BARRELBACK '19 MOTOR STRINGERS

Designs for inboard or outboard power

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martin63
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BARRELBACK '19 MOTOR STRINGERS

Post by martin63 »

I'm about to construct the motor stringers for the Barrelback '19. Plans call for nominal 2 X 4 of mahogany or fir. Couldn't I get straighter and stronger stringers from laminations of 3/4 inch meranti plywood? Does anyone see a disadvantage to this method?

Thanks for any input.

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billy c
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Post by billy c »

use the forums search feature and type "motor stringer" and the radio button all terms.... there is lots to read on this
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Dave Grason
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Post by Dave Grason »

I have the BB19 plans. Page 4 is describing the stringer as being cut from 2" nom stock with 3/8" ply layer to one side. That ply layer is going to hugely increase the strength of the stringer. But that's only an addition and NOT the main source of the strength in the stringer. In our other discussion recently about frame members being made of plywood, the consensus is that plywood is too flexible to be used for frames. If that's true, I would also be hesitant to use it for the stringers as well even if you did laminate it up to the proper thickness. I would feel good about laminating my own solid hardwoods up to get the required dimensions for the stringers and then I would lamintate the 3/8" ply layer to that.

But here's something else that actually surprised me when I first saw the BB19 plans. The stringers called for are only 3 and half inches wide. As a basis for comparison, the Mist Miss plans call for 6" wide stringers and the Dragster plans call for even more width - 8 and a half inches. This is only my impression and nothing more, but it would appear as though Glen wanted to err on the side of safety while Ken might have been a: "Well, that's good enough" kind of guy. I don't know. But this could be one of the reasons why Ken always stated that the BB19 really should not be powered by more than a 6 cylinder engine. The performance of the 6 would be excellent and therefore, the smaller stringer would be just fine. Maybe a V8 would jerk those stringers right out of the boat. :shock:

Now I've been vocal on this subject in the past and here we go again. It seems that this counsel always falls on deaf ears on this forum and I don't know why. I've certainly noticed just how difficult it can be to find quarter sawn stringer material in the lengths necessary for the building of inboard boats. Most oak and mahogany that is available is flat sawn so that the tree can yield more board feet. If you take flat sawn wood and laminate THAT up to the dimensions, you will end up with a quarter sawn stringer. Think about that. I've posted these photos before but hey, it's no problem to post them again. This is the way VanDam does their stringers and it certainly looks like an excellent way of doing things.

Image

Image
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Eric Fritz
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Post by Eric Fritz »

been awhile since i have been on here, but thought i might offer a thought. the one part of a boat that is probably under the most stress whether slow or fast is the stem right? always pushing through something, unlike the sides or bottom which is merely sliding with, not against the water. its the stem that does the heavy lifting. what is the stem made out of? at least in the plans i have its is laminated plywood. if its good enough for that very critical part of a boat, i see absolutley no reason why it could not work elsewhere.

fritzy, ducking for cover......

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Dave Grason
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Post by Dave Grason »

Eric Fritz wrote: at least in the plans i have its is laminated plywood. if its good enough for that very critical part of a boat, i see absolutley no reason why it could not work elsewhere.
LOL, no need to duck for cover. :D That is really a very good question. The stem can get by with ply because it also has the plywood topsides and fairbodies coming to a point right at the stem. This gives an amazing amount of extra support. But take a look at some of the pro builders of wooden boats. They almost all use solid wood or laminated solid pieces for their stems.

Glen-L plans often call for plywood stems because the plans are designed for the amatuer builder. And before the internet made finding other things so easily, this was an acceptable method for a beginner to do in his garage at home.

BTW, I see this is your first post. Have you been lurking? :D Welcome to the forum.
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Eric Fritz
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Post by Eric Fritz »

lurking for awhile, yes. i forgot my password after all these years. i think my username was fritz. in 03 or 04, i got into restoring the big finned boats of the 50's. i sold the last one on labor day and am thinking about diving into a wood boat. i have the zip plans, actualy have all the frames built, but now need something bigger.

i understand your point about the stem, but couldnt the same be said for the frames? it seems that there is alot of cold moulding that is going on around it for support.

Nova SS
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Post by Nova SS »

I know these pics have been posted before but I thought I would post them again. They are from the Clarion Bots website. These frames are either cut from ply or they have the best joinery that I have ever seen. These boats when assembled sell for WAAAAAAAY over 100K

http://www.clarionboats.com/

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Image

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Dave Grason
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Post by Dave Grason »

Eric Fritz wrote:i understand your point about the stem, but couldnt the same be said for the frames? it seems that there is alot of cold moulding that is going on around it for support.
Yunno?, I just don't know.

Here is the other thread that covered this topic recently:

http://glen-l.com/phpBB2/viewtopic.php?t=6064
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jb4lcm
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Post by jb4lcm »

Here's my two cents on the use of ply.

Take a sheet of 4x8 3/4" ply cut a 6" wide strip. Now pick up one end of the 6" wide strip - holding it about waste high - and leave the other end flat on the ground (the 6" side parallel to the ground), then press on the middle of the ply and see how much it flexes. Then do the same to a piece of hardwood. I think that should give you a good idea of how unstable plywood is laterally. To me, it is just way too soft.

If that isn't enough, then think about this: Hardwood can be fastened to on all 4 sides (but not ends). Ply can ONLY be fastened on the two NON laminated surfaces. If you put a screw or nail into the laminated side of ply - what happens? It splits. No strength. Not good.

I use standard ACX ply for non-bending VERTICAL reinforced pieces such as bulkheads and the like. I also use ACX for soles (decking). I use marine ply to bend sides and bottoms.

Plywood has its strengths. But it shouldn't be considered for replacing all solid wood parts.

JMO.

JB
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martin63
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Post by martin63 »

Thanks for eveyone's input. this is my first post as I just started construction last month. I found the older thread after I posted. It's hard to argue with the designers advice so this time I'll just pony up the extra $$$ and use hardwood timbers.

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kens
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Post by kens »

you guys all missed the point about the laminated ply on a stringer called out in the plans. the ply layer on the stringer is to help stop the stringer timber from splitting. its not a strength issue, it is about a timber splitting.

upspirate

Post by upspirate »

Ken's right about laminating to the sides of the stringers to prevent splitting.

That's why it's used on some builds as a laminate on the keel too.

I think it'd probably be ok as frames once everything is together& bonded,but in building up the frame,it's important to screw the chines,shears,and battens to assemble the frames before the epoxy sets up and the ply is in place.

To do this,the screws go into end grain of plywood,and that splits the wood....NOT GOOD!!
Last edited by upspirate on Sat Feb 21, 2009 6:49 am, edited 1 time in total.

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Bob Perkins
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Post by Bob Perkins »

I used two layers of 3/4" mahogany for the stringers. I scarfed sections of them together to get to the length I needed, then laminated the halves together to provide the 1 1/2" thickness called for.

My only reservation on using the plywood in the same way is that you are asking the ply to be strong against its width (maybe 6--7 inches in your case..?) The ply is probably not as strong as regular lumber in that direction.

At the end of the day - the entire boat turns into what is essentially a tube - and is very rigid. So it may not matter too much once completed.. But I'm not a naval architect.

I agree that trying to find a single 15' straight piece of lumber and hoping it stays that way is not a good option. Some version of a laminate should be used to build up the stringers.

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Eric
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Post by Eric »

All of the frames and stringers are ply. Those boats are really nice looking and they seem to know what they are doing. I would trust the ply if these guys do.

http://www.clarionboats.com/photos.html

Nova SS
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Post by Nova SS »

Eric wrote:All of the frames and stringers are ply. Those boats are really nice looking and they seem to know what they are doing. I would trust the ply if these guys do.

http://www.clarionboats.com/photos.html
The pics I posted above came from their site. :wink:

These boats sell for big money. It makes me think they must know something we dont about using ply for these parts

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