Hydro/Runabout Info for New Guys

[Tim Maynard]

Greetings from Borneo! I've been working in Kalimantan, the Indonesian side of Borneo, for several years and will be returning to the States in December. I'm sure this is not the first request for info on this sport, just don't know where to find it.

I'm planning to buy or build a boat and have a LOT to learn! Can anyone direct me to info on Hydro and Runabout "theory" - basic racing knowledge.
Like:
Why do you run the props you do?
Do they run partially out of the water and why?
Differences between Hydro and Runabout handling?
Why choose one over the other?

Any such info would be great! I'm sure there must have been other "new guys" seeking such info - somebody ought to write a book! :) (I've got several on order at the moment, but it takes ages to get stuff here!)

Thanx in advance!
Tim

[Mark75H]

BRF is one of the best places to find your answers. All you have to do is ask.

Question #2 is probably the easiest to answer:

Running the prop partly out of the water reduces friction (drag) caused by the motor's lower unit (also called gear box or foot). The special props are used to allow the boat to run with the prop a third or more out of the water.

Differences between runabout and hydro handling: some runabouts handle exactly like hydros. Driving a hydro is like riding on the front of a bullet, you basically don't see much or any boat in front of you ... with a runabout you see some boat and feel more like you are in the middle of the vehicle. Some runabouts "roll" when they turn instead of running flat; all hydros run flat. Hydros run more on a cushion of air than runabouts do.

[Tim Maynard]

So what happens with engine RPM when the prop is partially out of the water? Do you run a higher pitch prop to keep RPM down? Larger diameter?
Tim

[pops67g]

There are probably several different philosophies as to set-up with the type of boats we run. However, you are right in thinking that more pitch or diameter or both at the same time will reduce rpm. What we want to do is find a prop that allows the engine to reach maximum rpm before the boat reaches the end of the straightaway. If all race courses were the same then the same prop and set-up would work for all of them. Unfortunately, this is never the case so we have to invest in several props for the same boat to have much chance of success over an entire season.

[JohnsonM50]

Im a beginner to Kneeldowns, dont race, [maybe some day] built a runnabout, am building a tunnel and bought a hydro and have a few motors. 1 thing I learned is props are very important to speed. Racing gearboxes are drive only with higher ratios even as high as 1 to 1 The tradoff seems to be in prop diameter with similar pitch. the props are shaped different and have thinned and sharp blades, alot of them are stainless steel or bronze, I havent seen any aluminum ones. My hydro is raceworthy in C and has a good history the other 2 arent so boat choice is important. I started in a runnabout, had a hard time with ride and handling but am gaining speed and making progress. The hydro Ive only run with a MK25 on an H lower and its a great ride, cant wait to power it up.

[Mark Poole]

I have a question also being a former OPC racer. Do kneel down hydros truly prop ride like a larger inboard hyrdo? Or, is the trailing edge of these boats always in the water with the idea being to pack and spill air out the sides behind the sponsons? I know for starightaway records with big motors they do propride but what about set up for heat racing?

[Mark75H]

Set up for heat racing they are tail draggers. (Back in the early 1960's there were a few larger class closed course outboard hydros that were true prop riders, but they fell out of favor by the late 1960's.)

I have seen one boat that consistently hopped the transom off the water (or maybe it was intermittently touching down) going down the straights ... at almost 100 mph. If you put your head down near the water you could see light under it from front to back as it came toward you. It was built a few inches wider than all the other boats in its class. As a joke, before I saw it run, I called it "the kite" (and even gave the owner a ball of kite string) . After I saw it go, I changed my opinion and called it "the hovercraft". Before I saw it run I was also stunned at how low he had the prop compared to my boat and others in its class ... after I saw it run, I realized the prop had to be that low - because that was where the water was

[Mark Poole]

Yes, given the weight of the engine and driver in such a small boat it didn't seem like they were flying the afterplane from the water. Knowing that I suppose the shape of the afterplane at the rear is critical. At those speeds a little rocker or hook could make a huge difference? Have you ever raced a hydro with rear sponsons?

[Mark75H]

Do you mean a 4 point hydro? No, I have not. Reds (Rufus) Hall of North Carolina has one and it seems very easy to drive. He races 350cc hydro at about 100 mph (99.9 mph if the insurance company is looking;) ). It is a more interesting feat if you know that Mr. Hall is almost 80 years old. He was able to drive it competitively with about 15 minutes of seat time.

Most outboard hydros have straight flat afterplanes; a few have a step up after the motor, maybe 1/4 inch, even fewer have a major hook or rocker. My opinion of the function of the major hook/rocker was to side step prop height rules ... letting the boat ride on a bump with motor in/under (called 'tuck' or 'tuck in') so radical that the prop height was technically legal, but functioned otherwise.

[Mark Poole]

Thanks for the info. What I would call a four point hydro would be like the outrigger style RC hydros that have four large sponsons. What I meant by rear sponsons on a kneel down was long wedge like pads that would help lift the stern and reduce wetted surface. Is this what Mr. Halls boat has?

[Mark75H]

I hope you can see from these pictures

[Mark Poole]

Those are sure enough rear sponsons! They are running a narrow afterplane too I see. What class boat (engine size) is the "2R" boat?

[Mark75H]

2R is a 44ci (~50hp) maybe 80 mph or so