Settle an arguement on boat speed please.

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knewboater

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Given the modern designs of ocean monohull racers etc, is it still the case that say a 60ft (water line length) racer can still only do 1.3 times the square of the water line length e.g 10.06knots in this instance, unless of course they are planing?.
 
A displacement boat will always be limited to the WLL, as it will generate a wave with the lenght/frequency and speed proper for that lenght.

Of course modern design may ease the dinamic transformation, at definite condition, from a displacement to a planing behaviour.

This is true for Motorboats as well as for sailing boats.

Speeds attained from modern day 60 footers are obtained really by planng.

Cheers
 
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high speeds definitly achieved by planing. I think the 1.3xsq rt WLL is economic cruising speed under engine the max displacement speed is prob 1.6 or 1.7xsq etc. I think

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Aye, the 'hump' we all know and love (well those mono-hulls among us) is generally experienced at 1.6 to 2.1.
 
Can someone please explain how my MAB of 25ft ... heavy Motor Sailer type with its Perkins can do over 9 kts without anywhere near rising out of water onto the plane !!!!!

Yes WLL does factor into theoretical max speed ... given small to adequate power engines ... but falls apart when bigger engines come into it ....

If anyone see's my old tub planing - can they please photo it and send me a copy ............. PLEASE ?
 
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... !!!!! ... ... .... ............. PLEASE ?

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I am sure there is a hidden code in there, somewhere /forums/images/graemlins/smile.gif

What is the wash like when your 25 footer is sliding through the water at a nippy 9kn? I wonder if there is still energy being lost, only it is driving water down; rather than the boat up.
 
The wash has to be seen to be believed .....

There are various on the forums that can attest to the sheer speed of my MAB ...

Biggest problem is that I have to shut all cockpit drains before opening throttle - otherwise you get very wet feet !

I have estimated wave height of my wash in still waters .... well away from others .... at near 1/2 metre or more. Honest - I use the throttle with great respect for others - it's only when last orders or tide is called - that it gets nugded up !!
 
Sheer brute force /forums/images/graemlins/laugh.gif

Nigel at 9 knots that only gives a multiplier of about 1.9 for the root of your waterline length., which is possible.
 
I am no expert, but I have wondered if the point is that maximum hull speed is in ‘perfect’ conditions, that is, totally flat water. It seems logical that as we operate in non-perfect conditions, factors such as waves will influence the maximum speed, for example the trough of a wave will break down the bow wave allowing the boat to get over it.
Or am I talking rubbish ?
 
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Sheer brute force

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My understanding is that hull speed is not a definite line, but rather a period where the force required to add speed increases expedentially until the hull surmounts the bow wave. So if sbc has a whopping engine, it should be able to squeeze a bit more speed than my 26 footer with its little 8hp through sheer brute force.

Alternatively, it may be starting to rise up to the plane, which would explain the following tsunami.
 
The rule you state is not an absolute, it is just a rough guide. Hydrodynamicists spend hours trying to make more and more efficient hulls and some racing boats well exceed this rule without planing. The effective wetted area is more important than the OAL and the big 60s are designed to present max OAL with minimal wetted area when heeled. They also effectively overpower the hulls most efficient speed with enormous sail plans that cause the boat (which is built lightweight) to rise in the water reducing the wetted area. This is not technically planing as to plane to windward would loose directional stability from the high aspect ratio dagger keels, but it does reduce the wetted area. Downwind, flying kites, they are capable of true planing however and once up over the "hump" are not restricted to speed by hydrodynamics at all, but still need directional stability to prevent broaching, which is why many have bow keel blades too. Cats have narrow hulls that make nonsense of the rule too.... Here the pressure wave between the hulls is the limitation, not WL length. Thats why racing cats have very wide beams and why trimarans, in the right conditions are the fastest vessels. Just try pushing a pencil through the water rather than a housebrick.
 
SBC\'s MAB .....

25ft Motor Sailer ..... 1970's build .... heavy !

sunrider25.jpg


As you can see not the most modern of shapes .... rather typical chunky late 60's early 70's style.

Engine is a Perkins 4-107 of approx. 42HP ..... cruise at 1/3rd throttle at about ~4kts ....
From full speed to dead stop in water is in less than a boat length ! I know ..... and galley gear knows it also !! Don't 'alf make the ar*e rise when you do it and bow dip !!
Needless to say engine unloads rather quickly when throttle opened .... but I'm not worried about loss of power from glazing etc. - as it's so overpowered anyway ...

Boat has bias to port when given astern engine - such that berthing stbd-to is more drift in than in.. kick astern, stop job. Berth port side to ... slap her up to pontoon, kick astern ... drop ropes over .... Bobs the proverbial.

The size of engine was not my doing ... she had a 4-99 ~35HP when I bought her ... and was told that was from new. I had to replace few years back and wanted to drop straight onto bearers etc. - 4-107 being basically same engine did just that.

Anyone for water-ski-ing ?
 
Re: SBC\'s MAB .....

Yes the "hull speed" is a point or more an area where the power required to increase speed becomes disproportionately large compared to the power required to acheive an increase in speed at lower speeds.
ie if you graph power versus speed it gets very steep in the region of "hull length".
However the steepness varies with the hull weight and type. So a narrow catamaran hull can more easily beat the hull speed limitation than Nigels boat. But it can be beaten with enough push.
If we continue to picture it as a graph the graph can go downwards at planing speed so some light boats actually accelerate as they begin to plane even with less power.

The only use for hull speed formular is to give you an idea of what speed is most economical and what speed more power and fuel will give less value in speed increase. It is also an idication of how longer hulls can go faster relatively easier.

In sail racing on the wind I see the hull speed as a point where sail power can be more usefully lost to pointing closer to the wind than in trying to increase the speed. olewill
 
Re: SBC\'s MAB .....

As has already been said the theoretical hull speed calculation has to do with the wavelength of the bow-wave pattern. Below the limit, the hull is supported by two or more crests - at the limit, the wavelength has increased to the hull's length, so the stern falls into a trough and the hull is essentially going uphill (up its own bow-wave), thus needing gobs more power to do so. SBC's MAB simply has enough power to motor uphill. I take it that at high speed the stern squats (hence the tendency to flood the cockpit) and it rides bow up. It isn't planing as such - more like pitching the plane of buoyancy nose-up from horizontal.
 
I would be interested to know what the fuel consumption is at various speeds, in litres per hour.
 
Consumption ...

I usually average about 2 - 2.5ltr/hr ... and that's at 4 - 5kts. average.
I can potter around at low speed with about 1 ltr/hr ... or hammer the life out of it at a never measured but let's guess maybe 4 ltrs/hr.

When planning - I look at 2.5ltrs/hr and 4kts .... and always end up with spare left over.

The surprising fact is that I do not use that much more fuel than a lot of others - even though my power is a lot higher. Using barely a 1/3rd of the engines potential lets it basically idle it's way through life. Like the bigger engine model of a car - potential is there for faster - but can be economic as it's working less.

Over a weeks South Coast / Solent cruising - I would use a lot less than a tank of fuel - tank =~ 70ltrs and that is a fair amount of motoring. Probably less than half. With about 1 months total use per year in actual days used - probably tank and a half total.
 
Re: Consumption ...

My Yanmar handbook quotes the specific fuel consumption in, from memory, grammes/hp-hr. Knowing the specific gravity of diesel you can get back to l/hp-hr. I have plotted the theoretical hp/fuel consumption curve for my 110hp Yanmar and it is very useful...with a few actual points from measured consumption I have a pretty good idea of where we are. We are 42' and I tend to cruise at 5.5 to 6kt under power alone, for economy.
 
SG of diesel .....

That can be averaged out as it doesn't vary that much to be real concern I wouldn't have thought.

We worry about it in our business as we have to be accurate in calculating quantities etc. ..... a few points on (we actually use density ... not SG which is only useful to old fashjioned Chief Engineers on ships now and the odd Yank !!) density can make a difference when calculating a 60,000 ton cargo of the stuff !!

When we calculate additives required - we are supposed to work it out for actual density ... but to be honest we have fixed an average figure into the spreadsheet now for each of the fuel grades ... gasoline, kerosine, diesel(gasoil) and fuel oil M100 etc. Not worth the effort to change it really.

I would assume the calcualtion you refer to would be based on the Calorific value of the fuel ... or Specific Energy to some. This can be calculated from various points taken in analysis in lab - as we do .... likewise Cetane. To determine fuel consumption - I would also assume that the equation assumes energy values based on the "SG" inputted.

Wish I hadn't started now !!!
 
Is this measured with a handheld gps or using your trusty in-hull paddle wheel?
Is it possible that the prop is dragging the water backwards at such a rate that the flow over the paddle wheel had become a flume and is moving more quickly than the boat which is still obeying the laws of physics?

just a thought
 
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