You want an airfoil that is self-propelling? That would be good.

Haha.... Ooops 🙂

Thanks

Lift is normal to the flow, right? (not normal to the wing's chord)

so with an wing inclined to the flow direction (angle of attack), then the lift is acting forward of the normal of the chord,
but not "forward" as in generating thrust.

another day in paradise, or is paradise one day closer ?

I took the question to mean that the summation of all the pressure vectors would point forward of the chord - one might define any arbitrary frame of reference and separate out that vector sum in some other way.

i took as key the term "sum."

but that's what I meant. Lift is "forward of the normal of the chord" when the wing has incidence, no?
this is why we have to take components of lift to put it into the structural axis of the wing, no?
but this does not create "thrust".

another day in paradise, or is paradise one day closer ?

A picture could be worth a whole bunch of words.

But I think the first three posts in this thread have opened and closed the question.

Thanks all

I made the mistake of posting without really thinking it through. I wasn't really thinking of it in that context but I should have been.

I have a well performing cruising catamaran, the water flow at the stern is flowing up the hull profile and is coming up at 7.5* to the waterline and I have been trying to figure out if I can claim something back by placing a hydrofoil in that angled water flow to produce some meaningful forward thrust.

A guy named Newton says that the best you can hope for is to make the drag caused by that wake feature less.

If anyone races this type of boat, even in the Friday night marina series, there is certain to be a body of internet discussion about how to do that.

You would think so but I have done a lot of searching and posting questions on various boating forums, both cruising and racing. There are very few people on those forums who have an understanding of hydrofoils in marine use. I am sure the ones that do are keeping their research to themselves.... ha, racers hey.... but in any event there are next to no boats using foils to assist in drag reduction without flying, it seems to be the domain of the high end boats that fully fly or the dinghy racers who also fly and very little on gaining that few % advantage. I find it hard to believe there isn't something meaningful to be gained in this.

I don't think you can generate thrust, but you can generate lift. whether it's worth the extra drag is another question.

you could try the "engineers with hobbies" forum.

another day in paradise, or is paradise one day closer ?

Yes, I was also going to suggest moving this discussion to forum1528: Engineers with Hobbies, because it seems a hobby question, not work related.

Regardless, it's a displacement hull. Water has to be....displaced.... as the boat moves through the water.

The flow pattern is what it is now because that is quite literally the path of least resistance for that water. Water is lazy like that, it will find the easiest way to get where it needs to be.

Trying to force it to do something else will increase drag.

To decrease drag you need to give it a lower energy option.

yes, but the foils can create lift to offset some of the weight/displacement.

another day in paradise, or is paradise one day closer ?

Quote (rb1957)

but the foils can create lift to offset some of the weight/displacement.

But not for free. Making lift makes drag.

Until you change the regime and transition to a hydrofoil Newton dictates that it is a losing proposition.

which I commented on ... "whether it's worth the extra drag is another question".

I agree with you that it's probably a losing proposition. I wonder if you'd lose even the small lift from the hulls.

another day in paradise, or is paradise one day closer ?

I'm assuming something like this:

I'd have more of a "pop" as the hulls break clear. They seem to work that way, needing some extra drive to break maybe the surface tension ?

another day in paradise, or is paradise one day closer ?

Is this not a little like the Blackbird land yacht ?
or the same as sailing (tacking) upwind at an angle to wind direction and making ground upwind

Leveraging the differential in the waterflow angle to the direction of travel ?

I expect enough turbulence is in the mix that surface tension is only a factor for drag rather than keeping the hull in the water.

Coandă may still have something to say - hence the step on float plane floats and hulls to prevent the low pressure at the rear from producing drag at higher speeds. Similar problem though the foil is in the air.

well, hull shape "clearly" has something to do with it ... see the different shapes for AC35 boats.

I think that "land yacht" works 'cause a propeller can generate thrust with -ve speed ... something a sail can't do.

another day in paradise, or is paradise one day closer ?

A sail can't run directly upwind because the air flow is collinear to the direction of travel so there is no percentage of force available to propel the boat directly up wind but mine can sail at around 3/4 of true wind speed at an AOA of 30° to apparent wind.

My thinking is.

The land yacht changes the AOA over the propeller by altering the mechanical drive by 90°, and then again with the pitch of the propeller.

I have waterflow at 7.5° to the direction of travel. I was hoping in a dense fluid like water, 7.5° might add a knot or two ?

your foil application is nothing like the land yacht.

The lift generated by the foil would be normal to the direction of travel (ie not thrust). It's possible that adding lift at the stern may well increase the boat's drag by pushing the bows into the water.

But who knows ... go try it and let us know.

another day in paradise, or is paradise one day closer ?

Sorry, just working through this and it's all new to me...

If lift is perpendicular to drag and drag is collinear to the flow and the flow is 7.5° to the direction of travel of the boat then shouldn't a significant amount of lift be propelling the boat forward If I put a foil in that waterflow?