Boat extension

Perhaps Alberts wishes were not clearly defined as he did not give us some more parameters

ie if he wants the additional length to have a bigger head, stateroom, salon, galley, etc, ie proportional to the 12% target that he suggested, then just multiplying the frame width by the 12% will get him what he wants if he has already purchased the plans and wants the current layout, only larger

But if he is wanting to change the interior layout ie leave the galley and head the same, but add another stateroom, changing merely the frame spacing could introduce issues inside that relate to having to install bulkheads that do not conveniently coincide with frame locations. So perhaps Albert can tell us if it is just a bigger boat with all room proportionally bigger or if he is wanting to install another room.

If he is wanting to add another room which might change the other rooms measurements this might prohibit attaching bulkheads to frames, then perhaps he should just find another set of plans for a longer boat and discard all the problems that his stretch will introduce. Many of the home builder plans have some frames that become bulkheads.

Those of have spent money building boats will probably agree that the cost of a set of plans are a miniscule amount of the cost of a finished boat.
So my suggestion to Albert is this

1) if you merely want the same boat only longer but with the same configuration, then just add the percentage point to the frame spacing as many of the contributors have suggested. This will be the least problematic solution.

2) if you are considering changing the layout, adding rooms etc, then pay a small price for a set of plans and have a boat designer make the construction much easier

 

First post in the thread:

I assumed this thread was about lengthening a design which had yet been built.

 

Paul Kotzeue, I agree with what you say. In some cases it will not be easy to make the extension. If the boat has a very deep keel the solution is more difficult, and expensive, than if you have no keel.
Barry, you are quite right but that's not the issue that worries Albert.
What I have tried from the beginning is that Albert knew that lengthen a boat is quite complicated in general and should not be swayed by the advice of those who told him that a few more distancing frames was enough, and simple. Even if this could be done, which is not the case of Albert, the greater distance between centers of ribs could lead to a review of the scantlings.
Anyway, I hope I have helped Albert.
Cheers

 

TANSL, why is it not possible to lengthen the boat Albert is interested in by increasing the space between frames/stations?

 

I think there are many things I'm not understanding. The boat is built, so the frames are "welded" to the shell. How can you change the distance between frames?
I'm sure to be wrong because I can not understand it.

 

My understanding is the original poster, Albert Jr., is planning to build a boat, not modify an existing boat. He asked his question to help him decide what plans to buy.

for example in in post #51 he said

Me considering the Potluck was true but as you continue reading the thread, you will see that I've changed my decision and am now prepping to buy plans for the Johns Bay boat 32​

 

You're right David, I have not read (or have forgotten) some of the pots and, therefore, I was wrong. I humbly apologize.
In any case, I still don´t see clear why, in a new boat, to change the frame spacing proportionally from an existing boat. Is not it better to draw directly the boat we want and give the frame spacing suits her?. An incorrect frame spacing can increase the weight to no avail.
If Albert wants a boat ded 39 ', is not it better to choose plans for a 39' tan to modify a 32 '?
Maybe I've stopped reading something else and, therefore, I'm wrong again. Any clarification is welcome.
Cheers

 

You're misunderstanding what is done to stretch a hull by increasing "station mold " (not frame) spacing. The new station mold spacing is not used as the new frame spacing (though it can be on some designs). It's only used to provide the appropriate length hull form. With dictates from the designer, the frame spacing (if it even has any) is typically left the same or reapportioned by the designer.

You should read the thread more carefully, possibly solving some of the disputes. He doesn't want a 39' boat, of which a plan does exist, but he wants a stretch of a 32' boat to 35 ' 10" (12% stretch).

Lastly and again, you only change the station mold spacing and you do not:

change the molds (or frames) proportionally.

 

Thanks PAR, now I understand. I was not thinking about the mold but at the boat´s drawings. Everything is clear now. Also guess what Albert wanted was to buy, to a certain designer, a boat of a size that was not among the standards of the designer. Is that right or am I wrong again?

 

Damn. Was all this argument caused by a lack of understanding that design station spacing and frame spacing may, and usually don't, have any relation to each other? That is fundamental to designing a boat. Some boats have no frames at all. My boats get along quite well without a single thing that could be called a frame.

 

Yes Tom, you're quite right. My lack of understanding has caused several pots uninteresting but we have also discussed various possibilities for lengthening a boat, its pros and cons and the factors that are involved in such a transformation. As far as I'm concerned, I apologize for wasting everyone time.

 

No one is forced to "waste" time on forums. Hopefully, all are better served by time spent here. Learning what does not work is often much more valuable than what does. Bruised egos are part of the process and while some are better at the bruising than others, I for one, am grateful for the back and forth that takes place here..

 

You are correct, of course.

The fore and aft curves of the hull will no longer be fair.

You will have a flat spot equal in length to the inserted sections.

This probably will not do for a yacht, were fair fore and aft curves are part of the aesthetic expectations of the design.

Hydrodynamicaly, it will probably not effect the boat much at all. In fact I have heard that when this is done to displacement craft, the displacement is increased but the speed remains the same with the same engine.

If you have a boat that is built without lofting and the designer only provides offsets for the bottom and side panels, along with the bulkhead shapes, lengthening the distance between stations to get a longer boat will probably not work. The reason for this is there can be quite a bit of twist in these panels at the ends of the boat.

Lengthening the distance between station lines on these panels may cause them not to line up with one another, when the boat is assembled, because they are most likely of different lengths and will come together at slightly finer angles, causing unpredictable lumps and bumps where none were before.

The bulkhead shapes would be off too, making it necessary to cut and fit them. Since they help define the boat's shape, in the first place, lots of luck with that

By simply duplicating the largest station and running a straight line between it and its clone, you avoid all this hassle. The straight insert will have no twist, or fore and aft curve, of any kind in it. It will also be the exact same length on all the panels. The old bulkhead shapes will still work. And the boat will be longer.

Work boats are usually lengthened to increase their displacement. This method is quite effective in doing that, as the increase in displacement is out of proportion to the increase in length.

So, this lengthening technique may not be the best in all circumstances, but may be the only one that will work in some.