Lenghtening the Chamberlain Dory Skiff

[Ray Frechette Jr]

Gorgeous Week was had at Wooden Boat. I was teaching an introduction to Marine Electrical Systems there and had my CDS on a mooring.

I got up and rowed at 5:00 AM and watched the sunrise, , again after supper and a few times again after dark.

The Dino Flaggelates were illuminating the water where my oars disturbed them and that was incredibly neat as I had only read about it before. Rowing under the dark night sky seeing the canopy of stars above and the milky way was also a treat.

All told I may have approached 100 miles of rowing for the week.

Whcih brings me to my question.

The CDS in 14 foot version rows easily and gets up to hull speed quickly. But watching Clint Chase rowing his Drake gives me the desire for a bit more length on my CDS to raise potential hull speed a bit.

No, Not planning on tying a rope on the back and and hitting the accelerator to stretch current boat, but I do have the mold boards to make a new one. And Walt Simmons makes multiple lengths of his wherries by just increasing spacings on the molds... Well I started thinking and I know that can be a dangerous thing, but if I increased all the station marks say 20 percent to get around 17' 8".....

So, what of the downsides?

I know I might want to bump up the scantlings a wee bit as well....

Now I know Clint would be ever so gracious to sell me plans for Drake and allow me to build my own , but I do get a charge of the old timey look to the CDS with it's glued lap construction and it looked very at home on the mooring amongst the Herreshoffs and Havens even if the brilliant turqoise color was a bit more ostentatious than the whites and dark blues and dark greens....


Drake also looked like it was comfortable in it's own skin and looked fast even before Clint got in and flipped the outrigger hardware and used those long oars and his long frame to accelerate the boat.

In my 14 footer I looked like I was rowing to go to a Miniature Golf Tournament compared to Clint....

[Peerie Maa]

New longer boats were often built on old molds by moving the molds further apart. However 20% means 20% more displacement, or less stability for similar all up weight. As you are an experienced rower, and the flare of the garboard is substantial it may well work for you.

[Ray Frechette Jr]

Well if I changed the beam as well as the length by 20% I could agree that displacment increases 20%.. But if just lengthening I don't think the displacment figure would be linear to length increase.

And by same token weight of frames would be the same but as I increase length I have more wood on the boat and if I increase scantling size I am increasing weight too...

From what I have dread increases of lengths typically increase stability for a given design and decreasing length hurts it. As a general rule I have read that increases in length is all to the good in terms of stability so long as the increases are of a reasonable amount and 20% is usually the upper edge of the reasonable amount scenario.

Also from what I have read increases in length to up to 20% also improves the aesthetic....

[James McMullen]

I betcha it will do nothing but good to lengthen her by a foot or two. I really enjoyed my CDS back in the day too, but not nearly as much as I like a longer boat now.

[Peerie Maa]

Believe me, 20% in any direction = 20% on displacement, 20 % in two directions = 1.2 x 1.2 x displacement. Even though your scantlings will increase, it will not compensate for all of the extra displacement down to the original waterline, so she will float on a narrower waterline, and lose stability on top of the relative rise in CoG over VcB. Trust me on this.

[Carlsboats]

I don't think a longer wheelbase will do anything but improve speed, carrying capacity, and stability. Look at the racing shells -- longer is faster -- and in the singles, the designers often do not increase beam at all when adding to the length. They make them longer to carry a heavier rower, but they don't necessarily have to make them fatter at the same time. As for the scantlings, it is not a given that they have to increase with LOA. Maybe the boat with existing specs. is hefty enough to be lengthened. There might be an issue with torque in the longer hull, but you can evaluate that by looking at the construction specs for other successful boats of similar type, in the longer length.

[Jamie Orr]

John Welsford's Joansa is essentially a stretched CDS, and it is reported to be a good rower. See http://www.jwboatdesigns.co.nz/plans/joansa/index.htm

Jamie

[David Cockey]

Believe me, 20% in any direction = 20% on displacement, 20 % in two directions = 1.2 x 1.2 x displacement. Even though your scantlings will increase, it will not compensate for all of the extra displacement down to the original waterline, so she will float on a narrower waterline, and lose stability on top of the relative rise in CoG over VcB. Trust me on this.

Stretching lengthwise will result in less initial stability for the same weight, but higher statibility at large heel angles.

[Ray Frechette Jr]

For the same weight may be key here.

In any event can anyone tell me how I am going to increase length without increasing the weight proportionately?

The extra wood that is making the boat longer is also going to raise the weight proportional to the increase in length is it not???

If I add 20% more boat the weight is also going to be 20% more is it not?

And if I increase scantlings slightly throughout. The weight will increase more than the increase in materials for the lengthening....

Why is it that shorter kayaks need wider beam for stability if lengthening a boat is going to decrease stability.

Advice here is running contrary to experiential knowledge of boats. Shorter boats typically need to be wider than longer ones...

[Daniel Noyes]

I dont think thats right about the weight,
your not adding 20% to the stem, transom, and frames, just the planking wich is generaly the lightest part of the boat... though there is the most of it.

at 16- 17 ft the Chamberlain dory skiff will be a very large boat, even if you do not widen it,
the design is a oar powered utility skiff, good for any job that needs doing. Drake is a performance rowing craft, closer to a canoe in size than the Chamberlain.
I have rowed my chamberlain back from the flats about a mile, with the tide and against a 25-30 mph breeze took alot of work and any extra boat would have been even harder to move against the wind... though it would make a rocket of a sailboat at that size

[Tom Hunter]

I think David has it right.

Making the CDS longer without widening it will improve the speed, and will make it a bit more tender. With the longer boats you don't so much lose stability as transfer it from initial stability to reserve stability. So your boat picks up performance but tips more easily. It will also be harder to roll over, because the reserve stability is higher.

Dan is right about the weight, but Peerie Maa is also correct about the displacement, your wieght will go up some, but the displacement will go up more. That is not a bad thing, it means you can take two girlfriends out for a row at once.

My experience is the fast rowing designs that have come up in the last 15 or 20 years (or is it 30 now?) all have low initial stability and high reserve, it seems to be a necissary trade off.

[patrice Hubert]

As far as i'm concerned i believe that if you increase 20%, the volume increase more than 20%. Take for instence a cubic brick of 1 meter side it give you 1 cubic meter volume. Now increase that brick to 20% the side will be 1,20 meter and the volume is 1,72 m3...
This rule has to be considered when lengnening a boat ( and when shrinking too...).

[Ray Frechette Jr]

I realize the CDS lengthened will be quite different than Clint's Drake.

Some of the reasons I have contemplated a lengthened CDS are

1.) a little increased speed potential. Currently it seems to be happy maintaining a 4-4.5 speed under oars. I have thought increasing hull to around 16-7 feet from 14 would increase speed to perhaps 5 comfortably maintained.

2.) Extra length would make tandem rowing much more of a possibility. While I have two rowing stations at 154 feet, tandem rowing is awkward.

3.) Longer length should give more time fro water dripping off oars to hit water rather than blow onto my wife when I row in forward station with her in back of the boat. Right now if rowing into wind drops of water land on her and she doesn't care for that much.

[Daniel Noyes]

2.) Extra length would make tandem rowing much more of a possibility. While I have two rowing stations at 154 feet, tandem rowing is awkward.

3.) Longer length should give more time fro water dripping off oars to hit water rather than blow onto my wife when I row in forward station with her in back of the boat. Right now if rowing into wind drops of water land on her and she doesn't care for that much.

great point on #2 doubles is very cramped and inefficient, the Chamberlain dory skiff as designed is optimized as a one person rowing skiff.

#3... you'll need different oars, a different rowing stroke, or different weather... goodness knows we can't change our wives!

[David Cockey]

Lengthen the boat by 20% and the volume enclosed by the boat increases by 20%. enlarge the boat in all three directions by 20% and the enclosed volume increases by 73%.

The displacement when float on the same waterline will increase by 20% if you lengthen the boat by 20%, and by 73% if you enlarge the boat by 20% in one direction.

Archimede's principal says the weight of the water displaced when floating will equal the weight of the water displaced. So to float on the same waterline the weight of the boat would have to be increased by 20% and 73% respectively.

If the weight isn't increased by 20% or 73% respectively, then the boat will float on a new waterline so that the weight of the displaced water equals the weight of the boat.

By same waterline I mean the position of the waterline relative to the shape of the boat. That would be the same draft for a boat lengthened only, but 20% increase in draft for a boat enlarged by 20% in all directions.

[David Cockey]

At slower speeds, probably 3 knots or less, the longer boat may have greater resistance due to increased wetted area. At higher speeds the reduction in wave drag will more than offset the increased frictional drag due to the increased wetted area.

[Ray Frechette Jr]

Maybe a consensus is that a longer CDS would;

1.) Have a slightly higher speed potential albeit with more wetted surface and drag so requiring greater input at slower speeds than the shorter one.

2.) be a better tandem rower.

3.) have more windage if trying to row in stronger winds.

4.) Need more weight to bring down to lines if construction method remains the same.

All in all, does it seem like a reasonable exercise to lengthen it 20%??

[Clinton B Chase]

my personal issue with a long rowboat is windage.
Get too long and the wind can push the hull around too much, making the boat a real chore to row when the wind picks up.
an extra 18 inches might be ok, but go too long and you can start to have trouble, If you cant make a good one person rowboat by sticking within 16ft then I think you are doing something wrong. If where you row strong winds are very rare, then going longer may be more of an option

Drake is 17'4" LOA and time and time again I watch my GPS and, with only a little more effort, I can go as fast upwind or across the wind as I do downwind. Only when it gets above 15 kts do I notice a brake on my speed. So for this sort of hull form, I don't think what Peter says is true. I would bump his rule of thumb up to 17-17.5'.

The CDS is quite nice, but I think I would go to a Gunning Dory or the like if I wanted a dory for rowing. In this case, I agree with Peter that too long is troublesome as a single. Dories have a knack for their sterns sliding off to leeward (griping) and the bow blowing off in a cross wind. This is partly due to freeboard, partly due to not having a skeg effect going on when the boat isn't trimmed correctly.

Ray, if I were you I'd really consider the Piscatigua River Wherry...think Drake in a dory shape (but still not as fast

Cheers

[Ray Frechette Jr]

Any links to that river wherry Clint?

Original thought to 16' 8" is that is a straight 20% lengthening. The frames are not equally spaced on CDS so straight 20% made the math easier n frame spacing.

But I can do the math to keep overall length at 16 feet if need be.

All in all I like the CDS well enough solo. Suits me well. A bit more speed is always desired....

One attractant to staying with CDS Is I have done the lofting and have the molds already.. And it is a 3 plank per side boat which speeds build time as well.

Shorter builds allow more billable time for paying clients...

[Peerie Maa]

Any links to that river wherry Clint?

Original thought to 16' 8" is that is a straight 20% lengthening. The frames are not equally spaced on CDS so straight 20% made the math easier n frame spacing.

But I can do the math to keep overall length at 16 feet if need be.

All in all I like the CDS well enough solo. Suits me well. A bit more speed is always desired....

One attractant to staying with CDS Is I have done the lofting and have the molds already.. And it is a 3 plank per side boat which speeds build time as well.

Shorter builds allow more billable time for paying clients...

13' 10" to 16' is a 1.15 ratio, messier but not impossible.

[Thorne]

I agree with Clint that a nice double-ended Chamberlain / Gunning / Doghole dory might be a better choice than a stretched CDS for pulling on ordinary waters. Remember the CDS may be 13.5' long, but the waterline is much less -- I've not measured mine but guess maybe 10.5'??? So waterline on a 16' CDS might be 12' or so??? And you might be able to use most of the molds you have, hard to say.

Here's a shot of mine balanced on a rock at Big Lagoon last year -

[Gary Davis]

David (and others) - referring to your last sentence in post #15, are you saying you can increase length (and displacement) by 20% without the need to increase draft? Or are you saying the increased displacement won't cause an increase in draft?

Thanks - Gary

[David Cockey]

David (and others) - referring to your last sentence in post #15, are you saying you can increase length (and displacement) by 20% without the need to increase draft? Or are you saying the increased displacement won't cause an increase in draft?

Thanks - Gary

Displacement is the weight of the water which the boat displaces, which happens to be equal to the weight of the boat. ARchimedes principle. The draft in reality will be determined by how far the boat needs to sit into the water for the weight of the displaced water to equal the weight of the water, not by what the designer specified the draft to be. The boat and water don't care what the designer said.

If the boat is lenghened by stretching it 20% while keeping the sections the same size and shape, then the immersed volume at a give draft will increase by 20%. So if the weight of the boat increased by 20% when it was stretched 20% then the draft will stay the same. If the weight increased by less than 20% the draft will be less. If the weight increased by more than 20% the draft will be more.

[hightop]

Ray, I have followed this and your thread about adding ballast to this boat because it floats above its lines, I am thinking about building this boat for my son & I. I drew it the other night with 2" extra between stations & got to thinking about the waterline problem you mentioned. If lofted with 2" less maximum bottom width overall,(1" lofted) I would think it would float better. It would seem that being slimmer, it would be more tiddly, but probably not, as it would sit deeper in the water.

I built a 12.5' rowboat last winter, but it proved a little small for the 2 of us, I've been thinking about either this or the Swampscott, but decided on this one because i have a trailer it would fit on better. My son is autistic, & rowing is about the only form of exercise he likes. If you or anyone else has an opinion about a width change, please chime in.

[Thorne]

I've seen very bad things happen in dories when bottom width is reduced to "make the boat faster". Not to say it can't be done, but you really want to exercise caution when playing around with an established design. The CDS was designed to be rowed through surf and carry loads of fish, hence the low freeboard and short waterline with raked transom. For flatwater rowing there are a lot of superior designs, but the CDS will get you into and out of more trouble as it can handle rougher waters.

Remember the design was made for solid wood, and boats were often left in the water for months at a time. My fir over oak CDS weighs over 350# rigged for sail, and has very low freeboard. When build lightly with modern materials the hull has a lot more freeboard, with the downside of a lot more windage also.

[johnw]

For the same weight may be key here.

In any event can anyone tell me how I am going to increase length without increasing the weight proportionately?

The extra wood that is making the boat longer is also going to raise the weight proportional to the increase in length is it not???

If I add 20% more boat the weight is also going to be 20% more is it not?

And if I increase scantlings slightly throughout. The weight will increase more than the increase in materials for the lengthening....

Why is it that shorter kayaks need wider beam for stability if lengthening a boat is going to decrease stability.

Advice here is running contrary to experiential knowledge of boats. Shorter boats typically need to be wider than longer ones...

Will you weigh 20% more? If not, the boat will float higher, giving it a narrower waterline beam, which will in turn make it faster, but also making the boat tippier, which I think is what Nick was getting at. On the other hand, the waterplane area will be increased by the length, which will make up for some of the lost beam in terms of stability.

But if the boat floats higher, that means you don't have to narrow the bottom to have a narrower waterline beam. The more I think about it, the more I think this might work. Not clear on why you'd want to stop at 16 feet, is there a difference in registration fees or something?

[hightop]

I've seen very bad things happen in dories when bottom width is reduced to "make the boat faster". Not to say it can't be done, but you really want to exercise caution when playing around with an established design. The CDS was designed to be rowed through surf and carry loads of fish, hence the low freeboard and short waterline with raked transom. For flatwater rowing there are a lot of superior designs, but the CDS will get you into and out of more trouble as it can handle rougher waters.

Remember the design was made for solid wood, and boats were often left in the water for months at a time. My fir over oak CDS weighs over 350# rigged for sail, and has very low freeboard. When build lightly with modern materials the hull has a lot more freeboard, with the downside of a lot more windage also.

Thorne, I'm not necessarily trying to make the boat faster, just making it a little longer so possibly both of us could row at the same time (although we'll probably usually just take turns) and to get it floating closer to the designed waterline. I'm already deviating from the plans by going glued lap, which apparently Mr. Gardiner didn't modify for, although the book details ply construction.

Would leaving the bottom width alone and maybe taking out a little rocker be of some use? Or should I just plan on using ballast?

[ahp]

Stability increases as the first power of length and the third power of beam.

[Ray Frechette Jr]

I deviated from Gardners Scantlings to reflect stock available on hand. The boat has 9 mm okoume bottom plank and 3/8 inch MDO garboards and 6 mm upper strakes.

Garner aslo called for double planking bottom. I also put sacrifgicial runners on bottom..

All up my boat lcealry weighs much less than Thorne's boat.

With addition of 90 lbs of ballast in the form of loaf pans filled with molten lead, the boat now trims down to Designed waterline.

I place ballast in when going out rowing, and put it back in van before hoisting boat onto trailer.

I can launch and recover single handed without immersing trailer in water.

Don't know exactly what the boat weighs , but clearly the trim ballast makes a world of difference in comfort and pleasure of the boat. Without ballast is is very tiddly and if you really try to power up with the oars in a current you have to fight to keep the boat upright.

I really can't say if changing the width an inch would be good or bad. I simply do not know enough about design to answer that one.

[hightop]

Thanks for your input! I think I will narrow it when I loft it, it seems to me it would have raise the waterline compared to the standard width, lowering the center of gravity, and assuming similar weights, the waterline beam would probably be pretty close, & then I'll plan on ballast to trim it from there. Of course, I've been wrong before....

[patrice Hubert]

Here my swampscot " Fred Dion" from J. Gardner book lenght 520 cm; a wonderful craft.