Random Insanity.

This would be the most extensive and demonstrating video of the boat. This is how she looks, aside from her original paint job being painted over.

I can't speak for the actual top speed, since I have yet to get her where I'd feel safe putting her on the water.

The original builder has said that she has made about 50 mph, measured by a GPS. I can neither confirm nor deny that claim.

In the interrim, I've calculated out her best speed to be almost 42 knots. However: that is strictly hypothetical, based on the reduction in RPM through the transmission and differential without any other factors being considered, so fair warning with that.

 

So, now that you know what you have, what is it that you really are wanting to do? First let's recap: You have the most efficient hull type to accelerate up on plane quickly, yet the least comfortable and least suitable for any type of chop. The rudders are oriented in the most disturbed and turbulent water location they can be and are of both the wrong plan form and sectional shape, for your high speed needs. Lastly, the least efficient propulsion type has been used. Essentially, you have a couple of huge egg beaters attached to little more than a barge, spun up way faster than they should be, with poorly shaped rudders, in the worst position possible.

All you can do is something with the rudders. New plan form and sectional shapes and of course putting them forward of the wheels. I'm not trying to insult you, but you have to realize what you have there. If you want to soften the ride, slow down and/or make a new hull. There's no strakes or other devices that will make that type of hull run softer. If you want to go faster, toss the windmills and try outboards or an I/O(s). Steering on that puppy is always going to be rather vague at best, simply because this is the way rudders are on high speed powerboats, especially ones that are beating the water to a froth, just in front of them. Moving them forward will help, but they'll still be operating in highly ventilated water. It'll be an improvement, but it'll always be a bit sketchy at speed.

I'd say leave it alone. Cooking along at 50 MPH is faster than most pleasure craft and the boat is unique. I'm surprised it's stayed together, judging by some of the welds I saw, so just have fun, fix it when it breaks and just learn to live with her quirks. This is the nature of owning an oddball.

 

The dynamics of the steering at 3:38 is typical of shallow flat-bottom hulls with little lateral grip on water: https://youtu.be/Wd-zjtScnaw?t=3m38s
That sideways drifting is vaguely similar to that of an airboat.
It tells me that skid fins could help improve the rudder response, because you would introduce a more biting and a certain pivot point. But you would lose the shallow-water capability.
Apart this point of disagreement with PAR, I agree with the rest of his considerations.
So - it's your boat and money, hence you decide.

 

The rooster tail would show up on radar !

 

I agree Slavi, though I'll bet the skids will still be more than what most would consider comfortable, even with some fins. At the very least, so tail wagging should be expected and spinouts in hard cornering could also become an issue.

Moving the rudders, employing a wedge style of section and experimenting with fin sizes and locations will yield some results, but it's still going to be a twitchy beast.

 

Luckily, the previously mentioned gyroscopic effect from the paddlewheels is of help there IMO. If you watch carefully the last video, you'll notice that the boat tends to bank inwards during turns, almost as it had a bottom with a slight deadrise. That is IMO gyros at work.
The rudder action wants to rotate the paddlewheels axis, which in turn creates a torque in accordance to laws of gyroscopic motion and rolls the boat towards the turning direction.

You can also call it "the bicycle effect". When you turn the handlebar of a bike, the gyroscopic effect of the forward spinning wheel banks the entire bike. It is almost imposible to turn the handlebar of a bike without making it tilt inwards.

It is a collateral (and, I bet, unintended) safety feature of this paddlewheel arrangement, which IMO decreases the probability of both spinouts and rollovers. And its effect should get more pronounced as the speed increases, because paddlewheels spin faster.

 

Looking at the lack of shape on those paddle wheel blades, I'd of thought performance could be much improved. A lot of energy must be used up just smacking the top surface of the water...

 

I disagree in the stabilizing effect the rotation forces, the paddles impart will help save it from a spin out. I do agree they are having a significant effect, when biting hard, but I also could easily see the paddles getting so far to one side in a skid, that they simply lose all bite and the skid is on. At some point, I'd think the paddles will be more side edge on, instead of leading edge and the sudden drop in resistance, would permit the engine to really spin them up, making the wake literally nothing but froth. It would something interesting to run some models on.

 

That's a good idea.

 

As much as I dislike them, I would wear a life jacket while operating it.

Also, if the engine hasn't been ̶m̶a̶r̶i̶n̶e̶i̶z̶e̶d̶ ̶m̶a̶r̶i̶n̶e̶a̶d̶e̶d̶ ̶ converted for marine use, be aware of the explosion/fire hazards.

 

Marinated?

 

So, having reviewed the posts, and the polite disagreements so far...

I'm no expert, but my plan would be as follows:

1) Add 2 skid fins. I'm not exactly sure, but I would suspect that an ideal fin would either shaped like a sled runner, or be a right triangle, approximately 1" x 6" by 6 feet, positioned roughly 8" from the chine, with the back of the fin between 1.5 feet to 3 feet ahead of the transom.

2) redesign the rudders, possibly into something with a trapezoid shape, preferably out of 1/8" steel, or aluminum or something with a narrow front profile.

Repositioning the rudders would seem to be excessively time consuming and not cost effective. Unless I'm missing something, which I probably am.

 

One skid fin is enough, IMO. If you can place it along the centreline, that would be great.

Skid fins have an area comparable to the rudder, but sometimes can be tad bigger (depending on the maneuvering speed, the desired turning radius and the acceptable level of speed penalty). The planform shape is also similar to the shape of the rudder, with a swept-back leading edge to prevent a fin stall (which could lead to spin-outs) and catching of weed and plastic bags.

Sled runner-type (if I understood your intention correctly) is not an option - it would only create drag, with no significant effect on steering characteristics.

IMO, the best solution for the rudder re-design would be to slightly increase the depth of the existing one. However, as said in a previous post, it will also increase the hydrodynamic load on the rudder stock, so my advice is to get a help from an engineer or a good boat designer for that job.

 

I don't think any plan form changes to the rudders will do much in their current location. The real problem is their current location, which is in boiling water. Also sectional shape changes will also has minimal effect, if they're left in their current location. Now, if they're mounted forward, all this changes.

A centerline fin is ideal and it's placement can be critical, so expect to do some exsperementing with it's fore and aft location, until you find the sweet spot.

 

I absolutely agree, the rudder is currently working in aerated water, which dramatically degrades its performance. The reason why I have proposed to increase the rudder length is to put some rudder area in the clean water flow below wheels.

Which brings me to another possible modification, and it is a relatively simple one. It can be done by anyone with fair metalworking skills:



It will not dramatically enhance the rudder effectiveness, but by exposing 50% more rudder area to the clean flow the boat behaviour will surely improve to some degree. How much? The boat will tell.

I see two main downsides to this proposal:
1) requires some cutting and welding of the existing rudder bracing, so one has to be sure that he is technically able to do it. There is no going back, once the job starts.
2) the modified rudder will protrude below the hull bottom, hence it will be exposed to groundings.

In any case, the previous advice to put a skid fin remains valid. This boat needs lateral resistance which it currently lacks. With skid fins the shoal water capabilities are compromised anyways, so the above rudder modification wouldn't make things any worse than that.