Aeronaut Spitfire

[Peter Morris 5Participant @petermorris5]

After a summer of relative isolation, I finally got to test the Spitfire on the water last week. Unfortunately, despite following (and double checking) the Aeronaut instructions and the excellent article by Ernie Lazenby in MB Winter 2018 Special Edition, I cannot get the boat to 'get over the hump' and up onto a plane. The balance point (130mm from the stern) is correct and I have set the Hobbyking outboard motor both flat and tilted back, but it just cavitates a lot and refuses to get up onto a plane. The prop is the standard one supplied with the motor. I wonder whether any reader has built and sailed the Spitfire successfully and is able to provide the benefit of their experience?

[Peter Morris 5Participant @petermorris5]

[harry smith 1Participant @harrysmith1]

Hi Peter

Try lowering the outboard, so the prop can catch more water.

Also check U Tube, you may see something that they have setup.

Which Hobbyking outboard is it ???

Harry Smith

[Ray Wood 3Participant @raywood3]

Hi Peter,

As I was with you yesterday at our lake, I agree with Harry get the outboard as low as you can with the steering cables still working ! I know you have a brass weight on the nose to get the c of g right but that must lift the stern & prop up relatively, it certainly sounds fast when it cavitates 😀

Regards Ray

[Peter Morris 5Participant @petermorris5]

Hi Harry

The Spitfire's powered by the Hobbyking 'CNC Aluminium Outboard' which has a 2630kV motor with a 30mm prop. Unfortunately, vertical adjustment of the motor is not catered for either on the motor itself or the boat – in fact the Aeronaut instructions stress that the centre line of the prop should align with the hull bottom at the stern. I am therefore loathe to carry out structural alterations at this stage! There are three criteria to be met – the alignment of the prop (as described), the balance of the boat at 130mm forward of the stern (this achieved by positioning of the battery etc) and the angle of inclination of the drive (starting with a neutral position, i.e. in horizontal alignment with the underside of the hull). There are some great videos of Spitfires on YouTube (all of which appear to perform really well) but none show setting up or trimming although the static positions of the boats and the way they sit in the water does seem to correspond with mine. This leads me to believe that the problem is more related to the prop, motor or perhaps battery (although I am using the recommended 2S Li-Po). Thanks for your interest Harry.

[Chris FellowsParticipant @chrisfellows72943]

Hi Peter

Interesting what you say about the alignment of the prop. When I was researching the position of the stern-drive for my Huntress build I read that the prop, or most of it, should be positioned below the bottom of the hull so that it gets a good, clean flow of water.

Yours probably isn't cavitating but aerating (I think that's the term) resulting from sucking air from the surface. The prop is effectively running in air bubbles and isn't providing enough drive to get the boat on the plane and if it did the problem would be worse as a significant part of the prop would be out of the water.

I'll have a look on YouTube to see what the boat looks like.

Chris

[Chris FellowsParticipant @chrisfellows72943]

Having had a look I see that the transom is very shallow and so the prop is near the surface of the water. Even the boats going well generate a lot of white water from the prop once on the plane but not too much when getting there.

It's probably pretty borderline as to whether the boat works or not and as has been said by Harry and Ray you need to get the prop lower. I'd try it without the bow weight as the stern will come up when on the plane.

The kV of the motor and a small prop and a shallow draught is a recipe for generating a lot of aeration and no go!

Chris

[ashley needhamParticipant @ashleyneedham69188]

I would have expected a larger prop at that Kv for essentially a surface piercing design (as indicated by the instructions saying that the centreline of the prop should be lined up with the stern bottom)

My pontoon boat runs a massive x50 prop on a 28mm motor and it won’t run on anything smaller due to rampant cavitation. Bear in mind that with a 30mm prop on the bottom, only 15mm of it will be in the water at any one time.

Have seen several boats flounder because the props were unable to grip sufficient water to overcome the mass of the boat due to too small a prop and overspeeding.

Is the outboard as specified or is there no specific outboard specified (that’s a lot it specs)

Ashley

[Peter Morris 5Participant @petermorris5]

My thanks to you Chris and Ashley – I think that you have come up with the answer! As it happens, I have a 47mm hydro prop in my 'spares' box and I shall be giving that a try at the next opportunity. I will report back….. Aeronaut recommend the use of their Aqua Race 60 (3000kV) outboard motor with a 29-31mm prop, but Ernie Lazenby had used the Hobbyking motors (successfully) in the boats in his article.

Peter

[ashley needhamParticipant @ashleyneedham69188]

Peter. A couple of mm here and there, a tiny variation in weight and c of g….it all adds up. Surprised there is not sufficient vertical motor adjustment.

Ashley

[harry smith 1Participant @harrysmith1]

Hi Peter

The hydro prop may work, but add power on slowly at low speed.

I had the old Hobbyking outboard of the F1 tunnel hull.

The outboard was on a deep vee and set as above.

Once set right to boat sat up on the prop in a straight line !!!

Harry

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