Hull speed

[Weldon]

Interested in and under power speed with 50hp is 20kn +. Does anyone know the hullspeed or speed under sail with main and jib and how much faster with main and genoa

[Catigale]

Theoretical hull speed is approximately 6.4 knots basedon 23 foot length at waterline. The actual hull speed depends on heel angle, loading etc

It doesnt matter which sails you use, except that in heavier wind with more heel, the speed will change per above.

My 2002 did 22 mph GPS clocked with a 50 HP Mercury BF with 3 people on board...now I have it loaded for cruising it tops out at about 15mph.......if you want to keep your boat at the 20 mph performance level I think you need a 70 HP or even a 90HP...others know more about this than I.

[Chip Hindes]

Just this past weekend, testing the new lower unit which was just installed, my X with 50HP Tohatsu did 18.5mph by GPS, with just me aboard, no ballast but fairly well loaded for cruising. I believe this is fairly typical.

Also, headed up the Hudson in about 20 mph wind, 150 genny only on a dead run, I twice touched 7.8mph (not knots), and was able to maintain over 7mph for several minutes at a time. Was hoping for 8, but never quite got there. Even so at just over seven knots that's well over hull speed. The only thing I don't know is if the tide may have been helping or not.

The myth of the planing Mac may not be a myth after all.

[Zavala]

Chip, were you sailing with or without ballast? I see you motored without, but couldn't tell from the post if you had filled the tank for the second portion of the trip.

[Chip Hindes]

You're right; the motoring report was on Saturday, down river, no ballast, against the wind maybe 4-6mph. Second portion of the trip, upriver under sail in estimated 20mph winds, was actually the following day, Sunday: full ballast, all foils and motor down.

If I'd thought of it, I might have tried pulling the centerboard, motor and one rudder and I might have hit the desired 8mph. Doubt it, though. Although it was an exhilarating trip and I briefly considered staying out a while longer than I did, I was really distracted by the concern that I was going to have to put my newly repaired boat on the trailer, singlehanded, in a 20 mph cross wind. Worked out fine though. First time ever in five years I have succeeded in motoring onto the trailer without incident.

Very early in my five years of owning my X, I sailed without ballast once, by accident. I got knocked down and had water coming over the gunnels. Really managed to scare the crap out of myself before I realized my error. I have never tried it since.

[Frank C]

The myth of the planing Mac may not be a myth after all.

I'm convinced it's no myth, but the word "planing" exaggerates the case. The Mac has (at least) a semi-planing hull, according to Dave Gerr's definition. Therefore, its theoretical hull speed well-exceeds that of a full displacement hull, even though the boat may never actually reach a plane under sail. The Mac's flat hull simply incurs less resistance than the 6.4 knot limit of a traditiional hull form.

Thanks to Moe, we have these animations helping to depict the differing behaviors of displacement, semi-displacement and planing hulls (Click here for original thread):
DISPLACEMENT =>

SEMI-DISPLCMT =>

PLANING =>

Whether Dave Gerr's formula exactly predicts a "truly 9-knot" hull speed for the 26X is immaterial. The compendium of anecdotes just on this website indicate the 26X can easily run to 8 knots. Further, in 20 knot winds with adequate sail controls & sail plan, it can attain almost that same 8 knots close-reaching at 60-off the apparent wind. Following is my own quote from an April 2004 thread about Mast Bend.

. . . may have tacked through 90 w/ my dealer aboard, but I'm more likely to see 100 ... (maybe due to raking mast forward to 88 degrees) . . .

Regarding speed upwind, Dave Gerr created a modified hull speed formula for semi-displacement boats. If you buy Dave's modified hull speed theory (quoted here on a Potter owners website), you'll recognize that the Mac's flat hull could explain an adjusted hull speed of about 8-to-9 KNOTS, largely due to her flat bottom. Not to say she's planing (which is impossible upwind [for this boat] anyhow) but she does not have any rounded hull to hold her within the typical wave pattern.

I've managed a close reach at 7+ knots on three different days on SF Bay. Each time was in whitecaps, heeled at 25*, jib and reefed main. . . . the first two days, it was a fluke to find the groove, and hold 7-to-8 mph for 10 minutes at a time before a roundup. On my third day though, I had it figured out. We managed to hold a port tack for 30 minutes, north-bound from Treasure Is. toward Angel Is, WNW winds of 18-20 kn in 2' chop. The boat felt like it was on rails, and the GPS read 8+ mph for that half-hour, kissing 9 MPH a couple of times ... it never touched 7 mph once we gained speed on that tack. Currents on that wide stretch of the Bay are insignificant. You can do the math. . . . it took me 3 years of reading and learning (numerous control enhancements), just the right conditions . . .

I deeply regret that I've only managed a handfull of sailing afternoons during the ensuing two years with the boat. However, even more sail-controls are in-progress. I'm hoping that the (half-completed) traveler can help me to pin down a consistency formula.

[Chip Hindes]

I didn't realize my comment was going to get me into the planing versus non planing argument again. Last time I avoided it completely because I believe it's a waste of time and misses the point. I still do, it still is, and it still does, so scratch my comment.

[Beam's Reach]

I like to argue as much as the next guy...or should I say the last guy...Hi Chip! ... but I think Chip's right. Debating the "theoretical" hull speed is pretty much a waste of time.

I figure that when I'm at WOT, I'm going as fast as I can without resorting to dumping gear overboard, and that gear is there to make my cruise either possible, safer or just more enjoyable, so it stays.

If I'm sailing, I'm going as fast as I can with the current wind and water conditions and my (admitedly limited) knowledge of sale shape adjustment and the gear on board as mentioned above.

So either way, I just enjoy the speed I'm travelling at. As others have said, don't focus on the destination or the speed at which you get there....have fun and enjoy the trip!

[Frank C]

I didn't realize my comment was going to get me into the planing versus non planing argument again. Last time I avoided it completely because I believe it's a waste of time and misses the point.

Hmmmm - please correct me if I mis-read something above, but I think the "point" was "hull speed" rather than planing. In fact, planing is mentioned only in exclusion, save for Moe's final gif, which is most interesting for the way it helps delineate "semi-displacement" behavior. Suppose I could hve been more explicit, that I was focused on hull speed under sail - which pretty much obviates any potential for planing. Sorry if the post implied an unintended debate.

It seems to me that, beyond the fact that "hull speed" was the originating question, there's a very good reason that semi-displacement hulls are germaine to the discussion of a Mac 26X under sail. That is, to help index one's own success in performance of the Art.

I like to argue as much as the next guy... (but) debating the "theoretical" hull speed is pretty much a waste of time.

If I'm sailing, I'm going as fast as I can with the current wind and water conditions and my (admitedly limited) knowledge of sale shape . . . So either way, I just enjoy the speed I'm travelling at.

Again, unless I mistook something above, I fail to see any argument. The original poster asked about hull speed, both while motoring and under sail. Granting that there's probably NO "precisely correct answer" to the question ... the traditional formula for "hull speed" is clearly inadequate for a 26X, by numerous accounts.

Almost by definition, Macgregor sailors practice a (severely?) "limited knowledge" of the art of sailing. However, that needn't reflect on the actual raw ppotential of these unfortunately maligned vessels.

I too, am quite satisfied to simply go with the flow when I'm on the water. But on rare occasions when conditions just happen to coalesce, it's quite a rush to feel that raw potential - and to appreciate the physics that enable it. Assuming that I must have misled ... I hope that clarifies things.

[Dimitri-2000X-Tampa]

A Mac has a planing hull, not a good planing hull since its also a sailboat hybrid but a planing hull design nonetheless. With a traditional displacement hull type sailboat, the hullspeed is the limit so even if you put a 50HP on a 26 footer, you would still only be able to do hullspeed (around 7.5 mph). But a Mac can go from displacement mode to semi-displacement to planing mode.

Getting back to Weldon's question, a genoa is faster in light or moderate winds but a jib will produce a faster hullspeed in heavier winds. My guess is the difference is somewhere around 0.5-1.5 kn by using the right headsail for the job.

[RobertKing]

Chip, when you got a knock down without ballast, How did you get her back upright. I have been knocked down on several times on little boats where you could easily stand on the center board and pull it up, I cannot imagine doing that on a mac 26?

[Chip Hindes]

Understand. I've sailed both Hobie 14s and Sunfish, and it wasn't a proper day of sailing if I didn't get knocked down at least once.

When I got knocked down on the Mac, I didn't manage to go completely horizontal or put the sail in the water. I let the main out instantly, shifted my weight as far out on the rail as I could get, and it popped back upright. Most of the water which had come over the gunnel drained out through the transom; got not much more than a splash below.

It was only the second time I had the boat out, and the first time in fairly heavy winds, so I was pretty surprised the first time it happened. Here's the really dumb part: I wrote it off to normal sailing behavior and my own inexperience with the boat. I was thinking to myself, man what have I gotten myself into, this isn't fun and this boat is dangerous. And within five minutes I did it a second time. At that point it finally hit me like a bolt from the blue, the fact I had completely forgotten the ballast.

Haven't forgotten it since.

With ballast, I've had the side windows in the water a couple times, once a little water over the gunnel, but the boat always has rounded up and popped back up on its own.

I admit my tendency is to wait far too long before shortening sails.

[Catigale]

I can tell you from my experience on Sunday that an Island 17 with only 7 feet of mainsail up in 25 mph+ planes over 3 foot waves quite nicely

I lasted 5 minutes before I almost rolled it, so I headed home - Abigail wrote a perfectly good page on how "Seagull saves Man" referring to my motor which brought me back to the ramp.

[Scott]

These numbers are easily reproduced and often exceeded under Genny and main, (notice sheet location) Minus 1-2 knots under Jib.

Turning from broad to beam reach without adjusting the sails we accidently got over 9 knots with the assymetrical.

[Hubert]

The most I get powered with by Honda BF50 is 30km(GPS) no Ballast, 12km flying spinnaker alone is the best I've had under sail