Formula for hull speed?

Seagreen

Seagreen

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I admit it. Too idle to trawl via google. What's the best formula for estimating max hull speed on a displacement boat?

If anyone wants to actually work it out for me, the hull has a WL of 29'6" and a displacement of about 10 tons, with a long full keel (think fishing boat hull).

TIA.
 
Seagreen said:
I admit it. Too idle to trawl via google. What's the best formula for estimating max hull speed on a displacement boat?

If anyone wants to actually work it out for me, the hull has a WL of 29'6" and a displacement of about 10 tons, with a long full keel (think fishing boat hull).

TIA.
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sq root waterline length in feet x 1 > 1.5

your boat poss around 5.4kts
 
There is no such thing as "hull spped" - only a point at which amount of power required to produce extra speed climbs rapidly.

As Sailorman says, the estimates are sq root of waterline length in feet times a factor somewhere between 1 and 1.5 depending on the shape of the hull. A long keel hull form might be around 1.25 giving you around 6.8 knots. The only way to find out is to keep on increasing power until more power produces no more speed - but of course many auxilliary yachts do not have enough power to get to that point.
 
Seagreen said:
I admit it. Too idle to trawl via google. What's the best formula for estimating max hull speed on a displacement boat?

If anyone wants to actually work it out for me, the hull has a WL of 29'6" and a displacement of about 10 tons, with a long full keel (think fishing boat hull).

TIA.
Click to expand...

Double the waterline length and take the square root. ish. In your case that's a tad under 8 kts. Ish.
 
My problem with this is, I've done 7 kts towing a pram dinghy on a run F3 to 4. This was averaged off the GPS taking the tide into account. I didn't feel the rig was over pressed nor was the hull feeling over-driven. Slipped along as smooth as you like, in fact. I may have had a little help from a slight (3ft) following sea, but not enough to matter.
Can you see my problem with formulas?

I even only had the working rig up; main, jib and staysl. No topsail or mizzen.

The formula gives me 7.33 knots. This when I'm towing 150lb of dinghy and a bit of a furry bottom, too. OK, this sounds like I'm showing off, but I'm really after a better, more accurate formula, or some measurement that more accurately mirrors the measured reality.
 
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Seagreen said:
My problem with this is, I've done 7 kts towing a pram dinghy on a run F3 to 4 towing a dinghy. This was averaged off the GPS taking the tide into account. I didn't feel the rig was over pressed nor was the hull feeling over-driven. Slipped along as smooth as you like, in fact. I may have had a little help from a slight (3ft) following sea, but not enough to matter.
Can you see my problem with formulas?
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The "formula" is only an estimation. There is not a definitive point as you can see from the different factors used. Factor of 1 gives you 5.4, factor of 1.5 gives you 8.1. You are somewhere in between. It all goes pear shaped anyway with a dirty bottom or a head wind/sea.

The only real value in estimating maximum displacement speed is in sizing the power plant. You will note that in the sizing formula for this, weight is a significant factor as this has an influence on how much power you need to get to that speed.

So stop worrying - you are not breaking any "rules" - just demonstrating you have a well matched hull and rig.
 
I accept that the formula is a guesstimate. Getting my 'Chappelle' out of its shelf means going into the loft and waking the kids.

Sometime this summer, I'll actually try her with a clean bottom and all her slap on in a beam wind, without the baggage in tow, and report actual findings.
 
Actually you don't need to go to all that trouble. Just Google Hull Speed and you will find plenty of explanations.
 
Seagreen said:
I accept that the formula is a guesstimate. Getting my 'Chappelle' out of its shelf means going into the loft and waking the kids.

Sometime this summer, I'll actually try her with a clean bottom and all her slap on in a beam wind, without the baggage in tow, and report actual findings.
Click to expand...

It will make very little difference. As someone else pointed out, the hull speed is the speed at which, for hydrodynamic reasons, the power needed suddenly shoots upwards. Compared to the additional drag which appears then, a bit of a fuzzy bottom or the occasional towed dinghy are lost in the noise.
 
The formula is a constant times root LWL. The value of the constant depends on hull form so for a tubby boat it might be 1.3 while a sleek racer might manage 1.6. Another factor is overhangs where the bow and stern waves efectively increase the LWL.

Once the length/beam ratio exceeds about 8, hull speed ceases to have an effect which is why slender multihulls can go very much faster than monohulls of a similar size without planing.
 
x 2 for cats ???

snowleopard said:
The formula is a constant times root LWL. The value of the constant depends on hull form so for a tubby boat it might be 1.3 while a sleek racer might manage 1.6. Another factor is overhangs where the bow and stern waves efectively increase the LWL.

Once the length/beam ratio exceeds about 8, hull speed ceases to have an effect which is why slender multihulls can go very much faster than monohulls of a similar size without planing.
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I have always wondered ,is there a different formula for cats? is there a multiplying factor ?
 
Rowing eights, at around 60ft, doing 15+kts would appear to have a constant of upwards of 1.9. And we had a heavy boat with w/l of about 50ft that would do 12kts, but this was with lots of sail and wind so probably outside the formula (and nerves)
A
 
Just musing about my #14 post. The hp of a person has been culculated as 0.1 hp continuous. Shall we say an olympic rower might manage 0.5 hp? over the distance.This means an eight has 4hp to power it's thin hull at 15 or so kts. The 35 ton motor/sailer that I mentioned with a 50ft w/l could also do 12kts under engine (flat out) So the 280hp of it's twin engines equated to full sail in a strong wind. Lots of factors ignored here.
Totally unconnected was a test by Car & Driver in the US on car aerodynamics. They found that the shape of the car only mattered over 45mph as the rolling resistance was the primary problem under that. The car tested, a Mustang (mid 80s) actually was worse off with the fashion obligatory rear spoilers and front 'aprons' than the standard car.
Just musing
A
 
MoodyNick said:
After I failed an interview for the National Physical Laboratory (Marine Division) in 1975, largely because of this very subject, I was told that max hull speed is the square root of 2xLWL. This is really only another way of saying (sq.rt LWL) x 1.4.
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The theoretical figure is when the distance between the centre of the bow pressure system and the centre of the stern pressure system are one, three, or five wavelengths apart. Exactly where the centres of pressure are located depends on the design of the boat, but are typically a little closer together than the waterline length. And the detailed formula is the square root of [distance between centres x gravity divided by pi] -- so long as all the measurements are in compatible units (eg all in metres).
As several people have said, it usually works out (in knots) to be about 1.4Xsquare root of waterline length in feet.
But in practice, what you see is a marked increase in fuel consumption and/increase in wave size at round about the "hull speed" point. But it's not like switching on a light -- the increase happens gradually. And most boats can exceed their hull speed if you pour enough power into them. Most motor boats do it as a matter of routine because they have to exceed hull speed before they can start planing.
 
my understanding is that the formula was arrived at after tank testing a cylinder ie a form with blunt ends, by fairing the shape different results were obtained, the starting point is 1.4 * sqrt waterline in ft but as others have said a long narrow hull form can give 1.9 or more, it why long narrow boats go faster than short wide ones, the 1.4 is "an average" or starting point,.
 
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