Inboard options 6.0 vs 5.7 vs Diesel

[Bushwacker]

Originally Posted by pelican

i'm not a huge fan of the stroker motors - seen quite a few problems internally with those . . .

I have to concur . . . when you increase the stroke on an engine with fairly short rods like a small block Chevy, the resulting rod ratio is far from optimum and it increases side loads on the piston. (To visualize this, draw a right triangle depicting the crank pin at 90 degrees to the CL of the cylinder, with the rod as the hypotenuse. Now move the crank pin further out to simulate the longer stroke and watch the angle between the rod and cylinder increase!)

An engineering colleague of mine learned first hand about what happens when you increase the stroke on a short rod engine! Jeff ordered a new 27' Seamaster in 1967 with a large custom fuel tank for Bahama trips and a 350 Chevy/MerCruiser II I/O. Now Jeff likes to run fast, and he ended up putting a rod right through the block of the 350 in about 150 hrs! So he then installed a 400 CI small block Chevy, which is the same basic engine but with a much longer stroke. He said it was much better, as he got about 300 hrs out of it, at which point it was burning oil and blowing smoke! So he pulled the engine, tore it down, and found that every piston was cracked due to the higher side loads with the short rod/long stroke geometry!

At this point he installed a single 454 big block engine! He had no more durability problems after that, and concluded that the only thing better than that 454 was TWO of 'em! So he bought a 28' Bertram, with TWO 454's in it! (which he had to install himself, since Bertram would only sell it to him without engines, claiming that it was only designed for twin small block engines, and that big block engines would screw up the CG of the boat, making it unsafe!) Jeff said no problem, you just have to go down to your friendly Chevy dealer and order 2 pair of aluminum heads; with aluminum heads they weigh the same as a small block engine with iron heads! That was a 50 mph rig with those engines in it!

[Highway12]

I have a 1979 23 inboard with a Yanmar 240 and it will cruse all day at 27-28 mph and burn about 8 gph. It's a very sweet rig. It used to have a 5.7 V-8 and the diesel outperforms in every aspect.

[jnamowitz]

I run my carb'd 350 around 3400 all day and get around 28 knots. last weekend I ran out to the Marquesas and back, around 70 miles round trip and took on 32 gallons of gas. If I slow down to 3200, I still cruise at 22 knots. not sure of my fuel burn at that speed.

[Islandtrader]

Originally Posted by Highway12

I have a 1979 23 inboard with a Yanmar 240 and it will cruse all day at 27-28 mph and burn about 8 gph. It's a very sweet rig. It used to have a 5.7 V-8 and the diesel outperforms in every aspect.

I don't doubt your assessment of a sweet rig (always liked the yanmar) However I am surprised at the burn rate?? Its the same as the V-8 and speed seems to be equal.

Just checking to see if you made a mistake in your gph.

[Islandtrader]

Originally Posted by Bushwacker

I have to concur . . . when you increase the stroke on an engine with fairly short rods like a small block Chevy, the resulting rod ratio is far from optimum and it increases side loads on the piston. (To visualize this, draw a right triangle depicting the crank pin at 90 degrees to the CL of the cylinder, with the rod as the hypotenuse. Now move the crank pin further out to simulate the longer stroke and watch the angle between the rod and cylinder increase!)

An engineering colleague of mine learned first hand about what happens when you increase the stroke on a short rod engine! Jeff ordered a new 27' Seamaster in 1967 with a large custom fuel tank for Bahama trips and a 350 Chevy/MerCruiser II I/O. Now Jeff likes to run fast, and he ended up putting a rod right through the block of the 350 in about 150 hrs! So he then installed a 400 CI small block Chevy, which is the same basic engine but with a much longer stroke. He said it was much better, as he got about 300 hrs out of it, at which point it was burning oil and blowing smoke! So he pulled the engine, tore it down, and found that every piston was cracked due to the higher side loads with the short rod/long stroke geometry!

At this point he installed a single 454 big block engine! He had no more durability problems after that, and concluded that the only thing better than that 454 was TWO of 'em! So he bought a 28' Bertram, with TWO 454's in it! (which he had to install himself, since Bertram would only sell it to him without engines, claiming that it was only designed for twin small block engines, and that big block engines would screw up the CG of the boat, making it unsafe!) Jeff said no problem, you just have to go down to your friendly Chevy dealer and order 2 pair of aluminum heads; with aluminum heads they weigh the same as a small block engine with iron heads! That was a 50 mph rig with those engines in it!

Denny no doubt what you said above is true...however lets look at 2 important facts in todays world....

1. ALL 454's suck gas big time.
2. Back when your friend did this, I am sure the price per gallon was not a factor.

The 383 stroker has more torque than the stock 454 and that is why the manufactures are using this as a replacement in boats that had the 454
in it.

I know pelican likes new and not remanufactured stuff...however I believe you can not buy a new motor unless it has a catalytic converter on it(not 100% sure). That was one of the factors that drove me to the 383 at the time of my purchase 6 years ago.

[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

[Highway12]

240 YANMAR @ 2800RPM is around 8 GPH 28 mph. Full cruise is 3100 RPM @ 11 GPH 33 mph Full RPM is 3450 @ 36 MPH.

28 mph for that hull seems to be the sweet spot and the motor is under only 60% load. Like I said the 240 Yanmar is almost bulletproof and in my opinion one of Yanmars most reliable motors. The only complaint I have is that she will vibrate at idle and needs a trolling valve for stripers.

[deepsushi]

Highway those numbers are great and I agree that the 240 is the peefect motor for the 23 inboard. Unfortunately, it is no longer available. However, I heard a rumor that it may be on its way back as a common rail motor in the near future. Yanmar recently re-released the lower hp jh 4 cylinfer series in common rail configuration.

[Highway12]

Sadly, the EPA took care of most of the mechanical diesels, If you look, you can find them on the used market. I was lucky and found a guy that had a pair in a 31 Bertram that was a little underpowered with only 400 hours on them. They are sleeved motors and are very easy to rebuild if you can find one. I've seen water taxis with the same motor with over 10,000 hours on them. If you can find one, jump on it because it won't be for sale long.