designing a fast rowboat

So you are talking about designing something along the lines of the mentioned boats but longer, rather than actually building one of those three boats? That could work. I reckon you ought to carefully weigh things up though or you wont end up faster than a guideboat. A Liz is a comparatively beamy thing when you look at what is in the water. What you would want is something the length of a Liz but with a midship section that was a narrower version of a guideboat section. That's about the only way you'll get the result you're after, because to get more speed you'll have to keep the wetted surface the same as the guideboat but increase the length.

 

Liz is no longer in the running, replaced by Annie, the 18' St Lawrence Skiff that Andrew Steever drew based on a model he rowed and thought was best. This particular SLS is quite narrow -- about the same as a GB -- and just over 16' waterline. She can be built as light as 90 pounds. She has been raced int he Blackburn and won it, beating out other GB's, Kaulbecks and Browns.

The oar arrangement will make a big difference. So, it looks like it will be the Finnish rowboat and Annie (maybe Annie stretched a bit as an option).

Annie can be seen http://home.comcast.net/~qboats/photo_1.html

The Finnish boats coming to town early 2011:

www.puuvenepiste.fi

Cheers,
Clint

 

That sounds like a better bet. Get both of them and race Annie if it's blowing a gale and the Finnish boat if it's flat. That should work.

I've been messing with that gig sorta thing as on reflection that lines were a bit crude in spots. Of course I don't want a double myself and I'm pretty comfortable in narrow boats, so I refined it down to something I think I could go for. A smidgeon over 20 feet long and with just enough beam to swing a decent pair of oars comfortably if rowing fixed seat with oars on gunwales, plus of course it's long enough to use a sliding seat effectively when I want that option. The sections are pretty radical (sort of Flashboat) but that's the only way to get a decent rowlock spread with low enough wetted surface to make it worthwhile. I know what it could handle and would be fine with taking it out in some fairly interesting conditions. I was originally thinking of a narrower and more extreme version that would have been sliding seat w/ riggers only, but since I want to keep some stability and carrying capacity anyway the small difference in resistance I could get within those constraints was hardly worth the extra loss of practicality.

I reckon I could do this one at 80 lbs all up, including thwarts, oars and painter. I had me an idea, I did. Over here glass comes in one metre widths. That's not wide enough to go from sheer to sheer and I don't fancy buying a stack more than I really need. So, the obvious thing to do is to strip plank and glass it to about halfway up the minimum freeboard, then drop on a varnished clinker ply sheerstrake. That wouldn't need glassing, which saves work and weight and general nastiness. Should also look quite cool if done well.

 

What thickness ply for the sheerstrake?

How does a glassed stripper compare with an all-ply hull for weight and strength?

(I'm getting ideas too ...)

 

Well since this particular design has stacks of flare the gunwales should take any impact (99% of the time anyway) so the sheerstrake is basically just to keep my jeans dry. That gives options.

1/ Windows are always cool. Gives a nice view and all that. Skin on frame boats work. Some of those Yostwerk baidarkas with the clear PVC covering look rather groovy and so I was thinking that probably the guys who make roll down awnings for marinas and cafes would have plenty of six inch wide offcut strips lying around. That would make a very light sheerstrake and you might be able to get it for the price of a six pack of beer.

2/ If being slightly less Wally and a bit more shiver me timbers I think you could go down to 3 mm ply (1/8"). Given that the strips are most likely 1/4" and then the weight of glass (6oz or 4oz) and resin inside and out, plus the area of both sheerstrakes being in the region of 15 square feet, I think a four or five pound saving is about what you'd get.

Not sure on the weight/strength comparison. I wouldn't use 3mm ply for the main part of the hull so I'm sorta cheating here. If I was using ply for the lot I'd use 6mm I think. Possibly 5mm but I wouldn't go down to 4mm as that would be too fragile without reinforcement IMO, at least for bottom planking. Ply is usually a bit denser than cedar but 6 oz glass laid up is about 1/10 pound per square foot if the glass/ resin ratio is good. I think the strength to weight should about balance out for a well built boat in either system, but the ply boat without sheathing is going to be more vulnerable to wear.

ETA: Oh Option 3 would be to not use ply at all for the sheerstrake and just grab any nice planking stock that was lying around. That'd work too.

Meh, Should mention Option 4 too I s'pose. If just wanting the look of wood with minimum weight, and effectively treating the ply sheerstrake as if it was just non-structural skin like the clear pvc option, then you could use 1.8mm gaboon ply. That weighs about three fifths the square root of bugger all.

 

green gig

The green gig looks very good to me, about what I was picturing in my mind. To give you some parameters to work with; I'm 170 lbs, my brother, 190 lbs. we row with 50" spread on the oarlocks. our stations are 4' apart, but 4-1/2' works better in rough water. I row from the forward station and my brother rows from the aft station. Just a couple of thoughts on weight: light is good for flat calm, heavy is good for tough weather. The fastest boat we've built was our 19' Jersey skiff. In both light and heavy weather we could really move her along. When it got nasty the heavy displacement of almost 650 lbs carried us through the waves with a very smooth pace. with a light boat in those same conditions it feels like stop, start,stop, start as the waves almost stall you out between every stroke. I notice this very much in my skin on frame boat--- very discouraging when I've got a mile or more to row like that. In any design, the engine is of prime importance. How much you put into the oars makes the difference. We've won races just by toughing it out and rowing harder and longer and smarter than the competition. We won last Novembers' Weir River Race by deciding to risk being swamped, getting in the lee of an island to bail, rowing a course that was longer but put the wind and waves over our stern and just finishing when only 19 of 42 boats finished

 

Well you can always carry extra ballast in rough weather without needing to lug the weight around all the time. It would be very easy to build in a water ballast tank if you wanted to. Could do that with any boat. Get the best of both worlds.

 

I just had a thought; why not a plexiglass sheerstrake? Hmm ... did I take my medication this morning?

I think I must fall into the “shiver me timbers ” category because I have tended to go for the thinnest I can get in the past. I would use 1 mm in places, like canoe decks, but that means leaving the boat shop and going down the road to the aircraft shop; since that stuff is ridiculously expensive and I am cheap it’s not going to happen.

I don’t like fooling with glass and large amounts of epoxy so I tend to use ply a lot to get the cross-grain strength, but it has to be thin to save weight and then it’s hard to handle in thin strips.

I agree with your conclusions on 3 mm ply; it's fine for a 12 ft canoe used pon a calm lake but not for a 18+ ft boat in open sea.

 

Been doing some more thinking. That always gets me into trouble. Wotcha reckon of this?

Turns out PVC is about three times the density of common boatbuilding softwoods. That means that even if you use 0.020" PVC (and I wouldn't want to go thinner) it will still weigh roughly as much as 1/16" plywood but without any of the structural properties. Not sounding so good.

As you say, messing with ply is a lot nicer than messing with glass and resin. If I'm going to buy a sheet of ply for a sheerstrake anyway then doing one strake is kind of waste since I'd be left with half a sheet. Of course I could always build two boats but that way lies madness. A good option then might be to only strip and glass to a couple of inches above the DWL and then clinker ply the topsides in two or three strakes. If I use 4mm Hoop Pine marine ply that's nice and light (6.4kg/14.1lbs per sheet) and it has about the same strength as 6mm Gaboon since Hoop marine ply is rated F17 over here and Gaboon is only rated F8. Weight and price are competitive too when strength is taken into account. BTW, Hoop Pine is a native Australian timber so you wouldn't see it over there. Not any more rot resistant than other plywoods but it has a good strength to weight ratio and takes a good finish. Being an Araucaria it doesn't have the hard rings/soft rings problems that plague finishing on some pines.

And before anyone suggests it no, I'm not keen on doing the whole thing clinker. I have a fetish about smooth lines underwater. I also kind of like the look of clinker topsides on a flush planked bottom,. Some Australian beach boats used to be done with clinker topsides and batten seam carvel bottoms. The reasoning was that laps in the bottom were more vulnerable to damage and clinker doesn't resist drying out as well as batten seam carvel, but of course batten seam carvel is more work and no stronger.

 

In Australia wider fibreglass is available from Duck Flat in S.A., I used 1.5m wide 6oz for my guideboat on 8mm paulownia strips with a 6mm ply bottom plank. However if I did it again I would probably opt for multiple layers of thinner glass, maybe 2oz, as it's easier to work with and you can vary the thickness for different areas.
So far the boat has held up well although I think I went a bit light by using durien for the gunwales. I would guesstimate the weight at around 25kg as it's slightly lighter than my 27kg plastic sea kayak.

 

-well, there’s your problem then ...

I suspect for the length of boat you are writing about, you will need most of the sheet.

I have to admit my first thought was, why not ply for the whole thing. However, apart from the smoothness aspect, stripping makes it easier to get a good hydrodynamic shape underwater where it counts. I like the idea of lapstrake above water, who’s to know? Out of the water and upside down it looks like a stripper, but again - who’s to know?

Although Hoop Pine is stronger when measured (I assume) in tensile strength, in the form of ply it may not have the same stiffness (varies as T^3) or resistance to cantilever forces (varies as T^2) as the thicker Gaboon ply of greater thickness.



You need to cut ply into many planks to make a hull with the same hydrodynamic efficiency as a good stripper. Problem with ply is it's so floppy when cut into narrow planks, and it needs to be cut to the plank development projection as it won't bend laterally unlike strips. Stitch and glue is a serious pain in the primary boat-rower interface when there are lots of planks. I am trying to develop an alternative building method for ply without using stitches or laps.

 

KJL38, thanks for the tip. I didn't know about it being available in 1.5 wide. The thing is that I just happen to have a 6 metre roll of 1 metre wide cloth lying around. 25kg is a nice weight for the guideboat. Got any pix?

AK, the F17 rating is mainly based on bending strength from memory. Would have to check the standards. Plank developments are no problem. On giving it more thought and measuring you're probably right about the amount of ply required though. Things are getting a bit tight.

 

Have you looked into "tortured ply", like in the Gougeon Brothers' book?

 

The picture above, originally from Gougeon brothers I beleive, was from
www.smalltrimarandesign.com

Here is another example, a rowing scull,
http://www.sageboats.com/stock/longlake.html

And here is Instructables on the building of a Tornado class catamaran,
http://www.instructables.com/id/Tor...do-Catamaran-Olympic-Trials-1967-AYRS-Report/

 

Noeyedeer, there are some pics at http://www.woodworkforums.com/f32/floats-guideboat-test-launch-84581/ and a bit more info from post 96 in this thread http://www.woodworkforums.com/f33/sea-going-rowboat-design-considerations-71942/index7.html

I like the look of the design you posted, do you have figures for the displacement and prismatic coefficient?