Was: Mounting Height For Motor Bracket, Now: Backing Plate

[sreidvt]

My boat is a 1974 O'Day 23-1 and I'm about to install a new, 4-position adjustable outboard bracket. My Honda outboard has a clamp bottom to cavitation plate distance of 22.5" so it is a long shaft model. I've been aiming for an outboard position that puts the cavitation plate about 6" below the water line. I've managed to create a dummy test mount that lets me try different mounting positions without putting any new holes in the transom.

My painted water line sits about 2" below the cockpit drain hole so I am guessing that the actual water line (depending on who is in the cockpit, etc.) will vary between the marked water line and 1" above that. Unfortunately, I have not yet ever had this boat in the water and so there is no scum line. I will be sailing the boat in Lake Champlain.

As the test mount sits now, position 3 on the bracket puts the cav. plate 3" below the marked water line. Position 4 puts the plate at 8" below that line. I'm thinking that I should set the mount for normal use at position 3 so that I can drop to the even lower (5" lower) position 4 if I need to do that when I am out in a very heavy chop.

What are your thoughts? Should I set this up so that position 3 is at 4", 5", 6"? It seems like setting position 3 for 6 inches would really bury the motor if I had to go to position 4 in a chop?

The paired distances available (below water line would be): 3"/8" or 4"/9" or 5"/10" or 6"/11". I'll reserve the top two settings on the mount for holding the motor out of the water when the boat is sailing or moored.

When this motor was mounted on my Mirage 5.5 the cav. plate was about 9" under water and it still could lift the prop in a heavy chop if I was up at the bow adjusting something. But the Mirage is a shorter and much lighter boat.

Any input welcome and appreciated.

 

[jssailem]

Golly. You are striving to decide inches below the water line.
When I set my outboard on my trailer boat I wanted it to be in the water deep but shorter than the rudder and the keel. This kept the prop in the water even when I was in surface chop of a couple feet. It kept the motor above any rocks or shore bottom so I did not shear off the prop.

For sailing I used a motor lift mount. This got the motor up and out of the water. This definitely improved my sailing speed. It was simple and sweet.

I think the replacement mount I bought is still up on the SBO items for sale. PM me if interested.

 

[sreidvt]

Thanks for your post John. I have the mount I need, fortunately, now I just need to decide on the mounting height. Wherever it ends up will certainly have the prop up higher than the keel or rudder.

 

[sreidvt]

I think I may have just become that person who poses a math question at a time when everyone is homeschooling.

 

[jssailem]

Yes it is possible. Math nerds are ok in my book.
I was trying to suggest that the level of precision may not be that critical to the performance you will experience.

Thrust is only effective if it is beneath the water. On an O’Day 23 your not striving to get it up on a power plane. So deeper is better. Too deep is below the keel, too shallow is at the surface. Just right is where you get full submerged shaft that you can lift out of the water so you can sail with out dragging the prop in the water, heeled with the wind in your sails.

 

[sreidvt]

Yes it is possible. Math nerds are ok in my book.
I was trying to suggest that the level of precision may not be that critical to the performance you will experience.

Thrust is only effective if it is beneath the water. On an O’Day 23 your not striving to get it up on a power plane. So deeper is better. Too deep is below the keel, too shallow is at the surface. Just right is where you get full submerged shaft that you can lift out of the water so you can sail with out dragging the prop in the water, heeled with the wind in your sails.

Hi John,

Thanks again for replying. You are probably right that I may be planning more precisely for this than I need to. But it's not costing me all that much time to tweak this and I like the idea of not having to make changes later (especially those that involved drilling new holes in the transom).

The 23, of course, is a displacement hull so no planing. Everything I am planning here starts with that. But I think there are at least a couple of downsides to going too low (even before reaching keel depth) and you just mentioned one of them.

1. (as you just noted) We want to be able to lift the engine enough so that it is fully out of the water when tilted in its top bracket position. There's no way I'll be able to keep the motor skeg out of the water with the boat heeling to port but that's fine.

2. We want to minimize the chances of the motor head getting swamped in a swell.

I am fussing with this now because if I get it right I won't have to deal with it again for many years. I tend to spend a little extra time at the start of a project to save time long term. At least that's the hope. Sometimes it even works.

The new provisional position I'm trying today (on the hard) puts the cavitation (anti-vent.) plate 1 7/8" under the hull bottom (as measured at the center of the hull) and 4 3/4" below the marked water line. That's in position 3. In the lowest position (4) that plate is 9 3/8" under the marked water line. When fully lifted and tilted the motor skeg is 5" above the marked water line. The bottom edge of the plastic mounting block itself is 7" above the water line in position 3 and 2.5" above in position 4.

I feel like that's getting pretty close but I am going to think it through a bit more before I drill into the transom itself and mount this for real.

Feel free to jump in (John or anyone else) with suggestions about tweaking this height.


Cheers,

 

[jssailem]

I know better than to "Jump In" on a guy who is about to drill a hole in his boat.

You should have seen the measurements and delays I did before drilling a hole in the side of my boat to drain the cockpit seat.

It had to go in the side. It had to align with the drain etc. Finally my friend said "John! - Just Do It!" Yes. yes... But it still gave me the willies.. That was 10 months ago and it is great.

I was reminded probably a dozen times.. John it is fiberglass. You can always patch it. Easy for a friend to say.

Take your time. When It is right you will be the one to drill the holes.
It is Fiberglass and you can always patch it.

 

[sreidvt]

I know better than to "Jump In" on a guy who is about to drill a hole in his boat.

You should have seen the measurements and delays I did before drilling a hole in the side of my boat to drain the cockpit seat.

It had to go in the side. It had to align with the drain etc. Finally my friend said "John! - Just Do It!" Yes. yes... But it still gave me the willies.. That was 10 months ago and it is great.

I was reminded probably a dozen times.. John it is fiberglass. You can always patch it. Easy for a friend to say.

Take your time. When It is right you will be the one to drill the holes.
It is Fiberglass and you can always patch it.

Hi John,

Four holes no less! <G> Actually, I don't mind drilling once I am fairly happy that I know where I want things to sit. But it sounds like you and I both take the "measure twice, cut once" philosophy quite seriously. I find that, in the end, it saves a lot of time. I think I may come down about 1/2" (that's not a typo) from my last test position and that should be good. If I get it right, I won't need to think about it again for many years.

Of course, the saving grace for fine tuning outboard mount height is that one can always change the height or position of the actual mounting board without making any changes to the bracket bolted on to the transom. But I'd rather not screw around with that either.

Thanks for the feedback. Cheers.

 

[jssailem]

Take pictures and share them.
When your ready turn on the drill and move it near the boat. If the boat jumps STOP...

 

[sreidvt]

That’s great. This boat should be used to it by now but I will watch for any sudden movements.

Cheers,

 

[sreidvt]

OK. So the bracket height is now set and the reference hole is drilled on the port side of the transom. This boat has no port lazarette (alas) so I also needed to cut a round hole in the side of the cockpit seat for a 6 1/2" screw-in access hatch. With that cut, we were able to reach in and determine that the overall thickness of the transom is about 3/4". I assume that's outer FG, 1/2" marine plywood and then inner FG. Sound right? This board seems to go almost to the area where the transom meets the outer hull side. Amazingly, it seems to be dry and solid.

The motor bracket is going on the port side of the transom and a swim ladder is going on the starboard side. My plan is to use 1/8" thick, roughly 18" x 24" (before cutting to match the transom curve), aluminum backing plates on each side. Obviously, they'll be bolted through but I also plan to attach them to the inner transom using West Epoxy and micro balloons. Positioning the port side one will be tough but we have a plan for that.

Any thoughts, suggestions, advice?

 

[Timm R Oday25]

Sounds about right . I cracked our transom on our Oday 25 the first season we owned her. I crawled in underneath the cockpit and rebuilt the transom. What you describe sounds very close to my experience . I used 3/4" cherry plywood soaked liberally in epoxy to reinforce the transom . I scored the plywood to conform to the curve . "If " it were my boat ,I would go to at least 1/4" aluminium.

 

[sreidvt]

Sounds about right . I cracked our transom on our Oday 25 the first season we owned her. I crawled in underneath the cockpit and rebuilt the transom. What you describe sounds very close to my experience . I used 3/4" cherry plywood soaked liberally in epoxy to reinforce the transom . I scored the plywood to conform to the curve . "If " it were my boat ,I would go to at least 1/4" aluminium.

Thanks very much Timm. I debated over whether or not to go to 1/4" aluminum. The 1/8" plates I ordered just shipped today but I will think about ordering two more and doubling each one. I think my boat is plywood reinforced across the whole transom. Was yours also? What year is your boat? Mine is a 1974.

 

[sreidvt]

By the way, I'll add the following in case anyone finds this thread in the future, when doing a search, and wants details. I ended with bracket position that puts the anti-ventilation plate 5 1/8" below the waterline at the second lowest position. If things get really choppy it can go down to 9.5" below the water line in the lowest position. The bracket is a four position Garelick 71056. My 5 HP Honda outboard has a clamp bottom to cavitation plate distance of 22.5".

 

[Timm R Oday25]

When researching how to repair the transom ,I was told the transoms on the Oday were not known to be of the highest strength. There was a thin sheet of plywood fiberglassed in. I wanted to be sure I would not ever have to go back in. That's why I went with a total of 1 inch ply . I added perhaps 9 or ten layers of heavy fiberglass cloth over . All through holes were overbored and filled with epoxy and redrilled to the minimum opening I needed .
The boarding ladder ,outboard bracket and rudder all combine to add a great deal of stress to an area maybe 2'by 4' .

 

[sreidvt]

You are right about that and I agree with your choice to make the transom very strong. Rudy told me that the early boats, including the 23-1, were among the most heavily built models O’Day ever made. But it is true that many things stress the transom and the plywood is not very thick in that area on my boat either. I just ordered two 1/4” plates to use instead of the 1/8”.