What kind of RPMs are you guys running?

[mnypitboat]

Just curious. I cannot seem to get much over 3k, but the boat jumps out of the water and up on plane and before we put the new short block and distributor in there it would do 56mph with the car GPS. Not sure what it is doing now, because I have not tested it against the GPS, but the RPMs still wont go over 3k or so.

The boat is a Tahoe Q7 with a 5.7L 2bbl Mercruiser and an Alpha 1 outdrive. Not sure what prop is on it right now. I seem to have lost the paper I wrote it on and the boat is at the marina.

The reason I ask is because I was out with a buddy with a Caravelle206 bowrider, about the same size as mine, with a 5.0 Volvo I/O. His has no problem revving right on up, but takes forever to get out of the water. His speedo at WOT reads about 53ish and the RPMs are, if I remember correctly, up around 5500.

My Chap had a Yamaha 5.7l I/O and it screamed, jumped out of the water, and the RPMs went right on up to 71mph. Its been a while, so I cant remember the exact RPM it was sitting at, but it was up there.

I am assuming it is just the prop, but I figured I would check with you guys. Do you think its ok, or should I seek a differant size prop?

 

[cr2k]

Re: What kind of RPMs are you guys running?

Where are the new short block and dist from? Auto, Marine?

 

[HT32BSX115]

Re: What kind of RPMs are you guys running?

The boat is a Tahoe Q7 with a 5.7L 2bbl Mercruiser and an Alpha 1 outdrive.

Howdy,


Generally speaking, that engine usually should turn around 4600-5000 RPM MAX. (Your owners manual will be specific)

The range is usually 4200-5000 etc........

Not sure what prop is on it right now.

That's fairly important. As is the drive ratio. If you do not have an owners manual for your engine/drive combination, it would help if you posted your engine serial number.


If you're REALLY turning only 3000 rpm either your tach is defective, or our engine has a problem. either way, without knowing your actual RPM and speed..........it's impossible to make a judgment on it.....


Regards,


Rick

 

[Bondo]

Re: What kind of RPMs are you guys running?

Just curious. I cannot seem to get much over 3k, but the boat jumps out of the water and up on plane and before we put the new short block and distributor in there it would do 56mph with the car GPS. Not sure what it is doing now, because I have not tested it against the GPS, but the RPMs still wont go over 3k or so.

The boat is a Tahoe Q7 with a 5.7L 2bbl Mercruiser and an Alpha 1 outdrive. Not sure what prop is on it right now. I seem to have lost the paper I wrote it on and the boat is at the marina.

The reason I ask is because I was out with a buddy with a Caravelle206 bowrider, about the same size as mine, with a 5.0 Volvo I/O. His has no problem revving right on up, but takes forever to get out of the water. His speedo at WOT reads about 53ish and the RPMs are, if I remember correctly, up around 5500.

My Chap had a Yamaha 5.7l I/O and it screamed, jumped out of the water, and the RPMs went right on up to 71mph. Its been a while, so I cant remember the exact RPM it was sitting at, but it was up there.

I am assuming it is just the prop, but I figured I would check with you guys. Do you think its ok, or should I seek a differant size prop?

Ayuh,... My guess is, yer Tach is bad...

Ya should be runnin' 4400/ 4800 rpms at Wot...

 

[mnypitboat]

Re: What kind of RPMs are you guys running?

It read the same RPMs before the new short block, but it is a Marine 5.7 Vortec. Heads were redone at the same time. The ignition has been completely replaced with MSD distributor, box, and high vibration coil. All Marine parts. But again, it read the same RPMs with the old engine and origional ignition from 01.

The outdrive is a standard 01 Alpha. Not sure the ratio, but it seems to me when I was gonna pick up a spare, all the ones I looked at were the same. The prop is 14.5x19. I bought the boat in a pretty rough state, so no owner's manual.

I guess I should see if I can find another way to check the RPMs. Damn, I was hoping someone would say, try this prop or something. I cant beleive it is propped wrong though, because it leaps up on plane and will do mid 50s in a 21ft bowrider with a 2 bbl carb.

 

[Don S]

Re: What kind of RPMs are you guys running?

I moved this over to the prop forum so you can find the right prop.
First, we will have to assume your tachometer is correct and that you are hitting the rev limiter. With both engine.
What you now have to do is drop in prop pitch until you are a bit over propped. Then you have some numbers to go by. Look at the sticky at the top of this (the prop forum) for the information you need to determine the best prop for your boat.
Propping is a trial and error thing. You can get close with calculators, but they can not consider how every boat is loaded, all the different hull designs, and all the other things that makes your boat yours.

 

[mnypitboat]

Re: What kind of RPMs are you guys running?

I moved this over to the prop forum so you can find the right prop.
First, we will have to assume your tachometer is correct and that you are hitting the rev limiter. With both engine.
What you now have to do is drop in prop pitch until you are a bit over propped. Then you have some numbers to go by. Look at the sticky at the top of this (the prop forum) for the information you need to determine the best prop for your boat.
Propping is a trial and error thing. You can get close with calculators, but they can not consider how every boat is loaded, all the different hull designs, and all the other things that makes your boat yours.

I am certainly not hitting the rev limiter at 3k, and am happy with the performance. I was just curious. However, from the rest of the replies, it looks like I may have a tach issue, and it should actually be reading higher. Are you sure the prop forum is really the right place for this.

 

[zagger]

Re: What kind of RPMs are you guys running?

Is your new MSD distributor a performance one that sends out dual spark pulse? Your tach might not be reading your rpm correctly. To hit mid 50's with a 19 pich prop and average slip+gear ratio you are spining close to 5k not 3K.

 

[craze1cars]

Re: What kind of RPMs are you guys running?

Pull your tach outta the dash and have a look at the back of it. Most likely there is a plastic screwdriver dial that allow you to select a setting for 4 cyl, 6 cyl, 8 cyl, etc...

Since you say performance is good, and it did the same thing with the old engine as it does now with the new engine, I'd bet your tach is simply set for a 6 cylinder or something simple like that.

Or at least grab a shop tach/engine analyzer, and temporarily hook it up and go for a ride to compare the 2 tachs.

 

[mnypitboat]

Re: What kind of RPMs are you guys running?

Is your new MSD distributor a performance one that sends out dual spark pulse? Your tach might not be reading your rpm correctly. To hit mid 50's with a 19 pich prop and average slip+gear ratio you are spining close to 5k not 3K.

It read the same with the stocker. These are numbers that help though. Kinda confirms the tach issue. Not a prop issue.

Thanks to everyone for their input. I actually was just curious, but it looks like I may have uncovered another problem. LOL

 

[milkman250]

Re: What kind of RPMs are you guys running?

Kinda off your subject, but I have a 13x19 prop on a 4.3 turning 4700 rpms and I only go 38mph on 18ft boat, no were close to the mid 50s, would my prop be messed up.

 

[steelespike]

Re: What kind of RPMs are you guys running?

You really need to start your own topic.It gets very confusing as posts start to pile up.
Is your speed by gps? What brand of boat?dry weight? May be a combination of things
38 instead of 50+ would take a monster prop slip number.

 

[mnypitboat]

Re: What kind of RPMs are you guys running?

Haven't had the boat out in a couple weeks. I know, I know, I am ready to go into convulsions. Plan is to take it out a couple times this weekend and then get back on track to take it out more often.

So I am working on getting my meter working so I can check the actual RPMs. I will report on Monday if all goes well.

 

[Robbabob]

Re: What kind of RPMs are you guys running?

Haven't had the boat out in a couple weeks. I know, I know, I am ready to go into convulsions. Plan is to take it out a couple times this weekend and then get back on track to take it out more often.

So I am working on getting my meter working so I can check the actual RPMs. I will report on Monday if all goes well.

MN,

YOu many have found another problem? Waiting to hear......

Food for thought... First of all, confirm something for me, , ,you quote, "Just curious. I cannot seem to get much over 3k, but the boat jumps out of the water and up on plane and before we put the new short block and distributor in there it would do 56mph with the car GPS. Not sure what it is doing now, because I have not tested it against the GPS, but the RPMs still wont go over 3k or so. "

Are you saying:
1* today it still jumps out of the water as it did before the engine change?
2* The old engine would do 56 mph with everything else the same?
3* The tach has the same reading with new and old engine?

Now, check the following:
4* This may be dangerous, but we'd like to know if the engine will rev higher than 3000.
5* Do you run out of throttle as you reach 3000 rpm? Check to see if throttle linkage is set correctly.
6* What is the HP differential? old engine, new engine
7* make sure timing is set correctly. Could be it is retarded (no pun).

Just can't see a prop change being the answer for up to 1800 rpm; that'd be 9" of pitch! If you've dropped in HP (have to doubt it, but have to ask), you may need a new set of lower gears.

Again, food for thought while we await what you may have found.

Good luck this weekend!

 

[mnypitboat]

Re: What kind of RPMs are you guys running?

MN,

YOu many have found another problem? Waiting to hear......

Food for thought... First of all, confirm something for me, , ,you quote, "Just curious. I cannot seem to get much over 3k, but the boat jumps out of the water and up on plane and before we put the new short block and distributor in there it would do 56mph with the car GPS. Not sure what it is doing now, because I have not tested it against the GPS, but the RPMs still wont go over 3k or so. "

Are you saying:
1* today it still jumps out of the water as it did before the engine change?
2* The old engine would do 56 mph with everything else the same?
3* The tach has the same reading with new and old engine?

Now, check the following:
4* This may be dangerous, but we'd like to know if the engine will rev higher than 3000.
5* Do you run out of throttle as you reach 3000 rpm? Check to see if throttle linkage is set correctly.
6* What is the HP differential? old engine, new engine
7* make sure timing is set correctly. Could be it is retarded (no pun).

Just can't see a prop change being the answer for up to 1800 rpm; that'd be 9" of pitch! If you've dropped in HP (have to doubt it, but have to ask), you may need a new set of lower gears.

Again, food for thought while we await what you may have found.

Good luck this weekend!

Your 'are you saying' bullets are correct.

4* This may be dangerous, but we'd like to know if the engine will rev higher than 3000.

not sure, never tried it without a load on the engine.

5* Do you run out of throttle as you reach 3000 rpm? Check to see if throttle linkage is set correctly.

No, it just stops revving. It revvs fast and hard right up to there
.
6* What is the HP differential? old engine, new engine

Same engine punched out .060. Vortec 350 with a 2bbl. The old engine ran fine, and I thought it was a knock, but after replacing it I found 2 bent valves. I think the old shortblock was fine. I thought, and so did 3 shops, that it was a bottom end knock.

7* make sure timing is set correctly. Could be it is retarded (no pun).

Timing is right on. I have checked it several times and took the distributor wrench out on the water and tinkered til it was just right. It runs like a scalded dog. My buddy has about the same size boat with a 305 Volvo Penta, and my boat runs circles around his, and I am out of the water and pulling about 10 boat lengths before he is even on plane. Then top end I am about 5-8mph faster, but his revs up to 4800 without issue.

I will find out this weekend if the RPMs I am seeing are correct. From what everyone says here, I am betting my tach is wrong.

 

[Robbabob]

Re: What kind of RPMs are you guys running?

Yup, Tach is wrong. Good deal

 

[mnypitboat]

Re: What kind of RPMs are you guys running?

Unfortunately I could not get my meter to work. Much like boats, when you let tools like this sit for a long time, sometimes they dont work when you go to use them. I havent had a need for a meter with a tach for probably 20 years, and my Fluke meter does not have this capability, at lest I dont think it does. I got out of the automotive stuff many years ago. I may have another option for testing, but we are thinking of selling the boat, so it may be the next owners problem. We will see.

 

[emoney]

Re: What kind of RPMs are you guys running?

Well then I'd just tell the new owners not to trust the tach and move on. I'd be willing to wager, and I'm no gambling man, that's it's a tach issue anyway. When you take the potential owners out for a seatrial, however, I would definitely mention it, so that they can get a feel for the way the motor sounds, etc. in the event they don't want to replace the tach.

 

[mnypitboat]

Re: What kind of RPMs are you guys running?

Absolutely. I am probably too honest when it comes to selling stuff. I dont want anyone thinking I tried to hide anything from them. I lose sales over that too, but oh well. I hate it when I get something home and find out the seller left something out.