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Old 10-25-2013, 11:55 AM
  #101  
dadkins
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Jeremy,
Why the two different size motors? I assume its just what you had laying around(I wish) but being brand new... and of similar displacement why not 2 of the same ?
I had some hair brained Ideas of doing a similar build a while back and chose the inline design because I have a .40 and a .15 and that obviously would not work side by side. It all started getting very complicated and I bailed - You are doing a better job than I could have ever imagined ! Stellar Work.
Dave
Old 10-25-2013, 04:41 PM
  #102  
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The answer is buried in the volume of text in this thread, but I'm happy to say this again because it's very important to what I've discovered.

Originally I used two different engine sizes because I happened to have a few aero engines not being used, and OS 15, an Irvine 20, a TT 25 and an MDS 40. The 20 and the 25 were the closest in power, so I used those believing it was a compromise. This turned out to be a happy accident.

In my head it made sense to put the bigger of the two at the back, because surely it would be pointless to put the small one there simply because the airflow over it from the front one would be faster and in greater volume than it could manage to meet, so it would add nothing. Expanding on that I figured if the small one ran the same size prop as the rear but with a pitch suited to its size, then the moving air would hit the rear engine in a pre-accelerated state, so I figured a heavier pitch prop than the engine should have on the rear could be run as it had fast incoming air to work with. In essence this worked out well. At the back of my mind was a real boat water prop system employed by Volvo Penta called 'Duoprop'. On that system a heavy pitch front prop feeds a contra rotating rear prop which has an extra blade. When I was on the shop floor I fitted loads and loads of Duoprop stern drives on motor cruisers, and latterly as a test engineer I'd regularly take them out on sea trials, and superb they are too!

So something at the back of my mind essentially came to fruition when I tested the principle, which was latterly found to be a form of 'Pressure Recovery', where the spiral or tangential airflow off the front prop which was largely lost force was re-directed to a straighter air flow, and where the front prop 'charges' the rear allowing it to reach peak RPM with a heavier pitch prop, thus more thrust. By looking at the exhaust smoke when I was running the engines very rich a clear narrow conical airflow is coming off the props, rather than the usual dispersion you see. This is actually visible on the second video of the test boat, where accelerated air is still visible some ten feet behind the boat in a comparatively narrow area

If you think about it, two engines the same, running two props the same, will have the rear one aiding the pressure recovery element, but it'll just be moving in air at a similar speed to what it would produce, effectively running a prop with zero pitch in respect to the differential between incoming and propelled air, i.e if it's hitting the prop at X miles an hour and is only capable of moving it on at X miles an hour then it's adding little or nothing.

So, when I decided to go for bigger engines for the project boats I had in mind a 40 and a 36, but in the affordable SC range the 36 is a different motor, and the 32 and 40 shared ABC type and an option for rear mounted needle valves, which makes it a little safer. I've had one clip off the test boat as a reminder that putting your hands between two aero engines so close together is dodgy ground, so I thought it OK. In reality the power difference is only slightly off line to the difference between a 20 and a 25, so not too shabby.

However, it's worth pointing out here that whilst true contra rotating props add power to a single engine, two engines lose any such advantage through increased weight, so I remain interested in doing this simply because I've wanted to for a long time, with the advantages I've proven of negligible, even immeasurable, torque issues, and that great sound. One other advantage I've found is that if you're less than careful with duration by configuring fuel tank sizes so that the rear runs out first the boat can be brought home without those tedious waits or recovery needs. Fact, I spend more time on the pond using this airboat than everyone else in the club, who seem to spend an inordinate amount of time waiting for dead boats to drift in, and the only time I've had to do that is when I flipped it in those heavy winds. The rear engine must stop first though, or at least bring it back on the rear very slowly, as the rear is totally dependent on the front engine for cooling at high RPM.

To me there is no reason why a 15 and a 40 can't be bedmates, you just won't be able to run the same diameter props. Something like an 8x4 on the front and a 10x8 on the rear will, I guess, still realise the two stage air movement, it's just that the 40 will not be fed quite so wholly.

I may find myself running different size props on the final boat too, but if I can safely get away with the same size I will. I guess you could run two engines the same cc with the different pitches and get the same effect, I've left that door open as my mount will allow the later fitment of another 40 easily, but then the weight thing would be less favourable. Dunno, there's still plenty to explore but I hope I'm in the right place for these particular boats. I must say I'm feeling encouraged by the new engines, the thrust off them is very good for the engine type and I'm not so nervous as I was when I first felt the weight of my extravagant mount. Also with the vibration issue looking good so far I'm heading back to pylon mounted throttle servos to make the installation a lot simpler. remember I have this alloy mount because I've found that other types don't last, even glass filled nylon radial mounts have let me down over time. But a well put together plywood mount would work well for some time, make it replaceable and there's a simpler solution to longevity right there. As I enjoy the act of building though I'll stick to my over the top resilient mount for these boats
Old 10-25-2013, 05:16 PM
  #103  
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Ooh! One other thing. You may have twigged from the water pickup pics that the test boat twins are gone. Following the final test run I inspected the engines and in particular I checked out the area around the prop driver. The test engines are both plain bearing, which means a floating crankshaft. So on the rear engine the thrust is pushing the crank in and only stops when the prop driver contacts the front of the crank housing. There's a thrust washer in there, I assume for electric starting, but it was looking a tad thin by now and so I've dropped that pair to save the engine. Engines with ball race bearings are different, they are axially loaded slightly when the prop is tightened, hence no end float so the bearings take the thrust load in either direction, it doesn't matter which way the thrust is going as the crank is supported the same either way, worth bearing in mind. I don't recommend using a plain bearing engine in pusher mode.

Interestingly, at least to me, I've fitted the test boat with the old MDS 40, a first export generation rough old thing that worked well enough some time back in the '80's but was never a keen performer. I thought I'd keep the test boat active for other tests, like the water pickup, and that it would be nice to try the MDS again to see how it got on with modern fuels. Off the top of my head, with the reduced weight and what seems to be more power than the twins coming from a much happier engine than I remember, this heavy test boat is about to take a bit of a step forward in speed. CoG is a bit too far forward now though, so we'll have to wait and see.

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Last edited by Jeremy_H; 10-25-2013 at 05:37 PM. Reason: grammar
Old 10-26-2013, 05:38 AM
  #104  
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This is short as it was a jury rig I should have done better, but here's the first run of the SC's together:

http://www.youtube.com/watch?v=mBu1hpAXjYw&feature=youtu.be
Old 10-27-2013, 11:51 AM
  #105  
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Scoop worked well at speed, plenty enough to continue with it. Need to tweak bore sizes to improve it, but at the moment so long as the boat is on the plane the lower tube feeds, the taller ones come in as speed steps up. Encouraging results really.

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Old 11-03-2013, 02:25 PM
  #106  
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I think I'm pretty close to knowing where to go with the foam construction, so thought it a good time to just mess with some of it to get my shaping hand in. Using the small piece of pink stuff I had I've shaped up this cut down version of the project boat. It's a deviation in many ways, but it shares a lot of the features of the final boat. Already it's brought a few things forward to think about. Using one small piece of 3/4" foam meant many joined parts, but joins will be in the final boat's to aid construction rather than this one piece way, so worth learning how to deal with them now. I'll glass this up too, to pick up on what might arise there on the proper ones too.

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Old 11-04-2013, 07:04 AM
  #107  
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Jeremy looks like you made some foam dust there... WOW that thing is sleek as hell ! Since you are doing the laminated construction - maybe take a trick from the surfboard guys and put a piece of thin plywood in between near the deck layer - to stiffen it slightly - but also to mount your servos, motor mount blocking,and stuff... that is a sweet looking hull and when fully glassed should be plenty strong.
Old 11-04-2013, 10:09 AM
  #108  
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Hi Dadkins. Yeah thanks, that's where I'm heading, it's what I was alluding too when I mentioned joins. A thin ply sheet would be pretty much the plan profile of the boat, but I'm pondering extending that to the inner sides of the sponsons in full side profile. There'd need to be joins in the ply above and below the deck sheet to achieve that of course. In my head this ply will provide engine thrust load transmission throughout the hull (via a plate I'll be experimenting with on this little one), provide strong edges and compression strength to the sides and bottoms of the sponsons, provide edges to profile down foam infills, and it allows the pre-profiling of a couple of foam parts off the boat such as the centre hull.

Coincidentally, as you mention them, I'm trying a resin intended for surf boards on this trial boat, I've never made a board but have watched those who do, I'm just not sure if it's properties are right for this, we need thixotropic resins really and this one is designed to flow out. Dunno, Epoxy is there is in the background as a default, but this board stuff sounds strong with specific foam adhesion benefits and easy to sand, but it's a Polyester base. One of few resins intended to be used on this inside out glassing method. We'll see, waiting for deliveries, I have a variety of mats from strong but rough 2oz CSM through cloths and tissues to fine weave bi-axial Carbon Fibre to play with when the resins arrive .
Old 11-08-2013, 07:19 PM
  #109  
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Most of the way through mounting the throttle servo, a decent metal geared ballraced mini from Hitec with an IP67 rating, properly waterproof which is what killed the test boat's. On for a better run this weekend.

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Old 11-09-2013, 09:31 AM
  #110  
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A short vid of one of the engine trial runs today, much better

http://youtu.be/Z01v-IEuvKw
Old 11-23-2013, 04:51 PM
  #111  
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Start of the first attempts to get my head around the glass over foam thing. With some CF in there for experimentation, I found this process tricky and I've got a lot to learn.

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Old 11-25-2013, 09:23 AM
  #112  
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Looks like you are doing pretty good to me - that cf is going to look good on the topside!
Old 11-25-2013, 02:57 PM
  #113  
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The CF is probably not going to be as good looking as it could be. Now that it's hardened the high spots on the weave on this particular mat are quite pronounced, so a lot of resin would be needed to encapsulate that. Here though I'm going to have to waste resin on an experiment or sand it down and that will breach the weave, if I can keep some fibres in the weave being sanded then it stands a chance of being OK. This resin is also not ideal for this, not clear enough.

So with those two in mind I think I'll revert to another way if I use CF on the final models, laying up thin sheets of CF off plate glass under pressure, this can then be epoxied to the hull as a sheet. Same result, better finish and far less weight/waste.

I glassed the underside of this model this evening. I had a few minor eureka moments, so whilst it's still a bit of a struggle there's hope
Old 11-25-2013, 03:00 PM
  #114  
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Regular glass cloth on the bottom ?
Old 11-27-2013, 02:54 PM
  #115  
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Originally Posted by dadkins
Regular glass cloth on the bottom ?
All over bar the carbon areas, I've done the top this evening, more mess lol! Regular chopped strand mat is stronger and easier to manipulate than woven stuff of course so a sound thing to use.

The carbon flatted down leaving the weave looking good though, so that's usable visibly now, generally though I'm finding this process a bit finicky and I'm hoping the final boat's greater size will make this a bit more pleasant.

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Old 12-01-2013, 01:29 PM
  #116  
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A great deal of sanding later and the boat currently weighs in at 1lb 11oz, here's some pics where I've used pre-paint panel wipe to show the carbon surface. In the last pic I've coated the whole thing in one last layer of epoxy to deal with a few voids, and added a carbon transom under a weight to see if I can get a flatter surface to rub down. So far I'm liking both the function and the way the look of the CF is turning out, so I think I'll go with it on the final model:

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Old 12-01-2013, 03:32 PM
  #117  
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Transom idea worked out OK, fine if it's a flat surface I guess:



And a view of the last coat of epoxy before a whole bunch more rubbing down for the cosmetic finishing to go on, once the transom is trimmed in:

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Old 12-06-2013, 06:32 PM
  #118  
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This will work out OK from here, I've learnt enough and I'll move on to the proper boats at last :-)

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Old 12-07-2013, 03:26 AM
  #119  
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Hello Jeremy.
Excellent, clean work. This construction is going to be a very good model.
Old 12-13-2013, 06:20 PM
  #120  
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Committed to this glass over foam thing materials are on the way for the final boats, so in between some more finishing of this little one to get my hand back in. Spray painting is something else I've not done for a while.

First I had to finalise the transom:



Whilst chewing the fat with a mate over what colour it should be he made my mind up on it being his football team colours. Whatever, decision made as red with white and yellow trim. So, to make a red look bright it had to be painted white first:

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Old 12-14-2013, 08:20 AM
  #121  
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Just pulled the tapes:



I'm planning on a bit of a hatch design oddity with this one, it's going to be under the cockpit, so that will have to be next for this one.
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Old 12-16-2013, 01:37 PM
  #122  
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Put together an engine mount whilst the machines were on the go:

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Old 12-19-2013, 09:05 AM
  #123  
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Hello.
Great engine mount. Have you made this element yourself or was cuted by waterjet ? ? ? How do you resolve rudder mount ? ? ? If carburetor is placed too low water line ? ? ? Painting is gorgeous.

Greetings from Poland
Marek&Wojtek
Old 12-19-2013, 03:53 PM
  #124  
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Thanks Marek.

I cut the shape of the engine mount plates on a bandsaw, smoothed with a Powerfile, then finished by hand. The slots and holes were cut on a milling machine, but I need to clean these before anodising it all black.

The rudder mount is being made now, I will show photos soon.

I have used this carburetor position before, but we must see how the boat runs to know if water will get into it. I do not think there is enough splash protection on the front of this boat, the deck is too short

For now I have setup the engine mount so that I can run the engine, I will see if it is OK:

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Old 12-20-2013, 02:44 PM
  #125  
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Engine and mount run fine:

http://www.youtube.com/watch?v=IcWGK6WZkyM

Last edited by Jeremy_H; 02-10-2014 at 03:37 PM.

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