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Dave,
I more than understand EVERY situation is different, and other than the old rule of thumb regarding screw vs motor size as when to add reduction gear or stay direct drive, there is no definitive answer without complex equations for fluid dynamics. Well beyond the average knuckle dragger such as myself.
So I'm asking a few questions here...chop away!!
On my DeBoer 688, I was going to run her with that same 12V 10A Redmond RV Blower motor I used in the 1:48 DeBoer SKIPJACK. Runs about 8k RPM no load at WOT. The SHARK has direct drive swinging a 3-3/4 OD 7 bladed screw purchased many years ago from good old Al Matava (I miss Al), I think made by the UK's "Prop Shop", I have NO idea what the pitch is and I probably don't want to know lol!!
The screw on SHARK is close to the diameter of the motor, hence direct drive works good enough. If you remember, I had posted previous numbers of static in water testing, and I know that I can simply run all day at 50% throttle drawing about an amp. Above that the curve shows amp draw increases exponentially. If I do go with this motor, I do plan on a 4:1 Gear reduction to swing that "big ass" prop.
I did do a current test on your 1:48 scale 688 Prop years ago. I only remember she jumped to 7A when started and settled to ~5A WOT as I held the motor compartment under.
So since YOU designed that prop, and know people who/have run her, any idea what would a good RPM range be? I'm sure something more than 10 RPM but less than 10K, in the water. I'm kind of hoping what you think the RPM should run at, in the wet, can help affirm my choice or aid in choosing an alternate motor, say brushless where one can determine. no load RPM at WOT for a given voltage and also spec the Power rating.
Is there a way to correlate (rule of thumb again) between no load RPM and and in water on a 7 bladed prop, will its RPM drop 10, 25, 50%??
Say I have a brushless motor, 630KV at 12.8 volts WOT, Max rated at 528W, (which I do have laying around lol). So WOT, no load, derated 20% for losses gives me ~6400 RPM, add 4:1 reduction brings it to ~1600 RPM BEFORE I hit the water. NO idea how much it drops at that point.
But if I can assume 30%, WOT in the water I'm at 1100 or so RPM, dropping to 550 at 1/2 throttle. See where I'm going with this?
Knowing this boat, it's performance, having yourself designed an fabricated the prop, is an RPM range 0-1100 reasonable? Are we close?
I'm sure I don't want to purchase a 20k rpm screamer suited for a pump jet. I'd prefer torque and acceleration the lower KV brushless offer if I go that route.
All of this is more of a thought experiment than anything else, things I muse about in the late hours of the evening while others watch "The Tonight Show". Is looking for a "base" RPM a good approach other than say, I'll just use this "big ass" motor with an ESC rated at 30A?
Regardless. I am going to run with 4:1 reduction just because I like even numbers and I'd say greater reduction is better in this case and the choice between the Redmond brushed or the Quantum brushless may come down to fit and ease of build.
Thoughts, comments, bodily deconstruction?
I more than understand EVERY situation is different, and other than the old rule of thumb regarding screw vs motor size as when to add reduction gear or stay direct drive, there is no definitive answer without complex equations for fluid dynamics. Well beyond the average knuckle dragger such as myself.
So I'm asking a few questions here...chop away!!
On my DeBoer 688, I was going to run her with that same 12V 10A Redmond RV Blower motor I used in the 1:48 DeBoer SKIPJACK. Runs about 8k RPM no load at WOT. The SHARK has direct drive swinging a 3-3/4 OD 7 bladed screw purchased many years ago from good old Al Matava (I miss Al), I think made by the UK's "Prop Shop", I have NO idea what the pitch is and I probably don't want to know lol!!
The screw on SHARK is close to the diameter of the motor, hence direct drive works good enough. If you remember, I had posted previous numbers of static in water testing, and I know that I can simply run all day at 50% throttle drawing about an amp. Above that the curve shows amp draw increases exponentially. If I do go with this motor, I do plan on a 4:1 Gear reduction to swing that "big ass" prop.
I did do a current test on your 1:48 scale 688 Prop years ago. I only remember she jumped to 7A when started and settled to ~5A WOT as I held the motor compartment under.
So since YOU designed that prop, and know people who/have run her, any idea what would a good RPM range be? I'm sure something more than 10 RPM but less than 10K, in the water. I'm kind of hoping what you think the RPM should run at, in the wet, can help affirm my choice or aid in choosing an alternate motor, say brushless where one can determine. no load RPM at WOT for a given voltage and also spec the Power rating.
Is there a way to correlate (rule of thumb again) between no load RPM and and in water on a 7 bladed prop, will its RPM drop 10, 25, 50%??
Say I have a brushless motor, 630KV at 12.8 volts WOT, Max rated at 528W, (which I do have laying around lol). So WOT, no load, derated 20% for losses gives me ~6400 RPM, add 4:1 reduction brings it to ~1600 RPM BEFORE I hit the water. NO idea how much it drops at that point.
But if I can assume 30%, WOT in the water I'm at 1100 or so RPM, dropping to 550 at 1/2 throttle. See where I'm going with this?
Knowing this boat, it's performance, having yourself designed an fabricated the prop, is an RPM range 0-1100 reasonable? Are we close?
I'm sure I don't want to purchase a 20k rpm screamer suited for a pump jet. I'd prefer torque and acceleration the lower KV brushless offer if I go that route.
All of this is more of a thought experiment than anything else, things I muse about in the late hours of the evening while others watch "The Tonight Show". Is looking for a "base" RPM a good approach other than say, I'll just use this "big ass" motor with an ESC rated at 30A?
Regardless. I am going to run with 4:1 reduction just because I like even numbers and I'd say greater reduction is better in this case and the choice between the Redmond brushed or the Quantum brushless may come down to fit and ease of build.
Thoughts, comments, bodily deconstruction?
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