Which prop?

[Chamelion]

I'm in need of a new prop. Current prop is 10 3/8" 12p and is well worn. I'm still getting good holeshot and top speed is about 30mph. I won't know the RPM at WOT until my tachometer arrives. I'm assuming my prop has lost some blade area, but unsure how much.

My motor is a 40hp merc 2 stroke. USA made 2002 model. Boat is an open alu boat and at a guess her weight when loaded up with fishing gear (not including myself) would have to be in the realm of 900-1100lb.

Interestingly the workshop manual suggests I should have a very different prop based on weight and length.

I would prefer aluminium (aluminum) as there is every chance I'm going to snag a rock bar or tree stump at some point.

Any helps appreciated, thanks.

 

[Bamaman1]

Re: Which prop?

You probably would be better served with a aluminum prop. If you were running a 115 or higher hp, I'd tell you to go stainless.

I used to go through a prop a year on my I/O boat, but luckily the Prop Shop guy was a cousin. When I got a new I/O, I switched to stainless. The engine blew up after 12 years, but my stainless propeller didn't have a scratch on it. They're just much tougher than aluminum.

 

[steelespike]

Re: Which prop?

We need you to answer the stickys at the top of the prop forum to be really accurate.Also "about30" isn't accurate enough
No need for fractions but more accurate.
Based on your present info your 12" prop at 30 mph guessing 10% slip produces about 5360 rpm.
It does appear your setup is pretty good.
To me the prop chart is very optimistic. For your motor to push a 19" prop to 5500 for 48 mph would take a special boat.
Speaking very generally often a glass boat of similar type will be slightly faster than an aluminum boat even though heavier.
Again very generally a flat bottom will be faster than a deep vee or more rounded hull.
You can figure the actual diameter if your prop by measuring from the center of the bore to the tip of a blade and then double
the number.

 

[Chamelion]

Re: Which prop?

Thanks for the reply.. I completely missed that sticky.

1) Performance issue you are trying to correct. - Blades on prop are VERY thin on the outer edges. Looks quite worn and wouldn't mind a bit of an increase in cruising and top end speed. Current hole shot is exceptionally good, so can afford to lose a little hole shot for speed.
2) Current prop manufacturer, model, aluminum or stainless as a minimum. - Alu
3) Current prop diameter and pitch (required). - 10 3/8" x 12p
4) Wide open throttle RPM and speed with an average load (very helpful) - Unsure on RPM, no tacho. Top speed 31mph with me in the boat. 29mph with 2 adults and 2 children.
5) Engine/drive make, model, year, and HP - Mercury 3cyl 3carb 967cc 40hp 2002 (de-rated 60hp).
6) Boat make model, year, length and weight - Quintrex Dory 4.1m (13.5'), 2002, hull only 160kg. Estimated total weight incl motor without captain or crew 1000lb.

Thinking about looking toward a 13p prop. Maybe in the range of 10-1/2".

 

[steelespike]

Re: Which prop?

A nice little rig. Assuming 10% slip at 31 mph your rpm is a just about perfect 5547.
The looks of the prop I think a new prop may lower the rpm but add a little speed.
You may want to wait for a tach so you can tell better how this prop works and then evaluate the results of a new prop.
It appears the boat has 7 keels good for strength and handling but probably a little slower. than a boat with fewer.
Probably a 13" prop would lower rpm a little but help the speed. Hole shot slightly slower.
Does the motor have a hydrofoil on it?

 

[jestor68]

Re: Which prop?

When I put your numbers in the Mercury Marine prop selector, it suggests either a 12 or 13 pitch in all types(al or ss); based on an operating weight of a little over 1700 lbs(family load).

You classic 40(3 cyl) has a 1.83 gear ratio.

Since you're losing 2 mph now between light and family loads, it might be a mistake to put more pitch on there. The slightly larger 10-5/8 X 12" SS prop would provide better all around performance, as well as more top speed. It would also stand up better to the "sand bar" boating that you appear to do.

 

[Chamelion]

Re: Which prop?

Finally got off my backside and ordered a new prop...

Based on info I gleaned from a cheap tachometer (and to be taken with a slight grain of salt) I was doing 300rpm over the max WOT rpm my engine is supposed to do. So I've ordered a Solas 11.1x13p prop to try. I'll see about getting a hold of a better tachometer in the meantime, but at the cost of $117.95 delivered to my door in Australia from the USA (same prop is about $250 here.. yep, we get gouged) it's worth a shot. If I later find I need less or more pitch I can sell it locally for the same price I paid without any hassle whatsoever.

Cheers.

 

[steelespike]

Re: Which prop?

Good deal,let us know the details for future reference.