Diam. or pitch? Updated!

[NW Redneck]

My F-I-L has a 22' Campion Haida with a Merc 250 (350 chev) S/N 4768270. WOT rpm is listed as 4200-4600. Currently running a 14.5 x 15 3-blade alum. prop. With this setup we're only getting 4000 rpm. Do we need to change diam. or pitch to get the rpm's up? From what I've read it seems we need to drop in pitch? It's a prety heavy boat, will reducing pitch only cause too much slip? Thanks for your help.

 

[walleyehed]

Re: Diam. or pitch?

Re: Diam. or pitch?

I'd look into the tach before I spent a dime on a prop..

 

[NW Redneck]

Re: Diam. or pitch?

Re: Diam. or pitch?

Ok, got some more info. Alpha one drive. I had the prop size wrong in my first post. Current prop has the numbers 48-78/16-15. I verified tach reading with a blue point automotive dmm. Tach reads 4000 rpm, dmm says 3986. Pretty darn close. What should I do to get WOT rpms up to 45-4600?

Thanks!
Colin

 

[walleyehed]

Re: Diam. or pitch? Updated!

Does this boat set in a slip, or do you trailer it?
My Merc book says 4400-4800 for all 5.7L Alpha1 drives, injected or carbed with 1.47:1 ratio.
I also show a 15P for 25-29ft boats at 5100-7100lbs....
Any idea what the boat may weigh?
Has a Tune-up been performed lately?

 

[NW Redneck]

Re: Diam. or pitch? Updated!

Thanks for your reply.
Boat is in a slip. Original label on valve cover says 4200-4600 rpm. Model: MCM 250 Serial: 4768270. It's a carbed engine, and has just had a tune-up including new distributor. Manifolds replaced about 1.5 yrs ago. Engine starts easily, runs smooth at all rpm's. Not sure of weight, but it seems to be a prety heavy boat. 21-22' with command bridge. Not sure of gear ratios.

 

[walleyehed]

Re: Diam. or pitch? Updated!

I think it may be possible that you have some bottom growth that is causing some issues.
The RPM figures have been up-dated, and I think I'd shoot for 4700-4800.
That size of boat with the drive you have should be spinning atleast a 17P at max RPM.
Any way to check the bottom...?
12-14 days is all it takes to start a growth that will affect speed/RPM greatly....especially over a period of time.

 

[NW Redneck]

Re: Diam. or pitch? Updated!

The boat was out of the water about 5 months ago where it had a fresh coat of anti-fouling paint. It's been used intermittantly for the first couple of months while some engine problems were sorted out (hence the new distributor!) and for the last 6 weeks I have been taking it out for quick runs once a week.

I cleaned the outdrive today which was starting to get some barnacle growth but otherwise clear, and the hull looked pretty and blue still. Just a little brown alge around the transom area that wiped off easily.


PS: What are the first two numbers for? I figured the second two as the diam and pitch correct?

 

[walleyehed]

Re: Diam. or pitch? Updated!

Well, if you feel the engine is up to par and the bottom is clean, we may be looking at a 13 pitch prop but thats listed as a starting point for boats of 5900-8400lbs and 26-34ft long.....


First 2 numbers are just typical of Merc...some don't have 48- in front...the middle numbers are 5-6 digit and usually are the style of prop and indicate what it may fit. The last 2 -15, -17, -19 is the pitch. usually the diameter is listed on the hub under the nut.

 

[NW Redneck]

Re: Diam. or pitch? Updated!

Thanks for all your replies. You're a prety busy guy over here! I'll try to get a hull weight before I try other props. Could there be something else I'm missing that's causing me to run 600+ rpm's slower than expected with the current prop? With the ignition timing known to be good, could it be a carb issue? Jet sizes maybe?

 

[NW Redneck]

Re: Diam. or pitch? Updated!

OK, a little googling turned up a hull weight of 5500lbs.
Not sure of year, but not much difference between '78-'82 and I know it's in that range. All listed at 5500.

 

[walleyehed]

Re: Diam. or pitch? Updated!

15P should be a good choice...the tach is saying other-wise..
The spark arrester is not full of belt dust or anything like that?
something is holding us back here...

 

[NW Redneck]

Re: Diam. or pitch? Updated!

Spark arrestor was cleaned in solvent and blown out with tune-up incl. plugs, wires, filters. I trust the readings I got with the DMM. I borrowed it from a friend who's been a mechanic for 20+ years and he's got good stuff. Blue point is top of the line.

If you feel a 15 should be doing better than this, and I trust your judgement, maybe I need to do a little more looking at the engine. I'm thinking of puting a guage on the fuel line to see if I'm getting good pressure at WOT. If that checks out I'll move to the carb and see if I can get specs on it and see if it's set up right.

Thanks again for your advice!

PS: Belt is new too.

 

[walleyehed]

Re: Diam. or pitch? Updated!

Check and make sure throttle plates are in the 90 degree, full-open position with the throttle WO static.

 

[NW Redneck]

Re: Diam. or pitch? Updated!

Good thought! Thanks.

Edit: OK, i just got my hands on the '74-'78 Mercruiser shop manual my FIL was given with the boat with a prop chart. It gives me:

Model: 250
RPM range: 4200-4600
Drive gear ratio: 1.32:1 (?)
Approx. boat length:22-27'
Approx boat weight:3700-5400
Diam:14 1/2
Pitch:15"
Blades: 3-Aluminum

I believe my current prop is a 16" diam?(16-15) Am I too large for a 15" pitch? And are the gear ratios they give right?

 

[walleyehed]

Re: Diam. or pitch? Updated!

My Merc book does not show that gear ratio....doesn't mean it doesn't exist, just has no info such as that.
All 5.7L engines have been up-dated to 4400-4800RPM.
The 16" prop should be a good choice for a heavy boat...better than 14-1/2" for sure, but that IS enough diameter difference to lower the RPM some...400-800? I don't think it would be that much.
I'm still concerned with bottom growth.....

 

[NW Redneck]

Re: Diam. or pitch? Updated!

Gonna have to wail until next weekend to do any further checks. Decided to play with MY boat today! (see new thread in this forum!) 8)

The bottom is pretty clean, but I'll take the brush to it anyway. I'll check the throttle plate position like you suggested and see if I can find a guage to check fuel pressure too.