Print

Please login or register.
1 Hour 1 Day 1 Week 1 Month Forever
Login with username, password and session length.

[Nige52]

Has anybody ever put a Graupner mini jet drive into an MFA Piranha?
It's just that I have both of these items sitting there waiting to be used and I'm considering doing just that. I am a little concerned that if I do, and cut out the hull to take the drive, it might not be any good and I'll have wasted both items.....

Cheers
Nige

[tjones27]

Not that i am greatly experienced with this, but i think that it would work as the hull of the MFA Piranha is flat, or close to it, so atleast you wouldnt need to do any mods on the hull. It would be very interesting to see if you do it. 

[Roadrunner]

A piranha is a deep v mono so if your fitting a single, central jet unit you will need to make quite a modification to the hull, possible look at ''stepping'' it at the rear, since the jet drive draws water from the base of the hull and pushes it through the unit. I doubt you would need to step the hull but its worth thinking about.
I for one would love to see this build, so got for it, the worst thing is you destroy a £45 piranha, the jet unit can be pulled out if its goes bad sine you shouldn't need to alter the unit.

Besides it being experimental no ones going to know the outcome but that half the fun right!?

[Nige52]

Hmm, the hull is deep V at the prow, but quickly flattens out to almost flat at the stern, without measuring the angle, I would hazard a guess at 10% or less....

Correction, using my Helix school angle thingy, the V is only 5% from flat...

[Roadrunner]

I was under the assumption that the whole hull was a deep V, but it looks like the designers though to shallow it off to keep it more stable at speed, i should see no reason why cutting out a hole in the rear of the hull to take the water inlet will cause any issues, it should be a good little runner, but i would consider looking into self righting just to be on the safe side, i've no idea how the boat will handle with a jet drive!

[Patrick Henry]

Nige...no, please. Tell me you won't...

[Nige52]

Rich, the more I look at it, the more I'm thinking why not........a bit of a waste not using all the bits it came with, but if it runs over a twig/branch, it won't pull the rudder or prop off...

[John Mk2]

Experimentation is the name of the game mate  If it handle's like a pig on roller skate's at least you tried ! But if it's a stormer 

[Nige52]

It seems to power this fishing boat ok 

http://www.youtube.com/watch?v=e9t3imj9QdA

[John Mk2]

This one's a bit of a handful  But it's lighter & narrower 

http://www.youtube.com/watch?v=SxEx3dWlaSE&feature=player_embedded

[essex2visuvesi]

You guys have got me thinking now...... Brushless Jet drive in a vietnamese Aquarama anone?

[John Mk2]

And why not 

[phillnjack2]

ahhhh
now ive got an old piranha and ive got a jet drive

but my jet drive is a big big.

now this just might be the best little recovery boat for me

i think i need to cut the hole out and have a bash

it aint much good for anything else, so i think ill have a go at that

ha ha

phill

[GG]

Not an answer to the question of the Piranha hull being suitable for the Graupner unit but I have recently built a model using the same unit.  This jet unit was clearly intended to fit on a flat and thin section of the hull.  No matter how I tried, a conventional "V" bottom and wood construction always looked uncomfortable.
In the end a simple flat bottomed hull based on the "Skimmer" airboat design was used.  This raised doubts about possible air ingestion into the jet unit and how it would handle at speed.  In fact it was quite successful once it was sorted out.  The handling was akin to an airboat (sliding through turns) and hovercraft ( delay before responding to rudder commands).
A potential weakness could be the small coupling supplied as a high motor speed is needed to obtain good performance.  But, it turned out to be a simple model to build with good performance and several racing together could produce an exciting spectacle in a public display.
And yes, the plans have been sent to Model Boats.
Glynn Guest

[Norseman]

http://www.kehrer-modellbau.de/
I bought 2x 28mm jet drives from them and the servo holders too

Dave

[triumphjon]

i too have been trying to finish off a hull for one of these jet units , albeit a dory hull  about 24 " loa , the hull is of balsa construction coated in thin glass cloth impregnated in polyester resin to keep it strong but remaining lightweight , im going to couple the jet pump to a 1000kv brushless motor running on 11.1 volt lipos  . jon

[Norseman]

I'm told that my jet drives will cavitate beyond 20,000 rpm; what about yours Jon?

Dave