Prop Selection

Model: HXT80-100-B
Wire Turns: 8
Resistance: 32ohm
Idle Current: 2A
ESC Required: 130A
Input Voltage : max. 48V
Kv : 130 rpm/V
Weight: 1570g
Shaft: 12mm
Voltage Range: 20-48v
Non Load Current: 2.0A
Maximum Power: 6500W
Equivalent: 60-80cc Gas Engine
http://www.hobbycity.com/hobbycity/s...ner_(eq:_70-55)


I want to run this on 145 amp sealed batteries. Need to know what prop would be best on a shallow v mono surface drive 1 seater 8ft custom wood runabout. I will be wanting how much rpm at the prop with a given pitch?

Im thinking about 5.75 x 9 for moderate load on this motor at 80% cruse. The prop will be 24in past the rear of the transome. 3deg down thrust and side thrust too.
Added another pic the second motor is good for 23hp two of these geared with 100 pounds of lithiums would spank some water.

 

Looks like a neat toy to me. I like it. The 8 turn thing will probably turn way too fast to make it's stated power. No doubt you will need a gear reduction. The only drawback I see is the need for big batteries with plenty of ampacity. The advantage gained by the light weight, and powerful, motor is seriously negated by the battery weight. The spinner on the shaft suggests that it was intended for aircraft use. If so it will have thrust bearings of some sort.

My thought is that a more moderate motor, wound for torque rather than speed, would be more appropriate for a boat. Maybe some others will pitch in here with brighter ideas than mine.

Q: how fast are you hoping to go with this boat and motor combination?

 

8 ft runabout? Sooo small!
Sorry for my ignorance, what does it look like? Is it like a kind of small kanoe, john boat or what?. Do you have any pic or drawing of the boat?

 

Yes just a single seater perhaps 9ft would be better, need keep it light. Here is a pic for scale of the size im going. I can rewind the motor for a lower kv 60kv per volt would help or I can gear the motor. I need to know what rpm/load the prop can handle and go from there.

 

Surface props will get a best efficiency of 70%. You could get up around 90% with fully submerged prop. Attached shows a first pass for a 10" prop. It will require gearing. Probably need to be made of carbon fibre to take the loads.

It would be run off a spring steel curved shaft to get the prop perpendicular to flow. You need this for high efficiency high aspect blades otherwise the blade loading is nasty.

The second design shows what you could expect from the standard Bolly boat prop (pictured). It would need steeper gearing but will be strong enough and is off the shelf for about USD50.

You will find a thin shaft and thin supporting strut for the prop would not add much drag at the 30kts you could expect from the set up.

Mount the strut off the transom and run the shaft though the keel toward the bow of the boat to get a nice curve. Set deep groove ball bearings in the strut to take the thrust load.

I have run as little as 1/4" shaft completely unsupported up to 700W and Mark Drella has used 3mm shaft for same sort of power level but higher rpm than I run. Torque rating goes up with 4th power of diameter so 10mm spring steel will easily handle the torque even at 1400rpm on the Bolly.

Rick W

 

70% vs up to 90% is lil tough to chew on but.. the electric motors can turn up more rpm then pistons. Outrunners typicaly produce 85% to 93% efficiency at high rpms. I want an 1.5hr run time at 80 load and atleast a hr run time at full throttle or it wont be worth going to the lake with it. I have found boats run flater on the water with surface drives least in the high speed model boat world and like the rooster tails and advantage of unloading the prop for higher rpm. Here is a pic stock prop on the right.. Left prop has rake and no ears the other has a bar cut and no ears as the pitch is greater and I wanted to keep the amp draw simular to the other prop I modified that has less pitch. Im willing to mod props as needed to keep the amps down and the speed up even if it means I have to leg it out longer to get to speed.

 

If you spin a tiny surface prop at high rpm I expect you will get efficiency well below 70%. You will be lucky to get 50%. Just means you have to a accept cruise speed around 20kts rather than something like 30kts possible with an efficient submerged prop. You may find it will not have the bite to even get you on the plane. These speeds are not particularly fast for a planing hull. The advantage of a surface prop comes in at higher speeds where appendage drag becomes significant for a submerged prop.

Most submerged boat props are depth constrained and heavily loaded so they have low efficiency. This is what surface props are usually compared with. You have the opportunity to use a very efficient submerged prop because the loading is quite low.

My preference is "go" over "show" but you are not alone in being impressed with lots of spray. You could even fit a big speaker system to get the sound of a V8. Have a frequency controlled voltage generator controlled by a pot on the throttle lever and just amplify the signal. Would only require about a 20W speaker to give really impressive sound.

Rick W

 

There is a recent thread on the topic of efficient surface props. This paper gives an idea of the state of the art for relatively low speed applications:
http://www.icmrt07.unina.it/Proceedings/Papers/c/44.pdf

The attached picture shows the scale of the props relative to the boat to get these efficiencies.

If you are running RC electric boats you could crunch some numbers on efficiency to get an idea of what is possible with the props you are using. It would be an interesting exercise.

Rick W

 

I figure start off with one motor and add another gang them up as the fever for more hp/watts kicks in. True on getting on plane. I would imagine I will have to settle for more ears/tounge on the prop to atleast have enough bite to plane all that mass/me. Here is a nice 23hp motor needs 72v 240amps to get that.. and can be geared to indure up to 30hp. I want lipoly lithium instant on plane and gone performance with this project in the long run. Till then I can live with seald lead acid batteries. H class 125cc outboards get a over 40hp amazing screamming beasts with 6in dia props Im guessing I can use a prop made to perform at 20 to 30hp in the long run and live with 8hp to start with just to run about on the lake. I cant seem to find surface props for high rpm in the 5in dia range. I have sent a email to Ron today see what he can do.

 

In the long run I want a prop cutting water at 14k plus with me going along for the ride taking advantage of high rpm electric motors and light lithium batteries.

 

I expect they would go much faster applying 40HP to well designed submerged prop with low drag strut. But if all do the same thing then they will never know what is possible.

Rick W

 

Correct 'Rick... I spent alot of time designing electric outriggers in my past and know in the smaller boat world the power to weight is extream and you can get away with surface drive for more stable running at outrangous speeds for there size caft. Modern brushless motors and lithium cells can do the same for larger boats without a passenger in a 8 to 9ft range with me in it im ready for the losses for now. I look forward to affording more power for the craft till then im willing to go with a prop in the 5in range knowing it will not function so well on less power the prop will be adjustable to sit deeper in the water. Just the trouble is cant find a prop in the 5in range.