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Calif Bill wrote:
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I would like to add an auxiliary outboard motor to my heavy 18-ft
fiberglass boat. I need it for (1) Backup "go home" power in case the
10-years old main motor dies (2) Trolling for fishes (3) Use it in
lakes that only allow no more than 9.9hp motor.

I would like to know how many horse power the auxiliary motor should
have. The boat is around 3000-lb (this includes everything like
people, fuel and cooler). It is 18-ft long and I figure that it has
16-ft at the water level. It is 12" draft. The hull is a semi-V. The
boat is a center console. I guess I am asking for a motor that can run
this boat right at the displacement speed (hull speed?).

Someone may say that bigger is better. But I cannot get more than
9.9hp motor because I need to use it in lakes that has
9.9hp-or-less-only limit. This sets the upper limit. I figure I
really don't need a large auxiliary motor because I will not run the
boat off shore (the boat is a "bay boat" and should not be used off
shore). And I will not run the boat when the weather is not good
because I am afraid of sea-sick.

I am hoping that I may be able to use something smaller than 9.9hp;
then I can save money on the motor and fuel. Moreover, I hope I can
use a small and light weight outboard motor that I can mount it in a
small row boat. If it is heavy, I probably will not be able to
dismount it at ease.

So, what I am asking a

- What is the displacement speed of my boat?
What horsepower of auxiliary outboard can run
my boat right at the displacement speed?

- What is the lowest limit (in term of horsepower) that
the auxiliary outboard should have without running the
risk of having the boat got stuck under normal wind and
current situation?

- What size do you recommend?

Thanks in advance for any info.

Jay Chan


I run an 8HP Yamaha High Thrust on a 21' 3400# boat and it gets hull
speed.
You want a High Thrust for kicker, and probably a 6-8 is fine.

Glad to hear that you have good result using a 8hp with your slightly
longer and heavier boat. I probably will stick with 9.9hp instead of
8hp because there is hardly any price and weight difference between
them (according to the spec of Nissan outboard shown in Cabelas.com).

I would love to be able to use a 6hp in my boat as an auxiliary motor
because it is almost half the weight of a 9.9hp motor. But I have a
feeling that it may not be able to handle unexpected weather condition
if I have it in my boat.

Jay Chan


Actually a kicker and a small boat primary motor are dfferent beasts. I use
to use the 15 hp Mariner 2 stroke from my 14' 200# aluminum boat as a kicker
on the 21' boat. Got about 5.5 mph out of the motor, a lot of prop slip
when trying to push the big boat to hull speed. My 8 hp Yamaha T-8 pushes
the boat about 6.5 mph, or close to hull speed, at about 2/3 throttle. The
difference is the high thrust has a bigger leg, form an about 25 hp motor
with more clearance for the prop and swings a larger, higher pitch prop at a
geared down speed. My T-8 is not a candidate to wap between boats as it has
power tilt, electric start which all add weight. The 15 hp mariner 2 stroke
scaled about 85# and did troll OK, just did not troll as fast as the T-8.