If LWL is so important how come some boats are faster than others... read on!

[ds797]

Ok so the basic principle is hull speed is around 1.37 x sqrt LWL (ft)

eg 36ft LWL = 1.37 x 6 = 8.2 knots (sounds about right for a 40-45ft boat (which would have a 36ft LWL).

Now I know we can have planing racy boats such as Ker, Mills, J/111 etc.... but...

If the LWL is the key performance indication, which is to do with displacement wave forms on the hull, then why would one boat be faster than another?

Ok, so SA: DISP ratio comes into it, so a heavy boat with a small rig will be smaller than a light boat with lots of sail..... IN LIGHT WINDS.....

But once "powered up", lets say 15-20kts wind, surely virtually any two boats with the same LWL *should * perform the same?

So why is it that a (say) Warrior 40, is slower than (say) a Starlight 39, which is also slower than (say) an Elan 40?

I have a few ideas, but seeing this is the Racing, Tech, and DESIGN forum.... lets open the discussion?!

Thanks!

 

[maby]

There's a lot of twaddle talked about this "hull speed" thing! People seem to think it's like the sound barrier for aircraft, but it really isn't - see the article on Wikipedia for example : https://en.wikipedia.org/wiki/Hull_speed. When the boat reaches Hull Speed, keep pushing harder and it will carry on going faster.

 

[lw395]

There's a lot of twaddle talked about this "hull speed" thing! People seem to think it's like the sound barrier for aircraft, but it really isn't - see the article on Wikipedia for example : https://en.wikipedia.org/wiki/Hull_speed. When the boat reaches Hull Speed, keep pushing harder and it will carry on going faster.

Very true.
It's useful as a rough indication of how fast a boat will go with not much power, but that's about it.
It's more relevant to older styles of boat where the waterplane is narrow at the back. Also the heavier the boat, the more it will stick at 'hull speed'.

 

[drakes drum]

Leaving aside the hull speed issue, there are issues of power and of hull and foil lift vs drag. So its a question of faster - when and under what circumstances? My last bilgie for example was as fast downwind as a Sigma 33 but upwind sailed a different course and so was slower overall.

I have heard it said that a length to beam ratio of 14to 1 as in a multi means the hull speed doesnt apply any longer but I doubt it is as black and white as that

What racer types usually mean when talking faster or slower is faster or slower under one set of handicap formulae. And thats what the designer goes for . . Can he find a way of going faster without affectibg the TCF?

 

[Woodlouse]

The biggest factor is wetted surface area which translates as drag. A boat with loads of area under the water is going to be harder to push along than one with less and so will on average be slower.

Secondly hull shape is important. A hull shape that will not plane will be harder to push than one that skims along the top of the water.

Hull speed is only a theoretical figure and relates to an exponential amount of force that is required to effectively push a boat over and beyond its own bow wave and simply put the easier it is to push a particular shape of boat the less force is required and therefore the more likely it is to exceed its maximum theoretical speed.

 

[Bodach na mara]

When the boat reaches Hull Speed, keep pushing harder and it will carry on going faster.

That is only true up to a point. Consider a tug with an incredible amount of power per tonne or unit of length. Once it reaches its displacement limit, using full power (as lots here do when not towing but returning to their berth) it will go only very slightly faster. All the extra energy goes into making large waves to damage the shoreline and potentially other nearby boats.

 

[jamie N]

How does the original formula equate with regard to an Americas'Cup foiler, where once lifted, the LWL is around about a metre?

 

[Tranona]

How does the original formula equate with regard to an Americas'Cup foiler, where once lifted, the LWL is around about a metre?

It doesn't. It is not really a "formula" but a quick and dirty way of estimating the speed at which a displacement boat starts to exceed its wave length and the amount of power required (whether from sail or motor) to increase speed rises rapidly. Planing and foiling boats do not have this constraint because they sit on the water rather than in it so do not make waves.

Most displacement boats operate well within maximum displacement speed and as has already been noted, power/weight ratio, drag and hull shape influence the speed in any particular set of conditions resulting in some boats being faster than others.

 

[Norman_E]

The other issue is that boat builders quote static waterline length. On some modern boats that is pretty much the same as hull length. On my old cruiser the hull is 45 feet, LOA is 46 ft 5 inches, but waterline length is quoted at 37 ft 6 inches! Once I am up to sailing at 6 or 7 knots the stern has become immersed and the waterline has grown to about 43 feet! So what is my hull speed? I am regularly sailing at over 7 knots in any decent wind and have seen 10 knots on occasion though I don't usually push it that hard..

 

[Tranona]

10 knots is not unusual for a 43' LWL. The "formula" gives 9.2. so you should easily be able to get continuous speed in the high 7's. low 8's and surges to higher speed. The extra 5' of heeled LWL increases the potential by .6 knot, which is why long overhangs were popular when rating rules penalised static LWL. This is where the Power/weight, or as it is normally expressed SA/Disp for a sailing boat comes in. Lots of sail and a breeze on the limit can generate enough power to overcome the wave making constraint. Often stains the trousers as well!

 

[cmedsailor]

I was in a race with around 35 boats participating, many 45-50ft included in the race and the 3rd fastest boat in real time (before any correction) behind a pure 45ft race boat (you know one of these "naked" racing machines) and a 74ft modern classic was a Bavaria 35 Match. I was very impressed how that boat could sail so fast (8+ knots). The LWL of that boat is only 10,09m if I am correct.

 

[RobF]

The biggest factor is wetted surface area which translates as drag. A boat with loads of area under the water is going to be harder to push along than one with less and so will on average be slower.

Secondly hull shape is important. A hull shape that will not plane will be harder to push than one that skims along the top of the water.

Hull speed is only a theoretical figure and relates to an exponential amount of force that is required to effectively push a boat over and beyond its own bow wave and simply put the easier it is to push a particular shape of boat the less force is required and therefore the more likely it is to exceed its maximum theoretical speed.

Couldn't agree more. A keen racer friend of mine noted that every Kg of boat weight equates to 1 Kg of water that has to be pushed out of the way (or pushed underneath for planing hulls).

Also, whilst the LWL notes the bow and stern wave in which a non-planing hull is stuck, the shape of some hull designs seem to be pretty effective in moving the stern wave aft.

 

[geem]

Couldn't agree more. A keen racer friend of mine noted that every Kg of boat weight equates to 1 Kg of water that has to be pushed out of the way (or pushed underneath for planing hulls).

Also, whilst the LWL notes the bow and stern wave in which a non-planing hull is stuck, the shape of some hull designs seem to be pretty effective in moving the stern wave aft.

I did some quick calls recently sourced from an American yacht designers website that suggest for my yacht for every ton of weight added we will slow by 0.2 of a knot. We are theoretically a 15 ton boat but weighed 19 in fully loaded live aboard trim when we lifted out. So 4 ton knocks off nearly a knot. Sounds about right for our boat. Surprisingly she still sails very well and isn't down on her marks so we must be within the Van da Stadt original design allowances.

 

[Woodlouse]

I did some quick calls recently sourced from an American yacht designers website that suggest for my yacht for every ton of weight added we will slow by 0.2 of a knot. We are theoretically a 15 ton boat but weighed 19 in fully loaded live aboard trim when we lifted out. So 4 ton knocks off nearly a knot. Sounds about right for our boat. Surprisingly she still sails very well and isn't down on her marks so we must be within the Van da Stadt original design allowances.

I would have thought that added weight will only slow a boat down in certain low wind speeds. If it's blowing 30 knots no amount of weight is going to stop a boat hitting its hull speed.

 

[lw395]

I would have thought that added weight will only slow a boat down in certain low wind speeds. If it's blowing 30 knots no amount of weight is going to stop a boat hitting its hull speed.

But added weight means you hit every wave harder, so you spend more time trying to get back to speed particularly upwind.
Downwind, a lighter boat will surf more in waves.
Weight is the thief of speed unless it's lead on the bottom of the keel or fat people on the rail, and sometimes even then!

 

[Buck Turgidson]

Like the sound barrier, hull speed is not the brick wall it was once thought to be. Light boat, lots of sail area and wind, bob's your uncle.

not sure what the lwl of a sunderland hull is but he doesn't appear to be stuck at 1.34sqrt of it.

 

[PuffTheMagicDragon]

The old formula based on LWL was OK at the time when boats were very similar in construction and still give a good indication of what to expect when applied to cruising hulls. However, when you enter the present zone of racing machines all of that goes out the window. Light displacement, shallow bodies, canting keels, wide transoms, etc., allow for far greater speeds than indicated by the old formulae.

This week we had Rambler 88 and MoMo going consistently at twice the true wind speed. Just now, SFS II is approaching the finishing line and is doing 17,9 knots on a broad reach in 11 kts of wind.

http://www.rolexmiddlesearace.com/tracker/#pt

 

[ianj99]

That is only true up to a point. Consider a tug with an incredible amount of power per tonne or unit of length. Once it reaches its displacement limit, using full power (as lots here do when not towing but returning to their berth) it will go only very slightly faster. All the extra energy goes into making large waves to damage the shoreline and potentially other nearby boats.

That's more to do with the prop pitch surely? The big props produce a lot of thrust for towing, but are not pitched for speed.

 

[geem]

But added weight means you hit every wave harder, so you spend more time trying to get back to speed particularly upwind.
I am not sure that is particularly relevant to our yacht. We have large sail area, relatively fine bow and so much weight we punch through seas up wind very effectively. A lighter boat would be stopped by what we keep going through. 6 ton lead keel means we can stand up to lots of sail and a 7ft 2 inch draft gives us good up wind performance. She is a powerful yacht. Weight is not always the enemy. I think too many people look at yachts as either light like a Pogo or heavy like a Colin Archer. There are many fine yachts in the middle ground that have attributes of both extremes. I like to think ours is one of them

 

[garvellachs]

Like the sound barrier, hull speed is not the brick wall it was once thought to be. Light boat, lots of sail area and wind, bob's your uncle.

not sure what the lwl of a sunderland hull is but he doesn't appear to be stuck at 1.34sqrt of it.

It the film it's planing - the excess power of the props drives it up and over the trough between its bow and stern waves. When we owned a little Ring power boat (75hp) it took a lot of throttle to push the boat and a skier up onto the plane; but then one could throttle back and coast along at 20mph or so. Modern sailing boats have enough sail power to do the same and get up on the plane. Noticeably, when cruising most of us reef when the wind strengthens, but downwind a modern sailing boat can carry a lot of sail and plane in a strong breeze. All business as usual in a dinghy of course. The alternative is to use a very long thin hull, which is the Turbinia strategy, so multihulls exceed hull speed but not by planing, just by creating reduced bow and stern waves?