Data on improved performance from transom extensions?

[CaptainPete222]

I’m interested in extending the transoms of my Lagoon 39 as I’ve heard they improve the cat’s performance. Has anyone who did the refit recorded any before and after speed data? It’s a pricey undertaking and I don’t want to move forward with the project unless it will result in meaningful improvement in the boat’s speed (>0.5 knot in 12-15) There is a fair amount of discussion in recent treads about the pros and cons of extensions and some limited anecdotal mention that speeds were improved, but little in the way of before and after data, e.g., 10-12 knots wind with AWA 90 degrees = 6.2 knots boat speed pre vs 6.5 knots post, all other variables the same. Vicprop.com has a calculator that shows minor improvement increasing my cat’s length from 39 to 41 feet (0.2 knots), but contributors to the other threads noted that it would be possible to obtain considerably greater speeds in ocean chop/seas due to reduced pitching and better bridging of the waves. My downwind ama transom will submerge slightly creating drag, so I think that is another variable to take into account. I know it’s a long shot, but thought I’d ask anyway. Again, it’s a big expense and i just want to get as much information as I can.

[Dave_S]

I can't help you with the numbers, however I would like to understand exact science of a longer hull.

I've been drawing up foils to run in the waterflow beneath the sugar scoops (where the water flow is rising at about 7.5° to the waterlevel) and also wondering what might happen if I ran one a metre (3') behind the stern would/could that be made to simulate a longer hull ?

These can be made relatively cheaply and are removable/interchangeable with new changes in designs.

[CaptainRivet]

Originally Posted by CaptainPete222

I’m interested in extending the transoms of my Lagoon 39 as I’ve heard they improve the cat’s performance. Has anyone who did the refit recorded any before and after speed data? It’s a pricey undertaking and I don’t want to move forward with the project unless it will result in meaningful improvement in the boat’s speed (>0.5 knot in 12-15) There is a fair amount of discussion in recent treads about the pros and cons of extensions and some limited anecdotal mention that speeds were improved, but little in the way of before and after data, e.g., 10-12 knots wind with AWA 90 degrees = 6.2 knots boat speed pre vs 6.5 knots post, all other variables the same. Vicprop.com has a calculator that shows minor improvement increasing my cat’s length from 39 to 41 feet (0.2 knots), but contributors to the other threads noted that it would be possible to obtain considerably greater speeds in ocean chop/seas due to reduced pitching and better bridging of the waves. My downwind ama transom will submerge slightly creating drag, so I think that is another variable to take into account. I know it’s a long shot, but thought I’d ask anyway. Again, it’s a big expense and i just want to get as much information as I can.

The big issue is your Lagoon 39 the worst sailing Lagoon of all times. See my other post on it. You can do what you want, that one won‘t sail properly. It’s no bashing, it’s really that bad, sorry….

Get a 380 or a 400S2, not great performer either but much better.

[CaptainPete222]

Originally Posted by CaptainRivet

The big issue is your Lagoon 39 the worst sailing Lagoon of all times. See my other post on it. You can do what you want, that one won‘t sail properly. It’s no bashing, it’s really that bad, sorry….

Get a 380 or a 400S2, not great performer either but much better.

Just responded to your other post. Yes, the L39 is a heavy boat, but extremely comfortable for its size. Better headroom than other, larger boats (I’m 6’ 2”), a king size berth (really), and it’s heavier weight makes it ride through seas more comfortably than similar sized FPs. My F31 tri can (literally) sail circles around any cruising catamarans out there, but I can’t convince my wife to join me. She likes the L39 and it’s comfortable ride, albeit leisurely pace, and having her come with is the top priority. :-)

[Sail IC]

Do you have good sails. Laminate sails and good downwind sails will probably improve performance more. Easier and cheaper.
I assume you already have folding props. If not, by far the best performance upgrade.
Slightly submerged transoms is in my opinion not that bad. Yes, a bit more drag but it push the Stern wave back, making the boat be longer than it actually is.

[rabbi]

Originally Posted by CaptainPete222

Just responded to your other post. Yes, the L39 is a heavy boat, but extremely comfortable for its size. Better headroom than other, larger boats (I’m 6’ 2”), a king size berth (really), and it’s heavier weight makes it ride through seas more comfortably than similar sized FPs. My F31 tri can (literally) sail circles around any cruising catamarans out there, but I can’t convince my wife to join me. She likes the L39 and it’s comfortable ride, albeit leisurely pace, and having her come with is the top priority. :-)

I'd get good folding / feathering props and an asym spi and call it a day.
Just stick with the sailing rule #1: Gentleman don't sail to windward


You can add a few feet but you won't change the character of the 39. It's a comfy motor-sailor that can sail OK going downwind.

If you want a Lagoon with a similar ride and volume but much better performance then I'd suggest a closer look at the 400.

[Dave_S]

What are the specification differences in the faster Lagoons compared to the L39. For such a popular brand there isn't lots of comparative information out there.

i.e. if you add 3' to a 39 how do the numbers compare.
Hull length/width ratio.
Total weight empty
Sail area etc...

Those numbers must give a rough approximation of how extentions will perform. 7.5% increase in length must be significant.

I am absolutely no expert: about as far from it as you can get so none of this should be taken as advice but I'm interested in how this would affect the boat. Clearly size matters and I get that to optimise the extra length there would be other changes, but I also think on it's own it will help, maybe a fair % of the full potential.

I'm not sure why the extensions are 2', I'd be more inclined to go bigger. If your going to spend the money it won't matter much between 2' and 3' or even a bit more and you want to notice the difference. Nothing worse than going to a lot of effort and ending up with "I think it feels better"

[Redreuben]

A cheapish mod for draggy sterns is to sharpen up any radius on the stern.
It helps the flow break away earlier.

[longjonsilver]

As i understand it, it is proportional to the square root of the load waterline. So 107.5*1/2 divided by 100*1/2 = 10.36/10 = 3.6% faster. So a 100 mile day becomes a 103.6 mile day. Like the guys said, get another boat, or better sails and prop.

[Factor]

Originally Posted by longjonsilver

As i understand it, it is proportional to the square root of the load waterline. So 107.5*1/2 divided by 100*1/2 = 10.36/10 = 3.6% faster. So a 100 mile day becomes a 103.6 mile day. Like the guys said, get another boat, or better sails and prop.

That is theoretical hull speed on a mono, Square root of waterline length in feet multiplied by 1.34. So 36 foot boat has theoretical hull speed of 8. knots approx. This relates to the length of the bow wave being created equall;ing the length of the vessel.

This formula does not strictly apply to multihulls, however the heavy the boat and the fatter the hulls, the closer they get to following the rule. a good light skinny multi will easily get past the theoretical hull speed, easily.

No disrespect to the OP but extending the hulls on a L39 is like lipstick on a pig. Better focussed by getting rid of as much wieght as possible, and getting the best sails, otherwise save money and just use engines, the OP bought the boat because of its luxury and comfort, enjoy it for those reasons.

[CaptainPete222]

Originally Posted by Sail IC

Do you have good sails. Laminate sails and good downwind sails will probably improve performance more. Easier and cheaper.
I assume you already have folding props. If not, by far the best performance upgrade.
Slightly submerged transoms is in my opinion not that bad. Yes, a bit more drag but it push the Stern wave back, making the boat be longer than it actually is.

I’ve got great sails and feathering props. The reason I was (no longer am) interested in the performance improvement, if any, from transom extensions. In another thread, a couple contributors with similar-sized catamarans claimed gains of one knot or more by extending their boats. Seemed pretty far fetched to me, but, if others saw similar improvement, I planned to explore modifying my transoms. Needless to say, a knot is a lot! Buying top of the line sails didn’t give me that much of an increase in speed.

[CaptainRivet]

Originally Posted by rabbi

I'd get good folding / feathering props and an asym spi and call it a day.
Just stick with the sailing rule #1: Gentleman don't sail to windward


You can add a few feet but you won't change the character of the 39. It's a comfy motor-sailor that can sail OK going downwind.

If you want a Lagoon with a similar ride and volume but much better performance then I'd suggest a closer look at the 400.

Exactly...
Thats what I told him already, L39 is the worst sailing condo lagoon has. Don't like to bash a boat but that 39 was the worst sailing boat I was on since I sail.... And it didn't shine in other departments either...
400S2 way better in all aspects, thats what my buddy realized fast too and sold 39 and got a 400S2.
39
I wonder how many times with 6.2 he has hit his head on the corner that the fully open sliding door down the owner hull present exactly at 6ft1...all 3 giants of us above 6ft hit their head incl me...nearly as perfect engneered as the cupboard on the entrance to the master suit on the neel 47...
Corrosion on top of the mast is also a common 39 problem because Lagoon mounted all stainless without isolators or teff gel/duralac...nearrly lost his mast, cost him 9k to fix as lagoon played the known game...

Ok 400S2 has the bulkhead problem because due to excellent french engineering no ventilation openings are needed in the bulkheads below the mast sure rott is a standard feature and welcomed by every boater irony mode off...

[CaptainPete222]

Originally Posted by rabbi

I'd get good folding / feathering props and an asym spi and call it a day.
Just stick with the sailing rule #1: Gentleman don't sail to windward


You can add a few feet but you won't change the character of the 39. It's a comfy motor-sailor that can sail OK going downwind.

If you want a Lagoon with a similar ride and volume but much better performance then I'd suggest a closer look at the 400.

I agree with the general goal of avoid sailing to windward if at all possible. However, while it may be able to avoid beating, good performance at 40-80 AWA is pretty important in my book. You will only get reasonably good performance (6 plus) close hauled in 15 in moderate chop and 18 plus in sea conditions. Because of slippage, I would run an engine even in a stiff breeze to make progress close hauled. About the only boat I'd happily sail to windward in ocean seas is a 50'-plus trimaran.

I've found that the keys to making the L39 perform fairly decently at 50 AWA and above are (1) recognize that the main and jib will only get the boat moving at 6+ in 15+. (I think Lagoon's marketing that the boat could sail with just the main and jib in less than 15 is a bit slippery on their part. Same goes for the reviews of the L39 that came out.) (2) For anything less than 15, the Code 0 is a mandatory sail, unless you are OK motoring. (In less than 10, even with the Code 0, you'll need to motorsail to get over 4-5 knots.) (3) The boat shines with the Code 0 from 50 AWA to 110 AWA in 15 plus. (Same with an asymmetrical spinnaker at 120-160.). By "shines," I'm not talking about anything to get excited about, just a reasonably good clip (8-8.5), especially compared to a monohull. (4) You can comfortably carry a full main and code 0 in 20, gusting to 24, because of the boat's weight.

[CaptainPete222]

Originally Posted by CaptainRivet

Exactly...
Thats what I told him already, L39 is the worst sailing condo lagoon has. Don't like to bash a boat but that 39 was the worst sailing boat I was on since I sail.... And it didn't shine in other departments either...
400S2 way better in all aspects, thats what my buddy realized fast too and sold 39 and got a 400S2.
39
I wonder how many times with 6.2 he has hit his head on the corner that the fully open sliding door down the owner hull present exactly at 6ft1...all 3 giants of us above 6ft hit their head incl me...nearly as perfect engneered as the cupboard on the entrance to the master suit on the neel 47...
Corrosion on top of the mast is also a common 39 problem because Lagoon mounted all stainless without isolators or teff gel/duralac...nearrly lost his mast, cost him 9k to fix as lagoon played the known game...

Ok 400S2 has the bulkhead problem because due to excellent french engineering no ventilation openings are needed in the bulkheads below the mast sure rott is a standard feature and welcomed by every boater irony mode off...

I think you don't understand the key performance issue with the L39: It's seriously underpowered with just the main and jib in less than 15. The fix is to (1) realize that, and (2) power it up correctly by using the Code 0. If you take a 400S2 with a full main and a genny and put it next to a L39 with a full main and a Code 0 in 12 knots, they will probably run even. If the wind increases to 15-plus, the L39 will pull ahead. There isn't anything special about the 400's hulls or weight. It's slightly less heavy, so that's a plus, but the L39's extra weight allows it to carry more sail safely, which it needs given it's weight.

Way too many people just don't understand how to power up their boats appropriately for the conditions and then they blame their boat's lackluster performance on the boat, rather than their lack of sailing skill. There is no question that the L39 is nothing more than a motorsailer in less than 10; it's just too heavy and there isn't any way to power it up with more sail to overcome the weight in light wind. This is obviously a big problem for anyone who sails in places where the wind is frequently light and they should only buy a L39 if they don't mind motorsailing. Where I am the wind is rarely less than 12 and on the rare occasion when the wind is light, I stay where I am for a day. The rest of the time I can power it up appropriately and very few other Lagoons/FPs/Leopards/Mantas (which I can track with their AIS) are going faster than me.

[CaptainPete222]

Originally Posted by Factor

That is theoretical hull speed on a mono, Square root of waterline length in feet multiplied by 1.34. So 36 foot boat has theoretical hull speed of 8. knots approx. This relates to the length of the bow wave being created equall;ing the length of the vessel.

This formula does not strictly apply to multihulls, however the heavy the boat and the fatter the hulls, the closer they get to following the rule. a good light skinny multi will easily get past the theoretical hull speed, easily.

No disrespect to the OP but extending the hulls on a L39 is like lipstick on a pig. Better focussed by getting rid of as much wieght as possible, and getting the best sails, otherwise save money and just use engines, the OP bought the boat because of its luxury and comfort, enjoy it for those reasons.

If what you're saying is true--and I think it probably is, then pretty much everyone who put transom extensions on their boats wasted their money. If it doesn't work (in terms of improving performance), then it doesn't work. It doesn't matter if the boat sails relatively slow or is a performance multi; the principles are the same. The reason I started this thread isn't just that many people have added transom extensions, but rather than some have claimed pretty amazing/unbelievable improvements in speed. I wanted to see whether anyone who sunk a bunch of time and/or money into extending their transoms had any hard before and after data. Apparently not, but that's not surprising. Wishful thinking is a powerful thing, which is why I wasn't interested in anecdotal accounts. I looked at the same formulas as you and they indicate an extremely small increase in speed by extending the length, so accounts by others that they saw increases of a knot or more didn't add up for me.

As I mentioned in another response to someone who contributed to this thread, the problem with the L39 is that it's heavy and seriously underpowered with the standard main and jib configuration in less than 15. If there wasn't a solution to this problem then, yes, I'd call it a dog (or pig) of a boat. But the solution is fairly easy: power it up correctly by flying the Code 0 all the time in under 20. That won't help the L39 in less than 10--it's a motorsailer in light winds, but the boat sails well when you have the appropriate amount of sail area up for the the weight of the boat--which is definitely hefty. Way too many people blame the boat rather than their own lack of sailing ability.