Rocker

[JamesS]

New member here. I tried a search on rocker for small boats and can find nothing on the subject. What i am doing is building a 8 foot (on bottom)(9 ft 6 in overall) stitch and glue kayak/dory mix boat similar to the clarkcraft Lark decked kayak. If i like the way it works i will attempt a larger 12 foot on bottom build. I have no designing experience so i just started making paper models until i got the boat to look the way i want it. So i ended up with a 8 inch on bottom with a 3.5 inch bottom width and 4.5 inch sheer width, transom with 1.5 inches at bottom, 2.5 inches at top. I made the bottom completely flat for the length of the boat and created flare of the sides by cutting the bottom sides of the boat with a arc.The shortest point on the side( freeboard?)is 1.2 inches.

Anyway, my real point here is i would like to know if i should introduce some rocker to the bottom. This boat is for some kids and i would prefer for it to be as stable as possible without looking like a fishing skiff. A cheap sail kit is probably a given at a later date. I know rocker is used for sailing and smoother rowing but is it necessary for such a small craft? Knowing the kids a proa or trimaran conversion may be prudent as they may use it for a diving platform if i am not there to keep an eye on them. We live on a large pond easily accessible from the back yard. I really don't see where i need any rocker at all but thought i would ask here. Any information about rocker would be helpful.

Thank You,
JamesS

[JimD]

http://www.clarkcraft.com/images/lark9.jpgLike this? A little rocker wouldn't be a bad idea. But only a little or else you'll have a drift boat yawing all over the place.

[John B]

If you have rocker , then you get a place to put a little keel for the tracking issue.
Row anything trimmed by the stern or dragging its transom through poor design and you'll soon realise that exit is what its about after all, and you get that from deadrise ,or rocker, or both.

[JimD]

John B makes a very good point. The only way you want to leave any transom in the water is if there is virtually no transom. Ie, a double ender, which is pretty much how canoes and kayaks get away with little to no rocker. If you're dragging a transom the deadwater behind the boat will kill you. It'll feel like you're dragging an anchor.

Something like this, perhaps, with a relatively flat run from the stem with most of the rocker aft, with a shallow skeg for tracking:

http://www.boat-links.com/Atkinco/Oar/images/Dancer-2.gif

[Tom Robb]

Short version:
Straight tracks better. Rockered turns easier.

Make them wear life vests under penalty of mortal sin and who cares if they use it to dive off of?
If it's for children, especially other people's, liability rears its ugly head. So why design it yourself when with a published design you can at least palm off some of the liability on deeper pockets.:rolleyes:

[Bob Cleek]

If you don't believe the kids are old enough to practice suitable water safety skills without supervision, a lifejacket isn't going to protect them sufficiently. If they need supervision a lifejacket or a well designed boat are not substitutes for an adult eye.

[John B]

We have sabot of about 8 ft for one of my kids. I think I gave it to her when she was 5. They are chined with both deadrise and rocker of course, but this thing tows like a pig. I've measured it and it tows at 6 or 8 knots like a garden shed with a sea anchor behind it.
However, its just rows beautifully.So much so that almost everyone who's rowed it feels they have to comment about it.

exit!

[Thorne]

Designing boats is a very complex business, so I'll second (or third) the suggestion that you build your first boat from plans -- there are a LOT of designs that meet your specifications as stated (so far).

And pro designers know how to add things like flotation compartments or locations for flotation, in places that make bailing/boarding a swamped boat possible.

A kayak-dory mix sounds like a Mactruck-Corvette mix: possible but unlikely and dangerous. Kayaks don't really sail, dorys don't really paddle, and the hull shapes are very different.

Not trying to discourage you from learning boat designing, as it can be very rewarding (I'm told). But you want to build these experimental boats for yourself first, so you only put yourself and your $$ at risk.

Here's a boat designed and build by a friend of some boatbuilder friends of mine. He didn't listen to anything they had to say, did it his way, and has a boat that won't sail, barely rows, and generates nothing but unkind comments ("floating coffin"). We rowed all the way across a windy lake to see if he was OK, as we all were convinced that he'd be drowned, but he survives so far.

http://www.luckhardt.com/ST07/images/17.jpg

http://www.luckhardt.com/ST07/images/6.jpg

[Todd Bradshaw]

That's truly one of the ugliest, most screwed-up-looking boats I've ever seen. Any good warden would arrest him for littering the waterways......

[Cuyahoga Chuck]

That's truly one of the ugliest, most screwed-up-looking boats I've ever seen. Any good warden would arrest him for littering the waterways......

Thank you, thank you, thank you. I am inclined to make comments like that but, my standing here is so minimal I can't tolerate the backlash.
Not only homely but his oars look too short. Probably a good cardio workout but hell trying to make any distance.
I'm all befuddled now. I'll have to go back to post #1 and find out what the devil we were talking about.

[Thorne]

Thanks Todd -- you help illustrate my point.

;0 )

My friend's friend's boat is almost a poster child of what NOT to do when designing a boat -- maybe we should take it on the wooden boat show circuit? Notice how he got nearly every detail completely wrong -- the kickup rudder that becomes windage being just one example. And yet the builder is **so proud** of the boat he's made.

---------------------

James, I'm trying to not rain on this particular regatta, and do think that designing and buildlng your own boat sounds really cool.

Personally as a new woodworker, I found that restoring a dory skiff was challenging enough. And I got to a lot of designing work anyway, building the sailing rig, centerboard case, yoke rudder, mast and boom, etc.

But if you design you'll want to do it right, and get some of the basic boatbuilding books (available from our kind hosts). Build the boat for yourself and test it completely before handing it off to any children.

http://www.woodenboatbooks.com/WBB/Design_files/0937822442.jpg (http://www.woodenboatstore.com/prodinfo.asp?number=325%2D108)http://www.woodenboatbooks.com/WBB/Building_Methods_files/0937822108.jpg (http://www.woodenboatstore.com/prodinfo.asp?number=325%2D075) http://www.woodenboatbooks.com/WBB/Building_Methods_files/0937822507.jpg (http://www.woodenboatstore.com/prodinfo.asp?number=325-111)

[JimD]

I disagree that you need to buy plans from a professional. This is an eight or nine foot dory for the kids. Just go to a bunch of different websites and see what they offer, and draw up something similar. You mention stability, so make it on the wide side. Make a couple cardboard models out of posterboard. And have fun.

[John B]

I disagree that you need to buy plans from a professional. This is an eight or nine foot dory for the kids. Just go to a bunch of different websites and see what they offer, and draw up something similar. You mention stability, so make it on the wide side. Make a couple cardboard models out of posterboard. And have fun.



Damn right. Just pull a bit of rocker in:D


edit: since those tossers at sony are closing down imagestation, go here for a look at a bit of experimentation on kids dinghies.Those pics will all be gone in month , but it might be of interest.
http://tinyurl.com/2nok9o
All mixed up but you can go from crank and difficult to a fairly useful little unit still doing good service and usually thrashed by 5 or 8 different kids a season.

[Thorne]

Don't think so, Jim.

Here is the Clarkcraft decked kayak -
http://www.clarkcraft.com/images/s62.jpg

Now imagine that with a sailing rig, and kids onboard. To my mind, anything that is that fully decked really needs to be carefully designed if it will be used by kids - this avoids them being trapped if it turns turtle.

Just my opinion, mind you, I'm certainly no NA.

[SV Papillon]

That's truly one of the ugliest, most screwed-up-looking boats I've ever seen. Any good warden would arrest him for littering the waterways......

Todd email me svpapillon@gmail.com
I have a couple questions for you
Jake

[Bruce Hooke]

Some gentle rocker (just enough to get the ends out of the water with a normal load on board) will make it easier to turn the boat and should not have any real impact on stability. If you are worried about what will happen if everyone moves to one end of the boat then what matters much more is keeping the boat somewhat wide right to the ends (as on a pram). In a narrow boat, even one with no rocker at all, there simply is not enough volume in the ends to provide much buoyancy if the people on board move too close to one end.

Bottom width really has much more impact on stability. This is why your basic flat-bottomed skiff does so well in this department. Dories and kayaks are great in waves if the people on board have a bit of skill, but if you don't know what you are doing the narrow bottom and resulting lack of initial stability can make the boat feel pretty unstable, and if you stand up even a dory can pitch you over the side if you are not paying attention.

Going the other direction...with short, wide boats it is very easy to create a hideous and pretty useless bathtub of a boat.

Edited to add...now that I see the picture Thorne posted, the decks would do a pretty good job of keeping people from standing too close to the ends. A bit of rocker on such a boat will do no damage to the stability and will make the boat more usable.

[Cuyahoga Chuck]

Ok got it.
Kayaks have far more developement time on them than any other boat. About 1000 years give or take. One thing you will notice is that without the science of hydrodynamics they figured out that "longer is better". Easier to paddle because you get more distance per stroke (glide) than a shorter boat. Short boats yaw with each stroke so they go thru' the water like a snake. A to B is much longer if you don't go straight. Being able to go straight is important if you paddle well out from shore. It's very easy to lose your sense of line and meander in directions you don't want to go. If you would try to paddle along with conventional kayaks you would have to work much harder to maintain their casual cruising speed.
I realise this is not what you asked but I thought you should be aware of the general rules of thumb that determine why a kayak is designed the way it is.
As for rocker, I doubt you need much if any because your bottom is so short. Rocker is introduced into keels to make boat turn more easily. The little boat in my avatar is 8' long. It has considerable rocker because it is a sailboat. If it didn't turn on a dime I wouldn't be able to tack properly. Being very short, it is plenty slow. Except in winds over 15 mph kayakers outdistance me without too much difficulty.
I don't think much of the Lark as a playboat for kids, either. Being decked over you don't get use of all the interior space and it will be more difficult to empty if swamped.
A much more ideal hull that would be better able to satisfy all your desires is an 8' pram like D4 that can be set up to row or sail. Being a pram it has much more interior space for all the tykes to crowd in and it has built in floataion chambers that make it almost impossible to sink. It won't give you the thrill of designing your own but, you will never match the naval architect who designed D4, anyway.
Even if you want to continue on your present project download the plans for D4. They're free and even contain a tutorial about how the thing goes together and two sail plans. Perusing the plans should give you some good ideas.

http://www.bateau.com/freeplans.php
Click on "D4 files".

[JimD]

...I don't think much of the Lark as a playboat for kids, either. ...
A much more ideal hull that would be better able to satisfy all your desires is an 8' pram like D4 that can be set up to row or sail. Being a pram it has much more interior space for all the tykes to crowd in Even if you want to continue on your present project download the plans for D4. They're free and even contain a tutorial about how the thing goes together and two sail plans. Perusing the plans should give you some good ideas.

http://www.bateau.com/freeplans.php
Click on "D4 files".

I second the above.

[Ian McColgin]

Make the SIx Hour Canoe. Nice little boat and you could learn abit about what makes a boat work.

Then for yourself, make a Glouster Gull.

[donald branscom]

If you don't believe the kids are old enough to practice suitable water safety skills without supervision, a lifejacket isn't going to protect them sufficiently. If they need supervision a lifejacket or a well designed boat are not substitutes for an adult eye.

Even adults need to use the buddy system.

[tonydezoc]

A very experienced old boatbuilder told me once that when they used to build workboats where the designer had drawn a straight keel the master shipwright would "introduce" a small amount of rocker when setting up the keel by using slightly shorter supports in the middle area but then make any corrections necessary with the moulds. The reason he said was that when a flat keel is aground on an uneven surface it can bend upwards but a keel that is even slightly rockered wont. Perhaps not entirely pertinent in this case but a good story nontheless.T

[JamesS]

Thank You all for the comments. I still have some questions and comments on your answers so here goes.

Jim D, What is a little rocker? 1 inch. 2 inches. Since i do not know how to design i do not know how much will keep the transom out of the water. I also must consider that the transom is only 14 inches wide at the bottom.

Tom Robb, please know the kids always wear lifejackets. The problem i have is other neighbor kids swimming out to the boat to play with them. I also use a boat horn to tell the kids when they have moved too far away from where i am. I have been letting them use a aluminum skiff to play out in the water with but now they want a little boat of their own.

Jim D #12 & #18, Thank you for that comment. I have not been able to find what i am looking for so i decided to try modifying something for myself. I guess what im looking for is a catboat with a narrow transom. I want stability along with the ability for them to get into the boat easily. On comment #18, Another idea for me to work on , thank you.

John B, Those are some lovely skiffs. Do you have any plans for them? Could i buy the plans from you? I have saved most of the pictures you have there.

Thorne #8 and #14, That is one wgly( double ugly)boat. I have been a house carpenter for many years so i do know what looks good. I doubt my boat will be ugly, it just may not be real pretty in my eyes. Who knows, since i am getting close to retirement, i may start building pretty little round boats. I have already owned a wood 22 foot greek sailboat so i do not want to get that involved in a large boat anymore. On the #14 comment i am not against closing in the cockpit and filling the inside spaces with those large air bubble shipping filler bags( i get tons of those things at work). I also plan on using swimming noodles for sheer cushions as i plan for the deck to be a bit lower in the boat. I may not even have a real deck on the boat just a seating deck about halfway up the sides.

I may even scrap this idea and build a larger version for myself at a later date. I just wanted a small boat for the kids to play around with and i let them decide what they would like. Guess what they picked? Yep, the Lark, of course. They have seen pictures of my old sail boat and wanted something like it. It looks like the pram style may suit them more in reality and then they could get 2 or 3 people on the boat.

Thank You All,
JamesS

[Tom Robb]

Yesterday I noticed that the playground at the grandaughter's school was paved with foam rubber mats:rolleyes:
Nobody wants their kid to get hurt, but is it doing the kids any favors to try to make the world risk free/idiot proof? I don't have the definitive answer, but I suspect that we hover over them way too much.
If you want to make playing with a boat utterly risk free, why would you let them near any boat?

[Tom Robb]

John B, I just looked at your kids' boats in tinurl. :cool:
I think you've got it just exactly right. Well done!

[JimD]

Jim D #12 & #18, Thank you for that comment. I have not been able to find what i am looking for so i decided to try modifying something for myself. I guess what im looking for is a catboat with a narrow transom. I want stability along with the ability for them to get into the boat easily. On comment #18, Another idea for me to work on , thank you.

JamesS, here's a very fat little dory to consider for ideas from Paul Fisher. Very narrow transom, lotsa beam for stability. Fit a daggerboard and stick a little sail in her bow and you have a little catboat with a narrow transom. Doesn't look as though he's made much attempt to keep the transom clear of the water, perhaps because the transom is so narrow. I'd turn it up at the stern another inch or so anyway, just where the skeg is, although if such a design were heeled over even a little the amount of transom in the water would be negligible:

10' GRAND BANKS DORYhttp://www.selway-fisher.com/GBDory10d1.gif
A simple ply Grand Banks Dory for stitch and tape construction using just 3 sheets of 6mm plywood—she may have stowage/buoyancy compartments in the ends and could take an electric outboard motor. LOA 10’ (3.05m); Beam 4’6” (1.38m); Approx. wht 60 lbs (27kg) in 6mm ply.

[John B]

James, No, there aren't plans. But I'd encourage you to experiment and just do it. The reason I replied on the thread with the album is so you can see someone who's been through the process and some mistakes that were made.
The first one was crank ... too pinched and not enough beam but 2 of the kids went through it .
#2 ( ScootII,the little flatty)was like a lttle kids trike and did great service . It had some small amount of rocker but dragged because of the wide stern.
#3 ( Zip)was a reaction to that and I overdid it a bit.. too full forward but hell, its been great. probably on its last season this year now.
#4 is a gem, fine for its job.. which is a one kids skiff. probably on its last season for the boy, it'll go to the girl next and he'll get something else.
We were out there varnishing them yesterday ready for the christmas cruise.:D