John Cobb’s Crusader

[ashley needhamParticipant @ashleyneedham69188]

Cobb Motor installation showing the sled that the motor cowl fits on, the ally sliders and the fixed ply exhaust duct, which will be hidden under the glued on rear hull cover, soon.

[Chris FellowsParticipant @chrisfellows72943]

Coming on well Ashley.

Chris

[Paul TParticipant @pault84577]

Ashley

Looking good but take care not to cross connect the wires or it runs the risk of disappearing up its own a..e posterior

Paul

[carl brothertonParticipant @carlbrotherton75833]

How long is your boat Ashley?

My own from memory is about 24".

Given that the duct is pretty much parallel, Why not push the fan right to the back.

There is a reason for the last question, assuming that your boat is anywhere near similar to my own, I would be looking to get the Lipo towards the back, for reasons of balance. My other concern is that to get reasonable performance I would need approx 20 mm sq. of feed into the fan. My air intakes are nothing like this. Even using the cockpit as a cheat intake, leaves a lot of area needed.

As you can guess I am watching your build with great Interest.

I am even more interested in will it work, in that it will plane.

All the best.

[ashley needhamParticipant @ashleyneedham69188]

Carl. Thanks for your interest. You are not the only person wondering if it will work!

Normally I would have plonked this in the water at the very earliest opportunity, but as it is, it will be finished before that.

The boat is 20 inches long and 9 inches over the sponsons. Thrust/weight ratio should just under 1:1, whether this is enough?? Other edf boats on the pond seem to go well enough with (I would estimate) heavier hulls, but there again this is an odd hull-form and may be draggy at low speed hindering getting any faster!

As it is, the 50mm edf is only 77g,and the centre of it sits approx 4 1/2 inches from the stern, and the Lipo gets to within about 1 1/2 inches of the rear.

Looking at my picture you can see that the (oversize) outlet, about 50mm diameter, has been placed as high as possible in order to allow the battery to slide as far back as possible but obviously the lower curve gets in the way towards the very end. The rear exhaust cowl is in two bits. A round top and a flat bottom, the flat lower so that the battery can go rearwards, it would stop much further forward with a completely conical outlet. The rear of the flat lower bit was heat bent to go ‘round’ at the end. A quick float in the bath has it sitting nose-up, so that’s good?

Ashley

[ashley needhamParticipant @ashleyneedham69188]

Inlets, and mine will be horribly oversized, and the cockpit will be empty (apart from a centre thin outline John) with a slightly oversized but hollow headrest, to double as a scoop.

Just looked at yours, and it looks fab. Unfortunately I err on the side of functionality and easy build so mine only just resembles the original, but, fingers crossed…!

It’s unfortunate, but with edf power, the bigger the fan..the heavier it is and the more air it needs. I had a very useful weight reduction going to the 50mm unit of almost 50%, however the stated thrust is almost on par with the 64mm unit inferring I would still need the same intake area for both.

Ashley

[ashley needhamParticipant @ashleyneedham69188]

Finally epoxied the rear cover in place and it has turned out better than I thought it might!

The really difficult bit is now to come, the cockpit area with all the scoops bits and so on.

The plan is: I will use the rest of the fibreglass “barrel” for the actual cover, make the air scoops and headrest scoop, glue them on the fibreglass and then cut the necessary openings underneath them. This hopefully will ensure the cowl remains in shape, as the stuck on scoops will be the reinforcement for the large areas I will be cutting away.

All the gubbins forward of the fan on the Bluebird is covered by a hatch and the interior is painted black .

Unfortunately I can’t manage that on the Crusader as the forward rudder linkage/servo is highish and the fan wiring has to come out waist level so I can slide the battery underneath. I think I will have to wait until everything is in place and thoughts at the moment are to use a shaped ally mesh cover, with everything in the cockpit painted black along with the mesh. This won’t really hide everything to a “bench” inspection, but will disguise the interior when on the water.

Ashley

[Paul TParticipant @pault84577]

but will disguise the interior when on the water.

False noses and wigs then

[ashley needhamParticipant @ashleyneedham69188]

er……

[ashley needhamParticipant @ashleyneedham69188]

I may need a false nose and a wig to disguise myself as I slink off after an epic fail..

Spent the morning fitting, or at least, getting it to fit, the cabin section fibreglass "tunnel". And it does, it fits luvvly!

Not sure how to hold it in place yet but I think I will do the inlets and so forth first and once everything has been cut away, see what's left.

Ashley

[ashley needhamParticipant @ashleyneedham69188]

Massive success today!

Having left arguably the most tricky bit until last, things have fallen into place nicely.

I always planned to use a bit of tube, cut at a very shallow angle, for the air inlets, and same for the head-rest, and tested the concept with a bit of PVC tube, which worked ok (but which had wall thickness far to fat) and in my minds eye I was always going to make a glass-fibre/resin tube for thinness and compatibility with the fibreglass cover, but….

……All that effort….I could see that several attempts were likely before I had a nice tube section, so I cast around for a suitable bit of tube.

And found a card tube exactly the right size. I painted it with dope and once nice and dry, cut it about a bit on the bandsaw and voila! two inlet covers and a headrest. Bit of light sanding after 4 hours rest and (they still need a bit of work) there is light at the end of the tunnel!! The card tube is very rigid, and tomorrow once the sanding sealer is fully dry, will be MEGGA rigid. Tomorrow I will glue the bits in place and the day after (where are we?? sat?? sun?? mon?? the days are eluding me at the moment) I will break out the Maplin (sadly missed) Dremmelly tool and cut out the fibreglass underneath the cowls to allow air flow and that will be that.

Do I talk too much??Probably, but hey…I have nothing else to do !!! (the Mrs is making tea and the plastic packaged stir fry mix which has been in the fridge for a week has gone off… daren't say anything…) and am drinking Bacardi and pineapple juice (nice).

Wow

Ashley

[ashley needhamParticipant @ashleyneedham69188]

Progress. For those in the know, all the sticky out bits are much too large, the cabin area is much too big and too far forward but this will hopefully give me the inlet surface area I need.

As has been said, building to scale does not work for an edf. Besides which, I am building easy. The inlets and headrest are simply tapered cones, and after I cut away the cabin area from the fibreglass cowl, I would seriously weaken it if it was further back (with the elongated shape.

Ashley

[redpmgParticipant @redpmg]

Ashley it may not be exact scale but its still a pretty close representation – and it looks good – looking forward to seeing the finished aricle

[ashley needhamParticipant @ashleyneedham69188]

Thanks for that Red. Another snap, with a bit more paint and the cockpit cutout. The top makes more sense now.

Ashley

[ashley needhamParticipant @ashleyneedham69188]

Have noticed a spray rail attached along the bottom of the hull , above the front ski.

This is not that apparent on a lot of pictures, but I can well imagine that it is a vital thing to stop water sliding up the hull side and into the big sucky inlets!

As the bottom half of the hull needs no further finishing work, now is the time to attach a bit of wood, or better still, triangular plastic strip if I have some.

UNLIKE the nose section, which despite the apparent reasonable finish suggested by the flat grey paint, needs considerable work. It has been left until last as I had to finalise the cabin bit first, and was left un-ezekoted thus has suffered a few dings.

Ashley

[Bob Abell 2Participant @bobabell2]

Looking very nice Ashley

The half nozzles really enhance the model

Am I right in thinking that the rudder is at the front?

I remember suggesting that to our old friend Paul, for the Brutus and the idea being…..Poo Pooed!

Is there nothing new in this world?

Bob

[Paul TParticipant @pault84577]

Hello Captain Bob

Your idea for a forward mounted rudder wasn't just simply dismissed as I did try to apply the concept to Brutus but the rudders needed to be fitted close to the propellers in order to enable low speed manoeuvring.

Ashley

You have probably seen this link but there will be others who might appreciate the film.

Paul

[Bob Abell 2Participant @bobabell2]

I would have thought that a front rudder would like a bow thruster?

We should have tried Bow Thrusters on Brutus?

It would have been great fun to play about with

Bob

[Paul TParticipant @pault84577]

Bob

For consistent performance conventional rudders require a controllable flow of water across their surfaces this is why they are normally mounted behind the propellers.

The propellers provide the thrust of water that enable the boat / ship to manoeuvre, this process is especially important at low speeds as with a quick burst of power the captain is able to maintain control over his vessel.

The axe bow on Brutus was too slender for fitting conventional bow thrusters but there was plenty of room amid ships so the pumps could have been fitted there and connected to the outlet ports in the bow with plastic hose.

Paul

[ashley needhamParticipant @ashleyneedham69188]

Paul T has already voiced an opinion that the forward mounted rudder on Crusader was (rubbish) ineffective not only due to the fact that it is not in the prop wash (as there is none!), but also as the rudder, far from being mounted at the actual front, is situated roughly 1/3 back.

Not quite in the middle, but might as well be. I can of course make an oversize rudder, but then I might have much too much rudder effect at speed?

We will have to see what happens.

Love the music on pathe clip. It does seem that the boat nose dives and I am sure I read that there was some doubt about the bottom face of the from ski being strong enough….

….Ashley

[Ray Wood 3Participant @raywood3]

Hi Ashley,

Is there any mileage in considering a skeg / keel on the bottom to give the boat some directional stability, the multi boats from away back used to have them to help the rudder work ??

Regards Ray

[ashley needhamParticipant @ashleyneedham69188]

Ray. You are quite right. I will be fitting a couple of blade fixed skegs to the inboard ends of the pontoons as the last thing to do.

I have found anything leaving/almost leaving the water needs some sort of directional aid. The Ecranoplan has one, the pontoon boat and Farnham have rear rudder blades already that do much the same thing besides simply steering

I realised the Walrus needed one (suddenly dawned on me) after having it wallow everywhere once almost off the plane and have fitted a very large `rear tailwheel` fixed rudder…yet to be tested!!

Ashley

Edited By ashley needham on 05/05/2020 21:16:48

[ashley needhamParticipant @ashleyneedham69188]

Momentous moment approaching as all the cutting and gluying/filling ahs now taken place and resisting the urge to continue work after tea, I am in a position for spraying Silver paint tomorrow.

Strakes, of a decent size, have been fitted above the front wedge, and I can see that they will be very much needed, as the front will throw up loads of water until such time as the front rises out the water, and the strakes will turn a lot of this back underneath. Ally skegs attached to rear of sponsons and finally a method of holding the centre (removable) section in place has been decided.

I will be using small servo screws. Not invisible, but when I thought of using magnets and bits of steel stuck in place on the inside I also though of the carnage they would reek if one bit came loose.

The screws are fairly unobtrusive, and will provide a secure and vibration free securement. A bit more fettling took place once the screws were tightened and the cowl now fits very nicely.

Just a couple of thin balsa wedges now needed to gap fill to get a better fit underneath the inlet cowls and I will be away.

Ashley

[ashley needhamParticipant @ashleyneedham69188]

Silver now, and in fact, I have also splashed some red on it!! masking straight lines is ok, but the curves (like the inlets) will have to be done free hand. Should be ok. I may slightly simplify the stripes.

[Dave MilbournParticipant @davemilbourn48782]

That's really looking the part now, Ashley. If it floats/flies as well as it looks then it will be truly remarkable.

Dave M

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