Transom Bracket Mounting Height

[ApolloHT]

Hi Everyone,
As you are aware, I am am a Newbie to this forum...but I can't believe the experience you all have with transom brackets, so I will give you background.

I am in the process of converting my I/O to a transom bracket and OB setup. I unfortunately only have a 1" thick transom, so I will be adding (3) 3/4 sheets of plywood, with mat between, and mat, WR, mat, etc. over the last sheet. I then plan to use some angle braces mounted to the I/O motor mount pads, and glass everything in. This part of my project I have under control, and will utilize my creativity to (probably over-engineer it) build a transom to handle a Suzuki 250.

Now comes the bracket...which I have designed with a 36" setback...maybe a little extreme for tilting a Suzuki 250, but my family will enjoy the large swim platform I plan to build with the bracket.

My question: What distance from the bottom of the keel, do I mount my bracket? I have been told the rule of thumb is to raise the motor 1" for every foot of setback...therefore, for a 30" shaft Suzuki, I should have my bracket (or the motor mounting pad) mounted at 33" above the keel. Is this where you all start? I can always go up with the OB by mounting it in different holds, but to go down, I will have to cut my new bracket. How do you avoid this when initially installing an outboard bracket...what is the real "rule-of-thumb" for mounting a transom bracket. Any help would be appreciated.

Thanks,
John

[strick]

You avoid having to lower the motor by not screwing up the first time [img]/images/graemlins/grin.gif[/img]
The anticavitation plate of the motor generally is even with the bottom of the keel on transom mounted motors. As you stated the rule of thumb is raise the motor 1 inch for every foot of set back. Use the anticavitation plate as your measuring stick. At 36 inches set back the plate should be about 3 inches above the keel. This is a rough estimate!!! 36 inches is a lot of set back plus you have a motor with 30 inch shaft so your motor will be sitting way up. Most brackes are at 29-30 inches set back. If you are building the bracket yourself then make a extra large flotation chamber like that used for twins.. Look at Hermco's site.

Strick

[ApolloHT]

Strick...

36" setback is not committed as of yet, I could always modify my design to be 30"...the setback distance is an open issue for me, although the Suzuki needs at least 24" to clear at full tilt. Of course I want to buy myself a little more room, so maybe 36" is a little overboard...but then, how is the setback distance measured? Is it measured from the motor mount, in a straight line, to the transom, or is it measured from the motor mount, to the bottom of the transom? I want to be comparing apples to apples, and maybe the dimension I am referencing from my design, is not the correct dimension.

In any case, if I gave you a 30" engine bracket, to mount on the back of your transom, and the 30" was measured per the correct way (as I asked about above), and you had a 30" shaft O/B, what distance from the plane of the keel, would you mount it? Knowing that you would be safe, and any future adjustment would require raising the O/B...not having to worry about lowering it?

Note: I have the manual (PDF file) of setting up the Suzuki DF250, and in this manual they provide a detailed drawing of all the dimensions of the O/B...so I don't really need the motor's anticavitation plate as a measuring stick, I hav it all laid out on CAD, per the drawing. So I know my dimensions are on the money...just don't know where to set my motor mount height for the desired setback, to get the floatation part of the bracket designed. I also plan to use my existing 24" trim tabs, so this will limit the width of the floatation part of the bracket slightly.

Thanks, john

[strick]

John-

It's measured from the transom to were the engine mount touches the bracket mounting area in a strait line. I wish there was an easy way to answer the second question but alot depends on the construction of the bracket. All references made to mounting height of the engine are in relation to the anticavitation plate and it's height above the keel.
I were brave enough to take on a big project such as building my own bracket I'd base everything on were the anticavitation plate needs to be at 30 inches behind the boat. Make a mark at 2.5 inches above the bottom of the keel and design the bracket around that. I'd go with smaller tabs so you can build as big a flotation chamber as possible.

Some reading material:
http://continuouswave.com/whaler/ref...eBrackets.html
http://www.stainlessmarine.com/insta...ketsingle.html
http://www.stainlessmarine.com/bracketlayoutsingle.html

Strick

[ApolloHT]

Strick,

Thanks for the info. My extensive internet research has led me to one of the sites already, but the two stainless sites are new, and appreciated.

Let me put this same question to you a different way. If the rule of thumb 1" per foot is accurate for installing a transom bracket, and measuring the setback distance as you described, wouldn't it be safe to assume...if I had a 13 degree transom, and the water flowing past the keel would in actuallity be more then the measured setback distance (because the of the angle of the transom, and the keel being further from the point of the motor mount) and thus, you would end up with a transom bracket which would be installed in a "safe" position (safe meaning...you would never have to go lower), and the only adjustment of the motor would be up? I hope you followed that way of thinking. Another way to look at it...the top of the transom bracket mounts to the transom above the keel...and with a 13 degree transom, this would mean the point at which the water is flowing past the transom is acutally a few inches further forward.

One other question...according to the specs, a 30" shaft Suzuki DF250 (weighs 591lbs) is only 11 lbs. more then a 25" shaft. So why should I be any more concerned about the size of the flotation bracket, then if I was using a 25" motor. In addition, I would think a 36" setback bracket would add even more flotation then a 30"...but then...it is an additional 6" further back...so I could be way off base.

Like you said...I want to do it right the first time, because having to redo it...is almost not an option. Bear with all my silly questions, it is good for me to bounce these theories off someone.
Thanks,
John

[strick]

I think I see were you are hung up. The 30 inch setback measurement is taken from where the motor mounts to the bracket in a straight line to where the top of the bracket touches the transom. Dont worry about the 13 degree angle causing the bottom of the keel to be farther forward. As I said before on notched transom boats the cavitation plate is usually level with the bottom of the keel.... on bracketed boats the cavitation plate should be about 1 inch higher then the keel for every foot of setback.. Thus a 30 inch setback bracket would place the cavitation plate about 2.5 inches higher then the keel. If you are worried about having to lower the engine and destroying your bracket then just mount the bracket 1 inch lower then you had planned. On the boat I have now I have the anticavitation plate almost 3.5 inches above the keel and the boat runs great.. no cavitating what so ever so there is some room for error.

My comment about building as big a flotation chamber as possible had nothing to do with the difference in the length of the shafts of the two motors....my point was mearely that you have a big heavy motor and the more flotation you provide for the motor the nicer the boat will set in the water. Huage trim tabs may or may not get in the way of the bracket...D&D marine used to have a great installation page with diagrams for there brackets but they have since limited access to the page.

Strick

[warthog5]

ApolloHT You have mail.

[warthog5]

ApolloHT Got your email and sent imfo.

[ApolloHT]

Warthog5,

Thanks for the info...It provides me with some valuable information, for installation. But, one question...when I reference the instructions for installing a single bracket...the dimensions are very clear for mounting, but there is no mention of what the setback is, for the bracket. Any idea what the setback would be, for those directions? Once I am able to determine the setback, then I can determine the ratio between setback and installation height.

Thanks, John

[warthog5]

I'm pretty sure it's 29in. I will measure mine tommorow for you.