What made Pilot Cutter so fast? Just rumors?

[bergius]

I found Gerr’s hull speed formula, as presented in Charles Doane’s The Modern Cruising Sailboat quite interesting. It looks at D/L as an additional component.

For our boat it yielded a slightly higher number for an unloaded vessel than the traditional hull speed formula (7.1kt vs 6.8kt). But if calculated again with a reasonable cruising load, it dropped to 6.4kt. This is coincidentally the max STW we’ve done in the past two years of cruising discounting surfing and other anomalies.

Sure, it is still a very theoretical speed based on a lot of guesstimating, but I found it interesting how well it has mapped to our real world performance. Heavy full keel double ender.

[sanibel sailor]

I need to disengage. This thread is not good for my mental health.

[TrentePieds]

No, no, no - We don't wanna do that till "someone" has explained what a "zero displacement sailboat" is.

Best if it be done with reference to Adam Smith, Carl Marx, Christ and the Prophet. Isn't that the least "someone" can do while preaching a new creed?

TrentePieds

[dannc]

Originally Posted by bergius

I found Gerr’s hull speed formula, as presented in Charles Doane’s The Modern Cruising Sailboat quite interesting. It looks at D/L as an additional component.

For our boat it yielded a slightly higher number for an unloaded vessel than the traditional hull speed formula (7.1kt vs 6.8kt). But if calculated again with a reasonable cruising load, it dropped to 6.4kt. This is coincidentally the max STW we’ve done in the past two years of cruising discounting surfing and other anomalies.

Sure, it is still a very theoretical speed based on a lot of guesstimating, but I found it interesting how well it has mapped to our real world performance. Heavy full keel double ender.

I too was thinking of Gerr's hull speed equation vs reality.

Even if Gerr's equation is correct, or even close to correct, meaning a given hull can exceed 1.34, the big issue is the power needed to get to that speed.

I have 3-5 equations to determine HP for a given S/L ratio and none of them match. They sometimes give close results, sometimes not, but one point where the HP equations agree, is when the S/L gets around 1.34 and over, the HP needed goes up fast. So even if a hull has the theoretical ability to get to a S/L ratio of say 1.4, will it have the HP to do so?

Sailboats might have a sail plan that would provide the HP in some conditions, but if one had to use an engine, even if the engine had the HP, the fuel burn will be horrible to get to that theoretical S/L ratio.

Later,
Dan

[NewAlexandria]

Here are the formulas for Max Hull Speed. Both are flawed, but much better than Speed to Length Ratio. Speed to length ratio has no meaning actually. Not only is the speed wrong in that formula, it does not take hull shape into consideration, so you won't even know the difference between different sailboats. Btw, the coefficient in Speed to Length Ratio is used freely in sailboat design books, rendering different results for the same calculations.

As a matter of a fact WestSail 32 and a Pilot Cutter will have the same speed according to that formula. But both boats are on the opposite ends of the extreme when it comes to displacement sailboats.

Speed formula 1:
(Square Root of the Water Line Length * Water Line Length/Beam) / 2

Sailboat: Westsail 32
(5.24 * 2.5 ) / 2 = 6.55 knots


For those that say the beam is not enough to compare boats here is another approach:

Example 2:
Square Root of the Water Line Length * [(Water Line Lenght/Displacement)*1000]

Sailboat: Westsail 32
5.24 * [27.5/19500)*1000] = 7.38 knots


The speeds are wrong, but ironically much better than using Speed to Length Ration formula. BUT what is not wrong is this: The percentage difference between boat designs. You will be able to see that one boat has 15% or 20% advantage in speed over another boat. The advantage is manifested by how easy it is to push the boat to that speed. And those % numbers are correct.

Don't post explanation about kayaks or 5 degree entry on the hull boats. I'm talking about real sailboats with real hulls where entry is like 20 degrees PLUS. Anyway, you will probably have another 20 posts how this is all wrong, and it is in terms of actual speed, but not in terms of comparison. You can now actually compare boats on paper when it comes to speed. Both formulas basically compare how much water a boat has to push, and how easy it is to push that water for 1 boat vs the other. It's simple, it's handy, and it's the closest thing that exists right now.

Btw, every line you draw now, you will be able to recalculate and see what you did for the boat in terms of her speed ability.

[NewAlexandria]

I forgot. The way I calculate the speed has one important aspect: stability. Because stability of a moving object in water is derived of two things: Initial stability (beam, design, etc) and speed. So even though one boat seems to have better stability on paper, or is "known" to have it, you can now actually calculate for yourself and you will be close.

So if it's 20% easier for one boat to acquire her max speed verses another, she will be more stable, then a more stable boat to begin with but slower.

Calculating the cut offs (speed vs stability) and the exact sweet spot is hard. They say it's not possible. I know it is possible. Once I can calculate it, I will know exactly how much stability to give up for speed.

[NewAlexandria]

On another note, this is all academic. It's a numbers game. It's not required to design a sailboat. You don't need any of this, you can buy any sailboat and be happy with.

[Chris 249]

So you are going to utterly ignore the fact that you're completely wrong about your claim that sailboats can't plane?

[Chris 249]

I also have no real idea why you think that increasing speed equals increasing stability. It often doesn't due to the way that when a displacement hull increases speed and gets close to hull speed, it creates a large wave trough in the centre of the hull. That reduces actual waterline beam and therefore reduces stability.

[double u]

Originally Posted by NewAlexandria

.... But before I do that let me show you how this world works ....

...finally, after all this time on the internet, I have found you!
(Troll or serious personality problem...?)

[NewAlexandria]

Originally Posted by Chris 249

So you are going to utterly ignore the fact that you're completely wrong about your claim that sailboats can't plane?

No one has ever attempted it. The displacement sailboat would have to be 256 feet long to even attempt it. You can calculate that yourself once you read up on planing of a sailboat.


As far as your stability post, it does not work the way you assume,but you're not 100% wrong. You say once close to hull speed the trough is big and reduces the beam, which reduces stability. You are correct that lose of beam loses some initial stability, but Total Stability is not lost. It is gained. Read:

An object when standing still in water has only Initial Stability. It does not have stability from the momentum going forward. But the second it starts moving it's Total Stability keeps "growing" because of the force forward. Now you have to overcome the force from forward momentum plus initial stability in order to tip the object. The stability from the momentum forward is greater than initial stability especially at higher speeds. Hence very often people say (even on this forum) Initial Stability does not matter. I actually disagree with that, but will write about it another time. Below is the explanation why you should not worry about "big through" taking away your stability.

The second the boat moves it creates 2 waves and a through. First, waves are small and through is small. Small momentum stability is gained. The faster the boat goes, the bigger the waves, the bigger the through. The faster the boat moves the greater the momentum force forward, and the harder it is to tip her as you need to overcome 2 forces and 1 of them is gaining. It is a progression. It is not linear. So that fast speed at close to max hull speed produces so much stability that "lost" beam has no meaning. Hence you can take a "boat" that is 1 feet wide, 2 feet tall, and 24 feet long and push her on the water, she will actually go forward, but if you only put her still and let go, she will tip over. That's why some boats are 2.4 and some race boats are 5.0. They both have stability. Race boats at speed are stable. Even though they "lost" half the beam. It's the speed that gives you everything. Push a very drunk guy forward who can't stand on his feet. Momentum force is stronger.

More, the "through", is no magic either, it is water displacement, water from the middle moved away to the Bow and Stern. So beam lost in the middle is gained at the ends. So one could even made up an argument initial stability is gained if he wanted to. And since no one can calculate it, you can walk around and preach it.

[Don C L]

What is your definition of "stability?"

[NewAlexandria]

Originally Posted by Don C L

What is your definition of "stability?"

I don't have it, I don't fully understand it yet, and I can't calculate anything in it. I did not get there yet.
Chances are my stability understanding is too simple, and there never will be a way to calculate it. I just want to find the compromise mathematically. The Length to Beam that is. The rest I will simply use what others have used in the past.

I'm doing this on a backburner, reading like 10 pages per day and drawing.

[Don C L]

Have you read "Sail Performance" by Marchaj? I highly recommend it. I think you'll enjoy it. It may have all the math you are looking for to help you.

[NewAlexandria]

There is more to stability like center of gravity, center of bouncy, center of gravity to the base of the boat, but I'm not including that in this.