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