Modeling the Angle of Slipstream

What modeling techniques are most practical for estimating the angle of the slipstream in front of the propeller? The application here is for adding ice horns and if they are not angled properly then obviously there will be added drag.

If modelling is too sophisticated, then perhaps building a small scale model?

 

looks like no one responed to this post.

If it were me, I would do some flow tests on the boat hull itself. you could mount an underwater camera to the side and put some brightly colored ribbons at various places up stream of the prop. Mount some standouts or blades to get the tell tales higher out into the slip stream.

how larger a boat are you talking about? If it is really large (like a full sized freighter) it might be less costly to build a model, or even do some computational modeling of the flow.

 

I would begin by trying to estimate the actual drag that the horns would create as a function of their angle to the direction of travel and the error in flow alignment. Since the load will vary in ice, the inflow twist will very also. The question is, what condition do you design for and which side of the of the box do you want to be in during off design operation. You also want to include the effect of prop efficiency due to the changed inflow conditions. Perhaps what you lose in the horns can be recovered by the prop through higher efficiency.

Now, having said all that, aren't they mounted aft for protecting things while in reverse? Is it the inflow during reverse, or the outflow during normal ice-free operations that you want to have aligned properly?

 

I assume these horns are like a heavy fin to deflect the ice (a drawing would be nice). If so, we use similar to deflect driftwood & logs. A simple, heavy fin mounted on the hull, for'd of the prop. They work.. sometimes.

ps: I did not know you had ice issues in Qatar.

 

I have a doubt here, after reading the OP.
Is the "slipstream" intended as the wake of the hull, or as the twist of the flow entering the prop disc, induced by the prop rotation?

 

The principle is similar for deflecting logs, driftwood, nets and ice, so yes, the location would be in front of the prop. So the concern was the twist of the water flow entering the prop disc, induced by the prop rotation.

Thinking more about it, for a cruiser traveling at 7 to 8 knots, the drag produced by a few metal horns is rather negligible. Maybe I'm thinking too hard.

PS - I really wish we had ice problems here in Qatar. Would be a lot more fun to live here.

 

Log deflectors fitted on boats with I/O's (sterndrives) must be several feet for'd of the props. The manufacturers call for anywhere up to 10' (3m). On a slow boat, flow disturbance is not such a big issue unless its a big hunk of pipe or similar.
You don't want ice. Even the thin stuff (6 to 12mm) can damage hulls.