Diesel Conversion 23cc
 

Greetings,
I am nearing an agreement to acquire a potter series straight I/B 23cc. I currently own a 240hp yanmar with a bravo 3 duo-prop outdrive. The engine and bravo 3 have less than 100 hours on them. As I would have to remove /sell the bravo 3 and refit the engine with a new bell housing a configuration to straight I/B is possible.
Can I get opinions on the suitability of repowering a straight I/B (currently 350 260hp) with a yanmar 240hp? Will this unit provide reasonable performance or would I be setting myself for disappointment? My expectation is a cruise in the mid to low twenties and a WOT somewhere shy of 30kts.
Thanks in advance for the wisdom.

I would first check the weight of what your pulling out vs what you are replacing it with.
The diesels tend to be heavier to just make sure you are not adding too much more weight.

As for the comparison you will want to look at the TQ curves for both and see what RPM will work best.
Prop choice will also be a big player on what the setup is capable of.

Thanks. The engine weight along with the required transmission will be heavier in the 200-225lb range.

The yanmar 240 is the ideal diesel replacement motor for the 23 IB. I would love to put that very motor in my inboard -- it is a shame that it is no longer available for sale in the US.

Several inboards have that motor in it and the extra 2-300 pounds is not an issue.

You are correct about no longer being available. My understanding is that the yanmar 240 didn't meet the US emissions specs and they stopped importing them a few years back...fortunately parts are in good supply because Mack Boring was putting them in everything.

I'm in the process of putting one in my 23 inboard now. That motor was designed to replace a big block V8. I had one in a 28' x 10' lobster boat and it cruised at 22 kts and topped out at 28 kts. It's an awesome little motor. My calculations show it should cruise in the high 20's and top out at 30 +- My motor is at my mechanic's now just to check it over. It should be in the boat in the next couple of weeks. I've gutted the boat and I'm in the process of getting the bilge the way I want it.

The 240 Yanmar is an excellent motor for a 23.

My bigger concern would the the engineering needed to install a straight-drive inboard in a board origianally made for an I/O. The easy fix is filling the hole from the Bravo3. The harder issues are engine install under the console, mounting transmission, installing the exhaust with a surge tube and 12" of riser elevation (lots of guys, including some here on CSC do not properly install an exhaust .... the consequences of their oversight can be significant), installing the rudder and hydraulic lines.

Lots of work and I'm not ceratin that the benefits of a straight drive inboard are worth the costs/effort/time over the performance seen with a 4stroke outboard.

I think Harper's boat is also a straight inboard like mine. There are no real issues installing the Yanmar. It has the same exact footprint as the 350 V8. Pretty easy exchange. It will even bolt right up to the the velvet drive transmission, and doesn't weigh much more than the old gasser.

Hard to beat 9 GPH at close to 30 kts. with the simplicity of the diesel straight inboard combo!

Just fired up my diesel (v6 tdi 225 hp) in my yard. Its a i/o hooked up to a bravo 1x. I changed it from a 350 mercruiser alpha. Only ran it last year for 5 hrs, need to make sure I have enough height from the static waterline. If not I'm going to have to get a riser made. The engine came with the exhaust pipe and bravo coupler so the height was predetermined. Anyone know of anything that will work such as a flapper ?

Highway, you are correct. The potter 23 is a straight inboard with factory tunnel drive / shaft etc. can you tell me what transmission and drive ratio you are running? Many thanks!

Yes, it's a velvet drive 5000 8 degree down angle 1 1/2 : 1 . I talked to Mack Boring in NJ and they sent me the torque converter and bell housing I needed. Watermen around here swear by them on their crab boats. My guess is it will need a 17 x 19 wheel.

It's taking forever to clean up the bilge. The glasswork in those old boats is a little rough to say the least, and I'm glad to get rid of all that waterlogged foam!

I'm cutting down the motor mount stringers and eliminating the boxed out section next to them, so I can add aluminum angle with backing plates bolted through the stringer for the motor mounts. The stringers were wet, but in good shape. No rotten wood.

Once I test fit the motor, I'll take it back apart and glass everything back up.

I have had both velvet drive and ZF trans.
Both great and hold up great.

If you are redoing the bilge try the bilge coat product. Worked great on my shamrock inboard.

Mac Boring sells a 6' exhaust extension for the motor. The extension plus a rubber exhaust flap on the transom and you will be fine.

My boat has a 22 - 27 kt cruise and will plane out at 11 kts.

My only complaint is low speed vibration from the small diesel. I'm going to try different motor mounts to see if it helps.

Sounds like a great swap but I bet you will miss the trimable drive.

Mercury now has this engine and they changed the riser. I just installed it with a back pressure relief. I just ran it in my driveway..Going to drop it in Sunday.

Hey Harper how did you make out with the inboard purchase?

just bought a 23 seacraft inboard with a 350 small block can you guys tell me the performance numbers for this rig

What's the hp rating?

as a general rule -- the gas inboards cruise 20 - 22 knots and diesel 27 -- 28 knots.