Row boat, take two

Hello again,

Been a long time since I built my first boat, a small punt for rowing. It has been fun to use, but its limitations have shown themselves in spades since. It is too small, and the transom drags like crazy, just as you guys warned. Plus, it is scary as hell when there are two in it and a cruiser goes by on the narrow river.

So, the time has come for a new boat. It will be built with economy in mind, and will be made from home improvement type plywood with epoxied glass tape seams and light framing. I have the 4 sheets of 4'x8' 5.2mm Lauan I plan to use (not ideal but available) in the barn, and the tape and epoxy will arrive next week. The plan is to scarf the sheets into two ~16ft long panels, and then cut out the pieces.

I'm making this thread to keep me focused (lots of other projects on the go) and to get suggestions from you folks. With that in mind, you'll find below the linesplan that FREE!ship spat out. Any comments? The waterline is set at ~300lbs displacement, which is probably pretty reasonable for the boat, my bulk and some gear all together.

I am playing with the idea of putting a fairly large "deck" on the front, really just an overgrown breast hook. It would help stiffen the boat, and keep things dry. I've also considered making a long "bench" type seat running fore to aft. It would be like a long enclosure you can sit on, with an access hatch so that it doubles as a locker and reserve bouncy. Maybe it could even be built to mount a sliding seat sometime in the future.

Anyway, time will tell.

Cheers.

 

That's a really nice design. It looks like it will slip right through the water. Best wishes on your project.

Oh, yeah, I looked at your previous build. That was a cute little boat too. But this one does look a lot more sophisticated in terms of sleek lines.

 

Cameron; That's a good looking start.

You could run some trial mods on the computer such as........ Try reducing the deadrise at the transom to reduce the reflex curve on the aft DWL. A tad more aft rocker to get the transom slightly above the DWL. The boat will squat when you stroke powerfully so you don't want the transom to drag. On the other hand if the transom is up the boat will not be as directional and you may have to use a small skeg. Opt for the skeg.

Fiddle with the area distribution curve by moving the lowest part of the bottom slightly forward of the mid section while moving the maximum chine width a bit aft of the mid section. The act of moving max depth forward also reduces the angle of the quarter beam buttock. That is often productive, especially if you are in surfing conditions.

It is easy to experiment with the lines on the computer but really hard to unbuild the finished boat.

You are looking good but hurry!, it's going to get too cold for rowing in Canada before long.

 

I have to hand it to you, some good work on that first one. Please tell me what box store can still get the Luan exterior glue, seems to have vanished around here. Just an unsolicited opinion, a Gheenoe may be the type of craft you are looking for. Narrow transom for a small motor, wide amidships for stability and a flared/modified canoe bow to cut the water. Your design reminded me of one, they row great as well!

 

Thanks for the comments everyone.

I'd never seen a Gheenoe before Pungolee, thanks for pointing them out. They look a lot like a flat back canoe, which are also very handy. Maybe the Gheenoe is wider and flares out more at the bottom for initial stability? Seems really cool.

I think your comments make good sense Messabout - especially the one about hurrying up! Don't even remind me that we're half-way through July. I've attached another image - I believe it shows that the centre of buoyancy of the boat is in front of hull centre? I think this is what you mean when you talk about moving the lowest point forward (displacement forward). I'm not super familiar with hull terminology, but I think you're also suggesting I lessen the V near the transom, to prevent squatting. Both of these ideas make sense - as does a small skeg. Personally, I suspect that in practice the boat may trim slightly forward, raising the transom clear anyway.

The silly part is that I printed the full size templates today - so I may build it as is. I'll try to take time on the weekend to make another computer version and see the result, even just for my own education. If so I'll be sure to post it. Thanks again.

 

The red line in your attachment appears to be the curve of areas that I mentioned. If the boat is imagined to be cut into bread slices, then the area of the underwater part of an individual slice is calculated. When all the calculations are done, they are laid out along the length of the boat at the station locations, the height of the mark represents the section area at that point, then connected so as to create a curve.

The curve has some interest for more than one reason. It does show the location of the maximum area but that does not necessarily mean that it is the center of buoyancy. CB calculations involve a few minutes of arithmetic, which is easy, but your program can do it in a jiffy.


You can move the location of maximum height of the curve by selecting either the lowest point on the keel or adjusting the location of the widest location of the chine, or both in some combination to affect the shape and distribution of the curve. Conventional designs usually have the curve symmetrical or nearly so. The shape of the curve does tell you something about the acceleration rates of masses of water as they move across the surfaces. Curve shape is an argued subject and one shape is not a fits all proposition. Depends on a lot of application variables.

Buttock lines: slice the boat length wise into bread slices that go fore and aft instead of side ways. The profile of the line represented by the outer surface of the bread slice is the buttock line. The run is that part of the bottom that rises toward the surface, in the aft sections................There, that is a very loose description of some of the terminology used on boats.

Keep on keeping on.

 

Messabout, thanks for the excellent explanations. When I read your latest reply I at fist baulked at the idea that CB and point of maximum displacement could be different. Then I imagined a "boat" with a bulb on each end and a narrow section in the middle - the displacements would not match the CB. Just like side-to-side on a catamaran. Thanks for making me think a little harder.

 

It can be worth looking at the curve of areas at different LWL. You may need to carry more weight? But what it does is give a pattern to the curve. If you have curves from being too light to way too heavy (so to speak) it might show an unfairness in the shape is generating different curves. I would agree with messabout in that the curve of areas and CoB are things people will debate and sometimes there are multiple solutions and no single right way. For example what works in perfect flat water may be poor on the sea in rough waves, I'm sure you can relate to that kind of thing.

I think that is (hopefully) what messabout is pointing you at, or similar exploration. As you have a rowing craft you need to balance the weight transfer from the stroke a bit. Worse if you had a sliding seat!. You can start your own exploration!.

Different craft require different curves, you can find references for large commercial vessels along with some curves used. Harder to find those for smaller recreational craft.

You may well find that using sections and waterlines give you all the data you need. Buttock lines become almost redundant. Diagonals are useful to check for fairness, though.

 

Cameron when it gets cold enough this winter, you are not likely to be out rowing. To entertain yourself during those dark winters, you can tinker with some boaty math. How to find the center of buoyancy without getting into messy calculus.

Think of a wide plank on which you have placed a bunch of differing weights, at different locations along the length of the plank. If you lifted the plank and placed it on a single sawhorse it would balance if you had selected the right place on the plank to rest on the saw horse. CB works pretty much the same way. In the boat case, the force on the plank (or boat hull) pushes up rather than down. Same deal.

Erect an imaginary vertical line at the front of the boat. Measure from that reference point, or base line, back to the first section. Let's say the distance is ten inches, the next section is 20 inches, the next at 30 inches and so on. Take the area of the underwater part of the first section and multiply it by the distance from the reference point (10 inches) Take the area of the second section and multiply by 20, the third section area multiplied by 30 and so on until you get to the aft end of the waterline. It is best to make a chart with the areas in one column, the distance from the base line in the second column, and the product of your numerous multiplications in the third column. Do this in Excel or other spreadsheet if you like. It is more fun to do it by hand on a scratch pad.

Add up the column that contains the areas, now add up the column that contains the products of all those multiplications. Simple so far, right? We are almost there. The last calculation divides the sum of the products by the sum of the area. The result will be the location of your CB as measured from the base line. That'll work for the plank balance point too.

Don't want to do the math? A cheap and dirty way to make a fair estimate is to carefully lay out the curve of areas, to scale, on a piece of cardboard. Cut the cardboard out along the area line and then balance it over a pencil. That'll get you pretty close to the real thing.

One of the things we have not explored is prismatic coefficient. Your program probably calculated that one for you. It is usually noted as Cp. Does it matter? well, yes it does, at least to the extent that it ought to be at least noticed. Cp is a measure of the areas of all the underwater parts when compared to the area of the center or main section. It is clear that the end sections have less area than the mid sections. The average will be less than that of the center section by a considerable margin (unless it is a rectangular barge)

A loose description of Cp is; the average area of all the sections compared to the area of the main section. That will make what amounts to a percentage calculation. A boat like yours will have somewhere between 0.50 and 0.60 for a Cp. The larger numbers imply that the forward and/or aft sections are fatter. Whereas a smaller number will imply that the fore and aft sections are skinnier. There is a body of thought that says that the speed of the vessel needs to comply with an appropriate Cp or visa versa. There are divisions of belief but in general a faster boat needs a somewhat higher Cp. Skenes Elements Of Yacht Design has some comparative tables that are a good starting point. Dave Gerrs book; The Nature Of Boats has some input about this too. Your boat, as shown, will probably fall into the neighborhood of 0.53 more or less.

Why have I carried on at such length here? That is because you have shown a gentlemanly willingness to listen. That is in sharp contrast to so many people who come onto the forum with their minds made up such that no amount of reasoning will change them.

Meanwhile.......get on with building.

 

Messabout, a CP of 0.5429 is the number spat out by the software for the hull as pictured in the first post. And the higher CP correlating with the faster boat seems to make sense, since it implies the middle is as narrow as the ends. This wouldn't hold true for the barge you mentioned though, would it? Seems like it might make more sense to say something like: Where a boat has a narrow beam, a high CP implies speed, but where a boat has a wide beam, a low CP is faster. Maybe I'm missing something, or is a square barge faster than one pointed at the ends? Looking at your definition again, I'm reminded that it is area, not width. So I need to think about depth as well.

Just for fun I pulled in the beam at the waterline by a foot or more and reran the calculation, returning something closer to 0.56. I always did say I wanted to make a rowing scull someday!

It is worth mentioning that the software also returns a longitudinal centre of buoyancy at 8.013 ft. I believe this is measured from the stern, so the CB is ~0.8 ft past mid-ships (total length of 14.4 ft). It also gives this as a percentage at 3.783%? I would have expected 55.6%? What is the 3.8?

Just wondering out loud here. Thanks for all your time and the info you've added to the thread - hopefully is can be of use to others as well.

 

Nicely explained messabout. Cameron, I'm not sure what the 3.8 is unless a length from the other end of the 8.013 which gives a WL of 11.796'?. As the WL does not start at Datum (stern but at max height on your drawing) maybe the total length of the WL is that? It is certainly not 14'.

As you already know, the longer the WL length the faster the boat at displacement speed. The Cp is useful to know and a valuable help, but can also be confusing. You need to get the shape (underwater) to slip through water with minimal disturbance, so the curve of areas and it's form do matter. A designer can make this curve any shape they wish - pretty much. Imagine it for a barge. It rises steeply, levels then falls pretty steeply. Of interest is the fact that despite their box like central sections the old Thames sailing barges actually had quite a bit of performance difference between some of the shapes. It is considered that the aft sections in particular - the way the water is let go of, were the most important part of this speed difference. Why do cargo ships have huge bulbous underwater bows? How about their curve of areas?

You may be surprised to know that generally more modern dinghies have a little more volume in the ends than their counterparts from 30 + years ago. However rather than becoming brick like in shape it is subtle and sweetly blended in. I would hazard that a lot of rowing craft have done the same, partly to reduce the pitching motion exerted by body movement from working the oars.

messabout's advice on the shape from his first post should be noted. I concur with his guidance on the form, so re read it and then rework your concept. You will find a good solution. One other thing of note, although it will not affect you too much. A semi circular midship section is minimal area and drag but in the real world pretty unstable athwartships. Flattening this into a U section (like your Dory type) gives more sideways initial stability, useful for getting into/off the boat. Deep V shapes that are narrow can make quite sea kindly boats but also can be prone to lying on their sides a bit. As usual you need a compromise that will suit what you need from this craft.

 

Yup, I have comments. First one is that Freeship didn't "spit this out". Freeship only did what you told it to do. Second comment is that really, without meaning to be rude, this doesn't look like a particularly good rowboat. Your comment about Freeship just spitting it out gives the impression that you don't really know why your design is that shape, or if it should be different in any way.

My suggestion is to study good existing designs and understand why they are the way they are. This will give you a better basis for making your own decisions. Alternatively, if you just want a good boat in a hurry, just build an existing design.

As just one example, you can get basic plans for a stitch and glue guideboat quite cheap (see http://guideboat.ca/). They would be as easy to build as anything you would draw, and would row a hell of a lot better than the design in your OP.

 

Hello again all,

I have attached another lines plan to keep the discussion going. I believe it incorporates at least some of the suggestions originally made by Messabout as I understand them (shift the point of maximum depth forward, and the point of maximum width back, add rocker at the rear and flatten out the V at the transom). It maintains the same displacement, but changes the CP to 0.5554.

NoEyeDeer, thanks for your input, and I realise you're probably giving good advice. Why start from scratch when something is already available? You'll probably appreciate that some people just feel the need. I've always been one, and it's true that it doesn't always lead to optimum outcomes (see my last boat for a good example!). That being said, I won't be doing it again if I hadn't done it the first time.

 

Guideboat.ca website seems to be down at the moment.

For anyone interested in proven designs for rowing boats, the mother lode is in John Gardners book: Building Classic Small Craft. The book has plans for a Rangely, Whitehall, Saint Lawrence skiff, Peapods and several others. These boats may look old fashioned but they are the result of a hundred years or more of development by the people who used them to make a living.

Other very functional boats that excel at rowing are the Adirondack guide boats.

Camerons design is far more sexy looking than any of ones mentioned here. Nonetheless, for pure joy of rowing, or even hauling some cargo, I'd go for the Whitehall or similar. They are beautiful in their own way.

Whitehalls are the result of development by old time entrepeneurs who rowed out to incoming ships in New York Harbor. It was a race to see who could get to the ship first. The ships routinely hired the rowers to take people or cargo ashore before the ship tied up. It was a competitive business that rewarded the fastest most capable boats. The Whitehall became the design of choice over time for good reason.

 

Ok then, rather than asking people to effectively design the thing for you, how about you explain why you drew it the way you did? If you have to think it all through again it might be beneficial.

So, what do you see as the strong points of your design?