120hp, why slow ?

Plasma George · Jun 4, 2010

So I bought a 20' 1992 Maxum (sig) with a Force 120hp outboard. I'm coming from a 18' 1988 Chaparral with a 3.0L I/O.

The Maxum is slower at top end, less acceleration, and much higher RPMs at WOT. I'm doing 5500rpm at 30mph, and that's that farthest the thottle will go forward.

The guy before me was 100% fishing, so he'd cruise around to his fishing hole, use the trolling motor and not care about power/performance. I plan to do tubing and other water sports in addition to some fishing.

Is there a way to get more speed with doing some engine mods, or a different prop ?

I notice this prop is smaller in diamter than the Chap. I have an old prop so I can see them side by side. I don't have the prop specs.

roscoe · Jun 5, 2010

Re: 120hp, why slow ?

You are already running at the recommended wot rpm, so the prop is right.

The fact is the boat is bigger, probably heavier, and you have less hp.

Check the floatation foam to make sure you aren't hauling around 400# of water in the hull.

If not, you need more power.

thompy · Jun 5, 2010

Re: 120hp, why slow ?

i agree alot of boat for that motor

Plasma George · Jun 5, 2010

Re: 120hp, why slow ?

That's what I thought.
Repowering this boat ain't happening, I'll probably be selling it later this year or next.

What would be the performance change with a larger diameter prop ?

The propellor on the boat reads, 13" dia, 19p.
I have a 14.25dia, 19p from the chaparral.
The internal splines look to be identical, so it may slide right on.

What say you ?

roscoe · Jun 5, 2010

Re: 120hp, why slow ?

Larger surface area on the prop ears means more resistance, means less motor speed, = poorer hole shot and less top speed.

thompy · Jun 5, 2010

Re: 120hp, why slow ?

it will fit but the hubs are different sizes, and like was mentioned to much blade area

AlanR · Jun 5, 2010

Re: 120hp, why slow ?

20 footer go Atleast a 150, Id recomend a 175 a Nice newer Yamaha VMAX would be ideal.

Netjunky · Jun 6, 2010

Re: 120hp, why slow ?

I have a 1990 Bayliner 20' Cuddy with a 120hp force. The prop is a 13 x 19p with 3 adults 2 kids we get up to 39mph. Not sure why you would only get up to 30 mph. Speedo correct?

Plasma George · Jun 7, 2010

Re: 120hp, why slow ?

roscoe said:
Check the floatation foam to make sure you aren't hauling around 400# of water in the hull.
QUOTE]What's flotation foam ?

I've had it jacked all the way up with the drain plug out to try to get every drop of water out. I once ran the engine backed down a ramp after in the ocean to flush it, and forgot the plug. I only had it in the water a couple minutes, but it took much longer than that for the water to stop draining out.....but eventually it did.

findinghomer · Jun 7, 2010

Re: 120hp, why slow ?

Netjunky said:
I have a 1990 Bayliner 20' Cuddy with a 120hp force. The prop is a 13 x 19p with 3 adults 2 kids we get up to 39mph. Not sure why you would only get up to 30 mph. Speedo correct?

its amazing how similar boats can turn props so differently. i have a 20ft maxum cuddy 125 hp outboard, i can only turn a 13-15 in pitched prop. in fact i just bought a new 15 in pitch i have to send back bc its cupped and i lost 800 rpm. it will only turn at 4500 rpm. our top speed is 32 mph.

jaybirddog11 · Jun 7, 2010

Re: 120hp, why slow ?

Floatation foam is the foam that is under your floor board. If water gets under the floor board and is not drained out, it will seriously compromise the integrity of the foams ability to do its job. It will eventually absorb water and will need to be replaced. This extra water weight can add hundreds of pounds and if your boat were to take on water you could sink. If your boat is stored outside and not covered, you should have the floor slope towards the drain so there is no standing water.

120hp, why slow ?

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