Extending a Lagoon 40

[Bunji]

Generally speaking I think it depends on the original design, early designs had to much rocker proven by the fact that more modern designs have much less, so if you have an older design there is much to be gained from an extension.
More buoyancy aft equals less pitching, less squatting under power (motor), increased speed and more room better access to hop on and off from tender, swimming, fishing etc

[GRIT]

Those extensions look great! I can't wait to see more pictures.

Well done. and thanks for posting.

Cheers.
Paul.

[CaptainPete222]

Jumping into this discussion late and only found it by Googling “Lagoon 40 transom extension” bc I was thinking about doing the same thing to my L39, which is identical to the L40. I don’t think the bow extensions are feasible without radically reshaping the bow to permit the added length. The bow of the L39/L40 tapers down to just a few inches at the waterline, so, as a naval architect pointed out a while back in the discussion, you just don’t have the same ability to extend as you do with the much wider transom. However, although the transom is wider, the “landing” on the sugar scoop is less than a foot from the bottom edge of the transom. Continuing the fairly steep angle of the bottom for more than a foot would leave no transom at all (see attached photo). The other problem with extending the transom of the L39/L40 is that you would have to have a dinghy that is a 290 or less. My 310 is too large and both the bow and stern get hung up on the landing. A longer landing wouldn’t work at all with the 310; I couldn’t launch or haul the boat in the factory davits. So, as cool as it might be aesthetically (see attached crude mock-up) and as nice as it would be to ride through the seas slightly better, I just don’t think it’s feasible for the L39 (or L40).

As to the belief that adding a few feet to the boat would improve speed, it wouldn’t. Maybe a small fraction of a knot. Better sails give you a much greater boost in speed. The Incidence sails that came with my boat are very poorly shaped and are quick to lose their shape. I got new North 3Di Endurance and the boat sails much better. A big factor in how the boat rides through seas is its speed, so better sails help in that department. Also, running a diesel for an additional knot or two makes a huge difference in the boat’s motion and therefore comfort. I know that saying that is blasphemy for some, but it’s a fairly inexpensive solution and far less expensive than buying an Outremer 45.

[jmh2002]

Originally Posted by CaptainPete222

Jumping into this discussion late and only found it by Googling “Lagoon 40 transom extension”...

From earlier in the thread here is what @Spiv did on his L380 and it all looks pretty damn good in terms of both form and function.

Hopefully he will also post a picture of the transoms all finished up at some stage too.

Bow: https://www.cruisersforum.com/forums...ml#post3346334

Transom: https://www.cruisersforum.com/forums...ml#post3467393

[Redreuben]

I agree with the above Lagoon approach, it does depend on the design but if you add buoyancy aft and not adding it forard could this not result in you being stuffed into the backs of waves downwind ?

[CaptainPete222]

[QUOTE=jmh2002;3541546]From earlier in the thread here is what @Spiv did on his L380 and it all looks pretty damn good in terms of both form and function.

I saw that picture and now see how far he had to bring back the sides of the bow extension to conform with the curve of the ama in that location. That’s not really an extension of the bow; it’s completely reconfiguring a large section of the boat. My point about a bow extension on a L39/L40 is that same amount of work would be required so that the curve/shape of the hull is maintained. I’d be interested to see how the extension is supported structurally. It would need to be pretty beefy to withstand the abuse that it would take in heavy seas. Possible? Yes. Wise? Not so clear.

I don’t know what the transom of the L380 is like, but there isn’t any way to extend the transom on the L39/L40 even close to as far without running out of transom, unless, of course, you do what he did with the bow and start much further forward and reshape the hull.

And there’s still the problem that by extending his transom he has to shrink his dinghy.

But, if what we’re really talking about is making the boat look cooler, I can’t argue against that. It would look cooler.

[spiv]

Hi there,
Yes, to extend the bows, you need to fair back and below to different extents.
It will vary with the length of the added bit.
I added 80cm at the deck and 130 cm at the keel and you can see from the pics that I had to fill quite a way back. I also went 3cm lower.
I didn't alter the existing bows just 'engulfed' them.
The addition below waterline is all foam filled and I believe it's stronger than the original.
The boat pitched less, pointed a little better and on average I think I got close to 1kn faster.

But the sterns will give the most benefit, they are also a lot easier to lengthen.
I added 90cm, no extra width, just depth.
The transoms are smaller, but I will board from the tunnel side.
My 2.9m dinghy fits as before, it just touches the hull up high where the cleats are.
I will not launch the boat until April/may, so we will all have to wait for a performance update, however, with a much smaller Coefficient of Finesse and improved Bruce number, I can expect a faster, more comfortable ride.
I cannot add pics from my phone, I will from my laptop soon.

[CaptainPete222]

Thanks for that additional information. One thing I find surprising is your comment that the bow extensions alone resulted a 1 knot increase in speed. I plugged the numbers into the calculator on vicprop.com to see what I could expect by increasing my L39 by 3 feet. The increase was only 0.3 knot. I suppose wind speed would be a very significant factor in the benefit one would receive; the added buoyancy would permit you to keep up more sail in higher winds. For winds under 18 or so knots, increased speed—assuming all other variables remaining the same—would presumably reduce substantially. In 10-12 knots, I wouldn’t expect more than a small fraction of a knot increase.

For me, having to drop down to an even smaller dinghy to facilitate a transom extension is a deal breaker. 310 is already too small for me, but that’s the biggest tender you can use on a L39/L40.

I have no doubt that the extensions provide a slightly more comfortable ride. Also looks pretty sharp.

[spiv]

Propellers calculators don't take into account

1. reduction in pitching (the sail keeps drawing longer)
2. Slicing into waves, rather than hitting them
3. Coefficient of finesse etc etc.

They are good at best in calm river waters under power.


My transom extensions did not reduce the distance between hulls, the distance gets bigger and bigger, hence there was no difference to the dinghy handling at all.


Drag from fat, submerged transoms is the biggest drawback of just about all cruising cats.
Except already slim racing boats, all boats would benefit from transom extensions.
Boats nowadays are designed as short and as fat as possible to minimize marina costs and maximize payload.


While I had a lot of fun designing and modifying Galaxi, if I had the money, I would probably buy an Outremer 45...... if not else for the modern ergonomics and layout.



I would recommend everybody to read this book:
Seaworthiness: The Forgotten Factor by C. a. Marchaj (2007-09-20)
More information: https://www.amazon.com/dp/B01K9APD3O...2DR4PEC66QFJKY

[CaptainPete222]

Being able to “bridge” and pierce waves better would definitely improve speed, so that greater-than-expected increase in your boat’s speed makes a bit more sense. Still seems high, but a knot is a lot and is making me want to read that book you suggest.

As to keeping the transom out of the water, that was one of my primary reasons for looking into the extension in the first place. Although my transom is a few inches out of the water at rest, it sinks 6-8 inches when the boat’s heeled over. Lots of drag, as you point out. I’m looking forward to hearing how your boat performs after it splashes in April.

As to the increase in distance between the amas, it wouldn’t be nearly enough to permit me to fit my 310 unless I notched it. Good point, though.

I also fancy an Outremer, but wouldn’t be able to avoid worrying always about adding weight if I had one. I’ve got way too many toys. Yes, I know I could just get used to having a performance boat that’s weighed down, but that’s never a problem I have with my L39, the heaviest cat per foot ever created!

I do, however, have a plan to turn this heavy gal into a performance cruiser: use the excess power my PV system generates to power two 6 kW pod drives. I love the way the boat presently moves through the water running one diesel while sailing and would like to do away with the noise and vibration, among other things, with the soon-to-be-added pod drives. I put my dinghy’s 6 kW outboard on a mount on my cat’s port ama transom and the added propulsion was awesome. Unfortunately it’s a short shaft and was catching air on port tack. So, awesome half the time. :-)

[jmh2002]

Transoms look nice, and more like how they should come from the factory

[malbert73]

Originally Posted by spiv

Propellers calculators don't take into account

1. reduction in pitching (the sail keeps drawing longer)
2. Slicing into waves, rather than hitting them
3. Coefficient of finesse etc etc.

They are good at best in calm river waters under power.


My transom extensions did not reduce the distance between hulls, the distance gets bigger and bigger, hence there was no difference to the dinghy handling at all.


Drag from fat, submerged transoms is the biggest drawback of just about all cruising cats.
Except already slim racing boats, all boats would benefit from transom extensions.
Boats nowadays are designed as short and as fat as possible to minimize marina costs and maximize payload.


While I had a lot of fun designing and modifying Galaxi, if I had the money, I would probably buy an Outremer 45...... if not else for the modern ergonomics and layout.



I would recommend everybody to read this book:
Seaworthiness: The Forgotten Factor by C. a. Marchaj (2007-09-20)
More information: https://www.amazon.com/dp/B01K9APD3O...2DR4PEC66QFJKY

Nice work! Yes immersed transoms are a huge drag unless you are fully powered up. I could imagine light air gains >>1 knot if you’re not dragging a transom in the water