designing a fast rowboat

Interesting that you decided to look at resistance in different trims.

What are you using for resistance predictions?

Is the hull fore-aft symmetric?

What speed/length ratio of Froude number?

I assume the trim angles you gave are at rest.

 

Michlet. Leo seems to get good correlations between prediction and experiment for similar types of hulls, so I figure Michlet is as good as I'm likely to get.

No. LCB is slightly aft of midships. Yes NA's, this also means several other things are slightly aft of midships too.

Around 0.4. Will test it a bit more thoroughly later.

Yup. Basic analysis to give a ballpark idea. Haven't tried to incorporate the full range of dynamic effects (that way lies madness).

 

The boat is likely to trim more stern down at Fn = 0.4, so the drag could be higher on average. Suggests trimming bow down at rest might result in lower drag though tracking not as good (except with a strong head wind).

 

Oh yes, there was something else odd. I'm currently enthused about doing an SOF boat, which of course ends up being multi-chine in practice. The 30%+ increase in wave drag when trimmed by the stern was for the multi-chine hull.

The best round bilge hull I have been able to get, with the same stability, theoretically has about 0.3% less drag when running level but has more added wave drag when trimmed by the stern: over 40%. The round bilge hull has slightly less added wave drag when trimmed by the bow, but overall is noticeably more sensitive to changes in trim. I would have expected the opposite, but there you go.

Oh and the multi-chine hull is a seven panel job. One narrow bottom plank (a la guideboat) and two chines up each side. I found this was almost identical in drag to the round bilge hull, and noticeably less than a five panel hull.

Why not six panels instead? Because I figure having a built-in bottom plank to stomp around on makes more sense than adding extra floorboards. Better strength/weight ratio and cleaner inside.

 

Well at Fn=0.4 the stern wave is right under the stern, so trim change should be minimal.

 

Were the differences in resistance between the multichine and round bilge halls primarially in viscous or residual/wave resistance? Any other significant differences in the shapes beyond the sections being polylines vs smooth curves?

I'm a little suspicious that Michlet is predicting significantly different resistance between the two, particularly trimmed.

 

Differences were in wave drag. The figures for level trim make sense as I would expect a tiny increase for a long and narrow multi-chine hull, where the chines do cause some kinks in the hull but not very big ones. The wave drag for level trim is about 3% greater than the round hull, which works out to around 0.3% of total drag. Wetted surface for both hulls is near enough to identical.

The slight differences for bow down trim also make sense when I look at the lines of the hulls in question.

The differences for stern down trim are a bit odd for sure. I'm not certain if they are real or artifacts. I am thinking that perhaps Michlet had some issues with flow around the chines when trimmed stern down, and that in practice the multi-chine hull might not have a lower added drag in comparison to the round bilge hull. Hard to tell without full scale testing of both, though.

The basic point, about stern down trim being significantly worse than bow down trim, appears solid.

 

Same effect as the 3M plastic film that Dennis Connor had on the hull of Stars and Stripes in the 1988 Americas Cup.

 

You should discuss the results, and particularly the differences with Leo. Michlet does not compute the details of the flow over the surface. It would be good news to verify that it can predict the differences between a multi-chine and round bottom holes of other wise similar shape - but - I'm skeptical. I've had some experiences with numerical methods which appears to resolving smaller differences in the input than expected and then it turns out the differences in results are due to the details of the numerical solutions. But then it may be correct.

One other thought about a Skin On Frame boat. The skin will be pushed in between the longitudinal stringers creating a small increase in surface area and larger angle changes across the stringers.

 

Yes, I'm somewhat sceptical myself regarding the stern down trim predictions and am not putting a lot of weight on the differences. My feeling at the moment is that for all practical purposes both hulls (round bilge and seven panel multi-chine) would have near enough to the same resistance.

ETA: By the way, while pigging around with this thing I did do some (not extensive) comparisons for three panel hulls and five panel hulls in level trim. Now experience tells us that a three panel hull can be made surprisingly good, but that at displacement speeds it wont be as good as a round bilge hull, due to it having more wetted surface and some chine drag. Michlet appears to say much the same.

Experience also tells us that, at displacement speeds, a five panel hull will be better than a three panel hull but still not quite as good as a round bilge hull. Michlet seems to agree.

Extrapolating from that, and eyeballing the differences in diagonals, etc on the relevant lines plans, it would seem reasonable that the seven panel hull would be getting so effin close that in reality you probably wont pick the difference if you're not in the Olympics. Michlet seems to think so too.

So, I'm fairly confident that the results being generated for level trim are sensible for comparative purposes.

Yup. Thought of that too. Wouldn't be much of an effect in this case because the panels between the chines are pretty narrow. Of course, with a bottom board inside it, the bottom panel will not deflect noticeably. Next one up (to the chine just below the DWL) is where you would get the most deflection, but the panel is only around 5 inches wide at its widest point anyway. With reasonable cloth tension the sag should be minimal. Hydrostatic loads aren't much.

 

Thanks guys for the cool info. I’ve been putting it off as I’m a mac guy and I hate to have to load up PC emulators, but it’s time to start playing with hull design software.

Thanks for that. My eyes are really starting to see the efficiency of the unusual Guideboat / 'Doryman' stroke, more and more.

I agree with your advice, and would love to row NoEyeDeer's design, but only in relatively calm water. My needs entail the bay and open ocean rowing, so I have limits on how long I can stretch my length, and if I'm totally honest, I don't think I could do better than Clint's Drake, which may be the perfect open water boat.

But I just can't help wanting something sexier. [from earlier in thread] Nathanael Herreshoff supposedly said "Half the reason to own a boat is to look at it.” And the Phoenicians supposedly said "Feast your eyes on a narrow boat; Place your goods in a beamy boat." Well, I want to feast my eyes.

So I'll have to get up early and do more rowing in morning calm, which would be good for me anyway, and I'll have to go slow, skilled, and careful in bigger seas, which I can also do. I do believe in the truth that "Seaworthy is mostly about the person in the boat."

And I'll be prepared. ANY boat I'll design will have plenty of internal buoyancy, and there's something else I want to do that hasn't been discussed much: water ballast.

I’m also very interested in water ballast for rougher water conditions (as used by peter E [post 639] and others). This seems a great way to increase stability without any penalty for smooth water, when you’d carry none.

I’d also like to find a better way to drain water from the bottom of the boat. The Anderson / Seasure Super suck self bailers don’t seem to be optimal designs. I haven’t been impressed with them on sailing dinghies, or rowing shells. Through hulls on thin-skinned rowing craft don’t seem a good idea anyway. I’d like to think there’s a better way to channel the energy of the oar and the acceleration of the boat into letting the water’s inertia splash itself out of the boat, but haven’t seen or heard of anyone who’s been able to do this.

Of course, I’d love to hear what others think about water ballast, pro or con, and if put where, ideally?

 

dory stroke

OK, real world stuff. I went out in my SOF with my GPS. Dory stroke added .1 knot with spoon oars which aren't the most effective for that stroke. Very encouraging to know that I can move the boat along a bit faster. Nordfiord is now almost totally faired. Two weeks should give me a painted boat with lowered thwarts ready to start sorting out final rowing stations. Can't wait! To DCN blues, did you see the new issue wooden boat? big article on San Francisco rowing; the Viking Class. Maybe you are familar with them. They look to be something you might want to build.

 

I didn't know about that, thanks. Looked into it, and apparently it was also on Oracle's USA in Valencia. Be nice if this did trickle down.

3M Drag Reduction Riblet Film microreplication:

http://tinyurl.com/ylbbpx8

Now, I want a rowboat with riblet film, and a tank of alcohol / polymer mix to inject into the water near the bow:

http://www.sailinganarchy.com/article.php?get=5154

If I'm not mistaken, Reynolds numbers analysis on breaking water friction provide greater benefits at lower speeds than higher. But we'd need a pro to weigh in for an opinion that carried any weight.

I've also considered designing a hull with built in slots that could take the kind of porous, pumice like stones you see in aquariums, running tubing up to a small scuba tank, and having a 'nitrous' button like Mad Max's on the oar handle. Push the button, blow fine bubbles along the hull's bow and keel, and let them dissipate surface tension along the hull as the flow carries them aft and their buoyancy carries them up. How obnoxious / fun would that be?

It works for supercavitating torpedoes, so why not a fast rowboat?

http://en.wikipedia.org/wiki/VA-111_Shkval

***Edit: Wow, for the past two months I've been reading this thread (and its links) and haven't ventured out into the rest of this forum. Now that I'm considering hull design software, I'm just exploring, and it turns out that everything I just posted above has been covered in depth at other places. No reason any of these technologies to reduce hull friction wouldn't work for fast rowboats, but they're better covered elsewhere, so feel free to disregard.

 

Thanks Jeff, already covered back in post #1049 and #1050 http://www.boatdesign.net/forums/boat-design/designing-fast-rowboat-14250-70.html#post461048 as well as #1083 http://www.boatdesign.net/forums/boat-design/designing-fast-rowboat-14250-73.html#post462448

I have conflicting thoughts about the dory stroke. On the one hand, I don't see why it couldn't work even on a racing scull. On the other, a smooth, soft touch technique probably has even greater payoff in reducing hobbyhorsing and loosing energy to transverse waves. For the right boat / oar combo, I could see that oar blade lift being an asset. For sure for Paul Dean...

 

As the number of panels increases the choice between chines and round bilge is more a choice of construction once the hydrodynamic differences are negligible for all practical - i.e., non-competition - purposes.