designing a fast rowboat

I wouldn't think that was too long at all. I ran a lot of options through Michlet and Godzilla recently. Note that I'm 5'8" and around 160 lbs. From memory you're about 6'4" and a fair bit heavier than me, so I wouldn't think you would want a shorter boat.

The results I got suggested that for best sustained speed in flat water I would want a boat around 6.5 metres long on the waterline. This was for a 250 lb displacement, and assuming I would be pushing the boat at about 6 knots on average. I think this is about the highest speed I could sustain over any distance with a fixed seat (based on previous experience in other boats combined with some reasonable calcs).

Note that my primary interest is in flat or not-too-lumpy water (large estuary, up to Force 7 when I feel like being daft). The rougher the conditions, the shorter the boat should be IMO. Not just because of the lower speed you will be sustaining but also because of windage.

ETA: Note that the length recommendation is of course very much dependent on using a midship section that minimises wetted surface. This will result in an extreme amount of flare if you want to keep the rowlocks on the gunwales and have a decent spread between them.

 

Of course you have to accelerate that weight with every stroke !
I wonder if part of the perceived benefit of the "extra" weight is the the inevitable increased under water plane keeps the boat from being blown around so much. - nick

 

nice discussion guyz

 

Last month there was an interesting discussion in this thread on the effects of stroke-to-stroke speed variation on overall speed and energy used. In case you missed it, it starts on page #55 and runs on for a few pages.

 

Here is my input to my favorit thread, that I have been following for a long time.

A compounded plywood rowboat:

Design goals:
Recreational fast fixed seat rowboat for one or two rowers made of two sheets of 4mm plywood (250 x 122cm) for flat water rowing. For this design I prefer performance and light weight to stability and sea-worthiness.


Specifications:

LOA 492 cm
Beam 114 cm
Estimated weight 32 kg

Displacement 135 kg (300 lbs):

Wetted surface 2,24 m2
Draft 11,4 cm
LWL 479 cm
BWL 63,5 cm
GM 40,0 cm
Cp 0,56

Displacement 225kg (500 lbs):

Wetted surface 2,79 m2
Draft 15,5 cm
LWL 484 cm
BWL 73 cm
GM 40,9 cm
Cp 0,59

1:10 scale model:

 

That's basically a logical shape, and I'm surprised by the amount of compounding you got away with. It looks like the panel shape needs a bit more tweaking (there's a bulge in the waterlines at the ends) and I think in practice you'll need some sort of skeg to get good tracking. Other than that it should work well.

 

- true, but we don't know what material was used for the model ...

 

The model is made of 0,4 mm birchply and is exactly in scale. The amount of compounding is achieved by using a method Dave Kuperstein describes in the comments to his Long Lake design: http://www.sageboats.com/stock/longlake.html

I made three models until I was to some extent pleased with the hull-shape. The bulge you noticed at the ends of the waterlines is still bothering me.

There is an excellent comment regarding compounded plywood construction in this book by Chris Kulczycki

Having built a compounded plywood kayak I totally agree with this comment.

 

Interesting; reminds me of my project to replicate J. Henry Rushton's 1896 canoe "Wee Lassie" in non-lapped ply construction. I started by converting the lines from the original cedar-planked lapped construction into marine plywood planks, butt-joined along the chines. The bottom planks are severely twisted, similar to your design.

I found the bottom plank and the first bilge plank together could be formed using a single plank, double width, so I built a full-size half-model (midships to one stem) of a pair of the wider bottom planks to see if it could be done. The planks bent OK but the keel had excessive rocker and was not a fair curve. The plank development required to fair out the keel line is strange with two odd kinks along the bottom edge.

These planks were not being bent in two directions at once; rather they were twisted like an aircraft propeller blade, so that the ply was compressed along the centerline and stretched along the edges. As the twisted plank was bent in to meet the stems, the centerline shifted across the width of the plank so that the opposite edges stretched to a different extent, producing stress concentration where the kinks of the developed plank would be located - effectively straightening the kinks to form a fair keel line.

I did not trust such stressed wood in a boat so I went back to the original bottom plank width. Now the plank development is a reasonable shape which forms precisely as predicted with far less effort. Rushton knew more than I did!

The trick when I build the boat will be getting a strong butt joint along the shine seams. I am still working on that problem ...

 

Looks great ! I hope you go ahead and build it so we can see if the whole thing works.
I should expect that full size sheets will behave a little differently and some modifications will be made (a fancy way of saying " punt "). Nice Models too - Nick

 

comments

A very good looking model, it shows a lot of work to me. I don't convert metric numbers very well, but, to me, it looks too short to get any real speed. I would suggest you upgrade your parameters on material in order to get up around 17' or even slightly more on the waterline. In flat water conditions you need to have the length or you will not be competitive. In the rough water I race in, shorter boats sometimes prosper simply because no one is making hull speed and the shorter boat has less windage and less wetted surface. You seem to have a definite use in mind, so, design to the use. good luck with your build. Jeff

 

I've been watching Flo Mo's posts at the Wooden Boat Forum and I think the boats may answer the need for a way to get people into decent rowing boats quickly and inexpensively.

For me, I am looking to go faster than ever in a fixed seat rowboat and have no doubt that I will meet that with the Finnish rowboats. I will not go shorter than 19'. If I were always going to be in flat water I might go 20' but I encounter chop enough and wetted surface goes up a bunch. I'm hoping to prototype a 19' ish Finnish boat in 2011. They're putting together the 19-footer but I think it is for someone who wants a shorter sliding seat boat. I see it as an ideal fixed seater.

The 6.5m is a beauty.


Finnish Rowboat by Clint Chase Boatbuilder, on Flickr

Nice narrow waterline


Finnish Rowboat by Clint Chase Boatbuilder, on Flickr

 

Regarding the "faster then ever in a fixed seat boat": it's something I've given a lot of thought to over the years. Really, the fastest fixed seat single would be a scull with the sliding seat pinned in place (and suitable changes to the rigging of course) or something very close to that. If you're going to impose additional restrictions, like rowlocks must be mounted on the gunwales, then you're going to have to force an extreme midship section if the boat is long enough for real speed. Flo-Mo's design is an example of this, even though it's still too short for sustained high speeds. It's a good length for relaxed cruising but not for putting your back into it. If it was made longer the waterline beam would have to be reduced, but the beam at the sheer is already minimal for efficient rowing so the midship section would be more extreme than it is now.

In many ways this doesn't make sense. It means the boat has to be wider and therefore heavier. Both of these factors make it harder to carry out of the water. It means it will use more materials and therefore be more expensive. It means the extreme flare will make the boat harder to handle with wind and waves on the beam. It also means that the boat wont cut into waves on the nose so well as you're forcing a wider hull through them.

There is a lot to be said for just choosing a midship section that has the basic characteristics required (immersed area, girth, BWL, whatever) and bringing the gunwales in at a natural width without worrying about the rowlocks, then just bunging the rowlocks on outriggers at whatever width and height you want them. You'll likely end up with a better all round boat that way.

tl:dr version: Outriggers make as much sense for a fast fixed seat boat as they do for fast sliding seat boats.

 

Who says you can't bend plywood in two directions at once?

 

It doesn't seem to be made with tortured ply. Looks rather like something (what?) made of developable strips of ply.