Tech question??

[Concept1]

I have a couple questions about yesterday. We were at the lake for New Years and just running around as usual. I noticed that on the way in I was running almost 5200 RPM's doing around 62 mph or so and fully trimmed. I had a couple people and 1 full tank and a half other tank so I didnt think it was too bad running that fast. For some reason i decided to trim a couple bumps down and we started to go faster and my RPM's dropped, after 2 bumps down we were almost 67 MPH ay around 4800 rpm's. Is that normal to run harder while being trimmed down a little? Also does that mean I should try and start experimenting with props with that info and try and get it up over 5000 again?

[phebus]

To properly trim for spped, you must watch your sppedo and tach in unison. Top rpm, doesn't equate to top speed. If you over trim, rpm will go up, but your speed will go down. What you need to do, is to trim up while watching your speedo, and when you reach your maximum speed, look at where your tach is. This will let you know if you are propped right. You should be up near the rev limiter, but not bumping it. If you are hitting the rev limiter, add more pitch, and conversely, if you are down on rpm's, reduce the pitch.
This being said, all this only holds true to the conditions you are running in at that moment. Cool air vs. hot air, dense air vs. thin air, load of boat, surface conditions, elevation etc., make it beneficial to have more then one prop.

[Concept1]

Yea thats what I was figuring, it was pretty surprising to see the speed go up and the RPM's go down but I guess it makes sense. Should buy maybe a 28for my next prop and send it and my 26 out to be labbed. I was watching it and i know where the trim was at for my maximum speed it was around 4800 RPM. Is the 28 the right step up for more RPM or should I be going down.

[Deano]

Isn't that right where the HO should be running? A good cat hull shouldn't need much positive trim, if any. Your hull is just starting to perform as designed around your top speed. I would get used to the feel around there. When you are comfortable, whipple that biatch and let her fly.
The only negative thing about speed is, you really don't know how your boat is going to handle in any type of emergency situation, unless you have been there and pulled it off. I see complete idiots on the lake all the time that think they know how to drive, while their boat is on the edge of outta control. Im not saying that you would be that person, but from what I have been reading. That is your first boat? Take it easy. There is ALOT to learn about the water and how boats react to it. I grew up on fast boats and still learn every time I am out there.
I would recomend flying over to the East coast and hooking up with Tres Martin. Take a weekend to attend his class. It cost less than a set of headers and you will gain knowledge from a veteran boat driver/racer that has taken boats to their limits and beyond.
Personally, I wouldn't dump money into a deck boat and try to get big numbers out of em. (been there done that) Enjoy the boat as is and as intended, then step up to a faster hull. Just my .02

[phebus]

If you plug your numbers in a prop slip calculator, you will be able to see that after you trim up to a certain point, anything above will just result in greater slip, and less speed. You will also be able to see how efficient your prop is in regards to slip.
If you are spinning your prop at 4800 rpm now, to realize the most speed, I would get it labbed, and get your rpm up near the rev limiter. At this time of year, with the cooler air, will get you the most rpm. When summer comes, you lose rpm due to the hotter air, and you will struggle to run in the proper rpm range the manufacturer specs for your engine.

[Deano]

If you plug your numbers in a prop slip calculator, you will be able to see that after you trim up to a certain point, anything above will just result in greater slip, and less speed. You will also be able to see how efficient your prop is in regards to slip.
If you are spinning your prop at 4800 rpm now, to realize the most speed, I would get it labbed, and get your rpm up near the rev limiter. At this time of year, with the cooler air, will get you the most rpm. When summer comes, you lose rpm due to the hotter air, and you will struggle to run in the proper rpm range the manufacturer specs for your engine.
Good advise. Isn't that the boat that was tested 74.9 mph at 4,800rpm? Pretty sure it is. Sounds like labbing the prop would get it right in the proper rpm range. Also sounds like the boat is very weight sensitive. With three on board and 75gallons of fuel he lost about 7mph.
Concept1, Phebus is right. This time of year you need to be bouncing off your limiter. Load that boat up with eight peeps, full tanks, coolers, etc.. in the summer and it won't touch the limiter without alot of slip. Maybe going down a pitch and labbing will get you an all around good performing prop.(empty and loaded) that will still hit 75 empty...thats haulin ass

[Deano]

one more thing Bouncing off the limiter means that you are watching your speed and rpm at the same time. Your speed is still climbing, or at least not dropping, when you hit the limiter. Get your boat going wide open with 0 positive trim. Then start bumping it up and watching your mph. When your boat stops climbing in speed, bump it up again. Keep doing it until you stop gaining speed, no matter where the rpms are....unless you are hitting the limiter and still gaining. Then you know your prop needs to be bigger, or cupped. It all depends on the load though. You gottta get a happy medium prop that is good outta the hole too. If you pitch too high, it will be a pig getting on plane and acceleration will suffer... Thats not a good thing for a deck boat that varies in load from 300lbs to 3,000lbs

[Concept1]

Yea thats what I wanted to know, I really didnt intend to dump a alot of extra money in it now, I do agree that the big motor I was going to do was a waste for me and chose against it. This is my first boat like this, I have some experience with bass boats and stuff like that from when i was younger but i know its not the same thing. This is why I wanted to post about these questions because again I was wrong. Im going to just send out the prop to get it labbed for now. I really dont want it to be FAST or out of control im just trying to learn as much as i can before i waste a bunch of money like when i first joined the site! $40,000 for an 850 was the plan before now its just enjoy it for what it is. Thank you guys both for the help, Ill do the prop as soon as possible and report after that gets back and see what happened.

[BADBLOWN572]

Make sure that when you are doing this you are using a GPS speedo or hand held for speed. If you are using a water pressure speedo, the MPH's can be off by as much as 6-7 mph and are never consistent. Just my .02

[blender over]

Make sure that when you are doing this you are using a GPS speedo or hand held for speed. If you are using a water pressure speedo, the MPH's can be off by as much as 6-7 mph and are never consistent. Just my .02
I hate the GPS Reality sucks LOL