Modifying plans...

[SculptorSam]

Hi everybody.

I don't know whether I should be posting this here or in Building but here it is. As a general rule, is it possible to shorten the length of a plywood boat, by about 20%, while maintaining the same beam? Not by simply cutting off the end but more along the lines of repositioning the bulkheads proportionately to the original plans?

The boat I have in mind is a very straight forward flat-bottomed design that is just a bit long for my tastes or my workshop. I don't know how everybody out there feels about modifications but when I see something I like, I immediately try to figure out how I can "improve" it. Thanks in advance.

Sam

[JimD]

Considering I have absolutely no credentials as a designer, 20% sounds like a lot but maybe insn't, depending on the design. What boat is it, SculptorSam? - jimd

[NormMessinger]

Probably depends on the design. The McGregor sailing canoe in Iain Oughtred's catalogue of plans can be built in three different lengths just by changing the spacing of the molds. Each has it's own stem and stern profile. The difference from the shortest to the longest is something like 13' to 17', but I'd need to check the plans to be sure.

--Norm

[SculptorSam]

JimD- For some reason I was hesitant to mention it but here goes... It is the AF4 Grande by Jim Michalak. Can be found at: http://www.duckworksmagazine.com/sto...ande/index.htm
It is actually a 20% scaled up version of his base AF4 but I like the wider beam of the Grande for more room overnighting with my wife and the ability to add more amenities. Now that I have tipped my hand, we shall see.

Sam
www.sculptorsam.com

[Tom Lathrop]

Sam,

Why don't you pose this question to Jim? He is in the best position to give you an expert answer. I can say though that making such a large reduction in the length of the AF4 will certainly give you a slower boat that does not handle as well as the original. When the beam/length ratio gets too large, a boat gets kind of indifferent as to where it's headed.

If you will go over to the Wooden Powerboat Forum on Yahoo, there are several builders of these boats there. Someone there can probably help you.

http://groups.yahoo.com/group/WoodenPowerboats/messages

[SculptorSam]

Tom- The decreased handling concerns are something to think about. I am quite the novice so my thinking was along the lines of: I like the Redwing 18 by Stambaugh but would like something simpler (mostly because I'm poor). The modified dimensions I was thinking of for the AF4 are actually the dimensions of the Redwing which is supposed to handle nicely. I did contact Jim and he offered to redesign it for me but once again, I am poor, and can't cover the custom fee. Sorry for dragging you all into my messy reality.

Sam

[Bradley]

Incredible sculptures Sam! Bold, rugged, definitive, they really make you want to go out and build something!

About your skiff, i would recommend that you keep the same width to length ratio as the design calls for. A flatbottom that is too wide will not take the seas very well.

A man with your talents should be able to adjust the plans to suit your needs.

Personally, i don't understand why plans are that important in the first place.

[DesignByBird]

Hi SculptorSam, good question for many to read !!There are many designs which can be extended or shortened - though extending is often the more satisfactory - primarily because of balance and trim. Whichever.., the problem can often be extreme in short waterline lengths.

If you shorten a vessel ; items like the motor has to stay close to where they were (relative to the prop / rudder / transom) – importantly you need to consider the revised weight distribution (along with tanks, batteries, etc.), but also shaft angles - as well as impact on the arrangement plan (and again - crew weight), including headroom..

Similarly the angle of running trim will vary according to ; the hull angle of attack and the after-body run - possibly resulting in a vessel which porpoises badly. In some cases ; what was designed as a planing or semi-planing boat simply doesn’t have enough running surface to get up on step - so you end up with pushing an inefficient planning hull through displacement conditions (there was recently another question posted regarding this - you might be interested in checking it out).

And if that is all not enough ; then consider displacement – the same load going into the boat but a hull with less buoyancy = wave and running resistance, draft (topsided), as well as reserve buoyancy and stability - each adversely effected..

And this is before we even start to consider the scantlings.! I would be very weary of shortening a vessel - certainly by this amount without considering each and every one of these factors. Only on longer hulls with almost parallel hull lines, as more commonly seen on inland waterway vessels, is this idea safely feasible..

And let’s face it.., if you're on a low budget you really don’t want to build a doe-doe, (how do you spell that ?? ) unless of course it’s as a sculpture !! - I would recommend that you enjoy the shopping and research entailed in finding a suitable and proven seaworthy design to your criteria.!

Hope that helps !!, Pete.

[Aramas]

I'm going to have to go against the trend here. I had a look at the design, and imo making it 20% shorter won't do any real harm. The run aft is dead flat, so no worries there. With a flat bottom it will still plane easily, and once planing extra length doesn't do much anyway (since it's not in the water).
Whatever you do *don't* reduce beam. In a planing boat beam is everything. It would be like chopping off the ends of an aircraft's wings - it reduces lift, and that's a *bad* thing.
Shortening it is likely to make it a little slower to plane, but should be faster with a given powerplant once it gets there.
Since I'm going to burn for heresy anyway, I would cut the 2' off the back, not scale it lengthways. That will give you the same entrance, accomodation and waterlines, and would mean that any 'peculiarites' can be remedied by gluing the extra 2' onto the back after finishing it. [img]smile.gif[/img]
I wouldn't normally advocate such things, but that boat is just a box. It's really not that complicated.
Edit - Ack - it's 5:30am here and I'm spaced out from working all night, so I was reading 20% but thinking 2 feet. *10%* (ie 2 feet) would be ok, but 20% (4 feet) is too much. That would seriously mess up the entrance if it were scaled, and wouldn't leave a long enough run if it was cut off the stern. Scaling it *might* work ok, but it might not and there would be no way to fix it if it had weird habits. You could scale it vertically by the same amount and keep the entrance angle, but would loose a lot of headroom in the cabin. It's such a radical change that it would probably be better to make like goldilocks and keep looking at designs until you find one that's 'just right'.

[ 06-01-2002, 03:46 PM: Message edited by: Aramas ]

[John Bell]

I built an AF4 last year. I'd agree that making the boat wider would not be a bad idea. The 18' version is perfect for a couple, but it gets bogged down with more than that aboard. At the very least, I'd make it wider at the transom to make it less sensitive to fore and aft trim.

I do like the boat very much though. She makes 18-19 mph with a 15 horse 4-stroke, and cruises all day at 14 mph at a crack over half throttle. And she never fails to draw a crowd...

Best,

JB