Repower with a 6BT 270 HP Cummins
This is a Repowering of a Mainship 34 by Jay Leonard
Repower notes: Spring 2000 repower of Attitude Adjustment (1978 34 M1) with a Cummins 270 Diamond (270 hp at 2600rpm).
Cost of engine….$15,423
This included:
1) Hurth 630A gear at 1.56:1 with mounting brackets
2) 4 engine isolator mounts (Barry Controls)
3) Block heater (thermostatically controlled)
4) Engine oil drain hose
5) Deduction of $350 for “no factory exhaust elbow”
6) freight
7) “start up” This is an onsite inspection by a Cummins tech to certify the installation for warantee purposes He spent 1 ½ days on the boat and inspected /tested everything imaginable. Normally approx $400.
8) No gauge panel.
9) No wiring harness
** Current LIST price for either the 220 hp or 270 hp bobtail is $16,611
Changes I had to make:
1) change fuel feed lines to ½ inch
2) Build new fuel tank selection manifold with ½ inch valves
3) New pick-up tubes, 3/8 ID plastic
4) Add a second fuel return and manifold. (originally only returned to the port tank. Used 3/8 ID hose
5) Addition of custom exhaust riser/elbow ($575) (Marine Manifold Corp. Paul 631-694-0714). I also built a brace to support the weight. Also need to buy from Cummins the mounting gasket and 4 bolts.
6) Change to 5 inch exhaust hose and thru-hull
7) Installation of 2 inch raw water intake, strainer, etc
8) Addition of two cooling system vent lines to the expansion tank; 1 from the top of the turbo housing, 1 from the rear upper on the cylinder head.
9) Replacement of the factory temp and oil pressure sending units with the ones from the old engine. This is necessary because I wanted to use the existing gages and panels in the boat.
10) Installation of the old oil press and temp alarm sending units per above.
11) Addition of a special Cummins restrictor fitting and shut-off valves to run coolant to the domestic hot water heater heat exchanger.
12) Re-connection of the tranny shift cable (I had to build a mount from a different direction).
13) Replacement of the lower station throttle cable (original was 1 ft too short to fit the new engine).
14) Wire engine to existing Mainship harness: ( via a junction box)
1) Wire into ‘R’ terminal of alternator for tachometer signal
2) Run ‘hot’ lead from ignition switch to engine harness (to keep the fuel solenoid engaged.
3) Connect sending unit wires
4) Connect warning buzzer wires.
15) Calibration of oil dipstick
16) Alignment of engine to shaft.
17) I also replaced the angle iron engine mounts and bolts. Upgraded from ¼ inch thick to 3/8 inch thick.
18) Added one 4” x 6” vent into the engine room (not powered) to insure that I had enough air circulation
19) ** Change from bronze shaft to Aquamet 22 shaft…done the prior year and not included in the cost summary
Other notes:
Engine bed stringers SHOULD HAVE been made thicker by approx 1 ½ inches on the outside edge for the length of the engine compartment (bulkhead to bulkhead)
I mounted the Cummins supplied coolant overflow tank to the front of the expansion tank using plywood and 2 large hose clamps
The tachometers will need to be calibrated by using a hand held optical tach
Prop size should alloe engine to turn at 2650 (or a little more) at WOT for max longevity. The Cummins B series does not like to be overloaded, esp in warm waters.
The change in shaft speed (because of the gear ratio change from 2.1 to the new 1.56) caused hull vibration directly above the prop. This was due to the fact that I have only 2 inches of prop to hull clearance. (10%) I reduced that vibration (to a very acceptable level) by thickening the hull on the inside between the rudder post and bulkhead forward of that. I also added re-inforcing ribs above the thickened hull. That eliminated approx 90+% of the vibration. The keel had been faired previously.
I choose the 1.56 ratio because I could not go larger in prop diameter, and I had the knowledge that another Mainship 34 repowered with a 220 Cummins and 2:1 ratio gear. His prop size ended up being 20 X 24. I knew I would need more pitch than that and was afraid to go to a 20 X 26 (or 27) because of prop dynamics of having too much pitch.
If you have the prop diameter to go square or just slightly more than square at 2:1 ratio, I would strongly recommend it in order to keep vibration to a minimum.
Results of sea trial for Attitude Adjustment May 2000
Cummins 270 B W/ 20 x 21 prop
1.56:1 reduction
RPM Boost Speed
600 4.5
1000 7.4
1100
1200 8.2
1300
1400 8.9
1500 0 9
1600 9.6
1700
1800 3.5 10.3
1850 4 11.5
1900
2000 11.8
2100 14 12
2150 14.5
2200
2300
2400 15.6
2500
2600 30+ 17.5
2630 18.3
Idle speed 597
Max noload 3041
max w/load 2630
Item Cost Comments Item Price
Engine +gear 15423 includes tax Engine + gear 2400
riser 575 Parts 1000
Bayside's labor 1800 was quoted at 800 old block 300
bolts,etc 57
exhaust hose 350 Total 3700
exhaust thru-hull 375 (Old Perk items sold)
angle iron mounts 20
1 1/2 hose 40
2 hose 50
fuel fittings 130
throttle cable 22
junction block 9
flex fuel hoses 50
misc 35 clamps
Throttle fitting 10
boost gage 95
oil 35
vent hoses 50
Anti frze 15
19141 GT 15441
Jay Leonard
11-8-01