Hull Speed

All U Get · Sep 19, 2012

Here you go, select any of the calculators and have fun deciding.

http://www.sailingcourse.com/keelboat/cal__hull_speed.htm

All U Get

Manureva · Sep 19, 2012

the longer the water line the faster the boat with some exception, the hull speed is supposed to be the max speed of a displacement hull (sailboat), in theory. my san juan 23 has a hull speed of 6 knot, but i had it going 7.5 knots a few time, and it did 9 knot once in a storm.... and generally it does 5.5 to 6 knot most of the time in good wind of 8 knot plus

Victory · Sep 19, 2012

First of all, it's called "Theoretical Hull Speed" not hull speed. It's a specific formula for boats that push water out of the way, rather than skim on top of it (full displacement hull) and is based on the length of the bow wave created by that boat (waterline length) It is so misunderstood and incorrectly used it has become meaningless in the world of modern sailboats.

Do not let "hull" speed affect your buying decision. Theoretical Hull Speed has been discussed here "ad nauseum".

FYI, the formula, 1.34 X the square root of a boat's waterline length, was most commonly used in the days of steam to determine the optimal engine size for the full displacement hulled vessels of the day. Once a full displacement hull boat exceeds it's theoretical hull speed it is going faster than the wave it creates.... thus it is "planing"..... the additional weight of the fuel needed and extra horsepower required to exceed theoretical hull speed increases exponentially and thus becomes unprofitable to operate commercially.

Although many sailboats are full displacement hulls, even more are NOT. Many more fall somwhere between full displacement and planing huills. Compare a Dana 24, a J-24, and a Melges 24. All the same length, vastly different in speed.

So.... if I were looking for a sailboat that had a good speed rating.. the last thing I'd look at would be theorectical hull speed because it ONLY considers waterline length.... nothing else. Instead, to make speed comparisons between similarly sized boats ..... look at their PHRF handicap numbers.

Whatever you decide to purchase..... it'll be plenty fast enough to have fun and get you across the pond.

Good luck.. Joe

Charlie Jones s/v Tehani · Sep 19, 2012

"If planning a few hundred miles along the coast"

I've found I average 4.5 to 5 Knots, no matter what.. This with about 7000 miles behind me in the last three years.

Find a boat YOU like, and go for it- the difference of a half a knot or so is irrelevant over the long haul, coastwise. MIGHT make a difference crossing an ocean, but most cruisers I've spoken to on smaller boats, average about what I do, day in, day out.

JohnShannon · Sep 19, 2012

Hull speed in knots is 1.34*sqrt(LWL) but it is in no sense a limit.
However, for a displacement hull, the power required to exceed hull speed rises very rapidly so it does form kind of a speed limit.

They are also semi-displacement hulls and planing hulls characterized by flatter aft sections these forms take more power to move a low speed but less power to exceed hull speed.

Fellow named Dave Gerr has published a bunch of stuff findable on the internet and I think Caterpillar (marine diesels) discuss this in the context of how big a motor for what speed and what hull form.

Going by the formula, waterline is everything, but a 25 ft boat is 5/6 the speed of a 36ft boat so the payoff in speed for extra length is modest specially considering the $ difference between those examples

Indysailor · Sep 20, 2012

Davy,
You might want to consider how well a boat sails to weather if you are thinking about "speed". For example, I was following an S2 7.9 last week and could see that it pointed higher than my boat, thus reaching its destination faster than I could (it's also faster but being able sail closer to the wind made more difference on this day).
My boat is known for it's ease of use and comfort, but not as a fast boat or as one that points particularly high, but it works well for us as a conveyance of beer and children across the lake. There are likely trade offs with any boat.

weinie · Sep 20, 2012

Indysailor said:
Davy,
You might want to consider how well a boat sails to weather if you are thinking about "speed". For example, I was following an S2 7.9 last week and could see that it pointed higher than my boat, thus reaching its destination faster than I could (it's also faster but being able sail closer to the wind made more difference on this day).
My boat is known for it's ease of use and comfort, but not as a fast boat or as one that points particularly high, but it works well for us as a conveyance of beer and children across the lake. There are likely trade offs with any boat.

Maybe you should switch to LIGHT beer?

Indysailor · Sep 20, 2012

Some trade offs clearly aren't worth it!

rgranger · Sep 20, 2012

DavyBones

That is not a newbe question. I think your reasons for asking are good ones. And based upon the background info you provided I think you are really more concerned with miles (or knots) made good in a single day. Mesange alluded to this same notion in his response. The truth is when cruising you normally find yourself in light winds trying to squeeze every bit of performance out of your boat. Some days it blows hard and things are exciting but more often than not you will be sailing in less than 12 mph winds. So I think it is okay to also factor in the boats performance in light air. If you have to fire up the outboard anytime the wind drops below 15 mph, .... well that is a different type of cruising entirely. Bigger boat = more comforts = heavier construction = less responsive in light fluky air. Everything is a trade-off.

Over the years I have gone back and forth with this myself. I had a Mac V21, worked my way up through three boats to a Coronadoo 25' which was a dream in a strong blow and had a lot of room inside. She was a dog in anything less than 10 kts of wind. And I missed being able to trailer to other sailing grounds. So went back down to a Mac V22 (which I love and she sails like a dream -- easy to tow & easy to launch and will do 5kt/hr in an 8kt/hr wind) but she is crowed when you get 4 people camping out in her for a 4 day weekend....so I'm now in the process of restoring a Rhodes 22 (a little bigger) and I'm thinking about maybe getting a Mac 26C. I'd like something a little bigger for the family. (btw the Mac 22 is for sale <evil grin>)

So look at the quality of the sails, does the boat come with a drifter? or other light air sails? And is she sturdy enough to handle a hard blow when it comes. Any boat will hit hull speed in a strong blow but not all boats do well when the air is lighter.

anyway, that is just 2 cents worth of advice. In the end you have to fall in love with the boat.

shemandr · Sep 20, 2012

On a "Few hundred mile trip" the difference between a 21 foot and a 26 foot boat with regard to theoretical hull speed would be drowned out by the sailing characteristics of each boat and weather. I would base my decision on other criteria. If you plan to sail this trip than the boat that sails the best would be my choice. It's no fun trying to sail a dog into an adverse current against the wind.
A few hundred mile trip to a destination to sail sounds like a trailor/sailor to me.

rgranger · Sep 20, 2012

Hey DavyBones

I know this is not directly related to what you asked but since you are still in the shopping phase of this adventure let me add that when you buy an older boat like these, you should consider the actual hull (boat) as free and you are buying the sails, outboard, trailer and other accoutrements. A coat of paint, some varnish and some light fiberglass repair is cheap and easy. If you start buying sails, an outboard and a trailer, you are into some real money.

Jackdaw · Sep 20, 2012

Hull speed should NOT be a factor in purchase. As others have noted, it simply states the obvious, that most things being equal, longer boats will be faster than shorter ones.

The difference is not as much as you might think, because the SqrtRoot function is the great humbler of numbers. Now that being said, bigger boats will spend a much larger percentage of their time closer to their Hull Speed. Its why only small boat sailors ever think or worry about it.

If you want to get an overall idea of how well (fast) the boat generally sails, look up its PHRF handicap, and compare to other boats you are looking at. Lower is better.

http://offshore.ussailing.org/Assets/Offshore/PHRF/High+Low+Mean+PHRF+Handicaps.pdf

lillia28 · Oct 8, 2012

Sorry I am a bit late to this party. While I agree with much of the above, I believe that Roger MacGregor considered the Ventures and early MACs to have "semi planing hulls", so hull speed in your case is not limited to "hull speed" . I know my old V 2-22 had a wide flat hull behind the keel. In any case, as you heel, the water line lengthens, so your "hull speed" increases, depending on how far you are heeled. Boat trim, sail trim and a clean bottom are far more important to speed than LWL. Plus as far as I can tell the 1.34 is sort of an average, good designs can go a bit above, and bad designs will be a bit below, so there's really a range say from 1.3 to 1.4 times the square root of LWL, depending on the designer. Then there is the design LWL, a boat that is heavily laden will be a bit deeper in the water and therefore have a bit longer waterline than the design LWL, and therefore "go faster"? or will the extra weight slow you down? There are so many variables, that you should just pick the boat you like and turn off the knot meter. These discussions, IMHO, are best held in February, during a snow storm, sitting in front of the fire drinking Port. Or is it Single Malt? I forget.

Lou

dscribner · Oct 8, 2012

Lou,

That is correct. The hull speed is point in a cloud. The hull speed on on 22 footer will never be reached by another 22 footer and passed by a 21 footer. All dependent on the design and characteristics of the boat. Just like "the Code", it's more of a guideline. But, there is probably a +/- 6% (or something) that would apply. Truthfully, unless you're racing for money or pink slips, does it matter? 4.5 knots, 5.5 knots, still wicked slow.

Hollyweirdos · Oct 8, 2012

($1500 Craigslist + Needs some work) * Sq rt of LWL = X

20% +- error at West Marine *( Storage costs + fuel) * X = 4999

4999 - 6months to fix = $4000cost

Go find a nice little boat and sail the heck out of it and dont worry about everything else!

Charlie Jones s/v Tehani · Oct 10, 2012

You mentioned cats vs monos.

Having lived aboard a 35 foot trimaran, and having sailed beach cats for years you should understand that in very light air, cats (and tris) can become "dogs" They have much more skin friction than an equivalent size mono.

Of course, when it does blow, they go like stink. I saw 19 knots once on my tri for about 5 miles, fully loaded for cruising. Of course, we were running for cover just before a huge blow in the Chesapeake and I was "letting it all hangout". We had come down the bay dead down wind, logging just over 14, for two hours. Winds in the 30 knot range.

You don't often sail 'em like that!!

finding41 · Oct 11, 2012

Hi Davie. Take a look at the 26' MacGregors. They all have good hull speeds and ROOM inside. The power sailors are surprisingly large inside with standing head room (Not a huge area but...) The earlier 26 D&S are roomier than the old 25' and not just because of the extra foot of length. The 26's all have a queen berth under the cockpit. They all have water ballasts and are light to tow.
Good luck.
Edit: I forgot the great light air performance! And heavy air too.

kenn · Oct 17, 2012

very late to this... but here's my two cents anyhoo

As shown elsewhere, "hull speed" is just a number derived from a theoretical formula that takes into account NOTHING except waterline length. So, disregard it completely when making your boat choice, other than keeping in mind that a longer boat in general should be faster than a shorter boat, in a good blow.

When considering the sailing abilities of similar boats, the following factors are much more important than hull speed:
- pointing ability. A boat that can point higher will be faster upwind. (already mentioned above)
- light-air performance. Since most of us aren't guaranteed 10+ kt of wind every time we venture out, it's nice to have a boat that will move well in a light breeze. Such a boat will do loops around heavier boats on a light day.