Propeller mod for Inflatable E SUP

As a side project to my E surf project have a small motor that will goes to a toy grade inflatable sup. Its a small outrunner, 650w only. At time of purchse no suitable propellers available. Will run it on 24v, calculated optimal pitch would be very low, probably only 2.5 inch. Trolling motor propeller would be fine, but dimeter far to large so had to ditch that idea. In lack of easy options decided to go for a more relaxed approch…

Idea to buy a cheap China Yamaha clone of plastic, cut the diameter and alter the pitch as much as possible.

Her the base propeller, 10 eur, 71/4 x 5. IMG_20210606_194628
You can see on the photo some of intended cutting lines, estimate that blade area went down about 40%, also pich a bit, cutted the prop from both trailing and leading edge.

Ultra high precission work:)


Here actually the only tools used, like said, this is not so serious. But actually was able to achieve rather symmetrical blades, thanks to the kids toy, the green stuff.


Still need to drill the 4 holes for the bolts fastening the prop to the motor, but otherwise its not far from done.

Diameter is about 142 mm. Still a bit worried the pitch is to high for the motor( will burn), but will test it first. If that is the case can try to bend the blades( less pitch) by heat, another option is to induce ventilation by purpose, that will increase slip an reduce load on the weak motor. Wonder if adding small holes close to the tip of the blades would create some ventilation, without totally destroy the flow? Well, lets test first as is.


Nice job cutting the prop, it’s 140kv so it shouldn’t be a problem, just don’t hook 12s battery yet, start with 6s.

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6S is fine, will not achieve any high speed with this, doing it just for fun. Let see…

Her little mockup how planning to proceed. The question is what tools, sledge hammer, angle grinder or what? Well think this time will use the hack saw, the blade is actually pretty good, a real Bahco.

Then will need some alu profile to attache the motor to the fin. The boltholes in the motor is supposedly M threads( 5mm out of memory) but they are a bit oversized, very loose.


Now “destroyed” the fin, now need to find suitable alu profile from my scrap box to fasten the motor to it. Did purchase also a attachment for the fin, the SUP already have one, but need this when will do a “outboard” rig to my dinghy so if this works can also use the motor fin there.


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That’s a great looking unit, will not even try to achieve that finish level! Mine is like 1/10 scale of that( 10% of power, price & quality). Only principal difference is out runner vs inrunner and the use of ordinary Chineese fin as a base for the motor so it will fit any standard inflatable sup without tools. Had both the parts and design ready long before seen your version here, finding a propeller was the main problem until finally decided to go for the plastic clone. The similarity is little funny, but guess not that many ways of doing a removable motor fin.

Your unit is powerful, have You consider doing a outboard style bracket, where you can attach the fin, think it would work well, nothing like that on the market yet?

To get the motor attached to the relative weak fin will use some angel bar from alu. Needed a hole for the small shaft sticking out of the motor. Same rough style continues, still a few holes to drill.

The drill in the picture, Bosch PBD40…i don’t get the intended use…it is almost useless for anything, flexing all over the place even when working with wood. Worst purchase ever, hate using it, don’t buy!

On top of the alu need to do some slightly more streamlined shape, maybe use some clay or something for shaping as a plug.

Found this

pipe in the garage, 50mm hm…lazy as i am, maybe can use this for something…Bloody work is steeling too much time from my hobbies:)


Good enough, strange, the pvc pipe went out of shape when cutting the space for the fin, maybe need to heat it or then just let it be. Now need to make a bulb, maybe clay or den xps, will be tomorrow.

This must be world record in slow build, found in aliexpress the dates for the purchase, july 2019 LOL


I did the same mistake than you with the pbd40 Bosch drill , hopefully I will find time to take it a part and reduce play …

yeah, sorry the hear that, total crap it is …Have been considering an outside support including a bearing just below the chuck to stabilize the drill bite, but would be difficulty to use. The chuck is also junk, i always drop the bit, always…!

The low-tech approach continues :slight_smile: Golf ball was perfect…not flow tested in computer… but the round bulb shape is better than nothing, will probably just throw some grp on top an let it be hollow. This is not adult content…

Here also some other parts for this project.

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Pictures not that great quality. ESC is 6s 150a watercooled, relay is 200a/24v at least on paper: :grinning: hope both will survive 30a… Will use probably 7s lifepo4 battery, not to far from 6s, hope magic smoke will remain inside!.

Did some propeller testing today, in the bath tube :joy: My old Metabo acted a s power source.

Tried first in 90L basin but impossible. In the bath tube can at least test a little before all water turns.
The flite prop is incredible, even wit the low rpm the Metabo can turn(rated 1350, but guess slightly less with load) surprisingly much power it can push. Second was the tohatsu 3.5 prop( 6 inch). The 2 blade tohatsu 1Hp propeller was a surprise, pushed really well for being so small. In theory this would work on the sup motor but not sure that motor has torque enough. The Tohatsu 1hp motor is a small cc , high rpm motor with small torque, but there is gearing so still probably more. The modified propeller did not push much, much less than the Tohatsu 1hp prop, but probably will not at least burn the motor.

Actually noted one strange thing during testing. The heavily cut yamaha 5 clone propeller push is really weak, weaker than the 2 blade Tohatsu. However, when spun at “wrong” direction, the pull is strong, stronger than than the 2 blade. Guess during “modification” have possible somehow altered the hydrodynamics so that i works better backwards??? One explanation might be that this design need more “clear” water than the others, but don’t think so.

Not a huge issue, will then test also to install it backwards, with e motor easy to change rotation.

Was still a bit puzzled with the test result of the cut yamaha plastic 5 clone propeller : went to the sea, deep water, still same result… relative weak push, but when run in reverse, much stronger pull:) 95% of boat propellers have a bad reverse compared to forward gear…haha my mod kind of invented a propeller wit opposite properties. Well… will drill through the hub so can use it either way, problem solved!

Here a picture what happened when tried to drill the hub on the yamaha 7 pitch to 12mm…not all projects goes as planned! Actually it went rather well first but the pitch on a ordinary hss 12 bite is just too much…should have used less force, it cut to the brass to hard ripping the rubber hub away. This is for testing purposes only for the e surf.