Boy I hope you don't ever intend to go into rivers and that the lakes
have no current. A 9.8 with that load might not even hold its own in a
stiff current. I remember watching my brother in a 15ft aluminum boat
with an older 25 Hp on it sitting in the Little Kanawa river near
Parkersburg, WV. The river was high and flowing fast, but not yet at
flood stage. He was sitting still with the engine WOT. Could not make
any headway at all and barely got it back to shore where we could tie
it back up to the dock. At normal current it would do OK and get on
plane with him in it. I couldn't see it doing much with 4 full sized
men in it though.

Yes, you are probably right.

Some old post said that a 9.8hp would push a 12-ft boat only at 12 mph
with 2 adult males and other gears. I would guess that the same engine
will only push a 18-ft boat at 6 mph or less with 4 adult males and
other gears. This would be too slow even for me.

On the other hand, someone else in another post said that his 9.9hp new
engine with a new prop could push a 18-ft boat at 20 mph (probably with
only himself in the boat). I guess this would mean around 10 mph if
there are 4 adult males on the boat instead of just one.

Seem like the best course of action a

- Get a powerful trolling motor and use it in
electric-only lakes. Electric-only lakes tend
to be very small anyway.

- Get a 9.9hp outboard engine and use it in
10-hp-or-less-only lakes (there are many
lakes like this around here (New Jersey)).
And play around with it with various number
of passengers to see how slow it can get,
and whether the speed is acceptable. I would
think that this should be OK because those
lakes don't have any current to speak of.
At that time, I will get enough information
about the boat/engine combo, and I will be
able to decide whether I need a bigger
horsepower engine to run the boat in sal****er.

- If I discover that 9.9hp is only good enough
for freshwater lakes and is not appropriate
for sal****er use, I will get a larger engine
(like 40hp) and use the 9.9hp engine for
trolling.

You see, I need the 9.9hp engine to run the boat in 10-hp-or-less lakes
anyway. This means it will not be wasted.

Jay Chan

On Fri, 16 Sep 2005 09:08:28 -0700, jaykchan wrote:


Actually, I am thinking of 9.8hp because many lakes in New Jersey
(where I live) have restriction to limit the engine to no more than
10-hp. And a 9.8hp 4-stroke engine is around 90-lb; two persons with a
carrier should be able to hoist it. I am currently leaning toward
getting a 9.8hp as long as it can push the 18-ft boat with 4 adult
males on it at 10 to 15 land-miles per hour. What do you think?


I think a 9.8 might be a bit "lean" for that, although I Hear You about
the size. Here, "under 10hp" means you don't have to license it. Then
again, there's lots of 15hp engines around with 9.9 "tags" on them...

As for speed, there's this problem called "hull speed". In most boats,
either you're below hull speed ("displacement mode"), or well above it
(planing). So either you're going about 5-6mph, or 15-20mph. 10 mph is in
that "in-between" area where you're not really doing either. So either you
get an engine big enough to plane the boat (probably at LEAST 25hp for
your boat and 4 adults), or stay in displacement mode at 5-6mph, and you
could do that with a 7.5.

In other words, a 9.9 probably won't go much faster than a 7.5, but a 25
would to MUCH faster. Make sense?

Lloyd

No.

To clarify.
A boat rated for a 100 horsepower motor won't perform very well with
less than an 80 hp motor on the transom.
a =E9crit :

Generally, less than 20 percent of the rated max hp will significantly
degrade performance.

I assume you mean an outboard engine will reduce its performance
significantly if we operate it below 80% of its max hoursepower rating.
Do I understand this correctly?

In other words, I need to run an outboard engine at or near its peak
horsepower rating. This also means that I need to buy an outboard
engine whose _max_ horsepower rating matches the cruising speed of the
18-ft boat that I intend to operate at.

I probably want to cruise at a leisure speed of 10 to 15-miles-per-hour
(here the miles are not nautical miles). What kind of horsepower an
outboard engine should have in order to push a 18-ft boat with 4 adults
males on it at the cruising speed of 10 to 15 miles-per-hour? Any idea?
Thanks. I appreciate your help.

As for the difference in noise between a 40 and 50 hp engine, it's
insignificant. Besides current engines are much quieter than previous
two-strokes.

Good to know this. This means I should concentrate on getting the
proper horsepower to push the boat instead of being distracted by other
factors.
=20
Jay Chan

I think a 9.8 might be a bit "lean" for that, although I Hear You about
the size. Here, "under 10hp" means you don't have to license it. Then
again, there's lots of 15hp engines around with 9.9 "tags" on them...

As for speed, there's this problem called "hull speed". In most boats,
either you're below hull speed ("displacement mode"), or well above it
(planing). So either you're going about 5-6mph, or 15-20mph. 10 mph is in
that "in-between" area where you're not really doing either. So either you
get an engine big enough to plane the boat (probably at LEAST 25hp for
your boat and 4 adults), or stay in displacement mode at 5-6mph, and you
could do that with a 7.5.

In other words, a 9.9 probably won't go much faster than a 7.5, but a 25
would to MUCH faster. Make sense?

Thanks and Thanks Again! This sounds very good to my plan.

When I operate a rental boat, I never put it in plane mode. Therefore,
I probably was running at 5-6 mph instead of 10 mph that I original
thought. Because I don't intend to operate the boat in plane mode, I
can use a low cost small engine, such as a 7.5hp that you have
recommended. Honestly, the price difference between a 8hp and a 9.8hp
engine is quite small comparing to the total cost of an engine.
Therefore, I will probably choose the engine depending on which one is
available on the used boat. If the used boat comes with a 7.5hp engine,
I will use it. If the used boat comes with a 9.8hp engine, I will use
it too. I am not picky as long as the engine is within the 10hp limit
that many lakes in NJ state parks only allow.

I will think about the 25hp engine later (may be for sal****er rse).

This should work out great for my plan!

Jay Chan

I think a 9.8 might be a bit "lean" for that, although I Hear You about
the size. Here, "under 10hp" means you don't have to license it. Then
again, there's lots of 15hp engines around with 9.9 "tags" on them...

I forgot to respond to this in my last post. The 10hp limit has nothing
to do with license. Here in New Jersey, many lakes in state parks only
allow a boat that is electric only or has a small gas engine that
doesn't go beyond 10hp. If the gas engine in a boat is over 10hp, we
cannot run the boat in that lake. This significantly limits the number
of freshwater lakes that I can run the boat on (something like from 10
available lakes reduced to only 2). And two of the lakes that I really
want to fish have that 10-hp limit. If I want to use a large engine, I
will have to use it in sal****er or those two freshwater lakes
mentioned above.

Currently, my primary plan is to run the boat on freshwater lakes.
Running the boat in sal****er is only an afterthought ("that would be
nice" kind of thing). Therefore, outfitting the boat with a gas engine
that can run in most of the freshwater lakes is the first priority. And
I am glad to hear that a 7.5hp gas engine should be fine for a 18-ft
boat with 4 adult males on it as long as I am OK with running the boat
in displacement speed.

Two years from now, I may have explored enough of the freshwater lakes,
and I may start thinking of running the boat in sal****er. At that
time, I may decide to get a 25-hp or 40-hp outboard motor, and use the
7.5hp engine for trolling.

You have helped me to make the right selection. Thanks.

Jay Chan