Back to Judy B 's Potter Pages

How Fast IS a Potter?
A New Look at Hull Speed

by Judy Blumhorst, Sept 22, 1999

Boat

LWL (ft) Disp D/L S/L Disp HullSpeed True HullSpeed True Hullspeed with cargo
Potter 15 12.00 475 123 1.85 4.6 knots 6.4 knots 4.8 knots w 700#
Potter 15 12.00 475 123 1.85 4.6 knots 6.4 knots 5.2 knots w 450#
Potter 19 16.25 1300 135 1.80 5.4 knots 7.2 knots 6.3 knots w 700#
Montgomery 15 13.25 750 144 1.76 4.6 knots 6.4 knots 5.2 knots w 700#
Montgomery 17 15.83 1600 180 1.64 5.3 knots 6.5 knots 5.8 knots w 700#
Compac 19 16.33 2000 205 1.58 5.4 knots 6.4 knots 5.8 knots w 700#


Introduction:

We've all heard that the hull speed for a P19 is 5.4 knots and for a P15 it's 4.6 knots. Why, then, do so many Potter skippers tell stories of sailing much faster than that? Are they exaggerating or lying?  Nope, they're telling the truth.  I myself have sailed a Potter 19 at sustained speeds over 6.0 knots.

There are three types of hulls: displacement, semi-displacemnt, and planing hulls. Semi-displacment hulls can go faster than a displacement hull.  Planing hulls are even faster.

If your speed is limited to the hull speed predicted by the formula Anthony Deane worked out in 1670 for British Men O'War, Hullspeed(in knots)=1.34*(LWL^1/2), you have a displacement hull. (LWL is hull length at the water line)

If your speed exceeds the theoretical speed for a displacement hull, you have a semi-displacement or planing hull. The Potter 15 and 19 fall into the category of semi-displacement hulls.  Occassionally, but not often, they may plane going down the face of a wave.

A hull is planing when it is supported by the water flow under her hull. When this happens, she is displacing less water, and this will be reflected by the wake. Watch the stern wave. When the crest of the stern wave is aft of the transom, you are going faster than "hull speed." When the stern wave flattens out, you are planing.  


Part 1: The Math

Caution! Part 1 gets technical. If you want to skip it, you can go right to Part 2 of this article.

Heavy boats with a large displacement to length ratio (D/L>240) must push a lot of water aside and they create big waves. They are restricted to the theoretical hull speed limit that is defined by the propagation of waves.

The above equation for a displacement hull can be rearranged, 1.34 = Hullspeed/(LWL^0.5), so that the 1.34 represents the ratio of speed to the square root of LWL.

So, the speed/length ratio for displacement hulls is 1.34. If a boat exceeds its theoretical displacement hull speed, the S/L ratio must increase. A semi-displacement hull has a S/L ratio of 1.45 or greater. A planing hull has a S/L greater than 3.0.

The two Potters have semi-displacement hulls. The P15's S/L ratio is 1.85, and the P19's S/L ration is 1.80.

But just how fast can a monohull boat be expected to go? It all depends on displacement -- more specifically on the D/L ratio (i.e., how heavy the boat is compared to the LWL). Naval architect Dave Gerr worked out the relationship, one of the great accomplishments in modern naval engineering. (David Gerr: Nature of Boats, McGraw-Hill; Offshore, Dec. 94, pp 29-33)

D/L ratio = D[in long tons, 2240 pounds]/(0.01 x LWL)^3.

S/L ratio = 8.26 /(D/L ratio)^0.311

The formulas show that lower displacements permit higher speeds without actually planing.  Everyone is familiar with Anthony Deane's original formula for heavy displacment hulls, and people are slow to catch that non-planing boats go faster than Deane's formula predicts, despite our observations that boats sometimes do go faster than they're supposed to.  

That's probably because most folks divide boats into two categories - planing vs. displacement.  The difference between the two is visually obvious.  However, most small sailboats don't get up an obvious plane, but nevertheless, they DO go faster than a displacement hull of similar LWL.  The Potters fits into this category -- the semi-displacement hull

We can re-write the equations to make the math easier, and then we can solve them in a step-wise manner:

Eq #1: D/L=(weight/2240)/(0.01 * LWL)^3

Eq#2: S/L = 8.26/(D/L)^0.311

Eq#3: Hullspeed = S/L * LWL^0.5

Equation 3 looks very familiar -- it's just like the traditional hullspeed formula, but the constant is no longer 1.34.  In Equation 3, the S/L ratio  replaces the constant allows us to take the displacement and water-line length of the boat into account.


Part 2 -- Conclusion:

Like many other small sailboats, Potters are semi-displacement boats. Plus, with their flat panels in the aft hull, they occassionally actually do get up on a plane.  They're really fast, if you know how to sail them fast and you don't load them up with too much gear.

A P19 with 700 pounds of crew and gear can theoretically reach sustained speeds of 6.3 knots, comparable to a 25' displacement hull with a 23' LWL. An almost empty P19  theoretically can sustain 7.2 knots, comparable to a 30+' displacement hull with a 29' LWL.  Clearly, an empty P19 can't sail because there's no skipper, but there is a point -- the less gear you put in your boat, the closer it can get to the theoretical limit of 7.2 knots, given enough wind.

In the real world, it's not uncommon to reach 6.0+ knots in a Potter 19 packed with weekend cruising gear.  And as every good Potter skipper knows, you need to keep the weight up forward, so the transom lifts out of the water and so you have enough momentum to swing the bow through the chop or waves during a tack or jibe.

A P15 with 700 pounds on board can theoretically reach 4.8 knots. With 450 pounds of crew and gear aboard, the theoretical limit is 5.2 knots.  In the real world, single-handed Potter 15's often reach 5.0 knots in the hands of a skilled skipper.


P19 Redwing, doing 6 knots on Monterey Bay

Our Potter 19, Redwing, doing 6 knots on Monterey Bay.  

Something funny is going on here.  Look at the stern wave.  The crest is aft of the transom.  Redwing's going faster than the old the hull speed formula predicts.  When the crest of the stern wave is aft of the transom, you are going faster than "hull speed." When the stern wave flattens out, you are planing.

Notice how the windward chine is out of the water, from the middle of the cabin all the way to the transom.  She's heeled over about 15 degrees.  With the P19's flat-panelled aft hull sections and the 15 degree deadrise, she sails on half her hull when she's heeling15 degrees, greatly reducing wetted surface and friction.  The transom is completely out of the water, for a clean exit with no turbulence.  The compound curve in the vee of the bow, like a powerboat's vee-bow, tosses the bow wave aside.  It appears to me that the stern wave actually starts approximately one third of the way back from the bow.  If you look back at your wake when you've got her hull trimmed like this, you'll see a glassy "half wake", indicating that she's riding on the leeward half of the hull only.


Comments by Dave Gerr
from the Trailer Sailor Bulletin Board, where I initially posted this information

Posted By: Dave Gerr

Date: 9/22/99 9:15pm

In Response To: (deleted)

Hi Folks:

Glad my somewhat more sophisticated hull speed formula has been of interest. It has proven quite accurate over the years and across many kinds of boats. The old rule-of-thumb, “simply 1.34 x the square root of the WL in feet” really isn’t accurate. The multiplier “1.34” is really a function of D/L ratio. That’s what my formula is all about. Keep in mind, though, that my rule describes the top hull speed that a hull MAY achieve without planing.

Three things here:

1) You need lots of power (wind for sailors, obviously) or you don't go faster, regardless.

2) Many hulls that have low D/L ratios do have faster hull speeds before planing, but if--at the same time--the hull has some planing characteristics, it may well start to plane--at least to some degree--before taking advantage, as it were, of it's higher non-planing hull speed.

3) There are lots of small sailboats that do a little of both in heavy air (in the right conditions). In other words, they get up on plane a bit, and they also take advantage of their higher non-planing hull speed at the same time for a small but noticeable double boost.

Hope this is helpful. We're swamped with work here, so I may not have time to answer more, but I'll try and peek in occasionally.

Cheers, Dave


Back to Judy B 's Potter Pages