Conversion and Restoration OMC Stringer to Whatever

[RiverNoob]

So I got this boat,
How I got it isn't important, (it was cool, so I bought it and paid too much for it)

I found out about omc stringers a little too late. and have already started parting out the boat, have a great omc chevy 305 with it, runs like a champion just off the factory floor.
I did a little bit of research before buying it, just enough to know there was a lot of help posts for repairing your stringer, but not enough to figure out you probably shouldn't buy one.

Anyway, I am planning on selling the motor too, but I want to convert and restore it. This boat is pretty large, it is 25 foot, Super Deep Vee, offshore type.
The hull is really nice looking, there is a little bit of rot on the bottom side of the transom hole where the stringer 800 sterndrive went in. and then there is just a little rot on the stringers, where the motor was bolted on. (around the bolt holes)

I would love to restore this boat and put it in the water,,, It would be cool. This boat is something to look at for some reason,
I realize it is more expensive than just buying one, and I'm ok with that. I would like to have one that I built up myself. prolly over the course of 3 years or so,,

My questions are:
1) Would it be easier to convert to a mercruiser drive, using a cheap donor boat for the mounting parts, or go with a bracket and convert to an outboard? Which way is easier and which way may lead to problems, engineering-wise?

2) When reinforcing/re-building the transom, I plan to use foam and not wood,, Does it need to be the same thickness? Is there such a thing as over-doing it?
What if I rip the wood out, put in foam, then glass, then another layer of foam and glass on the inside again? really bulk that thing up. Will that do anything bad like make the boat drive weird? flip over and capsize while I have some hot ladies on board and cause my face to become red? anything like that?

3) The deck is up at least 5 feet from the bottom of the hull, and the whole area is full of foam. Can I hollow out part of this area behind the cuddy and make a little lower deck area with a port-a-potty, and perhaps other ridiculous amenities? also what about sliding the fuel tank back a little bit? right now the front of it is right behind the cuddy area about 10 inches. It's crampin my style right there...bigtime.

Any tips would be appreciated, and also would like to make some friends on here with some boatin conversion veterans and any other knowledgeable folks.
Let me know your thoughts....

Also there is no need to mention how this doesn't make financial sense, I'm already aware,,, I need to know about things that don't make engineering sense.

Hey also- anyone who is aware of unique/weird/new propulsion systems/ideas, I'd love to hear about that too. If you figured out how to get 240 hp out of 50 gerbils in a wheel powering a drivetrain, please do tell and brag about your stunning engineering achievements.

Thank you,,

 

[zool]

Welcome to drydock, Rivernoob.

Ist off, what make and model 25 is it? and definitely open a free photobucket account and post some pics for us to see.

That said, it will be easier, safer, and most likely less expensive to convert to Merc or Volvo Penta, then bracketing a big outboard...it will also be less expensive to find a doner boat for a full drivetrain, all those little connecting parts can get expensive too, but you can keep the gm small block, and go from there if the motor is good.

Anyway you decide to go, you need to replace the transom, and plywood would be a better choice than foam, imo.

as for adding an aft cabin area under deck, you would need to assess the bulkheading and re-engineer some things, but that can be done too, most likely requiring replacement and repositioning of stringers and or bulkheads...

like I said, tell and show us what you have to work with, and we'll talk you thru most anything

 

[MTboatguy]

Foam for the transom? Not going to give you enough strength, if you are going to tear into it, why not do it correctly and use wood, like the factory did, you can build a new transom and seal it so it will last a long time, but it needs to be done correctly.

 

[littlerayray]

We love pictures take lots of em there's a wealth of knowledge on this site and we love seeing boats get restored good luck and have fun

 

[Watermann]

I would have to second the mercruiser upgrade with a good donor boat. I converted my Chief from an older MC keyhole to the newer Alpha One building new motor beds but it was tin work not glass. Having the donor to strip made the job very easy. All of these ideas and plans need to be inline with ones abilities and it will be a ton of work but very doable!

+ 1 here on quashing the foamy transom idea and in favor of some nice solid marine plywood.

 

[DeepBlue2010]

Agree with everything that was mentioned so far but also want to stress the fact that foam is NOT for transoms. The Transom is a key structural element of the boat designed to distribute the thrust force of the prop across a wide area on the back of the boat. Making the transom out of foam is asking for trouble.

 

[jbcurt00]

A 25ft boat w/ a cuddy AND a hull to deck height of 5ft?

Yep, pix and a make/model would be great.

 

[RiverNoob]

The boat is at a friends property right now. I'm a business person / yuppy apartment dweller.
I Will get some good pics for you guys soon as I'm back over. Since you guys are boat lovers I think you are going to like seeing this boat,, she's an eye-catcher with killer curves. The boat type is a 1978 Columbian, oh now that I look at the title it's 24 ft and 4 inches.... so specific.
Super Deep Vee,, I actually don't really recall ever seeing another boat exactly quite like it. Many similar ones, but never one just like this one, and haven't ever seen too many Columbians at all either,,, the ones I did see online were little 14-16 foot fiberglass skiffs.

Ok,,, So I'm gathering the foam transom was a dumb idea. So do Stringers need to be wood too then?
and my Chevy 305 runs like a champ,,.. and has a San Juan System on it. so you say I can still use it and all I need is the outdrive? outstanding!!

What outdrive would you guys recommend that I should look for you think? This isn't really a fast boat,, but any muscle that could be added with a certain outdrive might be good. This boat is quite tall and heavy set,, sort of a hulked out steroid version of a typical 19-20 foot fiberglass boat. It never looks as big in pics, as it seems to be in person, either,

Thanks guys...... I can' get there this next weekend because a friend is having 40th b-day,, but I will be back with more post questions and pics in a week and a half or so,,,

 

[bruceb58]

sterndrives.com used to make an adapter plate. Search for one of those. They used to be $700. Sure they are way more now!

I wouldn't even bother no matter how nice the boat is.

 

[Bondo]

Ok,,, So I'm gathering the foam transom was a dumb idea. So do Stringers need to be wood too then?

and my Chevy 305 runs like a champ,,.. and has a San Juan System on it. so you say I can still use it and all I need is the outdrive? outstanding!!

Ayuh,..... Welcome Aboard,.... I do these conversions quite often,...

Have done 'bout 1/2 dozen Highliner 222s, 'n got an Old Town Atlantis in the works, along with 2 more Highliner 222s,....

Yes, use plywood to rebuild with, not foam,.....

With yer motor, you'll be able to reuse the Long Block, 'n possibly the heat exchanger,....
Most all the other marine accessories will need to be swapped to match whichever drive you find,....

I've done All my conversions to Mercruisers,....
Mostly Alpha 1s, 'n Gen.IIs,....
The Old Town is gettin' a Bravo I,....

 

[Guest]

Hey Bondo! You old salt, You know of Hiliner 222s too? I thought I was alone!

I was going to say that I too , have rebuilt 2 rotten transoms (both Hiliners, a 74 Gypsy ( and 72 Adventurer that I own) One I kept a sterndrive (changed from old OMC Stringer to Volvo SX FWC) and the other converted to conventional outboard transom (200 OceanPro).

Bondo's right, you have to use plywood in conjunction with a good quality epoxy resin/cabosil and fiberglass cloth matte.

In between each "buttered up" plywood panel I applied (2) layers of glass matte and built up each transom 3" thick.

Its a messy job,and a little time consuming, but is not difficult and I would be happy to share my advice (as I'm sure Mr. Bondo would). One thing I can't emphisize enough, make sure you get ALL THE ROT out of the rear of the boat, if there is not wood left when you're done, you'll have to replace it. Saturate everything you replace with epoxy to prevent moisture absorption.

 

[TruckDrivingFool]

I think the OP's foam idea came from not wanting wood that would rot in the future, If that's the case search iboats/web for seacast.

 

[MTboatguy]

I know what he was thinking, it is not the first time I have heard someone think that foam would be a good alternative.

 

[RiverNoob]

Thanks for the tip,,, there is very little rot in the transom as of now, but I have gathered I should cut out the inside fiberglass area of the transom,, get every last fiber of the old wood out,, and then replace it with new, whole solid sheets...
This brings me to my next set of questions:

1) the current transom on the boat is only bout 3/4 inch, half inch plywood with 1/4 inch glass on both sides,, you are mentioning building up the transom multiple layers to 3" thick,, so does the transom thickness depend on the mercruiser drivetrain and what size wall it can fit in? or is there a good size range and you can do whatever thickness you want within that range? Also if you made each transom layer 3" thick, what was the total transom thickness?

2) Do you need to do the transom and stringers at the same time and does the wood need to be connected at all?

3) Can anyone recommend a good written or video tutorial?

thanks guys,

 

[zool]

1- Mercs require between 2 and 2 1/4" total thickness, same with the newer VP's, the older AQ series Volvos allow less strict tolerances, due to a lack of an inner shield. The keyholes can be different too, so you would want to secure a drivetrain, then build the transom to suit.

2- You really need to check the stringers and motor mounts for rot, this is done by drilling into the lower portion of the stringers in multiple places, and check the shavings that come out, check the motor mount pads at the bolts.

3-There are tons of transom, stringer, and mount threads here, some with vids, some with pics....

 

[ondarvr]

Foam can work well in a transom, but it needs to be a high density foam in the clamping areas of an IO or OB, the high cost of this foam is what limits its use in new construction. The rest of the transom can be lower density. Coosa board and similar poducts work well too, but again, the cost goes up. Other things can be done to strengthen the clamping areas, but it takes a little time to do it, and in production time is money.

A bracket with an outboard could work well, but the cost will make you cringe, good 250HP OB's aren't cheap, neither is the bracket.

If you have some rot in the transom and are going to need to do any work on it, it makes no difference in what drive you go with, you just seal the whole thing up and recut the hole as needed. Buy the best outdrive you can afford, make sure the model you buy can take the stress of a 25' ( minus a few inches) of heavy boat, not a 17' runabout.

 

[Scott Danforth]

the reason everyone uses wood is simple, it is cheap, easy to get at your local lumber supply, bonds well with fiberglass or epoxy, and if done correctly, will outlast your great grand children.

seacast will cost you about 2-3 times what wood will cost (BTDT).

foam will cost you about 4-5 times what wood will cost.

 

[Guest]

OMC "stringer" drives is kind of a misnomer. the reason your transom was only 3/4" thick is because the transom does not take thrust loading with this type of drive. The entire unit is on 4 "feet" and these abosrb the thrust.

My Hiliner hulls were also about 3/4" thick. As Zool states, most drives other than than the old OMC electric shifts require the transom for taking the thrust loads of the drive unit (like an outboard). As a general rule, the maximum is 3" thick. On a flat transom, you can use 3/8" plywood and "build" it up. Be sure you let each layer if wood/glass you apply fully cure before applying the next. Too much glass at once and you develop excessive heat and weak joints.

Also purchase an epoxy resin that leaves no "amine blush" after it cures. ( a by-product of the curing process) I recommend Aero Marine Epoxy for this. This will cut out extra work of having to wash off the waxy blush before the next layer is applied.

You should also invest in a Circular Depth Saw, sometimes called a "cabinet saw", this is a small circular saw with a 3" dia. blade that can be set at depths smaller than an inch. This toll will allow to cut the heavy glass laminate away from the rotted wooden core in one or two pieces. In my case, by cutting away the inner laminate of the transom, I could remove all all the rotten wood on the back side of the exterior transom. Sears sells them for about $115.

 

[RiverNoob]

Got it, I've been doing some research on the process, ordered a full face mask respirator for the job, and planning all the other materials, I will cut through the deck to get a better look at the stringers/etc next weekend, and I will get pictures to post then too. I have action camera's so I might also make some short walkthrough vids.

I need some advice for my donor boat selection. Mostly all I am seeing are Volvo penta 270 and mercruiser alpha gen 1 and 2's
with 120, 140, and 160 hp powerplants.

Are these setups sufficient/compatible for using with my chevy 305 small block? or would something like a bravo 1 or Volvo 290 be what I need to hold out for?

What would one of you guys who has done this many times look for primarily that will work with the chevy well?

Thank you,

 

[RiverNoob]

My chevy 305 block is a 225 hp by the way, V-8. I think its the same motor from the '78 Chevy Camaro, in a marine version.