Paddle wheel gearing and blade size

[kiwimodeller]

Got my paddler going yesterday and although I am very pleased with it there are a few things to sort. It goes at better than scale speed and had no trouble coming head on into a strong breeze so there is no shortage of power. When running at full throttle (see my other post about running at anything less!) there is a very large wave behind the wheels almost up over the gunwales and it appears that the wheels might almost be cavitating. The wheels are 6" in diameter and the blades are 3'' wide by 1" high. Present gearing is 4.75 to 1. I had thought to fit taller gearing as I have another sprocket which would give 3.375 to 1 as this should reduce steam consumption and help keep pressure high but I am a bit worried that the engine will be more inclined to stall if I manage to get it throttled back. Would increasing the wetted area of each blade have much effect on engine load and therefore revs?  Within the framework of the existing wheels I could probably go to about 3 &1/4" by 1 & 1/4" blades. Any input welcome, cheers, Ian.

[derekwarner_decoy]

Hi PD's .... Ian... you are asking more questions than the renound brain trust team [Sandy+Bogstandard & Bandit] can answer

Lets simply consider your 3" X 1" blade = 3" square inches...irrespective of the number of blades or imersion level or input   power ...[& are these Waverley Paddle wheels :?:  :hmph ]

If you increased the float surface area to 3 1/4" X 1 1/4".....you have increased the float surface by 35%.... which with ineffencies considered would require a 45.27% increase in input power to maintain the status quo

 :hmmm ...not sure :nose ... you could always consider 2 7/8" long by 15/16" floats which would equal an 11% decrease on the original loading & hence a 22% increase in engine speed which would be OK

Cavitation & resultant paddle/engine overspeed can always be a power consuming issue

Naturally, if you need re-confirmation of these calculations... just send the POST card to Bogstandard & marked for the attention of Bandit & Vinnie

[bogstandard]

Hi Ian,
It looks like you are just overpowering the model.
When I was running steam engines in model boats or if I look to design one then I try to keep it at about 50-60% throttle to give a good scale speed, the extra power is only used in an emergency for getting out of trouble.
I think what you are getting with your paddle wheels is ventilation (dragging down air because of overspeed) rather than cavitation which is usually related to propellers, going so fast the pressure around them drops and in fact the water starts to 'boil' thus causing an envelope around the screw, which then overspeeds because it has nothing to grip on to.
I found the ideal speed for my paddle wheels was between 140 and 160 rpm at a good cruising speed.
What you should be doing is regulating the power of the engine rather than trying to adjust the size of the floats.

John

[kiwimodeller]

Thanks for the thoughts, I knew cavitation was not really the right word for what was happening but could not think of any better way to describe it. I will try getting better throttle control on the engine and see how that goes as it would be nice to have the reserve power but I may also try higher gearing as that should have the effect of loading the engine more, pulling the revs down and conserving steam useage. Will keep you posted on progress. Cheers, Ian.

[kiwimodeller]

Tried the 3.375:1 gearing today and it seems better although there is still some ventilating and frothing going on. There is also a very big wave behind each paddlewheel. Speed seems slightly reduced but still more than adequate. Pressure seems to stay a bit higher too. I wondered if the wheels are not deep enough in to the water as there is only one complete blade submerged. Any reccomendations? She already needs a reasonable amount of ballast but there is plenty of room for more. Only real problem arrising today was that the reduced engine revs mean reduced water pump revs and the boiler water level is now quite a bit lower at the end of a run. It would not be easy to speed the pump up as it is driven by a worm and wheel reduction box so I will have to ponder on that for a while or else just start the run with the boiler level a bit higher. Cheers, Ian.