Sterndrive replacement and placement considerations

[hoxha]

Hello
I'm new here. So first I'll fill you in on my project: the overhaul of an old DDR eastern german patrol boat, known as the 'Grenzboot'.

Original build date: 1980. 32 foot long. 3,5 metric tonnes heavy (7,716 pounds) when made of very thick and heavy polyester. Multiple seperated compartments. Originally motorized by 2x140hp russian V8's. Fuel supplylines, hydraulic and electrical system where all made in a dual setup. So you would always have 1 working system if something should fail.

You have to look trough the clutter I think it's a great looking fastboat :-) I'm also on a quest finding the original building plans and information which is stored in German military archives. I'm loosely thinking of building a complete new one after we are finished with this overhaul.

Were replacing the durable aluminium but VERY thirsty V8's with two V6 Mercruiser 4.3 MPI and two Alpha drives.




I have a question about this engine replacement, see the picture:


We have to fill the holes and place the new and smaller Mercury transoms. But the motorsupports/stringers are placed slightly offcenter outward.

So, the new sterndrives will be placed more outward and a bit higher. Which I think is a fast solution but perhaps not the best.

Are there any things to consider before changing the placement of sterndrives+engines? Knowing the rest of the design, everything in this boat is perfectly tuned. And placing engines more outward and higher could expose them in a tight turn.

the stern is 5cm thick multiplex sandwiched between polyester.

Any insights, or formula's are greatly aprecciated!

[gonzo]

The most crucial positioning is the vertical. The anti-ventilation plate bottom should be about 2-4mm higher than the bottom of the boat.

[CDK]

The Alpha-1's come with a stencil plate printed on the carton; that should help you with the positioning. The manual also has a chapter about it; if you don't have the manual I can post the relevant pages here.

The engines stand on front supports only, you may have to laminate wooden blocks next to the stringers if they are not in the right position or weld offset engine supports. As Gonzo stipulated, the vertical position is crucial.

[hoxha]

Thanks, very useful! I've seen and heard some very global things about sterndrive height, but looked into it. I've decided to do some reverse engineering coming week. Measuring the whole back and enginebay of the boat and putting it in a CAD-file, for exact placement.

I've made some simple Sketchup drawings of the back of the boat. The old transoms are way bigger (russian) then the new mercruisers. So we have to make the hole smaller:


I was thinking of this solution because the multiplex is in excellent condition. Not even a smallest amount of rot.
1. removing a bigger square of the polyester sandwich on the outside
2. sawing out a smaller square under 45 degree angle
3. put new 50mm thick multiplex with same 45 degree angle
3. glue the wood together and layer it on the outside with blankets&epoxy
4. layer it on the inside
5. saw new hole for smaller, new transom.

advantages
-force of forward thrust is always 'pushing' the construction together
-we dont' have to remove the whole backside and replace it (safe $$$)
-construction should be as strong or even stronger

downside
-more work then simply cutting out new hole next to the old and put a big metal plate in the back and filling up the old hole.
-it looks and feels strong, no exact science involved.

@CDK: engines should be arriving next week, I expect all the information and templates to be included.