Where do we Go From Here?

[Archie Stanton]

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[Tim_Duggan]

Could we possibly look in to and discuss as a group, a way to maintain good cornering speed and stability whilst reducing the size and drag /or dependence on turn fins? Also drag associated with rudder blades and drive lines below the tub.

As mentioned earlier - hydrodynamic drag is huge. Reducing it even slightly would be advantageous, but it is my belief that it would also make aerodynamic improvements more obvious.

 

[dwilfong]

This has bin a very good discussion on airo and wings.

Why not get rid of the rudder and use a plane fuselage with small sponsons on the end of wings cut short. You could use the elevator to control rear running hight and the rudder to turn. Don't know if it would turn real good but would make a good SAW boat.

Saw a pic of a boat set up like this in Miami. Mort at Crown Hobbies pulled out some old pics of boats from the RAMS club.

David

 

[Martin Hamilton]

I think you should start moving on from the bait box style hulls especially from the engine back, Motors need to be cowled and continue all the way to the back of the boat so the pipe is also inside. This is slightly different or more difficult on a 21 where the header turns 180 degrees and the pipe is on the side (ok on the Valvola if you use CMB). This is precicely why i started turning all my .15 and later .21 motors 180 degrees 25 years ago and still continue to do so. Regards Martin

 

[Paul Traufler]

Hey guys maybe this example of "real world" stuff will help. We have all tossed a flat rock and skipped it on the lake at some time in our life, right? Well what if we take two rocks about the same size/weight/shape and threw them at about the same speed and nearly the same angle. (Just follow me here please.) One rock skips on the water surface the other is just a bit higher off the water and does not skip, which one would go the the longest distance from the release point? We all know which one it is from emperical data (that real world stuff we all see but do not always understand) that goes the greatest distance. What is the difference between the two rocks? (assume they are same size/weight/shape and thrown at the same speed) Water surface impact and drag from the water, yes it is a big deal in the drag department. It would make very little difference if we "aero shaped" the rock as once it hits the water a LARGE drag force is generated and slows the rock so it will fall to mother earth before that airborn one. I do not see many (any!) supersonic subs, but supersonic aircraft are all over the place, perhaps that 800 times denser than air water is the reason here.

 

[Archie Stanton]

? smokin rocks ?

 

[Martin Hamilton]

The whole point of a more aero design boat isnt that the more aero shape on a relatively slow thing (boat) compared with an airplane travelling at much higher speeds. If you take an aero shape boat compared with a plank of wood shape boat their probably isnt going to be that much difference in speed. But what your able to do with an aero shape over plank of wood shape is set and trim the boat up much better getting things like the sponsons to run off the water,stabalising rear end,smoother running boat. These aero shapes dont automatically give you this level of trim thats where the hard work starts getting the boats to run in this way. With the aero shape hulls + wings that we use if say the front comes up to much the aero shape of the hull + wing are working all the time so as not to let the front end come up to much resulting in blow overs.The more you can get the boat out of the water & still running stable the more speed your going to see, Far more than say with fitting a much more powerfull engine in a not so good design boat will ever give.Around 40+ years on the tethered boats we started playing around with the trim of the boats raising the tether point where it attaches on the front of the boat which lifted the inner sponson off the water around 1".This made a massive increase in speeds,It also smoothed out how the boats ran as well. Lifting the whole front is just a progression on from that where aero shape is nessesarey but not for cuttin through the air smoother as every one seems to associate with aero shapes. Every little bit helps though. Regards Martin

 

[Martin Hamilton]

I can remember at the world champs many years ago one of the Russian competitors was trying to break the world record in the then .29 (5cc) class. This was with an aero design + wing boat but it was a bit breezy but the water was still quite flat, The front end was coming up a little to much & keeping the speed just under the world record (to much wind resistance with hull at that angle + drive line of prop angled to much. A lake side very technical mod was made,A match stick was glued across the under side of the hull from side to side round about where the under curve at the front meets the horizontal part of the hull (just in front of where the engine sits). This simple mod interupted the flow of air under the boat to keep the angle of the boat at the desired level enough to exceed the world record. This was in theory a very simple mod but it was thinking about somthing this simple that made this much difference. By changing flow of air in this particular exanple your altering the pressures under the hull as well as on its top as well, And no these aero theorys arnt going to make much difference i wouldnt think at say 60-80 mph. But we are only concerened with whats happening at 120mph(.21), through to 135mph(.60) no nitro allowed.Regards Martin.

 

[TUNLBOTR]

Is there any way to create thrust vectoring on a propeller as a way of cleaning up the prop wash out behind.

similar to products at propguardmarine.com

 

[lohring]

Thrust rings increase propeller thrust at low speeds. They would only add drag in our application. Jet thrust from an axial flow pump is another possibility, but so far surface piercing props are more efficient for high speed boats. The improvements in high speed propeller performance have all come from getting as much of the drive system out of the water as possible. However, because we are not as strength limited as full size propellers, some blade shape changes may be helpful.

Conventional propeller shapes can be seen here:

http://www.henleyspropellers.com/propeller_range.htm#Mako

An interesting idea can be seen here:

http://www.zakpro.com/Tip_fins.html

Start seriously bending.

Lohring Miller

 

[lohring]

Water jet propulsion is intriguing. It should be possible to get almost everything out of the water and steer with the jet. Full size boats seem to be most efficient at speeds from 30 to 70 mph. However safety and shallow water running seem to be the main reasons water jets are used. Both hull and pump design would be major challenges. Is anyone interested?

Lohring Miller

 

[Chris Wood]

Hi Lohring,

I think it is something worth looking into but the main issue is MONEY.. it has to cost a fortune?

chris

 

[lohring]

I doubt that any of the hobby units would be good enough. Someone needs to build one and experiment. For now, I can't see how you would design a rigger, or any hull including monos or cats, that would keep the intake in the water without creating too much drag.

Lohring Miller

 

[Archie Stanton]

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[Archie Stanton]

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[Charles Perdue]

Been there.....done that....

 

[Archie Stanton]

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[Charles Perdue]

T-shirt????? In the south we call them wearable grease rags......

Actually steers pretty good. I underestimated the impact load on the steering linkage. I ripped the pins right out of the heavy duty clevises......twice.

Guess which way the boat went with no steering linkage.....

 

[lohring]

Cavitation in a water jet is a design issue. Read the discussion here:

http://www.boatdesign.net/forums/jet-drives/jet-jet-pump-waterjet-jet-drive-vs-prop-11254.html

If you look at the picture of Mike Bontoft's "outboard" hydro you will see that it is basically an articulated surface drive. I don't see why an inboard system wouldn't work well, replacing the rudder with a small skeg. It should have better maneuverability and lower drag.

Lohring Miller

 

[Ron Olson]

Jet pumps also require that part of the hull be in contact with the water so just there alone you're adding unwanted drag.

As far as articulated (steerable) outdrives go, with my limited experience with them the adjustments have to be dead nuts on or handling can get "interesting". You're also back to old-school hard shafts and U-joints with the majority of them.