LadyInBed
Well-Known Member
Is the waterline length measured down the centre line or round the bend of the hull?
<hr width=100% size=1>
<hr width=100% size=1>
Waterline Length
- Thread starter LadyInBed
- Start date
qsiv
Well-Known Member
Centreline - unless you're interested in sailing length - in which case you take the distance between perpendiculars of the inclined waterplane for the angle of heel you're interested in.
<hr width=100% size=1>
<hr width=100% size=1>
bigmart
Well-Known Member
I think you'll find that the waterline lenght is measured around the waterline. This is often used as a design feature on yachts where the waterline length increases as the boat heels & thus increases the potential boat speed.
Martin
<hr width=100% size=1>
Martin
<hr width=100% size=1>
qsiv
Well-Known Member
No - it is the centreline distance between the points where the hull enters / leaves the water.
The reason for this is that the wavemaking limit on hull speed is a function of the interaction between the wave created by the bow cleaving the water and the that of the stern. When the wavelength is exactly equal to the LWL the boat is said to have recahed hull speed. If speed increases the aft peak moves aft of the stern, and therefore the boat will start to trim up (hence the huge increase in power to exceed hull speed, as you are effectively pushing the boat up a slope of water, and also why the stern squats).
As speed increases further, and provided there is enough power, and the hullform can generate lift, the boat will start to plane. As the boat rises fully onto the plane, the boat will trim down (in the absence of complicating hydrodynamic effects), and as a result a slight decrease in power is noted, to maintain planing speed. This is best seen in an inflatable - the boat trims up aggresively as you exceed hull speed, but if you can get up on the plane you can usually back off the throttle considerably.
The increase in waterline length you refer to is simply that the length between the immersed bow and the point where the aft buttocks exit the water is physically longer when the boat is heeled than upright, and not specifically due to the rounded hull shape.
<hr width=100% size=1>
The reason for this is that the wavemaking limit on hull speed is a function of the interaction between the wave created by the bow cleaving the water and the that of the stern. When the wavelength is exactly equal to the LWL the boat is said to have recahed hull speed. If speed increases the aft peak moves aft of the stern, and therefore the boat will start to trim up (hence the huge increase in power to exceed hull speed, as you are effectively pushing the boat up a slope of water, and also why the stern squats).
As speed increases further, and provided there is enough power, and the hullform can generate lift, the boat will start to plane. As the boat rises fully onto the plane, the boat will trim down (in the absence of complicating hydrodynamic effects), and as a result a slight decrease in power is noted, to maintain planing speed. This is best seen in an inflatable - the boat trims up aggresively as you exceed hull speed, but if you can get up on the plane you can usually back off the throttle considerably.
The increase in waterline length you refer to is simply that the length between the immersed bow and the point where the aft buttocks exit the water is physically longer when the boat is heeled than upright, and not specifically due to the rounded hull shape.
<hr width=100% size=1>
bigmart
Well-Known Member
I bow to your obviosly greater understanding of the subject. The point I was trying to make was that, as I thought I had read, designers used the rounded hull shape to increase the waterline length in certain racing classes. This is done presumably to squeeze every advantage, that one can, from the constraints of the class regulations, as the longer waterline length increases the displacement speed.
I am not a racer or a speed sailor so I declare my ignorance of subject here for all to see.
Martin
<hr width=100% size=1>
I am not a racer or a speed sailor so I declare my ignorance of subject here for all to see.
Martin
<hr width=100% size=1>
LadyInBed
Well-Known Member
Have I got this right?
Am I right in thinking that the centreline length would be given in the specifications for the boat, but the dynamic length round the bulge, at any particular angle of heel, should be used for calculating the hull speed?
<hr width=100% size=1>
Am I right in thinking that the centreline length would be given in the specifications for the boat, but the dynamic length round the bulge, at any particular angle of heel, should be used for calculating the hull speed?
<hr width=100% size=1>
qsiv
Well-Known Member
It isnt really the 'rounded length' that they are using, it is the fact that the distance between the 'bow' and the 'aftermost part of the boat that is immersed' changes as the boat heels.
This is less true with more modern race boats that tend to have plumb bows and transom sterns. If, however, you consider an old boat (perhaps a nice Fife!), with long bow overhangs, and even longer stern overhangs, then interesting things begin to happen.
In particular, when heeled and sailing at any speed, the stern wave moves adt along the stern overhang. At the same time the bow tends to bury a bit as well. My fathers Edwardian racing boat was about 60 foot on deck, yet at rest had a waterline length of 'only' 37 feet (Hull speed 8.2kts). We have photos of her fully powered up, with the stern wave sucked right up to the bitter end of the counter - sailing length about 57 feet(Hullspeed 10.2 kts). The fact that the stern wave is being lifted up and along the counter obviously requires power (as water is being raised), but providing the rig develops enough power the resultant 25% gain in hull speed is worthwhile.
To a lesser extent this benefitsall modern boats where the transom is not immersed (and is why rating rules such as IRC) are very interested in the extent and shape of stern (and bow) overhangs, and taxes them as benefits.
<hr width=100% size=1>
This is less true with more modern race boats that tend to have plumb bows and transom sterns. If, however, you consider an old boat (perhaps a nice Fife!), with long bow overhangs, and even longer stern overhangs, then interesting things begin to happen.
In particular, when heeled and sailing at any speed, the stern wave moves adt along the stern overhang. At the same time the bow tends to bury a bit as well. My fathers Edwardian racing boat was about 60 foot on deck, yet at rest had a waterline length of 'only' 37 feet (Hull speed 8.2kts). We have photos of her fully powered up, with the stern wave sucked right up to the bitter end of the counter - sailing length about 57 feet(Hullspeed 10.2 kts). The fact that the stern wave is being lifted up and along the counter obviously requires power (as water is being raised), but providing the rig develops enough power the resultant 25% gain in hull speed is worthwhile.
To a lesser extent this benefitsall modern boats where the transom is not immersed (and is why rating rules such as IRC) are very interested in the extent and shape of stern (and bow) overhangs, and taxes them as benefits.
<hr width=100% size=1>
Aja
Well-Known Member
Martin
For more information you could try a 'Google' search on metracentricity. This design feature, greatly used by Robert Clark, aimed to create the 'same' hull shape in the water no matter the angle of heel.
Yachts designed with this feature lacked increasing weather helm as the profile of the heeled yacht remained the same.
Regards
Donald
<hr width=100% size=1>
For more information you could try a 'Google' search on metracentricity. This design feature, greatly used by Robert Clark, aimed to create the 'same' hull shape in the water no matter the angle of heel.
Yachts designed with this feature lacked increasing weather helm as the profile of the heeled yacht remained the same.
Regards
Donald
<hr width=100% size=1>
bigmart
Well-Known Member
Thanks I'll do that. Mind you I wouldn't have been brave enough to try before for fear of tying a reef knot in my tongue with a word like that. Definitely not something to try & say after a shot or two of falling down water.
Martin
<hr width=100% size=1>
Martin
<hr width=100% size=1>
Aja
Well-Known Member
Martin
Don't rely on my spelling!!! Just tried a search and didn't come with anything!
Donald
<hr width=100% size=1>
Don't rely on my spelling!!! Just tried a search and didn't come with anything!
Donald
<hr width=100% size=1>
bigmart
Well-Known Member
Me too. I will keep trying alternative spellings & search engines.
Martin
<hr width=100% size=1>
Martin
<hr width=100% size=1>
bigmart
Well-Known Member
Generally I do not indulge in intellectual thought about yacht design because I'm just too thick. You comments about Metracentricity got me thinking (always dangerous) What do they do with the keel? & doesn't that affect the handling as the boat heels?
Moving on from that. I have seen reports of these new jobs with swing keels that pivot sideways to increase the righting moment of the ballast. How would this layout figure in a Stix rating? bearing in mind it is always moving depending on the point of sail/conditions & how would you calculate the AVS in a meaningful way to compare with a conventional setup?
I must admit the thought of screwing up yet another EEC constuction edict is quite appealing.
Martin
<hr width=100% size=1>
Moving on from that. I have seen reports of these new jobs with swing keels that pivot sideways to increase the righting moment of the ballast. How would this layout figure in a Stix rating? bearing in mind it is always moving depending on the point of sail/conditions & how would you calculate the AVS in a meaningful way to compare with a conventional setup?
I must admit the thought of screwing up yet another EEC constuction edict is quite appealing.
Martin
<hr width=100% size=1>
peterb
Well-Known Member
Metacentric shelf
Try "Metacentric shelf", which is what the theory was known as. I just did, and got 119 responses.
<hr width=100% size=1>
Try "Metacentric shelf", which is what the theory was known as. I just did, and got 119 responses.
<hr width=100% size=1>
davidhand
Well-Known Member
qsiv is correct except that in the days of IOR boats they were designed with relatively long overhangs so that the bow and stern waves effectively increased the waterline length of the boat when in motion. But the rated waterline length was measured with the boat stationary hence you were legally cheating.
<hr width=100% size=1>
<hr width=100% size=1>
