View Full Version : Wooden Davits
11-19-2000, 10:26 PM
I have a question on building wooden davits for my 42' Sloop Shipping News. The reason for building wooden ones is so that I will not ruin her lines, however I need a plan or even some hints on wood type and dimensions. My plan was to use laminated fir but what thickness? The davits will need to hold a 9' ridged Zodiac with 10hp engine and fuel as well as a 15 man platform life raft on top Please find attached a photo of the boat and a rough drawing of my idea.
If you know of any one or any publication that could help I would be very grateful for any assistance.
[This message has been edited by Mike Randall (edited 11-20-2000).]
11-19-2000, 11:34 PM
Can't see the picture but it sounds like all that is going to have some visual impact whatever the davits are made of. Its a lot of weight hanging over the back of the boat too. I'd seriously consider putting the liferaft amidships or on the foredeck.
I noticed a neat arrangement on a plastic racing boat the other day. The davits are simply stainless tubes with a bend in them, which fit into a couple of sockets in the aft deck, just forward of the transom. Completely and easily removeable when they come to do the photo shoot for the cover of a magazine, but there and strong when you want to hang the dinghy off them.
11-20-2000, 06:02 AM
Your idea of laminating is on the mark, if youíre priority is to use wood? Fir is not the wood of choice for the tight radius you show on your drawings. Oak, white or red, and ash, is the wood of choice. Iíll explain how I just laminated the frames on my cat boat. The transom frame had a fairly tight bend. The key here is the tighter the bend the thinner the strips. Do a search and you decide on weather you need KD or air-dried? If you canít get air-dried donít fret, do a sample bend with whatís available.
This is how you do it (short version):
1. Obtain straight grain wood.
2. Obtain straight grain wood
3. Obtain straight grain wood. You got the point?
4. Rip it to 1/4 strips.
5. Make a mold to clamp to that matches one of the edges of your profile.
6. Stack your wood together locating your best pieces top and bottom.
7. Mark one end with a diagonal slash mark with a marking pen.
8. Steam the whole bunch and clamp into the form. Lots of clamps! No clamps? Ask and I can explain how to build a form without store bought clamps.
9. Remove about two days later.
10. Glue with appropriate glue, (do forum search)
11. Place back in forms and clamp.
Obviously what I left out are the form details. I can post a picture, but I have to shoot it first. The key to bending wood is to contain the outside fibers of the wood on the outside of the bend radius. In your case one outside radius will be against the form, the second one wonít. What you do is go to ďThe Big Orange StoreĒ, and buy some thin flat stock as wide and as long as your bending stock. By two pieces so you can stack them the same way you stack your wood to bend. Bend them in on the outside of the stack when you steam bend (step 8). They will take the bend because they are thin enough. It is easier to bend two thin pieces of metal than one thick one. What the metal does is contain the outside fibers of the outside piece of wood.
I think Fine Woodworking published a book on bending wood. The forum has lots of info also, do a search.
[This message has been edited by T.KAMILA (edited 11-20-2000).]
11-20-2000, 11:41 AM
I'd make a graceful pair of curves rather than sharp angles. In fact, this is about what I designed but never built for Goblin.
Start 5' or 6' foreward of the transom at toe rail level and have a concave curve raise the piece to the level of your pushpit. Let it attach to, rest on or even redesign the pushpit so that you have a post holding up the davits at the transom and you have an athwartships beam at gunnel level of your dink to lash her off snugly. You may also incorporate a gallows. At the transom reverse the curve to where ever it's to end.
This gives an easy to laminate shape of great strength and it looks quite handsome, especially if you put some kind of decorative knob at the forward end and a graceful taper to complement the reversing curves.
11-20-2000, 12:40 PM
A rough rule of thumb you can use for determining the maximum thickness of laminating stock to laminate to a given radius is:
thickness = radius / 100
This is dimensionless - works with either inches or millimeters. I found it a good formula for mahogany - for ash or oak, you might have to go to radius / 120.
I got the formula from a book on industrial laminating, and it's proved pretty workable. I'll also lucked in a large sale of 1/16 and 1/28 veneers way back when Palmer and Parker went out of business, which I've used for laminating tight curver - down to 4" radius.
If you want tough, and you can get it, you can laminate things like davits out of black locust. It has to be deoiled the same as teak.
11-20-2000, 12:53 PM
Mike, I IMHO use steel. Hide it under any wood you like. It will be lighter and cleaner looking. Wood is great but not always the best choice.
[This message has been edited by G. Schollmeier (edited 11-20-2000).]
11-20-2000, 02:33 PM
To disagree - When I was planning Goblin's davits that never happened, I talked to an NA. We didn't do finished scantlings, but he did not think that a steel and wood structure would be lighter than all wood. In fact, wood lams with next to bottom and next to top lams with graphite or kevlar might be considerably lighter than any steel or aluminum structure, and it would not fatigue or permanently deform with a little flexing.
We figured worste case to plop a forked light 'back stay' to hold it up.
Given how strongly Leeward's frames came out, the preliminary plan for loading about like you plan had the outer end about 2"x2", thickening to 3x2 at the post over the ransom and thickening a bit more towards the toe rail for artistry.
Remember, this was at prelim stages and had not had engineering done yet, but I did know that an unsupported 7 layer bow about 1-1/2 x 3 could absorb my 240# at the end of a 4' lever arm, so it's pretty strong.
You could make it a little large in all directions and then plane a little more off each year till just before it flexes . . .
The idea of the extra stay has an added benefit. Had some friends who put up an aluminum davits similar to what you drew. Then the found that water got in the dink, generally a pain even if they pulled the plug since it would still hold water on the other tack, so they made a frame and lashed up a canvass between the davits parts to keep out the water. Then they found that with a couple of air mattresses . . . .
Rumor has it that they did an outside run from New England to Florida and the two of them and the dog only came off that trampoline for necessaries.
11-20-2000, 07:17 PM
Seen the pictures now. Sorry, but it's ugly as sin, and is going to clutter up the aft deck. If you are determined to uuse wood I'd penetrate the aft deck and have the davits attached to the transom on the inside, then a biggish radius curving out to the ends. You don't really want the top horizontal, because you generally won't be able to snug the dinghy up tight. The block and tackle, and lifting straps or whatever will lose you some height. If the davits slope down the side of the dinghy will meet them and can be made fast there to stop it swinging around.
I'd be a bit inclined to rebuild the pushpit in heavier gauge and incorporate davits into a couple of the posts.
Consider the weight/momentum of the dinghy half full of water in a boisterous seaway. Make em strong. Minimise the horizontal extension to reduce the leverage.
11-20-2000, 07:29 PM
PS. Have a look at how your dinghy will sit if you haul it up so that its top surfaces are horizontal. On mine the V floor then slopes forward, so it doesn't drain through the transom plug hole. To make it drain I have to leave the back end hanging quite a bit lower than the front. That then creates problems in terms of swinging around. The best setups have saddles coming out of the support posts which the side tubes snug into.
11-21-2000, 10:07 AM
Thanks so much for all your replies. Keep them coming. I now think that my idea is possible and so we shall wait till after the New Year to try and swing into action. I am amazed that none of the wooden boat books that I have found actualy mention davits letalone building them. Are there any plans for this type of thing.
11-22-2000, 06:24 PM
Wm. Garden's design books have some nice looking davits drawn, especially on Union Jack on page 72 of his second book of designs. He's right over at Canoe Cove so you could probably even give him a call.
11-22-2000, 09:50 PM
I dislike Zodiacs. They're rafts, not boats. Scrap it and make a tender that tows well?
Failing that, sure laminate the wood, why not? As Ian says, smooth curves.
I saw wooden davits last summer on a colin archer cutter up north. They were overbuilt and looked a bit clunky. You could make a test sample and see how strong it actually is before deciding on the final size required.
11-25-2000, 05:04 PM
I have to bring down my split backs on boomkins (46' schooner) and thought it would be a great place for storage of the tender. I've used curved (steamed) white oak and they set back from the transom a few feet through the deck landing on a bulkhead with socket. They lean against a deck beam and have a large (well shaped with clam shell design carving) knee at the aft base, on deck of each arm. They have a cross brace and are dogged with chain to the transom. I figured, if I was going to have this set of contrary lines rising over deck level, they better be pretty. The hard dink, (only way to go, by the way) will be in tow 90% of the time and stored aboard. In rough conditions, she'd be stowed on deck where she'll be safe and will not contribute to weight in the ends. This is a fine place for a net and light gear in good weather, but no place for anything but the backs in heavy going.
"Boatbuilding Manual" by Robert Steward has a handy chart for figuring the loading and reach of cantilevered beams (davits) from ordinary steel pipe, worth a look.
[This message has been edited by SailBoatDude (edited 11-25-2000).]
01-05-2001, 09:19 AM
Get Bruce Binghams "The sailors Sketchbook,page 69, shows laminated davits.
Get something that rows well and mount it amidships......
01-05-2001, 09:21 AM
and personally....thats a bad place for your liferaft if you seriously ever intend to use it....one day when you need it it may not be there...or the dinghy...depending on the conditions.
01-05-2001, 09:30 AM
Stern davits are for convenience in 'long shore cruising only. Of course you put the dink or raft in the middle of the deck, properly chocked, if heading off.
Remember how Leavitt's push boat in stern davits was one more panic factor in her sinking.
01-06-2001, 05:33 PM
Another possibility....if you absolutely insist on putting something back there, is to rig a couple of folding stairs such as masteps side by side about three feet apart on the transome, about a foot higher than the gunnel of the dink when she's in the water. Then, you row up to the big boat, lower the steps, and using a neatly fabricated set of brackets on the gunwhale of the dink, lift her up a foot to drop an angle into the opening in the mastep, and haul the dink up flat against the transome.....This sounds good I guess, but I have a rudder hanging back there.
Powered by vBulletin® Version 4.2.1 Copyright © 2013 vBulletin Solutions, Inc. All rights reserved.