Comments on the P-15 Design

By

Judy Blumhorst, DC, Owner of P-19 Redwing

I've got a few comments I'd like to make about the P15 rig, pulled together from some very old emails I've written over the years.

The P15 is a stroke of "small-scale" genius, unique in its own way -- a super stable, dry riding, small sailboat. Every boat embodies a set of design compromises, and the compromises are especially critical the smaller the boat gets.

The P15 packs a ton of coastal-water seaworthiness and stability into a diminutive package that weighs under 1000 pounds fully loaded on the trailer and can be trailered behind just about any small car. It sails very well to weather, and is dryer than other boats many times its size.

Comparing the P15 to contemporary designs for high performance sloops might lead to some inappropriate conclusions about how to get the best performance out of the P15. So here's my perspective on the virtues and shortcomings of the design of the P15..

..

• About the need for a backstay:

On most sloops with tall masts and large sailplans, the backstay improves performance most significantly when sailing closed hauled or on a close reach, not when sailing on a broad reach or sailing off the wind.

On the P15, the side shrouds are anchored much further aft, which changes the geometry. The aft anchor point permits them to do the job of resisting forward forces on the mast. Without a backstay, you can use a larger roach on the mainsail (with that wonderful, pesky large batten!) which would hit the backstay otherwise. So you can pack more mainsail area onto the boat without lengthening the hull (which would increase the weight geometrically)

Here's the "theory" behind the comments: The apparent windspeed which jib "sees" when pointing into the wind geometrically increases the loads on the headstay.

The increased loads on the headsail while pointing create a performance problem that the rig designer needs to address: The higher forces tend to pull the mast forward, causing forestay to sag and curve to leeward. The sagging headstay changes the shape of the jib, making the draft of the fuller and increasing the power produced by the headsail -- which is the opposite of what you want for pointing. A deep draft in the jib makes the boat heel excessively in the gusts and robs you of a few points of pointing high.

If you're sailing at 4 knots of boatspeed into a headwind of 15 knots, the sail "sees" 19 knots (very approximately). At 19 knots of apparent wind, the sails are producing almost twice as much lift as they would when sailing on a true reach, when the true wind and apparent wind are both 15 knots. On a deep reach, where the boatspeed might be 4 knots, the apparent wind is 11 knots, and the sails are producing less than half the lift as they would on a reach in the same 15-knot wind.

So, you can see that the mast would probably need something to resist large loads on the forestay mostly when you want to point, i.e., sail above a reach.

You can solve the problem in one of two ways -- either add a backstay (like the P19 and many other sloops) or anchor the side shrouds significantly aft of the maststep (like the Potter 15).

The way to prevent the headstay from sagging when you're pointing is to increase the tension on the forestay. There are two ways to increase the tension on the forestay. Either shorten the forestay, or tighten the side shrouds.

Repeating what I wrote above, the side shrouds on the P15 are attached a significant distance aft of the mast base, so tightening them /pulls or tilts the mast aft, which increases tension on the forestay. P15 shrouds are anchored further aft than they would be on a sloop designed to have a backstay, so they CAN pull the mast aft. The side shrouds on the P15 are designed to provide resistance to BOTH lateral and forward strains.

The "art" of tuning the P15 rig consists of balancing the relationship between the forestay and shrouds. You need to get two things right: a) the forestay tight enough so it doesn't sag when closehauled, and b) the correct amount of mast rake (vertical angle of the mast) so you have just a few degrees of weatherhelm.

In contrast to the performance problems the rig designer has to solve for pointing, a slight forward lean of the mast when you're sailing a deep reach or a run doesn't cost you in performance terms. In fact, most sloops with adjustable backstays loosen the backstay on a deep reach, in order to slack the headstay and deepen the draft of the main.

The P15, when the rig is tuned right and it's sailed by a skilled skipper, can point like a bat out of hell. In the last two Cruiser Challengers, the P15 outpointed the micro-cruiser competition. The editors of Small Craft Advisor Magazine lavished praised on the Potter 15's windward performance. (The P19's windward performance earned equally high praise from the editors too)

Neither the P15 nor the p19 can claim to be particularly fast dead downwind or even on a deep reach (defined to be the point of sail just high enough so the jib isn't blanketed by the mainsail. When you've sailed too low to call it a deep reach, the clew of the jib drops a little, just as the mains starts to blanket the jib).

The Mark II design wasn't optimized for downwind speed -- the safety of having good windward ability, low heel for comfort, and heavy-air resilience were the design objectives and priorities.

In contrast, a raceboat design would emphasize downwind speed over heavy air capability and stability, compared to the P15.

Mast rake affects the weatherhelm because it moves the CE of the sailplan. On the P15, you change mast rake by changing the affecting length or tension of both the side shrouds and the forestay.

"Bending" the mast affects only the power of the mainsail by lessening the depth of the draft between the gooseneck and the forestay attachment point). On a sloop designed to have a bendy mast, you bend the mast by applying a downward compressive force via the backstay. Since the forestay is already tightened to the point that it won't stretch, the mast rake shouldn't change measurably.

If you have a mast that you bend with the backstay, you'd better have the mainsail designed specifically for the range of bend you plan to adjust in and out of the mast. Why? For a non-bending mast, the sail designer builds the mainsail with a relatively straight luff, and cut the panels so the draft is appropriate. For a mast meant to be bent, he builds in some curve in the luff. The P15's mainsail luff is relatively straight, to match the straight mast.

Bending the mast to "suck out" the draft is the most powerful means of depowering the mainsail on boats with bendy masts and curved-luff mainsails, but not on the P15.

I've only sailed the P15 a few times, but she's already got ample windrange capabilities just as she comes, right out of the box, straight from the factory. (And I've been lucky enough to sail perfectly tuned P14/5, Jerry Barrilleaux's "Breezy". That boat is absolutely a blast to sail!)

I'd guess you wouldn't get much more top-end windrange out of a P15 rig if you rigged running backstays. Maybe you'd see an increase in the top end of the windrange if you also changed to a new mainsail that had a curve in the luff, but that's a lot of work -- you'd have to beef up the chainplates, the hardware, etc.

Don't forget that in higher winds, the P15 has a lightweight semi-displacement dinghy hullform, not a displacement hull. She'll scoot right along in moderate winds with the simple rig she's got, if you learn how to trim her.

In closing... You have to sail to P15 to appreciate the design strength of the P15 -- it’s an incredibly stable hullform with an easy to manage sailplan. It's a boat built for simplicity, coastal seaworthiness, safety and ease of rigging. The P15s are a hoot to sail, and they'll get you home when the conditions get rough.

The P15 is a harmonious marriage of hullform and sailplan. It may lack fancy trimming options like a traveler or an adjustable backstay, but it certainly doesn't lack the ability to handle high winds with aplomb.

After you've sailed her a bit, you'll appreciate her unique virtues.

Fair winds, Judy B, 1985 WWP19 #266 Redwing, sailing on windy, choppy San Francisco Bay.

Judy B's West Wight Potter Pages