MacGregorSailors.com

Hi Guys...

I am currently having a diplomatic dispute with one of the guys at our boat club....i have a Mac 26s and use a Standard Horizon 180i Chart plotter that gives boat speed via gps.
After tensioning the rigging completly and doin some other polishing and stuff we got our Mac back out in the water. the wind was gusting up to 30 knotts and we used a first reef and half of the 125% genoa.

Here is the dispute... My gps gave me a speed at one point of 7.8 knotts....however the other guy says that there is no way a Mac 26s will do more than 6 knotts as the hull isnt designed for it....so can any one tell me if.....A, they have done better than 7.8knotts? B, whats the best a Mac 26s can do? C, is the gps reliable when it comes to speed??

Any info would be greatly recieved....

Cheers all......Madguy

I would think you could get that running with that wind speed. I dont know about pointing.

I should have said....we were also on a broad reach......but more importantly...what would you think the hull speed of a 26s is and is the gps reliable????

Thanks guys.....happy sailing....
Madguy

We have reached 9.5 mph two different ways in our M26: one was on a reach with our std A-spinnaker in a 16 mph breeze and the other time was running down wind surfing the wind waves in a true 35 mph breeze with only our 3rd reef up. There is my 70hp that makes it go 20+ and we do 65 mph up across the Cascade Mt passes, these are the things that most of the bigger boat owners I talk to are truly envious of.

ive hit 8 knots on a reach,we were close to reefing but kept going,i think that we may haave kissed 9 knots but i was too involved with watching the rigging to see where the speed peaked

I've never quite understood "hull speed"...I know I have been over 7.5 kts a couple of times...hull speed is below that I think.

c130king wrote:I've never quite understood "hull speed"...I know I have been over 7.5 kts a couple of times...hull speed is below that I think.

From what im led to believe,,Hull speed is what the designer intended the boat to be able to do.....but dont quote me on that cause im probly wrong...

Madguy

Hee hee

Hull speed, sometimes referred to as displacement speed, is a rule of thumb used to provide an approximate maximum efficient speed for a hull. It is only ever an approximation and only applies where the hull is a fairly traditional displacement design. It is usually described as a speed corresponding to a speed-length ratio of between 1.34 and 1.51 depending on which of the limited sources one refers to.
In English units, this may be expressed as:

where:
"LWL" is the length of the waterline in feet, and
"v" is the speed of the vessel in knots
The constant may be given as 1.34–1.51 knot·ft−½, or in SI units, 4.72–5.32 m½·s–1
The concept of hull speed is not used in modern naval architecture, where considerations of speed-length ratio and Froude number are considered more helpful. It is still used by amateurs in relation to traditional displacement hulls.

Background

Wave making resistance begins to increase dramatically in full-formed hulls at a Froude number of about 0.35, which corresponds to a speed-length ratio of slightly less than 1.20. This is due to a rapid increase of wave-making resistance due to the transverse wave train. At a Froude Number of 0.40 (speed-length ratio about 1.35) the wave-making resistance increases further due to the increase of the resistance caused by the divergent wave train which is added to the transverse wave train resistance. This rapid increase in wave-making resistance continues up to a Froude Number of about 0.45 (speed-length ratio about 1.50) and does not reach its maximum until a Froude number of about 0.50 (speed-length ratio about 1.70).
This very sharp rise in resistance at around a speed-length ratio of 1.3 to 1.5 probably seemed insurmountable in early sailing ships and so became an apparent barrier. On the other hand, these values change dramatically as the general proportions and shape of the hull are changed. Modern displacement designs that can easily exceed their 'hull speed' without planing include hulls with very fine ends, long hulls with relatively narrow beam and wave-piercing designs. These benefits are commonly realised by some canoes, competitive rowing boats, catamarans, fast ferries and other commercial, fishing and military vessels based on such concepts.

I have numerous sailing books. I know what "hull speed" is in terms of its definition. What I don't understand is how/why it is even used if a boat can relatively easily exceed this speed. Seems to be a fairly useless number.

Or am I missing something?

Jim

Around hull speed will be the most efficient speed of the boat. It is the point where you get the best speed for the least amount of effort. You also get your best fuel economy at this speed. That's about all it's worth, particularly for Mac hull shapes which do not have a displacement hull shape and have very low displacements.

In general you are at hull speed when the wavelength of your bow wave has reached your stern. Once you go beyond that you are transitioning to a plane.

It is in no way a speed limit. Any sail boat, even a big heavy keel boat can go faster than the calculated hull speed. This happens all the time in heavy winds and large seas as the boat surfs down a wave. In your case, you applied enough force to the hull to start to get beyond your bow wave. For light Mac's (even with ballast) it is not hard to do this. For a heavy keel boat you usually cannot generate enough force and need special cases like surfing to get beyond hull speed.

All Macs can do this under sail, and obviously the X and M do it all the time under power. It's just a matter of how much horsepower you can apply to the hull with either the engine or the sails.

Hull speed, as Scott describes is the physical limitation of the top speed of the boat, due to its inability to get up on plane and break through the bow wave. The limitation is strictly a function of waterline length. All that said, hull speed can be exceeded in certain conditions, typically downwind and using the waves to surf, thus breaking free of the bow wave. Multihulls don't seem to be limited by hull speed... I don't know why that is. The Mac X & M have planing hulls, so they wouldn't be limited by hull speed off the wind, though upwind, they function much like a displacement hull. My top speeds: Over 11 knots downwind. Over 8 knots upwind. Neither could be maintained for too long. Upwind, if you're doing 6.5 knots, you're pretty much perfect and maxed out. Incidentally, the hulls speed for the Mac is 6.45 knots, dreived as follows: Waterline length=23.17 feet. Square root of that is 4.813 times 1.34=6.45

Current can play a role as well. If you're sailing at hull speed of 6.45 knots and have a favorable current of 2 knots, you may sail at 8 knots+ The reverse is also true

Leon

"Hull speed" gives me an idea what the boat is capable of. When I start approaching hull speed
I know I've done pretty well in terms of setting and trimming the sails to get the maximum effort
out of everything. I've maintained 6.3 knots for a couple of minutes when the wind and waves
cooperated. My daughter and I hit 6.7 once but you could tell there was a BIG assist from wave motion.
In good conditions I can hold her between 5.8 and 6.1 knots for the entire afternoon. My problem is
I have too much wieght aft, she's a taildragger.
Laurie.

I agree that the hull speed is about 6.5 reaching in a 14 mph breeze Main and 110 jib about 50% +- Packed with 2 weeks of food, water, dobro, banjo, mandolin in heavy cases music books (in the bow for a heavier bow around the mast area) and gear towing a 10 ft dingy. The numbers all change when we hoist our A-spinnaker w/sock in 2 to 15 mph breezes we probably get 75% out of a broad reach. We love crusing.

The Admiral and I were out for an afternoon of fun last fri. marine weather broadcast was for southerly winds in the 10-15kt area. the actual wind was from west and was more in the 20kt area. we were running with the main only, out to the spreaders, and were moving at 6.0 kts according to gps. rudders down, 6" of daggerboard, engine up, and as all the others, a bunch of boat stuff. seems to me, that if I had some of the genny out, 7+kts would not have been a stretch. stew

Holy Crap! I didn't know I am supposed to know my hull speed. Thats it for me,I give up . I'm still trying to get out of the Marina without bumping into something and now its hull speed? I thought "hull speed" was somehow related to the size of my butt and switching seats while tacking. Man am I a loser!