Prop change

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Rockxx
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Prop change

Post by Rockxx »

Having run aground last week while motoring around in the dark, I find I'm about to have to change my prop. I have a single 318 in my 72, 28' express and am currently running a 14 x 10 prop. I have the original delivery sheet on the boat and this is the same pitch prop that was delivered on the boat. The chart that was on the original Marinette web site called for a 14 x 9. As I understand it, you lose 160-180 wot rpm with each inch of pitch increase. In order to expedite the change out I've purchased a reworked prop from a local prop shop and the closest prop he had was a 14 x 11. I was only Turning 3000 rpm at wot with the old prop so am figuring I'm dropping to the 2800 range with the new prop. Since I spend very little time at wot can anyone give me pros or cons regarding the pitch change I'm about to make. Prop is returnable if not installed so any help would be most appreciated.
Regards to all,
Rockxx
1972 28' express "Safari"
Single Chrysler 318
Center Hill Lake, Tennessee
jralbert
Site Admin
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Re: Prop change

Post by jralbert »

I recall the engine manual for my boat (yours may vary) called for WOT of 4,000-4,400 to avoid "lugging". I leave it to you, your mechanic, others on the forum to chime in on what this might mean for your plans. Could the shop not re-pitch it to 10?. Since you've bought the prop from him, he might cut you a break on re-pitching
-joel-
former owner 1988 '32 FB Sedan
Chesapeake Bay
twin 318 / 240 hp
Potomac MD
EWRice
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Re: Prop change

Post by EWRice »

If your engine is tuned right, 3000rpm is too low wot. You need to be able to hit at least 3800. I definitely would not put a steeper pitch on. If anything, I would step back an inch or so. I would double check timing and throttle cable adjustment before making any changes in pitch. Then talk to a good prop guy. Verify your tach is accurate, too.
Muskegon Lake
1972 32' Express flybridge
"AL13"
Twin 318s
On board air & prime 920
1963 Thompson Super Sea Lancer
Graymarine 327
1961 Alumacraft 12'
'55 10hp Johnson
spdracer
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Location: Port Clinton, Ohio
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Re: Prop change

Post by spdracer »

4000-4200 is a must assuming the engine is tuned and running correctly. You prop the boat to achieve that.
Chris Hamann
Lakeland Auto & Marine
Port Clinton, Ohio
barkleydave
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Location: Kentucky

Re: Prop change

Post by barkleydave »

Remember that WOT specifications were with fresh engine(s) half tank of fuel no fresh water and calm seas. No additional gear was on board during the test. (Most our M's are pretty well loaded with gear, spare parts etc;) Aso clean bottom!

My 29 had 9" pitch props when I got it. It was setup for trolling and propping it down slowed the idle speeds. Best I every did with my engines and normal gear, full fuel and water was around 3600-3800. I repropped to 11 pitch and gained 4 MPH at 1,700 rpms. Did loose WOT down to 3400-3700. With around 1,100 hrs on the engines and 29 years of use I am satisfied with the performance.

She will plane comfortably at 2200 rpms.

I seldom run above 2,500 rpms

safe boating,
dave
1987 Marinette 29 FB Sedan
Retired Boat Accident UL and USCG trained investigator
Retired USCG Captain
Rockxx
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Re: Prop change

Post by Rockxx »

Many thanks for all of the responses re the 11 pitch prop for my 1972, 28' single 318. It is my intention to never run my 45 year old engine at wot. I'm a 72 year old cruiser who knows how it feels to be over worked. Barkley Daves advice that he re-proped to an 11 pitch and gained 4 mph is exactly what I needed to hear, as 1500 to 2200 is were I feel the old girl is comfortable.
Regards to all,
Rockxx
1972 28' express "Safari"
Single Chrysler 318
Center Hill Lake, Tennessee
jralbert
Site Admin
Posts: 885
Joined: Fri Jul 11, 2014 9:15 pm

Re: Prop change

Post by jralbert »

The point of a wot is not that you would necessarily run at that rpm but that the engine is adjusted to take it - a kind of balancing. I'm sure the engineers had a reason to put that info in the manual. Then, when it's capable of hitting that top rpm, you can cruise at whatever makes you happy (which, for economy, is at/just above planing)
-joel-
former owner 1988 '32 FB Sedan
Chesapeake Bay
twin 318 / 240 hp
Potomac MD
spdracer
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Location: Port Clinton, Ohio
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Re: Prop change

Post by spdracer »

Nobody runs @ WOT for extended periods. The point is that if the engine can attain 4200-4400 RPM, the boat is propped correctly and in good mechanical shape. Propping boats to gain MPH at cruising speed or a specific RPM is not a good idea. It can create detonation which can damage pistons, valves, valve seats, and head gaskets. I would assume most people cruise @ 2800-3200 RPM just our of the 4bbl.
Chris Hamann
Lakeland Auto & Marine
Port Clinton, Ohio
Tranquilo
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Re: Prop change

Post by Tranquilo »

I'm looking for a 14x9 myself because I think my current prop is 14x11 or 10 (can't remember, only know its pitched too much)

I can't get above 3200 rpm and that maxes me at 15mph on flat days at sea. My 73 28 express in Ohio does 24mph in the same conditions and I believe is a very old 14x9 (maybe not original but it looks very old and beat up) but my max RPMs are 3600 on that one.

Maybe my engine sucks but I'll reprop before dropping a new engine. Just have to find the correct prop (its harder than I thought)
2x 1973 28 Express - Single 318
New electrical panels, water system and velvet drive
Miami FL & Catawba OH
Rockxx
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Joined: Fri Mar 06, 2015 12:39 pm

Re: Prop change

Post by Rockxx »

I haven't installed the 14 x 11 yet as have other problems at present with my motor. One damn thing after another. I'm about to start another post in order to try and find a set of heads for my 72 318.
1972 28' express "Safari"
Single Chrysler 318
Center Hill Lake, Tennessee
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