Expensive Hull Speed.

On displacement boats it is expensive to run with engine at hull speed S/L Ratio 1.34.
If you run at S/L Ratio 1.20 you save 50 % in fuel consumption.

 

Hello Kjell, excuse my ignorance, but what is " S/L ratio " ??. I know what hull speed is and understand if you back off the throttle to ?? , well untill she's just "happy" , she'll use far less fuel .
If hull speed is 100%, then at what % do you say we'd achieve a 50% fuel saving ?
Pete.

 

Sorry to borrow the thread , but Mr.KJELL, I got your letter CD, Thanks it work fine, I have my guy check out the program.................... you really honor your word and work............ It a pleasure knowing you.

Wellydeckhand

 

The usual S/L ratio 1.34 times the sq rt of the working LWL only works for Fat (3-1 & 4-1) boats.

Once the L/B (LWL to BWL)ratio gets over 6 or 8 the number goes out the window.

Thats why catamarans and trimirans go at fine speeds

3X the sq rt is not uncommon and doesn't require anywhere as much power as lifting a box out of the water on a plane.

"Cable & Wireless" and other boats have used the Long Skinney concept to great results.

For fat boats "unity" , the place where the skin drag & wave making drag are equal is usually at the sq rt of the LWL.
For longest range , this is the most common cruise speed.

FAST FRED

 

Awsome fred, yep it's the " counterintuative "guy .
I'm sure that's a real good answer you just gave me , trouble is, I don't know what " S/L " is !, yeah I should have been more specific with my question.
Maybe it stands for 'slow learner" he he !.

Speed / leangth ratio ?

 

S/L Ratio = Speed /Length relation. With a displacement hull the top S/L Ratio is 1.40.
What is a displacement hull? A displacement hull is any type of boat slim or fat, with a narrow stern.
Running with 12% less speed you are saving 50% in fuel consumption.

 

Or, putting it mathematically:
Speed in knots = (SL ratio) * (square root of waterline length in feet)
using the Old American unit system.

 

Thanks guys, I was right, that was a good answer Fast Fred, it all makes sence now

 

Kjell,
I received the CD, thanks a lot. Sorry I didn't call or e-mail you, but I have had some frantic days lately, and I forgot.
Focusing on the thread, for long range cruising under engine, keeping a Fn (imperial units) around 1.1 is claimed as the most efficient in terms of fuel consumption and passage time.

 

Looking in to numbers.
Running at 1.34 making 8 knots. In 10 hours you make 80 n/miles and consuming 30 lit/Gasoil.
Running at 1.20 making 7.1 knots. In 11 hours you make 80 n/miles and consuming 16.9 lit/Gasoil.
This is 43% les fuel consumption. This is a lot with the new fuel prices.

 

OK, now I am confused. If Speed in Knots equals speed divided by lenght times the square root of the water line length, I dont see how you can learn anything. If you already have the speed to put into the S/L ratio, what are you trying to deduce???

I have always used 1.2 * the sq.rt. of WLL but I assumed the 1.2 was a constant, not calculated from speed.

 

The S/L ratio is an indicator of percentage of possible speed. 1.34 x sq.rt. Length is the formula for wave speed. If the crests are 25 feet apart, the wave is moving at 6.7 knots.

Since most displacement hulls are speed limited by the distance between the bow and stern waves, they are limited to 1.34 x sq.rt. LWL

Thus you can think of a S/L ratio of 1.34 as 100%

A S/L ratio of 1.1 is 82% of maximum, or 5.5 knots for a 25 foot waterline.

As FF stated the wave speed "limit" only applies to hulls that are constrained by their waves. Long skinny boats move faster for the same power input than short wide boats. The short wide boat is more likely to plane easily and at lower speed than the long skinny boat however, thus the advantage of long and skinny is relevant for non-planing hull types.

 

There is no such thing as "theoretical hull speed". The boat will always go as fast as the power input will allow. The idea of a "theoretical hull speed" is left over from the early theoretical work by Froude and Havelock and propagated by some magazines in the inter-war years. What it best represents is the first or second discernable wavemaking "hump" for a Wiggly shaped hull. Other effects such as block, prismatic, midships coefficient and length-displacement distribution contribute much more the location of this hump which can vary between a Froude number of 0 and infinity (yes, there are some hull shapes that have no wavemaking hump....they are called infinite planks...).

However, as far as maximum economic speed of Kjell's example is concerned, the "economic" speed will depend on wether the hull has enough power to climb over the first or second wavemaking humps (actually, they are the third and fourth, but the first two are negligible for most real vessels) and into the hollows between/beyond them. The first hump for most displacement hulls occurs about Fn ~0.27-0.3, the second ~ 0.47-0.5. Most displacement hulls do not see a decrease in power in the hollows, but rather a "flat spot" where very little power is required to increase speed. Some hull forms like SWATHs and powercats have such deep hollows that they become uncontrollable in speed as they clear the first hump and launch themselves across the powering curve at the next upslope.

In Kjell's example his vessel is ~ 35 feet LWL. At a Froude number of ~0.3 (the third Cw maxima but the first we have to worry about) gives a speed of 5.9 knots. (Fn = V/sqrt(gL) so say V=.3*sqrt(32*35) = 10.04 ft/sec = 5.9 knts). If it can get over this hump, the next one is at say Fn 0.5 ~ 9.9 knots. Note that Vk/sqrt(L) = 3.355 Fn, so that the S/L= 1.20 term correlates to a Fn of 0.35 which is just over the first hump and into the bottom of the hollow. Additionally, S/L = 1.34 = Fn .399 which is actually just starting to climb out of the "hollow" between the two "humps", so you expect a increase in powering per knot at this point.

No magic, no mystery.

 

Well, maybe some mystery....when you dont have the formulae that you just presented.

But less of a mystery now. I think I will change from a 1.2 to a 1.27 S/L number for my calculations now.

That confirms my calculations of 7 Knots with a WL of 31 feet. My most recent prop calculations look like a 20x18 prop at 2500 engine RPM (reduction gear of 2.57) (Prop rotation of 996 RPM) will give me 35hp which should be much less stress on the engine than 3600 RPM at 45hp with the old prop.

 

Now make the length/beam ratio around 12:1, put two of 'em side by side, and blow all that hull-speed guesswork out the window again