heavy displacement repower dilemma

[mako]

Originally Posted by Salmoneyes

One of the main reasons we are looking for simplicity for this repower.

I think it is a matter of whether or not you are required by law to install a tier-3 (or whatever tier) engine. If not, if you have an older boat that pre-dates all that stuff, then obviously the answer is put in an old-school engine. There can't be a single person on earth who would prefer a computer controlled common rail high speed engine. Nonsense.

[hpeer]

Salmon,

A lot depends on how you intend to use the boat.

We have a Yanmar 4JH4-TE, we have done a lot of motorsailing, bashing to wind. I frequently run the engine at 1,500 RPM, and never cruise above 2,200. Above that I start to get some shaft vibration. I will run the engine up if I’m stuck, or sometimes when backing down on the anchor, or if making very tight maneuvering where I need a lot of prop walk. I’ve had no bad results from that use.

Remeber we have a boat similar to yours, 44’ @ 40,000 lb. 72 HP max. I don’t run at hull speed but a bit below, everything just works better there. If you are pushingbtoo hard you can get prop walk which make helming tiresome. Compromises, compromises, compromises.

Your truck could probably do 100mph, but how much running over 50 did you do? Long idleing I can see as a creating an issue, not much of that on a boat as long as you are not charging your batteries with the main engine routinely.

[Salmoneyes]

PeterS posted a formula for HP and Torque.. I played with it a bit and I came up with this

Our 126 Hp at 2800 produces 236.3 ft lb at 1700
at 1700 the engine produces 76.5 HP

that says at 1700 rpm we are using 60.71% of our engines capacity

At 2000 rpm, we are making 90 HP for 71.42%

If this is correct, than our 126 HP would allow us to run at or close to the torque capacity with a safety margin of 30% which is what is recommended by some of the dealers we spoke to.

Am I right in my thinking? Running at 70% keeps us in the recommended HP range for our hull and is using the engines torque efficiently?

[Jimmyhenry]

Originally Posted by makobuilders

Jimmy, at that loading your engine is running at about 22% power. Not good for long term longevity. Do you regularly run it up or anything else?

BTW, I love the metric system but still absolutely hate Celsius. Fahrenheit has it all over you guys!

Very occasionally. Did 5 days back from Noumea on motor at 1100rpm. Gave it a blast when we got in smooth waters and there was carbon bits everywhere.
Just done Tonga to NZ. No wind so 5 out of 6 days on motor again at 1100. Probably buggering the engine but hey. I have a spare in the garage!! Same motor.

[mako]

Originally Posted by Jimmyhenry

Just done Tonga to NZ. No wind so 5 out of 6 days on motor again at 1100. Probably buggering the engine but hey. I have a spare in the garage!! Same motor.

If you ever get to the point where you want to install that spare motor, then perhaps have an engine shop rework the injector pump (or perform other magic) to drop the power output in half. Then you'll regularly be loading your motor at 50-70%.

[Jimmyhenry]

Originally Posted by makobuilders

If you ever get to the point where you want to install that spare motor, then perhaps have an engine shop rework the injector pump (or perform other magic) to drop the power output in half. Then you'll regularly be loading your motor at 50-70%.

Thanks for that tip.
Got the second motor for very little $s. Only had 2400 hrs on the clock and looked it. Previous want a bigger motor in his yacht. Went 6 cylinder Yanmar 120hp. End result was it went the same speed. I think he was already over powered.