but it is easier to find mounts for a 540XL than a 45L.

RE: Underwater prop
 

Finding a fitting mount should not be a criterium for selecting a motor..., we are modelbuilders after all, aren't we?

Regards, Jan.