Inboard options 6.0 vs 5.7 vs Diesel

[OSEAD23]

Hi

I am considering refreshing my power on my 23 IB. My carb 5.7 is running Ok but i would like the economy of a MPI or diesel and I'm ready to step it up. some of my questions are:
Has anyone gone with a 6.0 crusader or similar in a 23 IB? i would think the 6.0 with all the extra torque would allow me to turn more pitch and get more speed. My crusader is a 270Hp will i feel a worthwhile difference with a 330Hp MPI 5.7? I have heard guys complain about the performance with new diesels like the yanmar BY any real world experience?

I would like to get a real world cruise in the 25 knot to 27Knot range if possible. today i feel like I'm pushing the 5.7 to hard to run continuous at that speed. Maybe it me babying it but i likes to run at 3k to 3200 rpms and thats about 22knots +/-. I top at 31knots today at 4300 rpm.

Jim

[77SceptreOB]

Beautiful boat!! Your WOT at 4300 rpms seems a little low for a SBC. Maybe you are swinging too large a prop?. On my standard 454 i was 4500-4700 WOT

[OSEAD23]

thanks

i have a few props and yes i have 16x16 that will run to 45-47 and a 17x17 that will maybe hit 4400. the bottom line though is the speeds are similar just i hit the cruise numbers i gave with the 17x17. the other prop everything happpens at a little higher rpm.. I know the 16 is better for the engine longevity

jim

[Briguy]

Well a diesel will cost 30k all said and done. Even a 454 mag will only run a knot or 3 more.
Yes increase your HP a little but don't expect miracles. Do what's cost effective.

[Islandtrader]

Can't say what difference an inboard will make. However on my 6.0 mpi stroker i/o merc.
At 3000 rpm I am pushing 27-30 mph. With a burn rate of 8.5 gals or 3.35 mpg.
Top rpm is over 5k an 50 mph.

For what it's worth no matter what the configuration is. A 350/383 will burn around 8 gals per hour. I have had 3 boats, 3 different setups and 3 different lengths, and have put on over 3,000 hours between them. When I go back and look at the logs...different cruising speed...but same burn rate.

[Outrageous]

I would prop for the best cruise / speed and run it at 3600 - 3700 RPM. It sounds like your are nursing it a bit. Diesels are great if running long distance offshore or putting on a lot of hours every season but hard to justify the cost if that's not the case. The 383's are a great upgrade over the 5.7 in the 23 IB and easy on the wallet in comparison to the diesel.

[OSEAD23]

I want to thank you all for the information. I have been leaning on going with the 5.7MPI for most of the reasons you all mentioned so thanks for that. I wanted to make sure I wasn't sacrificing additional performance potential with the other options. I know this wont be as fast as an outboard or an i/O but a few Knots are nice on a 60mile run each way as we often do.

I spoke to some one who has designed Hulls and race boats today and he mentioned that one of the reasons the inboard pocket boats runs slower is that the pocket and prop work together like an enormous pump sucking water into the pocket and essentially causing you to carry hundreds of gallons of water with you as you move along and at a certain point the horsepower and torque as useless in creating more speed or forward motion due to this. He did mention that ventilating the pocket is something that has been shown to increase speed as it breaks the suction in the pocket.

Again I am not looking to make the boat something it cant be just the best it can I dont get to use it enough on blue bird days and it would be airborne most times on most other days because of the sea conditions.

At this point its looking like a new 5.7 mpi

Jim

[Briguy]

How exactly do you ventilate a pocket?

Originally Posted by OSEAD23

I want to thank you all for the information. I have been leaning on going with the 5.7MPI for most of the reasons you all mentioned so thanks for that. I wanted to make sure I wasn't sacrificing additional performance potential with the other options. I know this wont be as fast as an outboard or an i/O but a few Knots are nice on a 60mile run each way as we often do.

I spoke to some one who has designed Hulls and race boats today and he mentioned that one of the reasons the inboard pocket boats runs slower is that the pocket and prop work together like an enormous pump sucking water into the pocket and essentially causing you to carry hundreds of gallons of water with you as you move along and at a certain point the horsepower and torque as useless in creating more speed or forward motion due to this. He did mention that ventilating the pocket is something that has been shown to increase speed as it breaks the suction in the pocket.

Again I am not looking to make the boat something it cant be just the best it can I dont get to use it enough on blue bird days and it would be airborne most times on most other days because of the sea conditions.

At this point its looking like a new 5.7 mpi

Jim

[Bushwacker]

Originally Posted by OSEAD23

. . . I would like to get a real world cruise in the 25 knot to 27Knot range if possible. today i feel like I'm pushing the 5.7 to hard to run continuous at that speed. Maybe it me babying it but i likes to run at 3k to 3200 rpms and thats about 22knots +/-. I top at 31knots today at 4300 rpm.

Jim

Hey Jim,

I'd suggest you install a vacuum gauge on that rig, as it will give you a real good idea of how hard you're pushing the engine. Although a limit of 5" Hg is typically used as "Max Continuous" on HD aircraft engines, very few marine engines have sodium filled exhaust valves, stellite valve seats or positive valve rotators like the AC engines, so a limit of 7" is probably more appropriate for your engine.

Although the tunnel drive eliminates some of the loss most inboards have due to the steep shaft angle, prop efficiency is probably less efficient than an outdrive with a prop running on horizontal shaft in cleaner water. So I'm not sure that you would necessarily benefit from a bigger engine as much as if it were a sterndrive. Similar situation to muscle cars in the '60's, where engine power easily overwhelmed the tire technology available at the time! Your inboard configuration (NICE rig BTW!) may not have the speed/efficiency potential of an I/O, but it sure is hard to beat the CG/ride benefits and simplicity/reliability of a straight inboard! If you run offshore very much, I would think a softer ride and much less maintenance is a good trade for a few kts of speed! Denny

[pelican]

here's my 2 cents:

5.7 merc,MPI motor - put the correct prop under the hull.FORGET THAT SPEED !!! you want speed ? buy a contender/regulator/yellowfin...you have an inboard powered boat,not a twin outboard powered rig.
attempting to raise your cruise speed by upping prop pitch/diameter isn't the way to go.
ride of the hull suffers - time to plane suffers,and you're killing the motor,along with killing efficiency of the motor - you're sucking fuel like a jet...
running a gas motor,that's overloaded is kinda like holding your feet to the fire...the motor is overloaded,you need to open the throttle further,to get rpms - this causes the exhaust temp to rise - leading to lots of potential problems: detonation,valve failure,etc...all of which spells disaster for the motor....

diesel sounds good - but in a smaller hull like your's,it's not cost effective to spend that kind of money on diesel.stick with gas,gas motors when set up correctly,are incredibly efficient and reliable
little more to it than just dropping a diesel in - you're gonna need to modify the exhaust,bigger diameter is needed.you're gonna need to add return line fittings to the fuel tanks - this could turn into a big pain in the butt - you've got wiring,batteries - lots more to it than meets the eye

i'm not a huge fan of the stroker motors - seen quite a few problems internally with those...
i'm also not a fan of "reman" or "long blocks" - I like NEW,as in NEW,complete motors


speed:

everyone is obsessed with speed :

truth is,20/22kts in normal conditions is a good speed - you're not beating up the boat,killing the motor and beating up yourself.the 23 center console inboards,these ride the best !

speed costs money - increasing speed requires increasing both hp and torque ... there's a huge difference between that old 270 crusader and a new MPI motor...I believe you'd be very happy with a new MPI 5.7,propped correctly...