Working out hull drag

Laminar Flow

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I'm interested in fuel consumption for sure, also just general.
I'm not talking about waves in particular but meant by "wave drag" that part of resistance that comes from climbing up your own bow wave. When you start planing, it's no longer accurate, right? So if I can get one equation that can match my dinghy and my big boat both, up to, say F# 7, I'll call it good. I've noticed vicprop in Canada do seem tto get the right anwers, but they don't show their work! : )
Irregardless of any other factors, such as stern shape or hull form, the biggest single factor governing resistance is displacement.
In fact, to get a rough estimate of resistance, it can and often is expressed as a percentage of displacement.

Dave Gerr has published a table in "The Nature of Boats" that correlates shaft HP required fora given speed and displacement.
For example: to reach hull speed you need 1 HP per 500lbs. This corresponds well with results from Vicprop which bases their calculations on available SHP, displacement, DWL, waterline beam and hull depth. With these data it is possible to fairly accurately estimate frontal projection and using a drag coefficient of about 0.4, specific gravity of water and velocity, make a decent stab at resistance. The Delft series has shown that hulls are more similar than not.

Resistance by waves (not what is also known as form drag, but sea waves proper) is indeed of consideration when specking HP or a corresponding prop. Larsson & Eliasson offer some calculation models for that and in their example of a 40', 8.5t sloop and it, fairly consistently, amounts to about 200kg more resistance for speeds of 7,8,and 9 kts in rough seas, or, in terms of overall resistance, an increase of between 60% to 30%. It should also be noted that while a heavier boat has more resistance in waves, it also has more momentum to push trough.
 

Daydream believer

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Working this out amused me for a few mins on a Friday lunchtime anyway. Have a good weekend.
Interesting.:rolleyes: When I had my lunch Friday I was struggling as to whether to go for the sausage & mash or the full English all day breakfast.
Did not need a formulae, the price was on the board; but the girl at the checkout seemed to get it right at the till, only getting mixed up with the coffee or tea conundrum. I went for the coffee . ;)
 

Wing Mark

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I'm interested in fuel consumption for sure, also just general.
I'm not talking about waves in particular but meant by "wave drag" that part of resistance that comes from climbing up your own bow wave. When you start planing, it's no longer accurate, right? So if I can get one equation that can match my dinghy and my big boat both, up to, say F# 7, I'll call it good. I've noticed vicprop in Canada do seem tto get the right anwers, but they don't show their work! : )
I think there is a lot of curve fitting and 'rules of thumb' going on.
Wave-making drag in my simple view depends on 3 things: Displacement, length, and how bad the hull design is.

I think in the real world , unless you are climbing out of the 'hull speed bathtub' then wavemaking drag can often be a lot smaller (and certainly a lot more predictable) than wave-hitting drag.
You only have to stand in the shallows holding a dinghy to appreciate the forces can be quite big.

Many years ago, I did the 'motoring back from France after a JOG race gig', with an owner who 'knew' his fuel consumption at 2000 rpm or whatever it was.
In the wind, rain and chop we got through about twice what he expected.
Price you pay for a memorable spinnaker run over there I suppose!
 

Laminar Flow

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Wave-making drag in my simple view depends on 3 things: Displacement, length, and how bad the hull design is.
There are other factors that influence form drag as well. One of these is the prismatic coefficient and for a displacement hull this is of some consideration. Each relative speed range has it's corresponding, optimal PC. The problem for sailing boats is that they need to perform well over a wide range of speeds and the choice of PC ends up being a compromise solution.
For low speeds a low PC of, say 0.49 has a lower resistance. At the higher speed of 1.2 a CP of 0.61 is the optimum. How does that work out in terms of resistance?

At nominal hull speed the resistance for a CP of 0.61 would be half of what it would be for a low CP of 0,49 and about 24% less than that for a CP of 0.53 as would be fairly common for a standard sailing boat.

Motorsailers often have a fairly high PC, because they can more readily "choose" their speed. The penalty for that are fuller ends. The good news: the drag penalty for going slow with a high PC is considerably less than for trying to go fast with a low PC.

The difference in resistance between bad and good hull design in regards to displacement craft is in reality surprisingly small.
In terms of all the currently propagated rules of hydrodynamics the Dutch with their traditional craft apparently got it all wrong, yet there are many instances where a two hundred year old, blunt-nosed and gaff rigged craft has put the current crop to shame.
Our friend who sails a Boreal 43 rather sheepishly told us how a Lemsterark out pointed and footed him on a beat and when they could bear off simply disappeared over the horizon. The German magazine die Yacht sponsored a race between a X-yacht and a Lemster. The X won, but only because she could be tacked faster over an olympic course, on a straight course the taditional boat would have won. The difference in pointing angle between the two boats was less than 1 degr.
 

wully1

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There is a much simpler and more accurate method:

1. in a flat calm sea motor at fixed revs for a measured distance - with a clean hull bottom of course. Record the speed obtained.
2. Remove the hull and repeat the test as above.

The difference in speed is the hull resistance.
 

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