Outboard motor Center of Gravity.Increasing 20" transom to 25"

[itsstock]

I am starting to rebuild my 1700 Nova(mustang) runabout, the transom was rotten and a little in the floor at the back that has now all been removed.

In the rebuild i was going to increse the transom from 20 to 25. This will raise the CoG of the boat but will this have any noticeable negative effects on the boats CoG, handling or any other issues?
I was thinking to try counter act this installing an under floor tank 80-100ltr (20-25 Gal) to replace the portable under the outboard well.

Previous motor was a Mercury 150 I6, looking to replace it with a later model Yamaha 130 V4 or step up to 150 V6.

 

[Bondo]

Re: Outboard motor Center of Gravity.Increasing 20" transom to 25"

I am starting to rebuild my 1700 Nova(mustang) runabout, the transom was rotten and a little in the floor at the back that has now all been removed.

In the rebuild i was going to increse the transom from 20 to 25. This will raise the CoG of the boat but will this have any noticeable negative effects on the boats CoG, handling or any other issues?
I was thinking to try counter act this installing an under floor tank 80-100ltr (20-25 Gal) to replace the portable under the outboard well.

Previous motor was a Mercury 150 I6, looking to replace it with a later model Yamaha 130 V4 or step up to 150 V6.

Ayuh,.... Welcome Aboard,.... Changin' by 5" is alot, on a little boat....

Have ya considered a jack-plate,..??

You'll also have some serious structural issues to deal with in such a build....

 

[itsstock]

Re: Outboard motor Center of Gravity.Increasing 20" transom to 25"

I am wanting more freeboard(think thats the term), the outboard well is very wide almost the full width of the boat. Also the well height is very low too. its much lower than the gunwale close to 8' lower.
Swamping the boat from the stern is my main concern.

As its a runabout, i am using it to fish, tube/wakeboard and dive(spearfish).
In the rebuild i am going to chop out the well and rebuild it keeping access from the water the main objective.
increasing from 20 to a 25 gives me an extra 5 inches of freeboard before i start mod'in the outboard well.
Also looking around at 2nd hand outboard there are far more 25's than 20's around.

I could do a Jacking plate and just mod the outboard well so i can get more freeboard. I see a few boat having 20 to 25 conversions done on them, keeping weight low makes sense but would a change like this be noticeable to the handling of the boat underpower and at rest?

 

[Bondo]

Re: Outboard motor Center of Gravity.Increasing 20" transom to 25"

but would a change like this be noticeable to the handling of the boat underpower and at rest?

Ayuh,.... Of course it will,.... it's Only a 17' boat....

Got any pictures,...

From yer words, it don't sound Safe...

 

[itsstock]

Re: Outboard motor Center of Gravity.Increasing 20" transom to 25"

I cannot find the pics of the boat with motor attached.
This is the Dumbie fit up of approx what i was going to do +5", on either side of the reduced well will become a step(s).
The front of the well height will be approx the height of the highest points

 

[itsstock]

Re: Outboard motor Center of Gravity.Increasing 20" transom to 25"

Oh well i thought that a few more people(s) might have there 2 cents to add to this or someone who has added a jacking plate to allow for a 20" transom with a 25" O/b might have something to add, comment or suggestion.

 

[bob johnson]

Re: Outboard motor Center of Gravity.Increasing 20" transom to 25"

dont chop up your boat just yet.....they make a bunch of guards that help keep the waves out.....

http://www.veveinc.com/shop/Back-Trolling-Splash-Guards/

bob

 

[73Steury]

Re: Outboard motor Center of Gravity.Increasing 20" transom to 25"

I did the same to me 17 foot Steury with no ill effects. Boat handle just as before. The local marina does it quite often. Use 2 inch aluminum "L" brakets on each side of the motor on the inside of the transom to transfer the wieght from the extension to the rest of the transom. If transom is good you'll have no problems.

 

[oops!]

Re: Outboard motor Center of Gravity.Increasing 20" transom to 25"

^^^^ the marina did it wrong.

you will need to take the new transom and fit it all the way across the back of the boat and glass it in.

by adding L brackets..you are transferring the stress to the bolts of the bracket to the surrounding glass.

you can just do a full standard transom rebuild.....but higher.

YES.....it will affect the handling of the boat.....it has to by laws of nature,

the addition of a belly tank is a good idea.

but why are we doing this in the first place ?..... did you blow the old short shaft motor?. are you worried about swamp with the back wave?

a jack plate is the standard solution in this circumstance because. if you continue with your plans.....you will need to add glass to the transom over the new extension.....then gellcoat it.
after the exterior is done....you will need to re design your splash well area.

this will include cutting the splash well out from the cap....making a mold of some sorts for the inside of the splash well, glassing that......fairing the new joint area. and gellcoating that as well as finishing the contoured gellcoat.

this whole project (albeit) a fun one.......is unnecessary with a 200 dollar jack plate

 

[oops!]

Re: Outboard motor Center of Gravity.Increasing 20" transom to 25"

Oh well i thought that a few more people(s) might have there 2 cents to add to this or someone who has added a jacking plate to allow for a 20" transom with a 25" O/b might have something to add, comment or suggestion.

sorry you didnt get more replyes right off the bat.......these forums move very very fast.....some times it only takes a few minits for a thread to dissappear off the front page.

but the guy that answered you (bond-o) is one of the best there is (and he is correct).....i would trust him to do anything to my boat any time.

 

[itsstock]

Re: Outboard motor Center of Gravity.Increasing 20" transom to 25"

Thanks for the reply guys,

Too late i have already chopped it all out/up. The transom was soft probably was a good seacast canidate cause it was mostly good timber, but its not available here. Seeing i am redoing the transom seems pretty easy to increase the height.

Swamping the back when beach lanuching/retrival was where i had a little problem, back waves seem to just lap into the well and drain out.
Old motor still runs fine just looking for some more reliablity, economy, noise reduction.

Splash backs would work but i am after easy access from the water into the boat when we dive and tube and the original well was hard to get over even with a dive platfom on the back.

Should i increase it to 25??? maybe it should be polled