Motor Mount Questions

S

Steve O

Active member
I'm hoping to add a mount to the transom of my duckboat. The 2 photos below illustrate how it sits on the boat now, and how it would sit on the transom if I fabricate a mount along the lines of the model shown.

For years, with 3 separate motors, a problem arises from time to time where the propeller will spin at high rpms without propelling the boat. I'm pretty sure the present setup (where the motor sits on a 90-degree transom is making the propeller suck in air, and resulting in this problem.

When I constructed this boat, I didn't know much. In retrospect I think I should have angled the transom.

I suspect that putting an angled motor mount on the transom will solve the problem. The model mount is angled 11 or 12 degrees, and with that angle the 3rd pin gets the anti-ventilation plate the closest to horizontal. I estimate this moves the propeller about 4 inches further rearward from the back of the transom.

I'd greatly appreciate any insights the braintrust on duckboats can share. So far, I have the following thoughts (thanks for anything you can share):
  1. How far rearward of the transom does the propeller need to be to avoid sucking in air
  2. How far rearward is too far
  3. Is 11-12 degrees a good angle for the motor mount
  4. What material is best for making this (I've read quite a few of Steve Sanford's posts on this. Clearly, he likes to use treated southern yellow pine. I believe his has dried for a long time, however, and I don't have any dried material. I agree with Steve's observation that epoxy doesn't stay well on white oak. I've had the same experience. My boat, being quite small, fits well in my garage, so it's only been exposed to the weather when I use it, so will I be fine not using pressure treated. If so, is the drying a must, and if so, for how long, and how).
  5. How to attach the wedges to the existing transom (It looks like Steve Sanford's Southbay Duckboat Motor Mounts run parallel to the transom so that his bolts that run through the transom and mount sit perpendicular at both ends, but I'll only be able to have one end perpendicular)
  6. Motor on Boat.jpgMotor on Bracket.jpg
 
If you are sucking in air, I think the issue would be the motor is too high, versus wrong angle or too close to the transom.

Is the cavitation plate at least an inch or two below the bottom of the hull?

Hopefully others will jump in!
 
Carl,
I just measured, and the ventilation plate is about 1.5-1.75 inches below the bottom of the boat. The engine manual calls for the ventilation plate to be between .2 and 1 inch below the bottom of the boat. So I don't think that's the issue. Thanks for your input, however.
 
They make transom wedges. They will run like $75-$150 and I would bet you could achieve the angle you want with them. honestly, you could probably buy some chunk of steel and grind it down to make the correct angle you want, drill out a hole for the motor mount to go through and throw it up there and let the motor be angled.

Just google transom wedges and there are a ton to choose from.
 
Thanks for the additional input. For some unknown reason, the Outline Dimensions diagram from the Service Manual for this outboard indicates a 12-degree transom angle (see below excerpt from that manual). Maybe that's what is common in Japan, where this outboard originates.

As for buying a metal, pre-constructed mount, I may in fact have to go that route, but so far I'm having fun trying to come up with a solution I make from wood. At this point I think I will have to try to expirement until I (hopefully) get it right.

IMG_3569.jpg
 
Came across this article:

https://www.boats.com/how-to/the-ou...peed-with-outboard-engine-height-adjustments/

on the relationship of motor depth, boat speed, motor heights, motor angles, etc.

The part I found most useful is the following:

There are quite a few variables in play when considering optimal engine height on the transom. A key factor is how far aft the propeller is located from the bottom of the boat. Water flowing from under the boat rises so if the prop is further away from the transom, it may be possible to mount the engine higher. All outboards are different in this regard. For example, a Mercury Verado positions the propeller about six inches further aft than a Mercury OptiMax outboard. A boat rigger’s rule of thumb is that the motor can be raised one inch for each eight to 10 inches of distance between the transom and prop. As the prop moves further aft, it’s also likely to be in cleaner, “harder” water, and be more efficient. This is one reason performance boaters use a set-back device between the boat transom and the motor bracket. And on some high-performing fishing boats a jack plate will be used to set the motor back and also allow for vertical adjustments on engine height on the fly.
Click to expand...
 
I also didn't know much when I built my boat and built the transom at 90 deg. I use wedges to be able to get the moter trimed properly. My outboard is bolted on not clamped. Looking at your transom it does not look like the mounting bracket for outboard can handle a thicker transom and wedges most likely will not fit. In my opinion you are on the right track with what you are doing. As an added bonus of setting the motor farther back from the transom you should see better performance . You should be fine as long as the cavitation plate is still underwater.
 
Thanks for the encouraging word, Neal. I think you're right about wedges not fitting on this setup.
 
Have done some more digging, and it looks like the 12-degree transom angle shown in the above illustration is used because it is the most common.

Tweaked the model a little, and it appears that a 15-degree angle works best for my boat/outboards combinations.

Went rummaging through the garage, and found the board shown in the photo below. Best I can recall is that it is a piece of white oak left from when I constructed the boat back in 2008-2009.

This looks like a beautiful, solid piece of wood, and I think Zach Taylor's plans in his book Successful Waterfowling call for oak motor boards, so I think this is what I will try to use:
Board Photo.jpg
 
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