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Old 04-10-2016, 04:41   #1
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How Short Can A Short Mast Actually Be?

I've read some opinions from experienced sailors who are proponents of short masts that a mast doesn't have to be any higher than its boat's overall length; i.e. a thirty foot long vessel with a thirty foot high mast. For purposes of things like easier maneuverability, lighter weight, and less rocking because of high center of gravity, they say. Okay, to a boating noob like myself all that seems reasonable, I suppose.

But if the vessel were 1) not a racing boat needing great to reach high speeds, 2) super lightweight, and 3) equipped with a super shallow draft , is there any good reason the mast couldn't be even shorter? Say, a thirty foot long boat with a twenty foot high mast, or a forty foot long boat with a twenty-five foot high mast?

Maybe equip the short mast with something designed from the start to be simple and aerodynamic like a Wharram Wingsail Rig?

Please either confirm my hopeful suspicions or crush my naive optimism.
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Old 04-10-2016, 05:08   #2
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Re: How Short Can A Short Mast Actually Be?

You can make it as low as you want but the boat will be slow.

The lower, the slower

A fast boat might be 17' long with a light weight 30' carbon fiber mast and 7'-8' boom for example

Longer boom, shorter mast equals low aspect ratio. More power, less speed
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Old 04-10-2016, 05:17   #3
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Re: How Short Can A Short Mast Actually Be?

Sail area equals horsepower.

Nice to think you are not in a rush but the reality is that you don't want to limit yourself because speed can mean the difference between being able to out run or out manuver an approaching storm. Speed equals safety.

Split rigs such as schooners, ketches, and yawls carry their horsepower spread out over two or more masts which are likely shorter than a sloop rig on the same size boat. Another thing to consider is that there is more wind aloft than down low next to the water. Having a taller mast will get your sail area in that breeze.

Speaking of ketches, have you seen whats on Yachtworld lately listed for $178k?
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Old 04-10-2016, 05:25   #4
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Re: How Short Can A Short Mast Actually Be?

Sail area to displacement is an indicator of how fast the boat will be.

https://www.google.com/webhp?sourcei...cement%20ratio

You're going to need boomkins and bowsprits to make up for a short mast so that your sail area won't be too low.

The other reason for a tall mast is upwind performance. You want a high aspect ratio sail if you want to be efficient upwind. Which means a tall mast relative to the length of the boom. Not what you'll get with boomkins and bowsprits with a short mast.

https://www.google.com/webhp?sourcei...l+aspect+ratio
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Old 04-10-2016, 05:38   #5
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Re: How Short Can A Short Mast Actually Be?

Frankly, the theory (if it is one), is pure BS. With a mast that short, even on a relatively light boat, you can't fly enough sail to get her to move well until most other boats have long since reefed. IE; She will sail poorly in winds under 20kts. A more normal rig height is between 1.25 - 1.35x a boat's LOA, & a bit more than that on racers. And this has been the case since about the early 1970's on most boats.

There are statistics used as yardsticks of a boat's performance characteristics. One of them being Sail Area (to) Displacement Ratio (SADR). And for a monohull to move well in light air (under 10kts), she needs an SADR which is a minimum of 20, & 25 is far better.
As with an SADR of 20, she'll probably sail at 1/2-2/3 of windspeed down to about 6-8kts of wind. And with an SADR of 25 she'll sail at wind speed down to 4kts+/- if she doesn't have a lot of wetted surface area.

On multihulls, typically the boat's Bruce Number is used to discern how well she'll sail in light air. And to move well a boat needs one of 1.3 or better.
Bruce Number = square root of sail area (in sqft)/cube root of a boat's displacement (in pounds).

Both of these numbers need to be calculated with a boat fully loaded. Meaning all of her gear onboard, full tanks, & accounting for the weight of the crew & their gear as well.

In order to achieve such performance numbers, usually you need a spar that's at least 1.3x a vessels overall length. And by overall length, normally that includes bow sprits, & boomkins, etc.
These figures can be a bit lower for super light boats, boats which have high performance sails. Such as full batten, square top mains, or battened jibs, etc. Or with boats which have very low wetted surface areas (relative to most boats of their size).

And when calculating such numbers you can include the area added by a jib's overlap of the shrouds/spar, as well as a mainsail's roach. But without this kind of sail area, a boat will be a slug in light winds. And a huge preponderance of sailing is done in winds of less than 10-15kts. Often times far less.
Which is also part of the reason that sail areas on boats have continued to increase even as boats have gotten dramatically lighter over time. As without light air performance, you're stuck with either turning on the engine, or waiting for wind.

If you wish to see for yourself if what I'm saying is true, look up a boat's racing rating, & then compare it's sail area to it's displacement. And even do the SADR calc's yourself if you like.
You can look up a boat's specifications on www.sailboatdata.com (which also lists boats SADRs) & find her racing rating at PHRF New England - Handicapping - Base Handicaps

And if you look at a lot of boat's spec's on sailboatdata.com, you'll notice that many designs have optionally taller rigs. Which usually were added to boats after some of a particular design was built, & they discovered that she lacked for light air performance.
Also, it's quite common to add a taller mast to a boat if she loses her rig due to a mishap, so that then she'll sail better in light air. And taller rigs are also added by folks with some extra $ who simply want better performance than is available with the stock (short'ish) rig.

Or, for example, it's common to have taller masts in areas which have lighter winds as their norm, like Southern California. And boats designed for (or built there) usually have taller masts than elsewhere. Since in SoCal winds of less than 15kts are the norm, & often they're under 10kts (or less).
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Old 04-10-2016, 05:57   #6
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Re: How Short Can A Short Mast Actually Be?

Damn spectator boats! Was actually the Card, which sadly passed away in the Whitsunday Islands not too long ago.

Whitbread Race - The Card's final resting place
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Old 04-10-2016, 06:42   #7
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Re: How Short Can A Short Mast Actually Be?

Roll moment of inertia goes down when the mast is shorter. A decent roll moment of inertia can be useful to help resist the impact of large breaking wave trying to capsize the boat.

In some instances a boat with no mast can be more prone to capsizing. Though on the other hand too tall and too heavy a mast can make her excessively tender.

Its a delicate balance. Give me a reasonably tall rig anyday.

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Old 04-10-2016, 07:06   #8
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Re: How Short Can A Short Mast Actually Be?

Quote:
Originally Posted by Delancey View Post
Damn spectator boats! Was actually the Card, which sadly passed away in the Whitsunday Islands not too long ago.

Whitbread Race - The Card's final resting place
Sorry Delancy, I must have deleted my post on that Whitbread boat losing her mizzen mast while you were typing. I deleted it because I thought I was being a bit snarky/a know it all. So my apoligies.


On the tall mast thing, if it's not obvious, I'm with snowpetrel More sail area is better. And needs be you can always either reef, or fly smaller sails.

There is, however, a catch to taller masts & weight aloft. Which is that having more weight up there lowers a boat's ultimate stability. So that her actual (final) resistance to capsize is lower for each extra bit of weight up there.
But the polar moment of inertia thing which snowpetrel mentioned is definitely true. So that if your boat's motion is a bit too fast or whippy, hoisting some weight aloft will reduce it. In that she'll take much longer to roll from one side to the other vs. before you added the extra weight aloft.

Also, with a lighter spar/total rig & rigging weight, a boat won't roll or pitch as much in terms of both distances & angles involved, than were she to have the heavier (taller) rig. A benefit of which is that she won't be so inclined to bounce the air out of her sails, or move her sails enough via rolling/pitching in order to detach the air flow patterns from them. Which is why racers, & performance cruisers/sailors are so concerned about minimizing weight aloft.

Plus which, for every pound of weight aloft that you save, you can save several times that amount in the keel. This while still attaining the same performance levels. So lighter is a big perk in that regard.
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Old 04-10-2016, 07:21   #9
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Re: How Short Can A Short Mast Actually Be?

Quote:
Originally Posted by Snowpetrel View Post
Its a delicate balance.
"Balance" is the key word here. You simply cannot isolate a single element--like mast height--and make any sense of it all by itself. All of the different design elements have to be balanced. A boat is a combination of hundreds of different design decisions, all of which work together. When considering any one aspect of a boats design you absolutely MUST keep in mind how it affects, and how it is affected by, all of the other aspects of the boats design.
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Old 04-10-2016, 08:43   #10
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Re: How Short Can A Short Mast Actually Be?

The mast can be very short if the boat is very light or else if you agree to go slow (short mast+heavy boat).

However, with a short (and presumably light) mast, all other things equal, she will roll your guts out, so it is only easy with a light cat or else a very beamy light mono.

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Old 04-10-2016, 13:25   #11
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Re: How Short Can A Short Mast Actually Be?

Hm... The appellation "wingsail" sez it all. The "secret" is that the wingsail encases the mast in a "pocket" that is meant to ensure that the airflow over the first foot or two abaft the luff will be nicely laminar just as is the airflow over an airplane's wing. When the airflow over that forepart of the wing is well behaved (laminar) the aircraft stay aloft. If that airflow (for whatever reason) goes turbulent, the wing will stall, and the aircraft will drop precipitously.

Similarly, if the airflow over the forepart of a boat's sail goes turbulent, drive will be reduced. The reason that headsls in modern high aspect ratio rigs produce more drive than does the mainsail (when going to weather) is that the stay onto which they hank produces less turbulence over the critical part of the saii than does a fat, cumbersome mast.

In certain flight regimes, low airspeed particularly, a "fat" airfoil with lotsa curvature produces the most lift, and since only the forepart of the wing contributes you increase the LENGTH of the wing if you wish to generate more total lift.

PRECISELY the same is true for a "wingsail". High aspect ratio, i.e a tall mast, is efficacious. A short mast is not.

I hope the above will set you to thinking. The aerodynamic aspects of rig design are fascinating, but in isolation they become mighty abstruse. What is efficacious aerodynamically may very well be detrimental in respect of other design criteria, and indeed in respect of some operating criteria.

So before you make decisions about the rig, define, with precision, what the boat is FOR, define where you will sail her and in what weather regimes and evaluate your own abilities. What I have kept before me for many years is that in 20 knots of wind I can comfortably handle 400 square feet of sail in one piece. Beyond that, things can become rather like work :-)!

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Old 04-10-2016, 17:59   #12
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Re: How Short Can A Short Mast Actually Be?

If I remember correctly you posted a couple of days ago about having a very light trimaran built for you, to be power only. Some posters recommended you add a sail. I have been thinking about doing something similar, building a catamaran. However I started out thinking of building a sailing cat but lately have thought about making it just power. Now, I am thinking of a power cat with a short rig. Why? Seems as though the best cat sailing is mostly with the wind, up-wind, people just like to motor. Had you stated your plans for a trimaran, the replies might have been somewhat different. To boil this all down, if you are not a hard-core sailor, not above firing-up the old diesel, a short rig on a sailing boat could be viable. Especially on a multi-hull. Yes I know multi-hulls need more sail for speed, but on the other hand it is pretty common for multi-hulls to need to reef anyway when running with the wind.
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Old 05-10-2016, 03:14   #13
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Re: How Short Can A Short Mast Actually Be?

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If I remember correctly you posted a couple of days ago about having a very light trimaran built for you, to be power only. Some posters recommended you add a sail. I have been thinking about doing something similar, building a catamaran. However I started out thinking of building a sailing cat but lately have thought about making it just power. Now, I am thinking of a power cat with a short rig. Why? Seems as though the best cat sailing is mostly with the wind, up-wind, people just like to motor. Had you stated your plans for a trimaran, the replies might have been somewhat different. To boil this all down, if you are not a hard-core sailor, not above firing-up the old diesel, a short rig on a sailing boat could be viable. Especially on a multi-hull. Yes I know multi-hulls need more sail for speed, but on the other hand it is pretty common for multi-hulls to need to reef anyway when running with the wind.

Thanks for thinking of me and my obviously newbie fumblings. And yes, you're quite correct in your assumptions. And no, I'm not a hardcore sailor, so I'd feel no shame in firing up the diesel. Thanks for your reply, it helps my sense of nooby cautious optimism!
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Old 05-10-2016, 03:28   #14
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Re: How Short Can A Short Mast Actually Be?

Check out the boats of Sven Yrvind.
He sailed from Sweden to Canada, to West Indies and even rounded Cape Horn in boats 4.8 meters (18 feet) boats.
Search him on youtube and you will see some really small masts. But they worked.
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Old 05-10-2016, 07:23   #15
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Re: How Short Can A Short Mast Actually Be?

Arab dhows......very short masts with huge sprits ,ruled the Indian Ocean and Arabian Sea for many years...still in wide use today.
Many smaller dingy type craft with spritsail rigs,etc .,etc.. Check out the crab claw rig,no
less authority than C. J. Marchaj gives this rig the highest marks....maybe better than all the others....short mast used by "primitive" Pacific Islanders to sail circles around the early Western explorers.

..........luv you all............................mike............... ..................
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