I dont think that changing to a larger size exhaust hose is a good idea, an extra 3/8 dia is a large increase of water volume and a lot more for the exhaust gas to displace. Some here may no more about the physics but the engine was designed by an expert who selected 2" for a reason.

 

Your solution of fitting one hose on the other is not bad. Another possibility is to use a standard pipe reducer and nipples. 2 inch pipe is actually 2.375" OD and 1 1/2 inch pipe is actually 1.9" OD. Finding them in materials you like might be an issue.

 

I've seen fiberglass adapters made for marine exhaust systems that are used to connect two different diameter hoses. You might do a search online, or check Fisheries Supply in Seattle (I think that is where I saw them).

Here you go, found them. Scroll down to "reducer couplings".
http://www.centekindustries.com/fittings.html

 

You buy the bronze shaft. Ship it to me and and pay to ship it back. Shoot me dimensions you want and I'll turn you some on my lathe.

That size bronze shaft might be found at a yard as a bent prop shaft.

Going to be an expensive piece of metal and much of it will end up as shavings.


Sent from my Kindle Fire

 

On my Cal 28; I used copper sweat plumbing pipe of what ever size I needed and sized up with adapter to go in Plastic muffler . Used hose, double clamped, Works fine! All out of plumbing supply house.....Dale

 

When I bought my boat, the exhaust outlet from water lift was made of a smaller diameter hose slid into a larger diameter hose with two hose clamps (the od of one hose was the id of the other). Did not leak water or exhaust but I never liked the connection. I found some bronze fittings at west marine and made a hose to hose connection and fitted that. That has worked well. My water lift is two inch outlet and the od of the connected hose was the ID of the 2 inch hose, west marine had these fittings in bronze.

 

I would think a 2-1/2 inch fitting would work. buy a 2" pipe to hose fitting and a 2-1/2 inch pipe to hose fitting (bronze) and connect with a 2-1/2 x 2" pipe reducer.

Reducer available here (COUPLING REDUCING BRONZE 2 1/2" X 2" 124510
Item #: MTL-44-462)

As well as the tailpieces (hose nipples):
TAIL PIECE STRAIGHT BRZ 2" 116421
Item #: GRO-TP-2000
List Price: $60.00 / Each
Your Price: $48.99 / Each

--------------------------------------------------------------------------------

TAIL PIECE STRAIGHT 2.50" BRONZE 116422
Item #: GRO-TP-2500

:

http://store.hamiltonmarine.com/browse.cfm/bronze-pipe-fittings/2,808,51.html

http://store.hamiltonmarine.com/browse.cfm/bronze-pipe-fittings/2,808,51.html

Regards