Extending Stringers

Its repower time I'm going to give it a go at doing it my self. Only one way to learn.

I have a 4.3L I/O going with bracket and outboard.

I do know I should re-enforce my transom by extending my stringers. I plan on cutting out the mounts and the pad (I don't really know what purpose it served) at the transom. I'm not able to see the rest of my stringers as there is a bulk head. I don't plan on cutting out my floor as its solid.

How tall should I make my stringers I don't really plan on converting the area to anything for now just add a flat cover.

 

There is no need for you to extend the stringers. You only need to fill in the hole and laminate it properly.

 

So guess all those guys that add stringers just went overkill then?

Music to my ears a lot less work. I may just leave the mounts then and use it as something to work with for a small storage area.

 

If you think about it, the sterndrive is pushing at the transom. The connection between the sterndrive and the engine is via a spline sliding coupler. Longitudinally, there is no force applied. The bracket is going to apply thrust at approximately the same location.

 

True was more concerned if the weight of the bracket and outboard would be a concern.

But we are talking less than 1000 lbs and I would be getting a bracket with a swim step so the weight is distributed a bit more.

 

I doubt you will miss that lump of cast iron growling away down below eating up money.

Some bracket installations throw a lot of spray up around the powerhead, particularly the ones that are just a tubular frame with no spray deflector across the bottom. For every gallon of fuel swallowed approximately 15 gallons of atmospheric air goes through the engine (not as "through the engine" as with a two stroke mind), and what ever is contained in that spray will end up inside the engine and cause damage, mainly while at rest, and there will be little you can do about it. There were some popular leisure/fishing catamarans manufactured and sold here called Kevlacats that generated huge volumes of spray through the tunnel and the Yamaha two strokes that were fitted at the time had closer tolerances than the American outboards, and consequently were a bit quieter and ran a bit better, but they had no resistance to water entry and threw connecting rods regularly. Some of the old Mercs and Johnnyrudes would cough, fart and splutter, but get you home with a corrosion hole through a water jacket. Kevlacat sorted the issue by fitting ducting to the Yamaha lids at the air intakes so that they pulled their air from inside the boat, but Yamaha were moving a lot of replacement powerheads in the meantime.

 

Personally I would be concerned about the weight of the outboard especially since the bracket will put the outboard farther back. I am sure it can be done, I also hate I/O, just need some more investigating in my opinion.

 

@willy13 it is commonly done these days with the availability of well made large four stroke outboards. Those big lumps of cast iron were mainly installed because big two stroke outboards were extremely thirsty, using about a tenth of their horsepower rating in GPH at full throttle, so a 300hp could drink 30 US gallons per hour with the stick down.

It is common to have Mercruiser-powered moored boats taking water, with some sinking, over winter due to the universal joint bellows tearing or being cut, and allowing water into the hull, then the batteries go flat pumping it out and down goes the boat. It also takes out all the bearings associated with spinning shafts on the stern drive unit and other expensive components. I firmly believe that Mercruisers should not be moored, Volvo Penta is a better proposition for this, but they have their issues also.

As far as the leverage of the outboard goes, it will be vastly lighter than the sterndrive it replaces, and if the outboard mounting bracket has sufficient sealed air space within it will provide some reserve bouyancy, as a problem with some pod mounted outboards is that with large waves coming from the stern, particularly when heading astern, water can enter the outboard cowling; as the hull will not start to rise until the wave reaches the original transom unless there is sufficient bouyancy in the pod.

 

I'm still working with my outboard guy.

Plan is to remove the stern drive sooner than later and seal the transom hole. Then drunk it without the motor and bracket and see how she sits with people standing at the stern. I'm hoping it will be ok if not plan B is to jackplate the motor on the bracket to keep it out of the water more. But I've seen motors that were excessively high on the jack plates and they look like crap.

I can't do much weight distribution beside move the batteries to the front. Fuel tank is in floor down the center and I'm not cutting the floor as its solid.


I do plan on boxing out some of the bilge area where the motor was for a small storage. Assuming that would provide a little more strength all around but not counting on it.

 

I am aware its done all the time, and the builds I've watched involved beefing up the transom.

I don't see how a sterndrive is heavier than an outboard.

 

Well you could do a search and then I wouldn't feel obliged to do it for you.

He is swapping out a V6 Mercruiser, which are sized at 4.5l now but were 4.3l not that long ago, and the chart I found is for the 4.3l. It gives 840 pounds for the assembly, which is ~380 kilograms.
Mercruiser Sterndrive Engine specifications https://www.sterndrives.com/merc/sternspecs.html
I am assuming Suzuki for the outboard, and the 300hp XL model comes in at 639 pounds or ~290 kilograms, so the weight of a big man lighter than the sterndrive and, depending on the Mercruiser model, ~100Hp more powerful. The 200Hp Suzuki is 529lbs for the extra long model, so 300lbs lighter than the sterndrive for similar horsepower. I will trust you to find the 300 yourself if you want to check it.
Suzuki Outboard Motors I Inline 4 115-200 hp I Suzuki Marine https://www.suzukimarine.com/outboards/115-200hp/

I will tell you why, the sterndrive has a cast iron cylinder block, inlet manifold and cylinder heads, while the Suzuki is all alloy apart from the bits that need tougher metal, The sterndrive has two very heavy cast iron water cooled exhausts, a heavy assembly to attach it all to the transom and a heavy bevel drive gearbox that is needed because a sterndrive is a "Z" configuration whereas the outboard is a much simpler "L", The sterdrive has other components that the outboard does not need like a pair of universal joints and yokes to contain them. Petrol sterndrives were a necessary evil in the days when two stroke outboards were limited in output and very thirsty, but with four stroke outboards getting bigger and bigger (Mercury has a 600Hp V12) I feel that the market for petrol sterndrives will decline to the point they are no longer manufactured. Diesels will continue for a while longer as diesel outboards are still quite unreliable from what I have experienced and read.

 

On beefing up the transom, I agree, and I would put in a couple of knees between the old bilge and the transom, probably shape them to be a fishing rod rack or some such where they emerge from the new floor or hatch that will be replacing the engine cowling. He will be able to put seating across the stern now too if so desired.

 

Theres the confusion, I consider the "sterndrive" to be the part that is bolted to the transom, the part that puts forces on the transom. The heavy cast iron car engine sits in the hull of the boat. Not really entering into the equation of transom strength.

 

Totally agree.

 

Please stop making assumptions, it is dangerous in the marine environment. Firstly you didn't reference transom strength when you talked about weight, and considering it to mean the overall weight of the power unit is reasonable as this is far more applicable to the situation if the power of the two is similar. The engine is bolted to beds connected to the hull and transom, and also by two bolts to a bracket bolted firmly to the transom, so directly to the transom via the transom plate, and indirectly via the beds. On the terminology confusion, that is what is used wherever I have been in Australia and it is the only reference I have, the actual geared transmission part of the works is called the "leg" or "sterndrive leg" here, and the drive configuration, including the engine is called a "sterndrive". This situation seems to often occur due to many forums having an international audience. By the above logic of just considering the leg, we should be weighing the outboards sans powerhead also.