Transom modification insight

ADQdory · Posted: Wed Apr 23, 2014 1:13 am Post subject: Transom modification insight

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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thataway · Posted: Wed Apr 23, 2014 1:40 pm Post subject:

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.

potter water · Posted: Wed Apr 23, 2014 2:16 pm Post subject:

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).

thataway · Posted: Wed Apr 23, 2014 7:30 pm Post subject:

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...

potter water · Posted: Thu Apr 24, 2014 12:56 am Post subject:

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.

tomherrick · Guest

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.

potter water · Posted: Thu Apr 24, 2014 1:15 pm Post subject:

I like the sound of "the naval architect." You are doing it the right way.

tomherrick · Guest

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.

thataway · Posted: Thu Apr 24, 2014 7:58 pm Post subject:

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!

tomherrick · Guest

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.

ADQdory · Posted: Thu Apr 24, 2014 9:53 pm Post subject:

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 Shocked

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!

digger · Posted: Thu Apr 24, 2014 10:21 pm Post subject:

I really want to see the progress on Prairieboy also. Been waiting to see the new hull extension.

tomherrick · Guest

You and me both...

ADQdory · Posted: Fri Apr 25, 2014 1:02 am Post subject:

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?

tomherrick · Guest

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.