Author Topic: .90 cu in, 30,000 RPM, 7.2 HP custom built nitro engine  (Read 80547 times)

Offline strictlybusiness1

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Re: .90 cu in, 30,000 RPM, 7.2 HP custom built nitro engine
« Reply #375 on: April 08, 2019, 05:45:24 PM »
I have completed the fixture which holds the rudder blank for the 5/32" ID side inlet hole drilling & the side wedge milling. An extra piece will be clamped to the side of the blank when drilling the side water entrance hole. This gives a flat surface for the drill to start into.

Jim Allen

Offline strictlybusiness1

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Re: .90 cu in, 30,000 RPM, 7.2 HP custom built nitro engine
« Reply #376 on: April 11, 2019, 09:40:34 PM »
These photos show the new fixture clamped in the vise for drilling the side water entrance slot & milling the rudders wedge shape. The same fixture is used for both operations, only the rudder blank itself is inverted for the side milling operation. The rudder blank is turned end to end for wedge milling of opposite sides. A 5/32" ID hole is the largest hole possible in a 1/4" thickness full wedge rudder.

Jim Allen

Offline Admiral_dk

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Re: .90 cu in, 30,000 RPM, 7.2 HP custom built nitro engine
« Reply #377 on: April 12, 2019, 11:49:54 AM »
Beautiful rudder Jim  :praise2:

I must admit that there are a few things I can't get my head around here ….?

Since I assume that the horisontal hole is pointing forward, it looks like you got the thin part of the rudder profile at the front and the fat part in the back ….. This is the exact opposite of what you do to get the optimum flow in air and I would expect that you get "turbulence" at the back => drag ….?

Lohring explained why you made your mechanism to make sure you have a pull on the servo (only turning in the same direction) - fine - but I can't help think about why not just move the rudder horn to the opposite side of the rudder - that way you still only get a pull …. Is it because the horn then dips into the water when turning or ?

Best wishes

Per

Offline lohring

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Re: .90 cu in, 30,000 RPM, 7.2 HP custom built nitro engine
« Reply #378 on: April 12, 2019, 04:41:14 PM »
At the speeds model boats run, cavitation and ventilation always happens on the hydrofoils.  The low pressure side of rudders and turn fins drops to the point where air gets pulled down from the surface.  That's even true on the trailing edge of a wedge shape at zero lift.  An airfoil shape is considerably weaker and would still ventilate, giving no drag advantage.  All our foils and propeller blades use wedge shapes.  Without the thickness they would bend.  I personally have bent commercial aluminum rudders like Jim's.

High strength aluminum alloys help, but we ran hardened steel hammer knife blades on our 110 mph boats.  You couldn't run a low drag water pickup in them, though.  We ran a water pickup that was out of the water for the brief timed run.  It submerged during the slow turn around period, cooling the engine. The thin steel blade was good for around a 6 mph speed increase at 100 mph over a thicker aluminum rudder.

Lohring Miller

Offline Admiral_dk

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Re: .90 cu in, 30,000 RPM, 7.2 HP custom built nitro engine
« Reply #379 on: April 12, 2019, 07:37:54 PM »
Ahh - I forgot about cavitations and it's relevance here  :slap:

Thank you very much for clarifying this for me Lohring  :cheers:

Offline strictlybusiness1

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Re: .90 cu in, 30,000 RPM, 7.2 HP custom built nitro engine
« Reply #380 on: April 12, 2019, 08:06:19 PM »
Lohring, thanks again for helping with the detailed explanation. I still use a 1/8" thick, hardened & spring tempered AISI-O1 steel, full wedge, rudder on my gas rigger. I use a special designed dual prop blast water pickup that is adjustable in length & depth. Its depth is typically set at 5/8" above the water line at speed & the dual intakes are made of 3/16" ID steel brake lines. This same rudder has been mounted in its custom mount for more than seven years with the same precision machined pivot bushing & its 1/4", drilled out center, dowel pin pivot shaft.

Jim Allen

Offline lohring

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Re: .90 cu in, 30,000 RPM, 7.2 HP custom built nitro engine
« Reply #381 on: April 13, 2019, 04:01:30 PM »
There's an ongoing debate as to whether prop blast water pickups add drag.  The thin rudder has enough lower drag that any drag the blast pickup adds is probably minor in comparison to a thick rudder with an internal pickup.  The decision for us was easy.  We just didn't cool the engine for a few seconds at full power.  If you're trying to set two lap records, this would be a bad idea.  LOL

Lohring Miller

Offline strictlybusiness1

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Re: .90 cu in, 30,000 RPM, 7.2 HP custom built nitro engine
« Reply #382 on: April 13, 2019, 08:29:42 PM »
I'm sure if the prop blast type pickup is in the water at speed, there must be some loss of speed. I also wonder how much energy & speed is loss as a result of that prop blast hitting any part of the boat as it moves forward considering the 2 ft high 30 ft long spray of water?

Jim Allen

Offline lohring

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Re: .90 cu in, 30,000 RPM, 7.2 HP custom built nitro engine
« Reply #383 on: April 14, 2019, 06:05:34 PM »
I have no idea how to accurately test the drag of the various water pickups.  I do know that anything submerged has huge drag compared to things in the air.  Water has 800 times the density of air.  Think about sticking your hand out your car window at 60 mph.  Then, if you can find a fast enough boat, imagine putting your hand in the water at that speed.  Don't really try it. 

My favorite bet, and the very rare time I had better intuition than Mike Bontoft, was over streamlining sponson booms.  Even though he built a crude, leaf blower wind tunnel that proved that streamlined sponson booms had much lower drag than the round tubes, I bet that you couldn't tell the difference in an on the water test.  Mike bought me lunch after careful testing couldn't tell the difference at around 100 mph. 

I'm afraid that you would have the same issues with testing the drag of prop blast effects.  As long as you keep submerged things small and to a minimum, you can hang a rag on your radio antenna and I bet it won't matter as long as it doesn't touch the water.  Aerodynamic effects do matter, though.  A careful look at Joerg's 195+ mph hydro below will tell you a lot.  Look at the rear wings that lift everything, especially the water rudder, out at speed and really large fins to hold the boat straight.  Even at almost 200 mph it takes really large aerodynamic fins to equal a tiny water rudder.

Lohring Miller

Offline strictlybusiness1

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Re: .90 cu in, 30,000 RPM, 7.2 HP custom built nitro engine
« Reply #384 on: April 17, 2019, 10:23:49 PM »
Since I'm  planning on having my wife launch the 17" X 48" carbon fiber cat, it was necessary to mount two very strong aluminum handles. Their mounting areas are reinforced with a carbon fiber plate as well as an aluminum plate. They are mounted at the hulls designed balance point & glued with JB weld.

Jim Allen

Offline strictlybusiness1

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Re: .90 cu in, 30,000 RPM, 7.2 HP custom built nitro engine
« Reply #385 on: April 21, 2019, 12:28:11 AM »
I have mounted the custom 1/4" thick X 1.5" wide full wedge rudder with its 5/32" water inlet hole to the cat boat's transom. I also set up a 5/32" piano wire actuation rod with rubber boot. I'm also working on a custom, 3 piece, strut assembly made of steel.

Jim Allen
« Last Edit: April 21, 2019, 02:12:16 AM by strictlybusiness1 »

Offline strictlybusiness1

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Re: .90 cu in, 30,000 RPM, 7.2 HP custom built nitro engine
« Reply #386 on: April 22, 2019, 04:24:25 PM »
These five pieces make up the entire strut assembly.  The strut & its skeg are keyed before they are silver soldered to the hexagon shaped housing. After finishing, all the steel pieces will be protected from rusting with Precision Brand Tool Black. The brass sleeve soldered at the cable, shaft joint serves two purposes. It reinforces the cable shaft joint & it prevents loss of the prop shaft if the cable should break. Another simple fixture will be used to machine tapered leading edges where necessary. The prop shaft bushing is precision machined from B2 Bearium Metal & is a slip fit in its housing. It  is held in place with a 3-48 screw. Removing the screw allows the entire drive assembly to be easily removed.

Jim Allen

Offline lohring

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Re: .90 cu in, 30,000 RPM, 7.2 HP custom built nitro engine
« Reply #387 on: April 23, 2019, 02:59:16 PM »
Jim

Have you considered the double square drive setup?  Some of the electric record setters use an outboard propeller shaft assembly with a square drive on the engine and a double squared shaft.  See http://hyperprod.bizhosting.com/7_5_11_outboard_prop_shaft_assembly.html and http://hyperprod.bizhosting.com/square_drive_cables.html  They feel that the torque loss is less, but I haven't seen any test data.  They have pushed a P (14.8 volt) hydro to 140 mph, though.

Lohring Miller

Offline strictlybusiness1

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Re: .90 cu in, 30,000 RPM, 7.2 HP custom built nitro engine
« Reply #388 on: April 23, 2019, 08:47:30 PM »
Thanks for the information. I will be using a cable with one squared end (5mm, .1969") which will fit in the same size collet as the split collet used for 1/4" round cables. I already have the 5 mm square broach. The same precision machined clamp nut assembly & flywheel mount will be used. I will take up the prop thrust load with a machined teflon washer between the drive dog & the housing.

Jim Allen 
« Last Edit: April 23, 2019, 09:10:59 PM by strictlybusiness1 »

Offline strictlybusiness1

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Re: .90 cu in, 30,000 RPM, 7.2 HP custom built nitro engine
« Reply #389 on: May 27, 2019, 12:56:16 AM »
I have been busy machining & building the two systems added to the catamaran. These systems are not my design, but are the clever design of a very capable Australian model boat builder. Both systems, which are adjustable, can be used at the running site. The posted photos show the two systems which will be fully explained when all the parts are completed & mounted.

Jim Allen