Sizing Prop for Project Boat

I_5320

Seaman
Joined
Jan 3, 2011
Messages
59
Hello all,

Trying to zero in sizing my prop in completed Project Boat.. I?ve done some homework and some initial field testing.. Overall goal is hoping I can get this 50 hp to perform well enough to stick with it and not be tempted to up the HP.

Motor = 1980 Merc 50 HP Tiller (Canadian version if that makes a difference)
Boat Hull = 1985 Sylvan Super Sportster completely modified into fishing boat
Boat & Motor weight = 1,450 lbs.. Incudes everything on except fishing gear;
RPM Range = 5,000 -5,500
Gear Ratio = 1.83 (double checking this)
Weigh distribution = 12 Gal Gas tale in the Stern,, 3 Battery?s under fron deck platfom . ? plywood Deck
Current Prop = 10 3/8 X 14 3 blade Alum Merc
Prop condition = Blades look like they?ve been ground previously a bit and has lots of nicks.
Motor Height = Cavitation Plate 1 inch above bottom of hull. Was told this is just about right for this boat Appears not to be cavitating at all.

WOT Test = 4,950 RPM with me and 1 other 160lb passenger
Speed Test = No Speedometer and forgot about using GPS

Problem Symptom = Hard to plane with 4 persons.. Not as fast since I added all the crap.

Where I?m at:; = Bought a 10 3/8 X 12 Vortex 3 blade.. Was thinking to get close to 5,400 with me alone.

Questions.. Knowing how to rig up a shop tach now I can go it alone.

1 .Is goal to get as close to 5,500 RPMS as possible with just me and a light load?

2. Also ,I?ll be sure to check speed (with trim at optimum) when testing the 12 pitch.. Anything else I?m missing?

3. Last, is 10-11% slip considered good/normal? .If so is my math is right and the best I could expect with this motor/boat setup is about 30 MPH?

Mark
 

Sea Rider

Supreme Mariner
Joined
Sep 20, 2008
Messages
12,345
Some issues,

It's not same propping an engine to achive max wot range rpm with you alone and pretend same excellent water performance with 4 up including all that addedd crap. If you will be boating with usual 4 boatres, say family oriented combo, you'll need to dial a different prop to pull your revs up to around o near max wot rpm range for that engine to perform well.

The down issue as there's always one will be that when boating alone or with less boaters will achieve out of this world instant plane with huge over revving rpm, will need to watch tach with both eyes.

The best way to get all those 50 ponnies out of the barn instantly is a prop maximization that meets your current application needs along a engine/transom height optimization.

Happy Boating
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
To take a guess at your speed calculate, 4950,1.83 ratio,14" prop,12% slip I get (31.6) mph.
​ I stumbled on a new 40 hp 4 st. test; similar boat at 1520 all up weight.,2.00 ratio,14" prop 5950 rpm calculates to( 31.6) with 19% slip.
I guessed 12% slip if we use 19% we get about 29 mph.for your boat
 
Last edited:

I_5320

Seaman
Joined
Jan 3, 2011
Messages
59
Thanks for the replies.. All good stuff..

Let me go off and test the 12 pitch I have now under what i hope to be more normal with 2 persons and gear.. 'll update.. It may be I just have other prop for those trips.

Question on 4 Blade,, figure on -100 RPM's? Or does it depend on type?



 
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