Question on propeller piercing waterline

I’m considering a propeller for use on a surfboard (wave assist only) and was wondering how much thrust I would lose if half of the propeller was out of the water?

Just brainstorming ideas on how to reduce drag if I switch from a jet to a prop. It looks like the folding props are pretty expensive so I was thinking I could mount the motor so half of the prop was out of the water and get half as much drag when the motor is off while I’m on a wave. If I don’t lose half the thrust while the motor is on (spinning the propeller through half water and half air) this might be worth it.

Any thoughts?

There are special types of props that work this way. Look for long tail boat propellers (LTBP). Thaï boats use them for shallow waters.

Std - Improved LTPB

it could work i don’t really know. but its going to cavitate a lot, and the motor will be spinning at a higher rpm so you will probably lose efficiency.
do you already have the motor and electronics?

No I haven’t bought anything yet other than 2 kedean jets: the most powerful one and the smallest one. I put the most powerful one on a longboard and not impressed with the kedean jets for my use case. Given how complex jets are I might just go propeller and come up with simple ways to reduce drag.

i think at this level of power and the drag of the board itself , the drag of a motor and its prop will not be the problem

i like it some much that i post it again , such a nice built :wink:

EDIT/ wave assist only… what speed are you look for ?

Not sure what speed I need, just enough to assist duck diving waves, assistance padding over broken waves (whitewater), and assistance getting up to speed to get onto a wave.

The last requirement requires serious boost from stopped, but only for like 5 seconds. The first two just require a steady pace for around 10 seconds ish.

I’m happy to also paddle with the motor, so that might help, but after testing out the jet motor today o n waves I was getting lazy and not paddling much :wink:

2 small jets :
or 65151 with 80-100mm prop (cut solas plastic prop) 2/3 in the water at least maybe

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Hmm what’s the smallest jet nozzle diameter I could get away with if I used two? 25mm? 40? I was thinking maybe I could get away with using the RC jet setups on a surfboard, but I didn’t think they’d have enough thrust to move me. I don’t like needing two motors two ESC though

Most of smaller high speed boats have the propeller shaft close to the surface and you can have very good efficiency. They are not all cleaver props, also pretty ordinary looking props can grip okay in this mode. The problem is that usually they work well in higher speed, but hole shot is not necessarily good, and they are anyway deeper in the water at start. Long tail, like mentioned, works also this way, but operators seam to adjust the height in water constantly for optimal rpm( they are often direct drive, no gearing) so don’t think a surface prop is really workably in this application.

I would mount the motor and prop little like an outboard close to the stearn, and then have kind of a hinge & spring activated function so it can be kicked up from the water entirely when wanted.

5 years ago I strapped a 12v 33lbs trolling motor on the bottom of my first softboard which was pretty tiny for my weight/skills.
I’ll try to find some pictures and edit this post.
Powered by small lithium car jump-starter, 10A fuse with a cordless drill trigger, it was awkward, I had very little electrical knowledge back then, 200w~250w, sure got me to speed for paddling back from the shore and catching the swell but the problem was it was so heavy and bulky with no prop guard, no kill switch and thick cables everywhere.
Nowadays you have pretty much unlimited options, lightweight and efficient, for example this:
And this:
And this:

I would just do a jet drive for this, do a custom fully contained unit that sits in a cutaway as thin as the board, I’ve seen it done somewhere, just cant find it again.
Minimal drag, nice kick to get going and relative low noise.

That’s what I did for my v1:

Its heavy but it works. I’m probably overthinking V2🤡… now leaning towards a 65161 with 3d printed folding prop

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The issue with the maytech motor on a surfboard is that it won’t remain a surfboard anymore, if all you need is a paddle assist then it’s a great example of an overkill, big battery pack, drag, big prop, no carving, plus, a potentially dangerous limb chopper.
You can consider it as a 6hp outboard motor, sure, I’ll buy one or two in the future anyway for my efoil/inflatable boats, not for a surfboard.

I too would stick with the integrated jet drive.
I think its a very sleek and discrete way of implementing a wave assist.

If you now trim the back of your board so it is flush with the jet outlet your board would look perfekt. And the best thing is, it still keeps the slim profile of a surfboard. No big chunky motor underneath it.

In my opinion a prop behind the board is no option. It would be way to easy to fall backwards onto the prop. So if you want to build a prop driven board, the motor and prop need to be located underneath the board. That means the motor would sit probably 20cm below the board to make enough clearance for the prop.

what if I build a shroud that covers the top half of the prop? Realistically it’s way more likely to chop off my leash than my limb, but I see your point.

with a 12s1p or 14s1p (only using it for assist!), a 80-100mm folding prop, it should be light enough with minimal drag. With the right board shape (most of the esurf things you see are terrible shapes, would never be able to cave on a wave), i think it might work!

Well I guess it could work. I have no idea how a prop half in the air would behave.
The question is if it’s wort it.
Have a look at “waterwolf”. Maybe that could be an option too.

I still would prefer the jet drive though. It’s just so much simpler, more save and in my opinion more elegant as well

You want the prop half way out of the water because of drag right?
But if you use a folding prop is that even necessary?

And if I understood you correctly the keadan jet drive doesn’t produce enough thrust for a wave assist.
I don’t have a keadan, so this is just a wild guess. But maybe there are different more powerful motors that fit the keadan jet.

the problem is jets have more parts and variables to figure out: nozzle size, nozzle elevation, impeller, stator, intake optimization. I agree they are more elegant but i’m just a guy who can barely solder and draw things…

when I started this thread I was under the impression my only option to get a folding prop was to spend $1k, which is a no go. I wanted to use a regular prop but mount it 1/3 above waterline to reduce drag. However, someone on here 3d printed a folding prop so I’ll try that AND also keep it 1/3 out of the water as I don’t want the motor to be below the bottom of the surfboard – will spend time dreaming about it today. Might make a custom middle fin to house the motor