Repower Engine Size and Type for 40 foot Yacht?

[Snowpetrel]

Originally Posted by zeehag

my 35 ericson has a 20 hp yannie and runs well...my 41 formosa has a 4-108 perkins and i LOVE it....
good luck. dont forget the stringers and placement for engine--and re alignment and such other lil things to do when repowering...

Hi Zee, I considered a 4108, and if I could find one nearby I might even go for it, but the weight and size would be a struggle, and they might need some serious engine bed modifications. They are tough engines, perfect for a heavier boat like your Formosa. With luck the smaller kubotas will be able to be adapted to fit the old beds, need to do some serious measuring. I know the Beta 30 is a drop in replacement for the Bukh 20dv, so given no howls of outrage at me going for a smaller engine, maybe I should consider it.

As it stands the current engine need some serious alignment so replacing the engine will save me that job at least, Although not looking forward to that job with the new one! Might have to call in a few favors, my old man being a dab hand at shuffling them into place.

[S/V Alchemy]

Originally Posted by Snowpetrel

So at least torque wise it should be OK? need to run some figures and see if I need to go duplex or upsize the shaft to 1 1/4, providing the stern tube will take it.

Cheers

That depends on your ambitions and your definitions of "OK". I went from a typical 316 SS shaft to one made of Aquamet 22 recently because I have heard of SS fracturing over time or with the forces created by shifting feathering props (I have a 4 bladed Variprop on a repowered boat launching at the end of April).

30 HP and one-inch sounds fine and if you can "recycle", it will definitely keep your costs down, as reboring a stern tube is pricy. But I would run the numbers to make sure.

[banjoship]

In terms of placement of the engine, WM Diesel will custom make engine feet. I provided them with drawings and measurements of my existing engine bed. With the engine feet and mounts they provided, the engine slotted straight onto the existing engine bed.

In making up drawings, measure everything very carefully, look up the measurements for the engine and gearbox you have chosen, and work out whether the engine will be relocated. Take into account new output shaft, if used, etc and etc. Best to go through your calculations and measurements several times.

If keeping your old shaft, its a good time to take it along to a machine shop to check for true, and end for end it on refit. A flexible coupling might be a good addition??

I guess you are in the process of assessing engine performance curves, engine compatability with gearboxes, prop speed, etc. In my case, the new engine has performance curves are very similar to the engine replaced, so I kept the old gearbox and prop. The gearbox was completely rebuilt. Check that your existing fuel lines are adequate for the new engine. May be a good time to rebuild the fuel system??

And of course, there is the issue of how to physically get the engines out and back in the boat. The list goes on.

Lee

[Snowpetrel]

That depends on your ambitions and your definitions of "OK". I went from a typical 316 SS shaft to one made of Aquamet 22 recently because I have heard of SS fracturing over time or with the forces created by shifting feathering props (I have a 4 bladed Variprop on a repowered boat launching at the end of April).

Interesting, had forgotten about the shock loads folding and feathering props create when they go into gear. Thanks, shows why it is good to get outside input.

[Snowpetrel]

Originally Posted by banjoship

WM Diesel will custom make engine feet.

Very good to know, Still need to measure up. The quote from them was reasonable, and they supply a PRM box. Cheers