Motor Bracket / C-dory

Hello to everyone, being my first post I'll do a little introduction first before discussing the reason I joined your forum.

As a machinist of 35 years Ive built any number of projects and as a boat owner all of the upgrades to our boat came at my hand. Now I'm looking to add a motor bracket to the back of our 25 ft C-dory and thought it prudent to seek some input. It is my desire to fill the transom opening and laminate a glass bracket on capable of handling my 150 hp main and a 20 hp kicker. I've read a number of pieces on this subject and the more I read, the more questions I have. I look forward to some interaction of this subject.
Thanks from Mike in Wasilla Alaska.

 

Mike, The first thing you need to know is whether the C Dory 25 is capable of handling that much load that far aft without messing up the balance? The C Dory forum may be of some help but you need to get the advice of the builders first. How you fill in the transom gap is small potatoes.

 

I think the best handling boats and airplanes have all thier heavy stuff very close to the center of the vehicle. In the beginning outboards were quite/very small and light so they hardly changed a boats balance at all but now 4 stroke and large 4 stroke outboards change the CG a lot and to hang one out over the end of the hull dosn't seem like a good idea. Since the shape of the Sea Dory is so simple why not just add 2 or 3' to the stern. If the OB is going to be way back there I'd think some added support would be welcome. Would probably pitch and pound less too.

Easy Rider
of Thorne Bay

 

What about some drawings and measurements of what you are wanting to do .Also pictures of the actual boat .

 

This is a small boat but could give you some ideas !!

The old transom was not good , holes everywhere , rusty marks and a bracket that i didnt want , a spashwell that was a waste of room and a aft deck that wasnt good . so it had to go !
I changed to a 25 inch leg from a 20 inch i stepped the motor back 10 inchs and did a heap of other things inside as well , all in all its works a treat and i am really pleased with it .
The actual plant was built on my work bench over a wooden former and then transplanted onto the back of the hull . Its glass joined inside and out and really strong !. I simply cut bits of the old splash well that i needed and glassed everything in place , faired and filled and painted .
Inside the battery is mounted up off the floor , the oil tank is also mounted up and because everything is made of glass i mounted the fuel filter /water trap on the side of the bracket . Have rod holders down one side and water ski's down the other. Everything is easy to clean and easy to see and get to if need be
The 10inch step back has made the power trim a lot more responsive and boat better riding when trimmed .
I changed to hydralic steering because of the bigger motor and because it was compact and neat and tidy and that was heavenly move ! . From a boat that had a smallish cockpit and was sluggish with a old 70hp OMC its now one quick responsive little beasty that does 120km per hour on the flat , or a nice cruising boat to simply go fishing in !

 

Whats that non-skid stuff that you have there?

Looks nice.

Have you tried it out?

what kind of life span does it have?

where did you get it?

 

All of these boats are dories and have the limitation of low bottom area to deal with. The C Dory 22 dealers always insist on 100# of anchor chain in the bow to try lowering the too high bow. The 25 is a very similar boat and adding more weight aft does not sound like the thing to do. Forward visibility is already restricted and that would make it worse.

 

Alasgun,
I sure do like that other boat. And the extended splash well is a beautiful pice of work. I thought you were talking about one of those brackets that puts the engine much further back. Is your 150hp engine a 2 stroke? The lightness will help. I'm thinking w a 25" transom you may not need a splash well at all. I'd evaluate cutting out a big part of the transom and bolt on an overlaping thick lamination of ply the right thickness and do the glass magic you obviously know how to do. Or make the splash well inboard.

Easy

 

No, Easy, Those where pics Tunnels was showing off. Alasgun hasn't posted that I can see. I like your first response.

 

Well, I can see this will not be an easy sell. Maybe a bit more information may clear the air a bit.
I've attached a photo of the transom as a starting point. My thoughts were to fill in the transom cut out and laminate / build directly onto the transom a largish bracket capable of supporting my two outboards. Largish as in width! Added length would be what ever the minimum amount necessary to fully raise the big motor. It looks like 25 to 30 inches of length. In my simple mind I have it figured that a full width bracket configured to allow trim tab function would give greater floatation and easily hold up those to motors. Further I was wondering if the very reward section of that bracket had a semi fan tail shape it might aid handling in a following sea.
My goals in this upgrade are, more cockpit space a full transom and what ever performance benifits may come along with the deal. If there are none and the first two ideals can be acomplished, I'm fine with that.
I trust and respect the opinions of folks who already have the experience to make judgement calls in this matter and will value those tidbits if we go ahead with the project. Ive seen enough activity in this area to know it is possible and am willing to go to great length if necessary to end up with a quality upgrade.

 

The non slip is off cuts of the polypropolene carpet thats on the inside floor .:idea:
Doesnt hold water , doesnt rot, easy to get fish guts and blood off ,is soft to walk on , comes in a good range of colors and patterns , Easy to clean simply with a hose and scrubbing brush .
Its stuck down with bathroom grade clear silicone , i just did a strip round the edges and a few strips across .The carpet is soft on the feet and really hard wearing .
Its a easy way to add non slip and if it gets damaged in any way or what ever you can take it off and replace it with something else .
I covered the step on parts of the fenders on the boat trailer the same way and it been on there for more than 6 years and still looks as good as the day it was put on and they get a lot of wear and tear .
Its easy to remove using a really sharp knife , pick up the edge and cut the silicone along the painted surface and its off .
I have planed a pair of glass boarding plateforms each side of the motor well and those to will get covered the same way .

 

Tunnels,
That is the most beautiful OB mount I've ever seen. You should build them and sell them to manufacturers. The fact that the cavitation plate is a long way from the transom dosn't cause problems? This seems sooo much better than those long extensions but it probably wouldn't work on a production boat as they need to accommodate the tallest engines. Like my Winner that is about the same size has a very large well to accommodate the old tower of power Mercs. Thanks Mark for putting me back on course. I was so Ga Ga over the Tunnels engine mount.

Easy Rider

 

Well, obvious he is creative and meticulous but it looks so good because it matches the boat. It would not mate well with the C-dory.
To the original post; it is obvious that added buoyancy to support an outboard aft is a requisite. How much lengthening is hard to say but say the next time you bring your boat to Homer, we have a cup of coffee and discuss your project. I do this sort of thing (and have a few weeks available this fall) but "by gosh..." - I am not an engineer. I believe mods should look as if the factory had done them, with moderation and a goal of improving the total product, i.e., don't sacrifice fishing ability by sticking a bracket back there, increase deck space as much as possible while minimizing changes that alter the OEM feel. While at it, particularly in light of having a larger vessel (that holds water as well as keeps it out), real freeing ports are in order for when you get pooped in mean seas or when beaching. If the bow is a little high, as someone suggested, go with a tad more length to add buoyancy aft. Batteries, water, fuel can be adjusted later to trim. I like built-in tabs (planes, actually) as simplicity and never losing a fish to sharp metal objects is important to me. If a lengthening is taking place, it's a good time to look at that aft compartment, tanks, etc., as well.

 

Building a bracket /transom/spashwell combo takes a lot of planing . You have to look at what you want to achieve and end up with . My old boat (1975) has a good hull and quite sound . the nice rounded corners of the old transom were easy to mate the new to it after the reglassing inside and out . . each and every boat would be differant with the wants and needs that have to be taken into account . Like i said post some pictures and lets have a close look at what could be possible for the back of you boat . rather than a main and a auxilary it might be better to use a pair of maybe smaller motors and work round them with a step back and new transom design .
Remember the further the motor sticks out the back the more the back of the boat will go down by the stern at rest .Lots to cons ider !!

 

The cavitation plate is just below the level of the water coming out the back as it is away from the back of the hull so water pressure starts to climb the further the motor is back .
300 mm from the end of the hull = 6 mm the water will be above the keel line at 50 kmph , so 450 mm back the water is close to 10mm above the keel line . its spot on at 100 kmph and theres no cavitation when you lock the wheel hard round at speed either way !.
The picture is what i started with that was made on my workbench .
Something to help increase strength/rigidity is shape makes thing strong without loads of glass required !
Remember its a 115 v4 motor and a 25 inch leg so lots extra strain placed on everything with weight and leverage in all directions . Been in use in some pretty rough water and not a sign of a stress mark anywhere at all .