Transom Extensions

[ejlindahl]

Sandcrab. I remember that discussion too about the Outremer bow extension or maybe it was just an alteration to make a sharper, plumb bow. I only recall that the owner remarked on the reduction of spray coming off the new sharper vertical bows. I have no idea where that discussion was/is but I am sure I read it online.

[owly]

Originally Posted by ejlindahl

Sandcrab. I remember that discussion too about the Outremer bow extension or maybe it was just an alteration to make a sharper, plumb bow. I only recall that the owner remarked on the reduction of spray coming off the new sharper vertical bows. I have no idea where that discussion was/is but I am sure I read it online.

I can't see a spray reduction with straight bows... What I wonder about is the increase in displacement as the bows go down, reduced bow burying tendency with a traditional bow shape compared to a straight bow. At least one racer I know of installed sponsons above the waterline, but he was doing round the world racing.


What one has to be careful of I think is racing technology infiltrating cruising design. For example wave piercing bows may look cool and be great in racing conditions, but how about "survival" conditions. The majority of sailors never sail far from land, or experience survival storm conditions.... they don't build modern cats for the few who will face these things.

[Fore and Aft]

I have done a number of transom extensions on both monos and multis and the results have always been positive with one exception. That was a live aboard power cat had new bows and stern extensions and it made no diffference to its performance. The real issue was the boat was way to overloaded.
Cheers

[owly]

Originally Posted by Fore and Aft

I have done a number of transom extensions on both monos and multis and the results have always been positive with one exception. That was a live aboard power cat had new bows and stern extensions and it made no diffference to its performance. The real issue was the boat was way to overloaded.
Cheers

Overloaded is the rule, not the exception. The cat I'm looking at has just under a ton of payload. While that may sound like a lot at first blush, once you start honestly looking at what goes into payload, it gets eaten up very rapidly. Unfortunately as you go larger, the size, weight, and rig size increase far faster than payload. It's a real problem with catamarans, and worse with trimarans...... everything escalates rapidly, especially initial and ongoing costs.


H.W.

[Sand crab]

I forgot I started that thread 6 years ago. I think you have to pay to look at the article now but I used to have the mag.
http://www.cruisersforum.com/forums/...5-a-80814.html

[thinwater]

On a cat you need to aggressively clean out lockers every year. If you are good at it, each year you will gain a few things, but you will also pitch a few things, that in hind sight, where not used. The result is not just weight savings, but better organization, more useful gear, and more space.


Also, try to keep the weight in the center.


(After a fall cleaning. A nice, empty locker.)

[catsketcher]

Originally Posted by Seaslug Caravan

Phil, as one who has dabbled in design you really need to explain the ramifications of adding buoyancy aft to an existing hull.

Great when the displacement is shared equally between the two hulls.
Not so great when the sailing loads place more displacement into the lee hull.
Remembering the whole reason for the modification was overloading in the first place.

I have been involved in a few extensions and most were useful. The extensions on my little 7 metre cat were helpful but added very little volume aft. I could consider foaming up the aft sections forward of the old transom as this is the only way to increase volume in any meaningful way.

When you start a design the most important thing to start with is a weight study. This is really hard to get right and many nice designs started out with less volume than they had in later models. In the 80s the Crowther 10 had two sets of hulls as did the Mystery Cove 380. In some cases the difference was about 30% volume.

You need enough volume to support all the stuff we put in the boat and you also need to get the centre of buoyancy and the centre of waterplane area where you want. The CG will be above the centre of buoyancy as a matter of course. The boat will float so that it does. As we add weight to the back of our boats with dinghies and targa bars the GC shifts aft. The only way for the boat to make a reaction force is to rotate stern down.

Adding volume to the stern sections can move the CB aft to better reflect the real world use of our cats. The CG moves back and so moving the CB aft by foaming up the aft sections can get the hulls floating level again. Or you can just stick an extension on to smooth the water flow off the immersed transoms. If you look at some cats like the Perry 40 you can see the chamfer panel is at an angle to the water. It's not supposed to be. The boat started out at 37ft and had 3ft added on. The 3ft didn't rotate the boat back to the designed level but did smooth out the wake.

You also have to be careful about the waterplane area. When we add or take off weight the hull will rotate around the centre of the waterplane (which is the footprint of the hulls on the waterline). Some big bummed cats can have issues with lots of volume aft and little forward. This means that when one hull is heavily depressed (as in going hard to windward) it can go bow down as the sterns are harder to depress than the bows. This is not ideal. It is why some really good designers dislike adding wide sterns to cats even though there is lots of weight placed back aft.

Some designers have tried to play around with volume above the waterline to get the bows to stay level or even go bow up. I have found this a little tricky with my little cats and most designers seem to make the bows higher than the sterns and get extra volume by height rather than immersed volume at rest.

So it is important to be careful with stern extensions. If you only add volume to the immersed sections to get the boat floating more level, or add stern extensions that have low enclosed volume (steps and open sides help here) all should be good.

cheers

Phil

[yeloya]

We did extend one Lipari 41 by 2 inches a year ago for one of our aussie customer. It costed app. 10.000 e with extra hatches on the extensions.
I asked him what was different after the extension was made. His answer is below:

"
Well after 2 1/2 months of sailing in everything from dead calm to 45 knots on the nose my conclusions about the hull extension are:


Speed: hard to differentiate between improvements from the jib and the hull but she surfs easier down wind and carries the surf longer and upwind maintains the speed better because it is not as effected by the waves. I have been hitting 10 knots both on and off the wind which was rarer before


Ride: My wife who gets seasick in bad conditions says she has noticed a substantial improvement in the motion of the boat and it definitely has a much smoother motion through the water with a slower less violent hobby horse action


Other benefits: the additional storage is very useful, particularly on the hull with the additional hatch, but the big gain has come from having a decent boarding platform when getting in / out of the dinghy and also after swimming. This has made a huge difference and something I hadn't factored in


So in conclusion would I do it again - yes without hesitation. It has made her a very different and better boat for long term cruising so thank you for doing it for me.
"


Cheers


Yeloya

[Epicurean]

Did I get that right, €10K for 2 inches? €5K/"? €2K per cm?

It seems like you've done a lot. Is there any guide as to how much it costs?

[yeloya]

Originally Posted by Epicurean

Did I get that right, €10K for 2 inches? €5K/"? €2K per cm?

It seems like you've done a lot. Is there any guide as to how much it costs?

Ooopps.. sorry..

Of course I meant 2 ft..

Yeloya

[admiralslater]

I have seen a couple of Danson 45 s with extensions. The biggest advantage was the eas of getting into the dingy

[fxykty]

Originally Posted by admiralslater

I have seen a couple of Danson 45 s with extensions. The biggest advantage was the eas of getting into the dingy

The Outremer 55 is an extension of the 50 - hull lines reportedly exactly the same forward of the rear beam. Though I’m wondering whether a little was added to the bows as well to maintain the original CB. The extensions also allow shafts rather than sail drives and put the rudders right aft. Dinghy access is great, especially if you cut away the inner wall above the rearmost two steps.

[Simi 60]

If you're going to do an extension at least make it worthwhile and pick up a batten and follow those lines.

Crowther/Geoff Cruse foam kevlar epoxy super shockwave getting a makeover.

[jmh2002]

Great photo.

[mikereed100]

Something I would suggest to anyone considering extensions is to try to get the bottom "step" of the extension at least 1' above the waterline. If this step is lower it will easily submerge as the boat pitches in a lumpy anchorage. The noise this makes as it dips in and out of the water can be very annoying.