TLDI vs. 4-Stroke

[SAQuestor]

I’m considering building a boat to use as a water-borne weekend camper rather than as a sporty go-fast runabout or a fishing machine to run out 50 miles to kill some tuna.

This proposed usage will entail lots of slow poking about in shallows, coves and among the islands/islets and bays of the Texas gulf coast.

Essentially this means that while I need good low speed performance and reliability, I also need enough ‘oomph’ to get the boat up on plane and move right along from the launch point to those pokey areas.

Thinking about motors, specifically a TDLI versus a 4 stroke, brands and specific horsepower to be determined by the requirements of the boat design and “the best financial deal.”

My understanding is that the TLDI motors need some minimum battery voltage to even run, apparently it's a matter of the computer needing power, the air pump for proper fuel atomization, and their having inductive ignition rather than capacitive ignition. Whereas 4 strokes are much more forgiving in their required starting and running conditions. Comments?

My questions are specifically about 1) TLDI versus 4 stroke reliability where the operating conditions will generally be at less than cruising RPM, and 2) which type of engine will provide the better fuel mileage during the aforesaid operating conditions?

Thanks for your thoughts and comments.

Leo

[Bob S.]

Four strokes are really outstanding at lower and mid-range rpm's, particularly if you are propped right. Never owned a high tech 2 stroke, can't comment.

[marshmat]

Two-stroke outboards tend to be optimized for higher RPM ranges- while the new high-tech ones do idle better than the old ones, they are still very high-strung. For poking around like you describe, you probably want quiet too, so you don't spook wildlife and fish. The four-stroke option will tend to be quieter, as well as more efficient and reliable at the low end, while the two-stroke will burn more gas and make more noise but will accelerate better and probably give you a higher top end.

[Willallison]

We run a pair of Dfi Merc 225's on the back of a 26ft deep-vee.
With the rather notable exception of having 2 powerhaeds replaced at Mercury's expense (one due to a black-box malfunction, the other because someone left a bolt rattling around in the donk somewhere. It finally found its way into one of the cylinders... you can imagine the rest... ) we've been very impressed with them. They will happily chug along all day at displacement speeds and are very economical.
Most tests that I've seen indicate that direct-injected 2-strokes marginally outperform their 4-stroke counterparts in every area - except noise, which as marshmat says may be important (though not so much for tuna fishing...). Having said that, both types are evovling so fast that no matter which one you buy, there'll be a better one out soon!