View Full Version : Building My Own Davit System- Need Advice
07-17-2002, 06:08 PM
I have a 45' Chris Craft motor yacht (see photo of stern below), and I want to build a Dinghy Davit (Lift) for the stern.
Either I can make a unit that pivots from the waterline and lowers the small boat from the deck level down to the water, using a winch and cable, or I can make something like an Engine Hoist and use hydraulic. Help me think this through. I need to lift a 12' long wood boat which weighs 800 pounds including the 10 horse motor UP from the water approximately 6 feet and then pull it in toward the stern so that it can be tightly attached in the up position while driving the big boat through chop. I have already designed a pivot type, and it works with my 1500 pound winch pulling into position, but the winch does not have a brake so it slowly lets the boat back down. This is why I am investigating the hydraulic method.
The drawing below is the design that I have in mind, please offer any advice on these two methods. I need ideas. How has this problem been resolved before?
07-17-2002, 06:25 PM
Now, I'm no dinghy expert ... no wait a minute...
Anyway, your second drawing will let the boat right down on your swim platform.
The first idea is simpler and has less maintenance due to fewer moving parts. Plus, it will distribute the bulk of the weight lower on your boat which will translate into a lot less stress on your transom.
Make sure your transom is rock solid, first off.
You may just want to investigate buying davits design for the weight of your dinghy/motor and the beam of your dinghy in relation to your swim platform width.
BUT, I'm no mechanical engineer either. Hopefully more folks will assist hear.
And, please, someone correct me if I'm wrong in my thought process.
07-17-2002, 06:37 PM
I'm with Brad on this. Hydraulics seem like an unnecessary complication and I'd also be worried about leaks in the hydraulic valves slowing letting the boat down, but I don't know that much about hydraulics so take that with a grain of salt. If the problem with the first idea is that the winch won't hold the tension, couldn't you just rig up some sort of hook, lever or catch that would lock the whole thing in the up position. Then, when it was time to lower the dingy, you would just take up a little on the winch to take the load off the hook and then unhook and lower. It would seem better anyway to not have the winch constantly under tension.
The simple solution to the whole thing would be standard davits hanging out over the transom, but this would, of course, drop the dingy down on the swim platform...
800# of weight hanging high off your transom will create a definate change in the performance characteristics of the big boat.
12' and 800#? What is the dinghy made of? My 16' fiberglass skiff, with a 15HP motor, only weighs 575#.
Get a lighter tender, and use the 12' brick as a motorboat. Teach the dog to swim alongside, towing the dinghy. :D
07-17-2002, 07:38 PM
My brotherinlaw just cut up a well rotted 1956 38ft. CC connie and has a great set of aluminun traditional davits. I know he will sell them cheap. He is in Sierra Vista Az. Let me know if your interested..........Tom
07-18-2002, 09:57 AM
Donn, I have not put the boat on a scale to determine its weight as yet, but I am using the 800# as a safety margin. Sometimes the lifting system will be used to load the Great Dane from the water up to the deck of the boat. We would put him in the Dinghy at deck level, lower him and the dinghy into the water, take him to shore, come back and lift back to deck level. Kind of a Great Dane Elevator system. see photo
http://www.gngems.com/Catalina27/BoatDogCollage.jpg YES, it is a collage photo for fun, and the sizes are out of proportion, but you get the idea. The boat is of 3/8" plywood and epoxy, with a full size battery, and a 9.9 horsepower Honda engine which weighs approximately 100 pounds itself.
I wanted to use a larger than usual working load because the template I made for the winch powered lift system was welded square tube steel of 1" size. I lifted the boat fine with it, but as I had not built it for any loads but testing, it began to crack at a stress point, and would have broken through had I not quickly put the dinghy back into the water. I personally like the lift system best but am looking for advice from someone who may have experience with this situation prior.
Please contribute your ideas. I want to draw on the wealth of knowledge and experience here.
:D Love that pic, but I'd like to see one of him without the cone-head bandages on his ears.
Seriously...if the boat does weigh that much, and you hang it off the transom, your Chris will not perform the same. I'd still go with a lighter tender.
Drop the tender into the water, and raise and lower the horse with one of those harnesses they make for dogs. They're real strong, and have a handy handle on the back for lifting. I just lean over the transom and snatch my dog in by her handle, but she's only 60# wet.
An aft-cabin Albin that used to live down the canal had a great tender rig. They used an inflatable that was tied up on the swim platform, and carried the outboard on a bracket on the transom. No davits were needed...they just dropped the Zodiac into the water, and, standing on the swim platform, attached the motor.
Everybody else on the canal tows their tenders.
07-18-2002, 10:19 AM
For the dog lift Donn mentioned, I would have to have a crane just for "Scooby Blue Eyes". He currently weighs in at 150 and is only 18 months old. We expect him to top out at 185. He is a great friend so we want to include him on every weekend. I currently tow my Dinghy and prefer to, however I have a covered slip (only the best for my Chris Craft) and back her in. This makes for minor problems of the marina does not allow extra boats (dinghy) free floating in the same slip (yes they are jerks), however hanging from davits is allowed. I really do not want to launch and retreive the dinghy to a shore trailer every weekend..... spoils the Margarita hour.
I meant to use a winch to haul and launch the beast, not your back...like a lobster pot hauler.
Will the marina let you hang the dinghy off dock mounted davits?
Larry, looks to me like you simply need a larger boat built with heavier scantlings. Maybe an ex-commercial fishing vessel. I've been involved in a number of davit installations, usually on a larger scale though. However, the principles at play remain the same.
1. Your boat was not designed to deal with the stress, strain and leverage which will be imparted to your transom.
2. Which ever davit picks the aft end of the "dingy" is going to pick most of the weight of that boat. Therefore your davits have to be designed/built to handle most of that weight individually. This compounds the problems item 1.
3. Your davit system and transom also has have to be able to handle the "dingy" or the dog if the big boat is rolling around.
4. If you were to hire the services of a naval architect or marine engineer (probably wouldn't cost that much) to design this thing I think that you would be suprised at how elaborate ($$$) the mounting/support/installation details might end up being.
I think you and your dog need to join forces and convince your wife that you guys need a larger boat. Good luck.
07-18-2002, 10:53 AM
Thanks but, I like the 45' Chris Craft. Then I can brag I have a Big Woodie.
It sounds to me like the problems are these:
1. The marina won't let you float 2 boats in the same slip, and
2. You need a way to get the dog from the Chris to the dinghy and back.
Tow the dinghy when you're out, and when you're in the slip, stow the dinghy on mechanical dock davits if they'll allow it, or take the outboard off and stow the dinghy on the swim platform, with one of those gunwale clamp sets made for the purpose.
Put a tuna door on the Chris' transom, and teach the dog to walk from the dinghy to the swim platform and thru the tuna door.
07-18-2002, 11:02 AM
I'll bet your paying at least $2500-3,000 for your slip and you can't keep a tender in it?
I've got my 35-footer, 15' center-console skiff, AND my beloved Sea-Doo in a 45' slip.
Probably cheaper to move marinas than to come up with a davit system that, when you enter the dog into the equation, will be problematical ... especially after a few brewskies.
07-18-2002, 11:29 AM
Slip Fee- Try $4,900 per year. The only other marina capable of taking a 45' or larger boat (but allows more than one boat) is Holiday and they were $6,500 per year for a 50' slip. Unfortunately it would be with house boats and not covered. NO WAY.
Tuna Door- The deck is 6 feet above the waterline, and a door at swim platform level would lead into the rear Lazarette with the fuel tanks and no where else. The aft bedroom cabin is just the other side of the lazarette.
Dock Mounted Davits- Unfortunately, the Corp of Engineers which have jurisdiction of all shoreline, do not allow any permanent attachment to docks. They also had a problem with my stairs on the dock finger. I had to threaten them with a ADA (handicap access) suit based on my elderly father having access to the boat to get them to give me special permission. For some unknown reason, they do not allow Dock stairs, but they do allow dock boxes for storage. Go figure.
[ 07-18-2002, 11:33 AM: Message edited by: Larry Exum ]
07-18-2002, 11:36 AM
I think the main issue is still the weight of the dinghy. Weigh it again? Even with a 100 lb. motor, I just can't believe it weighs anywhere close to 800 lbs, unless it's full of water or people.
Whew...tough marina. Even if the dinghy is only 500#, RGM is right about the structure of the transom. Add the horse, and it's a major undertaking.
Answer me this...is there any open cockpit forward of the aft cabin? By open cockpit, I mean a clear lower deck with gunwales. If there is, you could use a mechanical pot-hauler in a flush mount rod holder to haul and launch the dog, and stow the hauler when not in use.
Tow the dinghy and stow it on the swim platform when in the slip.
07-18-2002, 11:45 AM
I will be weighing the dinghy next weekend before I begin to build the final version of the (Plan1) davits. There will be steel square tube, welded to template form as a skeleton, then covered with Mahogany planks, sanded, rounded edges and varnished. The look will be visible wood only, for a matching appearance to the big boat.
My best guess for the time being is as follows:
Honda 9.9 engine= 110 pounds.
Dingy (empty hull only) 350 pounds.
Battery, oars, misc - 50 Pounds.
for a basic weight of 510 Pounds. I may be high on the Dinghy but again.... I am using the 800# number so as to overengineer the strength of the davits. It would be tragic to see your own labor of love dinghy design fall off the davits and sink as you can do nothing but watch.
I put too much time into the construction of the dinghy just to make SWMBO happy.
John of Phoenix
07-18-2002, 11:48 AM
Larry, regardless of how you ultimately resolve this, PLEASE post a picture of the dog in the dinghy on the davits. :D
This is a BoatUS solution for inflatables...for $150. Make your own, fashioned for the dinghy. When you're coming back in, take the motor off the dinghy with the same mechanical pot-hauler you use for the dog, then pull the dinghy up onto the swim platform, and back into the slip. It doesn't have to be strong enough to hold the dinghy while racing across the lake, just enough to hold it out of the water in the slip.
I looked at a few pics of Constellations, and it shouldn't be too hard to find a place to install a flush-mount rod holder for the pot-hauler.
07-18-2002, 11:59 AM
Donn- Yes very tough. A Westrec Marina. No there are no lower entry points. See photo from July 4th weekend.
Here is a photo of my Chief Engineer "Priscilla" She is in charge of all the drawings of the davits. Here she is analyzing the stress points on the Cad software.
07-18-2002, 01:22 PM
Lessee....800 pound boat/motor.....three feet out from transom.....moment 2400pounds static at the anchor points.....no reserve.....er...ah...how well does your boat float without a transom?
07-18-2002, 01:45 PM
If I'm thinking about this correctly the hinge points will not be the issue. When the dingy is just clear of the water there will be some inward force because of the angle of the cable, but an outward curved transom should be able to take a good bit of inward force; and when the dingy is all the way up there will of course be some downward force on these hinges, but remember, those steps up the back need to be engineering to support a 250 pound person and the downward load on the davit at the back of the dingy should only be around 450 pounds, with the dog. These hinges should certainly be very solid, with large base and backing plates, but the real key is, I think, the winches.
As the winch or winches start to lift the dingy there will be quite a bit of force trying to rip them out of the deck. So, do you have something good and solid to mount them to? There presumably are some stern cleats in the area, which if they have been designed to handle storm conditions must be mounted to something pretty solid, so there ought to be something back there; but I wouldn't just slap those winches down anywhere on the deck without checking what they are attached to.
[ 07-18-2002, 01:52 PM: Message edited by: Bruce Hooke ]
07-18-2002, 03:42 PM
First, the low point transom Pivot Points are 1/2 inch steel plates welded (Same as you see on docks). They are thru bolted with 1/2 inch Stainless Steel bolts which are 4 inches long. They go through the transom, and then through a 2 inch thick backing plate of solid mahogany. There I use 2 inch fender washers to crank down the bolts. The backing plate is long enough to cover 4 planks in the vertical and is also side screwed to the transoms vertical ribs. The backing plate is also epoxy'd to the inner planks and ribs.
Second, the Winch (one at the moment, I may decide to add a second) is thru bolted to the deck and is also backed up by the same design backing plate of 12 x 12 inches (2 inches thick).
In both the Winch and Pivot locations, the force is sideways (parallel) to the plank direction, so nothing should "rip out" unless the whole deck or transom were to go. But if that happened, my surveyor would have really screwed up in his testing of the wood condition.
[ 07-18-2002, 03:45 PM: Message edited by: Larry Exum ]
07-18-2002, 11:50 PM
Looking at your designs there are similarities to gravity davit systems found on ships. Couple suggestions:
-Could be a single arm davit with a bridle on your tender to a hoist point, or to eliminate the athwartships motion two mounting points on the boat with davit arms to a single sheave or hoist point on the davit (more below on that). This would allow you to use one winch.
-Might consider using a wire which goes over a sheave at the end of the davit to the hoist point on the tender, coupled with a limiting device (line probably in this case) on the davit arm. This allows you to put the boat in the water and disconnect even if the boat is moving up and down--hook up, the boat rises until it contacts the davit arm, then both boat and davit arm continue all the way up and in.
If you've got access, take a look at a ship or two and see how they raise boats. Old merchant types probably the best, you don't want a launching tender ramp on your aft deck I'm sure!
07-19-2002, 01:31 AM
Larry, that is a task for a qualified NA or Marine Engineer, In My Opinionated Opinion.
800 Lbs. on end of a lever is quite a load magnified.
Once upon a time I was running a yard building Aluminium vessels. Charter fishing boats and Yachts. The Charter boats were in the 2 million range ( dollars that is ) and the yachts well...
rich boys and their toys if ya folla, though 10 million was not uncommon.
We had started on a nice 95 footer in the 10 million or so range when the owner came into the yard one day and announced he wanted to carry a full size 4 door Jeep Wagoneer on the boat deck!
That would be about 20+ feet above the waterline.
We did it but the engineer and I and then my prototype crew worked many hours assuring that the deck and the hoist could take the load.
And lets face it, a new Al. vessel is just a tad bit stronger than a vintage Chris Craft.
Please seek professional advice on this.
07-19-2002, 09:31 AM
$5000/year?!!!! wow. I have a whole new appreciation for my rinkydink $20/ft boat club.
It seems to me that the problems associated with hoisting the dinghy are wildly and unnecessarily (sp?) magnified by its unusually heavy weight. I know you just got done building this dinghy, but I suggest that it may be easier and cheaper to build or obtain a lightweight dink than it would be to contrive some mechanism for hoisting the heavyweight. but I digress.....
The Dog. why are we talking about an 18month old Dane as if it needed to be hoisted like some sack of potatoes? dogs can do amazing things. dogs can be TRAINED to do amazing things. get the beast to haul his OWN arse up into the boat!! I recognize that your Chris has a kingsize transom, but surely a Dane would only need one or two 18" square platforms to use as huge stairs, and up he'd be!
my (soon to be) inlaws regularly cruised their trawler with their 80lb labrador, who needed only a little boost from the humans to get from boat to dink and back. time passed, and both dog and humans lost strength, so they adapted. they taught her the "elephant trick" (yes, that was the command they used). picture a circus elephant standing on a footstool with all 4 feet. kissy would stand on a boatbox on the swim platform like that, and then up into the boat she'd go. Kissy died over this past winter at age 15 (near record-setting for labs), but just last summer she was still doing the 4'height from boat to dink and back on her own.
[ 07-19-2002, 03:56 PM: Message edited by: Sailortect ]
Puppy needs help, I agree. But it's not a hoist. What he needs is for those steps to the swim platform to have bigger landings put on them, so that he can use them to get from the swim platform to the deck and back for both transport ashore and for self-rescue. Squares the size of the length of the step, with "L" brackets underneath at the transom end, that hook into the slots in the steps, and a "U" shaped brace at the aft end, to hold up the aft end and prevent the "L"s from pulling out of the transom.
</font><blockquote>code:</font><hr /><pre style="font-size:x-small; font-family: fixed;">::::plywoodsquare::::
f (fitting to hold end of "U")</pre>[/QUOTE]You might have to add another step, in line with the third one down, but on the other side, for vertical clearance as puppy grows.
Now, the tender. Since you want to tow it once you're out of the harbor, it strikes me that what you need is a storage system, which might not be Davits at all, or at least not off the transom. Two other schemes come to mind; one is a pair of rails, from the aft edge of the swim platform to the top of the transom. Tie the tender to each of these rails, empty it, hook the winch to the far side, and pull. Tender tips up on edge, slides up the rails. Probably works better in a cartoon world. The other I have seen, on a boat a bit bigger than yours, but not much: a single
Davit, mounted on the deck near the middle of the coach roof. Tender brought alongside, cable let down to a bridle, tender hoisted (above the safety rails) with a winch, then the Davit turned 180 degrees and the tender lowered (still right side up) onto the coach roof. 'Twas hard to see around, I admit.
I, too, really don't like the idea of five hundred pounds hanging a couple of feet aft of your transom. Have you considered those Davits that are shaped like "C"s that pivot "overboard" at the base, putting the top of the C a couple of feet away from the boat, and then hold the tender inside the C when they're retracted?
07-20-2002, 11:58 AM
htom, Thanks for the ideas. The "C" design would work well if the Swim platform was sturdy enough to hold the weight of the dinghy (Weight magnified by 3) as it would then be Center of Gravity out on the end edge of the Swim Platform.
Using all yours and everyone elses ideas above to Noodle through all the options, last night as I was lounging by the pool, it suddenly came together.
The wife said she did not want to cover up the beautiful wood on our stern with our dinghy (especially after the graphics get applied next week). We (barely) have room to park the dinghy behind the big boat in the slip, so it would have to be lifted at least a foot, and also to satisfy the marina. I will build some fixed davit 4" thick posts at deck level, incorporating it into the framing of the deck. No leverage will be involved, just straight up lifting with TWO winches. This will also cure the question of rocking at anchor or while under way. The dinghy will be touching the Swim platform, but not resting but about 50 pounds of weight on it. The majority of weight will be supported by the support davits. When I pull out of the slip, I simply release the dinghy down fully into the water, push it away from the swim platform , and release the engine end of the support cable. The bow cable will still be connected, and I can tow it as needed with no worries.
Upon returning to the slip, to back in ... as I come to a stop befor the pirouette 90 degrees to back in, I simply winch in the dinghy up close to the stern and it will stay tight to the swim platform until I get situated in the slip. I will then connect the engine end of the winch cable and take the weight off the swim platform.
I may also plead with the marina to have them allow me to put a cradle attachment at the stern of the boat attached to the dock so as to be able to rest the weight of the Dinghy 18" out of the water on rails.
What a great brain trust all you guys are. I appreciate your commentary, and want any input on the latest variation.
07-21-2002, 02:46 AM
I like the first idea best. It is the simplest and the least messy if something goes wrong.
A couple of points. Notice how many coments you got regarding the weight? Standard engineering practice is to give the actual (or best guess) weight and then give the preferred minimum Factor-of-Safety (F/S). I would recommend a F/S of 3 for the davit and cable. The winch F/S can be less since if it fails nobody'll get hurt. As far as securring the dingy/davit in the stowed position, I would never trust a winch brake to hold an item for a long time. What you want is to have a separate securing line for the davits.
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