John Cobb’s Crusader

[Paul TParticipant @pault84577]

Wooooosh!

I presume this is a technical term but I cant locate it in any journal, the only reference that I can find goes back to a saying from a long lost southern tribe of beaker people which, roughly translated means 'wow look at that'.

[ashley needhamParticipant @ashleyneedham69188]

Wooosh! Is a modern Bushy park boaters term differing to the Beaker version for anything well fast, the longer the oooo, the faster the boat. I fully expect them to run out of breath when I try the Crusader…🚀

the beaker people were so named as they drank a lot of strong liquor. Consequently anything they DID manage to see deserved a “Whoosh!” Note correct spelling in this case.

Ashley

[Trevor HollowayParticipant @trevorholloway99134]

And as we know the amount of "woosh" you get out of a propulsion system is in direct relation to the amount of "Ooomph" you put in.

[Paul TParticipant @pault84577]

Unfortunately the quantity of Ooomph that I use to realise enough Woooosh usually ends in a FIZZ and BANG.

I am finding it increasingly apparent that mixing high octane fuel with a spark is resulting in an explosion, I am sure that this discovery could have huge implications in the scientific world and so I propose to write a paper about it.

Paul

Now where did I put that crayon

Ashley

The term strong liquor can not be applied to any liquid south of Watford Gap, for example Watneys Red Barrel being described as Beer is a gross misrepresentation of facts.

Edited By Paul T on 08/06/2020 16:21:34

[ashley needhamParticipant @ashleyneedham69188]

Paul. You have obviously never tasted "Molesey lights-out". A beer so strong it has to be consumed with a spoon, as it has so much body.

I thought there was only fermented Haggis juice that far north??

Ashley

[Paul TParticipant @pault84577]

Ashley

We have beers that are more than 1% proof, we have tasted Molesey lights-out but we know it better as Manchester tapwater.

Up here int' north we all suspect that southerners are soft but drinking beer with a spoon? that is so sad, do you take your own spoons or do the local councils provide them.

Whilst there is nothing wrong with 40% proof Haggis juice (that we call single malt) we prefer our northern beers, so strong we drink it out of pint glasses with no spoons in sight, proper beer and people who aren't afraid to drink it.

Paul

[ashley needhamParticipant @ashleyneedham69188]

First test:

talk about Bluebird first, as also edf powered.

Tried out on 2s Lipo and went really well. The 64mm edf producing great amounts of whoosh, surpassing expectations actually (on 2s). Not too much water splashed about. The sound of the edf was to die for, exactly the right high pitched whistle/woosh sound. Turning slowly around at the far side of the pond, the sound and the water being kicked up by the jet was simply captivating. It looked for all the world like the Coniston videos. Pushing the throttle forward the boat zoomed towards us nice and flat (battery well forward) andattained a good planing speed.

Recovering the boat and fitting the 3s battery I motored out and all of a sudden the boat took a nose dive. I thought perhaps I twitched the throttle or something, and continued. It was apparent the boat was not going as well despite the increase on power and whilst limping back, conked out. Back on shore it was full of water and in retrospect I think it hit one of the huge carp wot are in the pond mating, and hence the nosedive.in hindsight I should have bought it straight back, but there we are. I had forgotten to put the internal cover in place that may have prevented that, but simply forgot as the boat has sat on the shelf for at least 2 months or more.

After drying my tears I put the 2s in the Cobb. First off it is obvious that the smaller 50mm unit produces much less thrust than the 64mm one, despite the not too dissimilar claims on the specs, although these figures are for 3s operation. Anyway, motored out to find it would hardly steer, it would go right, a bit, but not left. This of course bears out other bods build experiences. I also noted that it was steering opposite to stick commands?. So switched the right stick over so the rudder went over to the last on right stick etc, and lowered the rudder. This made the thing steer although not very well. I opened the throttle but it was very sluggish. The nose stayed up ok (an issue with other modelsseems to have been difficulty in getting the nose to rise)  but there was insufficient thrust to get the back op. It wallowed and then conked out, water everywhere.

The Bluebird will fly exactly as planned with a 3s in mid position and the internal cover in place, I have no doubts.

The Cobb is going to need full power to rise up and sit on the sponsons and that’s just going to have to be a leap of faith. Shut eyes and whack the throttle open. It does not help that the cabin is a mess of wires as I don’t know where everything needs to be until it performs properly and this is I am sure impeding air into the fan.

it may be that my decision to use the smaller 50mm unit will bite me in the arse regarding available power?

As I see it,the way forward is to tuck everything in with the 3s and try and waterproof the esc and receiver with plastic bags (temporary, obviously I can’t keep the esc wrapped v

If it won’t rise on full power I will be a bit perplexed, but the other Cobb I saw on the pond belonging to Iain also took ages to plane.

Ashley

Edited By ashley needham on 16/07/2020 11:26:41

Edited By ashley needham on 16/07/2020 11:28:21

[Paul TParticipant @pault84577]

Hi Ashley

Your problem with the Cobb is simply down to power as this hull design requires a lot of grunt to get it out of the water, once the surface adhesion has been broken the Cobb needs less power to keep it on the plane. There is a thrust to weight calculation but it is very complex as it involves water & air temperature, water surface adhesion and humidity

You can reduce the water adhesion by getting the 'wet' areas of the hull and sponsons as smooth as possible then apply a silicone polish, reduce the weight of the model as much as you can and try pushing more power through the 50mm unit or go back to the 64

Back when I designed my Cobb I had the advantage of jet power as opposed to EDF, with masses of thrust the problem was stopping the boat from becoming airborne.

The Bluebirds design is totally different and requires far less power.

The rudder is next to useless so I added two more on the back of the sponsons.

Hope this makes sense

Paul

[ashley needhamParticipant @ashleyneedham69188]

AThanks Paul. At this size, putting rudders in the sponsons is a no go. If push comes to shove I might be able to put a “rudder on a stick” out the back.

It was sort of steering, and I was going slowly so I wouldn’t expect too much anyway.

Thinking about it, The reverse stick thing seems to be like car steering, when you turn right in a car, the wheel turns clockwise from the wheel centre, if you were on a boat the rudder turns anti clock from centre. The forward rudder is obviously behaving like a front wheel….? I think I will make a deeper, longer rudder.

I will have a static play with the wattmeter to see the effect of all the wires in the inlet to see if I am doing myself in…power wise. I have a 50A esc which is quite big and the wires do not want to bend much. My meter will show me how much current I am drawing, so perhaps a slightly smaller less powerful esc can be fitted and ease the space dilemma.

Ashley

 

Edited By ashley needham on 17/07/2020 15:45:18

[HowardParticipant @howard79726]

With an interest since my teenage years in Speed Record breakers (water & Land) and have been following your Crusader project. Although lured to the 'dark art' of International One Metre yachts in recent years, I would now like to build an RC model of Bluebird K4, Malcolm and later Donald Campbells' forerunner to the jet-powered K7 but I know of no accurate drawings or suitable references. I could make my own Vectorworks drawings from a reasonable general arrangement drawing with outline dimensions. Following on from the subject of this thread I was wondering if anyone could point me in the right direction – I think a couple of companies used to make kits or models of the boat, but sadly no longer. I'd appreciate any help.

[Chris FellowsParticipant @chrisfellows72943]

There's a good book on Ebay at the moment.

Chris

[ashley needhamParticipant @ashleyneedham69188]

Sorry chaps but stuff has got in the way of much high speed testing !

Bluebird is running really well on the new 54000kv motor on 2s, battery rearwards, it shows no tendancy to lift…it just goes very fast a Colin can corroborate.

I have left the Cobb to concentrate on Bluebird, but what with some weed, wind and now cold weather, testing will have to resume in the spring.

Unfortunately although i did a drawing for K4, it is not scale. It was always going to be an 'easy' build so has been simplified and the weight has been kept very low, as anyone who has hefted it can attest. Working edf powered craft cannot be true scale as the original jet inlets are far too small….so.

As per the b/w snap on "TODAYS BOATING", it does not stop you producing the goods as a stand off scale model!

Ashley

[carl brothertonParticipant @carlbrotherton75833]

Ashly, I have lost track, how long is your Crusader?

[ashley needhamParticipant @ashleyneedham69188]

About 550mm long and 140 ish wide (from memory as I sit at the pc.)

Ashley

[carl brothertonParticipant @carlbrotherton75833]

I have been considering the practicality of modifying my own Crusader. I love the idea of DF, although had issues with would it provide enough thrust. From following the TN small jet series in RCM&E, I suspect that the fan that he has used, just might.

I came to the conclusion that my 480 long version could just about accommodate a 50mm fan unit. The unit would be right at the back, almost, The triangular rear end would need to be opened right out.

The real problem was creating an inlet that approximated to about 1900 mm^2.

What has really killed the idea for me, is that the motor would need a 3s Lipo with a high C rating, with a high enough capacity that the run would be above a few secs. Then leaving the boat stranded without power. Such a Lipo would I guess be to heavy for my model, it may either sink, or never get onto the plane.

I am beginning to think that the K7 Bluebird is a better bet. The intakes are quite big to start with, and possibly could be enlarged without to much really obvious deviation. Also the forward sponsons, probably are where the buoyancy is needed to carry a Lipo. Both the forward and rear planing surfaces appear to be quite generous. Which might indicate that getting the CG right may not be as critical, as with the Crusader with its limited forward planing area.

Edited By carl brotherton on 02/12/2020 15:09:12

[ashley needhamParticipant @ashleyneedham69188]

The Bluebird was developed on the back of the data from the Cobb boat, hence the different layout.

My K7 has the battery at the rear….reports of nose lifting, at least on mine, seem not to apply. HOWEVER, it only takes a few differences to give a totally different hydrodynamic performance.

I don`t think you can use an edf without substantially altering the forward area. Jets appear to be a different matter. The inlet area for an edf has to at least equal the area of the fan unit or you will lose power. I have a 2200 mAhr 2s Lipo on mine and it lasts for a good few fast runs before starting to bleep.

Weight is the key…there is nothing of mine, it is very light.   My Cobb weighs 720g including 3s Lipo.

Ashley

 

Edited By ashley needham on 02/12/2020 18:16:10

[John W EParticipant @johnwe]

hi everyone

I realise this is a very old topic.   However, did Paul T ever produce a set of plans for John Cobb’s Crusader?  Does Paul T ever appear on this Forum these days?   Ashley, how did your Crusader end up performing?  All of these questions are because I am completely rebuilding my Crusader model.   Model number 1 suffered so many faults and distortions through being baptized at the lake many times – and it sucked in so much water through the air intakes and flooded the internals.   So, as I say, I am completely rebuilding a new model and doing a lot of research – so – this is how I came across this thread – tally ho old boy – here we go.

John

[ashley needhamParticipant @ashleyneedham69188]

John. Only just trawled through the forum to see this.

Basically I didn’t get it to go. It was very wet and I didn’t have enough power

I could shoehorn a larger edf in but couldn’t be bothered. The boat would still be very wet, and there’s no way to stop water getting in the hull due to the air inlet requirement.

I am keeping it for the moment as it’s in a box I might use but basically if anyone wants it they can have it for free if anyone wants a challenge. Snaps on Today’s Boating. I can supply pics and measurements if interested

Ashley

[ashley needhamParticipant @ashleyneedham69188]

[John W EParticipant @johnwe]

hi Ashley,

Personally, I would hang on to the old Crusader – because – someday you may just think Ah, I will have another go at it.  My number 1 Crusader at this present moment is living in the garage, minus all of its internals. Like yours, it was more of a torpedo, shall we say, than a speed boat – even though I ran it on a 90 mm EDF and 4 x batteries producing 12s – so I could distribute the weight around the hull, still though, it never worked.   What I have learned though, through research, is that the ‘real’ Crusader had similar problems of dragging as they call it ‘the rear end’ was to actually wrap ropes around the hull and around the out riggers to try and break up the water resistance under the hull (water suction).   When I first saw these photographs, I thought it was something to do with lifting the hull out of the water – but in reality it was to do with something else.

The other thing I have learnt is the weight to power ratio – on the real Crusader it was something like 1.3 to 1  and on the new one I am making now, I am hoping to get all up weight of the hull something like 4.5 lb with a thrust of something like 8 lb (hopefully) on 8 s batteries.

I know on the ‘old’ model of mine the hull weight itself was well over 6 lb before I even had fitted the batteries and fan.

I know at the moment I have spent more time building fixtures and jigs to keep everything square and right in the hull to prevent twisting.   I am building this one from 2mm lite ply and balsa wood.

John

[John W EParticipant @johnwe]

just a topic update …. the first Crusader which I built, turned out to be far too heavy, so, I built a new one.  This one is slightly smaller, hence weighs half the weight of the original one I made.   This ones hull  & motor work out at about 4 – 5 lb with batteries say about 5 1/2 lbs.  So, we took the Crusader for a test about a fortnight ago – that test turned out quite disastrous as I had the centre of gravity too far forward – she turned out to be a bit like a submarine.  I returned home and literally dug out an area underneath the fan housing so I could locate the batteries there and further aft – so I could move the centre of gravity as far aft as possible.

This morning, back down to the lake – running on 6s batteries x 2 with a 90 mm 12 blade fan.   It was a marvelous improvement nearly up on the plane and working as she should do.    I replaced the batteries from 6 s and upgraded them to 8 s – in other words, more power….and it worked really too well – the rudder was extremely sensitive and she did as the real one did – she took off – literally airborne – skewed to one side and flipped over – and hey ducted fans do no work under water.   Some club members rescued the model for me.    Everything seems to work and I will give it more time to dry out.

Then we will do more tests in the warmer weather.

Two images which came from my son’s mobile phone – one of them is basically where she is just flipping over – because once she flipped over it was all hell let loose 🙂 the rest of the mobile phone images were of the ground.

 

[ashley needhamParticipant @ashleyneedham69188]

Well done son! Persistence has paid off. Great work. Only one snap but it’s on the money.

I have room at the moment so will keep mine until it just has to go. As you say, if it’s there i.might revisit it in the future.

Ashley

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