Any fellow hydro design enthusiasts out there?

[Fear the Duck]

I've always been fascinated by hydroplanes, and I've designed several of them myself. Currently drawing up plans for an RC hydro, and I have already completed several sets of scale drawings for river racing hydros. There are a million and two things that can be done to a hydroplane to make her faster, more stable, turn better, plane quicker, etc etc etc, and I've always just looked at fast boats, noted little differences, cross-referenced with what I knew of their handling characteristics, and mix-and-matched to tailor to what I'd like to drive. I'd love to just chew the fat with other people who are into the design or construction aspects of hydroplanes, if you don't mind. Not wanting to start any arguments, of course, I just wonder if anyone would be willing to share their idea of a "fast" boat, a "quick" boat, a "good all-around" boat, or whatever their perception of the ideal hydroplane is, along with what design traits would characterize it.

[Fear the Duck]

Quick clarification: I understand that there will be plenty of trade secrets and aces up sleeves. I'm not asking for disclosure of these, but simply talking about general design aspects. No specifics of measurement or techniques/materials used to give her your trademark edge.

[Sonny]

Yeah sure man, I'll do a little bench racing with ya. I love this stuff and The science behind a good working hydroplane is intriguing. How heavy is the motor? How many mph do you anticipate the boat going? Can I assume that you are refering to an outboard?

[Fear the Duck]

Let's assume we're working with a c stock. The object is to win, with whatever driving strategy you idealize. To me, I'd make her a middle line runner, sacrificing out-of-the-hole acceleration for a top end that has to be carefully maintained. I always thought that a flat boat is a fast boat, so she'd have as little rake as I could give her without turning her into a torpedo.

[sam]

That would work if all courses and all drivers were exactly the same.

On a shorter course, you need the acceleration; on long courses you need speed.

The most you can hope for is a boat that is flexible and responds well to different props and set ups.

The characteristics of the way the boat digs in to grab for the turns and then lets go of the water to pop up free to accelerate might still be found on the boat capable of the highest speed.

The first scenario you have presented is more of a prop/set-up scenario than a boat design scenario.

[B Walker]

I divide hydro designs into 2 categories: below 70 MPH and above 70 MPH because below 70 MPH the problem is getting the boat out of the water and above 70MPH or so the problem is keeping it on the water.

All the boatbuilders I have talked to that have been building wooden boats for 20 years or more all complain about quality of todays wood at a reasonable cost. That is why the largest area of innovation today is in materials, not design.

I find very little difference between the top CSH makers in terms of design. Until there is a change in the way we race I dont think there will be much change in basic design.

There are no secrets in hydro design because boat racers all like to talk too much. So ask away

BW

"Building a boat is just a series of compromises." Ralph DeSilva

[Sonny]

C stock hydro, With Yamato: 36 inches across the planing surface, 75 inches after plane (from back of sponson to end of planing surface), chisel cowl, a tunnel, and a top deck that crowns early so she bounces up and off of the corners. Oh yeah...and plenty of air trap.

[sam]

How far is the steering wheel from the transom?

Placement of the driver's weight is a critical factor

[Sonny]

How far is the steering wheel from the transom?

Placement of the driver's weight is a critical factor

AAAhhhh...good one, I don't know. That one will take someone smarter than me. The good new is that that person wont be hard to find.

[mdaspit]

C stock hydro, With Yamato: 36 inches across the planing surface, 75 inches after plane (from back of sponson to end of planing surface), chisel cowl, a tunnel, and a top deck that crowns early so she bounces up and off of the corners. Oh yeah...and plenty of air trap.

What's a chisel cowl?
Why a tunnel?
Why does the top deck get you out of the corners?

[B Walker]

Most hydroplane design terminology comes from aviation design rather than boat design.

Important design factors in a racing hydroplane:

Center of Gravity: balance point of a boat with the motor on it. 24-36 inches from the back of the boat is a common number.

Cross Section: Amount of air displaced by the boat measured at the widest point. Thinner is better.

Chisel cowl is a cowl with a point on it rather than rounded. Rounded cowl tends to direct air up over the driver (poor airflow), chisel cowl tends to direct air around the sides of the boat (good airflow because the sides are smooth).

A tunnel reduces the wet bottom (drag)

An early crowning deck, often called forward lift, gets the boat out of the water faster (accelleration) because all the air behind the crown pulls the boat up (negative lift or downwash)

Scince a hydroplane is basically a wing an understanding of airflow and the principles and components of lift are essential.

Basic wing design explanation

http://www.allstar.fiu.edu/aero/airflylvl3.htm

BW

[mdaspit]

Once again... Thanks

[Fear the Duck]

So thinner is better, but couldn't more lift be achieved in theory with more surface area? A wider boat would channel more air into the tunnel and provide more lift from the deck. Perhaps a plus for a heavier driver, or does the increased coefficient of drag from moving a larger object through the air negate any potential benefits?
I've noted that, more often than not, the crown of the deck is right over the back end of the sponsons. Does that mean that moving the crown forward requires moving the riding surfaces forward as well? If so, then is it possible that the acceleration has more to do with the position of the forward riding surfaces in relation to the prop than with the lift created by the deck? One particular boat stands out in my mind: a river hydro with a deck that was very nearly flat, characterized by mind-blowing speed and extreme linear instability.

[B Walker]

So thinner is better, but couldn't more lift be achieved in theory with more surface area? A wider boat would channel more air into the tunnel and provide more lift from the deck. Perhaps a plus for a heavier driver, or does the increased coefficient of drag from moving a larger object through the air negate any potential benefits?
I've noted that, more often than not, the crown of the deck is right over the back end of the sponsons. Does that mean that moving the crown forward requires moving the riding surfaces forward as well? If so, then is it possible that the acceleration has more to do with the position of the forward riding surfaces in relation to the prop than with the lift created by the deck? One particular boat stands out in my mind: a river hydro with a deck that was very nearly flat, characterized by mind-blowing speed and extreme linear instability.

You have given some good examples of the compromises a boat builder has to make.

There is no relation between the crown of the upper deck curve and the length of the sponsons. It is just easier to put the curve there because of the framing on a traditional style modern hydroplane.

Alot of newer boats are now doing a "split deck" design and have moved the deck crown significantly forward while reducing the cross section at the same time. A B&H or a McCourtie are good examples of the split deck style.

BW

[Dave M]

Yes

but couldn't more lift be achieved in theory with more surface area?

Short answer - yes, you are on the right track.

If you ever wind tunnel a hydro of today you might find that decks don't have a lot of effect on handling until a speed reaches a certain point. At that certain point it becomes absolutley critical. Deck design plays a very small role in less than 100MPH boats in my experience. It is basically a moot point on the J and A classs.