Extending Jeanneau SO 54 Stern by 1.8 meters

[zamibc]

Hi

I got an offer to extend the length of a brand new Jeanneau Sun Odessy 54 by 1/8 meters. The use of the extran length is to provide a spacious swim platform, some additional storage, etc.
It may increase speed (LWL) but I am not sure by how much.
Nothing else will change.
I appriciate your input very much, especialy from stability, performance, safety/

Thanks

Zami ben-Chorin

[Eric Sponberg]

Zami,

Generally, in a swim platform extension on a sailboat, all of the structure is above the waterline, so it will have little effect on performance and stability.

What is more critical is how the structure is built so that it is strong and blends in well with the hull. For only 1/8 meter, this should not be hard to do--that it not very much extra length.

Eric

[MikeJohns]

Did you really mean 125 mm or 1.8 m for the spacious swim platform?

[zamibc]

1.8 meters

sorry for the confusion

zami

[Eric Sponberg]

Zami,

Even at 1.8 meters, this is just over 10% of the hull length, and again, most of the structure will be out of the water. Adding the weight aft will cause the boat to trim differently--a little more by the stern, and in some conditions may cause the new stern extension to partially submerge. This may help marginally on overall length on the waterline, which may have a fractional difference in speed, allowing for a slightly higher hull speed. But sailboats rarely travel at hull speed, so the benefit will be of little consequence. Again, the most important factors are going to be to keep the weight of the structure as light as possible without sacrificing strength and durability. The blend of the addition into the hull should be seamless and natural-looking. That is, the lines of the extension should fit well with the lines of the hull.

Eric

Thanks Eric
The other question is the influence of the hull extension on steering? the ability to stay course in marginal conditions? what will happen if the extension is partly below water line, it will add to bouancy, will you have to add to keel weight? what will happen to sailarea/displacement ratio? to hull length / beam length?
Is the whole exercise recommended?

Regards

Zami

[Eric Sponberg]

Zami,

Typically on stern extensions on sailoats, the amount that the new extension is in the water is usually very small and will have negligible effect on steering and maneuverability. You should not have to add weight to the keel, and there will likely be neglibible affects to sail area/displacement ratio and to length to beam ratio. Adding a stern extension is not going to make a big difference in sailing characteristics, but it will likely make a big difference in useability, which is what the owner wants. There is nothing too sacred with keeping design ratios constant--they are what they are, and a stern extension is not going to change them so much that you would have to care about them. The proof of the pudding will be if you can design and build the stern extension at a price the owner is willing to pay. If it is reasonable, then he will go ahead and get that much extra enjoyment out of the boat for the money that he spends. Properly done, a stern extension should add value to the boat and not affect the design and sailing characteristics appreciably.

Eric

[MikeJohns]

If you work out the weight of your laminate for the extension, add the stores that will be added and find the displacement of the new bit then the designer will be able to calculate the trimmer ballast rquired to put her back on her lines.

Some of these boats are pretty wide at the stern and narrow at the bows, the extension aft will add to the tendancy to force the bows under when running in a sea. To windward it will only improve performance.

I feel that a 1.8m extension is a bit too long not to involve the designer or to engage an engineer. A simpler option would be to add a good base for a large folding swim platform. Platforms that hinge up make life easier and cheaper in the marina and when they are raised they make it less easy for for people to board your boat stealthily.

[yachty4000]

I dont accept this part of the boat out the water. In wave pattern or near hull spead this extensions going to be well and truely in the water and needing to be part of the integrated structure.

Also for example if it built to ABS standards or most other standard you structures now void as you just altered L the thing all the calcs are based on! Remember they dont see it as an extension but a new boat design!

It will also as said be quite a bit of weight but this isn't such a problem. I dont think i'd do this without talking to the designer first as he has all the hull models to check how it will trim. And be able to make up templates for extension quite easily.