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Posts: 247
Location: Uxbridge Ontario | Hey guys, I have a 2007 Merc Optimax 75hp on a Lund 1750 Outfitter ss. This weekend I was having trouble hitting peak RPM. I usually run 5700rpm and a top speed of 39 - 41mph on gps; But for whatever reason the boat would only top 52-5300rpm and run about 35-36mph. I really noticed a heavy nose plow through the water and the boat just didn't want to smooth out or get out of the water. Trim was working fine and I was playing with it to find a swaet spot. I can usually run quite a bit a trim but was cavitating the prop trying to pickup the nose.
Anyone know why this could be? I have never had this issue.
The only thing I have done was install a new Sonar transducer, and was actually running with less gear in the boat than usual.
For my wallets sack, I really hope my buddies Nitro 929 with a 225 opti making my boat feel slow LOL.
Any suggestions?
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Posts: 32886
Location: Rhinelander, Wisconsin | Where did the new transducer get installed, and which model/brand is it? |
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Posts: 247
Location: Uxbridge Ontario | Installed about 17" to the right of the engine (looking at the back of the boat) and along the bottom edge of the hull and not directly behind an stake. Tiny little roost behind the transducer at WOT that need slight to adjust yet.
New finder is a Humminbird 788ci. |
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Posts: 3480
Location: Elk River, Minnesota | Hiya,
First...a question. How hot and humid was it when you ran the boat? If it was really hot and humid, the loss in RPM's would be expected. When the air is as juicy as it has been (at least here in MN) makes for a "wet" mixture of fuel and air which will not burn as efficiently as when one runs their motor in, say 70 degree temps and normal humidity levels.... My Yamaha 90 2 stroke is very finicky in this regard as well. Like you I can normally get to about 41 mph at 5400 rpms with my normal load, but on days like this past Tuesday, it would be hard pressed to get RPM's up to 5100.
The other issue that takes place with the warm temps as well is that with water, it expands, therefore your boat, although unable to tell by just looking at it, sits lower in the water, even while at speed which increases the wetted surface of the hull. More wetted surface means more friction, which robs speed and forces the motor to work harder to keep a specific speed.
When you combine those two elements, performance is robbed pretty dramatically.
Not sure if this is the case with your boat, but something nice to know nonetheless...
Steve |
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| Could be a stupid question, but did you check to make sure there was no water in your boat?
Did you silicone up the holes real well for the transducer? Maybe you are taking on water or something.
I have personally had this happen, no idea I had water in the boat, but it would definitely cause issues similar to what you are experiencing.
Hopefully its an easy fix such as this. Good Luck! |
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Posts: 247
Location: Uxbridge Ontario | Good theory Steve, never though of that. And it was pretty "stinky" here in Ontario too. Makes sense I suppose. I hope to take to boat out again tonight, and it is drastically cooler and less humid than it was last weekend.
Captain,
Yes, I checked the boat for water a few times. It really felt as if i had a few hundred extra pounds in the boat. When I installed the transducer laced the drill holes with a ball of silicone before running the screws in.
I also opened up the floor last night an siphoned out as much fuel from the tank as I could get. Going to run some new fuel as well. Just to be safe. Kept this other stuff for the lawn tractor.
Thanks guys.
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