Mercury 9.8 slow top speed!

[fastmike22]

I have a 1978 mercury 9.8 (110) and i think the top speed is to slow. Its on a fiberglass 14' center console bass style boat. With two guys probably 380 lbs total and fishing gear it will only do about 7mph. The engine runs good revs good and i just had the carb rebuilt. Do you think this is to slow for a top speed also? I dont think im getting prop slip either.

 

[Laddies]

Re: Mercury 9.8 slow top speed!

A lot depends on the hull and it's weight and bottom design

 

[Scott Danforth]

Re: Mercury 9.8 slow top speed!

what RPM are you running? the 9.8 WOT is in the 5800-6000 range

 

[anzomcik]

Re: Mercury 9.8 slow top speed!

That speed sounds about correct for a 10hp motor. You have to have a very very very light rig for a 10hp to plane out. If you can plane out with a 10hp you should be 15-18mph range.

But 7-9mph with a 10hp on a "normal" boat is par for the course. If the motor runs good, then smile and enjoy it. If you want more speed, you only real choice is to buy a bigger motor.

I have experienced a few things when i ran a 10hp for a main motor, chances are if it doesnt plane with propper set up, then in almost every instance putting add-ons to the motor or boat will not do anything except waste money. Also to reach your WOT range will require a prop that manufactures do not make (like a 5 pitch)

WOT is usually ment for when your planed out, but you are not planing out so you will never reach WOT (the way mercury is looking for the intended rpm range) No sense in beating your head against the wall in the area with a 10hp

 

[fastmike22]

Re: Mercury 9.8 slow top speed!

Im not sure what rpms im running but they seem to be up there. The boat is getting up on plane but then thats it. It actually holeshots pretty good but as soon as it feels like its on a plane thats all she's got.The boat is also a tri hull.

 

[anzomcik]

Re: Mercury 9.8 slow top speed!

How are you measuring the speed? Because I do not know of a boat that planes out at 7mph. Every boat I have owned has a plane speed of about 12-17mph (min speed to hold plane), it would depened on the hull and the load.

But none the less, if the motor runs good, starts nice and idles nice and reves up nice then for that setup you are doing well. reason i say that is because you are useing 10hp, if you had a larger motor 50,75,100hp then WOT throttle and gps speed, gear ratios, motor heighth prop pitch, number of blades on prop, alum or SS prop... All of that makes a difference, but my finding is 10hp generally is you have what you have and all the changes in the world tends to change very little with the performance with 10hp motors

 

[fastmike22]

Re: Mercury 9.8 slow top speed!

Well i guess it is what it is. Im going to try a solo run with the gps and see what it does. Thanks for all the imput!