14m LWL hardchine sailing hull-what performance loss?

Discussion in 'Boat Design' started by Lyle Creffield, Jul 1, 2006.

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Lyle CreffieldJunior Member

Hi

My question is simply what speed loss is suffered at varies points of sail at wind speeds less 15kn (actual)?

I am interested in a mono hull of 14t disp. beam 3.9m

The vessel i am comparing is the Radford 14

What is the velocity loss between a single chine and round bilge vessel?

Hit me the negatives of single chine construction

Thanks

lyle

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tspeerSenior Member

Basically, increased wetted area. By rounding the chine, you reduce the wetted area with very little loss in volume or righting moment.

However, there are single-chine boats that are famous for their light-air performance. The Ben Seaborn-designed Thunderbird comes to mind. So there's no hard-and-fast rule.

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GuillermoIngeniero Naval

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Lyle CreffieldJunior Member

Wetted surface area

Hi Tony

Yes wetted surface area is one of multiple causes for performance reduction
in a hard chine hull

To use the example i put forward 14m LWL 14t disp. 3.9m beam (possibly 3.7 on the LW) 14m x 3.7m x.27m = 14t, WSA = 37.65square metres (plus appendages + or -) = 2.744 squared by 5 (6 sides in a cube), (cube root of 14 = 2.744)

Max hull speed = 8.14kn = 1.2 x 6.78 (square root of 14m or 46 feet)

I understand that at 50% plus of 8kns (arround 4kns), friction of hull surface is secondary to wave creation

What difference at a speeds 4, 6, and 8kns would one extra square metre of WSA make that is to say a WSA of 37.65 equals 4kns 38.65 square metres equals ? kns

What i thinking is that the cross flow turblance (over chine) of the yawed hull is a greater detriment to velocity

lyle

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GuillermoIngeniero Naval

Lyle,
Vortex generated at chines and increased surface area go against speed, certainly, but on the other hand the roll dampening will be better than in a rounded hull, and chines, making bottoms flatter, may contribute also to the creation of lifting forces (depending on hull forms) when reaching-running in certain conditions. So it's not easy to give an overall opinion. Every boat is a case.

It's not easy to answer your question about the increased resitance per each aditional sqm when located in a hull, because several types of resistance happen. If we are just talking of frictional resistance we can follow Froude's model, to try to figure out things (There are other models, but this is the simplest), but I'm sot sure if this is what you are asking for.

Froude found experimentally that the frictional resistance, Rf, of towed planks could be expressed by the relation:
Rf= f*S*V^n
in which f is the coeffcient of frictional resistance, S is the wetted surface in square feet, V is the velocity in knots and n is a number nearly equal to 2.

The values of both f and n depend upon the length of the plank and on the character of the surface,

For the same length of plank, same kind of surface and same speed, the frictional resistance increases directly with S, the wetted surface. Simplifying things we can say that a surface increase from 37.65 to 38.65 (whatever the units) would roughly increase frictional resistance in more or less a 3%

If we keep constant the wetted surface, Rf increases with (roughly) the square of velocity. Increasing speed just from 4 to 4*1.03 = 4.12 kn (The same 3% amount of increase as for surface), Rf would increase around 6% (Dividing sqares of velocities)

And increasing speed from 4 to 6 kn would increase Rf 2.25 times and increasing from 4 to 8 will increase it 4 times!

For a cruising boat like the Radford 14, hull roughness (slime, barnacles, etc), probably will be of more relevance than a 3% increase in wetted surface (constants n and f increase with the roughness of the surface)

The Radford 14 has a Lwl of 13.24 m (43' 5"), so hull speed will be around 9.5 knots, corresponding 8 kn to a S/L ratio of 1.21 (Imperial units) a zone where residual resistance begins to rise steeply. 7.2 knots (when motoring) will probably be closer to the optimum 'passage time - fuel conpsumtion' binomium.

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Lyle CreffieldJunior Member

Hardchine performance loss

Hi Guillermo

Firstly, thank you for your informative response

You are most correct in the LWL re Radford14

If i have undersood you correctly:

A given breeze would drive a hardchine (allowing one square metre extra) at 4kns or 96kn in 24hrs

A round chine eg Radford14 at 4.12kns or 98.9 kn in 24hrs (additional 3kn + or -)

At 7 kns a hard chine would travel 168 nm in 24 hrs

Where a round chine would have a speed of 8kns or 192kn in 24hrs (+24nm)

Please forgive my rounding i acknowledge there are many factors contributing to performance however i am trying to get a handle on hardchine negatives

By way of explaining where i am coming from
i see boat builders putting to much emphasis on asthetic as opposed to functionality i have read the the recomended readings on hard vs round chine (boatnet) and conclude comfort and stabilty can be achieved in a hard chine vessel

So does a reduction 24kn in 24hrs mean that much?
WHY?
I estimate 500hrs to construct a 14t 14m hard chine hull from NC file cut material arround 10mm alloy hull (unlimited life with care) plate, mininum frames, nil paint save underwater @ a cost of \$20 000 for protective paint on a steely that has arround 20years commercial life

I have browsed your site which is great-thanks again

The Radford14 has much to recommend it i drew up a similiar vessel 20 years ago based on the Adams45 for up to 6 people Graham Radford has done a top job

cheers

lyle

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tom28571Senior Member

Paper answers may give an advantage to the round hull but races aren't run on paper. Ask all the round boats that have been skunked by boats like Ragtime over many years. All of speed is not determined by resistance calculations.

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GuillermoIngeniero Naval

Lyle,
Let's work with an S/L ratio of 1.05, where frictional resistance is still the main contributing factor, and residual resitance has an small and linear (almost flat) behaviour. Correspondent speed for the Radford 14 is 6.9 kn. Let's round it to 7 kn.

At 7 knots you have a theoretical 24 hours run of 168 miles, as you said. Increasing surface by a 3% will increase frictional resistance by a 3% and we can asume the increasing in residual resistance is negligible. Simplifying things we can asume speed will decrease in the same amount, so coming down from 7 to 6.8 kn (+/-)

A difference of 0.2 kn will make the boat lose 4,8 miles in a 24 hours run. You are the one who has to decide wether this is acceptable or not.

And remember we are talking from a very theoretical (and simple) point of view. Behaviour of boats at sea, with varying wind and waves, makes a lot of other factors to come into consideration.

Just talking from the point of view of frictional resistance, other factors, as hull roughness, may be of greater importance. Investigations on hulls with 25% of its surface covered by barnacles 0.118 to 0.157 inch high have been found to increase frictional resistance more than two times. Also a number of observations indicate that the frictional resistance of a submerged surface may increase with time of immersion in the absence of macroscopic fouling. This effect is attributed to the slime film, formed by bacteria and diatoms, which rapidly develops on surfaces exposed in the sea. Although with proper paintings some of this slime film will be peeled off by the boat's movements at sea, the increase in frictional resistance will remain in the order of a few percent, the same order we get for our considered increase in wetted surface.

On top of this, when motoring, fouling in propellers is of greater importance than hull fouling. In some practical cases has been found that up to a 2/3 of the increase of fuel consumption due to fouling was due to its effect on the propellers (twins).

In my opinion, I wouldn't be very concerned on the hard chine drawbacks for a cruising boat, as told before. The advantages of an easier amateur building process, some roll movements reduction and maybe even some benefits at higher speeds, probably will overcome the disadvantages.

Cheers.

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Lyle CreffieldJunior Member

Hi Guillermo

again thankyou

.2 of a knot loss i feel is quite exceptable

yes this is a over simplified guide when sailing theory approaches infinity

but it is what i sort a basic answer to a very complicated question

thanks

lyle

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