Cosine Wherry - repositioning thwarts for 1 rower?

[Thorne]

The previous owners of my Cosine Wherry had moved the thwarts around to install (horrors!) folding plastic seats and a trolling motor.

(see http://www.woodenboat.com/forum/showthread.php?t=88385)

I'm trying to reposition the center thwart, and will have to build the other two as they were trimmed to be narrower by the previous owner (who was not the builder).

Question is this -- how do you determine where the center thwart should be in a pulling boat?

Should the rower's center of gravity be over the middle of the boat's waterline, i.e. thwart just forward of the middle of the boat on the water? I assume the middle is not determined by measuring the actual length, as parts of the bow and raked transom are out of the water.

I know that in most boats you want a little more weight aft than forward when rowing, giving the skeg more directional ability and leaving the bow a bit higher for boat wakes and swell.

I don't plan on setting the wherry up for doubles rowing, and may pass on the third thwart entirely. But I do want to have the forward and center thwarts correctly located, so that I can row alone from the center thwart, and row from the forward thwart with a passenger in the sternsheets.

Looking at the marks on the rail/riser on each side, it looks as if the builder had placed them all differently from what is called for in the plans.

*PLANS:
Not that the plans make it easy, but from what I can tell the plans call for this (all thwarts = 10" wide):

A bow seat (sheets?) forward edge maybe 12-13" from the bow.

First thwart starting maybe 43" from the bow.

Second thwart (middle rowing seat) starting maybe 77" from the bow.

Third thwart starting aprox 111" from the bow.


*MY BOAT:
(all thwarts 9" wide)

No bow seat/sheets.

First thwart 38" from bow.

Second thwart 72" from bow.

Third thwart 90" from bow.

The overall length of the boat is 168", half being 84". The waterline length is probably 162", half being 81".

So I'll guess that the center of the boat's waterline is roughly 85" aft of the bow (leaving off/adding 4" for overhang of the bow). If correct, that puts the AFT EDGE of the designed center thwart at 87", with the center of the rower's weight probably a bit aft of that.

That means that if I put the center thwart back in my boat as the builder placed it, it will be roughly 6" forward of the designed location, and possibly throw the balance of the boat off.

That means that the forward thwart will be 5" forward from what the plans call for, but that may work well since I weigh more than most passengers.

Any thoughts or experience on this issue?

TIA

[htom]

Take the center thwart out, put in a stuffed-full duffle bag lengthwise, moving it fore and aft until the boat is sitting well in the water. Now sit on the bag, moving your self fore and aft on the bag until the boat is again sitting well as you row (or wiggle.) Mark the place where you were sitting, put the thwart there.

(Works especially well in a swimming pool with a huge mirror. Be inventive.)

[john welsford]

Have a look at the rowlocks, if they are in the original position then measure 13 1/2 inches toward the bow. That should be the after edge of the rowers thwart for a rower of average build.

JohnW

The previous owners of my Cosine Wherry had moved the thwarts around to install (horrors!) folding plastic seats and a trolling motor.

(see http://www.woodenboat.com/forum/showthread.php?t=88385)

I'm trying to reposition the center thwart, and will have to build the other two as they were trimmed to be narrower by the previous owner (who was not the builder).

Question is this -- how do you determine where the center thwart should be in a pulling boat?

Should the rower's center of gravity be over the middle of the boat's waterline, i.e. thwart just forward of the middle of the boat on the water? I assume the middle is not determined by measuring the actual length, as parts of the bow and raked transom are out of the water.

I know that in most boats you want a little more weight aft than forward when rowing, giving the skeg more directional ability and leaving the bow a bit higher for boat wakes and swell.

I don't plan on setting the wherry up for doubles rowing, and may pass on the third thwart entirely. But I do want to have the forward and center thwarts correctly located, so that I can row alone from the center thwart, and row from the forward thwart with a passenger in the sternsheets.

Looking at the marks on the rail/riser on each side, it looks as if the builder had placed them all differently from what is called for in the plans.

*PLANS:
Not that the plans make it easy, but from what I can tell the plans call for this (all thwarts = 10" wide):

A bow seat (sheets?) forward edge maybe 12-13" from the bow.

First thwart starting maybe 43" from the bow.

Second thwart (middle rowing seat) starting maybe 77" from the bow.

Third thwart starting aprox 111" from the bow.


*MY BOAT:
(all thwarts 9" wide)

No bow seat/sheets.

First thwart 38" from bow.

Second thwart 72" from bow.

Third thwart 90" from bow.

The overall length of the boat is 168", half being 84". The waterline length is probably 162", half being 81".

So I'll guess that the center of the boat's waterline is roughly 85" aft of the bow (leaving off/adding 4" for overhang of the bow). If correct, that puts the AFT EDGE of the designed center thwart at 87", with the center of the rower's weight probably a bit aft of that.

That means that if I put the center thwart back in my boat as the builder placed it, it will be roughly 6" forward of the designed location, and possibly throw the balance of the boat off.

That means that the forward thwart will be 5" forward from what the plans call for, but that may work well since I weigh more than most passengers.

Any thoughts or experience on this issue?

TIA

[Thorne]

Htom -

Well, I coulda figgered that one out! I was looking for an answer to the theory of placing a rower in a boat. Just don't want to wait until a good day, or possibly make some stupid error.

I'd like to follow the plans, but am curious as to why the plans place the seat where it is, and also why the builder placed his offset as he did -==- perhaps he also tried a floating test and found his boat balanced differently???

John W - Which "original"? The rowlock base has been moved by the previous owner, but the spacer blocks between inwale and outwale are still drilled for where the builder put them.

If I should position the center thwart according to the plans, I'll have to move the oarlock position also to a new, third location == where the plans spec'd them.

[Yeadon]

One thing to note ... will you always be alone in the boat? Where will you put a passenger? What if you want to tandem row? Tandem rowing is pretty darn effective when bucking a current.

I'd consider dual rowing stations ... you can move forward when rowing into a hard wind, and move back when rowing with it on your quarter or stern.

None of this answered your original question, of course.

[Thorne]

Yes, as I described above, I'll have rowing stations at the first and second thwarts, allowing me to carry a passenger in the sternsheets while rowing from the forward thwart.

My current boat is set up for possible doubles rowing, and I've never rowed it that way in several seasons of active use. But I'll reserve the ability to set up the third thwart with rowlocks.

[JimConlin]

I have a set of Cosine Wherry plans here.
The 'official' locations of the thwarts and rowlocks is as follows:
Thwart #1 (aft edge): 58" aft of bow.
Rowlock #1: 70" ditto
Thwart #2 92"
Rowlock #2: 104"
Thwart #3: 126"
Rowlock #3: 138"

As a check, the boat is supposed to be 14' LOA.

[Canoeyawl]

I would float it and while sitting on a temporary thwart have someone you trust advise you to be certain the trim is good.
Or, put someone your weight in that position and you watch and make the call. Or both – then have a beer and do it again - lol
So many rowboats are inadvertently trimmed a bit down by the head and the rower is constantly making corrections for it.
For me it is hard to watch, but ignorance is bliss I guess. This detail is pretty critical for your comfort.
I always have three sets of oarlocks (one at each thwart) in a boat to adjust trim for passengers and gear.

[rbgarr]

As far as any 'theory' is concerned, you'd have to ask the designer what his was, if any.

I'd clamp a board lengthwise between the fore and middle thwarts and slide fore and aft until I felt the boat was balanced for me. Reaching for the oars in their current position may be awkward but it's not a show stopper. Then decide where relocated oarlocks would be set.

[JimConlin]

As far as any 'theory' is concerned, you'd have to ask the designer what his was, if any.

IIRC, the 'theory' behind the design was some mumbo-jumbo about the mysterious cosine function. In spite of that, It looks like a nice boat.

[Thorne]

Thanks everyone!

The "theory" I was asking about is the general one of where to put a single rower. As Canoeyawl points out, it can be easy to get it wrong and have the bow too heavy.

I'll pitch it the other way , with the boat dead level or a teensy bit low aft when I'm on the center thwart, and hopefully not too stern-heavy with the woife in the sternsheets and me rowing from the forward thwart.

Jim - how are you getting your numbers? The plans in the book don't list distances from the bow to the thwarts, only from the bow seat/sheets to each thwart, rowlock, and finally to the sternsheets.

What number are you using for the distance between the bow and the front of the bow seat? Looks like 10" to me, so when you add the 10" for the bow seat + 20" for the distance to the front thwart, that gives me 40" from the bow, not 58"...

[Steve Paskey]

Theory? You want theory?

Forget the waterline length. It means nothing for this problem. As I understand it, the center of gravity for the boat, oars, AND rower should be positioned directly over the boat's center of BUOYANCY, which is not necessarily the midpoint of the waterline length.

That should be the point at which the trim is level. If you move the center of gravity forward, she'll begin to nose down. Move it back, and she'll begin to nose up.

The center of buoyancy will depend on the boat's underwater shape and volume with the rower aboard. You could try to calculate the center of buoyancy from the plans, and then try to calculate the center of gravity for the boat itself. Or you could just do what htom said, which is a practical way of finding the right point without doing any calculations.

Of course, I could be totally wrong...

[Steve Paskey]

Oh ... and the center of buoyancy could change slightly depending on whether the oarsman is 120 pounds or 250, but let's not go there.

[JimConlin]

...Jim - how are you getting your numbers? The plans in the book don't list distances from the bow to the thwarts, only from the bow seat/sheets to each thwart, rowlock, and finally to the sternsheets.

What number are you using for the distance between the bow and the front of the bow seat? Looks like 10" to me, so when you add the 10" for the bow seat + 20" for the distance to the front thwart, that gives me 40" from the bow, not 58"...

OOPS! I slipped up, a couple of times.

On that same page, the topmost drawing shows the spacing of the stations in the boat. The first two intervals are 9" and they're 12" until the last one which is 18". By my arithmetic, sta. 8 would be at 90" from the bow.
The third drawing shows the locations of the various parts vs. sta. 8.
I neglected to count the little seat in the bow.
So, the list should be:
Front seat (aft edge): 22"
Second seat(aft edge): 52"
Rowlock: 64"
Third seat(aft edge): 86"
Rowlock: 98"
Fourth seat(aft edge): 120"
rowlock: 132"

Your humbled...,
Jim

[Ben Fuller]

The CB works most of the time, except in boats that have relatively flat bottoms and pronounced stems or nothing in the way of a skeg. So for example, a flatbottom skiff with a big skeg is tolerant of bow down trim while a 14 foot Rangeley or Adirondack guide boat is not. My ducker is an interesting example: absolutely symetrical with relatively fine ends. The forward station has the locks in the middle of the boat which works fine with a passenger. I row from the after set of sockets which gives me some stern down trim as my butt is probably a foot aft of amidships. Hundreds of happy rowing miles and 30 years says its just right.

Another way to test things is grab some ballast. A couple of five pound water jugs is good. Then go for a row in your existing set up. Row beam on to a ten knot breeze or so. Start adding ballast aft until you no longer gripe up into the wind. With your weight, the ballast weight and the distance between them, figure out the CofG or the pivot point of the see saw, that will tell you how far back you need to move your seat.

[Thorne]

No worries Jim, I had the same problems myself and had to run the numbers about 4 times and got different answers each time. Would have been really nice for the designer to provide this basic info, but he did a good job with the rest of the book and plans.

At this stage I intend to go with the thwarts as laid out in the plans. Was just curious as to how this was determined "in theory", and why the builder shifted the thwarts forward...

Thanks again, everyone.

[Cedric Rhyn]

Hooboy you guys overcomplicate things, some of you need to get out and about a bit more. There are two reasonable replies in this thread, Jim Conlon and John Welsford. One can assume that the original designer had all the centers and positions worked out, and all that is needed is to find a reference point such as JW suggested, that being that if the rowlocks are in the original position then the seat will be positioned as he says, and Jim has been good enough to give all the relevant information as to positions of rowlocks and thwarts.

Cedric Rhyn

The previous owners of my Cosine Wherry had moved the thwarts around to install (horrors!) folding plastic seats and a trolling motor.

(see http://www.woodenboat.com/forum/showthread.php?t=88385)

I'm trying to reposition the center thwart, and will have to build the other two as they were trimmed to be narrower by the previous owner (who was not the builder).

Question is this -- how do you determine where the center thwart should be in a pulling boat?

Should the rower's center of gravity be over the middle of the boat's waterline, i.e. thwart just forward of the middle of the boat on the water? I assume the middle is not determined by measuring the actual length, as parts of the bow and raked transom are out of the water.

I know that in most boats you want a little more weight aft than forward when rowing, giving the skeg more directional ability and leaving the bow a bit higher for boat wakes and swell.

I don't plan on setting the wherry up for doubles rowing, and may pass on the third thwart entirely. But I do want to have the forward and center thwarts correctly located, so that I can row alone from the center thwart, and row from the forward thwart with a passenger in the sternsheets.

Looking at the marks on the rail/riser on each side, it looks as if the builder had placed them all differently from what is called for in the plans.

*PLANS:
Not that the plans make it easy, but from what I can tell the plans call for this (all thwarts = 10" wide):

A bow seat (sheets?) forward edge maybe 12-13" from the bow.

First thwart starting maybe 43" from the bow.

Second thwart (middle rowing seat) starting maybe 77" from the bow.

Third thwart starting aprox 111" from the bow.


*MY BOAT:
(all thwarts 9" wide)

No bow seat/sheets.

First thwart 38" from bow.

Second thwart 72" from bow.

Third thwart 90" from bow.

The overall length of the boat is 168", half being 84". The waterline length is probably 162", half being 81".

So I'll guess that the center of the boat's waterline is roughly 85" aft of the bow (leaving off/adding 4" for overhang of the bow). If correct, that puts the AFT EDGE of the designed center thwart at 87", with the center of the rower's weight probably a bit aft of that.

That means that if I put the center thwart back in my boat as the builder placed it, it will be roughly 6" forward of the designed location, and possibly throw the balance of the boat off.

That means that the forward thwart will be 5" forward from what the plans call for, but that may work well since I weigh more than most passengers.

Any thoughts or experience on this issue?

TIA

[Thorne]

Well if you carefully read my posts above, the rowlocks are NOT in the correct position according to the plans -=- the builder put them in differently. I'm curious as to why the thwart is 5" forward from where it should be.

Then the second owner moved the thwarts and central rowlocks again to another position -- neither / none of them where the plans call for 'em to be.

So I'm not trying to make mountains out of molehills, just trying to understand why the builder not put the central thwart in the location called for in the plans, making sure it is not because he built the boat a bit differently from the plans or something similar.

And I'm trying to get the correct distance from the bow to put in the thwarts. This distance is not marked in the plans, and as you can see from my and Jim's posts, calculating it is not easy. For all I know, this could be a common problem with these boats, and I've heard of folks shifting thwarts around in them but don't know why.

[htom]

Looking at the drawing, it would seem that all of the placement dimensions are based on the forward edge of the stern sheets (whose dimensions are not given on that drawing.) My guess is that the center thwart and rowlocks were moved forward to compensate for the weight of the trolling motor.

[Thorne]

Did a test launch at Richmond Marina -- the boat is FANTASTIC to row! What a difference 240 lbs makes between it and my dory skiff!


OK - I put in a temporary front thwart and rowlocks where the plans call for it to be placed, plus moved the center thwart to the position that the builder used. Front of front thwart was at 40" (maybe 3" aft of the builder's location), and front of center thwart was at 72" (same as builder's location).


Again, from my reading the the confusing plans, the front edge of the front thwart should be 40-42" from the bow, the front edge of the center thwart should be 74-76" from the bow.


Looks like the plans are correct -- surprise! The front thwart worked well with the wife sitting in the stern sheets and the boat sat dead level, but with anyone heavier it should be better with a slightly bow-up balance. Might be tempted to shift it a few inches aft to get that balance with her aboard.


With Mary aft and me on the center thwart, the bow was well up as it should be. I really enjoyed rowing with the boat in this trim, and the skeg / wineglass transom makes it track very nicely.


The center thwart in the builder's position was too far forward, as you can see from the photo, resulting in the bow being slightly down. Shifting it another 3 or 4" aft should get a better balance with the stern being slightly down.


Here I'm perched all the way aft on the 9" center thwart, making the balance a bit better but still not right.

[Thorne]

Here's a shot of me from today -- Like the Kirby's hat?

You can see the El Funko temp oarlocks clamped to the inwale.

[htom]

Spice looking at photos, said she wants one. But then instantly said "I think you should wait just a few months!"