2008 Mercury 75hp fourstroke / rpm probs

[mikesnredding]

Hi, I just bought my new Crestliner 1650 Canadian last friday, Me and the wife took it out saturday and ran it. The boat has the standard 13 x 17p prop. We recorded a max speed of 32mph at 5000 rpms. this boat only runs 20mph at 4000 rpms and at 3500 totaly drops off plane. I'm not sure if this is a prop issue or the fact that the cavitation plate is 1" below the back of the boat, it's only slightly below the actual keel, but on either side it's a full 1" below. The Stealer wants to mount a 15p prop on it to raise the rpms but would'nt it be better to just bring the cavitation plate up to the bottom of the boat or higher? this boat requires constant trim while making any throttle changes. I went with a 75hp for the little extra oomph but am now wondering if I should;nt have stayed with the 60hp and got the same performance. thanks

 

[Silvertip]

Re: 2008 Mercury 75hp fourstroke / rpm probs

If this Canadian 1650 is the boat I just checked, it is rated for a 115 HP motor max. The rule of thumb is one should not power a boat with an engine that is less than 75% of its maximum rated horsepower in order to maintain acceptable performance. 75% of 115 is 86 HP so you have a terribly underpowered boat with the 75 HP motor. A 60 HP engine would not likely have even been able to plane the boat. Unfortunately your dealer did you a disservice by selling you an underpowered boat. I can't imagine that Crestliner even suggests 50 and 60 HP motors for a boat this size. A 15P prop will help get the rpm up, but it will not likely increase speed. It will help hole shot but it won't help staying on plane. Nauticus Smart Tabs (trim tabs) would help in that regard.

http://www.crestliner.com/boats/boat_model.asp?TID=345&BID=186

 

[mikesnredding]

Re: 2008 Mercury 75hp fourstroke / rpm probs

Thanks, i know my dealer has sold a ton of the boats with the merc 60hp. I at least wanted 75 for fuel economy. I go back to the stealer on thurs to try a 15p prop. Isn't a 15p prop more for pontoons?