Shroud affecting performance

[AAAkings]

I am wondering the kind of performance loss a shroud would produce on a standard prop. (A rough percentage wise would be good enough for my purpose).

The reasoning behind this is I am planning on making a motorized surfboard/kayak and was looking at a prop system due to the cost/size issues limiting my use of a jet driven system. I am willing to increase the engine size in order to accompany the loss of performance for safety purposes for being so close to the prop.

Does anybody have a different option for me? I am hoping to keep the engine cost around $500 and figure all the rest of the components will quickly add up.

[jehardiman]

What type of vessel, what type of use, and what speed? Your original question is too vague becuase the selection criteria for a shrouded prop, an open wheel, and a jet drive are so very different and not likely to be traded off against each other on a single hull. For example; a shallow water speedboat leans toward a jet drive, a medium displacement cruiser an open wheel, a log bronc wants a shrouded wheel.

FWIW, you only use a shrouded wheel for low speeds and high loads when you you can't fit an open wheel of the required size. Without a wheel diameter restriction, a shrouded prop is always less efficient than an open wheel and should not be fitted.

See these discussions

propulsion of kort nozzle
just a question?
Prop efficiency?

[AAAkings]

As stated in the first post, it will be intended for a motorized kayak or surfboard. It will be light weight, 200lb + motor weight which will be determined once a propulsion method is chosen.

The speed I am looking at achieving is around 35-40km/h. I did read those forums hence why I realize that it will decrease the performance for my purpose but safety is a major concern being so close/so easy to fall onto it. A jet drive unit will prove to be to large for my project unless I completely manufacture one (From what I have gathered it is not worth the trouble and cost). Hence why I have expressed interest into a prop.

I hope I have answered all the question you raised and if I could get some recommendations for a different method or if I would be able to achieve what I am after with a prop.

[jehardiman]

Ok, I just saw the other post. For a surf vessel that implies beaching so just go ahead and build a pump jet, it's not that hard. While it won't be as efficient, it would be more in tune with a small air cooled ICE powerplant at the speed you want to go.

Otherwise, what you are looking for is a propeller guard. The guard must be strong enough to support beaching loads and proctect the prop from ingestion of debris. You could attempt a shrouded prop, but whether it is an accelerating or deccelerating nozzle depends on the power-torque curve of the engine rather than on the prop. Remember that a pump jet is just a specialized accelerating shroud.

(Edit: Doh...I just saw the same thing in the first post...Bad Engineer, no cookie)

[AAAkings]

I don't plan to beach this watercraft, I am fine swimming it out to wear I can ride it without worrying about running aground. Where I am from it is seldom that there are waves suitable for surfing, so this would be used for simple enjoyment on calm days.

From what I have read it is not really a do-able project to build one. I consider myself to have excellent machining abilities (with help from my instructors, but have access to cnc machines which I could see being beneficial). I am guessing I would need to go back to the jet-drive forum to try and get information required to build such a drive unit?

I tried to google what an ICE power plant unit is but couldn't figure out what you meant?

If I do decide to go with a prop design encased in a guard/shroud, what horsepower should I look at? I am guessing a 9.9 Horsepower engine should be more than enough to propel it at my goal speed?

P.S. I want to thank you for all the information and putting up with my ignorance in this subject.

[jehardiman]

A jet drive itself is simple, the problem is 1) reducing the losses to make it comparable to open wheels and 2) the possible complexity of making it vectoring and backing. For your school project you don't need to deal with (2), and (1) can be solved by throwing hp at it (i.e. the common small boat solution to powering). A jet drive is nothing more than a specialized prop pumping water out a venturi nozzle. It just has a much small diameter and higher blade area ratio than an open wheel. Both suffer the same caviation issues.

ICE = internal combustion engine, and yes, 9.9 hp should be enough to to drive a kayak at the speed you want, but maybe not a surfboard because a surfboard is actually designed to have drag on the hull.

[yipster]

Surfboards come in heavy long old hawai to very small lightweight sinkers that only fload under speed.
Think it was on discovery channel I saw a board with small waterjet build in that may be worth googling up.

[AAAkings]

What size of engine would you recommend for for a surfboard then? The one has a 45HP engine in it but it seems like that is complete overkill. but then there is this one which only uses a 12HP. http://www.justwin.com.tw/products_detail.php?id=24

I actually got the idea from this video here. There are a few different models all with different specs. http://www.youtube.com/watch?v=qXDw1dJv950

Thanks for the help so far guys, its been really appreciated.

And in terms of an engine I was suggested to use a Balta 39cc engine, it is able to get a pocket bike up to and over 50mph, any thoughts?

Edit: Now that it is mid way through September (abstract due on the 30th), I am planning on making it a kayak opposed to a surfboard for ease of fitting an engine and venting issues. (Prop more suitable than before at least).