Princess Sharpie 26 & 22

[Greg Luckett]

I am wondering what the design hull speed is of the PS26 and the PS22. I am guessing that about 8 knots for the PS26 and 6 knots for the PS22.

[ChrisObee]

I am wondering what the design hull speed is of the PS26 and the PS22. I am guessing that about 8 knots for the PS26 and 6 knots for the PS22.

Mr physics says 6.2 and 6.8 knots. 

[Travis Votaw]

I saw 10.3 mph on the gps for PILGRIM at last years Texas 200.  What's that, around 8.5 knots?

[ChrisObee]

I saw 10.3 mph on the gps for PILGRIM at last years Texas 200.  What's that, around 8.5 knots?

1 mph = 0.87  knots.  

10.3 mph = 8.9 knots

[Greg Luckett]

I saw 10.3 mph on the gps for PILGRIM at last years Texas 200.  What's that, around 8.5 knots?

Travis, that may have included current? It seems faster than the hull speed for a 22 foot sharpie design, which I think the PS22 and 26 are. My Catalina 22 wing keel would do about 5.5 knots relative to the water but sometimes the GPS would report 10 to 11 when moving down stream on the river. Its current was running 5 to 6 knots.

Graham is off teaching so I do not expect him to pipe in here yet but his input would be good.

[Greg Luckett]

Mr physics says 6.2 and 6.8 knots. 

Mr physics?

[Hirilonde]

The approximate hull speed of any boat is the square root of the waterline length times 1.4.  For a displacement hull to go faster requires a serious increase in power and even so there is a limit to how much faster it will go.  A non-displacement hull will start to lift out of the water at hull speed and start trying to plane.  How well it succeeds is a matter of design, weight, wind and skill of the crew.

I would think the P22 and P26 are neither a displacement hull nor a true planing hull.  So it will exceed its hull speed though getting on a full plane and significantly exceeding it will be nearly impossible.

[Greg Luckett]

Yes, this is a good rule of thumb and as you also point out, does not necessarily reflect what should be expected from a specific design since there are other factors that can add or subtract from the hull speed. It is just an educated guess on my part from other boats I have known that makes me think the PS26 should be about 8 knots and the PS22 about 6. My 19 foot lightening could do a lot faster but I never had a means to measure it.  She would really zip along and surf the waves sometimes too. The heavier Catalina 22 with a displacement hull, never did that. It did bob like a cork in the chop at the river mouth.....what a ride. 8)

[ChrisObee]

Mr physics says 6.2 and 6.8 knots. 

Mr physics?

Its what I say when I'm reporting the output of a physics related formula.  The hull speed of a boat in knots is equal to the square root of the lenght of the water line, in feet, times a coefficient that represents the hull form of the boat.  The coefficient is most often 1.4 but might be bigger if the boat was long and  thin like a sculling shell or a hobie cat.  Its rule of thumb really.    

[Greg Luckett]

Chris,

That was what I thought. I do not have a table of the coeffecients.

A PS22 is pretty close in hull length and width to the Catalina 22 but I do not know how the actual square footages compare. The PS26 is longer, close to the same width as the PS22, and ought to be several knots faster based on the length to width ratios. The Core Sounds are very similar to the Spindrift 12, mostly just longer and only slightly wider, and those hulls looks more like planing hulls and like my Lightening.

[Travis Votaw]

The 10.3mph on PILGRIM registered with the GPS was under full sail with the miz staysail flying.  I think we were in one of the shallow bays on that trip without too much current, if any.  The old girl was pretty well maxed out for my comfort and we soon took in the staysail and tied in a reef.

I don't think the hull would ever plane, but it is fairly light displacement and has a pretty flat run run, so I believe her hull speed could exceed the standard calculation method if pressed enough.  Just my opinion - maybe Graham can weigh in at some point.

This pic is from last year's TX200, and as you can see, she is moving out pretty well :smile:.

[JeffM]

I started getting interested in the physics of this hull speed business recently (I teach physics to 9th graders).  As best I understand it, the underlying principles are:

1. the boat makes waves as it moves through the water--the more water it has to push out of the way, the bigger the wave.

2. as the boat moves faster, the wave does too; the faster a wave moves in water, the longer the distance between its crests.

3. As long as the boat is long enough to span two or more wave crests, it rides about level, but once the boat (and so the waves) are moving fast enough so the crests are about as far apart as the waterline is long, the stern drops into the trough between, and the boat is left sailing "uphill" on its own bow wave.

4. Boats can "beat the math" to the degree that they are skinny (eg cats and tris) so their "hills" (bow waves) aren't very tall, or if they can plane because they have hull shapes that generate lift (e.g. Core Sounds and such).