Angle of attack

[achris]

As a side track to the 'Airplane on the treadmill' thread and so it doesn't get highjacked I have to ask if anybody here uses angle of attack when doing prop calculations for boats. The reason I ask is because, years ago, I read the merc prop book and they talked a lot about angle of attack, rather than 'slip'. As an interesting exercise I wrote a program, in basic, to calculate the angle of attack given all the usual parameters.

Does anyone use this method rather than slip?

cheers.

 

[Bass Man Bruce]

Re: Angle of attack

I assume you mean calculations to predict boat speed based on prop pitch/size, boat and motor. I don't really see how anyone could calculate this as one versus the other. It seems to me they all must be included.

cheers.

 

[jimyoung]

Re: Angle of attack

Is this another Teaser?

 

[oops!]

Re: Angle of attack

chris ive wondered about that

the aoa is totally different in most speed modes of a boat... until we use the trim....
even center engine boats have a weird aoa.
its gotta have a great deal to do with slip, but i dont think anyone uses thoes facts when calculating.
i read the slip charts. and it doesnt look like it

oops

 

[achris]

Re: Angle of attack

I was talking about the angle of attack for each blade to the water, not the propshaft down angle. According to Merc the ideal aoa is 4 degrees plus or minus 1 degree. I have run a lot of boats through that program I wrote and unless there was something screwy with one of the figures (speed by speedo, not GPS or the tacho was miles out) they all came in about the right area. The aoa does away with the slip question altogether. The slip will vary from boat to boat and from prop to prop. aoa should always be 4 +/- 1.

Cheers,

Chris............