Why is Calculated Hull Speed important even on a planning hull???

[Keith Brown]

Rainy City posted the equation for calculating hull speed. Which brought to mind an incident that happened on Lake Minnewanka in Banff National Park about 50 years ago. A tin boat around 16 ft ran out of gas at the far end of the lake. Our captain decided to tow the boat and it’s three occupants to the boat launch. Our captain started out slow and everything was going smoothly as he gradually started to increase our speed. I was watching the towed boat, and everything was progressing smoothly, until suddenly and without notice the boat we were towing started to veer wildly, swamping the boat and the three fishermen.

I’ve been wondering WTF happened to that boat, until I took the B4 course from Oak Bay Power Squadron a year ago. It was simply a matter of we had exceeded the tinners Hull Speed, which is VERY CRITICAL TO STABILITY when not under it’s own power. The B4 class and I had a good laugh when I described the scene with those three drowned rats and a tinner half full of water, even now when I think about it I can't help but smile. In the interest of brevity here is the equation.

1.34 x square root of the water line length of boat in feet = Maximum Hull Speed in knots.

E.g. 1.34 x square root of 16ft (4) = 5.6 knots hull speed.

P.S. Do your calculation now and enter the maximum tow speed in your "Navigators Personal Reference Book"

 

[Max123]

Its interesting how slow hull speed is. For my 18ft centre console it calculates out at 5.8knots/6.6mph (maybe a tiny bit faster because it has a pod, which increases length to about 20'). Just putting the Main in gear gets about 4.5mph, and I am still at zero wake at 6.5mph. I had always thought that hull speed would have at least some bow wave/wake.

I was playing around with my new kicker the other day, which is a small 6hp unit that I use as an emergency backup. It lets you know exactly where hull speed is. At 1/3 throttle its doing 6mph, half throttle its doing 6.5mph, full throttle... 6.5mph but more noise (this is with a low pitch/high thrust prop). Unless you have enough power to plane, hull speed is a brick wall. Its funny because I had thought about getting a 9.9hp kicker, but I expect I would get the exact same performance. Likely better control in wind/waves - but no difference in top speed.

 

[turbomack]

Its interesting how slow hull speed is. For my 18ft centre console it calculates out at 5.8knots/6.6mph (maybe a tiny bit faster because it has a pod, which increases length to about 20'). Just putting the Main in gear gets about 4.5mph, and I am still at zero wake at 6.5mph. I had always thought that hull speed would have at least some bow wave/wake.

I was playing around with my new kicker the other day, which is a small 6hp unit that I use as an emergency backup. It lets you know exactly where hull speed is. At 1/3 throttle its doing 6mph, half throttle its doing 6.5mph, full throttle... 6.5mph but more noise (this is with a low pitch/high thrust prop). Unless you have enough power to plane, hull speed is a brick wall. Its funny because I had thought about getting a 9.9hp kicker, but I expect I would get the exact same performance. Likely better control in wind/waves - but no difference in top speed.

and somehow 25 horse kickers are vastly popular. i have towed boats in from offshore before usually 22-25 ft. pangas and i can get them planning but can get a bit sporty in the swell if we both get surfing down the front of a wave.

 

[Pineapple Express]

Its interesting how slow hull speed is. For my 18ft centre console it calculates out at 5.8knots/6.6mph (maybe a tiny bit faster because it has a pod, which increases length to about 20'). Just putting the Main in gear gets about 4.5mph, and I am still at zero wake at 6.5mph. I had always thought that hull speed would have at least some bow wave/wake.

I was playing around with my new kicker the other day, which is a small 6hp unit that I use as an emergency backup. It lets you know exactly where hull speed is. At 1/3 throttle its doing 6mph, half throttle its doing 6.5mph, full throttle... 6.5mph but more noise (this is with a low pitch/high thrust prop). Unless you have enough power to plane, hull speed is a brick wall. Its funny because I had thought about getting a 9.9hp kicker, but I expect I would get the exact same performance. Likely better control in wind/waves - but no difference in top speed.

Remember, the equation is a rough estimate. Also it takes in to account water line length (the length of hull in the water at rest) not overall length.

 

[BC Coast Pilot]

Rainy City posted the equation for calculating hull speed. Which brought to mind an incident that happened on Lake Minnewanka in Banff National Park about 50 years ago. A tin boat around 16 ft ran out of gas at the far end of the lake. Our captain decided to tow the boat and it’s three occupants to the boat launch. Our captain started out slow and everything was going smoothly as he gradually started to increase our speed. I was watching the towed boat, and everything was progressing smoothly, until suddenly and without notice the boat we were towing started to veer wildly, swamping the boat and the three fishermen.

I’ve been wondering WTF happened to that boat, until I took the B4 course from Oak Bay Power Squadron a year ago. It was simply a matter of we had exceeded the tinners Hull Speed, which is VERY CRITICAL TO STABILITY when not under it’s own power. The B4 class and I had a good laugh when I described the scene with those three drowned rats and a tinner half full of water, even now when I think about it I can't help but smile. In the interest of brevity here is the equation.

1.34 x square root of the water line length of boat in feet = Maximum Hull Speed in knots.

E.g. 1.34 x square root of 16ft (4) = 5.6 knots hull speed.

P.S. Do your calculation now and enter the maximum tow speed in your "Navigators Personal Reference Book"

This is a displacement vessel formula for hull speed, which is simply a base for a block shaped vessel. IMO,

Remember, the equation is a rough estimate. Also it takes in to account water line length (the length of hull in the water at rest) not overall length.

I would not use it in this application as the towed vessel will simply plane with horsepower/speed applied as it sounded like it did in this case. There are other factors at play here. A better use of this base block formula may be to compare a 400ft ship to a 36ft tug
Square root of 400=20×1.33=26.6knots
Square root of 36=6x1.33=7.98 knots
These numbers can then be engineered by applying co efficients and of course.....more horsepower

 

[Eden Island]

Hull speed is important information even for guys with planing hulls. In the event you're coming home on your kicker it leads to this equation:

Hours to home = Distance from home / Hull speed [eg. 25 miles / 6.25 miles per hour = 4 hours]

Or about 3 hours later than you thought. Ask me how I know.

 

[zurk]

its very approximate but mostly true. for weird displacement hulls like mine my designed max hull speed for a 40 ft racing cat is 17 knots. 8.47 knots is the calculated hull speed. and mine can do 20 knots in real world f5 conditions (which is terrifying BTW and should never be attempted - dont ask me how i know).

 

[BC Coast Pilot]

Hull speed is important information even for guys with planing hulls. In the event you're coming home on your kicker it leads to this equation:

Hours to home = Distance from home / Hull speed [eg. 25 miles / 6.25 miles per hour = 4 hours]

Or about 3 hours later than you thought. Ask me how I know.

You're equation is just speed, not hull speed

D/S=T

 

[BC Coast Pilot]

its very approximate but mostly true. for weird displacement hulls like mine my designed max hull speed for a 40 ft racing cat is 17 knots. 8.47 knots is the calculated hull speed. and mine can do 20 knots in real world f5 conditions (which is terrifying BTW and should never be attempted - dont ask me how i know).

The formula for your vessel is highly engineered no doubt, with many coefficients applied and Simpsons rules, as a displacement based hull it is indeed subject to the base block formula unlike the planing hulls that are discussed in error here. I've been on a Cat at 14.5 knots but 20 I'd be terrified too