Does this top speed sound right?

[Josh in Pa]

I have a 1992 GW Invader, 20 ft bowrider. 350 mag with 4 barrel, 21" ss prop. Engine was rebuilt several years ago, and starts and runs really well. Usually tops about 4400 rpm wot. The other day I was out with gps, and with 4 people onboard, managed 50 mph. Never really checked it before, but thought this was kind of slow? I've never weighed the boat, but it is pretty small for 20 ft, and has a fairly flat bottom. In pretty good shape, always kept inside, I doubt its waterlogged. I see lots of claims for similar style boats (baja 208, sea ray pachanga) of 350 mags running 65 mph? Is 50 typical, or should I do some investigation?
Josh

 

[spdracr39]

Re: Does this top speed sound right?

It does seem a little slow and your WOT rpms are a little low. You could probably benefit from a 19 pitch prop to raise the speed a little. Proper trimming of the outdrive will make the biggest difference and a lot depends on the hull design and the water and wind conditions. My 21' with a 305mpi will run about 52 with four good sized adults in perfect trim and water but it is a newer and most likely lighter hull design.

 

[QC]

Re: Does this top speed sound right?

Important to note that the 1991 and 1992 350 Mag was only rated for 240 hp at the propshaft. I had a very light, and flat (16 degrees) 20 footer with a TBI 250 propshaft Merc and she barely bumped 60 with a 21 inch pitch empty and almost 5000 RPM. I don't think you are very far off, and the 19 may be the answer. I would retest empty, and work the trim as good as you can. Also, never forget those guys claiming 65 are lying, even with a modern MPI at 300 hp

BTW, with a 1.5 ratio I am getting 14% slip. I bet you can improve a few MPH with prop and trim work.

 

[LippCJ7]

Re: Does this top speed sound right?

Well I thought I had something to add until I read QC's post, so lets just say ditto QC, especially the lying part, always seemed funny how 65 MPH suddenly looks like 55 MPH when a 25' boat with the same horsepower and a GPS is right next to them...

 

[haulnazz15]

Re: Does this top speed sound right?

Nothing wrong with that speed or the WOT he's getting. He can try different props, but most runabouts with a 350 V8 will run right around 50mph depending on the weight of the boat and the people/gear in it. It hasn't really changed too much in the 70 years they've been making V8 I/O runabouts. There are a few lighter boats (Stingrays, etc) that will eek out a few more mph, but 50-55mph is the top end for most 20' runabouts, unless you have a specifically-designed boat built for more speed.