# Ratios for yacht/motor Yacht

Discussion in 'Boat Design' started by Laranjo123, Jul 31, 2011.

1. ### Laranjo123Previous Member

Ahm i would like to know what are the ratios of L/B for motor yacht....Actually all ratios other than L/B, D/B etc....

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### FAST FREDSenior Member

The LB ratio will vary from country to country on smaller , 30- 50 ft boats.

This is because the contemplated use for many is as a cottage, so performance under way is second to how much volume is aboard in a slip at lowest annual cost.

FF

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### rxcompositeSenior Member

Your professor is setting you a trap. From previous posting, your S/L ratio is 1.1 (Fn 0.33). This falls into the corresponding hollow or dip in the wave making resistance coefficient. See the Ct chart posted below. If your prof asked for an S/L of 1.34 (Fn 4.0) it will fall on the next upper curve where it is rising. The standard (reads easy) Taylor’s formula for wave making resistance in lbs. 12.5Cb*V^4/L^2*displacement in LT) will work as it averages the humps and hollows but continue rising after an S/L of 1.1 making it inaccurate at higher speed. I believe he (the prof) is pushing you to further to isolate the wave making resistance and the frictional resistance. I use Taylors for wavemaking and E.R. Froude for frictional resistance for a quick analysis at S/L up to 1.1 Havelock is a more complicated wave making resistance formula but includes bulbous bow in the equation.

Since Ct is expressed as Ct=Rt/1/2pSV^2, and Cw is also Cw= Rw/1/2pSV^2, it can be rearranged to read Rt (or Rw)= 1/2pSV^2*Ct (or Cw) the standard friction line. If you want to investigate further at lower speed, use the Grigson’s algorithm. Try to search for “An Extension of Grigson’s Algorithm by Leo Lazauskas”.

I have included the published data of NPL hull to give you to have a head start. Be cautioned because even if you get the optimum hull lines as far as resistance and wave making is concerned, you still need to get the highest efficiency in powering. That is swinging the largest prop possible with a given draft. What good is a slippery hull if your powering is inefficient?

The Sea Power downloadable demo version 3.3 http://www.sea-power.net/ has a Mercier-Savitsky powering program. Although the formula was originally developed for studying planing hulls, it can be also used for fast round bilge semi displacement craft. The Atx/Ctx ratio is included in the inputs. After fiddling for maybe 50 to 100 times, you will get an efficient hull coefficients, ratios, dimensions and possibly prop specs. If you like calculation, attached is the formula.

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### BATAANSenior Member

Note minimal wavemaking and wake: Vessels such as this often were 10:1 L/B ratio.

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