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

[avi8r]

Originally Posted by charmc

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.

thanks

Yeah, it was just a smaller version of what rambat shared with us. Just a steel wedge that I had lathed to a point at front and welded on the turning pin.

It was by far best effort yet and was reasonably stable for such a small craft

[avi8r]

Originally Posted by SaltOntheBrain

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.

Sounds like a pretty exciting project! Be careful and enjoy blazin a new approach.

I hope to have it sorted out by May, as I want to enjoy riding it this summer, not just working on it.

[skullhooker]

avi8r, great project. Glad you're ok. As a barefoot water skier, I know what the water feels like up to about 45 mph.
Above that, you're like a human cannon ball!

How are your knees doing? I used to jump skis in the surf and had to give that up, couldn't take the compression. I imagine hitting a chop at your speeds would feel like a 20 foot wave jump.

[rambat]

Before this forum is downloaded by your next of kin as evidence in the case of your untimely departure. I must state that there is no more dangerous of an environment than high speed transit over the surface of open water. We lost about half of the early 100+ mph offshore race boat club members in the late 80's.

I officially recommend remote control of your water rocket until the set-up proves stabile in chop, I am sure you could rig it. Even a gradual increase in your top speed as training will not prepare someone for a unknown tipping point in the set-up dynamics.

[RatliffFranklin]

Originally Posted by avi8r

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.

Before you even think about getting back into the water, get one of those drag chute jackets Fed Exed to you.

[RatliffFranklin]

Originally Posted by rambat

Before this forum is downloaded by your next of kin as evidence in the case of your untimely departure. I must state that there is no more dangerous of an environment than high speed transit over the surface of open water. We lost about half of the early 100+ mph offshore race boat club members in the late 80's.

I officially recommend remote control of your water rocket until the set-up proves stabile in chop, I am sure you could rig it. Even a gradual increase in your top speed as training will not prepare someone for a unknown tipping point in the set-up dynamics.

Radio control on the repaired Tempo Alcoa saved Les Staudacher's life.

[avi8r]

Originally Posted by rambat

Before this forum is downloaded by your next of kin as evidence in the case of your untimely departure. I must state that there is no more dangerous of an environment than high speed transit over the surface of open water. We lost about half of the early 100+ mph offshore race boat club members in the late 80's.

I officially recommend remote control of your water rocket until the set-up proves stabile in chop, I am sure you could rig it. Even a gradual increase in your top speed as training will not prepare someone for a unknown tipping point in the set-up dynamics.

I really appreciate your sentiment, and know that it comes from genuine concern, but if I have to wear drag chutes and remote controls to be safe, I will just slow it down a bit and enjoy the sunset.

This ski isnt being built for high speed runs or any records, just my everyday rec ski... 90% of its life will be spent cruising at speeds under 50mph...

I just like taking a walk on the wild side every now and then.

Last project I did was take my wifes 21 cougar and remove the one 280 engine and install 2 engines with 150 nitrous each..... 900hp on 900lb (bare hull) boat. Very seldom did I take it over 115, but it was nice having the acceleration and ride an occassional wheelie

If this feels ok, I willl sqeeze the throttle a bit, but most time will just enjoy the ride doing something different. The information you have given me on rudders has been very valuable and I appreciate that and the concern.

my last project

[SaltOntheBrain]

On second thought, maybe you oughta go all out on your speed quest.

Don't worry, if anything happens, we'll comfort your widow.

Just curious, does she still have the boat.

Just kidding, the one I have is all I need and I hope she doesn't see this post.

Lance.

[avi8r]

Originally Posted by SaltOntheBrain

On second thought, maybe you oughta go all out on your speed quest.

Don't worry, if anything happens, we'll comfort your widow.

OMG hahahah she just read this thread and what you wrote and asked if you would post a picture

[rambat]

I appreciate your approach to the speed issue and it is surely some type of a first in the PWC world. The other project looks fantastic and the boats not bad either.

[SaltOntheBrain]

Why would she want me to post a picture?

Does she like short fat guys?

You're a lucky man, having a wife with a sense of humor. That sounds like something my wife would say.

Lance.

[SaltOntheBrain]

Here's one...

I Keep mine well fed.

Lance.

[Frosty]

Im looking ---- I dont see it--- So what did you make up in the workshop last Sunday then?

[avi8r]

[quote=SaltOntheBrain;187555]
Don't worry, if anything happens, we'll comfort your widow.

QUOTE]

she wanted your picture because she said if someone else is comforting her , he had to be good looking and rich... ( i guess this time around, being funny just wont be enough) lol

[charmc]

Originally Posted by avi8r

she wanted your picture because she said if someone else is comforting her , he had to be good looking and rich... ( i guess this time around, being funny just wont be enough) lol

Beautiful, smart, and has a sense of humor ... If I were you, I'd be wearing that safety gear.