Why are my rudders so ineffective ? (pwc converted to propeller)

Safety

While we're on the subject of falling off, it may be a good idea to invest in a personnel drag chute of the type still worn by some drag boat racers.

 

And some prop shrouding.

Ever seen what a cuisinart does to meat?

E

 

Is there any head/neck protection you use or could use? If you can get one that might resist "bucketing" (snapping neck like a wishbone when you fall at 100mph). Looks cool. Fast, maybe too fast for me.

 

Safety

That's what personnel drag chutes are for.

http://www.lifelinejackets.com/chute_jackets.cfm

 

Yes, I use a lifeline jacket with kevlar , neck collar and helmet on speed runs. Also have a prop gaurd that blocks me from hitting prop in the even of a dissmount. That chute looks pretty cool also, i will read up on it more .

David

 

SC Rudder

There is a specific cavitating rudder shape which is a narrow wedge with a flat trailing edge, Its widest dimension. It is the same profile from a section of the cavitating propeller if straight. If I can find the design we used on a 100+ mph offshore boat, I'll post it.

 

rudder improvement

I have a consideration for you--if you're using the water inlet and out for engine or exhaust cooling, and you're using a surface prop and what will probably be a cavitating rudder, why not eliminate the water line from the rudder and tuck it into a high-pressure wet area (like the tips of your hull skegs)? Then you can either turn the rudder into the thinnest plate possible to cut drag (find someone who can work up some titanium for you, or consider carbon fibre) or go to a really slender symmetrical NACA profile; you're looking at only 100 mph, but you're in a much denser medium than air, and some of the fighter plane fin and rudder profiles would work nicely for you. If you're offset enough to be out of the bubbles from the surface prop, you could actually generate some lift if you had the right shape to keep the laminar flow attached. You could also try some of the dimpled surface material that intentionally creates microturbulence to keep flow attached--my BMW has it on the side mirrors to keep flow attached and cut down wind noise. Steve Killing describes such things as 'dillets' as opposed to fillets.

If you're using the water lines for something else, ignore this--or explain to us sailboat guys what you're doing.

Oh--another thing--assuming you were actually getting some/any lift or pressure differential flow on your rudders, adding those fins you tried could help accidentally helped you keeping flow from detaching and creating tip vortices of the end of what would be some pretty high aspect ratio wings, and maybe that's why they worked better--you added winglets. Your latest rudder drawing, however, reverts back to a very low aspect ratio, so you're likely to see a strong vortex and little or no lift--you're just building a drag plate/swash plate. And as was pointed out, dragging is very inefficient way to turn something. At really high speeds with a viscous fluid like water, if you could actually use a NACA profile with the proper Reynolds numbers calculations that could provide a laminar flow, you would need only a touch of lift to port or starboard to turn your craft. As a matter of fact, moving your steering fast or far would stall your rudder, (which would also happen with a high-lift contoured profile, which is why I suggest a really slim NACA profile) and render it pretty much ineffective.

I helped out on an amateur rocket design about 10 years back; the guy built an 8 meter single-stage rocket that was trying for 100,000 feet altitude but only made about 73,000 feet. Our biggest difficulty was stability, and we designed fins with such a slim profile and subtle curve you could barely see it, but it was enough, given the speed of the thing, to keep laminar flow before hitting the sound barrier and get the rocket into a longitudinal 'spin profile' that kept it stable. If we could keep fins 'lifting' at 700 mph in air, you oughta be able to do the same in water at 100 mph. Except you've got that surface prop muddying--I mean bubbling--the waters. It's a complex problem... and I wish you the best of luck.

If you need NACA profile information, I've got it--virtually all of them-- and could calculate station thicknesses for you and your CNC mill.

 

Rudder design is something I know very little of. I just built what I thought would work, and then kept making it bigger until it did work.

I spoke to a hydroplane racer today on the phone, and he described to me a 1/2 thick piece of steel that was 1/2 thick in back and went to a point in front like triangle. As far as what size I would need, I just can only geuss unless someone could calculate what I needed ( depth , width , and legnth)

I do use those pickup lines for cooling. When I first did this project, I had the cooling water pickup on the hull, the problem is, where I ride has a lot of trash in the water and within a few miles, it would stop up my filter (after rainey day and leaves all over top of water)

Moving the water pickup to the bottom of the rudders eliminated 99% of the trash in filters.

So I think the plan will be to use 1 rudder leftside mounted, and if you have a NACA profile or close up photos of design that you think would work, I will have access to a CNC machine this Sunday.

David

 

David
The triangle makes sense with the fact that it is aerating. It is just deflecting flow not a true lifting foil so no point in a nice NACA shape. Might be benefit in thinning the trailing edge but if it is relying on drag to counter side walk then it needs to have drag anyhow.

Rick W.

 

Rudders

"...a 1/2 thick piece of steel that was 1/2 thick in back and went to a point in front like triangle..."

I see no particular reason why that wouldn't work as is.

 

ok, 1 rudder wedge shape

Any idea how big? and how would a torque tab work out on the back right?

Also, How far back can I put the pivot point ? I would like it as far back as possible to help with steering force, but not so far back its twitchy. Would 65% behind pivot and 35% in front be good or too much?

 

How about this design, its 3/4 wide in back and tapers down to 1/8 in front.

Having it angled up the backside looks like it would cause a lowpressure and hopefully offset some lift (I want as little lift from rudder as possible)

 

Rudder

I'd set your rudder about two feet aft of the prop, and be prepared to make some adjustments in steering ratio.

 

This rudder in picture looks ok to use? ( i see it on ebay for sale)

So please help me understand, why would you put it 2ft aft of propeller rather then just on left side?


Here is my thinking...
the further back it is.. the more leverage it will have on ski to push nose down from lifting of rudder. I need more bow lift , not less

If mounted on left side in clear water, the subframe to mount it would be easier, lighter, and inherently stronger then mounting so far back as well as being more esthetically pleasing on such a small craft.

If mounted on the left, the drag of rudder would help offset the drift to right.

Most all hydroplane racing boats run offset rudders with surface drives, why do you feel in this application its best to put aft?

thanks David

 

If you can get hold of 'High Speed Small Craft' by Peter du Cane you might find some useful information. Though the book is old it does have lots of good stuff.
My instinct is that whatever your prop is pushing backwards is driving your machine forward very effectively, if you can direct that stream in some way you should achieve steering. I would try a pair of rudders quite close together and quite close behind the prop. The rudder in your pictures is a fairly conventional high speed rudder; in a 'normal' fast-ish boat you would have one of these behind each of two props - the rudder would be below a planing hull though. It is the sort of proportions that should work - unlike the miniature barn door further back in the thread.
A rudder is a lift device, it should not need a much angle of incidence at your speeds to be effective.

Apologies for a somewhat rambling post.