shaft size

On the 56' Hatt I run (74,000#) we have 2 1/2". Are there no shafts on to compare?

horsepower is your most important consideration when picking shaft size....how much HP do you have? I have seen 2 1/2" shafts in everything from a 75' Jim Smith with 2000hp engines (splined shafts/no keyway) to 2 1/2" shafts in a 45' cabo with 1000hp c18's

Pushing a 60' boat with 24" props, really? (not being a wise guy, just surprised).

As other poster has pointed out the largest factors are HP and shaft length; as well as number of supporting struts. Generally on higher speed vessels with 2100+ HP the shaft may be 4 inch. 1700-2000HP 3 1/2 inch. 1250-1650 is 3 inch. I base this on sportfish models I've worked on. IDK about 60 footers with 120 HP.

Too big is much better than not big enough. Well, you have a lot of reduction and swinging a large wheel (for the HP of engines) so it is going to put more stress on the shafts. Shaft length and number of struts are another consideration with shaft size. What model detroits do you have, and are you sure of their HP.

Sounds like 4-53's and I agree 2.5 is far too large unless the length is the reason. However it is a non planing vessel so reducing the shaft diameter will do little to increase speed or efficiency which would pay for the change.

It seems odd to change what was designed for the boat. Could you explain your reasoning or what you're hoping to attain?

Considering that most manufacturing standards today are based on 'how little can we get away with' I'd be hesitant to reduce the size of a shaft from that recommended by the manufacturer unless I was highly qualified to make such a determination. Would hate to jam the throttles to beat a swift current or touch a piece of flotsam and have a shaft snap like a twig.

And you are going to spend 10,s of thousands to change the struts, propellors and the coupler as well as buying new shafts on a boat that is decades old????

Call Frank and Jimmy's in Ft. Lauderdale, give them all of your specs and they should be able to calculate what shaft size you need. Other than that, why waste our time with your dispute with someone else. The engineers who built your boat decided on that shaft size for a reason, and you are obviously not more knowledgable on the subject than they are.

Shaft size determination can be scientific or it can be whatever the design team at the builder decided on. On the Donzi they decided to go with a 4 inch shaft on the 65's with 16-92's rated at 1450 HP. They "overbuilt" the whole boat so they went with a oversize shaft. At 1450 HP Viking goes with a 3 inch shaft. There is no real reason a sub 300HP engine needs a 2.5 inch shaft. Sometimes the builder does thing that defy reason. Just look at the threads about the lack of ease in boarding some newer boats and putting the helm electronic in the direct sun...