Typhoon hull speed

[Ned Crockett]

Can someone explain hull speed to me? Yesterday it was blowing about 15 when I was out on the Rappahannock River (Va.). Ladybug was "boiling" along and I decided to check her speed on my GPS. It was consistently over 5 knots, the highest being 6.2. I understand that by surfing down a wave hull speed can be exceeded, but my speed was steady at 5.5 and higher on a broad reach. How can a boat with an advertised hull speed of 5 knots achieve this? Could the GPS be giving false info? Is the 5 knot hull speed for a Weekender incorrect? Does hull speed change as conditions change?

Ned

[Chris L]

Ned,

Hull speed is based on waterline length while the boat is sitting level in the water. As the boat heels, the waterline length increases and so does the speed. Your GPS is probably giving correct info. My 25 has a hull speed around 5.5 knots but given the right conditions I can easliy get 6 knots. Chris

[Al Levesque]

The hull speed is that limit above which increases in speed need disproportionate increases in force. We often have conditions where the forces are sufficient to exceed hull speed and that makes for glorious sailing.

On the other hand, if using gps to measure speed don't forget to factor in any possible current effects. Tidal currents easily add a knot with hardly any notice.

[Chris & Dale Schnell]

I agree w/Al. Here in the Cape Fear river we can reach 5+ knots of current at peak flow (in either direction) so if you're following the prevailing current w/a stiff breeze on a good beam reach you can fly. Having a paddle-wheel knot meter (like we had on our CD30) to measure speed thru the water will help to bring a clear picture of your speed, but we're not putting another hole in our beautiful little daysailer just for that. In fact, until we get a hand-held GPS it's going to be "how long did it take us to get here, which direction is current flow, and swag at what it will take to get back". This is most important when calculating the "libation factor" and remaining supplies to return to port.

[wingreen]

I think Al's right about this. Imagine a really powerful boat towing your Ty through the water. You can easily exceed the hull speed if the boat towing you has enough power, but not without expending enough power to tow you that fast.

A few months back I posted about a day where I had my Ty out on a small craft advisory - force 5 - 6 winds on an almost perfect beam reach. We made it past the three mile marker in about a little less than a half hour, or slightly more than 6mph. However, think about all the force a 20 - 30 mph wind creates, and it takes all that just to make a 5 kt hull go 5.2 kt (I think the Ty's hull speed is actually just under 5 kt).

[Ned Crockett]

Thanks for the replies. I learn something each time I visit this site. Now I know that the actual speed of my Ty is a function of tide, wind, heel, and hull design. I always thought that hull speed could not be exceeded no matter the power applied. Now I know I can keep up with some of the 6 knot "big boys"!

[RIKanaka]

The max hull speed based on waterline length of displacement hulls is theoretical (approx~ 1.34 x square root of the LWL). Common sense tells you that of two boats with the same waterline length the one with the wider beam should have a slower max hull speed. Likewise, the one with the wider bow that creates greater resistance will also be slower, everything else being equal.

[mike ritenour]

Love that tide, wind and current stuff

11.8 kts in LaVida! What a rush!

Almost feels like a powerrrrrrrrrrrrr boat

Rit

[Neil Gordon]

The max hull speed based on waterline length of displacement hulls is theoretical (approx~ 1.34 x square root of the LWL). Common sense tells you that of two boats with the same waterline length the one with the wider beam should have a slower max hull speed. Likewise, the one with the wider bow that creates greater resistance will also be slower, everything else being equal.

Lots of factors affect boat speed but hull speed is only about waterline length. The limitation is caused by the waves the boat creates as it moves through the water. The faster you go, the further astern the series of bow waves goes, until the boat is sitting in a trough that it created. In theory, it can't go faster unless it escapes its stern wave. The more it tries, the more the stern sinks and the bow needs to plow uphill. Unless you plane, you can't (in theory) go faster.