hull speed CD 33

noah · Post by **noah** » Jan 16th, '08, 17:23

I would like to know the actual hull speed for a CD33 while under power.

barfwinkle · Post by **barfwinkle** » Jan 16th, '08, 18:09

In my personal experience with a 25D, if I have a good breeze I can sail just about as fast as I can motor.

If I remember correctly, take the length of the waterline x 0.312` or something like that to find hull speed. I know my 1GM won't push her any faster that that. Can't speak for multiple cylinder engines.

Fair Winds

Cathy Monaghan · Post by **Cathy Monaghan** » Jan 16th, '08, 18:47

There's no definitive answer. It depends on the engine, what RPMs you're running it at, the prop (2 or 3 blade, feathering, folding, etc.), the condition of the prop (clean, barnacle encrusted, etc.), the sea and wind conditions, and if you're towing anything behind the boat, i.e., dinghy.

Cathy
CD32 Realization, #3
Rahway, NJ
Raritan Bay

Dean Abramson · Post by **Dean Abramson** » Jan 16th, '08, 20:06

The hull speed is the square root of the waterline x 1.34. So my calculations say a 33 (w/ LWL of 24.5) would be 6.63 knots. So if you are motoring in calm water with a healthy engine, and a smooth bottom, you may be going near that speed, but probably somewhat less. Same with sailing. But if you are going downwind in a good breeze and there are significant waves, you can momentarily go faster than hull speed as you surf down a wave.

Of course, if you are going wit the current you can go faster OVER THE BOTTOM as shown on your GPS, but that is not just a hull-speed equation.

I would say that if you are going 6.5 thru the water, you are having a very good day on a 33.

Dean

Ed Haley · Post by **Ed Haley** » Jan 16th, '08, 20:19

The hull speed of a boat is (by definition) determined by the length of her waterline. To calculate it, the formula is:

1.34 X (square root of the waterline length in feet)

For a CD33, LWL is 24.5ft
(1.34)(24.5^1/2) = 6.63 knots

For meters, it would be 2.43(Length in meters^1/2)

Now, how fast will the engine propel her through the water is, as Cathy says, dependent upon several factors.

But her length will give you 6.63 knots while generating a wave underneath the boat of its waterline length. To climb over this wave, more and more energy is needed to climb the crest and start planing but a sailboat doesn't have sufficient power to plane. It just burns fuel at an excessive rate without gaining much speed.

for further details and illustrations, see http://www.cncphotoalbum.com/technical/ ... lspeed.htm

D Rush · Post by **D Rush** » Jan 16th, '08, 21:01

For a CD25,
LWL is 18.0ft
(1.34)(18^1/2) = 5.69 knots

that's my math homework for the evening

barfwinkle · Post by **barfwinkle** » Jan 16th, '08, 21:04

1.34 X (square root of the waterline length in feet)

Hey Ed I was close, I just forgot the sq rt thingy!!!!

Thanks for enlightening me.

Fair and Thawing Winds

Paul D. · Post by **Paul D.** » Jan 16th, '08, 21:22

We frequently motor at 6 knots by GPS and speedo. 5.5 is a little bit more common with any sort of waves. Our 33 can make this under sail too without difficulty.

I was once told when we chartered a Island Packet 32 that it was a "5 Knot boat" well since you asked for the actual hull speed and not theoretical, I would say we got a 6 knot boat, but then again I may be spinning the numbers a little as a biased owner.

Noah, where do you sail? Have you owned your 33 long and how do you like her?

Paul
below zero and getting colder...

SeaBelle · Post by **SeaBelle** » Jan 17th, '08, 08:14

Find a place away from strong currents. Using that approximate spot as a starting place, motor North for about 5 minutes at constant RPM, a RPM you often use, noting SOG from your GPS. Then sail a reciprocal course (South) for 5 min noting the SOG. Now East, then West. You'll see small variations due to Wind, Current, Waves, etc (importantly, RPM has been held constant). The average SOG is about what you can expect at that RPM at other times. Of course, the precision is not great. Remember the experiment trys to remove the effects of Wind, current, waves, etc. but at any other time you'll be subject to those effects because you will not be sailing reciprocal courses.

Ed Haley · Post by **Ed Haley** » Jan 17th, '08, 11:23

Just about every diesel engine has a Power-Torque-Fuel Consumption diagram that you can use to find the best operating RPM range for fuel economy. You can also find the range for the best power output.

If I have to power for any length of time I consider fuel consumption as a priority. With my Beta Marine BD1005 engine, I like to keep the RPMs between 1800 and 2000 depending on water conditions. According to the diagram, this will cause a fuel consumption of about 1 liter per hour and my measurements are pretty close. On flat water I get a consistent 5.3 knots. I like to keep it there.

If I'm in a hurry and want a faster speed, it costs dearly. To raise the RPMs to 2500 doubles the consumption. 2800 triples it. And speeding along at 7.0 knots would consume 5 liters per hour.

It pay$ to not be in a hurry. And 5.3 knots is a respectable speed.

Joe CD MS 300 · Post by **Joe CD MS 300** » Jan 17th, '08, 16:47

It's not exactly the same but you can also get a rough sense of what Ed was suggesting by making up your own chart of speed at various RPM's. It's something to do on a wind less and perfectly calm day. Chart the speed (in increments of 100 RPM's) using both the speed log and a GPS to get an idea of whether there is any current affecting your speed over ground. You'll be able to see where incremental increases in RPM begin to have less effect on your speed.

Tom Keevil · Post by **Tom Keevil** » Jan 17th, '08, 19:58

Any idea where we can get these diagrams for our old engines? We have a Universal 5424.

Ed Haley · Post by **Ed Haley** » Jan 17th, '08, 22:55

Good question, Tom. Can't say I've seen any for the old Universal engines even in their manuals. However, with 24 hp, your 5424 may have similar characteristics to the Beta 25 hp engine - similar but maybe not exact. See the performance diagram http://enginerite.com/images/engines/d9 ... rmance.jpg

Andy Denmark · Post by **Andy Denmark** » Jan 19th, '08, 14:36

"Hull Speed" as it continues to be misinterpreted is only one of a great many factors that contribute to a limitation, or accentuation, of the speed potential of a vessel.

While Ed is basically correct that "hull speed" is a way of quantifying wave generation based on the single parameter of waterline length, what it is not is a determinant of maximum attainable speed. Boats routinely exceed the speed determined by this equation without getting into a planing regime.

To selectively look only at the hull speed equation w/o considering other significant factors is analogous to saying that the wheelbase of a car is the sole limiting determinant for estimating its top speed.

Personally, I think that "hull speed" is a gross misnomer for no other reason than it confuses a lot of people.
________
Ford Telstar

Tom in Cambria · Post by **Tom in Cambria** » Jan 20th, '08, 10:16

The power/torque chart for the Universal engines is in the parts manual of all places. Second or third page inside the front cover. At least that's where it is for the M25 Universal engine. If you don't have the parts manual the local Universal parts dealer will probably make a photocopy of it for you.

The Cape Dory Board

hull speed CD 33

hull speed CD 33

In theory...

Hull Speed

CD25 = 5.69 knots

Thanks Ed

Try this experiment

Fuel consumption

Power-Torque-Fuel Consumption diagram

Performance Diagrams

Misunderstanding hull speed is not a top limit

Power torque chart