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The Right Size Outboard Gas Engine for a 18-ft Boat? - FollowUp
I end up getting a 18-ft fiberglass center console (SeaRay Laguna)
instead of an aluminum. The reason is that I really have no idea how
to fix an aluminum boat, and fiberglass boat looks very nice.That boat comes with a 115-hp Mercury outboard motor. The motor got
the boat on plane in just 5 seconds or so from displacement speed (plus
or minus, who was counting anyway). We had 4 adults on the boat, I was
expecting that the boat would take longer to get on place. I was
pleasantly surprised that the boat/motor combo worked quite well
together even with 4 adults on board. The boat got on plane and at cruising speed with 2/3 throttle at around
4,600 rpm. Putting the motor at full throttle didn't seem to increase
speed much and the motor only increased to around 5,000 rpm. This
means running the motor at full throttle didn't buy me anything. I posted this just in case someone wants to know what ended up
happening. Jay Chan |
jaykchan
Nov 7
2005
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| Yes, I understand your math. But when I saw the mechanic (who
conducted the sea trial) set the throttle at around 2/3, I saw the rpm
was at around 4,600 rpm (and that was what the mechanic suggested me to
run the motor at). And when he put the throttle all the way, I only
saw the rpm increased to 5,000 rpm (the tachometer can display more
than 5,000 rpm). I don't know why the motor rpm didn't increase
proportional to the throttle setting. This is probably one of those
things that I don't know about motor / rpm / prop pitch ...etc. Jay Chan |
jaykchan
Nov 7
|
| Yes, I understand your math. But when I saw the mechanic (who
conducted the sea trial) set the throttle at around 2/3, I saw the rpm
was at around 4,600 rpm (and that was what the mechanic suggested me to
run the motor at). And when he put the throttle all the way, I only
saw the rpm increased to 5,000 rpm (the tachometer can display more
than 5,000 rpm). I don't know why the motor rpm didn't increase
proportional to the throttle setting. This is probably one of those
things that I don't know about motor / rpm / prop pitch ...etc. Jay Chan |
jaykchan
Nov 7
|
| Yes, I understand your math. But when I saw the mechanic (who
conducted the sea trial) set the throttle at around 2/3, I saw the rpm
was at around 4,600 rpm (and that was what the mechanic suggested me to
run the motor at). And when he put the throttle all the way, I only
saw the rpm increased to 5,000 rpm (the tachometer can display more
than 5,000 rpm). I don't know why the motor rpm didn't increase
proportional to the throttle setting. This is probably one of those
things that I don't know about motor / rpm / prop pitch ...etc. Jay Chan |
jaykchan
Nov 7
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| If full throttle is 5000 rpm, then 2/3 throttle would be about 3300 rpm, which
is about my cruising rpm. |
John
Nov 7
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| I'd say you're powered about right. I have an 18 ft. fiberglass Chris Craft runabout, with a 140
mercruiser. I run it at about 3-3200 rpm. I could go up to 4000 but it doesn't prove to be much benefit. just
more wear & tear, and sucks more gas. I had a fin on my old lower end
and it helped plane quicker, but cut the over all speed. When I
installed the new lower end, I left the hydrofoil fin off, and it took
jsut moments longer to plane out, but seems like the top end was helped
dramatically, and the boat seemed to run smoother, probably because of
less drag. |
tschnautz
Nov 7
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| Glad to hear that your boat runs fast and smooth now that you have
removed the horizontal fin in the lower-unit of the out-drive. The
outboard in my boat doesn't have anything like that, and it gets on
plane just as well. I guess I don't need to worry about this. Jay Chan |
jaykchan
Nov 7
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