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The theoretical hull speed is just that-theoretical. One meter hulls can go substantially faster than that-without planning…
MORE: I’ve heard claims of 4-6 knots for "displacement " mono’s. It seems to me that the beam to length ratio is key: if the boat falls into a certain range then it will easily exceed hull speed whereas wider boats with the same sail area might squat. But as the boat gets narrower the tendency to pitchpole gets greater. Beam to length ratios of around 9/1 seem easily able to adapt to more power as long as the pitchpole tendency is addressed…The Bantock designed F100 is PREDICTED to exceed hull speed substantially and has a beam to length(wl) of 7/1 …
If a class has a very heavy displacement for its size beam will make less difference; the lighter a boat gets the narrower for greatest speed.
Doug Lord
–High Technology Sailing/Racing
Doug,
Where does the " Theoretical Hull Speed " come from then?. Why is it so often quoted, or was it formulated in the 1800s and is now hopelessly out dated?.
I have seen figures stateing it could be exeeded by as much as 50%, but how?.
If a Formula is 50% wrong, a new one is needed.
What formula do “the big Boys” use, if we can beat ours by “substantially faster” - -we should all be making Rockets!!.
Why, if a Hull can go faster than it should be able to, are The Rules of every Yacht Class in the World regulating Water Line Length?.
A VERY FAST 24" boat will not beat a One metre, no matter what is done to it!.
So why is the Rule wrong?.
OR, are we all fooling our selves?.
John.
Doug,
I KNOW you read my second Post, but you Edited your first one, to dodge the questions and points I raised in my second, but the points and questions are still there, so some-one will answer them.
Water Lines are the most rigidly enforced rule of Yachting, always have been.
IF a “Plank on Edge” so beloved of our Grandfathers, is SO superior to a conventional hull, where are they all?.
Or do the Computer Programs not accept them?.
John.
Theoretical speed limits sometimes come close but most times have little relevence.
Most sailing multihulls are displacement hulls but because of a combination of power to carry sail, narrow beam and light weight routinely go 4 or over times the so-called theoretical speed.
If you reduce beam, increase power to carry sail and reduce weight you will exceed “hull speed”. Then you have to worry about how not to pitchpole if the boat is a model…
Doug Lord
–High Technology Sailing/Racing
Doug,
Be carefull of what you say,ALL my posts are about a hyperthetical boat that is one metre long, conventional construction, remains the Same weight,etc.
I DID NOT MENTION a MULTIHULL !!!.
Go back and READ my post.
“IF, a One Metre Displacement Hull has a theoretical maximum forward speed, through the water of approx 2.2 knots, is it possible to increase this speed - -using the SAME Hull and by what approx percentage?.”
OK?
I know a Multi can beat a MONO.
I have been sailing One Metre boats for over 9 Years now.
Do try and stick to the Questions posted.
IF, you must answer my posts, don’t edit a previous one to try and get the correct reply, post a new one like every one else does.
John.
Mr. Jay Dee Sir:
- I did not edit my post to try to get the answer right-I thought of additional information that might help you which I added prior to seeing your second post.
- I was trying to give a frame of reference that you could use to determine the answer to the question you posed in your first post.
- I did not question your knowledge or lackthereof regardng multihulls ;I was simply reinforcing my point that a theoretical hull speed is largely meaningless depending on cetain factors. The point I was trying to get across was that you could get an answer to your question by considering the points I made relative to beam to length ratio power to carry sail and weight. By looking at the hull in your first post and determining its beam to length ratio you could see by comparision whether or not it was likely to increase speed if more power was applied or not.
There are a series ofcomplex relationships tat determine the answer to you question; I was trying to help you see them and apply them to the problem you posed…
Doug Lord
–High Technology Sailing/Racing
John
I think the drag force on an object goes up at the square of velocity (in air??? anyone…?) and so twice the speed needs four times the power. This changes a bit when a hull is in water since some hulls (especially displacement hulls) can have more drag at low speeds than they do at higher speeds. So can you get 10% more speed with 20% more sail area? I’d guess no. 20%more sail are needs more righting moment which needs more draft (foil drag) or more displacement (hull drag) Its kind of a catch 22. The question of why waterline length based rules reminds me of when an old lecturer once told me that all (generic) hulls do a boat length every 8 seconds (roughly) So if you can do 41’ every 8 seconds you’ll beat the guy who does 40’ every 8 seconds. I was taught this a while ago though (by an old guy!) and add things like ULDBs and skiffs and multihulls and all the other things into the mix and it quickly gets easier to find the exceptions rather than the rule.
One other thing, stick the fastest boats into some serious waves going upwind offshore and watch what happens. Big usually boat wins.
Just my two cents…
Troy
Tranth
The Squared thing is just an aproximation, A slightly more complicated form ads in a linear component. Bouth can be off allot depending on the shape of the object.
-Dan
I can not tell you this for sure. because i am not an expert. but from what I know. (my experience) a long waterline creates less( from drag my langaugea) for every hull. no matter what size and weight you will get 3 forms of drag. forst the drag from the hull entering the water. this is counterd buy a sharp piont. but there is still some restance there. the second. i call quarter waves. the water seperating for the hull. basicaly the hull pushing the water out. then the sterm wake . sucking your hull back. with a long waterline you can seperate these forces. and lessen the impact. you still have them but not all at once.
this is my idea and should not be considerd fact.
cougar
JayDee, Cougar, et. al.
The theoretical hull speed limit is set by wave drag. As a displacement hull passes through the water is displaces the water. This creates a wave of water moving away from the boat at the bow and back toward the boat at the stern. Since these waves are created on the free surface of the water, they create sympathetic waves at a fixed wavelength from the primary wave. Thus you will have a set of following waves behind the boat.
Now, since those waves travel at a fixed speed through the water, and since the source of those waves (the boat) is moving, the waves form a V shape angled back (V shaped wake behind the boat). The faster the boat moves, the narrower the V becomes. At a certain speed, the first sympathetic wave behind the bow wave will line up with the primary stern wave and the first sympathetic wave from the stern wave will line up with the bow wave. When this happens the waves will add to each other creating a much taller bow wave and a much taller stern wave with a trough in between. The boat is caught in the trough and in order to break out of that trough you need a lot of power to climb that bow wave.
The longer the waterline lenght of the boat, the more distance there is between the bow wave and the stern wave and the higher the speed you can attain before the V gets narrow enough to cause the bow and stern waves to add.
Here is a picture of my fairwind showing the bow and stern waves and the trough in between:
Download Attachment: 
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So that is where the theoretical hull speed comes from and why it is related to length. So how can we sail faster than this?
One way is to simply climb over the wave and start riding it. This is of course the classic planing.
but most displacement boats do not have that luxury. Displacement boats can however be designed to go faster than this theoretical hull speed. The trick lies in how big the waves are that the boat creates as it goes through the water. Since the waves themselves are related to the displacement of the boat, the more a boat displaces, the bigger the wave it will create. So a lighter boat for the same lenght will have relatively smaller waves. Since most RC boats have very deep drafts (at least as compared to their full sized cousins) they can create the righting moment they need with less weight.
But there are other tricks that designers play to come up with fast boats. The shape of the wave can be smothed out by the displacement distribution of the hull. In other words how much water is displaced in the first 10% of the hull length, the second 10%, etc. You can see that the bow wave on my Fairwind actually starts a good deal back from the bow. This is because the hull shape for that boat displaces a lot of water right at the keel and not much near the bow and stern. If you can smooth out this displacement function by displacing more water near the ends of the boat, you will get smaller and more spread out waves that are easier to climb out of. In fact those waves might be so small that you cannot even see them.
- Will
Will Gorgen
<blockquote id=“quote”><font size=“1” face=“Verdana, Arial, Helvetica” id=“quote”>quote:<hr height=“1” noshade id=“quote”> If you can smooth out this displacement function by displacing more water near the ends of the boat, you will get smaller and more spread out waves that are easier to climb out of. In fact those waves might be so small that you cannot even see them.
- Will
Will Gorgen
<hr height=“1” noshade id=“quote”></blockquote id=“quote”></font id=“quote”>
Will
This seems to indicate that a Spanish Galleon would be fast. There must be a trade-off to a fat bow and stern?
Thanks
Don
Vancouver Island
Don,
Everything is indeed a tradeoff. By increasing wetted surface area, you add drag. But if everything were simply related to waterline, then there would be no need for naval archetects. Understanding the tradeoffs is what keeps those guys in business…
And with respect to the Spanish Galleon, their speed was indeed slow but it was not limited by this wave drag mechanism. For a 200’ long square rigger, the theoretical wave speed would be 19 knots. But since they basically ran downwind (in the trade winds each direction) they would need winds significantly stronger than 20 knots to even have a chance of hitting that hull speed, much less going faster than it. I’m not sure what a typical speed for a galleon was, but I think it was significantly faster than the 7 knots that your average 30 foot keel boat is capable of…
- Will
Will Gorgen
Will,
I agree with you that the designers have ways and means to get around wave making.
But a Hull is moving through water, the water has to be displaced, that displaced water is stopping the Hull from moving.
How the water is displaced whether at the Bow, Mid or Stern, gradually or all at once, it still Has to be moved.
The Length of the hull seems to have more effect than anything else on how easily the water is moved.
Is THIS why the length of a boat is so closely regulated.
At a One metre championship I heard of, a VERY famous skipper had to sand down his Bow bumper a few millimetres because the boat was too long, and the bow bumper was NOT in the water!!.
Petty maybe, but it shows how the rules look at boat length.
The “Speed” of a One Metre is given as 2.452 knots, to increase the “Speed” to 3 knots needs a hull to be Five Feet Long, now just try getting that past the Committee!!!.
Yes, I know I am using the theoretical speeds, but just look at the increase of Hull length, to get 1/2 knot increase in speed.
John.
<blockquote id=“quote”><font size=“1” face=“Verdana, Arial, Helvetica” id=“quote”>quote:<hr height=“1” noshade id=“quote”>
How the water is displaced whether at the Bow, Mid or Stern, gradually or all at once, it still Has to be moved.<hr height=“1” noshade id=“quote”></blockquote id=“quote”></font id=“quote”>
I was thinking about this a while ago. Does it make any difference how far the water has to be displaced. For example if we have two boats of equal displacement(say 10 lbs.) and one is flat bottomed and wide. Say it only draws 1" of water. In rough terms this boat moves 10 lbs. of water but it only has to move it 1". The other hull is round and draws 3" of water. This one has to move some of the water over 3 times as far. Does this make a difference or does it all equal out in the wash. I think I can see where the shallow boat has to move MOST of its water 1" and the deep boat only has to move SOME of its water 3" but I thought I would ask anyway. I think hulls are way more complex than sails.[:D]
Thanks
Don
Vancouver Island
Isn’t this part of the difference between a planing hull and a displacement hull?
At planing speed, the flatter bottom hull doesn’t displace (or drag thru the water) as much as a displacement hull - thus a possibly faster speed?
The boat that with only 1" draft will have to be a lot wider at the water line. If the 3" (1/2 circle) has area of 14 sq. in. the 1" deep boat would have to be 14" wide. The water is moved sideways not downward creating waves. May help if you were planing. Would be slow pushing a lot of water otherwise. Surface area(circumference) would be 9.4 for 3" and 15-16 for 1"
Hello All,
From what I have read, but not understood all of it, a Planeing hull is almost flying, but the point BEHIND the boat, where the waves come back together, is its “Hull” Length!!.
That length is the limiting factor in how fast the boat will go.
To further increase its speed requires a lot more horse power, or, a Hull redesign.
Not a wider, or more of a V shape, but a different shape to go faster and fly further.
You must have see the Sport Fishermen, in their boats, fishing for Marlin etc, “shove a bigger engine in Bud, make it go faster!”.
All that happens is the Bow comes up a lot more than it used to do, and it looks like they are sailing up a hill!.
Hull speed wins again.
Our boats don’t plane, if they did we would all speed up, claims of this or that increases of speed need to be measured, accurately.
Just saying that doing XXX will speed up a boat has to be checked and verified.
We need someone with a least a Phd to fire some questions at - -and then have good think about it!!.
John.
<blockquote id=“quote”><font size=“1” face=“Verdana, Arial, Helvetica” id=“quote”>quote:<hr height=“1” noshade id=“quote”>Originally posted by hoj
The boat that with only 1" draft will have to be a lot wider at the water line. If the 3" (1/2 circle) has area of 14 sq. in. the 1" deep boat would have to be 14" wide. The water is moved sideways not downward creating waves. May help if you were planing. Would be slow pushing a lot of water otherwise. Surface area(circumference) would be 9.4 for 3" and 15-16 for 1"
<hr height=“1” noshade id=“quote”></blockquote id=“quote”></font id=“quote”>
O.K. Lets make it 1" wide and 14" deep![:D]
Thanks
Don
Vancouver Island
As far as a planing hull a hydroplane racer proved to me flatter is better. He attached a motor to an upside down table and it really flew. But a displacement hull is circles, circles, circles.