Repowering a 54' Donzi Sportfish

Here is the shaft calculation from the Western Branch Metals catalog: http://www.wbmetals.com/



As far as the Safety Factor goes, ABYC recommends 5.0 for ALL Diesel installations. It has been my experience if you go below 5.0 even to 4.0, you will experience premature shaft failure/breakage. If I had to design and be responsible for shafting, I would not go below 5.0 , even though you have been ok at 4.68. I could not recommend a sf below 4.0. The SF of 2.0 is for a gas engine installation, fyi.

If you haven't plan on the exhaust yet, I know a guy.

All the above numbers are based on AQ 22 HS (High Strength), not the lesser strength normal AQ 22.

AQ 22 HS has a yield strength in torsion of 70,000 psi for shaft diameters from 2-1/4" to 6". See the last column
Regular AQ 22 is at 63,000 psi up to 2-1/2", then drops to 50,000psi from 2-1/2" too 3", requiring an even larger shaft diameter.

For AQ 22:


I have no idea of the cost, contact them to see who is the distributor in your area.

My struts where replaced, don't know if it was possible to bore them. The shaft logs were able to be bored out to except the larger forward shaft bearings

More food for thought.

Keep in mind that there is a relationship between shaft diameter and other key geometry items in the propulsion system.

A selected Shaft Diameter has a specific Taper Length, and this needs to be checked with the propeller hub length when changing shaft diameter. If unchecked, and the propeller hub is a bit short in length for the larger shaft diameter, you will have a larger gap between the forward face of the propeller hub and the aft face of the strut barrel. You want this dimension to be around 1/2 of the shaft diameter, no greater than 1 x the shaft diameter.

SAE Propeller Shaft Geometry:



Cutlass bearings have a geometric relationship with shaft diameter as well. The length of a cutlass bearing is usually 4 x the shaft diameter, so a cutlass bearing for a 2-1/2" shaft is 10" long while a cutlass bearing length for a 2-3/4" shaft is 11" long and for a 3" shaft is 12" long. The strut barrel may or may not be able to take a standard length cutlass bearing, as it can be just enough to accept a 10" cutlass bearing, so you may have to machine an non-standard bearing length for a larger shaft diameter if this is the case. Get the tape measure on this. Their OD varies as well. For a 2-1/2" Shaft, a Cutlass Bearing can have an OD from 3-1/8" to 3-1/2" depending on make and model, a 2-3/4" Cutlass Bearing is 3-3/8" to 3-3/4" OD and a 3" Cutlass Bearing is typically 3-3/4" to 4" OD.

Cutlass bearings:

http://www.duramaxmarine.com/pdf/CutlessBearingMS.pdf

The best case is to use the existing strut with a larger cutlass bearing. I believe the Donzi 54 repower went up in hp significantly so the strut up-grade makes engineering sense in that case with the increase in power and speed. Your remaining in the same ball park, hopefully it works out for you after you inspect it at haul out.

From a design and engineering standpoint, the 1.971/1 gear ratio is desirable for the following reasons:
1. Reduces the propeller tips speeds, they are too high at 1.6/1 IMO.
2. Reduces the design cavitation number
3. Allows for a better propeller design selection - better blade loading
4. Should be smoother
5. Should be quieter/less cavitation
6. Should have an efficiency advantage at cruise, on both speed and fuel consumption
9. Will give you a little more "bite" during docking/maneuvering

PacBlue, that is some great info. Now I find myself rechecking that everything was done the way it should have been.

While you are currently in discussion with ZF, I would take it one step further and ask them what would be the optimum gear ratio for your repower? It wouldn't hurt and it would give you another engineering point to consider. They would have to have the usual existing performance data, weight, and current propeller.

Donzi most likely selected the original 1.6/1 ratio to keep builder costs down with a 2-1/2" shaft system. A 2-3/4" shaft is fine, maybe a little unusual to source, and a 3" shaft system would have been even more builder's cost. I am sure they did not sit down with their supplies to create an optimum system, surprising as it may sound.

Your trade-offs for this repower would then include:

1. Standard 2-1/2" System with a less than optimum 1.6/1 ratio.
2. A modified 2-3/4" Shaft System with an upgraded 1.971/1 ratio.
3. Open question to ZF - what is the optimum gear ratio and propeller match for my repower. This may in turn validate item #2 or give you something else to consider based on your repower budget, project timeline and the ZF response.

Great project!! Maybe I missed it, why are you adding only a 9kw backup generator, and not a 2nd 20kw?

Here is another place you may try. Commercial trade paper, may even be buyers there

https://www.boats-and-harbors.com/

Never dealt with these guys, but they have been around for awhile:

http://www.goldcoastpower.com/