Data on improved performance from transom extensions?

[thinwater]

I stretched my PDQ and blogged about the process (this is just one post):
Sail Delmarva: Search results for transom


The results? Greatly abridged (only performance), from the above post:



The Results

The dingy is lashed to the hull at 2 points.
There are good reasons to cu the inside edge low.

• 
• Boarding is greatly improved. This is a major plus for family members with mobility issues. But even for the rest of us, loading the dingy with "stuff" is simpler and boarding kayaks is now a breeze, even in lumpy conditions.
• Pitching. Certainly it must be reduced, but controlled measurement is impossible.
• Weatherlyness. Any reduction in drag, resulting in better water flow over the foils, has to help. The net improvement in VMG is more than the sum of the parts.
• Speed. Motoring at full throttle we have increased from 7.3-7.4 knots to 7.5-7.6 knots (measured by GPS, average 2 directions, both times with fresh bottom paint). The waterline length increase would suggest about 0.25 knot increase is possible, but since the hull form was not changed, 0.2 knots is more realistic and is what we saw. A small improvement, but worthwhile. The fuel savings at a constant speed to should pay for the upgrade in ... about 120 years.
• Length vs. speed. Would even longer be faster? I don't think so. I have not changed the entry and the extended transoms are out of the water ~ 2 inches at rest with this 2-foot extension. Other boats with transoms that drag more, or cruisers that load more heavily may find additional length would help. But since the rudders are not typically relocated the affect on handling should be considered before going too long. Though sail boat extension are generally successful, there have been stories of extended power boats that lost steering control.
• Load carrying. Though most of my sailing to this point has been with a rather light load, when we go cruising I notice the drop off in performance as the water line is pressed 2 inches lower at the stern. I believe the streamlining will help even more when overweight. If I had a very over weight boat or cruised full-time I would go 30 inches.
• Docking. Since the extensions are 50% out of the water at very slow speeds, just sort of rounding the transom, there is perhaps a reduction in both yaw drag and backing drag. The docking length is not affected, as the dingy still hangs ~ 18 inches past the transoms.We can't feel any difference.
• Handling. We've noticed no negative affects. More speed means faster tacks (positive), any drag aft should lead towards better balance (positive--comes out of irons faster too), and we haven't noticed any negative affect in quartering seas. It seems like it might be inclined to surf a little sooner, but controlled observation is impossible.
• Irons. Seems less prone to getting caught and comes out more quickly, simply backing up with the helm over. I think there is clear improvement, the result of length aft.
• Weight increase. About 35 pounds each, or about 35 pounds per foot of water line (the over-all figure for the PDQ 32 is about 230 pounds). I over built them, but I envision backing into a few docks (note as of 10/2013; I have. the pilings have dents, not the boat).

Was it worth the time and money? For me, yes. Though the process was an exhausting mid-summer exercise (we picked the 3 hottest days of the year--stupid), everything went as smoothly as could be expected with very few surprises and with the level of effort I expected; not my first FRP project.


----


In other projects I re-shaped and extended the keels, moved the genoa tracks, added roach to the main and up-sized the genoa. The overall impact was considerable and higher pointing, but only because I carefully targeted the changes at specific weak points. She ended up being pretty light, fast, and weatherly for a boat of that class. Balance and handling were improved by synergistic changes.



But for a Lagoon, I would only do it if the boarding platform was worth it. That will be the most noticeable thing.

[CaptainPete222]

Originally Posted by thinwater

I stretched my PDQ and blogged about the process (this is just one post):
Sail Delmarva: Search results for transom


The results? Greatly abridged (only performance), from the above post:



The Results

The dingy is lashed to the hull at 2 points.
There are good reasons to cu the inside edge low.

• 
• Boarding is greatly improved. This is a major plus for family members with mobility issues. But even for the rest of us, loading the dingy with "stuff" is simpler and boarding kayaks is now a breeze, even in lumpy conditions.
• Pitching. Certainly it must be reduced, but controlled measurement is impossible.
• Weatherlyness. Any reduction in drag, resulting in better water flow over the foils, has to help. The net improvement in VMG is more than the sum of the parts.
• Speed. Motoring at full throttle we have increased from 7.3-7.4 knots to 7.5-7.6 knots (measured by GPS, average 2 directions, both times with fresh bottom paint). The waterline length increase would suggest about 0.25 knot increase is possible, but since the hull form was not changed, 0.2 knots is more realistic and is what we saw. A small improvement, but worthwhile. The fuel savings at a constant speed to should pay for the upgrade in ... about 120 years.
• Length vs. speed. Would even longer be faster? I don't think so. I have not changed the entry and the extended transoms are out of the water ~ 2 inches at rest with this 2-foot extension. Other boats with transoms that drag more, or cruisers that load more heavily may find additional length would help. But since the rudders are not typically relocated the affect on handling should be considered before going too long. Though sail boat extension are generally successful, there have been stories of extended power boats that lost steering control.
• Load carrying. Though most of my sailing to this point has been with a rather light load, when we go cruising I notice the drop off in performance as the water line is pressed 2 inches lower at the stern. I believe the streamlining will help even more when overweight. If I had a very over weight boat or cruised full-time I would go 30 inches.
• Docking. Since the extensions are 50% out of the water at very slow speeds, just sort of rounding the transom, there is perhaps a reduction in both yaw drag and backing drag. The docking length is not affected, as the dingy still hangs ~ 18 inches past the transoms.We can't feel any difference.
• Handling. We've noticed no negative affects. More speed means faster tacks (positive), any drag aft should lead towards better balance (positive--comes out of irons faster too), and we haven't noticed any negative affect in quartering seas. It seems like it might be inclined to surf a little sooner, but controlled observation is impossible.
• Irons. Seems less prone to getting caught and comes out more quickly, simply backing up with the helm over. I think there is clear improvement, the result of length aft.
• Weight increase. About 35 pounds each, or about 35 pounds per foot of water line (the over-all figure for the PDQ 32 is about 230 pounds). I over built them, but I envision backing into a few docks (note as of 10/2013; I have. the pilings have dents, not the boat).

Was it worth the time and money? For me, yes. Though the process was an exhausting mid-summer exercise (we picked the 3 hottest days of the year--stupid), everything went as smoothly as could be expected with very few surprises and with the level of effort I expected; not my first FRP project.


----


In other projects I re-shaped and extended the keels, moved the genoa tracks, added roach to the main and up-sized the genoa. The overall impact was considerable and higher pointing, but only because I carefully targeted the changes at specific weak points. She ended up being pretty light, fast, and weatherly for a boat of that class. Balance and handling were improved by synergistic changes.



But for a Lagoon, I would only do it if the boarding platform was worth it. That will be the most noticeable thing.

Very useful and interesting information. Thanks. Glad you saw good results.

[Sail IC]

Originally Posted by CaptainPete222

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.

Even if the L400 is around 3000 lb lighter than the L39, it's about 1.5 feet wider. Similar RM. So you cannot really have more sail up safely.

But on any of the lagoon/FP/Lep, you can probably put every possible sail up in sub 30 knots and it will not capsize. People don't do that as things would tend to break.

I wonder why the shorter/narrower L39 got so much heavier than the more spacious L400.

[smj]

We owned a Seawind 1000 for about 6 months before adding 4’ to the sterns. The results were incredible.
Almost negated all pitching and hobby horsing which greatly increased speed. The added length put the transom above waterline which seemed to increase light air performance. The added length of transom made the cat surf easier and for longer durations.
As far as hard facts, that to me is almost impossible to come up with on a catamaran that sails well as the speed fluctuates by so much and quickly.
In the case of the Seawind 1000 I guess the extensions were a success as the Seawind company soon after came out with the XL, a standard 1000 with transom extensions.
Whether or not they would help significantly on a Lagoon 39, I have no idea.

[Factor]

Originally Posted by CaptainPete222

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. .

Nope, it does matter, a fat hulled heavy 12 meter boat with 1 metre transom extension will see little benefit, a slim hulled light 12 meter boat with 1 meter transom extension will see significant benefit.

[CaptainPete222]

Originally Posted by Factor

Nope, it does matter, a fat hulled heavy 12 meter boat with 1 metre transom extension will see little benefit, a slim hulled light 12 meter boat with 1 meter transom extension will see significant benefit.

Why? Not trying to be a pain, but seems like just about everyone has theories/opinions about things, but few have any data to back up those theories. Seems like some data would be helpful before people spend a big chunk of money extending the transom of their boat.

The other reason I ask is that the hull speed formula for displacement hulls came up with the same, very minor, increase in hull speed (0.29 knot) by increasing the length by one meter of both a 12 meter 5 ton boat that’s 3 meters wide and a 12 meter boat that weighs 10 tons and is 4 meters wide.

[CaptainPete222]

Originally Posted by smj

We owned a Seawind 1000 for about 6 months before adding 4’ to the sterns. The results were incredible.
Almost negated all pitching and hobby horsing which greatly increased speed. The added length put the transom above waterline which seemed to increase light air performance. The added length of transom made the cat surf easier and for longer durations.
As far as hard facts, that to me is almost impossible to come up with on a catamaran that sails well as the speed fluctuates by so much and quickly.
In the case of the Seawind 1000 I guess the extensions were a success as the Seawind company soon after came out with the XL, a standard 1000 with transom extensions.
Whether or not they would help significantly on a Lagoon 39, I have no idea.

I don’t have any doubt that the added length improved your boat’s motion, but where I’m having a problem is your and many others’ claims that they saw a significant increase in speed by extending their transoms. Obviously anyone who put significant time and/or money into building out their transoms would be biased and susceptible to something akin to placebo effect. I figured there are so many people who track their avg boat speed and average wind speed on passages that someone might have some actual before and after data.

[jmh2002]

Anecdotally, it's quite common for racing catamarans to be extended in this manner - they wouldn't do it if it's not faster.

But as @Factor mentioned, these are much slimmer hulls than your Lagoon so the effected is amplified.

If it was me I would do it anyway to improve the swim steps / swim platform, and then motion and performance benefits would be an added bonus.

But that's because these short truncated sterns and steps are a pet peeve of mine on a live aboard cruising boat that spends a lot of time at anchor.

I think the access to the water, as well as to the dinghy, is less than optimal and can be vastly improved.

[CaptainPete222]

Originally Posted by jmh2002

Anecdotally, it's quite common for racing catamarans to be extended too - they wouldn't do it if it's not faster.

But as @Factor mentioned, these are much slimer hulls than your Lagoon.

If it was me I would do it anyway to improve the swim steps / swim platform, and then motion and performance benefits would be an added bonus.

But that's because these short truncated sterns and steps are a pet peeve of mine on a live aboard cruising boat that spends a lot of time at anchor.

The formulas I used show a 0.29 knot increase in speed—so, not nothing and perhaps everything if you race. Not enough additional speed alone to justify the hassle and cost, but, as you point out, the improved area for platform would be really nice and improved motion through waves is always welcome. Unfortunately I’ve not been able to get much information about pricing to have the work done in the US.

[CaptainRivet]

Originally Posted by Sail IC

Even if the L400 is around 3000 lb lighter than the L39, it's about 1.5 feet wider. Similar RM. So you cannot really have more sail up safely.

But on any of the lagoon/FP/Lep, you can probably put every possible sail up in sub 30 knots and it will not capsize. People don't do that as things would tend to break.

I wonder why the shorter/narrower L39 got so much heavier than the more spacious L400.

Due to one of my friends who is a naval architect and our buddy with 39 consulted him mostly everything is wrong and too much off on the 39.... His words its actually hard to tell whats not f..up on this 39....nightmare from mast position, ratio jib to main, boom height, side stays, hull form, bridgedeck clearance... Its purely build around the beds. And because of that add 3ft of transdom the 39 does not even make a 400 performance wise, it would screw stuff up even more. On a seawind 1000 or PDQ32 or a fp 32 the setup is correct and the 3-5ft more puts them in 37-40ft class length where the pitch and hobbyhorsing is much lower in the waves and also adds another 300-400kg extra payload, all small add ons but the sum makes a big difference. The FP36 Mahe eg would also profit strongly from 3-4ft trandom extension.

Your 8-8.5kn in 18kn winds that you claim hardly reached by monos is easily reach by eg my buddies 44ft Dufour with performance rod rigg with standard dacron sails in the first reef sailed very conservative with jib&main. In 15kn with 2nd reef main/full jib it does etmal 7-7.5kn, heel 5-10 degrees so you can easier cook or sleep=night passage mode....

That all newest modern condos are massively undercanvassed so the dummest caterpiller charter monkey cannot capsize it is a well known fact but they still sail kinda acceptable as 380 or 400 Lagoon.

[smj]

Originally Posted by CaptainPete222

I don’t have any doubt that the added length improved your boat’s motion, but where I’m having a problem is your and many others’ claims that they saw a significant increase in speed by extending their transoms. Obviously anyone who put significant time and/or money into building out their transoms would be biased and susceptible to something akin to placebo effect. I figured there are so many people who track their avg boat speed and average wind speed on passages that someone might have some actual before and after data.

Your right, the boost in performance was just in my head, no speed gain at all. In fact if I were you i wouldn’t do stern extensions on your Lagoon 39 as it would probably make it slower.

[CaptainPete222]

Originally Posted by CaptainRivet

Due to one of my friends who is a naval architect and our buddy with 39 consulted him mostly everything is wrong and too much off on the 39.... His words its actually hard to tell whats not f..up on this 39....nightmare from mast position, ratio jib to main, boom height, side stays, hull form, bridgedeck clearance... Its purely build around the beds. And because of that add 3ft of transdom the 39 does not even make a 400 performance wise, it would screw stuff up even more. On a seawind 1000 or PDQ32 or a fp 32 the setup is correct and the 3-5ft more puts them in 37-40ft class length where the pitch and hobbyhorsing is much lower in the waves and also adds another 300-400kg extra payload, all small add ons but the sum makes a big difference. The FP36 Mahe eg would also profit strongly from 3-4ft trandom extension.

Your 8-8.5kn in 18kn winds that you claim hardly reached by monos is easily reach by eg my buddies 44ft Dufour with performance rod rigg with standard dacron sails in the first reef sailed very conservative with jib&main. In 15kn with 2nd reef main/full jib it does etmal 7-7.5kn, heel 5-10 degrees so you can easier cook or sleep=night passage mode....

That all newest modern condos are massively undercanvassed so the dummest caterpiller charter monkey cannot capsize it is a well known fact but they still sail kinda acceptable as 380 or 400 Lagoon.

A 44 Dufour is 6 feet longer than the 38' L39, so I'm not sure what your point is. Almost all 38-ish cruising monos I've sailed don't go 8-8.5 in 18 knots of wind unless the rail is buried and everyone is miserable. It's another day in the park in my L39.

I don't understand your comment about the "newest modern condos are massively under canvassed." It's true that my L39 and just about every other boat used in charter fleets are underpowered if all you're flying is what the charter companies give their customers. The extremely easy solution is to fly a Code 0 instead of the dinky self-tacking jibs. One would think this fairly obvious adjustment would have occurred to your naval architect friend.

Assuming you're quoting your friend's opinions of the L39 accurately, it's hard to say what's not f----d up about those opinions. "Ratio of main to jib?" Maybe he should look at some of the cutting edge Chris White mast foil cat designs which are basically no main and all jib. Great, fast boats. No massive, cumbersome mainsail to contend with. I'd really like to see what your friend could design for a smallish cat that doesn't total suck in terms of headroom, storage, and sleeping area. You can't have performance and at least reasonable comfort in that size range. And, by "reasonable comfort," I mean, among other things, not having to sit on the toilet to take a shower.

[smj]

Originally Posted by CaptainPete222

A 44 Dufour is 6 feet longer than the 38' L39, so I'm not sure what your point is. Almost all 38-ish cruising monos I've sailed don't go 8-8.5 in 18 knots of wind unless the rail is buried and everyone is miserable. It's another day in the park in my L39.



I don't understand your comment about the "newest modern condos are massively under canvassed." It's true that my L39 and just about every other boat used in charter fleets are underpowered if all you're flying is what the charter companies give their customers. The extremely easy solution is to fly a Code 0 instead of the dinky self-tacking jibs. One would think this fairly obvious adjustment would have occurred to your naval architect friend.



Assuming you're quoting your friend's opinions of the L39 accurately, it's hard to say what's not f----d up about those opinions. "Ratio of main to jib?" Maybe he should look at some of the cutting edge Chris White mast foil cat designs which are basically no main and all jib. Great, fast boats. No massive, cumbersome mainsail to contend with. I'd really like to see what your friend could design for a smallish cat that doesn't total suck in terms of headroom, storage, and sleeping area. You can't have performance and at least reasonable comfort in that size range. And, by "reasonable comfort," I mean, among other things, not having to sit on the toilet to take a shower.

I guess it depends on ones definition of “reasonable comfort” but our 39’ TRT 1200 has what we would describe as having “reasonable comfort”, and performance at least matching your F31.

[CaptainPete222]

Originally Posted by smj

Your right, the boost in performance was just in my head, no speed gain at all. In fact if I were you i wouldn’t do stern extensions on your Lagoon 39 as it would probably make it slower.

Well, if you're not susceptible to wishful thinking, then you're a pretty special person. Personally, I acknowledge my limitations and know that humans largely live in fantasy worlds of their own creation to, among other things, make themselves a bit happier. Because of that, I like data. And it's not nearly as hard as you suggest to get (unless you really aren't interested what it shows).

[smj]

Originally Posted by CaptainPete222

Well, if you're not susceptible to wishful thinking, then you're a pretty special person. Personally, I acknowledge my limitations and know that humans largely live in fantasy worlds of their own creation to, among other things, make themselves a bit happier. Because of that, I like data. And it's not nearly as hard as you suggest to get (unless you really aren't interested what it shows).

The only worthwhile data that you can get would be data from another Lagoon 39 that’s had stern extensions added, anything else is absolutely useless for you?