I am ready to cut the transom hole for my Merc III drive.
This is the big older drive with the 1 1/2" shafts. Can anyone advise me if I can use the mercruiser X dimension specs for this drive.
It is a lot deeper to the C/L of the 20"prop. Thanks, Barry

x-dim is meaningless without length of drive. it is all about getting prop where you want it. start from there, and go up. find spot where crank center would be, and start measuring from there in the engine compartment to see if engine fits under hatch. if, not, then you have to decide whether to compromise on prop position or raise top of boat for engine clearance. we don't know your hull, power, what you wanna do with it. you need to do some research on prop location for your set-up. can you use the spec? yes. do you want the prop that deep?

I am installing a Yanmar 315HP diesel with a Huber1450 Velvet Drive.
The hull is a semi-displacement V. The trany and Merc III will be connected with A jackshaft w/steady bearing (7').
I am looking for guidance how high to make the cutout in the transom. The Merc III is 25 1/2" C/L input shaft to C/L prop shaft.
Is there a general rule as to where the prop shaft or cavitation plate
should be above or below the hull btm.

sounds more like a TR drive.
when you say older drive, i think of the big E,and H, drives from mercruiser.

lets get a picture of what you got

Back in the day the general rule was cav plate 2" above the bottom. Then adjust either way depending on the application. I am thinking down an inch for that boat, maybe more. What are you going to use for a prop? They are pretty much extinct as far as the stock Mercury props.

No, There was a III clubfoot, which came before the III SSM series. When the SSM came along, it used the III upper and gimbal. It was sort of a good idea, a production heavy-boat drive that could honestly handle 500 hp or more. I don't know why they never became popular, as I wasn't even born yet when those drives came out.

http://www.mercruiserparts.com/Show_...LY%2C+COMPLETE

i did not know that,
those have to be some pretty scarce parts

Yes, probably the lowers. The uppers should be easily had. When MBAM checks back in on this thread, he should be able to tell us everything there is to know about them.

nobody here does any homework on prop locations on semi-displacement hulls. i would think deeper. they usually have props sticking out the bottom with rudders behind...you are not gonna be worried about bow lift, stern lift, what happens when prop blades break the surface. i think you want top of prop below the keel at the stern for cleanest water flow over the prop. that being said, you wanna ask this over at the hull truth. these big old drives never became common because with a big heavy boat, a straight shaft works as well and is cheaper to build and WAAAAYYY less maintenance. big heavy boats don't tend to be pulled up on trailers at the end of the day...

Thanks all for the replies.Although this is not a go-fast boat,the install that I am doing is not unlike a TR, just bigger. The reason for the conversion is that I cannot match the existing shaft angle and keep the Yanmar diesel below the saloon hatches. I have 3 Merc III's on hand, some parts are still available but install info is scarce or not available. I was looking for guidance as to the height from prop C/L to btm of hull. Thanks,Barry