New transom

[btbldr2]

Ok, here I go, my first forum question:
My current project boat is getting a new transom - whoop-de-do, no big deal. Where I'm encountering a problem is finding any kind of formula to help me determine optimum offset for any given length boat vs. HP as well as degree of (transom) angle to be factored (assuming normal deadrest angle at 5 - 12 deg.). The present setup was already at max down trim angle and that was with already having 2 1/2" of offset from the jack plate where I'm thinking an 18 footer like mine would benefit most at about 5 to 6 inch (maybe more? say 7" to 9"?) offset which should bring me back out on the trim also... Or am I missing something? I want to stay to the conservative side while still getting the very best performance/economy from the family cruiser. Yea I realize, any offset is better than none, but I'm shootin' for the best.
I am also raising the height of the transom so the jack plate won't really be necessary, but I'd just assume keep it for the extra contact area.
Any thoughts?

 

[Capt'n Chris]

Re: New transom

There is no formula and your hull deadrise has nothing to do with your transom angle. 13 to 14 deg. will catch it. You you have to remember; however, is that outboards all have varying degrees of tuck. Mercury used to have the least.

Captain Chris
SternMate?

 

[Knightgang]

Re: New transom

It took me a minute to figure out just what you were trying to ask. I agree with the Cpt. The angle of the Transom does not increase/decrease performance. Put it back in at the same angle it was stock. I would raise it as you suggest so that you really do not need the jackplate. However, I can see why you might still want to keep the jackplate, no problem. Having the higher transom will also increase its strength...

 

[btbldr2]

Re: New transom

Ok, I didn't get accross the correct idea.
I was talking offset motor mounts
- you know, where you mount the outboard motor farther back (away from the hull edge) which allows it to be mounted higher, which also puts the prop in more "solid water" and the whole works has less drag.
Just the mount attaches to the transom and then the O/B goes on the mount. Setback mounts come in many sizes, so there must be a way to figure from the size of a boat (or a weight range?) how far back would be correct. I mean they go from a mere 5 inches up to 32 inches prefabbed and even more by special order. Some are composite (good looking) while others look like an erector set on steroids.
They even make them as an addition to/for jack plates which does make sense because one of the purposes is after all, raising the motor.

I was asking too many questions at once- for now, just figuring that one out would be good.

I tried getting answers from a distributor which came out pretty much like "They all work - Do you want one for over 200 hp or under?"

Is that all there is to it? - How much hp? If that's it, why so many different amounts of setback? - or is that just for aesthetics?

I should mention also the amount of setback will help me decide if the transom needs to be built any stronger or not.

Happy sailing...

 

[Knightgang]

Re: New transom

You are refering to an engine bracket...now I get it. A few comments on that;

1. If you are going to add an engine bracket, you might want to consider building a full transom and closing in the stern of your boat. The only reason a boat has an open stern (our smaller craft types) is for an outboard motor to be attached and be able to function. So a fully enclosed transom would be a consideration.

2. I have been told that for every 6" of setback, the engine should be raised 1" up the transom. So, if you are going to put on a 30" bracket, then your outboard cavitation plate would be raised to 5" above the keel.

3. Engine brackets come in several models, as you are probably already aware. Some are for single engine only, others have built in swim platforms, and some have some foam inside and are designed to help add flotation tot he stern of the boat since the engine weight has been pushed back as well.

If it were me, I would close the transom fully, pick out the bracket I liked, then place it and adjust my engine height accordingly...

 

[btbldr2]

Re: New transom

You are refering to an engine bracket...now I get it. A few comments on that;

1. If you are going to add an engine bracket, you might want to consider building a full transom and closing in the stern of your boat. The only reason a boat has an open stern (our smaller craft types) is for an outboard motor to be attached and be able to function. So a fully enclosed transom would be a consideration.

2. I have been told that for every 6" of setback, the engine should be raised 1" up the transom. So, if you are going to put on a 30" bracket, then your outboard cavitation plate would be raised to 5" above the keel.

3. Engine brackets come in several models, as you are probably already aware. Some are for single engine only, others have built in swim platforms, and some have some foam inside and are designed to help add flotation tot he stern of the boat since the engine weight has been pushed back as well.

If it were me, I would close the transom fully, pick out the bracket I liked, then place it and adjust my engine height accordingly...

That's the kind of answer I was looking for... Thank you very much.
Now, going from there, I've seen some very attractive composite units which look good (way better than those brackets) and apparently spread the load accross the transom. Should anything be done to the transom in the way of reinforcement? Or is that why you suggested closing the stern?
The reason I asked about a formula, was so I could figure the least amount of setback that would give the most effect. There has to be a point here where the law of diminishing returns kicks in, yet I don't want to go too short either. Or does this seem like a waste of time on an 18 footer? I mean as far as performance and economy are concerned.
I saw a newer 20 footer set up like that, but there was no one to answer any questions.
It didn't look like the motor could be adjusted vertically so I thought there must be numbers to figure the new height.

Thank you.

 

[ondarvr]

Re: New transom

While there is a guideline for height related to setback, there is no hard formula, it still involves fine tuning after its done. There are just too many variables involved, hull design, weight, HP, prop, speed, type of use, etc, all play into how it will it perform and the setup needed. Closing in the transom makes it stronger, plus most people build up the area where its through bolted for the bracket. A little extra glass always gives you a better feeling about this modification also.

 

[btbldr2]

Re: New transom

So, I went to the marine supply this morning and there it was, a boat with a setback motor mount that really looked good, though still like an add-on. What was done looks more like an inset hull extension (the sides grafted in a couple inches in from the hull - ditto the bottom with the bottom angling up rather than going straight back and then the rear which was really another transom. I couldn't get any info as to the differences this change makes as the owner said it came to him that way. The original transom and splash well had been left as original. This way of mounting seems to make more sense to me than just putting a bracket on the original transom concentrating all the stress in one small area, plus as mentioned above, allows for extra floatation.
Also noted, the second (call it the outer transom?) was, definately on a bit more of an angle than the original. Visable, but not too pronounced.
Most important question of all though, is this a practical mod to do on such a small (18 foot) boat? Would the gains be appreciable enough to warrent all that extra fabrication? Still need to figure out the weight all this will add too.
Right now I'm thinking about 18" for the setback...
-I'm giving myself a headache with this- ignorance is NOT bliss

 

[bigredinohio]

Re: New transom

Are you talking about a jack plate? What type of boat do you have?

I have very little knowledge other from what I've read in the Checkmate forum but I know they beef up their transoms to 2.25" (3- 3/4 ply) and add knee supports. I want to say between 6"-12" offset is normal for a boat around 18'. Maybe go over to the Checkmate forum to see if they have your answers.

Here might be some useful reading:
http://www.marinepartsman.com/installing-an-outboard-on-your-boat.html

 

[Bob_VT]

Re: New transom

You do realize...... that 18" of setback is going to require lengthened controls to include steering?

That 18" could set you back far more money than you think.

 

[btbldr2]

Re: New transom

You do realize...... that 18" of setback is going to require lengthened controls to include steering?

That 18" could set you back far more money than you think.

Yes, thank you. That's already been resolved (from a donor boat) and a very good point.

So far, my biggest problem has been getting the numbers. The people that make these things are just all over the map with regards to mounting height, so in the end, I believe I will be keeping the present jack plate and mounting the engine so at it's lowest adjustment point the anticavitation plate will be just level to maybe 1" above the bottom of the hull which will allow a 5" maximum that I will be able to go up from there. Even considering the 40 mph rule, I shouldn't ever need to go more than that(?), or does that work differently with setback installations?

I'm just guessing here, I'd really like to hear from someone or better yet, several someones who've done this and can provide before and after info.

By the way, it's a lightweight hull (V) with a 140 hp engine

 

[btbldr2]

Re: New transom

Are you talking about a jack plate? What type of boat do you have?

I have very little knowledge other from what I've read in the Checkmate forum but I know they beef up their transoms to 2.25" (3- 3/4 ply) and add knee supports. I want to say between 6"-12" offset is normal for a boat around 18'. Maybe go over to the Checkmate forum to see if they have your answers.

Here might be some useful reading:
http://www.marinepartsman.com/installing-an-outboard-on-your-boat.html

Believe it or not, I have that page in my boat folder, but thank you.

Meanwhile, your project is lookin' sweet!
You'll have plenty to be proud of when she's ready for water.

 

[NSBCraig]

Re: New transom

You keep saying jackplate.

Do you mean bracket or jackplate they are two different things- a jackplate is adjustable.

Also what boat are you talking about?

Does it have a pad?

.....notched transom?

 

[ondarvr]

Re: New transom

This is a link to another project here that included mounting an OB bracket, it may answer many of your questions.

http://forums.iboats.com/showthread.php?t=271420&highlight=scarab

 

[The Enforcer]

Re: New transom

btblder2,
I've got an 23' Checkmate with a 2 foot gil bracket, hanging off the back is a 250 HP Rude. My transom was beefed up from the factory to handle twins (special order).
I'm in the process of replacing the transom and floor, the pics are over on the checkmate forum under the name cmpulse170. Anyway my transom is 2" thick and the knee braces are also 2" thick. Underneath the sunpad is open so you can get to the batteries and oil resevoir. Sorry for the pictures being huge. Here's some pictures of my Checkmate:

http://i264.photobucket.com/albums/ii194/cmpulse170/100_1119.jpg
http://i264.photobucket.com/albums/ii194/cmpulse170/100_1120.jpg
http://i264.photobucket.com/albums/ii194/cmpulse170/100_1140.jpg

 

[Knightgang]

Re: New transom

Ok guys, he currently has a Jackplate on his boat. He wants to add en engine bracket and possibly use the jackplate on it if he needs to raise the engine more than the bracket rise to gain more optimum performance...

btblder2,

There is no formula for getting the optimum setback for performance. Most of the bracket installs that I see and read about it for one of two reasons, mostly one reason overall. They are closing the transom and adding brackets to add to the seaworthyness of the boat for offshore fishing. Many times, the owner is upgrading from one to two engines, therefore the bracket makes more sense.

Still, decide you setback, build your transom and then start adjusting the engine height for performance. Like I said, about every 6" setback will raise the motor 1" up from keel, on average. You may have to fine tune this once installed and you check performance...

 

[btbldr2]

Re: New transom

Knightgang you nailed it, that's exactly correct.

The old transom is a half transom which lacks about 4" in height to even mount the O/B without the jackplate to raise it. With the plate at it's upper limit, the anti-cav plate is just shy of the bottom of the hull. I have a good 15" between the top of the old transom and the bottom of the cap. (plus it's notched!)

The transom was getting replaced in favor of a full transom because this boat will see most if not all it's time in the gulf. Also, the original transom was only 1 1/2" thick. I am presently planning on a 2" thick replacement with knees for extra support.

Thanks for the pictures Enforcer! A pic is definately worth a thousand words.
I like that bracket too. Does it allow for vertical adjustment, or was it a "best guess" situation?

I'm just plodding along right now during work season. When I get back to it full time (April or May) I'll put up pics. I pretty much know what I want because I've already owned this exact boat (one open and one closed bow, but otherwise except for power and condition, identical) It's a whole nuther story how this one came to me.

Smooth sailing...

 

[The Enforcer]

Re: New transom

Knightgang you nailed it, that's exactly correct.


Thanks for the pictures Enforcer! A pic is definately worth a thousand words.
I like that bracket too. Does it allow for vertical adjustment, or was it a "best guess" situation?


You're Welcome!!! The gil bracket is fixed, the motor bracket is mounted in the second hole from the top. My setup is "best guess" but I'm planning to add a hydraulic jack plate in the future. The current setup gets 65 mph on the GPS. Good luck with the rebuild, I'll be following it.

 

[Fl_Richard]

Re: New transom

I have a Wellcraft Coastal 238 that has the transom design your considering from the factory. (I think)

Here are some pictures to add to the discussion.

I think the idea is similar to what your considering.