Old boat, prop selection for cruising performance

whiskeyRichard

Petty Officer 3rd Class
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Sep 16, 2018
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96
I have a 1980 24 foot SKV day cruiser with a mercruiser 260 and alpha one out drive with 1.47 drive ratio (picture attached) . Not sure what the boats weight is, but for these types of boats, it is a heavy one.

WOT throttle, measured with the tach in the dash is about 4500 RPM which seems correct based on the data plate on the engine.

I am currently running an aluminum prop that is stamped 15p (picture attached).

I am looking to maximize speed at 2600-3000 RPM's and the ability to get to that speed quickly. Would a different prop make a difference for me? In my reading, a SS prop may help some, but would a different pitch or number of blades help as well?

Also, I am curious as to why there are wear marks on the prop in the two locations shown in the attached picture. Does that indicate some sort of off balance situation?

Thanks for the info,
wR
 

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Last edited:

QBhoy

Fleet Admiral
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Mar 10, 2016
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I can tell you that the local merc dealer came up with the idea of trialling the 16x16 prop on similar boats on my lake. Had great success with it. Specifically targeted the usual 25-30ft cruisers with the 5.7. The usual bayliner 2655, 2855, maxum, etc etc. Gives incredible acceleration out the hole with the big diameter blades and not too hard on the top end. Great for cruising. But...most of these boats with that engine, alpha drive and 1.47 ratio were 17” black max standard props. Occasionally a 15” on the larger end of the scale.
 

alldodge

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Mar 8, 2009
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Stainless props weight more but blades are thinner. Being thinner will reduce some drag and and can increase efficiency. The aluminum props will flex under load where stainless does not. If you hit something with aluminum, in most cases its the prop that needs replaced, stainless it can be the drive.

So stainless less drag but blades will not flex so rpm may stay the same but could go down. Going to a larger prop (pitch or diameter) will lower rpm at WOT and cruse, but thins also causes the motor to work harder.

A motor burns very little gas reved up in neutral. If a motor is correctly propped (WOT 4400 to 4600) it will burn less gas turning 3600 rpm doing 30 mph, then if it was turning 3000 rpm doing the same speed.

For quick hole shot, pitch is reduced, and if a bit to much then you can over tach. Its better to have it on the over tach side then be a few hundred rpm under WOT range
 

jimmbo

Supreme Mariner
Joined
May 24, 2004
Messages
12,944
'Blade Flex'. a great little ditty by the marketing department that has helped sell millions of SS props and propshafts. If there is any flex, I doubt is is greater than 1/100",
 

Scott Danforth

Grumpy Vintage Moderator still playing with boats
Staff member
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Jul 23, 2011
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47,302
Propping so you lug the motor simply costs you more fuel in the long run and kills the motor.

your boat motor will burn about 0.4# of fuel per HP used per hour of use. no getting around BSFC.

if you want fuel economy in a boat, get one with sails or oars

so prop your boat so that you hit the WOT RPM at WOT.
 

Sea Rider

Supreme Mariner
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Sep 20, 2008
Messages
12,345
What's the min-max wot rpm range for that engine ? If max is 4500 wot revs engine is already revving at the top as usually loaded which is great. Personally would stay with current prop. Throttle to max and as soon combo is on plane reduce throttle to whichever rpm yopu like to go cruising at.

Blade Flex ? Probably if running a extremely high HP engine with an alum props which seems are not available for that particular purpose as all others are usually SS.

Happy Boating
 
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