Thrust area

Hi gang,

Im getting ready to cut my parts out of plate aluminum and tweaking the plan as I go. I can put it on paper all day long, but really need to see and hold it for a good grip on the plan

Anyway, my question of the day is: How much area is the flow from a prop in reverse? My design has a 45 degree down deflector because my transom is true 90 degree and wouldnt work in reverse worth beans. Does the column stay pretty much intact in the first 24-36"? So with my 15" prop, is the 16" ramp enough to get by with?

Thanks, Chris

 

Frankly, I didn't understand a single thing about what you're describing and asking. Not necessarily your fault, it could be just a matter of language barrier.
Do you have any explanatory pics or drawings to show?

 

A. I get home I can post one. Basically, my transom is flat top to bottom. So in surface mode, the reverse wash will hit the stern and state the thrust.
So I have my SD support structure coming up and out at a 45 degree angle. The reverse wash should hit the angle and follow down under the boat, giving me better reverse. In theory at least.

My clumsy question is how wide does that 45 degree surface need to be to be effective?

Is that any better or still mud?

 

What type of boat?

What propulsion system? Outboard, I/O, inboard, jet?

What is an SD support structure?

 

Oh. It's posted in surface drive and I took that for granted. Sorry

20' by 7' slight V aluminum sled with new trimmable surface drive build project underway.

 

Yes, much clearer now.
Your reasoning works when going in reverse, but will imo hamper the planing characteristics of the craft, unless the deflector is engaged only in reverse and is lifted up when moving forward. You need a nearly-vertical transom to get a good handling characteristics and low drag in planing regime.

In theory, a mid-way solution could be a fixed deflector which doesn't extend all the way down to the transom edge, but leaves a vertical area for a flow separation around the bottom edge. See the attached explanatory drawing (which also shows how I understand your question). And in this case, I would make it as wide as I can, in order to prevent the plate create excessive pressure drag in reverse. The flow area is more or less same as the prop disc area, but once it finds the plate obstructing it's path it will want to expand around it, hence it's better to make the plate wide.

But I am not an SD expert, that's just theory and it might be wrong. If I were you I'd hear a second opinion from folks with a significant real-life experience with surface drives construction and setup. One of them is Baeckmo, who can be contacted through this page: http://www.boatdesign.net/forums/profile/baeckmo.html

Cheers

 

I am stuck with the 45 degree off the bottom of the boat. What if I add a small strip at the base so that there is a sharp trailing edge? The 45 degree section is only 16" wide out of 7 foot width. How does that work on most boats that have a 14 degree transom. An outboard wont hang on a vertical transom properly. Or at least I couldnt get it too on this one.