Boat Speed in Freshwater 90 HP Mercury with 17' Whaler Montauk

[SmallTauk]

I picked up a used 2000 17 foot Boston Whaler Montauk with a 90 HP Mercury Saltwater outboard about 2 months ago. I've been using it in saltwater since buying it and the performance has been great.

This past weekend I trailered it up to a lake in western Maine for a two week vacation. At full throttle (4500-5000 RPM) the the boat seems much slower in the lake compared to saltwater. The engine is temperature is fine.

Does anyone have an idea of why the boat could be running noticeably slower in the new conditions? I've seen some posts that state a boat could run slower in a lake (higher elevation, less buoyancy), but supposedly only a couple of knots. Is this normal or could I have an engine problem. If an engine problem where would I start troubleshoorting?

Smalltauk

 

[fishingdan]

Re: Boat Speed in Freshwater 90 HP Mercury with 17' Whaler Montauk

Too many variables to guess why you were slower. I have never noticed any significant performance differences between salt and fresh. I usually do both in any given week. Extreme changes in elevation could have an impact, but I doubt anything in Maine would qualify as high elevation.

The folks on this board will need more info to help you out. Was the load the same in the boat (fuel, people, gear, coolers, etc)? Was the engine trim the same?

Your full throttle rpms seem a little low. You'll need to look it up, but I'm betting that you should be turning closer to 5500 rpm at full throttle.

 

[scrobo]

Re: Boat Speed in Freshwater 90 HP Mercury with 17' Whaler Montauk

This is just an educated guess...

Do you think it may be a buoyancy issue? Saltwater has a higher density than freshwater so things float better in it. Maybe the boat is sitting lower in the water because of this?

 

[SmallTauk]

Re: Boat Speed in Freshwater 90 HP Mercury with 17' Whaler Montauk

Elevation at the lake is 700ft (so not a tremendous change) load was the same. I filled up the tank with regular unleaded (engine is oil injected) before launching. Tried various RPMs and trims, just seems generally slower than with the same configurations in salt water. I'm going to see what comes up here on the forum as well as trying out with higher octane gas on the next tank and see what happens before I take it out of the water and get it checked by a mechanic.

 

[JB]

Re: Boat Speed in Freshwater 90 HP Mercury with 17' Whaler Montauk

My son had a 2000 Montauk with a Merc 90. It would go 41mph with a 21" prop with two aboard. When he loaded it with kids and toys it would only do 37mph, so he changed to a 19" prop and got 39mph in both situations. Don't remember the rpm he reported.

That boat and engine combo is very sensitive to load and balance. Perhaps you had it loaded differently in Maine than offshore.

 

[Texasmark]

Re: Boat Speed in Freshwater 90 HP Mercury with 17' Whaler Montauk

Agree on watching the 90 loading up.

I have a 90 on a light boat and with just me and tackle I can do 48-50 gps, no problem with a 24P prop, at 800 ft and 5600 rpm's....did it just last Friday....in July.

I filled the livewell with water to take a fish home to the farmpond (which needs to be restocked as we had a drought the last two years....monsoon this year, and all the big fish died) and the performance of the boat went to pot.

Don't know how many gallons of water I had but it was at least 50 or so. So that's a couple hundred pounds or so aft of where I sit.

I didn't hook it all the way up to see what WOT would do, (wanted to get the fish home intact......16" Black Bass, caught 1st cast that day) but it obviously made a difference.

Told me that if I were going to do that very often I would downsize to my 21P and stay off WOT if I were light to keep the rpm's down.

Mark