Keel Rudder question

[JJA1A]

Would a Keel rudder be effective on a pivot keel? How do I calculate the proportions?

[rbgarr]

I doubt it would be effective because the rudder post angle would change (or the rudder would cease to turn!) depending on how much the centerboard was raised or lowered.

[JJA1A]

I’m not planning to use the keel depth to change the balance point so would it be effective if fully extended when the keel is in use?

[john welsford]

If I understand the question, Its likely to be too close to the center of the boat.
You need the rudder to be at least somewhat toward the stern to be effective.

JohnW


Would a Keel rudder be effective on a pivot keel? How do I calculate the proportions?

[Peerie Maa]

Possibly a dumb question, but are you going to use the boat at any time with the keel up? If not why have a pivoting keel?

[JJA1A]

This boat will be rowed and sailed and may have a small outboard motor. So I want to be able to retract the keel and rudder. Take a look at the attached picture. My question is what is calculation for the length beam and keel when the keel is a wedge?

[James McMullen]

If you're thinking of putting that contraption on the Gardner Swampscott dory you were talking about earlier, you're on the wrong track. That "keel" you've drawn will be pretty much useless for real sailing, and very inconvenient and in the way for rowing too. I recommend you build according to Gardner's plan instead, including the centerboard and rudder he's drawn for it.

[TerryLL]

The pic you posted looks like a skeg-hung rudder. This arrangement is fairly common and works well when properly sized for the hull and properly positioned for the rig. But a skeg is not a keel, and a centerboard is still required if the boat is to sail properly.

Here's a pic of a skeg-hung ruder and centerboard on a dory. This would be the Alden Indian.

http://www.woodenboatstore.com/images/400005.jpg

[JJA1A]

Thanks for the information about the Alden Indian.

[JJA1A]

Learning about the Alden Indian is a big help and I appreciate it. In all likelihood I will build Gardner’s Swampscott Dory as designed. Despite my artistic inability I have tinkered and designed for 30 years. Every design has been based on modifying an existing item using a principal of engineering. My notion is that the skeg-hung rudder that replaces Gardner’s centerboard and rudder is retractable so the boat can be rowed. Gardner’s centerboard is 40” long x 12” wide or 480 square inches. The rudder is 198 square inches. If toward the center of the boat the skeg is 40” tall and just forward of the stern a rudder with a surface are of 198” is connected to the skeg will it control a 16’ boat with a 96’ sale area? I respect clever ridicule as much as the next guy but I’m looking for a well considered answer from experienced boat builders.

[TerryLL]

Check out the Bolger Sweet Pea. It has a long and narrow retractable keel something like the one you describe.

[JJA1A]

I have read the Sweet Pea is based on a Surf Dory. Less flair and straighter sides than a Swampscott Dory. How do you think a Swampscott would behave with a retractable keel of similar dimensions to the Sweet Pea?

[rbgarr]

Poorly.

[Daniel Noyes]

Learning about the Alden Indian is a big help and I appreciate it. In all likelihood I will build Gardner’s Swampscott Dory as designed. Despite my artistic inability I have tinkered and designed for 30 years. Every design has been based on modifying an existing item using a principal of engineering. My notion is that the skeg-hung rudder that replaces Gardner’s centerboard and rudder is retractable so the boat can be rowed. Gardner’s centerboard is 40” long x 12” wide or 480 square inches. The rudder is 198 square inches. If toward the center of the boat the skeg is 40” tall and just forward of the stern a rudder with a surface are of 198” is connected to the skeg will it control a 16’ boat with a 96’ sale area? I respect clever ridicule as much as the next guy but I’m looking for a well considered answer from experienced boat builders.

The Gardner swampscott dory is an excelent, well thought out design that can be rowed and sailed very well as designed... what is the objective of your suggested modifications?

[JJA1A]

The change is driven by my desire to find a boat that does everything well and I know that is not practical. From time to time I may want to retract the keel and put a small outboard engine on the boat. I realize there are balance issues and I may have to broaden the stern slightly or cantilever the weight for this to work.

[htom]

Boat that does everything well ... doesn't exist and can't. Physics gets in the way. The Gardner Swampscott dory does two things rather well -- sail and row. That's far more than most boats do (most of them struggle to either row well or sail well, and doing both is an major accomplishment, which is why the Gardner Swampscott is so well regarded. That design could be tweeked a bit to make her an even better rower or even better sailer, but a move in either direction would diminish the other form of performance; tweeks to add motor power are going to mostly diminish both rowing and sailing performance.)

[Daniel Noyes]

The change is driven by my desire to find a boat that does everything well and I know that is not practical. From time to time I may want to retract the keel and put a small outboard engine on the boat. I realize there are balance issues and I may have to broaden the stern slightly or cantilever the weight for this to work.

As designed the Gardner Swampscott dory has a "retractable keel" (known as a centerboard) and removable rudder, I see many negatives and no positives in trying to build a combination keel and rudder.

If you want a Sail and Oar and Outboard motor boat you are looking in the right book but wrong design.
The 14' semidory on page 231 of J.G. The Dory Book
or page 63 of J.G. Building classic Small Craft (vol.1)

Dan
http://dansdories.googlepages.com (http://dansdories.googlepages.com/)

[JJA1A]

Thanks for mentioning these Daniel. I read about this boat a few weeks ago. It looks like a fine boat but Gardner was certainly less passionate about it than the Swampscott. Have you ever used this semi-dory and if so what do you think about its performance in each category?
Have you built a cantilever to shift weight to the best load bearing point of a boat?

[Daniel Noyes]

Ive never used one but had one in the shop this spring for repairs. The wide bottom transom should make the boat harder to row but it will handle a small outboard motor, should sail fine as long as the motor is left ashore... thats the catch 22 as long as the motor is on the transom sailing and rowing ability is severly diminished.

you could clamp a eletric trolling motor to the rail of the Swampscott dory and motor along at a comfortable 4-5 mph

Dan

[ewsponberg]

Would a Keel rudder be effective on a pivot keel? How do I calculate the proportions?

JJA1A,

My design Project Amazon had a large trim tab built into the back of its pivoting keel. It worked extremely well. In fact, the trim tab qualified as the emergency rudder for the 1998 Around Alone race, conducted by test in front of the race committee. You can see photos on my website:

http://www.sponbergyachtdesign.com/ProjectAmazon.htm

To make it work properly, you need a relatively fat keel section, in this case, it is a 21% thick GA(W) aerofoil section. The trim tab is the last 30% of the chord length. You can download a complete description of Project Amazon in a paper that I wrote for SNAME's Marine Technology journal at:

http://www.sponbergyachtdesign.com/Articles.htm

It is the third entry down in the list of pdf downloads. They're free, and that article gives a complete description of Project Amazon and all its features, including the keel and trim tab sections.

Eric

[andrewe]

IFIRR, Sweet pea had a removable keel, not a retractable one I saw the proto. in Dynamite's garden and there was no slot. The idea of a long shallow retracting keel/skeg makes me wonder about the structure of the casing.
A