rpm issue

I have a 1996 Astro 18.5" with a mercury 150 efi that i just bought. The boat runs real well . Old tach was bad installed new quick silver tach. The problem i'm having is the boat only runs at 4900 rpm and gps at 56.9 mph. The boat is fast enough for me but the engine redline I belive is 5400 rpm. It doesn't matter if I have 2 people in the boat or 1 with a full tank of fuel or almost empty same mph and rpm. Motor trimmed up all the way watching rpm and gps to see if i'm losing mph but i'm not. And the boat diggs pretty deep on the hole shot and doesn't get on plane to fast. It has a trophy 25p prop

Re: rpm issue

Do you have a jackplate?

Re: rpm issue

Dhadley said:

Do you have a jackplate?

Click to expand...

Yes. I have a jack plate with a 4in setback

Re: rpm issue

How high is the motor? A 4 blade will like to run pretty high compared to a 3 blade of the same blade configuration.

Re: rpm issue

I'm thinking you may want to try coming down 2" in pitch if everything else (like height) checks out OK.

Bob

Re: rpm issue

Dropping pitch as a first effort is the common error made in most cases...ALWAYS maximize what you have, first, or it could end-up costing more than just 1 prop...
My first question, beings you say you trim and trim with no loss in speed, would be..do you have the lower unit with water pick-up in the nose of the gearcase? and, do you have a water pressure gauge?
And I would follow with Dhadley and say let's get this sucker up until we see some venting or water pressure issues so we know where we are as far as Heigth...

Re: rpm issue

no i'm not losing any water press. I keep a close eye on my water press. and temp gauge. water press. is at 30 psi and temp at 140deg when i,m trimmed all the way up. and i,m not leaving any rooster tail whats so ever so i'm thinking the motor is to low and that might be why I have super hard torqe pull to the right. I messured from my pad hight to the nose cone and get 5 1/2 in

Re: rpm issue

walleyehed said:

Dropping pitch as a first effort is the common error made in most cases...ALWAYS maximize what you have, first

Click to expand...

While I agree that dropping pitch can be a "band-aid" approach - note that I was careful to say after "everything else checks out OK - like height"

Re: rpm issue

Don't take that wrong, Crownie, but if we're not perfectly clear on what should be done first, we can go back to the old fassioned way and just use pitch, as most will understand that, but few understand there are many other ways to increase RPM/speed (efficiency), and that's why "I" am here...atleast that's my goal. Not eveyone will catch the part about "everything else checking out", because most people looking for help "might" not know what everything else is....
Sometimes the details take longer to type, but if we don't, we really aren't helping much...some people have a better grasp than others...you can "read" it in the first post....nothing intended by that either....

Re: rpm issue

I see what you are saying - I was just counting on you other guys to fill in the blanks, because I could see you were on board already.

Still - IMHO, in the end, he is going to find he is a little over-propped.

Bob

Re: rpm issue

That may well be the case..but let's think about a little detail Master Dhadley taught me....when elevating an engine and losing drag, of course we begin to gain speed...well, something else that happens is we are finding ourselves with less wetted surface as speed increases and although we have the same HP, the load or drag lessens with speed due to lift, so LESS hp is required to maintain. At some point, it is very possible to make a good jump in speed/RPM due to this.
The first time I ran a TXP, I was kinda upset at the performance until I started dialing in the set-up...when I got the combo right, I was 725RPM higher than where I started with it, so is gaining 600-700RPM possible? You darn right it is...especially when we don't know how far off it is without actually seeing or running that boat. It "could" be that it's close now, but sounds like it's quite low to me due to available full trim..first indication...
In the long run, yea, it may be over-propped, or even improperly propped.