Tad Roberts

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Ironbark

Construction: cold-molded
Length: 28' 0"
Beam: 9' 10"
Draft: 3' 8"
Displacement: 16,250 lbs
Power: 62 HP Perkins 4-236
Cruising Speed : 6.8 Knots
Builder:

Comments

Ironbark's looks are also entirely in keeping with her performance, no tricks here! The deckhouse and accommodation is very limited for her weight and cost, but a large deckhouse can visually overpower a graceful hull; get the proportions wrong and the boat starts to look top heavy.

Ironbark carries 2600 lbs of ballast in the form of lead pigs down on top of the keel between the floors. Ballast? in a powerboat, how come? Because it lowers VCG (vertical center of gravity) and lengthens GM. GM is the distance from the CG (center of gravity) to the metacenter. The metacenter's height is dependent on waterplane inertia, a narrow, fine waterplane gives a low metacentric height and a rolly boat. A wide, full waterline gives a tall metacentric height and a stiff boat. As the boat heels the VCB (vertical center of buoyancy) moves about the metacenter, and righting arm is the distance between a vertical line connecting VCB and metacenter, and CG (center of gravity). The longer the GM, the faster a large righting arm is developed, and the faster the boat returns to upright. There is no real agreement on how much GM is enough, Francis S. Kinney in Skene's Elements Of Yacht Design, mentions a GM of 2'-2'6" for small power cruisers. Ironbark is right in the middle of that with a GM of 2.25'. The CG two inches above DWL allows for additions of weight up high, a heavier mast and stabilizer poles, or a dinghy on the cabintop.

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