Rocker Changes

[Obie]

I'd like to increase the rocker on a boat I have already built. How do I go about that?

[Dolly Varden]

you dont

[JimD]

Originally posted by Dolly Varden:
you dontSince 'dont' is a contraction of 'do not' it requires an apostrophe like this: 'don't'.

[Thorne]

Obie -

You've cross-posted this very similar topic on both Building and Design forums, so you'll need to check both for answers.

Again, you'll need to be much more specific as to the boat design, materials, building method, etc. But the burning question is "WHY?"

Grammatically correct or not, DV's answer is probably correct for an existing boat.

[ 03-21-2006, 04:00 PM: Message edited by: Thorne ]

[Steve Paskey]

Seems like it might be feasible (though difficult) on a flat-bottomed boat with a single hard chine. I can't imagine doing it on a round-bilged hull.

[ 03-21-2006, 03:56 PM: Message edited by: Steve Paskey ]

[Dolly Varden]

Originally posted by Steve Paskey:
Seems like it might be feasible (though difficult) on a flat-bottomed boat with a single hard chine. I can't imagine doing it on a round-bilged hull.how?

[Thorne]

Well, the manufacturer recommends first unscrewing the rocker supports,then gently detaching the rocker from the boat's hull...

;-0 )

http://www.a-rocking-horse-to-love.com/store/files/images/large/d_155.jpg

[Bruce Hooke]

It seems a very odd thing to do, and whether it might conceivably be a reasonable thing to do with a flat bottomed boat would depend a great deal on the details of the construction (I agree with Steve that trying to do this with a round bilged hull would be very hard). One approach would be to remove the existing bottom then cut away a bit of the sides, more towards that bow and the stern and none in the middle, to increase the rocker, and then fit a new bottom.

Another approach would be to leave the existing bottom in place and add a new "false" bottom (along with extensions of the sides of the boat down to the new false bottom). The space between the new and old bottoms could be floatation. This would add a bit of weight of course, and the floatation compartment could become a nice place for rot to get started.

Mind you, I'm not saying any of this would be a good idea, I'm just throwing out a couple of possible approaches to the problem.

[mmd]

I will echo Thorne's question:

Why?

[Bruce Hooke]

And I'll third that question...

[Thorne]

Bruce --

That's a great idea to make it part of a false bottom -- I kept thinking about chainsaws, crowbars and lots of epoxy, but your method would be practicable (as much as an extreme retrofit of this sort can be) for converting a flat skiff for river use. Floatation and/or impact protection all in one!

;-0 )

[ 03-21-2006, 05:51 PM: Message edited by: Thorne ]

[Dolly Varden]

it would be more practical/less insane to build a new boat

[ 03-21-2006, 05:51 PM: Message edited by: Dolly Varden ]

[bainbridgeisland]

Oh boy, I hate to admit this. I have added rocker to a boat as folows:

Remove the backbone assembly aft (keel and floors), remove the inwalls (the boat was not decked), remove the knees aft, remove a wedge of planking on either side of CL aft, twist the planking to meet at CL (planking is not very rigid in torsion), install a new keel, refair the sheer line, replace the other structure.

I don't recommend this and don't plan to ever do it again.

[ahp]

I have posted on this subject before. Some years ago at the Mystic annual boatbuilders show-and-tell one amature builder told how he increased the rocker of his boat. It was perhaps a lapstrake boat about 18 ft long. My memory is hazy. He partly filled it with water while it was supported at the ends and left it that way for several weeks. It seemed to work but it sounds risky.

[Thorne]

Sounds like Ye Olde Methode of forming a curve in a cotton canvas sail -- sew it up dead flat, lay it out tensioned tightly on its side, and fill with wet sand for awhile...

;-0

[Todd Bradshaw]

Don't believe everything you read or hear. Most cotton sails were cut with shape using very similar techniques to those used on modern sails. Since this post also showed up on the stripper section of the Wooden Canoe Heritage forum, there is a good chance that it might be about a canoe. If so, you can often make small changes in the amount of rocker in an existing canoe by widening (more rocker) or shortening (less rocker) the thwarts which spread the gunwales. It's not ideal as it may throw the gunwale tops out of level and at some point it really starts to distort the whole design, but it can be done.

If you're building a stripper, you can gradually lower the heights of the forms and stem forms or stem pieces as they sit on the building strongback (building up-side-down) as you approach the ends of the boat and fair their positions with temporary battens before starting the actual stripping process. This will also increase the up-sweep in the gunwale/sheer line on the ends of the finished boat, but excess end depth can always be trimmed off before attaching the gunwales if desired. It's not unusual to see canoes that actually have fairly short stem profiles to reduce windage if the rocker is so great that the stems aren't even in the water. It happens on whitewater boats and goes all the way back to some regional variations of old birchbark canoe designs.
http://www.madrivercanoe.com/images/boats_lg/Outrage_rx.jpg

This is the only photo of a Crooked Canoe that I could find with a quick google search. Many of them didn't have so much up-sweep in the gunwales at their ends, since it does little other than catch wind. In a lot of cases, and in various regions, highly rockered models were actually no deeper at the stems than they were in the middle and the rocker was the only reason that the gunwales ended up being curved upward at the ends.

http://www.rootsweb.com/~nbcarlet/MP51NascapeeCrookedCanoe.jpg

[Obie]

Thanks folks for the advice. It is not an existing boat, just a design thus far, but it was interesting to see the advice on how to change an existing boat !

[mmd]

"I'd like to increase the rocker on a boat I have already built." - Obie "It is not an existing boat, just a design thus far..." - Obie Well, which is it - an already-built boat, or just a design?

[P.I. Stazzer-Newt]

This is a demonstration of why you should not cross post.
Over here (http://www.woodenboat-ubb.com/ubb/ultimatebb.php?ubb=get_topic;f=1;t=013336)

So it seems to be a Solo canoe - All Todd's points apply.

I would note that how you dial in the rocker might significantly impact the handling.

<center>Summary</center>
<iframe src="http://www.woodenboat-ubb.com/ubb/ultimatebb.php?ubb=get_topic;f=1;t=013336" height="480" width="640"></iframe>

[Bruce Hooke]

Originally posted by Obie:
Thanks folks for the advice. It is not an existing boat, just a design thus far, but it was interesting to see the advice on how to change an existing boat !Then why did you waste people's time by asking about an already built boat?

[AngWood]

Maybe you meant, "I already built one to this design, and I'd like to build another, but with added rocker."

[Obie]

I'm sorry to have troubled you-all with questions that I "should" not have asked, nor to have "wasted" your time, nor to have "cross-posted"....jees ! lighten up ! why not be pleasant to others, especially newcomers?

[mmd]

Obie, I asked which of your postings had the correct statement - was the boat at the design stage or already constructed - because the status of the vessel impacts greatly on what advice I would be able to offer you. My next two questions would be; "what type of boat is it?" and "Do you have any pictures?".

[Wes Kisting]

Obie,

I don't think the folks here meant to be rude. Your original post was unclear/ambiguous, and so a few folks were understandably exasperated by the confusion.

I'm sure you can get some good advice if you just clarify your needs and are willing to be patient.

The cross-posting thing is considered bad etiquette because no one wants to have to post their comments twice to be sure they get heard. If you start two separate threads, it causes two separate discussions that invariably end up repeating or overlooking sentiments which get better expressed on the other thread. Better to keep it all in one place. Again, just an issue of practicality. I'm sure no one has any hard feelings... just a bit frustrated is all. Be patient.

[Bruce Hooke]

My apologies for the tone of my previous post. I just got a bit annoyed that it was looking like I spent a bunch of time answering a question that may not have been the real question. For future reference, more details in a question are a very good thing. :D Also, if you do really have two seperate but very similar questions (as it would appear if you did in fact intend to ask both how to change the rocker in an existing boat and how to change the rocker in a design before building a new boat to that design), it would be very helpful if you could explain why both questions matter to you.

[Thorne]

I wouldn't feel too bad -- Obie has posted on two of the forums here, both posts with information so minimal as to be confusing, and only given the minimal answers to a series of highly-informed questions.

I asked him for specifics on what model of boat, type of wood and building methods, what waters he'd be using it on, etc.

and he replied,
"It's going to be a solo wood-strip tripping canoe. The design now has no rocker. I'd like to have an inch or so of rocker and am willing to trade the slower speed with manuervability. A similiar model is being sold by a manufacturer and they have added rocker to their model."

So the model is "solo tripping canoe", the method and materials "wood-strip".

Dunno about you, but that doesn't give me enough to go on to provide any sort of accurate answer.

His feelings may be hurt, but he certainly can't be surprised at the response -- this is a give-and-take democratic forum, and you gotta either fish or cut bait.

If he won't make the effort to give specific answers to specific questions, he can expect a certain amount of crankiness...from ANY group on ANY forum. Heck, even he might get cranky if we insisted on asking him something vague like, "How long is a canoe paddle?"...

[Dolly Varden]

you gotta either fish or cut bait seems to me hes done some productive trolling already smile.gif

can you say oyster?

[ 03-23-2006, 10:42 AM: Message edited by: Dolly Varden ]

[Thorne]

Well, personally I wouldn't call it trolling, as he has answered some questions and responded a bit. Probably more a factor of general inexperience, combined with the difficulities of expressing oneself in writing.

Face it, we live in a nearly post-literate world. If it wasn't for email and web forums, nobody would write at all, or nearly so. I work with a team of 5 techs, NONE of whom could write more than a sentence or so without grammatical or conceptual errors -- and several are college grads.

The real issue that I have with his two posts is lack of followup. As a relative newbie to this forum, I've certainly posted some gormless questions -- but have "owned" the questions enough to valiantly attempt to clairfy them -- or realize and admit that I've asked the impossible without providing much more specific information.

My topic on cutting down marconi sails to spritsails was a perfect example of one that shouldn't have been posted -- DOH!

;-0 )

[ 03-23-2006, 11:20 AM: Message edited by: Thorne ]

[P.I. Stazzer-Newt]

I don't think that was Oyster, I'd assumed that it was a straight question.

And it seems that Todd has divined the intent and answered it. Just use the original sections (molds, forms...) and allow the sheer to come up (down as its built inverted) - if it looks really ugly then trim off any excess.

[Todd Bradshaw]

If anyone cares to get back to the original question, it should be noted that increasing rocker in a canoe hull does not necessarily cost you speed unless you take it to extremes. Some of the fastest tripping boats I've ever paddled had 1.25"-1.5" of rocker and were noticably faster, side-by-side, than similar boats with dead flat keel lines. Rocker is only one design factor out of many that determining how the hull moves through the water - especially when you're paddling at cruising/touring speeds for extended periods of time, rather than a constant, high-cadence racing-style pace. Whether adding more rocker will make for a better or worse boat is hard to say without trying it or doing a lot of figuring (and then trying it), but you might not automatically lose noticable speed.

[Wes Kisting]

I'd have to agree with Todd. I prefer un-rockered designs myself, as I am a sea kayaker and accustomed to little to no rocker in the boats I paddle... but some rockered designs are pretty quick. Speed depends on a lot of other factors, too, not rocker alone.

I don't think Obie's answers ("solo tripping" and "wood strip" construction) were all that cryptic. He sounds like a novice who lacks a working vocabulary for the concepts he's trying to ask about. (We've all been there.) It seems he's talking about a cedar strip design built over a series of forms, and if that's the case...

It IS possible, if done carefully and conservatively, to adjust the vertical alignment of the forms SLIGHTLY to introduce a MILD amount of rocker into a design without catastrophic results. The problem, of course, is that doing it this way effectively "bends" the whole hull shape (introducing a bit of distortion--perhaps negligible, perhaps not--through the midsection), whereas a design with deliberately-introduced rocker would lift the ends but preserve the desired hull shape in the mid-sections (i.e., less distortion). Obviously, the shear will also be affected as the stations are moved.

I think it would be preferable, if one had the proper know-how, to plug the existing design into a program like FreeShip, to visualize the effects of the introduced rocker and make sure you're not "overdoing it" so to speak. The KAPER drag program built into FreeShip won't necessarily give a dead-accurate indication of the real-world results, but it should give a better idea than just guessing as you go, and should indicate any serious warning signs.

More interesting to me is Obie's mention (if I understood it correctly) that at least one manufacturer is already building the design with some rocker added... If it were me, I would begin by taking measurements off that design, if it has proven successful. If it's a smaller manufacturer, you might even be able to call them and squeeze out some information about how they modified the rocker for production. Some manufacturers guard their designs jealously, but a few can be surprisingly helpful and volunteer a lot of useful information--if you catch the right person, on the right day. (Good luck... but worth a try.)

I'm assuming (perhaps wrongly), Obie, that you have paddled the rockered version made by the manufacturer, and that's why he wants to mimic it. If not, I would strongly suggest that you DO paddle the rockered version first before you try to replicate it, to make sure it performs as you're expecting it to. (In other words, don't just assume the rocker is desirable because someone else is building it that way in production.) Try it out, and take measurements to get a sense of how much the original shape has been altered.

That's not a complete answer, but it's a place to get started.