What is "Hull Speed" Anyway

[Dockhead]

Originally Posted by Polux

As it was said lighter boats can not only reach more easily hull speed as they can somewhat bigger Hull Speed than what is given by the traditional method. It should be used a bigger factor than the one that is used for a heavy or medium weigh and this without really planing. I have some papers about that but don't know if I can find them or if I put them on the garbage

For having big 24 hours sustained speeds you have to sail on the trade winds, out of that the winds are normally too variable to allow a very high 24 hour mileage.

Even so on a Bavaria 36 I made a 7.3kt average speed between Minorca and Sardinia. Hull speed measured by the traditional way is 7.4kt.

However on the first 2 or 3 hours and on the last 3 hours I had weaker winds and did not sail at over 6.5kt sometimes less. During the night I made during several hours a speed between 9 and 10kt sometimes going over 11kt, near 12.

Note that I was not using a spinnaker and I was sailing alone with my 14 year old son...and when I went to sleep for a while, much against my son's will, I reefed abit more the genoa diminishing the speed to between 8 and 9kt. If I had a more experienced sailor than my son I would not have needed reefing and the average speed would be superior.

With the Comet 41s with my wife aboard I have to sail always in a very defensive way (not to get divorced), going less faster than what I would do if I was sailing alone, even so this year between Samos and the little Cyclades I averaged 8.3k on a boat with 8.0 hull speed (by the traditional method) and the final two hours or so where sailed doing about 7.5kt.

On other occasion (this year) I got a blow near Hydra and just with a very small piece of genoa out made during an hour or so between 11 and 12.5K with a nasty sea with waves, in complete safety. I could have gone way faster but my wife was nervous about that. (35 to 45k wind). If the wind had lasted I surely could have maintained that pace for many hours.

Some people would call these stories "outlandish", but it all sounds right to me. Your boat is similar to mine in D/L, and my experiences are much the same as yours, although you have gone much faster than I have, in terms of knots above hull speed.

A key question in surfing at such speeds must be rudder control. Some boats threaten a broach, and you just don't want to be at those speeds. My boat has an extremely large, like garage door sized, semi-balanced rudder (a full spade would be better), which is certainly causing unnecessary drag at normal speeds, but I sure am happy to have it in such cases.

[Dockhead]

Originally Posted by Polux

. . .For having big 24 hours sustained speeds you have to sail on the trade winds, out of that the winds are normally too variable to allow a very high 24 hour mileage.. . .

Well, it depends on the latitude. Up here above 50N we have weeks at a time with 20+ knots of wind.

One of the delightful things about Channel sailing, is that the prevailing wind runs ALONG the Channel, so crossing the Channel is usually a BEAM REACH . With 20+ knots of wind . So we get to a lot exhilarating, hard, fast sailing. Come join us sometime.

[Polux]

Originally Posted by Dockhead

Some people would call these stories "outlandish", but it all sounds right to me. Your boat is similar to mine in D/L, and my experiences are much the same as yours, although you have gone much faster than I have, in terms of knots above hull speed.

A key question in surfing at such speeds must be rudder control. Some boats threaten a broach, and you just don't want to be at those speeds. My boat has an extremely large, like garage door sized, semi-balanced rudder (a full spade would be better), which is certainly causing unnecessary drag at normal speeds, but I sure am happy to have it in such cases.

Not quite in what regards D/L, yours is 201.4 mine is 172.18 and the one of Kenomac's boat is 226.29.

28.6 points between my boat and yours, 24.89 between your's and Kenomac's boat.

The "sail power" of my boat is (proportionally) also much bigger than your's (the data on sailboat data is wrong about the sail area).

I found out the papers that establish a hull speed in function of D/L by David Gerr and some very interesting comments about it, but it's late here, I will post it tomorrow, if I have time.

And by the way, that old formula for hull speed is really old, it was made by Anthony Deane in 1670 to calculate the speed of British Man O'war. A notable feat!!!

[Dockhead]

Originally Posted by Polux

Not quite in what regards D/L, yours is 201.4 mine is 172.18 and the one of Kenomac's boat is 226.29.

28.6 points between my boat and yours, 24.89 between your's and Kenomac's boat.

The "sail power" of my boat is (proportionally) also much bigger than your's (the data on sailboat data is wrong about the sail area).

I found out the papers that establish a hull speed in function of D/L by David Gerr and some very interesting comments about it, but it's late here, I will post it tomorrow, if I have time.

And by the way, that old formula for hull speed is really old, it was made by Anthony Deane in 1670 to calculate the speed of British Man O'war. A notable feat!!!

Wow, that's something new -- 1670!!

But you have wrong information about my D/L -- it's 188, based on the as-built survey, certified by the designer, Bill Dixon.

What you are right about, however, is SA/D, which is 16.5 on my boat, quite a bit less than yours. A compact rig made for these latitudes where we have a large percentage of days over 20 knots of wind. In light wind, say in normal Med conditions, your boat will be much faster than mine, but up here, I will run away from you, as you will have to reef.

[44'cruisingcat]

Originally Posted by Dockhead

What you are right about, however, is SA/D, which is 16.5 on my boat, quite a bit less than yours. A compact rig made for these latitudes where we have a large percentage of days over 20 knots of wind. In light wind, say in normal Med conditions, your boat will be much faster than mine, but up here, I will run away from you, as you will have to reef.

That would depend upon where the reefing points are. It's possible that a boat with a reef in still has a higher SA/D.

Just for fun I ran the numbers for our boat.

D/L = 79, SA/D = 28.

[El Pinguino]

I was just sitting here thinking...sometimes I sits and thinks ... other times I just sits.....

I've said it and I think others have as well..... 'There we were... scooting along at a knot or two above theoretical hull speed ...flat water... just a one metre sea on the quarter...' .

Actually I would really just say 'Jeez we wuz flying... you should have seen us...'.

Moving along... a one metre sea and a 12 metre boat ? ... that is not a flat sea. It is probably enough to modify your bow wave enough so that you are effectively surfing or at the very least the 'flat water rules' no longer apply....

Hull speed for 24 hours? I dream of stuff like that....

[storyinframes]

I was on a 25 foot boat and she did 11 kn off Scarba and Luing. Many people tried to convince me that it was tides and not your yanmar.

[Polux]

Originally Posted by Dockhead

..
But you have wrong information about my D/L -- it's 188, based on the as-built survey, certified by the designer, Bill Dixon.

From 201.4 to 188 goes a huge diference. Your boat displacement on the RCD shipyard data has a displacement of 44132 lbs and a 46.08'LWL

Obviously the LWL is the same for all the Moody 54 regarding the weight for having that D/L the boat would have to weight less 3000 pounds. Your boat for the way it is built is already a relatively light boat and unless your boat was built in another way, namely using vacuum infusion processes, epoxy resins in conjunction with carbon spars and rudder there is no way to take 3000 pounds from the standard boat.

What probably happened is that a different weight was used by Bill Dixon and not the RCD weight that regards minimum sailing conditions, the one that is normally used for determination of the SA/D, at least in Europe. There are several other weights that can be considered on a sailboat (light weight) and probably was one of those that Bill Dixon considered for the D/L.

Originally Posted by Dockhead

What you are right about, however, is SA/D, which is 16.5 on my boat, quite a bit less than yours. A compact rig made for these latitudes where we have a large percentage of days over 20 knots of wind. In light wind, say in normal Med conditions, your boat will be much faster than mine, but up here, I will run away from you, as you will have to reef.

I am not interested in discussing this again with you. Last time you were adamant regarding LWL to be factor to be considered in what regards speed.

Now you seem to have changed of opinion considering that after all D/L is important in what regards hull speed and boat speed.

I don't know in what conditions my boat will be faster or slower than yours but the best indication to the speed potential of a boat is the IRC or ORC handicap numbers and the one on my boat is not smaller than the one on yours. I have many experiences regarding sailing faster than much bigger boats, less performance oriented than mine but more than yours.

If you really like to sail fast you should consider other type of cruising boat, like this one for instance:
JPK 45, l’appel du large

[Dockhead]

Originally Posted by 44'cruisingcat

That would depend upon where the reefing points are. It's possible that a boat with a reef in still has a higher SA/D.

Just for fun I ran the numbers for our boat.

D/L = 79, SA/D = 28.

Are you trying to give me cat envy again?

The lack of ballast means you have a different relationship with the water.

As to SA/D -- a reefed sail of x m2 of area is not equivalent to an unreefed sail of the same area -- not even close. Especially a headsail.

[dcstrng]

Originally Posted by billgewater

My two bob's worth.... So there.
Merry Xmas.

Thanks -- you just made Hammitt understandable for a layman... Gets my vote !!

[Dockhead]

Originally Posted by Polux

From 201.4 to 188 goes a huge diference. Your boat displacement on the RCD shipyard data has a displacement of 44132 lbs and a 46.08'LWL

Obviously the LWL is the same for all the Moody 54 regarding the weight for having that D/L the boat would have to weight less 3000 pounds. Your boat for the way it is built is already a relatively light boat and unless your boat was built in another way, namely using vacuum infusion processes, epoxy resins in conjunction with carbon spars and rudder there is no way to take 3000 pounds from the standard boat.

What probably happened is that a different weight was used by Bill Dixon and not the RCD weight that regards minimum sailing conditions, the one that is normally used for determination of the SA/D, at least in Europe. There are several other weights that can be considered on a sailboat (light weight) and probably was one of those that Bill Dixon considered for the D/L.



I am not interested in discussing this again with you. Last time you were adamant regarding LWL to be factor to be considered in what regards speed.

Now you seem to have changed of opinion considering that after all D/L is important in what regards hull speed and boat speed.

I don't know in what conditions my boat will be faster or slower than yours but the best indication to the speed potential of a boat is the IRC or ORC handicap numbers and the one on my boat is not smaller than the one on yours. I have many experiences regarding sailing faster than much bigger boats, less performance oriented than mine but more than yours.

If you really like to sail fast you should consider other type of cruising boat, like this one for instance:
JPK 45, l’appel du large

I am also not interested really in arguing with you, particularly since I don't have any serious disagreements with you.

Yes, LWL is very important, for reasons we discussed last time (M54 is 47.1 not 46.08). But D/L is also extremely important, and as we were discussing in this thread, the whole concept of "hull speed" changes with different D/L. Both of these metrics are really important to speed, because both of them together is what determines resistance and therefore how much power is needed, given similar hull form.

And as to cruising faster -- it seems that you and I have a similar passion for speed -- yes, a different boat could be better, and I am planning to change the boat pretty soon. I seriously considered a Chris White Atlantic 57 catamaran, but I finally settled on custom building something out of aluminum. I will not go the way of large form stability (as in Pogos or other boats you like), but rather more the Dashew way, which is a much longer and lighter boat, with a rather fine hull (I seem to recall you also like this type of boat). Dashew's Beowulf is very close to my ideal boat; only just a little large; I want to stay within about 20 meters.

The choice of this type of boat rather than that is really just down to the kind of sailing -- long distances and high latitudes make you want something easier to manage -- something you can sail from the pilothouse -- and up here berthing is cheap and a longer boat is not really a problem. If I were sailing in the Med, I might prefer a Pogo-type boat. Or the catamaran. Probably the cat, actually.

[Polux]

So, about hull speed we could consider 3 cases, displacement hulls, semi displacement hulls and planing hulls. Sailing boats with a D/L over 240 are considered displacement boats were the traditional formula will prove approximatedly correct.

Regarding planing hulls in what regards cruising with boats loaded and all, to the generality of sail cruisers does not make much sense to talk about it, even if in some rare exceptions, giving the right conditions, some can plan, but they are still a very small minority, all of them performance cruisers.

You know that a boat is sailing at semi displacement speed when the crest of the stern wave is aft the transom. You know that a boat is sailing at planing speeds when the stern wave flattens out.

Dave Gerr explains:

"My hull speed formula has proven quite accurate over the years and across many types of boats. The old rule-of-thumb "simply 1.34x the square root of WL in feet" really isn't accurate. The multiplier "1.34" is really a function of D/L ratio. That's what my formula is all about."

As Gere explains the diference reside on the 1.34 factor, that is a fixed one for heavy boats but that varies for medium and light sailing boats, the lighter the bigger will be the factor.

The factor is called Speed Length ratio (S/L) and it is calculated like this:

S/L=8.26/(D/L ratio)^0.311

Then... Hull Speed = S/L*LWL^0.5

(LWL^0.5 is equal to the square root of LWL)

Anyway there is no better way to judge the speed potential of a sailboat than to look at one of the two accurate handicap ratings, IRC or ORC, taking into consideration that the boat has to have the same draft.

Only if you cannot find information about that, this formula will give some information even if not so complete or global as the one expressed by a complete handicap rating. Note that for instance on ORC there are two different ratings one for inshore waters and other for offshore waters that has in account the difference in sea conditions.

Here a ORC certificate for a XP44 (there was one making a circumnavigation with the ARC):

http://www.regatacopadelrey.com/insc...dos/lat909.pdf

You can see that the XP44 is able to sail at over traditional hull speed (8.37) with as little as 10kt of wind and that with 20k wind can go 2.5k over wind speed. With more than 20k the boat will enter in planing mode going many knots over wind speed.

As I tried to explain once, without much success, it is not so much the ability to plane with high wind speeds that makes this type of performance cruisers much faster than non performance cruisers (even bigger ones) but this ability to go one or two kt over theoretical hull speed with easiness and not much wind.

It may not look that much but the 1.1kt over hull speed that the XP44 can make with only 14kt of wind will give it the same hull speed of a Hylas 56 and probably the Hylas cannot sail at hull speed with 14k of wind.

A much bigger difference if we consider stronger winds, with 20k winds the Xp44 can go 0.3kt faster than the hull speed of a Hylas 70.

With bigger wind speeds the Xp44 will start to plane and the difference will become huge but those are conditions less frequent than weaker winds.

I hope all this as contributed to a better understanding of our sailboats. If not I have just lost a lot of time an effort LOL.

It is time for me to prepare our family gathering, so I will not be around for a while.

Wishing all a great Christmas.

Paulo

[Dockhead]

Originally Posted by Polux

So, about hull speed we could consider 3 cases, displacement hulls, semi displacement hulls and planing hulls. Sailing boats with a D/L over 240 are considered displacement boats were the traditional formula will prove approximatedly correct.

Regarding planing hulls in what regards cruising with boats loaded and all, to the generality of sail cruisers does not make much sense to talk about it, even if in some rare exceptions, giving the right conditions, some can plan, but they are still a very small minority, all of them performance cruisers.

You know that a boat is sailing at semi displacement speed when the crest of the stern wave is aft the transom. You know that a boat is sailing at planing speeds when the stern wave flattens out.

Dave Gerr explains:

"My hull speed formula has proven quite accurate over the years and across many types of boats. The old rule-of-thumb "simply 1.34x the square root of WL in feet" really isn't accurate. The multiplier "1.34" is really a function of D/L ratio. That's what my formula is all about."

As Gere explains the diference reside on the 1.34 factor, that is a fixed one for heavy boats but that varies for medium and light sailing boats, the lighter the bigger will be the factor.

The factor is called Speed Length ratio (S/L) and it is calculated like this:

S/L=8.26/(D/L ratio)^0.311

Then... Hull Speed = S/L*LWL^0.5

(LWL^0.5 is equal to the square root of LWL)

Anyway there is no better way to judge the speed potential of a sailboat than to look at one of the two accurate handicap ratings, IRC or ORC, taking into consideration that the boat has to have the same draft.

Only if you cannot find information about that, this formula will give some information even if not so complete or global as the one expressed by a complete handicap rating. Note that for instance on ORC there are two different ratings one for inshore waters and other for offshore waters that has in account the difference in sea conditions.

Here a ORC certificate for a XP44 (there was one making a circumnavigation with the ARC):

http://www.regatacopadelrey.com/insc...dos/lat909.pdf

You can see that the XP44 is able to sail at over traditional hull speed (8.37) with as little as 10kt of wind and that with 20k wind can go 2.5k over wind speed. With more than 20k the boat will enter in planing mode going many knots over wind speed.

As I tried to explain once, without much success, it is not so much the ability to plane with high wind speeds that makes this type of performance cruisers much faster than non performance cruisers (even bigger ones) but this ability to go one or two kt over theoretical hull speed with easiness and not much wind.

It may not look that much but the 1.1kt over hull speed that the XP44 can make with only 14kt of wind will give it the same hull speed of a Hylas 56 and probably the Hylas cannot sail at hull speed with 14k of wind.

A much bigger difference if we consider stronger winds, with 20k winds the Xp44 can go 0.3kt faster than the hull speed of a Hylas 70.

With bigger wind speeds the Xp44 will start to plane and the difference will become huge but those are conditions less frequent than weaker winds.

I hope all this as contributed to a better understanding of our sailboats. If not I have just lost a lot of time an effort LOL.

It is time for me to prepare our family gathering, so I will not be around for a while.

Wishing all a great Christmas.

Paulo

Thanks for this, Paulo; it's very much in line with what we were previously discussing in this thread, and it's something I think we all, or nearly all of us agree with -- that hull speed is not a concrete wall, and that the lighter a boat is, the more easily it can exceed the theoretical hull speed. That was the main theme of this thread, actually.

I think we've only just scratched the surface of this question and I think that plenty more good discussion may be had around it.

[Polux]

Originally Posted by Dockhead

.... but I finally settled on custom building something out of aluminum. I will not go the way of large form stability (as in Pogos or other boats you like), but rather more the Dashew way, which is a much longer and lighter boat, with a rather fine hull (I seem to recall you also like this type of boat). Dashew's Beowulf is very close to my ideal boat; only just a little large; I want to stay within about 20 meters.

The choice of this type of boat rather than that is really just down to the kind of sailing -- long distances and high latitudes make you want something easier to manage -- something you can sail from the pilothouse -- and up here berthing is cheap and a longer boat is not really a problem. If I were sailing in the Med, I might prefer a Pogo-type boat. Or the catamaran. Probably the cat, actually.

Yes no reason not to like the Beowulf even if I would not chose that. Size for size in what regards speed is not as efficient as a beamier boat, it is more difficult to handle and it sails with a lot more heel (more even than your sailboat) and that can be very tiresome on extensive sailing. That is not by accident that the shape of Beowulf hull was discarded in what regards ocean racing and fast cruising for efficiency reasons. However It should be a great boat to sail upwind.

I believe the more practical and cheapest way would be to modify one of the existent fast aluminium cruisers produced by French shipyards to have a pilot house. An American to whom I helped to chose the boat (without knowing) invited me for the trials of his new live aboard boat, I think this year, maybe you are interested in seeing how the boat sails. It is a Cigale, I don't remember if a 16 or a 18.

There is also a relatively fast (much less fast than the Cigale) aluminium pilot house on the market. Don't know if you know it:

http://www.futuna-yachts.com/explore...20Brochure.pdf

I would say that the boat can be one and a half to two tons lighter if that unballasted swing keel is substituted by a swing keel with all the ballast on the keel . They exist on the market with the right dimension since Allures is using them them on their boats (and Garcia) as an option.

The hull is designed by one of the best Na and with two tons less the boat would have a 149.4 D/L.

[Dockhead]

Originally Posted by Polux

Yes no reason not to like the Beowulf even if I would not chose that. Size for size in what regards speed is not as efficient as a beamier boat, it is more difficult to handle and it sails with a lot more heel (more even than your sailboat) and that can be very tiresome on extensive sailing. That is not by accident that the shape of Beowulf hull was discarded in what regards ocean racing and fast cruising for efficiency reasons. However It should be a great boat to sail upwind.

I believe the more practical and cheapest way would be to modify one of the existent fast aluminium cruisers produced by French shipyards to have a pilot house. An American to whom I helped to chose the boat (without knowing) invited me for the trials of his new live aboard boat, I think this year, maybe you are interested in seeing how the boat sails. It is a Cigale, I don't remember if a 16 or a 18.

There is also a relatively fast (much less fast than the Cigale) aluminium pilot house on the market. Don't know if you know it:

http://www.futuna-yachts.com/explore...20Brochure.pdf

I would say that the boat can be one and a half to two tons lighter if that unballasted swing keel is substituted by a swing keel with all the ballast on the keel . They exist on the market with the right dimension since Allures is using them them on their boats (and Garcia) as an option.

The hull is designed by one of the best Na and with two tons less the boat would have a 149.4 D/L.

That's an interesting looking boat, and you're certainly right about its being much cheaper to modify a series built boat, than to do a custom build. I don't have the money yet for my custom build so I still have some time to study this. One of my target numbers is D/L below 150, so this seems to tick that box

One way I do NOT look at this is "size for size". I don't know why we would ever compare size for size unless we have some hard constraints concerning berthing (maybe in the Med?). The cost to build a boat is closely related to displacement; Beowulf for all her length is only 27 tonnes. So I don't think that is less efficient than a 27 tonne boat which is 60 feet long. For racing maybe it makes sense, but for cruising I don't see why we should torture a design to get performance the hard way out of a shorter hull, if we can just spread the same materials out over a longer hull, and get performance the easy way.

As to heeling -- Beowulf will heel more than a very beamy boat of similar size, but I'm not sure Beowulf will heel more than my boat. Beam is the same as my boat but righting moment derived from form stability will be greater in proportion to her length (assuming same prismatic coefficient, etc.). Her D/L is far less than my boat, so far less power will be required per unit of length and thus as a function of form stability derived righting moment. Center of effort of the sails will be much lower due to the ketch rig, so less heeling moment there. Due to the much greater fineness of the hull, there will be much less wave-making resistance also, so even less power required. Less power required means still less heeling moment. I believe Beowulf will heel much less than my boat does, but the wild card will be ballast stability, which may be compromised by the undesirability of scaling draft up proportionately to length.

The Beowulf approach does not compete with Pogo-type boats on a "size for size" basis, but if you throw away that constraint, I believe it works extremely well. If something like this could be slightly scaled down to about 20 meters, I think it could be ideal for me.