Small boats

[peterCAPitALL]

Here is my take on a workable air drive for small boats, this is a shrouded 39" x 24" prop I had on a hover craft with a belt drive, as the belt sapped some power it would not rev up so I now made it direct drive. My calculations tell me that I get the same air penetration with a 24" pitch at 3600 r p m as with a 32" pitch at 2700 r p m. That is a 13 hp. Honda motor, I think I will try it on my 10 x 5 boat some time.

[Corky]

Very nice !! Is that shroud shaped like a venturi ??? How much more effeciency do you think the blades pull with a shroud like that ??? Small honda engines have proven themselves dependable for years...I'd take one of those over most other brands out there....

[peterCAPitALL]

Very nice !! Is that shroud shaped like a venturi ??? How much more effeciency do you think the blades pull with a shroud like that ??? Small honda engines have proven themselves dependable for years...I'd take one of those over most other brands out there....

Venturi, with a small 2" throat outside the tips for this small motor, it possibly increase thrust by 25-30% over open prop.The design of the "duct" inside and outside straighten out the air flow so it move more air than the same size open prop.

[Papee]

My question would be is the drag this shroud added be worth the effort?

[peterCAPitALL]

My question would be is the drag this shroud added be worth the effort?

By all accounts the drag only become a factor at higher speed and low trust cruising speeds over 60 m.p.h. Most small Hover craft use them. Ducted fan airboat see www.airboatuk.com

[kjkuebler1]

Have you tried this setup yet?
How did the boat run? What is the shroud made from?
If it works I may steal the idea.
I have a 27hp kohler v twin and it pushes my 14X48 jon boat under 10mph

[dr.peck76]

I love this idea. The ducts also quiet the fan noise substantially as well as increase thrust, from what I've read. My question is, How critical is the shape and distance from the fan tips for this to work as intended? Obviously, the better it is executed the better results you would achieve, but if a guy put a flat shroud 3-4" away would it work at all? As I understand these ducts have a bell-mouth cross-section.....

[Flathead]

I like the idea too! I spent some time researching this when I built the Fatcat, although I was more interested in noise suppression. "shrouded propellers" is still and interesting topic and I'm surprised there seems to be very little information on the subject as it pertains to airboats. Even a modest 10% more thrust would be significant in my boat. I can't tell if your duct is a straight tube or if is cambered. I's also love to hear results if you decide to try it! It would be really easy for me to add a shroud to the Fatcat seeing how my cage is already circular in design......

[peterCAPitALL]

I have not tried this on my boat yet, but judging from the push on the dolly it may have more thrust than my 45" x 32" on the boat. It's made from fiberglass, with a 40 mm. (1-1/2") bell mouth and 5 mm. (3/16") gap from the tip of the prop.
Some references suggest that you can get as much as 60% increase in thrust with a well designed duct (shroud),the ideal would be to have a odd number of blades to avoid resonance.I am working on a three blade model for a bush wacker motor.

[WBS]

I posted some info links under thread "thrust Improvement". You may find some info there of interest.

The best performing fan/thrust ducts see to be those from BBV hovercraft in the UK.