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Slocum's World Voyaging Yawl

'SPRAY'

Slocum
Click Image Above for a Much Larger Inverse Sail Plan Image

Spray Original Sail Plan | Spray Original Hull Lines | Spray Original Body Lines

Article & Screen Shots Copyright 2011 - 2013 Michael Kasten
 

QUALITIES OF SLOCUM'S SPRAY

First, it should be known that I am a fan of the original Spray - i.e. the one that Joshua Slocum sailed around the world. However I am only a fan in a limited sense. Allow me to elaborate...

What the ORIGINAL Spray was able to contribute most notably for Slocum's voyage was excellent directional stability, i.e. outstanding and apparently infallible course keeping ability. This inherent self-steering ability was the result of several factors:

This combination allowed the vessel to behave somewhat like a dart, with the weight forward and the feathers aft. Alternately you can imagine a tear-drop shaped bomb with fins aft - the ideal shape for stability in a free-fall environment.

This made the Spray rather poor to windward, but superb off the wind. The Spray was not slow, as many might think. On the contrary, the Spray was capable of quite fast sailing when off the wind or on a reach. This was due to her shoal draft, far forward CB, long straight run of the buttock lines aft, extreme stiffness due to her excessive beam and shallow draft, and having a low aspect but very ample gaff yawl rig. In other words, a slippery shape and plenty of sail area...!
 

DRAWBACKS OF SLOCUM'S SPRAY

As a long distance voyaging boat the Spray did have several drawbacks, the most important of which were excessive beam and extremely shoal draft. This combination resulted in the vessel being just as stable upside down as it was upright. In plain words, the Spray had zero self-righting ability.

It should be noted that the wooden oyster smack that Slocum named the "Spray" was presented to him as a gift, i.e. for free. At that point it was a rotten old hulk which Slocum then proceeded to restore on a very limited budget.

Due to Slocum’s extraordinary talent as a master mariner he had quite a successful voyage despite the limitations of the vessel, which was never intended for sailing offshore in the first place, but rather intended as an inshore “dragger” with ample sail area for that purpose.

This is a prime example of a commonly shared characteristic amongst many of the well known long distance sailing accomplishments... they were not the result of long arduous planning to create the "ideal" voyaging boat. Instead, the most spectacular individual sailing achievements occurred when a determined sailor encountered a more or less suitable craft that they could acquire inexpensively and put to sea in without too much fooling around. In other words, they were opportunists who made expedient use of what was ready to hand... It is a good lesson!

Slocum's spectacular voyage with the Spray was no exception.
 

FAITHFUL REPLICAS OF SLOCUM'S SPRAY

The above comments apply to the ORIGINAL Spray, as rebuilt by Joshua Slocum, which has subsequently been reproduced by others such as Pete Culler and Gilbert Klingel, often with good success. Most of them were highly faithful to the original.

I was fortunate enough to go aboard a very good replica of the Spray in Hawaii in 1979, built by Bob Carr in Vermont, and single handed from there to Honolulu. He had been some 180 days from Panama to Hawaii, possibly a record for the slowest passage for that route..! However Bob had encountered long periods of calm, and a number of storms. His mains'l was blown out, and he was nearly down to the bottom of his "bean barrel" even though he did catch a lot of fish..! It was impressive to see three large garbage pails in the store room: one for beans, one for rice, one for wheat.

A friend, Michael Colfer and I helped Bob Carr paint the boat's bottom while he was hauled out at Keehi Lagoon, near Honolulu. We heard several good stories during that time and we were well fed, albeit on chicken stew of questionable vintage. There being no refrigeration aboard, this was cause for concern. Like many before him, Bob Carr sailed without an engine so he used kerosene for lights and for cooking. We were fairly certain that the only electricity aboard was inside his flashlight and transistor radio...!

Another friend, George Maynard, built a faithful replica of Slocum's Spray in Noank, Connecticut, Slocum's original locale. Maynard named his vessel "Scud" and subsequently sailed it around the world with his wife and young children - a daughter and two sons. Maynard's voyage, like that of Slocum and Bob Carr, was accomplished without the aid of an engine or any fancy electronics...

Recently I have read about a sailor who purchased Maynard's "Scud" and who has once again circumnavigated in the vessel, also without an engine. Quite a number of Spray replicas have achieved similar success. Others have met with shipwreck or loss without a trace, as in fact happened to the original, which took along Mr. Slocum himself.
 

... AND THE UNFAITHFUL

In recent years the name “SPRAY” has been used by a few boat designers in order to take advantage of name recognition for marketing purposes. Most notably, several steel "Spray" models have become popular among amateur steel boat builders despite those designs not being at all faithful to the original Spray design. This is so in terms of the rig and the hull shape as well as other parameters, but most notably it is so in terms of their appearance.

I regard these so-called “Spray” designs as being extremely crude attempts, especially in terms of capturing the beauty of the original Spray. In my opinion these “Spray” poseurs have little if any relationship to the original except to say that they have borrowed the "Spray" name and that they are fat, overweight, and total dogs to windward - nothing else has any semblance to the original.

Those vessels have not improved on the very poor ultimate stability of the ORIGINAL Spray. As insult to injury, the structure of these vessels is inordinately complex, in other words poorly adapted to amateur construction, as is typically their claim. I am quite sure you know the vessels of which I speak... I do not need to name names.
 

MODELING THE ORIGINAL SPRAY

Having said the above, I have always been curious about the SPRAY, in particular to understand and possibly replicate its excellent course keeping ability for use on other designs. In order to know more about the SPRAY, I went to the trouble to model the design so that I could investigate its hydrostatics, stability, balance and sailing performance in greater detail. Images that show the results of that modeling effort can be seen in the following links and the image below:

Original Spray Above Aft | Original Spray Above Forward

Slocum's Spray - Kasten Marine Design, Inc.
Click Image for Larger View
 

PARTICULARS & COMMENTARY

Particulars of Slocum's Original Spray are:

Commentary: Looking at the above, a few comments are in order.

REFINEMENTS..?

My conclusions are that the original SPRAY model would benefit greatly by being made deeper and longer, with possibly slightly greater freeboard, but done without changing the favorable attributes of the hull. With that accomplished, the ratio of beam to length will be brought more in line with traditional cruisers, as will be the ratio of beam to depth.

These changes would be introduced primarily for the sake of enhanced large angle stability (greater length and depth), but also to improve the windward ability of the hull (greater depth & outside ballast). In so doing, ideally the Center of Buoyancy would not be moved aft, but the CB would be lowered as would the CG. This combination would preserve a measure of the vessel's excellent stiffness without degrading its voyaging capabilities.

After I created a good NURBS surface model of the original SPRAY, I then made the above mentioned modifications to see what they might offer. Though my mods may not be easy to detect in the images linked below, they have indeed produced the desired result...

Modified SPRAY - Kasten Marine Design, Inc.

Modified Spray Perspective Forward | Modified Spray Perspective Aft

THE MODIFIED SPRAY MODEL

Length on deck is increased to 48 feet, and the WL length to around 41 feet; beam is unchanged; depth of hull is increased and draft is 4.72 feet; displacement is increased to 22.9 long tons; and sail area is increased. As a result of having been made longer but not wider, the Displacement to Length has been reduced to around 329, which is still a robust hull shape but not overly heavy.

The Prismatic Coefficient is down to .64 including the keel and stem, and .66 for the hull body only. This is still a bit too high for optimum performance at typical sailing speeds, but is indicative of a high speed potential and ample buoyancy in the ends for better support in heavy weather. The Center of Buoyancy remains approximately amidships, therefore is unchanged. The fore and aft distribution of displacement is unchanged, i.e. still a full bow and fine run aft.

The rudder was made wholly external, which required that the transom rake be changed slightly, and that the deadwood be extended slightly farther aft. The aft deck has been given an overhang just sufficient to house and protect the rudder, and to create a convenient landing for the mizzen mast.

In the image above, you will notice that I have also raised the aft deck to the height of the bulwark top, then introduced a toe rail around the aft deck. Raising the aft deck is not at all necessary, but it does provide advantages in terms of reserve buoyancy and self righting, as well as considerably greater interior space aft.

Along with those hull refinements, a few improvements will ideally also be made to the rig – not to change it from Slocum's Yawl rig, but to increase the relative size of the rig for the sake of improved performance. Ideally the SA / Displacement ratio would be above 18, preferably closer to 20. This implies a Sail Area of around 1,500 sq. ft. or more. I have proposed a sail area of 1,700 sq. ft.

The amount of lead from the CE to CLR should stay the same, i.e. approximately 9%, or possibly it could be reduced slightly due to the reduced beam ratio. By increasing sail area, a higher CE will result, but this will be offset by the lower CG inherent in the deeper hull. The result of having less beam will be an increased amount of heel vs the original SPRAY. The result of having a lower D/L ratio, increased sail area, a deeper hull and external ballast will be better performance on all points of sail, especially to windward.

All of these changes are possible without having to degrade the inherent excellent tracking of the original Spray design on a passage, nor to adversely affect the traditional aesthetics of the original.
 

THE ORIGINAL SPRAY vs THE MODIFIED SPRAY...

In The Rudder magazine, Vol. 21, 1909, there is an article by C. Andrade that attempts to analyze the Spray. It is interesting to note that the article was written roughly a month after Slocum was lost at sea... It is also interesting to read a fairly elaborate but rather arcane analysis using the extant naval architecture principles of the day...! Anyone curious can read the article on Google Books.

In any analysis of the qualities of the Spray, it must first be pointed out that:

1. To my knowledge the vertical center of gravity of the original Spray has never been documented.
2. The lines of the Spray as published by Slocum in Sailing Alone Around the World are not accurate. In other words, the three views do not match…!

Others have published their own version of the lines of the Spray, including Weston Farmer, Pete Culler, and in the above article, C. Andrade. I have not checked those versions of the lines against the original, nor against my own model of the Spray, but I will say for certain that the lines that Slocum published are not accurate.

SLOCUM'S ORIGINAL SPRAY

The model I've created of the original Spray is as close as I can make it according to Slocum's published lines. In my analysis of the original Spray model I have assumed the VCG to be approximately 8 inches above the WL. This is only an assumption, since I have not taken the trouble to actually calculate the VCG, but it is an assumption that's based on having calculated the VCG of other heavily built wooden craft, so it is probably not far off.

Using that VCG, the enclosed stability curve is derived. This curve takes credit for the volume of the deck houses, as well as the floatation of the masts. It is evident that stability is lost at 100 degrees of heel, after which the vessel is stable when inverted.

Slocum's Spray Righting Curve - Kasten Marine Design, Inc.
Click for Larger Image

If I lower the VCG to the WL height, then the righting moments that I calculate actually match those calculated by C. Andrade fairly closely. It is therefore evident that C. Andrade made the assumption that the VCG is located at the WL, which I believe to be much too optimistic, especially considering the inside ballast of stones and cement..!

However if the VCG were to actually be located at the WL (highly unlikely), then the point of vanishing stability would be approximately 116 degrees – quite a difference..! In other words, the righting curve derived by C. Andrade is likely to be in error by that amount, or possibly even more (say in the event that I have also been too optimistic with regard to the VCG).

A range of positive stability of 100 degrees or less is considered to be inadequate for a sailing vessel. In the EU, a Stability Index is calculated, called the STIX value. With the VCG located 8 inches above the WL per my assumption, Spray earns a STIX score of 25, whereas the EU requirement for all ocean operations is a minimum STIX score of 32. Thus the Spray fails the STIX criterion by over 20%. Ideally there will be a substantial margin of safety above the minimum score, i.e. a STIX score in the mid to upper 40's or greater.

Regardless of all the many discussions of the Spray and her admirable qualities, and notwithstanding the obvious achievements of the vessel, the stability characteristics of the original Spray are plainly inadequate.

THE IMPROVED SPRAY MODEL

I have gone to the trouble of modeling the original Spray so that I could see what improvements could be made starting with the original Spray lines. I wanted to modify the model in such a way as to preserve the good qualities of the Spray, but to improve the stability range and sailing performance.

Using the modified Spray model that I eventually settled on at 48' length on deck and 41' WL length as described above with a deeper hull body, assuming the VCG to be 3" above the WL, (possibly too conservative, i.e. a higher VCG than would actually be the case), the stability range increases to 121 degrees, and the STIX score comes to 43.6.

Assuming it is possible to lower the VCG all the way to the WL (possibly achievable with the deeper hull, deepening the keel farther, and by using external ballast), then the stability range comes to 126 degrees and the STIX score is 49.1.

Improved Spray Righting Curve - Kasten Marine Design, Inc.
Click for Larger Image

This is a dramatic improvement, even though I would like to do even better with the range of positive stability…

My primary goals in making such tweaks to the SPRAY have been to see what could be done make the design safer and more seaworthy and to show that such improvements could be made without screwing up the excellent traditional aesthetics of the original, nor to adversely affect its excellent tracking ability and its consequent value as an ocean voyager.

The updated model preserves the balance and course keeping of the original, and makes a variety of improvements for the sake of performance on all points of sail, and would be a much safer platform for voyaging on the briny deep…!

OTHER CHANGES...??

The question arises, "Would I make additional changes...?"

Yes. I would prefer to reduce the beam some, but without changing the distribution of displacement. Displacement would decrease, and the resulting model would have a more modest D/L ratio. With that change made, I'd prefer to reduce the prismatic coefficient to between .57 and .60 for the sake of improved all around sailing performance. This combination would result in a more comfortable ride, and would actually increase the range of positive stability...!
 

CONCLUSIONS...

Many are attracted to the SPRAY simply on the basis of it having been the first vessel to carry a human around the world, sailing single handed. It is quite an achievement to say the least. What must be realized however is that the voyage was accomplished by a determined and very experienced professional sea captain, not by your ordinary beach bum.

Just like many other world-voyaging sailors who have also achieved extraordinary sailing feats, Slocum was an opportunist who made excellent use of what was available to him, in his case a freely provided old wreck which he rebuilt on a shoe-string, and which he then quite stunningly sailed into permanent fame and an honored place in our history of the sea.

Before Slocum left on his historic voyage, dockside wags said of the SPRAY, "It'll crawl...!"

When he returned, his dockside advisors would only deign to say, "You were lucky...!"

Luck or no, one cannot deny Slocum's extraordinary achievement, which was accomplished more or less in spite of the tool that he was given, i.e. the SPRAY. In other words, it was Slocum's WILL and long experience at sea that allowed him to achieve this feat, rather than it having been some sort of caprice conferred upon him by magical attributes inherent in the vessel on which he sailed.

Copyright 2011 - 2013 Michael Kasten
Helsingborg, Sweden