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

avi8r,
so during your excursions, have you got any more "U-tube" to impress us with? Well done sir... Rativied by Guinnes records yet?

 

106! Summer's coming

 

Oh, man, you're positively nuts!!!

 

I would give up on the rudder idea unless you want to redesign the hull. What you need is a a thruster for high speed and rudder for low. See if you can channel water or air to one side or another. It might also work by shifting weight (including your body). Have you consider doing the stearing at the air level (airplane rudder deal).

 

One more thing! Maybe some kind of trimtab rudder foot operated might work at the speeds you are attempting. When I was a kid I used to run downhill in a shopping card and used my foot combined with my weight (touching= breaking left and right to change direction).

 

I have not read the entire thread but most of it. My opinion is that the problem is the prop walking indeed. The prop pushes the back of the pwc to the left so the pwc has the tendency to go right all the time. This happens because the prop doesn't push the stream of water exactly at the back of the pwc but a little bit to the right due to friction between the prop blades and the water. If there was no friction the stream of water would go exactly straight to the back of the pwc. To solve this problem the best solution for me would be to "redirect" the stream of water before it hits the propeller, a little bit to the left. So after it hits the propeller it will go straight to the back. In simple words put two fins shaped like inverted shark fins in front of the propeller to "redirect" the stream of water slightly to the left. They must go as deep in the water as prop goes. The spacing between them should be one third of the prop diameter. After this the two rudders you initially have put on the pwc will work fine. The spacing between the rudders should be a little bit more than the propeller's diameter. The axle or the rudders should be not vertical but slightly inclined backwards. Please let me know the top speed you will achieve.

 

A superstructure that arches over the prop and places a modest rudder behind the prop will most likley be the best solution.

 

I have not read every page of this thead, so maybe someone else has mentioned "torque fins".Model boaters use this ploy almost invariably on fast, surface drive, boats. It consists of a small vertical fin placed somewhere aft and well offset from the center line. For racing hydros the fin is on the right side because they always turn left around the race course. The fin is adjustable fore and aft such that you can put a bit of incidence in the foil/fin to counteract the prop walk. Many of the fast models have a tremendous power to weight ratio. I expect you have that too. Esecially when you used nitrous. The models are prone to "blowovers" so they also use horizontal trim tabs and avoid rough water like the plague.

Several of us think that you may be out of your mind, but our participation in this thread suggests that we have some envy. Make sure your insurance is paid up and then go for it dude!

 

What a great project, well done Avi8r!!!
my 2 cents...
I would be nervous about a bronze casting if the rudder is mission critical.
These are not permitted on many racing classes..

If you want to drop the torque / prop walk you could always get a gearbox ( from god knows where but I'm sure Weismann could whip you up something) run a step up ( so you can run a smaller pitch)and opposite rotation than the engine?
It does look like a lot of prop for such a small hull to me?
What is your prop slip ratio now?
Is the prop shaft above the running waterline?
You could also make a cradle that the engine sits in and the skeg is attached to ( so you can move the whole lot around)to allow alteration of both horizontal and sideways shaft angle

You could always try making alterations to the hull as I guess its symmetrical and maybe make alterations only to one side

envious
Powerabout

 

That may be something I will try next. I see big advantage of low speed rudder effectiveness....at idle the rudder can direct prop thrust At highspeed, it seems like it may even have less drag as the rudder would be traveling through aireated water if behind propeller?

Do you know of any prevaling reasons why rudder should be in front of or behind the prop at extreme highspeeds?

I added a stabilizer torque tab... ( see picture when I was epoxying it into hull) I choose to put it in the center since I turn equally left and right. I tried it out and it helped rudder effectiveness to a small degree, but I like it being there to help protect the expensive stuff from debri on the water. I also think it would effectively help directional stability.

You suggest an interesting way to tackle this problem..... An adjustable engine cradle.. hmmmm interesting.. Hopefully I can get it resolved without having to do that, but it does present an option if I get stumped.


My prop slip is about .06 with the 10in 3 and 4 blade props I have been running. I really need more propslip in midrange to help allow the engine to climb into highrpms for best performance. With my high surface drive ( propshaft 2inches above bottom of ski) I needed this big of propeller to reach down into the water for planning. I have since gotten a 8 5/8 2blade custom menkens propeller. It works great while on plane, but I have to get towed on plane as it ventilates badly and wont bite water enough to get going. I moved propeller shaft back 4inches further and this helped tremendously, but still sent it back to Dallas to add some cupping to help acceleration out of gate.....

While prop is sent off.. I added a propshaft support and moved the rudder 2inches closer inboard. Its still offset to the left, but not as much so. The ski is tracking relatively straight now with the FIN on bottom and longer propshaft.
I just need to weigh merits on having rudder in front (offset to left) of prop and behind it....
extended propshaft and 2blade prop


added driveshaft support, moved rudder in 2inches and moved waterpickup to back of rudder ( and filled in rudder to increase structure stability) when I determine where and shape of rudder, I will get a nice one made. Right now everything is still in roughed in functional mockup testing.
4blade


and 3blade.. these customs props arent cheap

 

Wasn't it OMC that did this exact same thing to trim their out drive back in the 70'-80's?

They had a hydraulic lift cradle inside the boat and a large corrugated rubber bladder that allowed for a flexible seal. All the thrust from the out drive went to the engine cradle and then to the stringers so the transom saw no thrust.

 

Good one avi8r, keep flying, keep safe, best wishes....

 

Thats right they did.

 

avi8r

Anyone got a photo of an old Unlimited Hydro? as they usually had a single rudder offset from the prop and a little behind it.

The further the rudder is behind the boat the more leverage it has over the hull.

 

Thanks guys for support and suggestions.. I certainly appreciate the help

Below are some a few of the props i've tested...

2 blades 3 blade and 4 blades configurations

Pitches ranging from 12 - 17in pitch

Clevers (tail lifting) Choppers (bow lifting) and drag props

sizes ranging from 8 5/8 to 12 in

and also different materials including brass, bronze, mild steel and stainless steel to in effort to find perfect blade that flexed when needed to and stiff when it needed

Now... I have a propeller that accelerates pretty hard which most likely will be my everyday recreational rider and one that ventilates badly out of the gate, but pulls like freight train 70mph plus....