Sail Trim Question

[Peggie Hall HeadMistress]

You've confused me, Rich...

Every boat I ever owned was planing hull boat. When the boat is on plane--FULLY on plane--there's no bow wake and very little stern wake. It's only when the boat anywhere in between idle speed and full plane that there's wake...increasing as rpms increase until the bow "falls over," then immediately reducing to a stern wake so flat it's not even noticeable except visually (scares the hell of sailors who only see a fast boat without noticing how little wake, though). It's the big displacement boats and planing boats that are running with their noses in the air that make the most wake.

We've always considered "hull speed" to be the max speed through the water at what can be considered max idle speed...iow, the fastest an individual planing hull can move before its bow starts to rise and its stern starts to dig in and make wake. And calculated it using 1.3333 x sq. rt. of the WLL.

You're saying that's inaccurate...but if the boat ain't makin' any wake, how do you calculate the speed of any planing hull that's moving within "idle speed" range?

 

[FourPoints]

You're saying that's inaccurate...but if the boat ain't makin' any wake, how do you calculate the speed of any planing hull that's moving within "idle speed" range?

You are correct, the 1.34 * sqr rt of waterline applies to all boats, even a planing hull, at any time it is not planing.

Different Hull shapes will move up to hull speed with less power than others (a powerboat with a 22' waterline probably cannot accelerate to its hull speed of 6.2kts using a 8hp engine, but my sailboat can). The formula is universal for anything that floats, and as mentioned before, simply calculates the speed to waterline ratio that will cause the bow wake to join the stern wake, creating an uphill route the boat must travel if it is to go any faster. But regardless of what type of hull you have hull speed still dictates the fastest the boat will move without a significant increase in power. The differences comes in with how much power it takes to transition from displacement to planing, and how efficient the hull makes that transition.

A sailboat with a planing hull is designed to create a minimal bow wake with a small narrow bow, and then have a large flat bottom (think pumpkin seed shape) combined with light displacement, it will easily ride up onto, and then over the bow wake without an excessive amount of additional power.

Powerboats that have planing hulls do so by sheer might and raw horsepower. They do not have efficient low speed planing hulls like planing sailboats, they have high speed planing hulls that at speeds at, or slightly above hull speed, generate huge stern wakes and lift the bow way up in the air, until the boat slowly claws it's way up and over the bow wake and then accelerates down the face of the wake where it gains enough speed to ride ON the water instead of IN the water. With the deep V in the bow, lots of hard chines to deflect spray (and generate additional lifting force), this equals a very small wetted surface on the power boat

Could any fin keel sailboat plane? yes, if you can somehow generate enough thrust to acheive enough speed to ride on the water, but for 99% of the sailboats out there that will never happen.... Not to mention that the drag of the keel at said speed may very well cause it to be ripped off in the process

 

[steelcastle]

Ok, so would I hit 12 knots on a Mac25??

 

[kloudie1]

Ya can if you are surfing .. Surfing down the wave front where ya have gravity to help drive you, or ya can do it with a 70 horsepower (approx) outboard.. and no sails

 

[Joe]

I often manage to exceed my hull speed of 6.2kts .......

..... and most of the time i can sail the boat right up to 6.1 or 6.2kts, and no amount of trim improvement will get me past that limit.

Well, either your boat does exceed theoretical hull speed, or it doesn't... what is it?
Do you see the problem here? Do you see why using theoretical hull speed as a sailing performance tool is IRRELEVENT? The formula is for full displacement hulls.... which your Hunter isn't.... and was most commonly used to determine optimal size effeciency for steam engines...

Assuming you were measuring your speed with a log (speed through water) rather than a gps (speed over ground), this is exactly why theoretical hull speed is a poor measurement of boat performance... because it only considers one factor...... the speed of the bow/stern wave....and for a full displacement hull at that.

 

[Joe]

Ok, so would I hit 12 knots on a Mac25??

Sure...... pull up the centerboard, dump the ballast, set the spinnaker and head downwind in a 30 kt blow. Uh... keep your weight back and hang on.... oh, and make sure your rig is strong. Just like dinghy sailing.

 

[HEINZIR]

I don't know either. The Mac carries water ballast, so it is not a "centerboard" boat like a laser/470/49'er/Rich's scow, etc.

While 12 for a Mac sounds high to me - maybe with an asym? Maybe Mac owners could chime in. RichH is really good - who knows how fast he could get it going...

OC

FWIW, the Mac 25 is not water ballasted. It has a swing keel of about 600 lb, not unlike the Catalina 22. All of the older Macs (17, 21, 23, 24, 25; also known as Ventures) were swing keel boats. Water ballast was only used on newer (from 1987 on) 26' models, although they did make a 19' PowerSailer model for a short while. The 26D and 26S are regular sailboats with water ballast while the 26X and current 26M are the hybrids with huge outboard motors.

 

[OldCat]

FWIW, the Mac 25 is not water ballasted. It has a swing keel of about 600 lb, not unlike the Catalina 22. All of the older Macs (17, 21, 23, 24, 25; also known as Ventures) were swing keel boats. Water ballast was only used on newer (from 1987 on) 26' models, although they did make a 19' PowerSailer model for a short while. The 26D and 26S are regular sailboats with water ballast while the 26X and current 26M are the hybrids with huge outboard motors.

I stand corrected. The range of sailing speeds for different types of Mac would be an interesting discussion.

If exceeding 1.34 * sqrt(WLL) is semi-planing - then I guess we are semi-planing. We got soooooo far off topic .

OC

 

[steelcastle]

So, Rich.You still haven't explained how I would be able to hit 12 knots on a Mac25.

 

[kgw]

Surfing down a big, scary wave, or more likely, in your dreams!!

So, Rich.You still haven't explained how I would be able to hit 12 knots on a Mac25.