Hull enlargement question??

[jackason]

Hello,

Okay, I have a hull enlargement question. In theory, couldn't you enlarge a very small hull design
(14' x 6') with minimal alterations to the original design by doing the following.

Resize the building stations width but not the stations height (as to not make the boat taller)and respace the stations accordinally to the same matching percentage to achieve the increased Hull length.

So a given Hull 14' LOA and 6' Beam at a 25% increase would be now 17.5'LOA with a beam of 7.5'. wouldn't that be the best way to enlarge a hull while trying to try the original hull design intact.

I relize you can't just respace the stations with the original diamensions and call it a day to gain a longer boat. I also relize there are other structural/scantling issues etc. but this question just pertains to the actual hull.

I'm sure I'm not the first rookie to ask this same question, thanks in advance.


-CJ

[Gilbert]

Hi CJ,
The short answer is you can do anything you want in the way of changing or stretching a design. You can respace the stations, you can expand the length and beam as you suggest, or you can expand the length, beam and depth. Or you can expand the length using one ratio the beam another and the depth yet another. Your choice just depends on what you hope to accomplish. And then the proof is in the pudding as the old saying goes.
Hope this is of some help. I'm just trying to encourage you against someone who is bound to tell you "Oh, My God No, you can't do THAT!" Well, you can if you want to.
The fact that you are a newcomer to boat design etc. warrants that you exercise your best judgement as you proceed. This actually is good advice for old hands too.
Cheers,
Gilbert

[PAR]

Re-spacing the station molds of an existing design is commonly done to gain some additional length. 10 to 15% increases are very common, but beyond this, you start running into trouble. Proportional scaling up isn't practical as you end up with an overly beamy, deep boat. As boats get larger (longer) they require less beam in proportion to their length. Other elements of the design are similarly effected. Once you start moving things around, loads of stuff can happen, stability, trim and balance concerns crop up because the weights of things aren't where they were calculated they would be, plus a number of other issues.

Boats in the size range (17' - 18') you're interested are plentiful, it is the most sought after size in this country. What is the plan you wish to enlarge? Most designs have a cousin or bigger sister from the same drawing table. I have several 16 to 20 foot designs. There are many available on line (some free) quite a few of relatively low cost.

If you can live with a foot less boat, then re-spacing you station molds at 15% will bring your 14' boat to just over 16'. Leave your station shapes alone and plank her up. This is the safe way to do it. The moment you start playing with new station shapes, you're designing a new hull and should be knowledgeable enough to do so with a reasonable level of success.

[jackason]

well the plans that I have are for the Betts/Brewer 14' SDI a.k.a. So-Du-It sailboat. I purchased these plans 3/4 years ago while I was single and had planned on sailing solo most of the time. That was just before I met my wife and had a son.

So I shelved the plans figuring I'd need to built something somewhat bigger to accomidate at least 3 people on for a day sail and 2 on a overnight/weekend trip If I even did build a boat after all.

After looking at power boat plans for small trawlers and mini tugs recently I decided to look at the plans I still had for the SDI. Although I haven't ruled out building a power boat, while looking at the SDI plan I relized I still wanted to build a sailboat....decisions, decisions.

Regarding the SDI, What I found was that even though I still wasn't crazy about the A-frame roller mast sail setup I still liked the rest of the boat alot especially the ergonomics and high cockpit sides. Anyway I still haven't seen another design that is similar that I like let alone larger.

Being realistic I know I don't need a boat any larger than 20' or so (power or sail)and want to be able to trailer. That is the motivation to enlarge the hull 25% to gain 17.5 LOA.

As far as I know that boat was a one time design with no bigger or additional versions. Thanks for the other info though. I'll keep researching the subject.



-CJ