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

Just as an interesting aside, have a look at the Kitchen Rudder. It could be an interesting addition to your setup as it is more closely related to vectored thrust as opposed to a standard rudder. It also would help shroud the prop, you would just have to see how much drag there is and if you could carry it near the water surface and use some hard water and some spray to turn.

E

 

Rudder

I don't think having the rudder beside the prop will help rudder authority.

And judging by the photos you might have more than enough bow lift right now.

 

what happens is with the surfacing propeller, it grabs large amounts of air and puts it in the water just in front of the rudder rendering it inefffeective.

If you see the rudder wings (little ones on bottom) what would happen is at higher speeds over 80 mph, the rear would lift and I would lose prop thrust. I added those to rudders to pull tail down and lift bow...

I am hoping not to have to add wings to next rudder.

I know its hard to see whats happening by just the pictures, but I am open to suggestions and constructive critisism. Its just that when I first did a rudder, I put it behind the prop and it was undrivable after getting on plane... I would turn rudder nearly lock to lock with very little effect and it turns out the surfacing prop was rendering it ineffective.

At idle speeds, having the the prop thrust made it very much effective.

http://en.wikipedia.org/wiki/Kitchen_rudder thats cool

 

You're definitely right about the offset. The pictures I posted in #28 here show both Slo Mo Shun and Miss Budweiser using offset rudders. Any aspect of control that persists for 50 years and a doubling of speed is well proven, IMHO. Slo Mo's rudder is bolted to the transom and its shape is similar to the photos you posted in #42. I think you may be right about the shape helping to minimize lift. Miss Budweiser's is set back about 18", but remember that the forces there are orders of magnitude greater (much larger boat, several thousand hp). You could try your existing left side mount (on the transom), with a rudder of that shape extending 4 - 6" below the bottom of your prop, and see if it's enough. If not, try a setback mount, perhaps 6 -8". If needed, you can experiment with setback lengths until you get the effect that feels best to you.

 

Rudder

By aft of the propeller I didn't mean directly behind it. I meant similar to an Unlimited hydroplane configuration.

 

In any configeration flat plate rudders will only ever give you a maximum of 15 degrees of steering each way from straight ahead.More often much less with increased speed.They are like aircraft wings and stall beyond 12 to 15 degrees and create a breaking effect with no control at all.In an aircraft you would crash.Trim tabs added to the rear of a flat plate rudder should add another 5 degrees of steering each way from centre.

 

http://cgi.ebay.com/ebaymotors/HIGH...011QQitemZ320195754218QQrdZ1QQsspagenameZWD1V

I ordered this rudder off ebay. When it comes in, I will install on the left side like slo mo shun and see how it performs.

Thanks very much for comments and suggestions.... feel free to let them continue because I am always looking to update this project trying to fine tune

Tom, the stalling idea is most likely why my rudder didnt get much more effective the more I turned it .

 

Just to help get some terminology straight--because your new eBay rudder is an airfoil of sorts, it will have 'lift'--generated by differential water pressures that will be lower on one face of it and higher on the other. This is lift that you WANT--because it's what will turn your craft to one side or another; the lift generates your turning force. This is not the same as the lift that causes your stern to rise up at 80mph. That kind of lift could be a number of factors--air getting under your hull, the wing shape of the hull itself, effects from the surface prop pushing the water down (and therefore the hull up) and so forth, plus the vortex at the bottom of the rudder may have an effect as well.

As I mentioned in my earlier post, try to stay with very slight movements of the rudder at high speed, because just a few degrees of 'angle of attack' will cause the water flow to separate from your rudder and stall it, which mean it's now not providing any lift to turn you, and only generates drag (and not enough, apparently, to turn you).

I suggest you stick with the left-side mount in order to keep the rudder well away from the prop wash. Underwater prop wash is one thing--and you'll have prop walk from the wash hitting the rudder, but at least you're in a fairly homogeneous liquid stream--but you're running a surface prop and the rudder, if behind the prop, wouldn't see a viscous enough fluid to generate the turning lift you need; it would be turbulent and air-filled, I assume.

 

Rudder wedge

Here is a quick model of what I recall using for our Superboat, it was out of stainless and made longer than we needed for "fine tuning". At high speeds near 100 mph anything protruding in the water will be forced up, including rudders. This size was for dual rudders on a 50', 3200 hp offshore boat. I am glad to see you showing up those little waterjets used on all jetski's; jets are great for swimmer safety but for best planing speed its hard to beat a supercavitating prop set-up. I do worry about you losing high speed stability, the flat running trim of the jetski could cause an abrupt change of direction with the slightest passing wake or wave. It would happen so quickly a chute would be useless. Consider a bottom flat wedge (aft)to ride on, it concentrates the lift aft and if done properly will allow almost the entire keel to ride clear of the surface. By the way I have some 19" Levi drive props for sale if anyone is interested. (480x600mm P&S)

 

Thats a very intuitive thought you have about jetskis and how they are prone to bow steering. On many skis,this is very prevelant, the GPR like I am using was choosen for its bottom design and that its less likely to occur. Ride attitude is something i have to be very mindful , because when a ski drops bow, it is in blink of an eye and your in the water. It happened to me with jetpump. That is the reason I am working to keep bow up in all higherspeed runs.

 

Stability

As long as it keeps the bow from plowing in, it sounds like the extra aero drag from keeping the bow up is a good trade.

 

RPMC (Rotary Power Marine Corporation) tried to introduce a supercharged 13B rotary for marine use a few years ago. They put on in a Kenner bay boat (among other boats) with a surface drive and had two rudders aft, outside the propwash.

I am pleased for you in your success thus far. I wanted to try a similar project, but with two kids, I don't have the money for such a project just yet,. But your success encourages me that my idea was viable.

Here's where my ideas differed: I intended to build a Hickman type inverted vee roughly the size of a PWC, using the Rotary I mentioned. I would have used a long shaft, though, with a partial tunnel behind (Above) the prop with my rudders 12 to 18" aft of the prop.

Heres what I'd do:

1) Move rudders aft. More leverage, cleaner water for better bite

2) lengthen shaft to get more water to prop. That larger diameter prop is probably a good Idea, too.

3) put a skeg in the middle of the hull. Sharp leading edge, square trailing edge. Flat bottom on skeg with rear edge 1/2" lower than front, water pickup on aft edge of bottom of skeg. you'll get a little lift, clean water pickup, directional stability.

4)Make your rudders look like rudders, not like boards.

5)Rudders work better with a hull over them, so I intended to mount mine on hull extensions on either side of the "thrust tunnel".

I was hoping to hit 150 mph with the design I hoped to build.

I'll be wishing you the best and following this thread to see how you work out your problems. You have the power, keeping it hooked to the water without disaster is the trick.

I can't tell in your pictures...How thick is the leading edge of your rudders? If they aren't nearly knife-sharp, i'd guess that to be your main problem. at above 30 or 40 kts, they are probably super-cavitating. (Running in a pocket of air, NO BITE)

Someting else that might work...if you decide to extend the shaft, mount a u-joint at the transom and then angle the shaft to the left a couple degrees, then put a strut ahead of the prop with a cutlass bearing.

You have roughly 5 years to get this right. Both my kids will be grown by then, and if you don't have it figured out by then, I'll have to do it myself, and you'll have already worked out a lot of the bugs for me.

Best of luck!

Lance.

 

How much does the PWC motor weigh?

Avi8tor, that is wicked!

I want to do the same thing to an inflatable catamaran hull - we usually use 50-70Hp outboards at 70 - 85Kg.

How much does the PWC motor weigh?

 

I had an accident...

I was just making some passes testing a new wedge style rudder this weekend experimenting with different depths in the water. Its supported by 2 pillow bearings on top and bottom of mounting plate and they failed at 94-96 mph

It was with almost no warning went from making a decent pass to all of a sudden I lost steering and rear jumped around to left , it snapped back the other way and the right throttle grip came off in my hand as I was pitched into the water. I hit on my right shoulder and back. It knocked the breath out of me, but no real damage done, just sore. I climbed back on ski but it wouldnt restart, so I was towed back. Turns out the throttle linkage was tweaked and efi skis wont start unless throttle is closed.

I had already ordered a new rudder last week which ups tracking will put it here weds , and instead of using bearings, it uses bushings and is actually a real high performance rudder instead of one I made and welded.

Will have it back on the water this weekend and will make sure that I make the leading edge sharp like razor. I really liked how it was doing this weekend, so I think its getting closer

The PWC engines are light, but not sure how light. I havent ever had mine out, but I understand that one strong guy can lift it out by himself when intake and exhaust off it... They make a 150hp 2 cylinder for gp800r, you may want to look at that motor for your inflatable project.

 

Good that you are OK

94-96 mph with a homemade rudder; great! From your description would it be fair to say the steering was stable up until the failure?

Glad you weren't hurt.