Transom modification insight

A

ADQdory

New member
I am wondering what people have done to their early model C-Dorys to help gain deck space. I have a 1981 angler and am thinking about glassing in the transom hole, cutting the engine well out, and building a motor bracket much like beer thirty's but out of aluminum. As far as tank placement, I like catch 22's saddle tanks and would like to build tanks like his.

Any advice or even pictures would be helpful
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Quite a few boats (not necessarily C Dorys) have had this modification. The positive is that it gives a much more useful cockpit, and putting in the saddle tanks will help with the weight distribution. Bringing the transom height up to the level of the gunnel will make the boat safer and more seaworthy.

The engine weight will be further aft, and there may be some change in the handling characteristics. You can search around on the internet and you will see many ideas. The Hull Truth is one forum where I have seen this modification discussed. I Boats and Boat Design Forum also have some in-depth discussion of building up the transom and adding an engine bracket.

I personally think it would be a great idea, and hope that you document this with photos and a blog.
 
Without reading all of the material on this, I would think that the transom would have to be "doubled up" because of the torsional load the motor bracket will place on the transom. You are creating a cantilevered structure that will pull on the top of the transom and push in on the lower bracket mount. Then you hang a big o'l motor back on that. I would hope that those who've done it have done the mechanical and stress engineering professionally before making such a dramatic modification. I would also think that the modification makes moving the tanks to saddle type and as far forward as possible, a requirement, not just an option or consideration.

I'd definitely spend the few grand to have a structures guy model any such modification to your boat.

(Space craft engineer coming out in me....sorry).
 
One of the points made by folks who have done the bracket modification is that the bracket spreads the load out over a larger area of the transom, and creates more attachment points (Thru Bolts). One of the issues with an older glass/cored transom is that you don't really know the condition of the core. Potter's point is certainly well made--and it would not hurt to over engineer the new structure...
 
Yes, condition of the core and overall transom structure is critical. There is a surprisingly large increase in the stresses on a base structure (transom) supporting a cantilever structure, (bracket), as the supported load, (engine and associated steering components), is moved further away from the supporting structure by even a few percent. That modification is not, what we use to say in the space business, the "LAR" method of design. (Looks About Right). Not saying it can't or hasn't been done well, but it is certainly a trickier job than hanging a bracket on the back of the boat. (Not that anyone would treat it as a simple mod in the first place).

In any case, good luck. One of the ONLY drawbacks to the 22 cruiser is the small cockpit area of the boat. But my wife and I fit fine in the cockpit, even with an occasional evening visitor or two who wish to share a drink and lies.

I suppose because of my engineering background, I'm pretty much limited in my imagination when it comes to modifying the design of a car or boat or airplane. I've been involved in the structures design and analysis and modeling and compromising business and can accept that when I buy a product that someone put a lot of thought into making it fill the bill as well as possible and that they have intelligently made the necessary compromises. At least for me, I just don't change the basic structure of things that come from a factory.

I also think vanilla ice cream is the best flavor and I love my white bread.
 
I'm definitely in this camp. My hull extension project took a pause for some unforseen life circumstances, but I'm about ready to crank on it again. The upside of the downtime is that it gave me time to reconsider aspects of the project, particularly the potential of a bracket. Mine would essentially be a 30" continuation of the hull which would make it a 28.5' C-Dory. I already have two 24-gallon saddle tanks from Moeller. My transom has a new 1.5-inch plywood core and the bracket structure would push directly on the existing sole and sides with a full-width cored top and center brace. Based on the calcs the naval architect did for the four-foot extension, I expect to gain another 500 lbs. buoyancy with the extension as well as a serious launching pad for our kayaks. I will have to fill in the transom cutout but that's cake at this point.

The other gain is a longer cabin. Another 30-inches on the old Classic cabin would make a small shower/head possible as well as a real galley and still have over eight feet fore and aft in the cockpit.
 
Well, he did only look at the four-foot extension, not this new brainstorm. While I'm not too concerned about the construction or having the bracket fall off the boat, the doctor mentioned changes in handling characteristics. I can believe that. Planning on hiking it up to a 115HP from the old 90HP, but it'll be a long way from the bow. I hope I don't end up needing tabs to keep things in trim.
 
Tom,
I have wondered what happened to your project. I just reviewed the album.

Any chance you could start an new thread and let us know How the project is going.

Thanks!
 
Sure, Bob. As soon as it gets going again. A few small projects to get out of the way first, but the boat is next; then a few weeks to cure the exterior epoxy laminations before gelcoating will give me time to paint the house...

I'm really enamored with the idea of a 'bracket', although the term makes it sound like a bolt-on accessory; this will be integral to the hull. I really need to get a clear idea of how the boat can be expected to handle before I commit. Other than not being able to turn around in some of those Pine Key canals without four boathook handlers fending off, I wonder about how stable the boat will be rolling. I haven't done the LWL measurements yet, but generally the length to beam ration is gonna be around 3.8:1; good for speed/economy but I'm not sure about stability across the beam. Yes, it's a dead flat-bottom boat, but... Perhaps I'm making much ado about nearly nothing, but I want to KNOW before I go.

I should probably quit talking about my boat on AQDory's thread and take it to an new one anyway. Hopefully, commentary here on my boat will assist AQ contemplate their project.
 
Thanks for all the replies. I had not thought about having it engineered, but that is a good point. I did ask the local boat builder about some ideas and out of curiosity what it would cost, and he gave me a ballpark between $3,000-$5,000. And with that price I think I will manufacture it myself, since I have built aluminum boats before.

I am concerned about the condition of the transom core since one of the current motor mounting bolts was over torqued by a previous owner and has since crushed the fiberglass on the inside of the transom. Not sure if it's just surface damaged, or if it goes deeper I'm afraid to dig.

I have looked at your album Tom and am impressed with your project. I would mind doing that as well, but I don't think I could trust my fiber glassing skills enough to take the boat out on the water after :shock: Also I don't have the shop/yard space for a project that large.

One concern I do have about doing what I call a "bracket," is when I make the buoyancy chamber, do I go full width , or just a little more than the motor width. I would want to make a full width swim platform so I have something to climb onto after scuba diving. The concern I have with making the buoyancy chamber fully width, is would it make the boat too bow heavy after. Since I'm sure the new Suzuki 90hp aren't much heavier than the old Evinrude 88spl that is currently on there.

Whoa, that's a long post. Thanks for all the advice though!
 
Tom, have you installed the moeller tanks in your c-dory yet/before? If so, how did they fit? Also, do you happen to know how much they cost?
 
Nope, the tanks are in storage and the boat is upside down. I don't remember off the top of my head how much they were, but I can look it up and post later. I think they were somewhere between $250 and $300 each.
 
The question is are you going to make your "buoyancy chambers flush with the bottom of the hull, and close to full width? Or is it just king to be the width for a bracket--lets say 20", and then a swim step on top? Look at the Tom Cat--not really much buoyancy there --and none when the boat is running.

I had a trawler, which had a 30" hull extension/swim platform--and I think it was a factory experiment--since they built two with single 150 hp diesel, and two with twin 150 hp--The hull extension, had large holes aft so it filled with water when at rest, or lower speeds--but would drain, when the boat got up on a plane. Problem with the single diesel boats, they never really got on a plane, and drug a lot of extra weight around. You might as well make the hull extension flotation…It also takes care of some of the engineering problems, buy a lot of area which is attached, and you are not really loading the transom, as you would as much with a bracket.
 
I always wanted to remove the motor box, close the transom notch and put the motor on a bracket on my previous 22' Angler Classic.. It never got done and now it is my son's boat.
My 26' ProAngler CD is built that way and it provides a bunch of room on the rear deck. About an 8'x8' deck.
My friends 26' ProAngler came with a motor box and motor on the transom. Not very much room on the rear deck compared to mine.
Both of the ProAnglers are in my photo album to see the difference.
 
I think for this summer, I will hold off on this project, at least the extension and glassing in the engine well. But will try and achieve everything else in prep of it, the design, make up my mind, and build/buy saddle tanks.

Back to trying to figure out the design, I would like to make the buoyancy chamber flush with the bottom of the hull, what I'm trying to figure out though, is if the buoyancy chamber is the full width of the stern, would it make the stern of the boat too buoyant? I am repowering the boat with a new Suzuki 90, which is only 341lbs per the manufacturer, and I currently have a 1988 evinrude 88spl on there which is only 301lbs roughly. I like to pack at least 30 gallons gas, but would like to increase to 40 gallons (two 20 gallon saddle tanks). So maybe a full width buoyancy chamber wouldn't hurt.... I am trying to avoid making the bow sit even lower while at rest/slow speeds making it harder to get on plane. Also making the bilge pump in the stern near pointless.
 
I was visiting a friend today who has a 22 foot Panga which is very similar type of hull to the C Dory22. He has the motor on a bracket and a 12" setback hydraulic jack plate: total setback is about 18". He will get the boat up on plane, and then raise the motor until the antiventillation plate is just on the surface of the water--claims to pick up 5 knots of speed, by manipulation of the jack plate. He runs everything from a 70 to a 110--both 2 and 4 stokes (He rebuilds engines as a hobby/business-and uses this boat as his test platform.
 
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