Union Jack extension - would like to increase the length by 15% or maybe 20%

My wife and I have decided to build the Glen-L steel hull Union Jack semi-displacement hull. Glen-L says we can extend the hull by 10% with no adverse effects, however we would like to increase the length by 15% or maybe 20%. We have been told that extending it by 10% means we do not have to increase the number of frames, only that they have to be re-spaced. If we increase it by 15% to 20% would we need to add a frame and is it feasible to extend to 15% to 20%. Would it affect the way the boat would preform. Any tips or advise would be gratefully appreciated. I am a proficient welder and mechanic but not a marine engineer so any help would be great.

Iceman46

 

Do not trust you in which say things without demonstrating them, because he does not risk anything. You risk your life and your money.
You will build the boat completely from scratch so it pays to do some calculations and create the appropriate drawings to build the boat you want. Increase the length by 15% or 20% can greatly increase the weight and therefore the necessary power, you will reduce the stability and can have some harmful effects that currently do not occur to me.
Make someone consider what you want to do.

 

Thanks TANSL
The boat is a glen-L boat plan specifically for steel with full size plans
There is a man in England that extended it to 34 feet with no problems and al I want to do is extend it to 35-36 feet.

 

You can do everything you can think of but you'd see whether it is correct or not. For example, you can extend your boat and have no problems except, perhaps, the balance period is so short that the navigation is very uncomfortable. One can sense that the elongated boat 20%, with the same engine not reach the same maximum speed.
What is clear is that how to navigate the ship will be different and, in my opinion, it's worth doing some investigation.
You will build this boat you both want but knowing well where are you going to arrive.

 

Union Jack Extension

My wife and I have decided to build the Glen-L steel hull Union Jack 31 foot semi-displacement hull. Glen-L says we can extend the hull by 10% with no adverse effects, however we would like to increase the length by 15% or maybe 20%. We have been told that extending it by 10% means we do not have to increase the number of frames, only that they have to be re-spaced. If we increase it by 15% to 20% would we need to add a frame and is it feasible to extend to 15% to 20%. Would it affect the way the boat would preform, and if so how? Any tips or advise would be gratefully appreciated. I am a proficient welder and mechanic but not a marine engineer or designer so any help would be great.

Iceman46

 

Generally, no. This is a standard practice with some designs, that is to "stretch" it by respacing the station molds or frames. 15% is commonly considered a max stretch, in the best situations, so 20% isn't wise. The problem is the structure can't tolerate the loading with the length enlargement, without redoing the scantlings, recalculating weights, centers, etc. Call Glen-L and see what they'll say, though I can guess what the reply will be. Simply put, if you need a 37' trawler, you need to look at 35' - 37' trawlers, not up sizing a 31'.

This is a duplicate thread, please delete the other one.