SailNet Community banner
1 - 20 of 23 Posts

·
Registered
Joined
·
86 Posts
Discussion Starter · #1 ·
This is probably a dumb question.
I was doing a little research on my probable hull speed and I was wondering...

So, you've got a 90' monohull.. say 80' at the waterline. That puts your hull speed at about 12 knots. On points of sail other than a run, do larger boats need comparatively stronger winds to hit max speed, or is it all relative.

I seem able to top out in 12-14 kts of wind on a reach in my 30'er... barely besting 6kts. Can someone in a boat 3x as long barrel by me at nearly twice the speed in the same wind, or would they need a stiffer breeze to top out?

Thanks,
 

·
Freedom 39
Joined
·
1,156 Posts
Yes they could easily pass you at twice your speed with the same wind. Read about Steve Dashew's Deerfoot designs or the Maltese Falcon.

I crewed on a Santa Cruz 70' and recorded nearly 14kts beating into the wind and pointing quite high.
 

·
Senior Member
Joined
·
19,485 Posts
Horsepower (sail area) vs weight and wetted surface (drag) and overall length are all factors that dictate ultimate speed, along with the ability (or lack thereof) to break out of displacement mode and actually 'plane'.

Boats like the Volvo RTW boats are 70 feet long but routinely break onto a plane and can do well over 30 knots, clearly in excess of 'theoretical hull speed'.
 
  • Like
Reactions: BeejDeC

·
Freedom isn't free
Joined
·
3,137 Posts
Faster has you covered...
A smaller boat might hit your top hull speed with less wind... if they have a different shape, and their wetted surface gets bigger with heel, and their rig is powered up... Now if they are also much lighter and can plane, they might well be able to sail much faster than you as well.

Hull speed on my Capri 25 was somewhere south of 6 knots. I considered it a good day sailing if I could spend the whole day out above 6, and managed to do it a few times.

I'm by far NOT the best sailor and I saw double digits a few times with the boat (mostly broad reaching with some waves, or under the kite). The Capri isn't exactly a boat that planes either (fin keel, drag monster that it is).

Watch this video in HD... 22 footer (J/70) you'll be able to read his knotmeter.
 

·
Registered
Joined
·
712 Posts
As a rule... longer waterline means it's scientifically capable of a higher speed. But then the variables come into play: sail area, boat hull design, crew capapbility and even the sails themselves. I have a 31' boat with a 27' waterline, but know a Catalina 27 can go scooting by me. I have a heavy "cruising" hull, so in my case... the longer, faster thing doesn't apply.
 

·
Registered
Joined
·
86 Posts
Discussion Starter · #6 ·
Thanks all.

I'm not asking in a racing sense. I'm just talking about, all things being otherwise equal, two displacement hulls of differing lengths. My boat is 40yo, so I'm pretty sure it won't be planing, or that that was even a goal of the designers.

Even with my very limited abilities though, it pretty easily surpasses its theoretical hull speed. I thought that was impossible.

So, another trick question. I'm sitting pretty high in the water this year. No water in the on-board tanks, very little gear on board. Am I faster this way, or would I do better to fill the water tanks, install the stove, add a few spare anchors... basically get heavy enough to buy myself another 4" of LWL?
 

·
Registered
Joined
·
2,704 Posts
So, another trick question. I'm sitting pretty high in the water this year. No water in the on-board tanks, very little gear on board. Am I faster this way, or would I do better to fill the water tanks, install the stove, add a few spare anchors... basically get heavy enough to buy myself another 4" of LWL?
I'm the first to admit that I'm not an expert here so this is just my understanding.

The hull speed determined by waterline length is a top speed that under normal conditions will not be exceeded. I believe even if you're under-loaded and sitting higher in the water, you still won't exceed hull speed.

So being lighter will allow the vessel to be more responsive and reach upper levels of speed quicker, be lighter to handle but theoretically will still not exceed hull speed.

Loading it down to the waterline makes the boat less responsive but will (theoretically) not make the boat faster in terms of top speed than a lighter-trimmed boat.

Just think about how much waterline length is lost by loading lighter - 4 inches is about 1% of your LWL - would you notice 1% loss in boat speed? Probably not. (Maybe this is not a linear relationship - just saying).

My boat has a theoretical hull speed of 8.7kn but I have seen it do 14 knots - down a 30 foot wave in a storm!!! Not for the faint-hearted (including me).
 
  • Like
Reactions: BeejDeC

·
Master Mariner
Joined
·
9,225 Posts
The fallacy of trying to determine maximum actual speed possible and that supposedly possible by waterline length, is that as speed increases, the boat sits deeper in the water.
I was caught in a Med storm with way too much sail up and the boat was literally decks awash as she hurled herself before the wind. At times only the cabin top was visible from the helm, the hull and deck completely submerged. Being before electronic navigation and common installed speedometers, I have no idea what our speeds were, but to say I was in control most of the time, would be stretching the truth some. After that experience, I could see how it was certainly possible for the clippers to sail themselves under, with a loss of all hands.
 

·
Senior Member
Joined
·
19,485 Posts
So, another trick question. I'm sitting pretty high in the water this year. No water in the on-board tanks, very little gear on board. Am I faster this way, or would I do better to fill the water tanks, install the stove, add a few spare anchors... basically get heavy enough to buy myself another 4" of LWL?
I think esp in lighter air you're better off 'light' and 'short', I think the tradeoff of wetted surface will do you more good than the couple of inches of WL you might gain by loading her down.

Work out the theoretical hull speed of WLs 4 inches different and I suspect there's not a lot in it - never mind how irrelevant that calculation may be ultimately in the 3 or 4D world we sail in.
 

·
Registered
Joined
·
4,721 Posts
'was certainly possible for the clippers to sail themselves under' Back when stuff broke and saved the ship . Now dacron sails and modern materials can easily drive a vessel to trouble The Bluenose off Burmuda caught a 90mph burst and would have made the big dive before anybody could react but the main sail came out of its bolt ropes . Less displacement and the right shape would appear to allow the hull to yield, plane or just get dragged without depending on prudent seaman ship or gear failure for survival. At that point are you calculating the square root of your WL?
 

·
Registered
Joined
·
3,262 Posts
Bee,

Assuming things like SA/D and wetted surface are similar larger boats actually have an advantage in light air. Because of the longer waterline they operate in lower drag portions of their speed/drag curve which allows them to go faster. This starts to accelerate the apparent wind which again allows them to sail faster year again. And the beneficial spiral keeps on going until drag bleeds away any additional speed.


As for adding weight to go faster... No boat I am aware of is faster in light air with more weight aboard. Even if there is some advantage in waterline leingth the power:weight ratio always favors reducing weight first.
 

·
Registered
Joined
·
81 Posts
I am working through the mechanics of this and trying to come to some understanding. We have a Hunter 290 which is a reasonably fat cruiser and we have not had a lot of opportunities to compare our speed to other boats other than motoring. It does seem that bigger boats travel faster under power than I am happy doing and I wonder if they can loaf along at 7 knots under power compared to my 5 knots. Under sail I love to work on the trim and really maximize speed but I still wonder if a 38'er would be blasting by with the same wind.

Would they if they were also well trimmed? Assuming 10 knots of wind speed which would put neither of us anywhere near our hull speed. Are things like folding props, good sails and not carrying any excess weight going to matter more than the length?
 

·
Registered
Joined
·
898 Posts
My longer lighter boat will blast by you, at hull speed, in 10 knots of true wind. So, yes. And under power too, of course, but it burns much fuel. Longer boats can do circles around shorter boats upwind and down. They are also much steadier when the seas get up.
 

·
Mud Hen #69, Mad Hatter
Joined
·
415 Posts
We had a Catalina 34 Mk II that is by no means a racing boat - but no slouch. In the next marina over was a Luders 16, which was maybe 60 years older than ours. Fractional 3/4 Bermuda rig. Little slip of a jib. We had about equivalent SA/D.



LUDERS 16 sailboat specifications and details on sailboatdata.com

But he could out-point us by almost 20° to wind in wind under 10 knots. I have no idea how, and his speed was about 4.5 kts (16' waterline . . . until he heeled a bit & a beam of 5-3/4') but he could walk a LOT of boats going upwind in light air.

So, of course, you never took him on going dead upwind unless it was blowing 15 or better. ;-)
 

·
Registered
Joined
·
45 Posts
Generally speaking and all else being equal, hull speed = square root of WL length X 1.34.
30 foot WLL boat = 7.33 knots
60 foot WLL boat = 10.38 knots, or 41% faster. Note the 60 ft boat won't be twice as fast because the formula is based on a squaring function.

In practice, the WLL will not be the same one 30 ft boat to another because of hull shape. As one boat heels -maybe even to nearly the same degree of heel as the other boat- the shape of the hull will change the effective WLL somewhat, as will changes in its overall wetted surface contribute to more or less drag that the other boat may experience.

Once u begin to reach that 'real hull speed' -which will differ boat-to-boat even if the WLL is the same between boats- it takes very large increases in power (sail or motor) to drive beyond that 'real' hull speed. Indeed, once you get to about 90% of that theoretical hull speed, that last 10% begins to require more than 10% more power, hence, 90% of theoretical hull speed is about what can be practically achieved wih reasonable power, be it wind or mechanical.

Case in point, the DDG my son was in for nine years:
- two turbines, full speed, one shaft trailing: 24 kts
- four turbines, full speed, both props powered: 35+ knots, not 48 knots
Twice the power, 10 to 12 knots more, not 2X more knots.
 

·
Registered
Joined
·
3,554 Posts
Thanks all.

I'm not asking in a racing sense. I'm just talking about, all things being otherwise equal, two displacement hulls of differing lengths. My boat is 40yo, so I'm pretty sure it won't be planing, or that that was even a goal of the designers.

Even with my very limited abilities though, it pretty easily surpasses its theoretical hull speed. I thought that was impossible.

So, another trick question. I'm sitting pretty high in the water this year. No water in the on-board tanks, very little gear on board. Am I faster this way, or would I do better to fill the water tanks, install the stove, add a few spare anchors... basically get heavy enough to buy myself another 4" of LWL?
The theoretical hull speed is just that: theoretical. In reality there is more than just waterline length at play. for example my boat has a 26 ft waterline while at the dock, but once the boat gets moving the waterline increases as the stern wave fills in the transom area. When a displacement boat is really powered up it gets locked into the trough of its own wave. If wave conditions permit it may break loose and surf down a larger wave, allowing it to exceed it's theoretical speed. I was in a long distance race once where a tug boat slowly overtook the fleet and we were able to ride his wave for a long time, again breaking that theoretical speed limit, and allowing us to hang with bigger faster boats for a while!

As for the weight, loading it on to increase your waterline would be of no benefit. The same boat with less weight will accelerate faster, and will require less power to reach it's maximum speed.
 

·
Registered
Joined
·
81 Posts
So I understand the hull speed relationship to length and I certainly get that my boat will not exceed 7 knots except in the most rare circumstance. However, my question was related to less than hull speed situations, is the longer boat generally still faster? Modify my question to 8 knots of breeze and a broad reach to eliminate pointing ability. will the longer boat still be much faster?

In addition, assuming this 38'er has a 40 horse Yanmar compared to my 20 horse, and we are both poking along at 2000 RPM, will he be going faster and or "fastereasier" than me because of his length?

Dan
 

·
Senior Member
Joined
·
19,485 Posts
So I understand the hull speed relationship to length and I certainly get that my boat will not exceed 7 knots except in the most rare circumstance. However, my question was related to less than hull speed situations, is the longer boat generally still faster? Modify my question to 8 knots of breeze and a broad reach to eliminate pointing ability. will the longer boat still be much faster?

In addition, assuming this 38'er has a 40 horse Yanmar compared to my 20 horse, and we are both poking along at 2000 RPM, will he be going faster and or "fastereasier" than me because of his length?

Dan
Generally the longer boat will be faster than the shorter boat in such conditions, but it's not entirely and 'apples to apples' situation.

There are some 50' boats out there that are literally tanks, overloaded, heavy displacement, inadequate sail area that will be outsailed in nearly all imaginable conditions by a light, spritely 24 footer.
 

·
Freedom isn't free
Joined
·
3,137 Posts
I seem able to top out in 12-14 kts of wind on a reach in my 30'er... barely besting 6kts.
In those winds I could walk past you in my 26 foot S2. Especially downwind. Upwind it might be a tie. A reach? Forgetaboutit, and my waterline is only 22 feet.

SHNOOL who thinks hullspeed is just a suggested limit.
 
1 - 20 of 23 Posts
Top