Whaler Central - Boston Whaler Boat Information and Photos

Posted by John Fyke on 08/09/08 - 4:46 PM

Anyone have an idea of how much difference it would be going to a 13-1/2 by 22 pitch from a 13 by 19. I know I would be increasing a 1/2" diameter but I'm thinking going up 3 degrees might be like going up 1-2 degrees.

Edited by Tom W Clark on 08/10/08 - 7:01 AM

Posted by joninnj on 08/09/08 - 7:51 PM

John..

That is good jump up in pitch in one shot, with the slight increase in diameter, you may drop 500 RPM or more depending on the prop.

What kind of prop today? and what are you thinking about? Also what engine and boat?

Posted by Tom W Clark on 08/09/08 - 9:05 PM

John,

It really depends on what 13-1/2" x 22" and 13" x 19" propellers you are talking about. There is far more to propeller design than just pitch and diameter.

As a general rule you can expect to loose about 150-200 RPM for each inch of pitch increase on a typical propeller on a large outboard so long as the propellers are the same model of prop. All bets are off once you start mixing types of props.

Exactly what 13" x 19" propeller are you running now?

Exactly what 13-1/2" x 22" propeller are you contemplating?

What are your goals?

What did you mean when you wrote: "I'm thinking going up 3 degrees might be like going up 1-2 degrees."? That makes no sense.

Edited by Tom W Clark on 08/09/08 - 9:07 PM

Posted by John Fyke on 08/10/08 - 5:35 AM

Boat is a 15' with a 70hp merc. Currently running an aluminum 2038p19 (13 by 19) Quicksilver. Looking at a Mercury LaserII 13-1/2 by 22 Stainless. Would like more top end.

"What did you mean when you wrote: "I'm thinking going up 3 degrees might be like going up 1-2 degrees."? That makes no sense."

I figured going up a 1/2" in diameter might offset a degree in pitch as far as rpm's.

Posted by joninnj on 08/10/08 - 5:58 AM

John,

That is even a bigger jump. The Laser is great prop, it will allow more trim and will not break loose of the water thus giving a bit RPM trimmed out, but you will not make up what you will loose. At even money WOT, with 0 trim you will loose a lot of RPM...:o

I did something like this on my V6 Merc (175) started with a 21 aluminum, Performance was lousy. I experiment with a few props, basic stainless, saw some improve with increasing trim and no cavitation. Then got the Laser, even better..:p. Added a jack plate and raised the engine about 3 inches up, and was able to turn 5800 RPM, from 5300 with aluminum but this was a very different boat with 2 1/2 times horse power.

This may not apply to a whaler with 70HP...:|

What is your goal, more top end end? Low end?

Also Very important, What does your prop/engine spin/RPM today? This is the base line to compare to.

Edited by joninnj on 08/10/08 - 6:27 AM

Posted by joninnj on 08/10/08 - 6:36 AM

Then 22 is not the right prop for you. You did not mention the engine type? Assuming the the normal range for your rig is 4500 to 5500 you would be in the middle of the range. I would not want drop RPM especially if you do water sports...

Try this link, Mercury prop selector. This may give a general idea on what to choose. Seems fairly accurate if you plug in the numbers you know.

http://appcenter.mercurymarine.com/pr...pSelector/

Edited by Tom W Clark on 08/10/08 - 7:05 AM

Posted by Tom W Clark on 08/10/08 - 7:04 AM

John,

Propeller pitch on outboard motors is quantified by the inches of travel for one rotation of the propeller without any slip. Degrees measure angle and since boat propellers have blade that have a different angle on any given spot on them we do not try to quantify the blade angle in degrees.

Posted by Tom W Clark on 08/10/08 - 7:13 AM

"I figured going up a 1/2" in diameter might offset a degree in pitch as far as rpm's."

Actually the larger diameter prop is the Laser II so the increase in diameter would *theoretically* add to the reduction in RPM caused by the increase in pitch rather than offset it. But the aluminum prop and the Laser II have different blade designs with the Laser II having less surface area per blade.

The Laser II also has much more cupping of the blades so your RPMs are going to drop far too much for your application.

You do not tell us what year your Mercury 70 is but let's assume it is a mid 1990s or newer with the 2.33:1 gear ratio and a 5500 RPM redline.

If you are only turning 5100 RPM now you do NOT want to increase pitch you should really reduce pitch or find a prop whose using allows more RPM.

If we knew the top speed of your boat now with the 19" aluminum at 5100 RPM, we could calculate how efficient it is and see if there is potential for more speed.

Have you measured your top speed accurately with a GPS in a two-way test run at WOT?

Edited by Tom W Clark on 08/10/08 - 7:14 AM

Posted by John Fyke on 08/10/08 - 9:44 AM

#11

The motor is a 1987 and for the year the rpm range is between 4750 and 5250. I'm still working on getting an accurate gps reading. The last time out the gps would cut out and restart at 37 mph. Getting 45 mph off the motors water pick-up.

Posted by Tom W Clark on 08/10/08 - 10:25 PM

#12

John,

If you are getting 37 MPH out of an aluminum 19" prop at 5100 RPM you are doing pretty darn good with only 6 percent propeller slip.

Pitot speedometers are mostly worthless; there is no way you are doing 45 MPH.

The best prop you could buy for top speed is probably the Stiletto Advantage 4.25 in the 17" pitch. You would need to run this with a Flo-Torq III hub kit which will add some expense but will cushion the shift clunk of the heavier stainless steel propeller. Those mid 1980s gear case were not that robust and do like the standard Flo-Torq II hubs as much as the old rubber pressed-in hubs.

Edited by Tom W Clark on 08/10/08 - 10:25 PM

Posted by Bake on 08/11/08 - 6:12 AM

#13

I had a 15 sport with a 70 yamaha. There is no way your boat is going to preform worth a fart with a 22 pitch anything. I found a 17 gave the best all around preformance whith no top end gain going to a 19. It will take you a long time to get on plain with a 22 pitch prop.