RPM's don't make sense

[mburgholzer]

I just installed a new 350 / 325hp at 5400rpm engine. Max rpm I am getting is 3500. My prop is a 17P which doesn't make sense. The tach has been checked and is functioning correctly. My outdrive is a Alpha1. Does anyone one have a idea? Thanks for any help.

 

[TilliamWe]

Re: RPM's don't make sense

I just installed a new 350 / 325hp at 5400rpm engine. My max rpm I am geting is 3500. My prop is a 17P which doesn't make sense. The tach has been checked and is functioning correctly. My outdrive is a Alpha1. Does anyone one have a idea? Thanks for any help.

You're right it doesn't make sense.
Are you sure about that tach? I mean really sure?
Is the bottom of the boat fouled with growth? What is the boat, exactly?
What 350 is rated at 325 hp at 5400 rpms? That's a pretty high RPM for max hp in a marine engine. 5000 is more like it.

 

[45Auto]

Re: RPM's don't make sense

Something is wrong with your motor.

Compression, ignition timing, firing order, carburation, cam timing, etc, etc.

You'll have to find what's wrong with it. Start with basic engine troubleshooting.

 

[Hansolo99]

Re: RPM's don't make sense

Also what is your speed? My friend is having the same problem, but he is having the tach checked tomorrow by a tech to make sure all is working as it should....

 

[mburgholzer]

Re: RPM's don't make sense

Boat is a 23' Wellcraft. New Jasper engine tweaked a little. This is right off their spec sheet. Bottom of boat perfect no moss, speed at 3500 rpm is 48mph. Still a little slow out of the hole. Engine is running as smooth as can be both in idle and running. I am thinking about double checking the tach.

 

[steelespike]

Re: RPM's don't make sense

Does the motor sound like it's wound tight? 3500 is loafing.And you won't see 48 at 3500 w a 17" prop.

 

[mburgholzer]

Re: RPM's don't make sense

No it doesn't sound like it is loafing. That is why I think I may have my tach checked again. How would I do that? Last time it was at a repair shop and they said it was fine. Thanks again.

 

[mburgholzer]

Re: RPM's don't make sense

PS I know 48 is good with a 17p but it still does not have the jump out of the hole. Which is what you would think with a 17p prop. That is why I think maybe the rpm's are right. I have been out of boating for 20 yrs and am starting to wish I stayed out of it.

 

[steelespike]

Re: RPM's don't make sense

Yes 48 may be right but not at 3,500 with a 17" prop.

 

[Hansolo99]

Re: RPM's don't make sense

Yes 48 may be right but not at 3,500 with a 17" prop.

I agree that is the same problem a firend is having at 43 MPH with only 3900 RPM. He has a 4.3 V6 and the speed is right where it should be....

 

[180shabah]

Re: RPM's don't make sense

Yeah, 17" prop, 48mph, and 1.5 gear ratio(should be anyway) 12% prop slip(guess) all adds up to 5k plus not 3500.

23' wellcraft probably has fairly steep deadrise - I wouldn't expect it to "pop" out of the hole with any prop. Tabs will help though.

 

[TilliamWe]

Re: RPM's don't make sense

speed at 3500 rpm is 48mph...

As already pointed out, that is NOT possible. Your tach is wrong.

 

[xeddog]

Re: RPM's don't make sense

I have seen a lot of discussions regarding prop pitch, but what about prop diameter. What if someone put a larger diameter 17p prop on there?


Wayne

 

[180shabah]

Re: RPM's don't make sense

I have seen a lot of discussions regarding prop pitch, but what about prop diameter. What if someone put a larger diameter 17p prop on there?

Diameter is secondary, it makes a difference at WOT, but it is fractional compared to pitch. Besides, this is an alpha - max prop diameter is only 15".

 

[Hansolo99]

Re: RPM's don't make sense

As already pointed out, that is NOT possible. Your tach is wrong.

Just got back earlier today from the boat mechanic and YES the tach is wrong in my buddies boat. A quick check can help find the real problem because as stated earlier it does not add up.

 

[180shabah]

Re: RPM's don't make sense

solo - You really nered to start a new thread for your buddy's boat before this gets too confusing...

 

[Hansolo99]

Re: RPM's don't make sense

solo - You really nered to start a new thread for your buddy's boat before this gets too confusing...

Whats confusing his tach seems to be wrong.....

 

[180shabah]

Re: RPM's don't make sense

There is nothing confusing about the tach being wrong. In a tech forum though we try to keep it - one issue = one thread so that the "conversation/responses" don't get confused.

Not trying to ruffle any feathers here, but it sounded like you might be posting follow ups on your friends boat, and if this is correct, I am suggesting that out of respect for the OP it needs to be discussed in a seperate thread.

If I read more into your post than was there, I appologise.

 

[TilliamWe]

Re: RPM's don't make sense

If I read more into your post than was there, I appologise.

I think he was "comparing" this guy's tach to his friend's situation. Kind of a "it happens" thing.

 

[180shabah]

Re: RPM's don't make sense

I think he was "comparing" this guy's tach to his friend's situation. Kind of a "it happens" thing.

Thanks for keeping me straight TW.

I found his thread about his buddy's boat.