Help me pick a stainless prop for my Sylvan Super Sportster 16V with Mercury 70

[GoldDuster360]

Hi guys,

I have a 1983 Sylvan Super Sportster 16V (StarCraft Super Sport), modified V, 5% or less deadrise at the transom. Boat weighs approximately 1,400lbs fully loaded with people and gear. It has a 1983 Mercury 70 3cyl (no power trim or tilt), with a 10-3/8 X 13 pitch aluminum propeller, Quicksiver, has small nicks on all three blades closest to hub and very little cupping on outer edges is present, very minimal wear/damage to blade tips, would rate prop at 90% to a new one, no vibrations. Engine is in excellent tune with recent spark plug change and is propped per the engine chart in the manual which suggested 29-35mph performance.

Holeshot and planes satisfactory and at full throttle will do 34mph (GPS) at 5750rpm when fully loaded. Cruises nicely at 3850 to 4,000rpm, around 18-20 knots. Boat operates satisfactory and am tempted to buy a new Quicksilver prop and use the old one as a spare, boat works ok as is but there must be room for improvement.

Only issues are the prop occasionally ventilates or breaks loose momentarily when crossing wakes or if there is allot of wind. Nothing bad but would require manually moving engine down one notch to prevent it which usually causes it to plant the bow down a bit more than we would like. From what I have read so far, an aggressively cupped prop might reduce this.

Because of the manual tilt deal we sometimes get a little close to the bottom when looking for a place to anchor, manually lifting is a pain so sometimes we kick up a little sand, not enough to really damage anything at idle but it seems like a good idea to have a stainless prop for a little more abrasion protection and hopefully pick up some economy/performance with the thinner blades.

Based on above, what stainless prop and pitch would be a good replacement for my aluminum one? Engine Max RPM range given in the manual says 5300-5800. What isolators (rubex, mercury, etc.) provide the smoothest shift into gear with a heavier prop and provide strike protection for the lower unit due to stronger stainless propeller? I am looking at a Stilletto in the same pitch and diameter but suspect if aggressively cupped may require a notch down in pitch. This is my first boat and have tried to get a good handle on everything but I could use some advice from the experts before laying out some more greenbacks into my hole in the water.

Thank you in advance,

Bob/Goldduster360

 

[SkiDad]

your speeds are really nice ! i believe the merc vengeance / quicksilver silverado is same blade design as your prop but in stainless - i would drop 1" in pitch if you want to maintain rpm - i would go down 2" in pitch on stiletto or something aggressive - well unless you want to get your rpm down.

staying the same pitch will bring your rpm down some and "might" give you more speed. Don't expect a huge change - i have 3 15 pitch props and they run within 2 mph of each other - the stainless is not the fastest top end but accelerates the best.

if you want to look after market - look at powertech props - they can hook up up as well

 

[Frank Acampora]

There is a persistent myth that stainless props because of their added rigidity drop engine RPM more than aluminum props and require a drop in pitch.

Try not to buy a stainless prop without testing one first. Stainless props are all over the place compared to stock aluminum props. Some that are almost stock equivalent are about 200-400 RPM faster and may require a higher pitch. Some aggressive props are 200-400 RPM slower and may require a drop in pitch. Some turn faster and give better top end while some turn slower and give the same top end. For example: On my Force engines the exact prop in stainless will turn 200-400 RPM faster and deliver 1-2 MPH more. Aggressive props like a Ballistic will turn about 400 RPM less yet give the same top speed as a stock stainless prop. In fact, different design aluminum props in the same pitch will not turn the same RPM on a given engine.

SO: Unless you get someone who really knows your engine and boat combo or has the same combination rig, buying a stainless prop can be a bit of a crapshoot.

 

[GoldDuster360]

Thank you Frank and Ski Dad for your opinions, definitely gave me some food for thought. Looks like anytime you are changing brand/blade type it's kind of a crapshoot. Since my goal was to keep the same general performance level when switching to stainless it might be wise to purchase one of the same make and model in stainless and see what happens, worse that happens is I exchange it for another one

 

[GoldDuster360]

Ok, I picked up a Stiletto Triad model 802313 (Same as Turbo Hot Shot-Precision Propellers) 10-1/2 x 13 SST prop.


It replaces my quicksilver 10-3/8 x 13 aluminum prop


Thinner Blades, better cupping and slightly larger in diameter with a longer exhaust barrel






It has the odd style hub isolator with three notches to hold the torque, it has plastic aft nut which I wasn't real fly with, also this means rubex and flo-torque will not interchange:

The black washer with the two holes in it is not used on the Mercury install-Kit fits Suzuki-Yamaha too.







Prop self centers on the brass spline tube protruding from plastic isolator


If this aft washer cracks it looks like the metal aft washer from the rubex kit would work. I actually got an extra hub with purchase because previous owner cracked first aft washer when attempting to torque to 55ft lbs per manufacturer specs. I torqued mine to 30 and then moved until I could engage lock tabs.


The quicksilver weighs around 3lbs, about half the weight of the Stiletto.



My prediction: Prop will lose maybe 150 rpm's and gain 2-3mph top speed with less ventilation.

 

[GoldDuster360]

Ok, we took it out this morning and here is what happened:

35-1/2MPH was our best speed, 5600RPM, so we lost about 150RPM and gained about 1-1/2 mph. At 4,000rpm we go 23-24mph, about 1-2mph faster than before. Boat does come out of the water a tad more(bow and stern) and seemed to do much butter in the chop after the wind picked up. Ventilating has been greatly reduced at same engine height setting. Over all very happy with my shot in the dark on this one. Also, prop engagement is still smooth in spite of nearly doubling the weight of the prop. Happy Camper!

 

[steelespike]

I guess your thoughts were right on.! Thanks for getting us the results.

 

[NYBo]

Nice Duster!

 

[GoldDuster360]

Nice Duster!

Thanx, Duster did well on our 50 mile round trip with boat in tow. Turns out a 360-4bbl with viscous fan package mated to a torqueflite, 3:55 w/sure grip and super stock springs makes a perfect tow vehicle! Way less stressed than our Odyssey.