How Fast should I be going??

[Aquaman-PSD]

I have a 19' fish and ski model with a 7'6 beam. I have a 93' GT 200 on the back of it. It has a Turbo prop on it ( i know this isn't a prop section but FYI) The motor sounds great at WOT and seems to be running fine. My speedometer doesn't work but from experience I would say that the max speed I can get from full throttle and trimming before I start to porpoise is around 60 or 65. This seems a little slow to me considering I have a friend with a 94' GT 150 that will do around 75! When I bought the boat it came with the Turbo prop that is on it and a Raker. Maybe thats the problem but I won't know till I change them. Still does this speed sound slow to anyone? I am not sure what the normal RPMS for this motor is and can't find it in my Clymers though I know it is probably in there. I am going to change the prop, has the compression checked, and clean the spark plugs this weekend. Any other thoughts?

 

[hidef]

Re: How Fast should I be going??

Do you have a Tach? If so what RPMs are you turning at full thottle? Max RPM is 6000 operating range for that engine is 5000-6000. The best way to check speed is with a GPS but knowing how fast you are going isn't as critical as over revving the engine.

 

[Chris1956]

Re: How Fast should I be going??

Achieving 60+ MPH in a boat is not an easy task. One must have a hull capable of that speed and a motor with enough power. I recommend you use a GPS to clock your speed. Use a tach to dial in the prop and trim to reach max RPM.
If your friend is not using a GPS to measure his top speed, I would tend to doubt his boat goes that fast.

 

[Aquaman-PSD]

Re: How Fast should I be going??

I agree that it 70 mph on the water is, as we say in KY "boot scootin" The only way that I "know" he is running that fast is by being next to him on a jet ski assuming my speedometer was correct and he was passing me easily. They are bass boats so they are very low profile and made to run pretty fast on water as smooth as glass. I will check out my tack the next time I am out.
Also I would consider 200 hp on a boat that weighs less that 3500lbs as plenty of power to reach 65 but I'm new to all this so take that for what its worth.

 

[Chris1956]

Re: How Fast should I be going??

Water driven speedos are very inaccurate. I run a Merc 135HPV6 on a slight vee hull. The entire setup weighs in a 1K pounds. I do not think I can reach 70MPH. To go 75MPH would likely require 200HP.

 

[Aquaman-PSD]

Re: How Fast should I be going??

does anyone know anyway of measuring speed without GPS? I don't have it. I guess I could mark off a mile stretch of the river and have someone time me...

 

[Bigkat650]

Re: How Fast should I be going??

Its pretty easy to get your hands on a GPS these days. A lot of people have them for your car, and any smart phone has GPS capabilities. It would probably be easier (and more accurate) to borrow a GPS from someone then it would be to try and time how fast it takes you to run a mile at WOT.

 

[hidef]

Re: How Fast should I be going??

Got a calculator? Know how to do math? Very simple to figure out your theoretical speed.

Engine RPM / Gear Reduction = prop shaft RPM

prop shaft RPM * Prop pitch * 60 / 5280 / 12 - prop efficiency loss % = speed

The hardest part is figuring out the prop shaft speed. Once you have that the rest is easy. For prop efficiency loss % it is a guess a cupped stainless steel raker prop is more efficient than a three blade aluminum. Figure 15% for the aluminum prop and 10 % for a High performance Stainless Steel prop.

 

[Aquaman-PSD]

Re: How Fast should I be going??

wow i am really ashamed to say that i didn't think about a GPS for a car...:facepalm: that would probably be easier that the math. The math behind it all is pretty interesting, but %10 prop slip sound high for a high proformance prop? I thought you should shoot for under %5?

 

[Aquaman-PSD]

Re: How Fast should I be going??

Hey all a little update. I check my RPMs last night on the river. When trimmed out all the way it reached a maximum of between 5600 and 5700 RPM so that sounds like it is within spec. I still haven't gotten a chance to change the prop so when I do that maybe it will change. I will keep you posted. I will say that last night for whatever reason it seemed like the motor was running like a scolded dog! maybe it was just me but it felt like I was flyin!

 

[Chris1956]

Re: How Fast should I be going??

A 2" pitch change on the prop will change your max RPMs by +/- 400