Flat-turning vs Roll-up turning

Questions about modifying a design

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Number19
Posts: 4
Joined: Tue Apr 23, 2013 5:12 pm

Flat-turning vs Roll-up turning

Post by Number19 »

The 13' Class CD - Racing Runabout is a classic design highly popular in the 50's and, in turning, generally "rolled up".

My plans locate the fin along the centerline but does not locate it athwartship. The written instructions make a general comment that the exact location may be determined experimentally.

Researching, I find that the fin is located off-centered, port side. There is usually a fin brace located between the keel and the first stringer where the fin should be located. These boats are designed for left turning.

Modern Class D runabouts are often designed for "flat turning" and the fin is located on the portside chine.

What I've been leading up to, is, I would like to prepare my CD for flat-turning rather than roll-up turning, if this is at all possible; and I'd like it to turn both left and right. The intended use is not for circular racing but rather for marathon racing.

Obviously, the first thing one thinks of is to relocate the fin to the centerline of the boat. Is this correct? Would this produce flat-turning?

One might then consider using two fins, portside and starboard, at the chine. I have no idea what effect this would produce.

I would appreciate comments on fin placement and flat-turning in a Class D runabout.

Mike Sea
Posts: 43
Joined: Wed Oct 07, 2009 4:20 pm
Location: North Canton, Ohio

Re: Flat-turning vs Roll-up turning

Post by Mike Sea »

A boat design from the 50's can not be made into a flat turner without a redesign. A typical flat turner has a narrow bottom to decrease drag while turning. They are normally designed to turn one way left but could be setup to turn right.
Placing the fin in the center on CD will let you turn right or left but it will not be a flat turner, it will still be a roll-up boat. You do not want to use two fins, that is a flip waiting to happen.
One thing to remember about the current flat turning race boats is that for the most part they were designed for closed course racing on smooth water and they are very light and fragile.

Several years ago I built a 4 passenger runabout patterned after a racing runabout. This boat is not a race boat but it does have a rather large turning fin in the middle, flat bottom and non-trip chines. At 20 mph it will sort of turn flat but at 42 mph it will rollup on the chines and is a great handling boat.

Are you considering building the Glen L CD or do you have the boat?

If you haven't built the Glen-L CD, I would suggest you looking into Hal Kelly's Madcap or one of the new design marathon boats on Sorensen website.
Hal Kelly was noted for designing marathon race boats and his last two runabouts were Madcap and Foo-ling. I have the A-B Foo-ling and it is a great rough water boat. Free Madcap plans can be obtained at Boat Sport Magazine Archives.

Number19
Posts: 4
Joined: Tue Apr 23, 2013 5:12 pm

Re: Flat-turning vs Roll-up turning

Post by Number19 »

First let me say that I'm just tossing ideas around in my head. I know most of these are not viable, or I would have found something in my research, and I know that putting the fin on the hull's centerline is likely my best option.

I'll also explain that I'm new at this and I'm 64 years old. I can still drive the boat from a kneeling position, for now, but I don't know for what duration or for how much longer. I was first considering a sitting position, cross legged Indian style, with a removable Kayak backrest. I was told, and it's easy to see, that driving one of these things under race conditions is a grueling and demanding workout; and I was told that in rolling up, you brace your knee in the chime.

I was a bit misled in early descriptions of the "utility" class being exactly that, a boat you could use for family recreation and then on the weekend, race it. I'm well aware than the modern design is strictly racing, which is why I was attracted to the early design.

I don't mind the roll up turn if it can be accomplished in a less demanding style.

As you can see, I'm exploring my options while trying to learn how all this works before I actually put boat in water.

At first, I was going to build the Glen-L design and purchased the plans. But I then realized that Glen-L did not sell this boat as a kit. So I've decided on Madcap.

Your response was quite informative. Is there anything more you can add about rolling up under non-racing conditions and whether a sitting position is at all doable. Thanks.

George

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