How fast should I go with a V6 135 bhp?

[LarsMW]

I have a 21 feet Gulf Craft Swan bow rider and a last generation carb V6 135 2-stroke Mercury outboard. I will not go any faster than 32 knots and I feel this is way slower than I can expect (the former owner claimed 48 knots...) It will reach around 5300 rpm... Can anyone give some pointers? The boat is freshly painted and drained for water

 

[jerryjerry05]

135hp at 5300
The motor should be able to get about 5600 at wide open.
You using the trim? (had to ask).
Is that an acyrate speed compared to a GPS?
​Is the speedometer pickup tube(pitot tube) down in the water?
The tip not broken?
​What kind of load do you have?
​1/2 tank of fuel, cooler, 1-2 people(normal sized)

​Do a compression and spark test.
Post the results.

 

[LarsMW]

Hello and thanks for getting back to me.

I have not tested the trim very much (will do)
Speed is with GPS - on board dial is out of order at the moment
Fullt tank of gas 1,5 adults
New sparks fresh from service

 

[Scott Danforth]

to get optimal performance, you should start with your hole-shot with motor trimmed all the way in, then once the boat comes on plane, start trimming up until the bow starts to porpoise, then trim down just a touch. you should pick up a bunch of speed.

if your trimmed in all the way, your bow will plow, you will loose speed and lug your motor

you should be operating the trim button as often as you operate the throttle

 

[jimmbo]

48 knots is like 55mph. A 135 is not going to push a 21 footer 55mph. 32 knots is 36mph. If the boat is indeed plowing there will be an increase in speed as positive trim is applied, but I doubt it will be an increase of 19mph. 38 knots or 43mph would sound like a good speed for that combo.

 

[ajgraz]

What prop have you got on there now?

I'm with jimmbo, you'll probably never get that boat and motor over 40-45mph with any kind of load in the boat and a prop that still allows decent control.

 

[jerryjerry05]

Was the road service guy for a new boat dealership.
​Went on a repair: too slow poor handeling etc??

The new owner was in his FIRST boat ever.
​27ft. 454 with trim tabs and of course T@T.
​The sales guy that took them for the shakedown didn't show them anything.
​Within minutes I had the boat up on plane and moving and level.
​The owners were setting there with the biggest smile you could ever want to see.
​They had NO idea how to handle a boat or run one and no idea how to take care of it.

​I talked them into a 1 day training run .I charged 100$ back then 1992.
​Spent the day instructing and showing how to handle their boat.
​They gave me a $200 tip on top of the $100 .
They bought lunch too.

 

[flyingscott]

I agree you are not going to get much more speed out of that set-up. You can optimize it to get a little more but i wouldn't expect a lot.

 

[LarsMW]

Ok guys: I have trimmed up/down back and forth and done "all the tricks in the book" Max speed is 36 knots... Still only 5300-5400 RPMs at max. What can I do to get more RPMs (and some more speed)?
Jerry: I will pay you 500 bucks if you come to Norway for a lesson (Including lunch!)
The prop is a Mercury Vengeance (steel) and it looks more less spot less

 

[ajgraz]

Looks like you picked up 4 knots at same rpm vs the original post, so sounds like your trimming worked for you.

How is the boat performing overall? Hole shot, "holding" the water, control, etc.?

What is your prop pitch? Engine model number? Approx wt of your whole rig (hull, motor, batteries, fuel, gear, people)?

As to speed: we can assure you higher WOT rpm, but higher mph is no guarantee.