The Rebel E-SURF build

Can guarantee this will be a slow thread, and it can go any direction:) Will take some time and efforts but in the end most of my projects will get there, sooner or later…
First stage will be slightly off the topic here but i really need a easy test board and a base line to compare for further development of the e -version.

So what I will do first:, will run some tests using 3.5 hp outboard motors, that’s about 2,6 kw and trying to optimize hull and propeller around this power, at least for now. I have a small inflatable boat that clocked close to 24 kmh with that power, will get out of comfort zone and try to reach 30 kmh with this power. That will be a challenge, if even possible…

The test platform then? I have a inflatable sup…got an “great” idea(maybe not). Why not use that as plug?
Had some old polyester, light plywood and some epoxy …here we are. Some comments of the build:

  • very stupid idea…next time will just use xps/ eps foam as a core
  • will put zero effort on finish, will leave it at epoxy primer and not smooth anything with putty, this is just a tool for testing. Actually one spot is slightly smoothed, the spot where transom and bottom meet. To “let the water go” so to say, this must be sharp and is one of the most important place to get right when you are over hull speed.
    Actually the sup shape is far from optimal, so to improve the “design” laminated the plug only partly of the full length to get a sharp stern. Still the sides curves in a bit but don think that matters too much in this case

Its easy to make some quick and dirty mods on the hull case needed (add some shape) as its pretty sturdy. Its huge so it also weights a bit guess 25 kg. There is a removable bolt on transom for the motor, will take some more pictures later.


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Nice , always wanted to have my boat in my backguard , but I bought hard access house and I just sold my boat …

Nice project , will follow …

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From this morning, final coat of primer and will leave as is. Looks pretty bad but no point spending more time on the hull at this stage, its now water proof enough. The four bolts are for the removable transom board, easy to attach anything. Will glue some EVA foam on deck so its not so slippery

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Removable transom now need some paint and its almost ready to go. Need a few grab handles also, will probably only sit during testing the outboard anyway. Will do them simply from hypalon so the board is still simple to throw on a roof rack.
It was a bit tricky to get the spun hub back on the 7 pich propeller, when drilling the 8 pitch clone to 12mm was really careful not to do same mistake. So later this week…interesting to see is the 30kmh a dream far away or not. Think will do testing about 5 clock in the morning, no crowd, otherwise someone might think i need mental medical help:)

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To be on the safe side, as the transom is relative high over deck, wanted to install some removable supports( from deck to transom) . Made some preparations in the stringers for the threaded metal inserts to be epoxied in place. There will be only moderate pull forces on these.

DIY handlebars from hypalon. Much more work than planned…but think they will do the job! Also here at " good enough" standard, no perfection.


Almost there…

What do You guys think, best guess for max speed, test day tomorrow?

Standard 3,5 hp, 2stroke. surf 25 kg i guess, motor 13kg pilot :joy: 75kg.

I think the prop is 7 pitch( plastics crap), at 5500 with some slip should go 24kmh. I dont know if this have a rev limiter or not, if not, hope for 6000 rpm.

Total fail ha haa… The board is ok, removable transom works perfect. Slow speed rather stable. Only issue was that propeller started to ventilated before getting on plane, its the original tohatsu 7 pitch plastic prop. I know from before that these are really bad design for planning speed, flex too much, ventilates too easy, need to keep very dep in the water to work properly. Was a bit too ambitious hoping that having the cavitation plate flush with bottom would work as with proper propellers…Without planning, speed was only 10kmh. Have done 23-24 kmh with this motort so a bit to improve.

Left the surf on the roof rack, will cut the transom 3 cm an try again.
Will also try to mod quickly the Yamah propeller to fit the Tohatsu, the issue is the pin in the shaft, Yamaha don’t have that. one.

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Nice , you don’t want to try to add a small foil wing at the back just under the board ? , work great on under power jet board

Thanks, what size you are thinking of ? Any pictures ? I should have also somewhere a hydrofoil intended for small outboard motors. But the 3,5hp should be sufficient for planning without any of this, at least done on light dinghy and inflatable boats. The shape of the surfboard is a bit narrower so for planning at these speeds, kind of worse than a shorter and little wider dinghy, but hope still doable.

New try in the morning, with 3 cm cutdown transom, will take along also another similar motor but that has little faster gearing, this in case will get it to plane… hope also to have the yamaha 8 pitch clone ready, that must have better grip than the plastic Tohatsu…

Wish to get speeds closer to Waterwolf MPX-3 by outboard, then will move on to phase 2, doing it electric…have all parts needed.

A bit gutted cutting a perfectly fine transom :slight_smile: but that was quick. Hope this is enough. Need to go with the plastic propellers tomorrow, haven’t figured out yet best way to get the yam clone to work on this shaft. Diameter is all sorted but the pin is still an issue to sort.

Found an old, little unusual 2 blade Tohatsu prop also in my garage, this might work on a e foil also, don’t know the pitch, suspecting 6 inch. Never even used this, came with an spare part motor.



The transom mod heleped a bit. With the 3 blade propeller still problem with ventilation but got it on the plane, about 17 kmh. Then changed to the odd 2 blade plastic. felt much better, and did almost 20 kmh.
To get clearer water for the prop added a piece of wood behind the transom. This way got the propeller about 25 mm further stern. Still not perfect but now was able to do 21-22 kmh. I operate in a speed restricted(a safer place to do this than open sea) area so did not push my luck any further, only a few short runs. Attached a short video.
Would be very surprised i this is the final speed, with a better propeller there must be some more to gain. Also considering the hull is just a random modification from a inflatable SU
P, far from optimal, think this power is enough for close tp 30 kmh speeds. It really is tougher to get this type of shape planning as easy as a wider hull, but no can do about that. A short video from this morning.

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After a semi successful run at 22kmh will continue the rehearsal. This motor have 1/2.15 gear, will now try with similar but with 1/1.85 gear, should be faster… Here preparations for doing the yamaha prop compatible to Tohatsu shaft . Will fill the hub with JB weld marine, hope its strong enough. Had a package that was unusable, to stiff( dated 2017) but heated the resin tube in water, got it somehow workable again. Used a 12mm alu shaft and drilled a hole for the pin to use as a plug. Hope will get it loose:) after the JB has hardened, only protected by thin film… Have high hopes for the 8 pitch Yam propeller, not sure 3.5hp is enough to get full rpm for it though.

IMG_20210805_150049

To minimize the risk of ventilation did cut the transom another 6mm, can always lift the engine case needed. A word of safety, in the background the other 3.5 hp test motor, it did not have a kill switch lanyard, added one, would be extremely foolish doing even a short test without. If can not achieve speeds closer to 30 kmh wit this, will use the 5 hp motor, that will for sure go 30+ but the weight is already 21 kg and high, will be a handful to handle… Guess as using twin 6384 on the e version, that will be comparable to 5 hp 2stroke

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Do you have a link to that 2 blade propeller?

Unfortunately not. Its a pretty old Japanese prop, manufactured long before internet time. Not available anywhere, did not even find a picture on the web. After checking, believe pitch is close to 7, diameter is 172 mm. Relative small blade area compare to any of the 3 blade standard props, but worked well in this application. Possible would work rather economically on a efoil also? Some comparison can be made little later, i have the Flite prop… On a surf though, its little different as no foil. If promising maybe worth scanning to digital format if someone has such equipment?

Now we have a Yamaha clone that fits a Tohatsu 12mm shaft and pin. Made a mistake when banging out the plug so needed little additional filler to fix that but should be OK now. Don’t know will the JB weld hold up for 3.5 HP but guess its hard as any plastic so why not? Need still to drill the hole for the cotter pin keeping the propeller in place…not the easiest spot to drill( no straight surface and off center) but hope it will be ok.

Next test Sunday morning.

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After some careful measuring crossed my fingers and started drilling, probably one of the most scary free hand drilling’s have done. The stars was in the right position this time… so to my big surprise the new hole is located pretty much spot on, not sure would like to repeat this…only loss was two 2 mm drill bites, did not even drill my leg:)

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So had further test runs today. The idea was to test the 2 blade and the Yamaha clone with the motor having 1/1.85 gear. The good thing was that the modified Yamaha propeller as such worked on the outboard, and it was immediately noticeable that blade area and pitch is higher, much higher speed on idle. The bad thing was that the outboard suffered some technical problems, was not running properly under load, so unfortunately had to change motor back to the Tohatsu with 1/2.15 gear. So put the 8 pitch Yamaha prop on that one instead. All fine, engine had to work harder to spin that prop, but the propeller started to ventilate loosing grip totally at higher speed( when starting to plane). Transom is now 40 cm, so should be fine but the grip on these propellers are not that great( to say the least), so will continue cutting until get the Yamha to work properly. The disadvantage of shorter transom is obviously more drag. Its interesting though that the 2 blade has much less ventilation issues than the 3 blade ones. Believe the hull shape is also contributing to the ventilation, a slight v would probably be much better, not to mentioned the rough shape of the gear case on these small motors. To be continued…

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Is it called ventilation or cavitation?

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