Deck scantlings?

[Norm Bernstein]

Hi, my 15' x 6'6" catboat project is proceeding nicely, but I've been doing a bit of 'thinking ahead' and wanted to ask about deck scantlings.

This is the first boat I'll have built with a partial deck, and I need to get a handle on how to construct it. The boat is 15' x 6'6" and will be decked over, except for a cockpit with curved coaming. I want the lightest deck construction that will safely support a 250 lb guy standing on the foredeck.

The deck will have a slight crown to it (about 1" or so), so I figured that the deck itself would need to be laminated from two layers of plywood, perhaps 4mm or 6mm, and covered with one layer of 10 oz fiberglass set in epoxy. The deck framing (beams and carlins) would likely be made from 3/4" x 1.5" fir or spruce. How close together should they be? Does anyone have any suggestions? What do similarly sized boats use? Thanks in advance!

(see the project at:
The Ellipticat Website (http://www.marisystems.com/ellipticat)

[Jon Etheredge]

3/4" x 1 1/2" sounds a bit light for the deck beams. Especially if they are spruce. The fir would be stiffer. Increase the molded dimension (the depth) of the beams if at all possible. Are at least some of the deck beams going to be supported by the top of the centerboard trunk?

I built the deck of my 13' x 6' catboat as follows:
</font> All deck beams are sided 3/4". The material is sassafras and they are sawn to shape.
</font> Deck beams in the way of the c'board trunk are molded 1 1/2". This dimension was chosen to allow the beams to be fastened to the top of the trunk.
</font> The remaining deck beams are cut straight across the bottom and with the crown of the deck this gives a depth of approx. 3" on the centerline.
</font> All deck beams are on 8" centers
</font> The decking is 5mm 5-ply occume marine ply with an overlay of 1/4" vertical grain pine planks and 1/4" sassafras covering board set in epoxy.
</font>Here is a picture of the forward part of the deck:
http://www.imagestation.com/picture/sraid91/pb26e4e047231297bf10610fac13f1525/fa5f87d5.jpg

There is no noticeable spring in the deck of my boat.

My guess is that you will be fine with 2 layers of 6mm ply. You could possibly use 4mm if you've got enough stiffness in the deck beams.

I think the glass cloth will do much less to increase the stiffness of the deck than other factors. I think you could probably go with 6 ounce cloth to save a little weight (it will take less epoxy to fill 6 oz. than 10 oz. cloth). If you plan to paint the deck then you might want to leave the weave showing so it will look more like a traditional canvas covered deck.

I think you can probably use beam dimensions similar to what I used if you use fir. But the longest beams really need to be supported by the centerboard trunk.

Glueing all interfaces between the layers of decking and the beams will contribute greatly to their strength.

[Lucky Luke]

You people are doing magnificent work smile.gif smile.gif smile.gif !!!... I would just question about the hallyard pulleys on deck: just screwed down into the beam?????? and very small diameter sheaves too... :confused:

[ 12-11-2003, 01:44 AM: Message edited by: Lucky Luke ]

[Nicholas Carey]

Checking Skene's Elements of Yacht Design (you do have a copy, don't you?)...

Mr. Skene (actually Francis Kinney) recommends for 1/4ins. plywood decks that the deckbeams be spaced on 5-3/4 ins. centers for sufficient stiffness.

Mr. Nevins' scantling rules base the cross-sectional area of the deckbeam on the cube root of the displacement in cubic feet with the yacht in load condition.

Assuming your 15' cat, in load condition, weighs in at less that 3000 pounds or so, Mr. Nevins' handy table suggests that your deckbeams ought to have a cross-sectional area of 3/4 sq. ins and ought to be spaced on 6 ins centers. Mr. Nevins goes on to say that the siding of the deck beams ought to be not more than 65% of the molded depth...and "There shall be partner beams and hatch beams whose siding shall be 1-3/4 times the siding of the main beams."

Mr. Herreshoff's scantling rules work off displacement in cubic feet, among other factors and are a little more complicated. For the sake of argument, I'll assume your 15' boat in load condition displaces about 1000 pounds -- that's probably excessive, but...

D = displacement in cubic feet. Water has a density of 62.427 pounds per cubic foot or so, making your D (in cubic feet) at 16.012. The cube root of 16.012 is 2.521.

Looking it up in the table...well. We kinda fall off the low end of the table which has its entry value at 3.0. We'll just use that. Best to over-engineer smile.gif

Using 3.0 as the entry value, we get directly

Factor I = 5.91
Factor III = 3.35

To compute the other factor we need (Factor IV), we need B, the beam, exclusive of moldings at 55% of LWL aft of the fore end of the water line. Let's call that 6-1/2 feet.

B = 6.5 feet.

Factor IV is the 4th root of D*B, or 3.194. Looking that up in the table, we get

Factor IV = 3.59

D-sub-h (depth of hull from top of deck beams to the rabbet line at 55% of LWL aft of the fore end of the water line. Let's call that 3 feet, for the sake of argument.

Mr. Herreshoff says that Factor I provides your frame spacing (read 'deckbeam spacing') directly in inches.

The molded size of your deckbeams should be 0.28 * Factor IV, or 1.005 ins.

The basic siding for your deckbeams is 0.17 * Factor III, or about 0.567 ins. At the partners, your deckbeams should be sided 0.27 * Factor III or about 0.771 inches. At breaks in the deck, such as at either end of the cockpit, the deckbeams (and I'd say the carlins) should be sided 0.30 * Factor III, or about 1.000 ins.

The camber of your deck (it adds considerable stiffness to the structure) should be between B/36 and B/24, or a radius of curvature of 9B and 6B. That yields a total camber over 6.5 feet of between 2-1/4 and 3-1/4 inches, or

In a nutshell...
Deckbeam spacing: 5.91 ins, about 5-29/32 ins.
Deckbeams molded: 1.00 ins, about 1 ins.
Deckbeams sided: 0.567 ins, about 5/8 ins.
&mdash;at the partners, 0.771 ins, or 3/4 ins.
&mdash;at breaks in the deck, 1.000 ins, or 1 ins.
Deck Camber: 1-3/8 to 2 ins per 4 feet.

Mr. Herreshoff recommends that your deckbeams be made of white ash, chestnut or butternut.

Hope this helps.

[Art Read]

"...I would just question about the hallyard pulleys on deck: just screwed down into the beam?????? and very small diameter sheaves too..."

Actually, those were fairly common fittings for these type of smallish daysailors. I think you'll see the exact same turning blocks on most of the famous "Beetle Cats". Also many somewhat larger designs from the same era. As for support, they are usually thru-bolted into solid backing blocks at the partners. At least mine are...

http://www.imagestation.com/picture/sraid73/pa757f4c40ebd767ab30377a504955fd0/fb7450c8.jpg

[Norm Bernstein]

Thanks John, Luke, Nick, and Art for your replies... tons of information to chew on!

Actually, I'm glad I asked the question, because I don't think I would have been nearly conservative enough on the number of deckbeams. To save costs, I was planning to saw the deckbeams from fir poorch flooring, which is tough, light, clear, and cheap. It comes in 1" x 4" nominal size (3/4" x 3.5" or so), and with a crown of only 1 or 1.5 inches, I can at least have lots of depth to each deckbeam. I guess, from the comments, that the design could have used more crown, but I left only 1" of margin at the transom to cut a crown, and wouldn't want it to be disproportional at the deck.

Also, I wasn't planning to support the lip of the deck on the forward edge of the centerboard case... but I could add a vertical support, which will help.

As for the decking, it sounds like 2 layers of 5mm okume will work fine. I like the photo of the deck with the pine plank overlay, but it might be overkill for this experimental project... I'll go with a fiberglass surface and anti-skid paint.

The Ellipticat website (http://www.marisystems.com/ellipticat)

[Lucky Luke]

Art, your Dark Harbor is superb, everyone appreciates that, but....:
hallyards will chafe tongue.gif tongue.gif tongue.gif ...not only on these small sheaves, but on this bronze bar at the edge of your cabin!
That's not cricticizing, Art, she is a really cute little one (I saw all your pics...welll..most!), and I see taht her deck had some salt water on them now smile.gif smile.gif smile.gif !
Wish you nice sailing, and enjoying a lot of splicing on those hallyards :D

[Stephen Hutchins]

Laminated 4mm oakume 1"x1" spaced on 8" centers with top of deck radius of 21 feet. 4mm oakume ply overlay epoxied inside and out, covered with dynel set in epoxy. The beams can be made most efficiently by ripping a full sheet of 4mm in half and half again, laminate the four pieces over form and then cut to one inch sides on table saw. epoxy the overlay underside and while it is still wet, lay over and fasten to beams. This is the deck I'm using on a 16'-3" x 6'-4" double ender. It's the lightest I think we can get away with for the money-time spent.

[Stephen Hutchins]

Oops, laminate thicknesses should be 6mm for both the beams an the deck. -Too much on my mind, I guess.

[Jon Etheredge]

I would just question about the hallyard pulleys on deck: just screwed down into the beam?????? and very small diameter sheaves too...
On my boat, the deck blocks are through bolted wth bronze machine screws, nuts, and washers. There is mahogany blocking under the deck that the bolts pass through. The blocking is fastened between two adjacent deck beams which serves to spread the fastening load over a large area of the deck.

The deck blocks themselves were purchased as rough castings from Rostand RI which is, I believe, the same place that Charlie York of the Beetle Cat company buys his blocks. They are, at the very least, a close match to what has been used on Beetle Catboats since the 1920s(?). They work fine for the throat and peak halyards on my little gaff rig and since my boat is a pseudo-Beetle Cat they look right too.

I assume that the "bronze bar" on the forward edge of Art's Dark Harbor is half oval stock that will actually decrease the chafe of the halyards. And it protects the edge of the cabin too. Very nice work Art!

Norm:

I'm not sure if you realize that if you use the same crown on all deckbeams, your deck may have a funny, duckbill shape near the bow. This is a fairly common mistake that people make on these small boats. Sometimes it works out but often it doesn't.

There are several ways to avoid this problem. If you have lofted the boat then you can lay the centerline of the deck out on your profile (waterline) view to determine the crown needed at each deck beam. Otherwise, you can use cheap plywood to mock up a temporary deck beam with the desired crown at the aft end of the deck. Then lay a batten on that beam and the stem head. Spring the batten to a sweet line for the centerline of the deck and measure the actual crown at each location where you want a beam. I believe you will find that the beams will need a bit more crown near the bow to keep the deck line from looking too flat near the stem. Don't ask how I know this ;)

[Norm Bernstein]

Jon, I think you're pointing out a little geometry problem that was recently written up in WoodenBoat... and I'm aware of the problem. I was planning to use the author's suggestion of a device to draw the crown separately for each deck beam... but I like the lamination idea suggested by another person who replied... going to have to think about this a bit!

[Art Read]

"...but....:
hallyards will chafe ...not only on these small sheaves, but on this bronze bar at the edge of your cabin!"

"...I assume that the "bronze bar" on the forward edge of Art's Dark Harbor is half oval stock that will actually decrease the chafe of the halyards. And it protects the edge of the cabin too..."

Jon is correct, that is half oval stock at the leading edge of the "cabin" trunk. I worried over the best lead for the halyards quite a bit, but this seems the best solution. When in use, I'm standing in the cockpit, behind the little trunk cabin, and the pull on the halyards is a direct line to the turning blocks. The lines themselves don't contact the "edge" at all. No chaffe then. Under way, there is concievably some extra friction where they pass over the half ovals, but with the relatively low strain of the gaff rig, I haven't noticed any problem. Hasn't even "polished" the half oval yet... As for the size of the blocks themselves, well, we had the exact same set-up on our old, somewhat larger, family daysailor for over fifty years... Never had to spilce her halyards in all that time. Never even needed to "end for end" 'em! The sheeves you see above are the same size as all the rest of the #5 blocks I've used, why should they cause problems that the others wouldn't?

What I DIDN'T anticipate is the akward lead of the jib sheets over the cockpit coaming. When triming the jib after each tack, it's a long reach to leeward across the cockpit to get at the cleat mounted outboard on the side deck. (For some reason, people seem "hesitant" to sit to leeward when she's got her rail edge near the water!) So instead of a straight pull aft, like I'd assumed, they tend to get hauled in over coaming with just a turn around the cleat and then tied off. I can see now I'm gonna need to put a small, bronze chaffing strip on both sides of the coaming in way of the natural pulling angle on those sheets. They've already "sawed" through the varnish and are starting to wear at the wood... :rolleyes: Oh, well, that should be an easy fix before sailing weather comes 'round
again.

(This picture shows how one might be tempted to haul the sheet in across the coaming...)

http://www.imagestation.com/picture/sraid82/pd8b307a35bea58cb455ebe5f695bf004/faea6dbd.jpg

Norm, my plans, not having a centerboard case at all, show a oak compression post just forward of the mast. I "beefed" mine up a bit with a small knee at the base, but it was really pretty easy to make and adds a LOT of strength. Of course, with your mast so far forward, I'd assume you'd want to position yours aft of the mast, perhaps just forward of the "break" of the foredeck?

(You can see the compression post and the mast partners/turning block backing here...)

http://www.imagestation.com/picture/sraid16/p0a1c3a74a1691fcf5c963fe0df68d43a/fdf4264f.jpg

The "powderhorn" phenominon Jon speaks of is real, but at least with my deck, even using a constant camber for all my deckbeams, I was able to make it "eyesweet" again with some judicious use of battons and lots of elbow greese and abrasives. Would have been easier if I had lofted it with that in mind, but on a smallish deck, it really wasn't too hard to "fix". Keep it mind when you dimension those beams though!

[Art Read]

"...I guess, from the comments, that the design could have used more crown, but I left only 1" of margin at the transom to cut a crown, and wouldn't want it to be disproportional at the deck..."

I belive the crown on my deck is 2 1/2 inches in 5 and a half feet. It seems to me that most catboats I'm familiar with seem to like a well crowned deck too... Are you planning to 'glass and paint your transom? If so, it seems like maybe a little "surgery" to add some beef to the top edge before laying the deck could "even things up" without detracting from her looks or structural integrity? Epoxy is wonderful stuff! ;)

[Norm Bernstein]

Art, that's not a bad idea. I could cut off the 1" margin from the transom, and then mount a piece on the top edge to allow me to give the deck a full 2 1/2" crown. The transom outside will have already been glassed, but it won't be any big deal to fix that afterwards... I'm going to consider it.... thanks for the suggestion.