designing a fast rowboat

Exactly!

 

I read somewhere, and it's easy to imagine, that the 'guides' at these hunting lodges were in competition with each other, and that after breakfast, all the hunters/fishers left at the same time, and there was some definite racing (one can easily imagine the egos of trophy hunting sportsmen) to beat the competition to the prime locations. So the boats would, to some extent, be indeed designed for speed competition.

And this is part that's hard to grasp: the guideboat design became something not just fast for it's limitations (being portaged), but fast outright.

Scull blades would have drag if dipped deep, true, but they are mounted lower than guideboat oars. And GB oars, at the much steeper angle, don't look as though shaft drag is much of an issue at speed.

The Steever book covers all this in depth, and well. And another factor is that blades which are too large (or made of wood that is less dense) have enough buoyancy that the rower has to expend energy to get them submerged enough to bite cleany, and over time, that adds up. Advantage: guideboats.

He also shows clearly that a GB at speed, accelerating with the 'double surge' of a faster than normal peak acceleration benefits from the limber oar because the higher stroke rate keeps the boat near hull speed. The aerolift initial bite (effective because of the 2 degree thole pin set angle) provided enough surge to lift bow of the boat onto its bow wave, greatly increasing required energy. The delay from a limber oar lets the thrust happen just below that inefficient speed, and is thus effective.

More detailed analysis also talks about how basically, the guideboat's fast thin oars create less swirl in the surrounding water than any racing sculls short wide blades, and this is more efficient.

It's weird to find myself making these arguments. I want to build one these classic hulls, and put 'better' equipment on it (sliding seats, riggers, longer oars), and I'm learning more and more about how good the original designs were, and how, for my purposes, I'm not going to make my build as superior as I thought I'd be able to. In the delivery of Aretha Franklin, R E S P E C T - that's what I've got for 19th century boat builders / designers, and the masters who taught them, Ad infinitum.

-NoEye, I think you'd enjoy the book. I'd bet dollars to doughnuts (frack, we're in the 21st Century, and now, doughnuts are worth more than dollars) Steever would influence your thinking. The book backs up all its assertions with much more authority and persuasion than I'm capable of paraphrasing.

 

Fair enough. I suppose they would have raced sometimes, but they would also have spent a lot of time not racing.

Oh I don't find that surprising at all. Making the thing as narrow as possible reduces wetted surface, and that is a huge advantage for racing. Mind you, it's also a huge advantage even at low speeds, since wave drag virtually disappears at low speeds and almost all the drag is from wetted surface. Reducing wetted surface is a win/win proposition for any speed you happen to be travelling at, even if you'e just lugging a fat city slicker and a pile of dead critters at two knots.

Reducing wetted surface is such an advantage, and the guideboats have done such a good job of it, that they are still likely to be faster than all other traditional boats even if their rowing position is slightly less efficient. This argument about wetted surface is something I have been trying to get across to Clinton, not because I want to discourage him from building a nice boat but simply because if he wants a fixed seat boat that will beat guideboats then it's not enough to just pick anything that looks nice.

Example: which is prettier? A Saint Lawrence River Skiff or a carbon fibre Olympic scull? Now, which is faster? This is not a silly comparison, because the principles that apply to the comparison between an SLRS and the scull are exactly the same principles that apply to the comparisions between a guideboat and an SLRS or any other traditional boat.

Yes I agree. I thought of that after I posted. It would largely depend on freeboard compared to loom length. The steeper the angle the looms have to be at to get the blades immersed, the more vertical travel you will get on the blades.

The boat I was thinking of in particular was my old boat. That was 12 1/2" deep amidships (about the same as a guideboat) and had the bottom of the oarlock jaws about 10" above the water (they were raised about an inch above the gunwales) with outer looms being 3 1/2 feet to the inner end of the blade. If I submerged anything more than the actual blade there was a very definite increase in resistance from dragging excess shaft backwards through the water.

So use use carbon fibre cleavers.

ETA: One other thing here. Yes, you would have to exert extra force to submerge the blade. Steever is forgetting though that the same bouyancy would help extraction of the blades at the end of the stroke. If you also take that into account then the difference in energy expenditure is going to be virtually nothing.

Hang on though: a limber oar is a device for reducing stroke rate.

Ok, I can see what the argument is. It's plausible.

Except that in practice we know that racing sculls are very efficient. They used to use longer and narrower blades and ditched them, precisely because they were demonstrably less efficient.

There are several things going on here. First you have the lift generated by moving the blade vertically at the beginning and end of the stroke. For that you would want a long, narrow and fairly straight blade, just as such blades are most effificient for sculling over the stern with a single oar.

However, that is only going to be a small portion of the stroke. For the remainder of the stroke, a blade like a modern cleaver is going to be more efficient. You also have to bear in mind that modern blades also generate lift at the beginning of the stroke, except that they don't do it by vertical movement but by horizontal/radial movement combined with a highly curved tip. The force generated is largely lift at the catch (when the oars can be as close as 30 degrees to the centreline) and changes to drag at mid stroke (just as a guideboat oar would).

Oh for sure, they did an amazing job given what they had to work with. That doesn't mean we still can't learn anything new though. I'm fairly sure that when guideboats were new there were people arguing about how the old ways were better, and I bet they had all sorts of reasons why.

How much do they want for it?

 

There's another thing about guideboats that interests me, and that's the position of the locks on the gunwale. I never see them at the lowest midpoint, and it may just be they set the positions for dual rowing, and solos row further astern than they would otherwise, or they like the additional height above waterline. I don't know.

Damm, of course that's exactly right, and I'll have to check, but I think you may have caught him there.

*Okay, I checked, and it's my paraphrase that's off. He actually complains about buoyancy that can delay the dig of the oar. Not the energy.

This is the thing. They’re more efficient due to a different effect. In this effect, disturbing water, they’re not. And the tradeoffs make the old way much higher in efficiency, and closer to racing sculls, overall much better than I’d previously credited them.

Well said. Steever talks in depth about how that curved blade needs to rowed differently, and gives a different power curve.

$15 plus shipping, but no tax if shipping out of state...

 

The location of the locks is dictated by the seat positions which are in turn dictated by the balance points. They generally have two lock positions, for solo rowing where the seat is at the lowest point of the gunnel so the lock is further along, and for carrying a passenger the seat is close to the end of the boat so the lock is also away from the center. Despite this the locks are close in height to the lowest point as the sheer is fairly flat through the middle of the boat as you can see in this short clip I found on youtube. http://www.youtube.com/watch?v=tr4hZBhpAgc&NR=1

I've been thinking about what you want to achieve with your boat and I think a guideboat is a bit short for a sliding seat and may pitch too much with the weight transfer. Perhaps something like the design Noeyedeer posted on page 63 of this thread would be better.

 

The position for solo rowing is pretty close to the lowest point of the sheer. There wouldn't be much in it with regard to height. Maybe 1/4 to 1/2 an inch. I've also read that guideboats row better solo if trimmed slightly by the stern, which makes sense to me given my experience in other double-ended boats. That would tend to drop the oarlock height slightly.

Ok. Would have to be a pretty bouyant blade to have a noticeable effect though, and it would still help with the recovery at the end of the stroke. So even if we accept that it delays the dig of the oar it will still help with extraction. If the speed of extraction is supposed to generate extra useful thrust...........................

Hmm. I'm thinking that if you have a large blade like a Macon or cleaver then it may appear to cause more disturbance during the middle of the stroke, when all blades are only generating drag. However, basic physics tells us that a large blade moving a large mass of water with little slip is still more efficient than a smaller blade moving less water at a higher speed (ie: more slip). Even if the visible "disturbance" is more the large blade will still be better in that situation.

Pretty cheap. Might grab a copy if they can ship over here at a good price.

 

I've seen 16 foot boats with sliding seats and they do seem to pitch an awful lot. The old Kittery Skiff springs to mind. Even one of those Herreshoff/Gardner boats with a fixed seat pitches noticeably when a solo oarsman is puting his back into it.

Those boats (the Herreshoff/Gardner ones) are pretty fine ended though. There is a fair bit of flare in the topsides but the portion that keeps you afloat is fine ended. They actually squat pretty badly in a sprint and are really only suitable for speeds of up to five knotsish. A guideboat has a higher prismatic so would be better at high speeds, but I'd be inclined to go with one at least 18 feet myself if performance with a sliding seat was wanted.

 

I've been following this thread since the beginning, & now I have a question for you guys. Last year i built a beamy 13' rowing skiff to get my inactive 11 year old exercising, which has worked out somewhat, I get him out a couple days a week, however, as most of you will know, with a design like that, your efforts are pretty unrewarding speedwise, so for myself I decided to build something faster, although not for racing.

After reading Skiffs & Schoolers several times, this weekend i drew up an Otter like Bateau for plywood. I went 22" on the bottom for a little more initial stability, raked the stern post similarly to the Culler design for full sections aft, but straightened the stem somewhat for sleeker forward sections. I also sprung the chine with the maximum width at the approx oarlock area, 20" aft of center, which also slimmed the forward stations.

I'm at 17'7" OAL, approx. 15'6" LWL, but I'm looking at all the waterline I'm giving up with the raked stern, & wondering if there would be an advantage to making an extension, like a rudder, with no depth, to lengthen and streamline the stern. What are your opinions?

I live on a river less than a mile from its mouth into the Chesapeake Bay, so it's often choppy, if that makes any difference.

 

I would agree. My boat(page 75) is a straight keel 16'7"lwl loosely based on Herreshoff's original swedeform, not Gardner's later symmetrical, drawing. The ends are too fine for a sliding seat and it pulls a strong transverse wave over 5mph on my gps.
Regarding oar length, a lot of snap from the arms will give the tip speed you need with 8' oars. I tend to grunt with my back and core to achieve the same tip speed with a longer oar at a slower stroke. I don't claim to be as fast or faster.

 

Sounds basically ok.

No advantage. If you want to extend the waterline it has to be an extension that has some useful volume. Just sticking a fin on the end will not help wave drag at all. It would make more sense to just extend the whole boat.

Note that depending on how fast you want to go, the waterline you have may be adequate. For a cruising speed of around four knots there wouldn't be any advantage in going longer. You'd only want more length if you wanted to optimise for sustained speeds higher than that.

 

You would be better off starting a new thread about that.

 

Boat-builders say the darndest things!

Here's a good one from the so-called hydrodynamic experts Fluidesign Composites Inc.

"The performance of a boat is determined by hull design. The more wetted surface in the water, the more surface tension, the less efficient the hull will be. Along with an already hydrodynamically advanced hull, the hull of our boat was refined even more to reduce the wetted surface and surface tension to an area smaller than other competitive models of similar size."

So much bs, so little time...

Leo.

 

Awesome. So if you have a hull with a lot of wetted surface (supertanker, for instance) there will be a lot more "surface tension", so you should be able to do a JC and walk beside it.

 

Or, the new paint tech from 2020 will really help:

 

Something that might be interesting: last night I got curious and ran some resistance predictions for varying states of trim. This is for the 21' beastie I'm messing around with, which may end up with a sliding seat. The results will probably translate reasonably well to a lot of other fast rowboats.

My initial assumption was that you'd set the boat up so that when the slide was fully forward the boat would trim level, and then when the slide was fully aft the boat would have stern down trim. Seemed sensible because it would help ensure good tracking.

The maximum angle of trim worked out to around 0.5 degrees in this case (would be more for shorter boats), which equates to the stern being about 2.25" lower than the bow. What I found was that there was a quite dramatic increase in wave drag when trimmed by the stern. The increase in wave drag was over 30%, and there was also a few percent added to the skin friction, which meant total resistance increased by over 7% (which is quite a bit). For shorter boats that have proportionately more wave drag, and will trim through a greater angle, the added resistance would be even greater.

Funnily enough, when I made the oppposite assumption (that the boat would trim level with the slide fully aft) and ran it for a bow down trim of 0.5 degrees, wave drag didn't increase nearly so much and the increase in total resistance was much less: a bit under 3%.

IOW, for this hull at least, trimming it down by the stern caused about 2 1/2 times as much added resistance as trimming it down by the bow. Obvious conclusion is that to minimise resistance the stern down trim should be kept as small as possible, consistent with the boat being able to hold a course.

Even more obvious conclusion: sliding riggers rock. Not only will they result in the boat's speed at any time being closer to the average speed, but they'll also minimise the added drag penalty from running out of trim.