Question re: Modifying Boat Designs

[WI-Tom]

From what I understand, many designers agree that it's often OK to stretch an existing design by around 10%, keeping the same beam and increasing length only.

I've also seen examples of boats that were built by increasing the total size of a design, increasing both length and beam. I'm thinking in particular of a Bolger small boat design--a peapod?--which the builder had built large enough to include a small cabin. Maybe a 150% increase overall?

I've never heard of anyone trying (much less succeeding) to take a design and increase the beam, while keeping the designed length the same.

As for me, I'm not one to modify a hull at all, though I've changed construction methods, interior details, and techniques at times. I'm keenly aware that changing one aspect of a hull creates a cascade of rippling consequences, and never wanted to get caught up in that.

But I recently met someone who is interested in taking an existing design (18' long, 4' 8" beam) and increasing beam to around 5' 6" while keeping length at 18'. I've never heard of that kind of thing being done, so:

What says the WBF? Is such a thing possible? Common? Advisable? Ludicrous?

Tom

[David G]

Sorry, but the answer to any general question about modifying a design is...

"It depends". From the scant bit of info you've shared, red flags are flying.

[WI-Tom]

Sorry, but the answer to any general question about modifying a design is...

"It depends". From the scant bit of info you've shared, red flags are flying.

The design in question is Don Kurylko's Alaska, which is based on a traditional whitehall hull. This:

Alaska Plans.jpg

The person in question is an experienced builder (kayaks, canoes, a Caledonia yawl, a GIS). But the idea seems a bit iffy to me.

Now, with more info, what says the WBF?

Tom

[Edward Pearson]

Myst is 5’7” x 18’3”. Tell him to build that and stop vaccilating.

[Jack Loudon]

I'm no naval architect, but think the increased beam would turn out okay, though I wouldn't do it myself. It would still be a narrow boat by most standards, and the long waterline and balanced ends would minimize potential hull imbalance issues.

There are plenty of shorter fatter boats out there that sail well. I had a friend once who was building a Flicka, but spaced the mold stations too close together so the boat ended up at 18 ft rather than 20 ft, on an 8 ft beam. He finished it anyway and it sailed fine. It even looked okay, though he lost the standing headroom. The Moore 24 ULDB sailboat originally had a very narrow beam (around 6 ft when it was called Grendel) and the builders forced the mold sides apart to increase the beam to over 7 ft. The sides turned out asymmetrical, but the boat was a very successful ocean racer. This is a long way of saying that people increase beam in non-conventional ways and still end up with good boats.

The bigger question is WHY your friend wants to do this modification. Would he rather be rowing or sailing? His modification will gain capacity and sail-carrying power, and lose some rowing efficiency. But it will never sail as well as a boat designed primarily as a sailboat and secondly as a rowboat, rather than the other way around. To paraphrase Bolger: good sailboat will almost always row better than a good rowboat will sail.

[Todd Bradshaw]

I added about six inches of beam to my fur trade canoe, in the form of a long cigar shaped addition starting at the bottom of one stem and ending at the bottom of the other stem. Unlike the big "ride" canoes that you see in places like Disneyland (with cross-sections like a big Grumman) it was an authentic design. The real boats were built with narrow bottoms and flared sides and nearly always carried a big load down low, thus increasing the waterline beam and initial stability. Since I wanted to be able to use it comfortably with only the paddlers in there, I figured that it would be a good idea to increase the beam a bit. As it turned out, it worked fine and was worth doing.

HB1.jpg

[Andrew2]

As mentioned above, why not find a design that fits ones ideas? Bulging out an existing one has loads of problems as opposed to stretching a bit. Your friend has experience in builds, so why is he on this path?

[Jack Loudon]

As mentioned above, why not find a design that fits ones ideas? Bulging out an existing one has loads of problems as opposed to stretching a bit. Your friend has experience in builds, so why is he on this path?

What are the loads of problems? Certainly different characteristics inherent in a slightly beamier boat, but problems? I say this because I believe that if common sense is used, the results won't be nearly as dire as some suggest.

[woodpile]

Have read the 3:1 length to beam is a good starting point, my experience is narrower beams tend to run a chop better, wider beams like to pound.

[Canoeyawl]

I know of a No Mans Land boat that was build to Chapelle's lines at 18'+ and then a friend took the same molds and re-spaced them equally to give a 16' oal boat.
That was a great little boat, it sailed very well, powerful for a boat that size and moved under oars just fine, it had a 2hp outboard in a well. All in all it was at least as good a boat as the original and maybe better in some respects. (It fit in his garage)


The increased displacement may be interesting, and depending on "how" and "where" the beam was increased it might about double the original boat, but a bit of beam is usually a good thing.

It is going to be a completely new design.
Let's have a look at the overlaid plan views and sections (w/waterlines...)

[Thad]

Using the model for the Columbia Lifeboat, NG Herreshoff had molds made for boats from 10 to 16 feet (at least) which then were spaced to make boats from 9 to 19 feet, with varying proportions length to width by stretching or compressing mold spacing, sometimes adding strakes for depth. I would think Alaska would be susceptible to such treatment, whatever Don would think. Whitehalls were traditionally not built to a plan but a process, the proportions varied to fit the proposed use, with deadrise and wineglass stern shape generally maintained.

[Stiletto]

Given that the fellow proposing the idea is an experienced builder, I think he probably has a fairly realistic view of the level of risk involved, both in performance of the materials and the finished boat.

[neil.henderson]

Sorry, but the answer to any general question about modifying a design is...

"It depends". From the scant bit of info you've shared, red flags are flying.

Very true - try watching the recent Tally Ho video regarding centre-of-buoyancy and centre-of-gravity. Altering proven designs has its risks, but perfectly ok if you accept that there is a process with steps which should be undertaken.

Regards Neil

[David G]

What are the loads of problems? Certainly different characteristics inherent in a slightly beamier boat, but problems? I say this because I believe that if common sense is used, the results won't be nearly as dire as some suggest.

It depends.

Some boats are rather basic. Some are engineered to the nth degree. For the former, small mods are not usually significant. For the latter - mods are likely to tip a design that's pushed hard in specific directions over into 'non-viable'.

[Woxbox]

Isn't Don still with us? Why not ask him?

[NickN]

I like the idea, and I'll throw out this thought.

The profile edges and the sections are provided in the study plans that can be downloaded for free. Insert them as background in delftship and rebuild the lines over them, then use the scale function to expand all points laterally by 1.18? Could keep the keel plank the same width (omit those control points from the scale-up). Might not be perfect, but it will provide a lot of hydrostatic information that I imagine would be close enough to be useful if done carefully (Prismatic, wetted area, displacement, entry angle, waterline beam . . . - and that's without adding weights and getting into stability). Seems like a decent starting point if serious enough to take the time.

Expanding beam can change the look of the windward sheerline when heeled, so you could view from all angles to assess aesthetic changes too.

[stromborg]

Myst and Alaska, interesting to compare the two side by side. What, one wonders, does the builder hope to accomplish by increasing the beam of the Alaska? Is it the rig? Wouldn't it be easier to change rigs than re-engineer the hull? Maybe it seems like a fun project? (Can't argue with that)

I'm not on one side or the other, just curious about the the thought process. For most of us building these boats is a romantic flight of fancy so you should build what strikes your heart.

[Autonomous]

Let me (over?) simplify Mist vs. Alaska as sail-n-oar vs. oar-n-sail. IMO the rigs are secondary to the hull forms.

[dbp1]

Let me (over?) simplify Mist vs. Alaska as sail-n-oar vs. oar-n-sail. IMO the rigs are secondary to the hull forms.

But isn't that significantly a function of beam? i.e., what is wider-Alaska?

Seems like the most _sensible_ approach would be to at least start with the lines for Myst; if they want to do the internal furniture, rig, etc, from Alaska, then make that be the fun project.

[stromborg]

Let me (over?) simplify Mist vs. Alaska as sail-n-oar vs. oar-n-sail. IMO the rigs are secondary to the hull forms.

You're right, the rigs could be more or less interchangeable, though without digging too deep Alaska looks to have a lot of unreefed sailed area for a skinny hull. Reef early and often? It also looks like adding beam to Alaska pretty much gets you Myst.

Still curious about the thought process.

[Jack Loudon]

Still curious about the thought process.

Me too, especially since the requestor has previously built a Caledonia Yawl, which seems similar to Alaska in size and row/sail function. I have to think he has well-thought-out reasons to want a beamier Alaska, rather than something else.

I'm currently finishing up a designer-authorized 25% stretch of a stock-plan row/sail dinghy, done because my specific needs couldn't be met by any existing designs. So I know the feeling of finding a design that is "perfect, except for one thing".

[NickN]

I don't know about the comparisons with Myst. That is a much larger boat. Alaska disp LWL = 425 lbs; Myst disp LWL = 1200 lbs. 12 inches more beam isn't going to add more than a couple hundred pounds displacement to Alaska, I'd guess (?). Still only half the size of Myst.

As already suggested, seems like it would just make for an Alaska that's better able to carry sail, but doesn't row quite as well, with the benefit of a bit more room in the boat. I agree it would be interesting to know.

[Autonomous]

Alaska at 425# to Myst's 1,200# is significant. It's been said one average man can reasonably row 1,000#.

[Autonomous]

You 'spose any Alaska builders have installed a third mast step ahead of the daggerboard? She does seem to have a lot of sail area/displacement.

[WI-Tom]

You 'spose any Alaska builders have installed a third mast step ahead of the daggerboard? She does seem to have a lot of sail area/displacement.

That third mast step is specified in the plans. When it's time to reduce sail, you can step the main, reefed main, mizzen, or reefed mizzen alone in that step, and it should balance like that. I sail with mainsail alone (no mizzen) and use only that mast step, like this:

44.JPG

Seems like most Alaska sailors are down to mainsail alone, in that step, when the wind hits 15 knots. It's a slender little pulling boat with lots of sail area in the full rig of main (85 sq ft) and mizzen (49 sq ft). With mainsail alone, I might be a tad underpowered in ultra light airs, but that doesn't bother me because the boat rows faster then anyway, especially to windward.

The one disadvantage of that center mast step is that it's almost 12" lower than the forward step, so the full mainsail comes down all the way to the sheer when using the center step--visibility to leeward ain't great (though it's OK when sitting on the keel, as I often do). Simple enough to just lift the sail from time to time to peek under, though.

Tom

[WI-Tom]

Alaska at 425# to Myst's 1,200# is significant. It's been said one average man can reasonably row 1,000#.

It's worth noting that Alaska can be loaded up to 1,100 lbs displacement with no trouble at all. The plans show only a 2" increase in draft fully loaded like that if I remember right. You can haul a LOT of stuff in a whitehall, and it will still row gracefully. Takes a bit more time to get up to speed, but then the momentum/inertia kicks in and it's easy to keep moving, even through a moderate chop.

Loaded for long trips, I can row my Alaska at 3 knots with moderate effort for a long time in flat water. You could carry lots of fresh water for a long ocean trip if you had to.

Tom

[WI-Tom]

Me too, especially since the requestor has previously built a Caledonia Yawl, which seems similar to Alaska in size and row/sail function.

I disagree--I think they're very different. Alaska (as designed) is far easier and more pleasant to row than a Caledonia Yawl. And Alaska is a MUCH smaller boat, lower freeboard and much lower volume. Two people in an Alaska is about all you want, unless you park a third person way up in the bow ahead of the center mast step (that's actually a good place to put a novice non-sailor, where they have a comfy backrest and don't have to duck the sail at each tack). I think the Caledonia Yawl might comfortably carry 5, maybe 6 people sailing.

The Caledonia Yawl in question is rigged as a gunter sloop, so I think the builder wants some kind of lug rig for faster rigging to facilitate daysailing. But more beam for sail carrying capacity than a standard Alaska.

I'll admit, Alaska feels pretty tender. In high winds it'll sail along with the gunwale right at (or just under) the surface of the water. Easy to scoop up a bunch of water if you're not paying attention, though the final stability locks in right there and it's not easy to tip any farther than that.

Tom

[AJZimm]

I’m with those who are asking why the person wants to modify Alaska, and wonder what they are looking for that the Alaska doesn’t provide.

Having built an Alaska and rowed and sailed it for a few years, I would say Tom has nailed its characteristics. It’s a fine boat for what it is but I wouldn’t modify the design to achieve whatever it is the person wants to achieve. When I decided that the Alaska didn’t do what I wanted it to, I designed an entirely new boat and didn’t modify the existing design.

[NickN]

I got curious and made a model in Delftship last night from the limited, public information on the Alaska. Obviously the result is not an Alaska, but I'd say it provides a means to compare original and widened versions of a hull fairly similar to Alaska. I considered posting my original and expanded lines, as I find it interesting, but thought better of publicly vandalizing Mr. Kurylko's work. I hope it will be inappropriate to share my findings in more abstract terms, though.

Expanding the beam of my model (including keel) by a factor of 1.22 to get 5'6", I got, approximately:
- a corresponding increase in waterline beam
- a 20% increase in displacement
- a 17% increase in wetted area
- .003 drop in Prismatic Coefficient
- 3 degree increase in entry angle

I don't see any deal breakers in the above, myself. The real downsides seem to me (1) the altered look of the boat (surprisingly different in my opinion, with a good bit of powderhorn. To my eye the widened version wants the stem to be several inches higher with more curve in the forward part of the sheer) and (2) the need to use and store longer (10' or 10'6" ?) oars.

Now I can exit this rabbit hole. Hope someone finds this useful/interesting.

[NickN]

I designed an entirely new boat and didn’t modify the existing design.

I think modification is possible, but personally I agree with this way of thinking.

[Matt young]

It looks doable to me. In the end it depends on the builder/modifier. If he has the skills to make sure his modification looks good, then it should work. Plenty of crafts in that size range. Of course it could go wrong, that would be on him.

No boat design is written in stone, otherwise we’d be in dugouts.

[Canoeyawl]

I got curious and made a model in Delftship last night from the limited, public information on the Alaska. Obviously the result is not an Alaska, but I'd say it provides a means to compare original and widened versions of a hull fairly similar to Alaska. I considered posting my original and expanded lines, as I find it interesting, but thought better of publicly vandalizing Mr. Kurylko's work. I hope it will be inappropriate to share my findings in more abstract terms, though.

Expanding the beam of my model (including keel) by a factor of 1.22 to get 5'6", I got, approximately:
- a corresponding increase in waterline beam
- a 20% increase in displacement
- a 17% increase in wetted area
- .003 drop in Prismatic Coefficient
- 3 degree increase in entry angle

I don't see any deal breakers in the above, myself. The real downsides seem to me (1) the altered look of the boat (surprisingly different in my opinion, with a good bit of powderhorn. To my eye the widened version wants the stem to be several inches higher with more curve in the forward part of the sheer) and (2) the need to use and store longer (10' or 10'6" ?) oars.

Now I can exit this rabbit hole. Hope someone finds this useful/interesting.

Yes, very interesting, thank you. Having watched an Alaska turn turtle in about two seconds (no fault of the boat, a radical change in wind conditions) it could be a good thing. Talking to the designer would be wise... I'll bet he is interested, and may even post here about it.
A very different boat it will need to be 20% heavier to be "on it's lines" (What is that in pounds? a bit larger picnic another person, maybe an outboard in a well?)
Some heeling moments would be interesting.
Personally, I like it.
You would almost want to build a mock-up. Maybe half of it, I have done that, half frames and battens to represent planks would give a good idea of appearance. That can be done upright, tweaked until you like it and the offsets transferred to the lofting.

[Autonomous]

I see the Alaska as inspiration, it was the first serious sail-n-oar boat that caught my attention. For me it is too lean though. (See what I did there?)

My novice concern is how much do you adjust the hull and rig scantlings when you add beam and displacement to a proven design.
Clean sheet vs. corrupting (?) a sleek vessel? Aye, there's the rub.