Extending a Lagoon 40

[smj]

Here’s the story of the Woods designed catamaran that started of at 28’, then had the sterns extended to make her 32’, then cut the boat down the middle, added 5’ for a final length of 37’! Maybe that’s taking it to far?

https://www.time-for-a-catamaran-adv...rade-projects/

[Shrew]

[QUOTE=smj;3345705]..... cut the boat down the middle, added 5’ for a final length of 37’!

IMHO, cutting down the middle is the only thing that would make sense. Otherwise, I would think the rig would be unbalanced with the mast too far forward or aft. This would then require that the entire cabin be redesigned as well.

To me that would be a kluge. When you're done, in the case of the OP, how much cost difference to simply buy an L450?

[Sailing August]

Naval Architect & Marine Engineer weighing in here.
Such a modification would easily cost > $100k. In my opinion it is not worth the effort and cost. The custom modification will also decrease the value of your production catamaran. Save the $100k and apply it to the purchase of a different catamaran.
Catamarans are selling very quickly. Im sure your production 40' Lagoon will not stay on the market for very long. Take the sale proceeds and the additional >$100k you were going to spend and buy a newer, bigger and more optimized design. Just my humble opinion. Good luck and please keep us posted.

[Cheechako]

This has got to be the worst case of 3 foot-itis I've ever heard of.... really?
$100k will not be enough to do this.

[smj]

Originally Posted by Sailing August

Naval Architect & Marine Engineer weighing in here.

Such a modification would easily cost > $100k. In my opinion it is not worth the effort and cost. The custom modification will also decrease the value of your production catamaran. Save the $100k and apply it to the purchase of a different catamaran.

Catamarans are selling very quickly. Im sure your production 40' Lagoon will not stay on the market for very long. Take the sale proceeds and the additional >$100k you were going to spend and buy a newer, bigger and more optimized design. Just my humble opinion. Good luck and please keep us posted.

As someone that’s had this done and knowing many others that have also had their sterns extended professionally your $100k estimate is WAY of. A professional job may run $15-$20k and should come out looking like part of a factory build.

[mikereed100]

Sure, it can be done, but is it worth it? If you pay to have it done it will be crazy expensive. If you do it yourself it will cost you weeks or months of your life, and still be expensive. This seems to be a solution in search of a problem. I have sailed in company with a Lagoon 40 and it's performance surprised me. Also, consider that it is after all a cruising boat. The extra 1/4 or 1/2 knot is always welcome, but not necessary to enjoy cruising. In fact, I find that I spend as much time slowing our boat down as trying to make it go faster. Better to just slap a couple of folding props on it and go sailing.

[btompkins]

Don't know if you all follow this on facebook, but this guy has been doing his own sugar scoop addition somewhere down south.

It's been interesting watching him to say the least! I'm rooting for him.

https://www.facebook.com/groups/1427...0000160014922/

[longhorn]

There's a Lagoon 420 named "The Norm" that was extended 5 feet. Shows up for sale on some sites so it might give you some ideas / inspiration.

[jim King]

It seems you'd be money ahead by just buying a used Lagoon 42 rather than trying to slab on 2'. it would be less costly and give your Lagoon 42 a much higher resale value rather than trying to sell a modified Lagoon 40.

[captlloyd]

Originally Posted by smj

Here’s the story of the Woods designed catamaran that started of at 28’, then had the sterns extended to make her 32’, then cut the boat down the middle, added 5’ for a final length of 37’! Maybe that’s taking it to far?

https://www.time-for-a-catamaran-adv...rade-projects/

I too was going to recommend this video, it shows what can be done, and this guy is an amateur. ( but a damn good one). I would guess that he had less than 5 grand in the modification. However I would not recommend this to anyone who is not very good with their hands. This modification was not done to improve sea going characteristics, it was done to provide salon access to each Hull. The couple have sailed this boat in blue water extensively. Impressive!

[Paul Howard]

I extended the stepped hull transoms on my Manta catamaran by two feet (Eric Lerouge design) and she sailed much better for it.
The boat used to squat and the bottom step was underwater. I made a mold in place by screwing some battens and arborite to the hulls, extending the upward curve of the underwater hull shape, waxed it up and laid up some solid glass, then added some interior framing with foam, fiberglass and epoxy panels, added a top (floor) part that extended across the top of the lower step to the new hull transoms and had about a 36" long boarding step. I faired in the outboard side so the profile was now further sloped rather than rounded and left the inboard side low for boarding from the side. I removed nothing, never cut into the existing part of the boat, so the extension could be torn off and the watertight integrity of the hulls was still intact.
We sailed that boat about 60,000nm, heavily loaded and she leveled out nicely, as lots of the storage space and tankage for water and fuel was under the cockpit and aft lockers, dinghy in davits, etc.
We had about 500kg of sealions resting on them in Argentina and Uruguay and never a problem. I sold the boat about 5years ago, she is still sailing with those extensions.
The only problem was that the rudders were not moved back and the longer extensions would hit the dock if not careful when pulling away.

[Clivevon]

You mentioned better performance. Hull speed is usually calculated by the formula: 1.34 knots x the square root of the waterline length in feet.

According to that formula the hull speed of a 25 foot waterline boat would be 1.34 x 5 = 6.7 knots. 36 foot waterline would be 1.34 x 6 = 8 knots so only 1.3 knots more from an 11 foot increase in waterline length.

Lets say your waterline length is 37 foot or so = 1.34 x 6.083 = 8.15 knots hull speed. Increase waterline length to 40 foot gives you 1.34 x 6.325 = 8.48 knots. So potentially a whole third of a knot more... For a fanatic racer I can see it - not for a cruiser.

[Cadence]

Originally Posted by Roddu

Anyone got any views on extending a Lagoon 40 by a few feet to improve seaway and performance? Both bow and stern, or one of these only?
We really don't need more interior space, but are looking for a bit better performance and better seaway. From experience (chartering) i get this if I go a few foot bigger, but that quickly adds +$100k to the bill, and space I really don't need. I understand that the boat will still be under-rigged with a too small main and jib....but I am wondering whether from a boat-building perspective just adding 2-3 foot to bows and stern might make sense?
Also, if anyone has a view on whether the stern extension is even feasible or makes sense (re location of saildrive and inclination of below-water line stern), please let me know as well!
Thanks!

I think altering the architect's design is folly. JMHO

[smj]

Originally Posted by Cadence

I think altering the architect's design is folly. JMHO

Don’t think I’ve met a perfect architect, or a builder that fully builds to an architects specs.

[Paul Howard]

Originally Posted by Clivevon

You mentioned better performance. Hull speed is usually calculated by the formula: 1.34 knots x the square root of the waterline length in feet.

According to that formula the hull speed of a 25 foot waterline boat would be 1.34 x 5 = 6.7 knots. 36 foot waterline would be 1.34 x 6 = 8 knots so only 1.3 knots more from an 11 foot increase in waterline length.

Lets say your waterline length is 37 foot or so = 1.34 x 6.083 = 8.15 knots hull speed. Increase waterline length to 40 foot gives you 1.34 x 6.325 = 8.48 knots. So potentially a whole third of a knot more... For a fanatic racer I can see it - not for a cruiser.

As per the theoretical hull speed, with their long and narrow hulls, catamarans are not bound by hull speed in the same way a monohull is. In my former monohull, surfing off the waves the speed might go from 6 to 8knots, but with the catamaran the speed can go from 8 to 10 or 12knots on a surf even when heavily loaded for long term cruising.
Also, a buoyant transom with a clean run aft will sail faster than an immersed transom sucking water up behind it.
I have made passages of 20days or more on my catamaran and even if my extensions were only to increase my speed by about one-third knot it would save nearly a day off the passage.
After the extensions were in place and buoyancy added aft, the lower step of the transoms no longer had green and slippery growth on them that was difficult to clean off.