Aeromarine Thunderbolt 45" Sports Hydro

[drwayne]

Hey, Nice set up. One comment on the CR props - I switched from two RH drives on my 1:1 boat and repowered with CR Alpha drives. I have had a couple issues that I cant seem to resolve - 1) cavitation above 60MPH. Never had this with the two RH drives. 2) Handling; boat does not seem to track as well at low speeds, wanders and hunts much more than with the RH drives. Havent really noticed much of a positive difference after the switch plus was a $3,500 addition. I think that for aircraft such as the P-38, it was a positive addition because of the huge power/Torque of the 1600HP supercharged engines and the CR tended to offset this. But this doesnt make sense to me either as the Right hand rule would suggest spin would result from this?. I have yet to try it on my 1/8 scale thunderbolt (expresscraft). Maybe the scale torque of these Castle motors, like the p-38 will make it a worthwhile endeavor.

I am Looking forward to your results, please fill us in with the benefits/disadvantages of the scale down implementation.

Hi
Full size twin prop drag hydros run CR.
They're well overpowered, same as this build above....3500Hp/3000lb Vs ~20Hp/18lb(15KW/8Kg)
However, if CR doesnt achieve the required wide grin status, it's a flex, prop and 2 wire interchange to change.
CR props induce transom ride height change.. .. both RH is translated into yaw.. as CR they counteract and create lift/drag.. perhaps your CR set is 'hunting' as it raises/lowers the transom at low...
Seriously, show me any RC Hydro owner who builds for slow speed stability

wayne in response to the above post & being an bloody aussie you will know about sharkcats right???.
well yes I have a 20 ft boat it has mounted on the back 2x 115 hp johnsons props are non cr runs wot 60 mph + wot no issues and its a pure offshore bluewater fishing machine. and does not exhibit bad habits period.

Gday, mate.
Yep, they're a popular hull down under.
Having tunnel hull-water contact at transom where props drive does not compare handling/setup characteristics a hydro where sponson pads are ride surfaces.... and even struts/props themselves to a lesser degree.
Induced yaw ( propwalk ) on a hydro is amplified as there is no hull/water contact to avail transom track stability...
I note that rc petrol cats use a skeg strut to deter propwalk at speed... and many monos have offset stinger to accomplish same.

The Mk#1 40" hydro ran CR outwards.. and a screamer once trimmed.
X665 CR Prop centers set 78mm apart.. rudder amidships directly behind in center of propwash between props..powering to left or right no issues..

new phone 045.jpg
Fellow in Qld puts these out.. great hull at 1200mm....

[drwayne]

Experimentation and innovation are key to progression of the hobby.

[drwayne]

Counter-rotation disadvantage - poor turning on the race course. Virtually all twin nitro riggers run non-CR props. The twin fastest drivers in the US I've spoken to at races say that the boats do not turn well enough with non-CR setups. They were all adamant about it. But it is easy enough to try with FE and with the big hydro it may work fine.
.

Let's have a fresh look at this ...
Riggers use turn fin, rudder and prop to corner the boat.. with CR there is no added turn assistance from propwalk.
A pair of LH motors.. torque/props will always induce Left sponson lift under load..will always push rear of boat to left, thus aiding R corners on oval as they power around.. if you back off in a rigger , they dont turn so well !
Single engine sports hydro build instructions say to add weight to the left sponson to reduce prop steer ( heavier L sponson makes boat yaw/turn/drag to left, countering propwalk induced yaw to right).. handling inefficiencies.

CR centered at transom with rudder directly behind in the center of propwash from both allows rudder movement to focus the path of propwash.
The rudder is not relying on clean water passing it by to turn the boat, it has use of the accelerated propwash to create a thrust vector change.
Ive not seen riggers with rudder mounted in/near prop wash as this is not the done thing ( why not try central when twinned ? .. side mounted offers no benefit when CR Vs same IMO )
In this build, centralized between CR is a maelstrom of turbulated water.. which itself has no drive, but the turning rudder redirects flow from immediately adjacent props... vectored drive.
Rudder water pickups are set only a scant distance below the props lower edge, making total rudder length only some 4" total.

[drwayne]

12S1P feeding 2x Leopard 5692/730 to CR X665
40" Sports hydro Cf hull.. with a few aerodynamic concerns later solved by larger nose canopy/cowl to add downforce..

Nuthing wrong with CR props in a hydro......... btw, water was not calm..
(Do turn down the volume)

[Brushless1]

All good discussions. My reference above was to deep V hull (21 degree deadrise) and not hydro or cat. But, some characteristics may likely be shared by many different hull types. Lower unit type set ups (MHZ etc) likely share most in comon with the research below than direct drive cats or hydros. I was trying to find toe in/out for directional output of the twin props. My performance contacts (all two of them....) indicated the props should be set up to where the outputs cross at some length (related to prop size, power output, and boat lenght/hull type) behind the boat independent of speed. Seems to me it was like 1.5x boat length. But cant be for sure just now.

Other considerations learned from Mercury Marine Racing identified that for the Alpha lower unit design, there is a max speed at which they should not be pushed above (about 65MPH) because at higher speeds severe cavitation around the lower unit occurs which propagates from leading to trailing edge as speed increases. Eventually causing complete loss of control. Although none of them knew what would occur in a dual prop situation nor what that speed threshold really was. As you might think, having heard this, I continued to research the topic. Some of my research found on the following information on this subject:

"The amount the drives should be toed-in or toed-out on a boat depends on a number of factors. The most common considerations are hull type, drive type, propeller shaft depth and propeller rotation.

Generally speaking, twin-engine V-bottom boats with the propellers spinning out are usually set up with the drives toed-in slightly. It is common to set the nose of the drives about 1/8-inch closer together than the aft end of the prop shafts. The theory is that water exiting from the back of the hull is being parted slightly by the keel, so setting the drives at a slightly toed-in angle would result in them running more aligned with the water flow. That is only a theory. What we do know is that certain boats handle better with varying degrees of drive toe-in or toe-out.
If the propeller shafts are high enough so the propellers are surfacing, the side load created by the propeller blades seeing cleaner water in the lower sweep of their rotation will tend to push the drive in one direction. Similar to an automobile suspension, there is a slight amount of play in the gimbal ring and transom assembly pins and attachments.

The side force created by a surfacing propeller will load the drive and transom assembly in the opposite direction of the propeller rotation. In other words, if the propeller is a right-hand rotation (clockwise from the rear), the result is that the force is the same as turning (or flexing) the drive to the right, and vice versa". As I mentioned above, for me, this was not the case for my original RH drives set up.

"If your propellers are surfacing and turning out, the toe-in (nose cones closer together) will increase under load. And, if the propellers are rotating inward (port clockwise and starboard counter-clockwise), the force will tend to pull the prop shafts closer together resulting in toeing the drives outward. The deviation is related to the type and condition of the gimbals and transom assemblies, and how high the propeller shafts are when the boat is underway.

The result of the movement under load should be considered when deciding what the ultimate setup is relative to drive toe-in or toe-out.
Catamaran hulls that are running a high X dimension with the drive placement very close to the tunnel present additional criteria to consider. If the drives are close enough to the tunnel wall so that the sweep of the propeller blade is seeing clean water exiting from inside the tunnel, there is a tendency to neutralize the affect on the propeller created by water exiting from the angle of the running surface of the sponson.

The bottom line is to get some advice from the manufacturer or someone who knows a lot about your particular type of boat and setup. Then start from a setting and make minor adjustments testing the boat each time to gauge the affect on the handling and efficiency.

Always measure the toe-in or toe-out with the drives trimmed to a neutral position and the steering straight. With an assistant, measure from the front of the center of the nose cones or bullets compared to the center of the aft end of the prop shafts. If you run the boat and discover that the toe-in measurement has changed by itself, the indication is that there is significant slop in your transom assemblies, tiebar or steering. A change in the measurement also could be because the transom assemblies themselves are moving on the boat’s transom and need to be re-torqued".

[drwayne]

Excellent info.

I have no empirical or wrote knowledge of subsurface drive configurations ( until above ) .. the 600' or larger vessels Ive worked on were not the speedy variety.

I do understand shiny propellers work better on Frigates.

The prop position on these twin screw rc hydros presented here are as subsurface drive ( under hull, extended to behind transom )
The pic below is outwards subsurface props.. which similar to the rc Mk#1 example, became surfacing props after the hull rose from launch.
A clear indication of 'hunting' with the capture and release of excess water also visible.
It does not appear of concern to performance.

problemchild.jpg

[drwayne]

Have just spoken to Orange County USA.. ( remembering Im in Australia 15+1/2Hrs ahead )
Gearbox is ready to ship .. leaving Tues pm.

[drwayne]

The inside transom mounting bits, before they get mounted.
Shafts are 75mm apart
Prop tip separation is 10mm with these 65mm props...
Rudder leading edge is 10mm behind prop trailing edges

noting that the shafts are joined by the gearbox.. I could use larger props and set the props at 90deg to each other

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[Brushless1]

Small prop separation. Will be very curious how that turns out. Likely will reduce straight line wandering. Please share once this is together and test run!.

[drwayne]

Small prop separation. Will be very curious how that turns out. Likely will reduce straight line wandering. Please share once this is together and test run!.

The video of version #1 in post #19 has 12mm ( 1/2" ) separation.
However that unit ran 2 independent motor/shafts , this has a gear driven pair of props where they rotate synchronously.. same or any degree of separation is all a matter of testing ..
Yes, I will share

[iridebikes247]

that picture you posted of the roost off that boat is crazy. without the boat in the picture looks like a very nice line up of depth charges

[Timmy]

That is just freaking awesome. Ties in with my love of cars also! The real question however is, how much is that worth!!!!!?

[drwayne]

The hyperperformance gearbox has arrived.
Brilliant workmanship.
2x for these guys.

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The 2xshaft collets here were supplied by www.rcboatbitz.com.au

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[drwayne]

for those wondering how the box is being used.. this draw shows a view from above the install
Changing motor rotation will change inwards/outwards rotation twin shafts

motor-box-2shafts.JPG

[drwayne]

How much power is too much ?
I have thought over the 2028 power through this modded V-drive re fatigue and survivability, and have chosen to up the ante by upgrade to the transfer case available from same manufacturer.
I hope I have chosen wisely.

My partner's xmas presents will have extra bubble wrap padding this year.. 'size does matter'.