New RT178 owner with rpm & speed advise needed

[dan2116]

Just took out my new RT178 w/Merc 75hp 4 stroke and need advise . The boat will run GPS speed 36.4 speed with the trim up till the point the boat begins to porpoise? ( bounce ) severely until I trim down and stop the bounce . At that point my max rpm is just a bit under 5000 rpm with the speed at 36.4 . water condition was optimum with a little (2-3 mph wind ) . Factory aluminum prop installed and the motor is in the second from the top hole position. Dealer said the measurement they checked with the motor in its position is proper. Thoughts ??

[fishon396]

Speed is about right for that setup you may get a couple more mph. Mine runs best in the third hole from the top and a Spitfire X7 4 blade prop. It took me a while to figure out the trim and I was over trimming my boat.

[alanhore]

Speed is about right for that setup you may get a couple more mph. Mine runs best in the third hole from the top and a Spitfire X7 4 blade prop. It took me a while to figure out the trim and I was over trimming my boat.

17 pitch SpitfireX7?

[fishon396]

17 pitch SpitfireX7?

Yes that is the one 17 pitch. At the time I bought it Jacos Marine had the best price.

[AlumRanger]

Speed sounds about right, I'm not familiar with the 75 hp Merc, I'm wondering if 5000rpm at wot is correct for that engine. My Yamaha runs closer to 6,000rpm.

[dan2116]

Put a new Apollo 13 1/4" x 17 on my 75hp 4 stroke Merc ( RT178 ). W.O. still 5000rpm but picked up right at 4mph (gps) . Just a tick under 40mph with me ( 285+lb'r ) and 1/2 load of fuel. I know of a RT188 ( 115hp merc ) owner that dropped his motor 3/4" and made a world of difference for him. My motor is in the second from the top hole position.

[fishon396]

I think max RPM is 5500 on that motor. I rum mine in the third hole from the top but I am not sure what my RPM number is.

[dan2116]

fishin396 - What motor are you running and what is your best speed set in the third hole? Are you running a stock aluminum prop or stainless .

[G4bama]

I would not trust the console tach. Invest in a Vessel View Mobile module. Money well spent and peace of mind

[fishon396]

I have a 4 blade spitfire x7 it is not aluminum it is stainless or some kind of alloy. My top speed was 37 but I did not pay much attention to the RPM because of accuracy. I am going to try and figure that out this year with the vessel view mobile app.

[dan2116]

I have a MercMonitor installed . I went out today with my fishin partner and water was rough. Best was 37.5 @ 4950 max rpm with it trimmed up until it starts bouncing. 35-36 mph is a comfortable speed @ 4650 - 4700 rpm. I have read where owners have different motor placement ( 1rst , 2nd , 3rd whole mount ) Mine is in the second hole and was wondering if it would worth a try up or down to maybe gain rpm and speed.

[fishon396]

Mine came mounted in the third hole and some come mounted in the second hole. I hope to get my boat out in the next 10 days to get some numbers to try and figure out what my max RPM is and I will post them on here. Did you install your MercMonitor yourself I have wondered how hard it would be to install one.

[Tim Kelly]

I have a MercMonitor installed . I went out today with my fishin partner and water was rough. Best was 37.5 @ 4950 max rpm with it trimmed up until it starts bouncing. 35-36 mph is a comfortable speed @ 4650 - 4700 rpm. I have read where owners have different motor placement ( 1rst , 2nd , 3rd whole mount ) Mine is in the second hole and was wondering if it would worth a try up or down to maybe gain rpm and speed.

You really should try to get the rpm up another 1000. Sounds like you have far too much prop.

[dan2116]

fishon396- I had the dealer install it.

I agree with you TimK. The dealer checked the motor hight and said it was correct . If a prop change is made to increase the rpm , will there be a increase of speed? This is my first ever boat of this kind and know nothing about motor setup.

[fishon396]

I cam from a boat with a tiller motor so this is also my first boat with lots of things to learn. Lifting your motor one hole would be the cheapest thing to try. Like you I have a newer prop and think I may be on the lower side of the rpm range so I will be looking at my numbers when I get on the water. I like the 4 blade prop it did make the boat handle a little better and I finally figured out these boats do not need a lot of trim. I think I will be talking to a sponsor Marks Propellers a guy named Mark Croxton and see what he thinks about adding some cup or reworking the pitch a little or maybe another prop to get me to closer to 5500 rpm. I am not worried about speed 37,38 is fine with me I am looking for the best handling and the well being of the motor to run it hard every now and then.

[dan2116]

fishin396- In what way did the 4 blade prop improve the handling. I am with you as for speed . I could not afford the fast boats and am happy with cruise speed of 35-36 . My boat does not use much trim either. My last and first fishing boat for the past 24 years was a Lowe Lake Jon 14' w/Johnson 35hp tiller ( pull start no less ) set up with front trolling motor , floor boards and a stringer for live well :o)

ither.

[fishon396]

I seems like I can drive around on plane at a lower speed, I think it makes the turns better, and it feels like there is less torque steer. Now the less torque steer may be because I am learning the boat better but with the black max 3 blade prop it seemed like by the end of the day my arms were wore out from steering the boat. It felt like I was arm wrestling with the steering wheel at some higher speeds.

[dan2116]

Just an update. I called Mercury Marine and asked if running in the 4600-5000 range with the 75hp four stroke was in any way harmful and he stated that was right in the middle of this motors range and was just fine ( 4000- 5500 ) . I had heard that if you didn't run in the 5500 area it put undue stress on the mains . The word that the Merc tech used for that thought was malarkey !

[fishon396]

Thanks for the news I am glad to heard that from Mercury.

[BassNvlog]

I wouldn't trust the TACH on these boats either. I run an F70 Yamaha and have had the take past 6500 with no alarms or rev limiter kicking in. Yamaha told me that the rev limiter should kick in at 6300.