Boat extension

If one is to buy a set of plans and extend them to let's say 12% longer.
What would be the pros and cons ?

 

It depends on the design, but most can have the station molds expanded this much, simply by respacing them on longer centers. For example if you had 16' design with stations on 24" centers, you could (for example) add 3" per station, so they'd be spaced on 27" centers, making her 18'.

This is the safe way to get a longer boat, as the beam and draft remain the same, just the length is affected.

Again, it depends on the design as, some may not like this much, so we'd have to know which design you have, to make this determination.

 

Thanks for the reply, pm sent.

 

If the boat has a plywood or metal hull with developed surfaces, and the plans include unrolled panel shapes, then the panel shapes in general cannot be simply stretched but will need to redone.

 

Interesting, thanks.

 

Extend a boat is simple: cut by half the length and added a cylindrical body of the desired length . But there are more things to consider :
- Generally will decrease the depth so that the propeller no longer work in the initially planned depth.
- To increase the length in general decreases the longitudinal strength of the hull girder . You have to study this phenomenon and see how it affects the modulus of the midship.
- To reassess the overall scantlings.
- To decrease the ratio beam / length, decreasing the stability of the boat. Essential to check the loading of the ship.
- The position of the center of gravity of the float switch so that the trim of the vessel will be different. If planing hull , planing conditions change.
All this , and something else , you have to check before you decide to make the change. It is unwise to rely on other similar works because each boat is different to others. Note that if the boat is broken, it's your boat that breaks , not the neighbor's boat.

 

Didn't know there was so much that goes with changing the length and beam
of the boat.

 

These are issues that should be analyzed. Not to say that the extension can not be carried out.

 

Understood.
I will not forget this, thanks for all the advice everyone.

 

I general what DCockey says is true but: You can do that if you are careful and know what is happening to the station section lines. It does require relofting of the edges but it is not difficult unless the shapes of the hull are severely curved. Done it several times with no issues.

 

The only boat that is really easy to "stretch" is one where there is continous section of the hull that is a prism, or the projected lines aft form a prism. I have seen a few planing hulls that were successful in their original conception, which tempted people to 'improve' them by tacking on more length, usually with disappointing results.

 

All this ridiculousness from a supposed designer. I have no idea of what "adding a cylindrical body of the desired length" has to do with anything, unless lengthening a submarine, but stretching a design is a pretty easy thing. Designs have been stretched by respacing station molds for centuries. It's not difficult, nor especially problematic. I've done it several times as has any experienced builder for countless generations. As I mentioned, some designs can be a problem, but the lobster boat he has in mind, appears to be a good candidate at first blush. Naturally, you'll need to spile the planking, even if it was a hard chine (it's not), but this would be assumed by any experienced builder. Considering Tad's relationship with this designer, he'd be the best person to ask.

 

TANSL wrote this about the problems re extending a boat.

"decrease the ratio beam / length, decreasing the stability of the boat"

Depends how you look at it. If you extended a 16' typical OB boat say 100' I'd say stability would be much higher. Step aboard and it will hardly move. No more beam but much more boat holding the hull steady on her WL.

 

You're correct Eric, it will be more stable, but I've stop attempting to discuss typical method and techniques with [him] < reported post: personal comment removed from post 14 and response 17 >

 

Tansl states - To decrease the ratio beam / length, decreasing the stability of the boat."anoe
Tansl is correct.

A 16' OB boat extended to 100’ would roll over if hit by a large wave, its width that gives a boat stableness, this is common sense.