Cal 2-27 speed

[Stumble]

Originally Posted by amiller

This simple equation is new to me; probably because math was never my strong suit....

How does the equation change when calculating for a catamaran? I hear a lot of "fast" speeds quoted for cats on a broad reach, numbers that would seem to exceed this equation.

So is the equation valid for cats? Or do you multiple x2, or divide by 2, or....

The hull speed formula that everyone throws out is a gross simplification that assumes that all boats are roughly the same shape. It used to work because all vessels were roughly the same shape and thus everything worked out fine. But as hull shapes changed the simplification becomes less and less true, and eventually breaks down completely (cats, tri's, fast planing hulls).

The simplification is that the 1.34 used is not a constant, and really only applies to vessels with a maximum S/L ratio of 1.34. So instead of 1.34 what you should use is

max speed Length ratio=8.26 / (D/L)^.311

Where D/L = DLT ÷ (0.01 x LWL)³.
- DLT - Displacement in Long Tons (2240lbs)

This is the formula derived by David Gerr and it does work for catamarans up to a Length/Beam of around 15:1. Past this and wave making resistance seems to fall off a cliff and a whole new range of even more complicated formulas takes over. But there are a vanishingly small number of boats that get into this range. Even fast cruising cats don't meet this normally.

[svHyLyte]

Originally Posted by Stumble

The hull speed formula that everyone throws out is a gross simplification that assumes that all boats are roughly the same shape. It used to work because all vessels were roughly the same shape and thus everything worked out fine. But as hull shapes changed the simplification becomes less and less true, and eventually breaks down completely (cats, tri's, fast planing hulls).

The simplification is that the 1.34 used is not a constant, and really only applies to vessels with a maximum S/L ratio of 1.34. So instead of 1.34 what you should use is

max speed Length ratio=8.26 / (D/L)^.311

Where D/L = DLT ÷ (0.01 x LWL)³.
- DLT - Displacement in Long Tons (2240lbs)

This is the formula derived by David Gerr and it does work for catamarans up to a Length/Beam of around 15:1. Past this and wave making resistance seems to fall off a cliff and a whole new range of even more complicated formulas takes over. But there are a vanishingly small number of boats that get into this range. Even fast cruising cats don't meet this normally.

Greg--

You are correct of course, but in the case of the OP's boat, a 1970's design era Cal 2-27, the simplified proforma formula works quite well enough within a few 10ths of a knot.

That said, I think that if he goes into his "settings" on his GPS, he'll find that his speed setting is KPH, rather than Knots. If one divides his reported speeds by 1.852 one gets reasonable numbers for Knots.

As for the OP "calibrating" his GPS, I suspect he is merely resetting his trip/speed log to zero at the commencement of an outing.

The foregoing not withstanding, if he's "happy" with thinking he's zooming along I'm not sure what harm it does to anything save his credibility. One of my wife's friends has a bathroom scale that's so goofed up it under reports weight by upwards of 10%. (On her scale I weigh only 135#). Yet she beamingly proclaims that she only weighs XXX# and it must be so as that's what her scale reports. Where's the benefit in bursting her, or in this case, the OP's bubble?

FWIW...

[Sailor Doug]

I once had a fantastic spring. I Han redone the boats bottom and speed had improved greatly. Mid summer I found my gps was reading statuary miles not nautical miles. Had a great spring.



Sent from my iPad using Cruisers Sailing Forum

[Stumble]

Originally Posted by Sailor Doug

I once had a fantastic spring. I Han redone the boats bottom and speed had improved greatly. Mid summer I found my gps was reading statuary miles not nautical miles. Had a great spring.



Sent from my iPad using Cruisers Sailing Forum

I have my instruments set to read in kph rather than knots since I like the finer detail. But I always have to be careful that I convert it back before talking to anyone. My top speed on my cat is 42kph... Almost as fast as an AC cat, but converted into knots it's only 22.5kn or so.

[herculesboat]

I agree with With a 22.08' WL, your theoretical top speed is about 6.3 knots (1.34 x the square root of the water-line length).


________
speedboat

[Fog Bank]

Originally Posted by Family Time

Look guy's I really don't care if anyone believe that we went as fast as we did. I saw and felt it. However I do have another question. Why does the back end of the boat sink down in the water so much when going this fast.

I own the exact same year and model of boat, and what you are claiming simply isn't in the cards. In San Francisco Bay, in the slot, those speeds wouldn't happen. I say that as the owner of a 1976 Cal 2-27 that is sailed in the slot. What you saw and felt is not what you believe you saw and felt.

[CoastalEddie]

Obviously, this reply is a bit late. But, what the hell....

I also have a Cal 2-27. I can push her a little past theoretical hull speed, and even momentarily touch 7 kts on occasion (usually on a broad reach, and surfing down the waves of a following sea). But, 6.2 to 6.4 kts is about as fast as you can hope to sustain in this boat. Even that would be only under ideal conditions. Realistically, averaging 5.5 kts over a daysail is doing pretty damned good.

[goat]

I think the OP left after he realized his gps was set for kilometres per hour instead of knots.

Sent from a boat somewhere