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Go Back   SailNet Community > On Board > Gear & Maintenance
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  #11  
Old 03-10-2008
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The math seems sound to me. If you follow the link below you can see the bitter truth graphically. The speed in knots along the X-axis (horizontally) and the consumption in liters on the Y-axis (vertically). Now the values are for my ship and my engine, but the curve will be the same for any ship. A smaller engine than mine (90 HP) will enter at a lower level. As the ship is pushing close to the hull speed, the curve becomes vertical.


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Old 03-10-2008
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Craig,
I would conservatively plann on .5-.6 gallons per hour. Trying to figur miles per gallon there are too many vaiables ie cureent with or against, wind with or against, etc.

Like Cam said. Plenty of filters. Do you have a two filter system or just a primary. At least 1 alternaor belt. Habdheld VHF along with the regular one. A marine book with ssrvice marinas. Unlimited BoatUS Towing also.

Good luck to you. Hope to see you on the Magothy in a few weeks. VTW go to the Chesapeake forum on here and look at our Sailnet rondevous June 14&15 in Rock creek. We would love to have you participate.

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Craig, your hullspeed is 6,7 knots (through the water), squareroot(1,34*34). It is more or less imposssible for your boat to go faster than that. The question is, what is the maximum rpm for your engine? Because it is at this maximum that all the capasity is used. If 2100 rpm is the maximum rpm that your engine can bear, and the speed is between 5 - 6 knots, than either your engine is a wee bit too small, or you need to clean it under the waterline. (provided the engine is working properly and that the propeller is the correct size with the correct pitch).

Anyway, you should aim to go about 1 knot slower than the hull speed, that is around 5,5 knots in your case, that is good for the economi end the engine.
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Tjaldur-

Your hull speed calculation is a bit off. It should be 1.34 * sqrt (34) or 1.34 * 5.83 or 7.8 knots.
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The hull speed formula is based on LWL and not on LOA. I don't know what the LWL is for a Sabre 34 off the top of my head but using 30 feet as something close the hull speed will be around 7.3 kts.
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I too am getting ready to start north. On my trip south, in a Beneteau 36 center cockpit with a Yanmar 27 HP engine, I averaged 10 statute miles per gallon. My cruise RPM is 2700, which gives me between 7 and 8 SM/hour. I find it convenient to use statute miles on the ICW because that is how the charts are set up.

We are planning to leave Jacksonville the week after Easter. I sure hope the weather is warmer.
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Quote:
Originally Posted by sailingdog View Post
Tjaldur-

Your hull speed calculation is a bit off. It should be 1.34 * sqrt (34) or 1.34 * 5.83 or 7.8 knots.
Saildog:

You are right. I could not find the squarerootsign in this font, so in the haste I got the parenthesis wrong, thank you. (I was planning to wright the 1/2 behind the parethesis)
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very true...but he had the formula wrong.
Quote:
Originally Posted by SailorMitch View Post
The hull speed formula is based on LWL and not on LOA. I don't know what the LWL is for a Sabre 34 off the top of my head but using 30 feet as something close the hull speed will be around 7.3 kts.
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The speed formula is based on waterline length...not boat length and the S34 has either a 26'3" waterline or a 28'3" waterline depending on if it is a Mark1 or 2 respectively.
This gives 6.87 knots and 7.12 knots as hull speed on the two models.
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Thanks cam. I can do twice that on a good day.
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You know what the first rule of sailing is? ...Love. You can learn all the math in the 'verse, but you take
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—Cpt. Mal Reynolds, Serenity (edited)

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