posted October 15, 2006 05:54 AM         

Calculated And Measured Boat Speeds

The traditional formula for hull speed is:

Hull Speed (knots) = 1.34 x sq.rt.(Waterline Length (feet))

So for the Sandpiper with a LWL of 15 ft, the calculated hull speed would be 5.190 knots.

During the MLTs, boat speed was recorded twice to be 5.5 knots. So, the traditional formula may be too simple. The factor "1.34" is the Speed/Length Ratio and this number can be calculated from the boat's Displacement/Length Ratio. If the boat has an all-up displacement of 1928 pounds (2 crew with gear, outboard, gas, battery) then the Displacement/Lenght Ratio would be:

DLR = 1928 lb / 2240 lb / (0.01 x 15 ft)^3

DLR = 255

A DLR greater than 300 would be a heavy displacment cruiser. A DLR less than 150 would be a sport boat. At 255, the Sandpiper is a medium displacement cruiser.

The calculated Speed/Length Ratio:

S/L = 8.26 / (DLR)^0.311

S/L = 1.474

A S/L less than 1.34 is a displacement hull. A S/L in the range of 1.6 to 3.9 is a semi-displacement hull (fishing trawler). A S/L greater than 4.9 is a planing hull.

So now the hull speed for a Sandpiper is:

Hull Speed = 1.474 x sq.rt. (15 feet)

Hull Speed = 5.710 knots

Windy Island achieved 5.5 knots with a potential for 5.7 knots.

posted October 15, 2006 06:20 AM         

Windy Island's keel was full DOWN during the MLTs. So the question is: "What would have been the boat's downwind speed had the keel been UP?" Here is a calculated guess:

Boat speed is determined by 2 components:

- Frictional Drag: related to the wetted surface area, and,

- Residual Drag: related to the wave-making hull shape.

If Frictional Drag is related to the speed rasied to the power of 2 and if Residual Drag is related to the speed raised to the power of 4, then with the keel UP:

- 21.8% reduction in wetted area = 4.7% increase in speed

- 9.23% reduction in frontal area = 1.7% increase in speed

- 6.4% total increase in speed.

Instead of 5.5 knots, Windy Island could have achieved a downwind speed of 5.9 knots.

The fine print: these numbers will not stand up to scrutiny. They are posted here for entertainment only.

posted October 16, 2006 12:16 PM            

Crew weight well forward. Even when cruising we sit right up front. (It's also the most comfortable.) If you're a really keen racer, the crew doesn't even come back into the cockpit, they role over the top of the cabin.

One thing that I don't think your data tells you Dennis is the abilty of a small boat to excellerate faster than a larger boat. Every surge off a wave, every proper sail adjustment, every well done tack, every well done jibe, every wave manouvered over, makes a small boat proportionally much faster than her overall hull speed will suggest.

Skipper weight should be "always" forward in light and medium weather. Since most of us are carrying a bit more "moveable ballast" than our good looking crews this is critical. Get used to sitting well forward with a long hiking extension. As well, since you can also see the wind gusts, wave action and where you're going better from that forward position the boat is going to be sailing even faster.

Data aside, the best way to compare speed is side by side pacing. There are just too many variables to solely base your performance on a steady wind force, (which it never is) steady windy direction, (which it never is) steady course steering, (which you never do) and steady sail position (sails are always played).

Future Rendezvous could see a lot of pacing. It's a nice way to cruise to a destination. Good for the mind too.

I still think my bow ballast works better than the rock. I've got 250 feet of anchor line, 24 feet of heavy chain, and 2 anchors. Besides the godd weight position, I can anchor almost any where safely without fear of dragging.

posted October 16, 2006 10:04 PM            

Hi Darcy, thats a mouthful, and very true. If or when we should all get together again it would be fun to pace each other.

I don't race often, and when I have its been on someone elses boat. I always take something home that I can apply to the sandpiper.

I should join a club and get a little more involved racing. Someone is always in need of another person to help crew. Might be worth trying to race the Sandpiper but Its so outdated with the boats I see racing. Worried I would always be dead last.

Sid

posted October 18, 2006 05:00 AM         

While tied to the dock, most of the Sandpipers' transoms were 4-6" above water level, even with an outboard on the mount and the lazarette loaded. However, some in the fleet seem to be stern-down. It is hard to see from this rafting photo.

But here is a photo of Windy Island in the 1000 Islands. This boat looks to be particularly low in the stern.

Ab has never reported a water problem. So perhaps, the previous owner who augmented Windy's keel also added some internal ballast. Could there be a cast iron billet bolted under the cockpit or in the lazarette? It would need to weigh 200-300 lbs to get Sapphire's transom to settle 2" below the waterline.

posted October 18, 2006 08:14 AM            

Sid

posted October 20, 2006 01:00 AM            

Extra equipment in the stern seems to effect speed performance drastically (almost squared to moving weight forward). The stern acts like a lever. We picked up a lot of boat speed on my Brother’s Shark by simply changing to a lighter rudder. A few pounds out there in the stern section drastically slows you down.

Ab’s mast support would add a lot of leveraged weight to the stern. But, keep in mind we are cruising boats after all. Ab would be the fastest boat under a bridge.

If you look at the Rendezvous raft-up you’ll notice that the boats with high sterns (like ours) have crews standing on the bow. Ab’s boat has both people in the cockpit. If Ab were sitting right up at the cabin, Windy Island’s stern would be substantially higher. A foot or two forward can make a big difference.