Exceeding Hull Speed, Discussion?

[motion30]

My west marine dinghy had a hull speed of 2.1knots The last time I saw it it was doing 56.4 knots 6' off the surfacface of the water motor not running ( see issac)

[micah719]

Darn those dinghy thieves....but how did you know his name was Isaac?

[Dockhead]

Originally Posted by carstenb

This past summer I blew across the baltic with my Jeanneau 40.3 Sun Fast in 35-38 knot winds with 45 knots gusts. Only the Genua set. Knotmeter showed 9,5 to 10,5. Hull speed (theory) 8.5 knots. Proof of planing: We covered the 37 NM from Falsterbo to Ystad in just under 4 hours, including the time spent setting and taking down sails and motoring into the harbor. Average speed. just over 9 knots. Knock off about 1/2 hour for sails and motoring into harbor, then the average speed is 10.5 knots.

I have to say we really did fly across the baltic. The next day we did it again on a 48 NM stretch - just a mite slower.

I believe you. I've had similar wild rides in a new Bene 411, downwind in a Meltemi, and in other boats. Boats like yours will pick up their skirts and fly, in conditions like that. It is tremendous great fun.

[Dockhead]

Originally Posted by djmarchand

Hull speed, generally calculated at 1.34*sqrt(LWL) is just an engineering definition. That defintion is based on the theoretical speed of the bow wave. At hull speed the boat catches up and a trough forms. It takes a lot of horsepower to get out of that trough.

Any boat can exceed hull speed; all it takes is horsepower. But I would be surprised if a sailboat could exceed hull speed by 10% other than in ideal circumstances- ie 20 kts of wind on a beam reach.

David

But I think ideal circumstances are a run, not a beam reach. Because on a run the waves help you - they disrupt your bow wave - let you surf over it, riding the swell.

[Astrid]

Larger, heavier displacement hulled sailing vessels very often cannot reach, let alone exceed their theoretical hull speed as the bigger and heavier the vessel, the more it tends to plow through the waves rather than ride them. The clipper ship Cutty Sark, with a waterline length of 212 feet had a hull speed of 19.7 knots, yet her best ever logged speed was 17.5 knots--they simply cannot carry enough sail safely in wind strong enough to allow them to reach hull speed. In power vessels, the opposite is often the case, provided there is sufficient power available--the USN destroyer Ward of 1918 had a theoretical hull speed of 24 knots on a 315 foot displacement hull, but easily reached 35 knots

[perchance]

To get an idea of the theory see" Prismatic Coefficient".

[micah719]

A nifty resource for understanding all those numbers and ratios. Even I can understand some of it.....an interesting read.

http://www.sponbergyachtdesign.com/T...N%20RATIOS.pdf

[Darktiger]

Today I took my Tasman 26 out of antifuling and changed the old propeller 11x8 with a new one 11.5x11 3 blades. Engine 8hp diesel yanmar.
I flu 13km/h against the wind with 80% of throttle... very impressive. The hull speed is 12km/h or 6 knots. I have exceeded the hull speed for 15%.... crazy....

[GordMay]

Greetings and welcome aboard the CF, Darktiger.

[valhalla360]

Originally Posted by Darktiger

Today I took my Tasman 26 out of antifuling and changed the old propeller 11x8 with a new one 11.5x11 3 blades. Engine 8hp diesel yanmar.
I flu 13km/h against the wind with 80% of throttle... very impressive. The hull speed is 12km/h or 6 knots. I have exceeded the hull speed for 15%.... crazy....

Theoretical Hull Speed (THS) can vary a bit based on the hull shape and wave conditions. (Theoretical being a key term). Multihulls actually can exceed THS by a large margin because their hulls are so drastically different from your typical cruising monohull.

More importantly, hull speed is not a fixed point. If you plot power vs speed, you won't see a sudden bend point at a very specific speed. Instead, you will see the line curve upward. It's more of a rough area of speed were throwing more power at the problem offers little advantage in speed for a lot more fuel burn.

If you keep throwing more HP at it, you will keep going faster (assuming nothing breaks due to extreme forces) but the further you go, the more HP it takes for each extra 0.1kt of speed.