Top speed of boat seems slow

D.spencer · Sep 7, 2008

Hello all, I am new to this forum. I recently bought a 1975 terry bass boat with a 140 hp johnson. I took it out today and the best I could get was 37.5 mph as measured with my GPS. The boat is a 17' fiberglass and is rough as all get out(top side-not pretty). Don't know rpm's yet-tach is busted. don't know what pitch prop-there are no markings.A-V plate is about one inch above the bottom of the boat( as checked with a straight rod to verify). the outing today was with two fuel tanks ( 5 gal each),one battery,no gear- other than two paddles and life jackets. The trolling motor is not even mounted. Motor does not seem to be missing or anything. Compression was 120-130 psi on all cyls. Prop does seem to be slipping on the holeshot ( I felt a vibration on initial take-off,the smoothed out) motor seems to be very strong . Trimming seems to change speed very little-actually made it "porpoise" when trimmed up. no jackplate. Seems like a little slow (37.5mph) for a engine this large. what are opinions here?

kenmyfam · Sep 7, 2008

Re: Top speed of boat seems slow

Many factors to consider before deciding if it is slow. Set up is huge. Knowing the WOT rpm and prop setup is vital. How heavy is the boat ???? you may have waterlogged flotation foam in there and that will sure slow thing down. How solid is the floor ???? What year is the motor ??? is it a cross flow or a Looper ???

kenmyfam · Sep 7, 2008

Re: Top speed of boat seems slow

By the way, welcome to iboats.

D.spencer · Sep 7, 2008

Re: Top speed of boat seems slow

Dang, I forgot to say what year motor. 1977. not sure if it is a crossflow or looper. How do you tell? what is a looper/crossflow anyway? I will have to get a tach or fix this one-I realize that that is vital.

david_r · Sep 7, 2008

Re: Top speed of boat seems slow

your prop should be marked probably on the side to the motor so youll have to remove it to see. like kenmyfam says set-up makes a big difference...night and day diference. i thought the same about mine but the writing was itty bitty and barely legible.

dajohnson53 · Sep 8, 2008

Re: Top speed of boat seems slow

kenmyfam said:
... is it a cross flow or a Looper ???

kenmyfam: this is an honest question that I've asked before in other threads, but don't remember getting an answer to.

The question "is it a cross flow or looper" is regularly asked, but how is a "normal" engine owner to know this? I don't believe the common owner even knows there's a difference, and I don't believe that OEM owner's manuals even mention this part of the engine technology. Plus, it is probably not even relevant to the normal use of an outboard, including routine owner maintenance. Just my opinion having operated outboards for 40+ years without even being aware of this difference.

So my question is: how would someone know this, and why should they?

Again, an honest question out of curiousity.

tashasdaddy · Sep 8, 2008

Re: Top speed of boat seems slow

my question is this boat new to you, are you sure it is not holding an extra 500lbs, of saturated foam.

kenmyfam · Sep 8, 2008

Re: Top speed of boat seems slow

dajohnson53 said:
kenmyfam: this is an honest question that I've asked before in other threads, but don't remember getting an answer to.

The question "is it a cross flow or looper" is regularly asked, but how is a "normal" engine owner to know this? I don't believe the common owner even knows there's a difference, and I don't believe that OEM owner's manuals even mention this part of the engine technology. Plus, it is probably not even relevant to the normal use of an outboard, including routine owner maintenance. Just my opinion having operated outboards for 40+ years without even being aware of this difference.

So my question is: how would someone know this, and why should they?

Again, an honest question out of curiousity.

Here is a link to start with. More to follow. Hope this helps.

http://forums.iboats.com/showthread.php?t=108816&highlight=crossflow+versus+looper

kenmyfam · Sep 8, 2008

Re: Top speed of boat seems slow

Yes, the watersoaked foam thing is vital to know. You will learn a lot from answering that one.

kenmyfam · Sep 8, 2008

Re: Top speed of boat seems slow

Another link

http://forums.iboats.com/showthread.php?t=75353&highlight=looper+description

kenmyfam · Sep 8, 2008

Re: Top speed of boat seems slow

More info.

Crossflow-scavenged
In a crossflow engine the transfer ports and exhaust ports are on opposite sides of the cylinder and a deflector on the top of the piston directs the fresh intake charge into the upper part of the cylinder pushing the residual exhaust gas down the other side of the deflector and out the exhaust port. The deflector increases the weight of the piston and exposed surface area of the piston, also making it difficult to achieve an efficient combustion chamber shape. This type of two stroke has been largely superseded by loop scavenging method (below). With smaller size and lower piston speed the deficiencies of the cross flow design become less apparent. The last of the OMC (Outboard Marine Corporation now Bombadier Recreational Products BRP) V4 and V6 two strokes produced up to 1995 in their mid range were still cross flows. These were produced in the 90-115 horsepower V4 configuration in a 1.6 litre as well as the 2.4 Litre 150-175-200 Horsepower V6's. These engines remained extremely competitive on fuel use compared to their loop charged competitors due to advanced exhaust tuning by the manufacturer. These Crossflow engines produced more torque and horsepower by burning less fuel than the Japanese loop charged competitors. Eventually OMC shifted to the Spitfire series Loop charged V4 and V6's in their mid range.

The 235 horsepower 2.6 Litre crossflow V6 (1976 - 1986) still remains today as a very high output low weight engine compared to its much heavier loop charged 2 stroke and 4 stroke replacements.

The Crossflow design produces far more low down engine torque than the slightly more fuel efficient Looper design. Many a boater replaced their Crossflow 2.6 Litre 235 (Flywheel rated) for larger (propshaft rated HP) 2.7 Litre and 3 Litre 225 horsepower V6's only to be disappointed with the lack of low down torque offered.

During the late 1970s 1980's OMC successfully raced the OMC CCC engine. This was a crossflow, carburettored V6 2.6 Litre that out ran many of the oppositions Loop scavenged, fuel injected larger displacement competitors.

It lived on with the XP 2.6 until 1986 in a much more civilized form.

Cross flows are still to be found in small engines because it is less expensive to manufacture and allows a more compact design for multiple cylinder configurations. BRP still offer the 9.9 and 15 HP twin cylinder two stroke available through their Johnson brand as there is still no alternative to this popular lightweight high output engine.

Loop-scavenged

This method of scavenging uses carefully shaped and positioned transfer ports to direct the flow of fresh mixture as it enters the cylinder. Usually a piston deflector is not required conferring considerable advantage over the cross flow scheme (above). Often referred to as "Schnuerle" (or "Schn?rl") scavenging after the German inventor of an early form in the mid 1920's, it became widely adopted in that country during the 1930s and spread further afield after World War II. Loop scavenging is by far the most common type used on modern engines.

kenmyfam · Sep 8, 2008

Top speed of boat seems slow

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