Hull Speed

Figuring out hull speed for a displacement hulled vessel, such as a sailboat. How to cheat hull speed and go faster.

Puffin is a 67 foot Dutch steel hulled sailing ketch that I purchased in Italy. My wife and I have lived on PUFFIN for about a year following an extensive refit, and she is home. We are living the dream in Charleston, South Carolina.

[mllawhorn]

The first 90 seconds of this video is the simplest explanation I've ever heard about hull speed. Thanks very much and great job!

[ericmohler]

Just listened to a naval architect explain this. Nothing like the explanation you just gave. Common sense. Brilliant.

[rossphelan]

Thank you for being so clear and concise! You are a rare phenomenon on YouTube.

[Peterowsky]

Very informative without being overly long or boring.
Now I want more.

[nickaskham]

Superb explanations; you have answered several questions I've had for a while. Clear and concise, excellent, many thanks, Nick.

[yakovbok]

I’m a newb to boating, trying to get my head around planing a marginally powered 8’ inflatable.
This is a top notch explanation. Many thanks.

[rubenvicenteleuzzivazquez]

Soy diseñador y constructor naval. Y a mi juicio tu explicación es brillante no solo por ser cierta, sino también por ser simple y entendible para la gente sin conocimientos técnicos profundos en dinámica de fluidos. Lo bueno, si es breve, dos veces bueno. Saludos.

[srennielsen]

Im back. I was here around 2020 and now again in 2022. This is the best, down to earth, useable guide on the internet. Thanks from Denmark! I hope you are still enjoying sailing.

[davidkeffen]

Thank you Brad. What a simple and yet comprehensive explanation. You’re an excellent teacher.

[zobrombie]

The delivery of the information in this video is just outstanding. As a side note on the principles of shouldering water my grandad had a saying. 'You try to go any faster, and you'll just be draggin' the whole bay behind you."

[atakd]

No computer graphics to explain this? Just a whiteboard and a pen? Oh, and you know what you're talking about! Great stuff!

[tvannaman]

Great job, well thought out, to the point and presented without needless side stories, rabbit holes and chit-chat that is prevalent in many Youtube videos.

[lutzweb]

This is the BEST explanation I have EVER listen about displacement hull design, after year of sailing, I had never found SUCH A PERFECT beautiful simple clear explanation. Subscribed immediately, I will watch ALL your video. Thank you! That is what a sea-man should be!

[tobyhyman]

Hey mate, self employed marine engineer here. 18 years in the marine industry. Just learnt what hull speed really meant today. Thank you.

[TommyboyGTP]

The types of sails used really explained a lot. I live in Florida and i like to watch the sail boats and how fast they go. Now I understand they're trying to overcome their hull speed. Thanks for the explanation!

[stephenmosack]

a wonderfully simple explanation.  I spent years sailing as a kid... with some very intelligent guys, but I've never heard hull-speed explained like that.

[justapicker]

Not being an engineer does not mean that you don't understanding physics and fluid dynamics. You did an excellent job of explaining Hull Speed.Thanls for helping us understand.

[hkkabanossi]

Hi, nice vid and I agreed on most of it. The rule of thumb is right in here, where the estimated hull speed can be derived from the length of the waterline. But as some other(s) noted, the effect of clean bottom or the smoothness of lines mainly affect to the amount of turbulence, thus the amount of power needed to achieve the hull speed. All turbulence created needs energy and that is converted from fuel in the boats propulsion or from wind by sails. So basically, a dirty bottom only increases the needed power, but the boat should still be as slow/fast with that hull length.

To go faster than the hull speed, the bow tries to climb over the bow wave, but as some other noted, the effect of wave through at the stern is also in very important role! The stern has its own wake similar to the bow wake (wave) and the hull speed is actually the situation where these two waves interact in the same phase: the bow wave's second wave crest (first one at the bow) has lengthened all the way to the stern and increasing the stern wave which is easily visible from a high stern wave. If going faster than the hull speed, it would mean that the crest of the bow wave would go beyond the stern, thus making a hole under the stern, and sucking the stern down. Together with the bow trying to climb up, this increases the power need exponentially, thus making it impractical situation.

What comes to the effect of a bulbous bow and adding a kite sail (spinnaker/gennaker). There your presentation went wrong. The bulbous bow can in small boats increase the waterline length and make it a larger hull speed, in theory, but it also generates its own wave formation as you noted. And this additional wave is easily making more harm since it needs energy from the propulsion such as any turbulence. It is helpful only in large ships that does not operate even close to the theoretical hull speed, based on the waterline length. In those ships the limiting factor is the hull friction due to the large wetted surface, and the wave formation. Every wave you make needs its energy from the propulsion. In ships the bulbous bow is used to counter-effect the bow wave, thus making in result a lower wave. This is especially why it is not seen in sailboats that has to perform in different heeling angles (changing geometry). The bulbous bow is not possible to optimize for different angles and sea states.

As for the spinnaker/gennaker help: It does not in general lift the bow, in fact quite the opposite. The force resultant of the sail might point up 10-20 degrees from the horizontal level, but the force is still way above the waterline level, thus making a moment arm that is actually pushing the bow down using the rig as the moment arm. All the dragging force is under waterline and all the pushing force from sails is above the waterline, making a huge moment arm.

[Wiggles]

Best explanation I have seen. Brad you don't over complicate things and that makes you an excellent teacher.

[isabellforester]

i think this is the best explanation i could find on the internet! Great thanks!