17 vs 19 vs 21 Pitch?

[EZDuzIt]

I have an old, heavy 17' Apollo cathedral hull. She has the good ol' Mercruiser 165 (GM 250 cu in/4.1 liter) straight six. Currently she has a 15"x17 pitch prop on her. With just me in the boat, on perfect water and at WOT she's dead nuts on 4500 rpm and 40-41 mph. For an old, heavy boat that's pretty good. These old cathedral/tri hulls weren't known for their speed but far more for their stability. I'm just curious what a 19 or 21 pitch prop would do. I have a pretty good hole shot for such a heavy boat (about 3000 lbs). It's a legit all purpose boat. It does everything from trolling for fish to pulling tubes and water skiers. I know as I raise the pitch angle of the prop I'll lose some bottom end but gain top speed. I know the pitch of your prop is like the gearing in a vehicle. By the book my top WOT should be no more than 4300 rpm but I'm at 4500 rpm. My prop guy and boat mechanic both say that engine can handle that all day long, and it has. I don't often run at WOT because I don't have to just to get where I'm going. She's the happiest at 3200 rpm and 26-27 mph cruising speed, burning about 5-6 gph which for a boat in general is pretty darn good. Is it even worth messing with it? I've always been a fan of sleeper cars. If I can keep a decent hole shot but gain another 5-10 mph and get my WOT rpm's down a bit that would be worth doing. It would also kinda make my old girl kind of a sleeper. A lot of people are already surprised it moves as well as it does. Experienced thoughts and opinions would be appreciated. I'm just trying to get the best reliable performance and least fuel consumption I can out of it. Cheers.

 

[alldodge]

I know as I raise the pitch angle of the prop I'll lose some bottom end but gain top speed.

Not a rule of thumb but depending on the health of your motor you may gain maybe 1 mph top end but hole shot will suffer

Currently she has a 15"x17 pitch prop on her. With just me in the boat, on perfect water and at WOT she's dead nuts on 4500 rpm and 40-41 mph.

Manual 3 list WOT at 3900-4300 and if your tach is accurate then your over reving



Note you have only so much HP so moving to a high pitch prop just changes the RPM point when its all being used. Move it to high and motor is lugging and using max fuel. Move it to low and Max WOT RPM is reached to soon but your saving fuel because max HP has not been reached

 

[Chris1956]

If you change to the 19 P prop, your RPMs will likely drop by about 400RPM. You may gain speed, but it is not a certainty

Tri-hulls tend to plane pretty flat, also making it hard to trim the bow up for more speed.

Modular props are pretty cheap. It may be worth changing to a 19P prop. I doubt that changing to the 21P would be wise.

 

[Stinnett21]

What is your current prop, make, model, aluminum, steel? Depending on your budget consider going to one of Mercury's higher end props such as the Enertia. It comes in 1 inch pitch increments so you can get it dialed in better and this prop is a known top performer

 

[jlh3rd]

I don't see a 10 mph increase.....maybe 5. Props aren't magical . They're for fine tuning.The #'s you see are for the same boat, load , and conditions. Props were new Merc. 3-16x13 and a 3-16x12.
You can see the 12 pitch hit 22.5 mph@6000. I could hit 6100+ rpm which could get me 23+ mph but that's 100 over my limit.
The 13 pitch got me 25.5 @5800, the highest rpm I could get @WOT.
My boat is a pontoon, I got a 3 mph increase with 1 inch pitch difference.
Yours is a different hull, that's why I said you might get 5.
Not gonna get in between a guy and his boat/manual/ mechanic....your money, your boat.

 

[Scott Danforth]

Your pitched just fine.

 

[EZDuzIt]

Thanks for all the feedback! I've been thinking a 15"x19 pitch might be the way to go. I don't recall the manufacturer of my prop but I buy exclusively from Precision Propeller here in Spokane, WA. Very, very knowledgeable people in there with a 30 day guarantee. If the prop you buy doesn't perform the way you want it to and it is undamaged, they'll swap it out. All in all I'm happy with how my boat performs. It has a fair amount of bow rise but planes right out. That old GM "Workhorse" straight six has torque for days! I think a 19 pitch would probably put it nuts on and by the book, which I have, and mentioned what my max WOT should be vs what it is. But thanks for the info.

 

[jimmbo]

Any difference in Top Speed, will be low, far less than 5 mph.
Any Idea what Make and Series the current Prop is and how old is it, and is it cupped?

 

[QBhoy]

I’d say most 19” props will be around the 14” diameter size (off the shelf) for that set up. Something to take into account. Won’t have a huge impact, but maybe some. Should also be aware that you can occasionally see similar rpm results (but slightly higher speed) when going from a bigger diameter lesser quality prop, to a fairly good prop of a couple of inches increase. Just that you might gain more lift from the hull etc etc. best of luck !

 

[gbrowne63]

I would look for a used 15 or 14.5X18 so your RPM,s will be right on the money. face book market place or kijiji. It will probably be stainless.

 

[racerone]

did a test 40 years ago.-----Same boat / motor.----Same day within an hour.-------A 19 " RPM 5800 , speed 54 MPH.----21" speed 54 MPH at 5300 RPM----23" speed 54 MPH and 4900 RPM.

 

[jimmbo]

Did similar tests back in 76, with the old man's 90hp. Little or no Speed diff, just Rpm and Hole Shot changes

Edit: In 72, put a much bigger Prop(pitch) on his 40hp, boat would barely Plane, so there was a big speed change

 

[racerone]

Agreed-----Motor RPM / holeshot changes.----Just more proof that many folks to not understand the concept of POWER and LOAD.-----And electric car power consumption.-----

 

[Chris1956]

My speedboat (16 foot avenger w 1500 Merc) could pull two skiers slalom or on two (with deepwater starts), with a 21P prop. I changed to a 23P prop, and it could not get a single skier out of the water (deepwater start). the motor would not exceed 2000RPM pulling the skier. The takeaway is that motors have a power band, that we need to stay in for performance.

 

[Chris1956]

That same boat, when the engine was new, would lurch forward at 55MPH, when the throttle was firewalled. The top speed was in the late 60s.

 

[jimmbo]

The I6 1500, was not known for great Low End Grunt, and was a cantankerous thing between 2000-4000rpm but between 4000-6000rpm, that motor could sing, on a light hull

 

[cyclops222]

What is the SLIPPAGE of each prop ?

 

[EZDuzIt]

If you change to the 19 P prop, your RPMs will likely drop by about 400RPM. You may gain speed, but it is not a certainty

Tri-hulls tend to plane pretty flat, also making it hard to trim the bow up for more speed.

Modular props are pretty cheap. It may be worth changing to a 19P prop. I doubt that changing to the 21P would be wise.

Trimming it out is a very simple process. Raise the outdrive just until she starts to porpoise then lower it back down just a touch. That's as trimmed out as she gets but flat out you'd be surprised how well it rides on the water. That old straight six is a torque monster. The hydrofoil (whale tale) I put on her makes a huge difference in not only her hole shot but also how fast she planes out at such a low speed especially for such a heavy boat. If I slowly roll on the throttle, which is how I typically get going, she has a lot of bow rise but once I clear about 2500 rpm she starts to level out. That hydrofoil made all the difference in the world. I can almost stay on plane down to about 18-20 mph before she starts to plow water. That's another advantage to a tri hull. They sit so high on the water that it doesn't take much to get them up on plane. They also stop pretty fast. They're either planed out just skimming the top of the water or they're plowing through it, making a hell of a wake. Excellent boat for tubing lol! They'll also beat the hell out of you on a choppy day but they're super stable.

 

[Texasmark]

Agreed-----Motor RPM / holeshot changes.----Just more proof that many folks to not understand the concept of POWER and LOAD.-----And electric car power consumption.-----

"And electric car power consumption.-----"

Interesting nobody ever mentions what it took to get the battery charged, or built and its weight!

 

[racerone]

I often talk to folks about generating electricity.----You can buy a generator with 5 HP.----Yet where I worked there are generators as well.-------The most common guess ( 9 of 10 ) for most was 10,000 HP.----Yet the true number is 1,000,000 HP.-----So when an electric car uses 20 HP at highway speeds , somewhere 20 HP ( or equivalent ) must be generated to recharge that battery.