designing a fast rowboat

If the "two major differences" are two hulls instead of one it wont help. That has been tried before. The only way to increase stability is to increase the metacentric height. Doing that in practice, for this sort of boat, involves increasing beam. That increases resistance. If you have another solution that increases metacentric height without increasing beam I suggest you test it and publish the results as it will turn naval architecture upside down and make you world famous.

While we await those test results, I've been playing around with Delftship some more. I hadn't done much work with it before but it contains two algorithms for calculating resistance. With this sort of boat the Delft algorithm is getting near it's applicable limits and I wouldn't trust it for wave drag calculations. The KAPER algorithm should be more accurate as that is based on actual canoes and kayaks, which in terms of leading proportions are similar to fast rowboats. There are differences in freeboard and flare but the underwater stuff is pretty comparable (displacement/length ratio, prismatic, beam/draught ratio, etc).

And despite what American spellcheckers say I do know how to spell draught. Damned modern software. When I were a lad things were different.

Anyway the interesting bit, since this is a boat design forum and not a linguistics forum, is what happened when I played around with the length of the design. For a rough and basic check I simply stretched the boat longitudinally and adjusted the waterline height to keep the displacement the same. Having had more coffee this is not sounding quite so logical as a longer boat would have to be a bit heavier, so that would influence the results. However, both algorithms indicated a reduction in resistance with increasing length to an extent that I hadn't expected.

The total resistance from the KAPER algorithm with the length scaled to 100%, 110%, 120%, 130% and 140% came out as 100%, 97.1%, 95.4%, 94.1% and 94.1%. Corresponding waterline lengths were 5.96 m, 6,50 m, 7.04 m, 7.57 m and 8.10 m. Now as mentioned the longer boats would invariably weigh a bit more and that is going to increase both wave drag and skin friction. The longer boats could also be more of a handful under some conditions due to increased windage and the tendency of longer and skinnier boats to behave more like submarines if the water is misbehaving somewhat, as it tends to do. Because of these factors I think the 130% and 140% options can be dismissed as being of no real benefit. It does look though as if a waterline length of up to 23 feet (or 7 metres in French) could be beneficial.

ETA: Oh this was worked out for a speed of six knots.

 

Relax. Have a draught.

 

I might just do that. Mmmmmm. Beer.

Anyway I went and dug up the Mystic Seaport link for the Quitsa Pilot as I hadn't heard of that boat before. Nicked and expanded image from their site is attached. It's a tad blurry but that's the best Photoshop could do with the starting image.

The shape in the ends of the waterlines is actually extremely fine. Way too fine in fact for low resistance at any decent speed. Looking at the lines and taking an educated guess I'd say there is no way the prismatic could be over 0.5 and it looks rather less than that. The shape is best for very low speed/length ratios and the end sections have a lot of wetted surface for the amount of volume they contain. All in all, rather pretty but not the fastest shape.

 

Ok this is about the smartest thing I can do at the moment. All the numbers are good and it's not uggers. I've been over it from every angle I can think of and can't see any real way of improving it. Length seems to be optimum as far as I can tell and ended up at 22 feet. Designed displacement is 600 lbs. Waterline beam is as narrow as I'd be happy with for this sort of boat. Freeboard is about as low as is practical but the bow flare should help keep her nose up without too much windage. The hull should have some grip on the water but not be too broachy and it wont pound into a chop.

Assuming some sensible and not too optimistic heights, the righting moment is about 15 ft lbs per degree. This is just taken from the upright metacentric height and doesn't allow for increased stability with heel due to the topsides flare. In real terms that means if both crew members sit half an inch off centre the boat will heel about one degree, which with this beam would put one gunwale near enough to an inch higher than the other. With both blokes sitting a foot off centre she'd be getting close to getting the gunwale in the water, depending on the exact weight of said blokes and what the water was doing at the time.

So yeah, if it was me (which it isn't) I'd try something like this.

 

Pointed stern or transom

Why are most rowing boats for rowing exercise more or less pointed at the stern, that is, without an effective transom?

These boats are between 4.5 and 6.0 meters long and travel between 4 and 6 knots. This means a Froude length number between 0.30 and 0.45 and a Froude volume number between 0.95 and 1.40.

These are speeds that are normally considered as transition speed. And most modern boats have a beam at the stern to limit wave drag and aft trim. Why, then, have most rowing boats a thin stern when no other boats in this speed region have it?

js

 

In a long narrow rowboat with Fn=0.45 hartly can be described as a transition speed... Rowboat is not only about resistance it is also about directional stability. A rowboat with wide transom is heavier and need a skeg anyway. About 80-90 % of well designed rowboat resistance is friction (minimized wetted surface) and under 20% wave resistance (optimized LWL and Cp).

Terho

 

Dear Jurgen,

From your question I went to your website and was able to get an idea of where you are heading with your work.

My 2009 Design Challenge entry was a selected runnerup. You can see the finished boat on www.bmsyachtdesign.com. It would be interesting to establish communications.

Obviously you have an open mind!!!

Jerry

PS I spent many years working with ABB with their division in Finspong, even before ABB was formed and the group was ASEA. JDB

 

Wetted surface is a big part of it. This boat has a displacement/length ratio of only 27 so the wave drag is proportionally less than in yachts and other more normal boats. That hollow near the bottom of the transom isn't just for looks. By steepening the run along the centreline and reducing the width of the aft waterlines you get the same volume contained within a shorter perimeter as compared to a wider and flatter section. The volume has to be there to get the right prismatic and reducing the wetted surface is important for this style of boat. Even though it isn't much in the scheme of things it still trimmed a bit off the total resistance when I ran a comparison with an earlier version.

Another advantage is that it helps reduce drag in waves. Towing a transom through the water is a very effective way of stopping a rowboat in its tracks. You have very little power available and it's only available intermittently.

 

Transoms are not carried all the way down to the waterline as it would increase drag. Either the bottom is raised to meet the transom above the waterline as in a flat-bottomed skiff, or the transom is given a wineglass shape. The transom is provided to increase carrying capacity which is not an objective in an excercise or racing rowboat.

 

Just did a bit more number crunching on this and got some results I found interesting. It turns out that for this particular boat in the speed range of interest (ie: small changes around 6 knots) the total drag is varying almost exactly as the cube of the speed. This would mean I could assume that speed would be inversely proportional to the cube root of the resistance, except for one extra factor: not only does the same grunt need to be applied but if the boat is moving faster the grunt also needs to be applied just that little bit faster. This will mean more work for the crew and needs to be compensated for by a slight reduction in predicted boat speed. On that basis it seems logical that speed will be inversely proportional to the fourth root of the resistance (since power output of the crew wont change and power is work divided by time). That should be near enough to the truth for now.

Ok so back to the green beastie. It has a predicted total drag of 17.1 lbs at 6 knots, of which 26% is wave drag and the rest is skin friction. This is for a displacement of 600 lbs. I ran another calculation with the boat floating slightly higher for a 570 lb displacement. That gave a predicted total drag of 16.8 lbs. This is a reduction of 1.58%. Big deal huh? It adds up over a long race though. If we're talking about a six mile race, which seems to be on the money for a lot of these races, then that means 36,480 feet covered. This means you can estimate the winning margin in feet as: (1-(0.9842)^0.25)x36480

That gives a result of 150 feet. That's not much in terms of absolute speed but it makes a lot of difference at the end of the race. Clearly if it is possible to make the boat 30 lbs lighter it would be worth doing. Even 10 lbs is still going to give a forty to fifty foot margin, which is definitely enough to discourage the competition.

Another step would be to go for a slightly more radical design with less waterline beam. Green beastie has a designed Bwl of 2'11". Knock that down to 2'9" and you get reduced skin friction. You also get reduced stability. You get a much bigger reduction in stability than you get in skin friction, because boats are evil like that. The reduction in wetted surface is 4% and the reduction in stability is 20%. The stability is still manageable for a good crew in my opinion but it would be trickier to handle and would not be suitable for mugs in a blow. If you are nuts enough to go with this option the reduction in total resistance is 2.76%. That works out to a margin of 250 feet at the end of the race and illustrates a fundamental point about rowboats, which is that in basic terms the tippier they are the faster they go as long as you don't fall over.

 

For a displacement hull, skin friction predominates at low speeds and increases somewhat gradually as speed builds, whereas wave drag typically is much lower initially but rises in a dramatic fashion at the "hull speed" - quickly exceeding the skin friction. Thus a long boat will have a greater top speed but will be harder to push along at slow and moderate speeds than a shorter boat as a general rule.

I got a practical demonstration of this when I and a buddy compared my sleek new plastic kayak to my shorter homebuilt wood boat. They were comparable as far as stability went (neither too great) and when sprinting the longer boat had a clear advantage but as soon as we slackened off the shorter boat had the legs of the other boat. This was the same no matter who was paddling which boat.

Added to the fact that the homebuild woody is 1/3 the weight of the plastic dingus and 1/10 the price and gets way more ooohs and aaahs, there isn't much doubt which is to be preferred! However, the plastic boat is the best one to have if you plan to toss a boat off the back of a moving pickup truck, which I was tempted to do.

In the past I have tended to ignore skin friction but I don't anymore.

 

Yes that's definitely true. Very small boats, even an eight foot canoe, are delightfully easy to push along if you don't mind going slowly. The low wetted surface gives them low resistance. They fall down as soon as you need to go fast though. The wave drag on this design increases roughly as the sixth power of the speed at speeds of around 6 knots, whereas the skin friction only increases as a bit less than the square of the speed.

 

Hello after a while

It has been awhile since I have posted. Keep an eye out for some of these Finnish boats coming to the market here in the US. There is some cross-Atlantic business in the works.

My pursuit is on for a fast, strip built (meaning fast to row not build!) pulling boat (fixed seat but that someone could drop a rig into if they liked).

The contenders are:
--Herreshoff Rowboat
--St Lawrence Skiff
--Ken Basset Liz

Strip built because of a course I am designing and we need a good strip built boat, but it has to be fast, wicked fast. In fact, my goal in life is to take Guideboats off the top of the race listings!

 

My guess is the guideboats will still win. They have a narrower waterline beam and much the same length.

 

But with a longer waterline....