What is "Hull Speed" Anyway

[carstenb]

Our Sun Fast "hull speed" is just at 8 knots. Running from the Canaries to Cape Verde, we had one day where we made 193nm and 1 day where we logged 187nm or right at the 8 knots.

I can tell you all for sure that were a number of times during the day (and night) when the log was over 10 knots and stayed over 10 knots for a couple of hours

were we surfing - yep.

Was it fun?

Damned right it was fun

[four winds]

Sausage and bisquits here.

People have reminded me many times that the Gulf of Mexico is "not the ocean". Fair enough, but still a lot of water out there.

Coastal overnighters after waiting on ideal conditions mean that I have never in seven years experienced conditions that offer an opportunity for what is described here.

But a steady offshore wind of 25 knots and almost no seas, my boat is bumping its 7 knot hull speed and averaging 6.3 or so. With the rail in the water. Partially furling the head sail stands the boat back up, and it continues to peak at 7-7.2 and average 6.3.

Don't think I've ever surfed. Maybe the rare times a pop up thunderstorm was chasing around behind me.

I know, meh, but I'm in the best chapter of my life. Close to the natural world.

[Kenomac]

Originally Posted by Dockhead

What do 100 foot Maxis have to do with anything? As was discussed earlier, Commanche can sail at 40 knots. Nothing to do with anything we've been discussing. That's what we call a "red herring".

So do I understand you also doubt the truth of Dashew's video? What -- you think Dashew fiddled with his speed log in order to impress people with impossible speeds he wasn't actually achieving?

Well, Dashew certainly doesn't need to impress people with false speed claims, nor do any of the rest of us.

Beowulf achieves speeds well above "hull speed" for just the reasons we've been discussing. You've been provided with tons of objective information, to allow you to understand this. Beowulf has a normal displacement hull quite like mine except without the keel bulb. But she is narrow. And she is very light (D/L of about 60 -- like TJ's boat "Rocket Science").

As boats get lighter and narrower, they make less of a bow wave. That means less energy is transferred to the water by wave-making, and there is less of a bow wave to climb. That means the acceleration of resistance which is happening between 1.2 and 1.5 S/L is less steep, so takes less power to overcome.

That's why Beowulf is able to make 350 mile days for days on end -- although that's an average of 14.5 knots, 2.5 knots above Beowulf's theoretical "hull speed" -- and why Beowulf is able to surf for longer periods at more than double hull speed, as we saw in the video.

That's why Jim Cate can "frequently hit 12 or 13 knots, and occasionally over 15 knots" in his light 46 foot boat, and why Phil (Boatman61) could average 9 knots in a storm in his Hunter Cherubini, and why others reported similar experiences.

OK, those are the "outlandish" facts. You can believe that we're all just lying to impress you (!), or not, as you like.

Sorry but I won’t be going along with the “group think” just to be considered one of the smart people, nor will I be baited into an argument. Nice try.

Still waiting to be shown the similarities between a Moody 54 and a Dashew 70 something or planing Maxi.

[B00B00]

Its an interesting topic.
My interpretation of it is that its just the distance between the bow and stern waves. A very fine bow creates the bow wave further aft reducing your hull speed slightly but a blunt bow creates a bigger wave that can be harder to push past. But i think stern shape is the defining thing between planning and not planning.. Have you ever tried to drag a large round inflatable racing marker bouy (or big round fender) when relaying a racing couse or startline, its almost impossible to tow over about 3kts. They just completley bury themselves underwater with huge drag. The weight is not the thing but just the round shape sucking it down. And identical weight and size cylinder buoy just glides effortlessly over the top.

I was recently very suprised(and pleased) to see our 2007 beneteau oceanis 50 very capable of effortlessly breaking hull speed and planning while surfing. 200nm days are quite easy to do and this is just under autopilot with the family on board and only in 20-25kts of atlantic trade winds.
I did look at the hull shape while we were out of the water and thought there was a chance of planning as the stern does have a flat fun aft but i just didnt believe it could be possible.
For fuel economy, we can motor at 8kts burning 4.5L an hour or 5.8kts burning around 2L. Big difference and over TWICE the motoring range! To push over 8kts it goes quickly up over 6L an hour. So inshore we cruise at about 7.4kts and offshore its down to 5.8kts.
You can see at 5.8kts there is vertually no wake but over 8kts there is a large quarter wake and the stern is starting to drag a fair bit of water.

[Kenomac]

Here’s a video of a Nautitech Open 40 doing 18 knots which is very believable. To my “nine year old “ eyes, the Beowulf video doesn’t look like a boat traveling at 27 knots, maybe more like 18-19 knots which is believable for a very narrow hull which is more like a catamaran.... but not in any way similar to a Moody 54.

https://m.youtube.com/watch?v=2KWDE9G_k-M

[Dockhead]

Originally Posted by carstenb

. .. I can tell you all for sure that were a number of times during the day (and night) when the log was over 10 knots and stayed over 10 knots for a couple of hours

were we surfing - yep.

Was it fun?

Damned right it was fun

Two knots above "hull speed" for a couple of hours -- right in line with my "outlandish" story of doing two knots above hull speed for a couple of hours. Yep.

[Kenomac]

Originally Posted by Dockhead

Two knots above "hull speed" for a couple of hours -- right in line with my "outlandish" story of doing two knots above hull speed for a couple of hours. Yep.

But if I remember correctly, your Moody was doing it against the tide and current. You claimed to have covered 30 miles in 2:45 hours against a .5 knot tidal current.

We also need to standardize the method of measurement on this discussion... gps, hull transducer and with or without current added into the mix? Our boat has no trouble making 13 knots with a 4 knot current running with us, so I’ll assume others can do the same.

[Dockhead]

Originally Posted by Kenomac

Sorry but I won’t be going along with the “group think” just to be considered one of the smart people, nor will I be baited into an argument. Nice try.

Still waiting to be shown the similarities between a Moody 54 and a Dashew 70 something or planing Maxi.

No one except you was talking about Maxi yachts.

The Dashew 77 Beowulf has a normal displacement hull -- I posted a photo of it. It is lighter (D/L) and narrower (L/B) so naturally much faster than my boat -- surfing at more than double hull speed and making 24 hour runs of more than hull speed. But the point is that without changing to a different hull form (like a Maxi or an Open 60 or whatever), the relationship of a boat to her "hull speed" changes according to these parameters. That's why other people have experiences which are different from yours.

Originally Posted by Kenomac

Here’s a video of a Nautitech Open 40 doing 18 knots which is very believable. To my “nine year old “ eyes, the Beowulf video doesn’t look like a boat traveling at 27 knots, maybe more like 18-19 knots which is believable for a very narrow hull which is more like a catamaran.... but not in any way similar to a Moody 54.

So again -- you're saying that Dashew's video is fake?

Beowulf is narrow, but not "very narrow", and is nothing like a performance cat. Beowulf has L/B of 4.7; my boat, fairly narrow compared to more modern designs, is 3.4. But being heavier and wider, my boat is much more constrained by her hull speed than is Beowulf -- I have never seen more than 4 knots above hull speed even in a wild surf (far from Beowulf's double hull speed), and my daily averages are never more than one knot below hull speed (far below Beowulf's daily averages of hull speed plus 2). Still heavier boats than mine are still more constrained by hull speed, and that's actually the whole point of writing all of this.

That is a sincere attempt to explain it.

[jtsailjt]

Now that hull speed has been beaten to death can someone explain what their boats Froude number is and how that applies to a hulls maximum speed? We’ve all seen the bow wave build until it becomes more and more difficult for the bow to “climb” it near hull speed, whether you think that is closer to 1.34 or 1.5 times the square root of LWL, but I can’t visualize how Froudes number impacts a boats maximum speed? Any naval architects or fluid dynamics majors who can explain this?

Also on a somewhat related subject, when I’m motoring at speeds approaching my boats hull speed, the stern squats so the step on the transom that’s normally 8”-10” out of the water is submerged with water a couple inches deep swirling around on that step. But when I’m sailing at the same speed that’s not the case. The water level at the stern is higher than when the boat is not underway but nowhere near as high as when I’m motoring at that same speed. Why is that?

[Kenomac]

Originally Posted by Dockhead

No one except you was talking about Maxi yachts.

The Dashew 77 Beowulf has a normal displacement hull -- I posted a photo of it. It is lighter (D/L) and narrower (L/B) so naturally much faster than my boat -- surfing at more than double hull speed and making 24 hour runs of more than hull speed. But the point is that without changing to a different hull form (like a Maxi or an Open 60 or whatever), the relationship of a boat to her "hull speed" changes according to these parameters. That's why other people have experiences which are different from yours.




So again -- you're saying that Dashew's video is fake?

Beowulf is narrow, but not "very narrow", and is nothing like a performance cat. Beowulf has L/B of 4.7; my boat, fairly narrow compared to more modern designs, is 3.4. But being heavier and wider, my boat is much more constrained by her hull speed than is Beowulf -- I have never seen more than 4 knots above hull speed even in a wild surf (far from Beowulf's double hull speed), and my daily averages are never more than one knot below hull speed (far below Beowulf's daily averages of hull speed plus 2). Still heavier boats than mine are still more constrained by hull speed, and that's actually the whole point of writing all of this.

That is a sincere attempt to explain it.

Please ‘splain how your Moody 54 with a beam of 15.92ft is somehow less “tubby” and more streamlined than my Oyster 53 which has a beam of 15.25ft. How is your Moody more like the very narrow Beowulf as you claim?

[Kenomac]

Originally Posted by jtsailjt

Now that hull speed has been beaten to death can someone explain what their boats Froude number is and how that applies to a hulls maximum speed? We’ve all seen the bow wave build until it becomes more and more difficult for the bow to “climb” it near hull speed, whether you think that is closer to 1.34 or 1.5 times the square root of LWL, but I can’t visualize how Froudes number impacts a boats maximum speed? Any naval architects or fluid dynamics majors who can explain this?

Also on a somewhat related subject, when I’m motoring at speeds approaching my boats hull speed, the stern squats so the step on the transom that’s normally 8”-10” out of the water is submerged with water a couple inches deep swirling around on that step. But when I’m sailing at the same speed that’s not the case. The water level at the stern is higher than when the boat is not underway but nowhere near as high as when I’m motoring at that same speed. Why is that?

That’s because you haven’t installed a warp drive on your Tayana starship; therefore, you’re unable to attain warp speed. Don’t you no nuttin’?

[Colin A]

I have heard several Naval architects over the years call Hull speed a good rule of thumb, and as much art as science. I think a good general guide and rule of thumb is the best way to think of it. Even some old boats could blow by it and some struggle to meet it.
I was talking to a naval architect at the Work boat show in New Orleans a few years ago for instance discussed bulbous bows. He said the more study and testing they do the more they find that while they make a huge difference on some hull forms no so much on others. My take away has been that despite Naval Architecture being one of the older engineering disciplines we still have a lot more to learn.

[Dockhead]

Originally Posted by Kenomac

Please ‘splain how your Moody 54 with a beam of 15.92ft is somehow less “tubby” and more streamlined than my Oyster 53 which has a beam of 15.25ft. How is your Moody more like the very narrow Beowulf as you claim?

My boat is not narrower than yours, but it's a lot lighter.

Also I suspect your boat is perfectly capable of breaking hull speed and surfing downwind, and that you've just never done this because you're simply more cautious in strong weather. Which is good seamanship on your part, actually.

[B00B00]

Can someone post a link to the other thread please.

[1000 islands]

Very interesting reading and educating for a "sailor" but being a power boater some of the discussion has been a bit difficult to decipher.

Such as D/L & L/B.....

But as this is all regarding displacement hulls, how does hull speed get determined in a power boat?