Pondering Pitch

[JoLin]

It's the boat in my sig and avatar. Planing hull, approx. 9500 lbs as tested today. 4 barrel 4.3's putting out 225 hp per engine. Both engines are in excellent state of tune. Recommended WOT range is 4400-4800 RPM. Current props are Merc 'Vengeance' 3-blade stainless with 19P.

Calm water and light breeze for a change, so I took her out for a top speed run. Wide open, both engines maxed out at 4800 rpm (top of the range) and I hit a top speed of 33+ knots/ 38-39 mph on GPS. That surprised me. I didn't expect her to be that fast. I seldom cruise at anything faster than 3600 rpm/26-27 mph. The water is normally too rough for comfort at higher speeds, so keep in mind that top speed is not my concern.

The issue - I can normally stay on plane down to about 20 mph/3100-3200 rpm. It's iffy below that, and I'll usually slowly 'fall off' If I drop the rpm's any lower. That's problematic when I convoy with friends that have larger inboard boats and cruise at around 18 mph. I'd like to be able to stay on plane at a couple mph slower than I can now. In those conditions, engine efficiency and mpg aren't my primary concerns- it's about staying with the group.

On a previous boat (same goal- lower planing speed) I got good results from a Solas Titan HR 4-blade stainless and I'm leaning in that direction again. They're good props and cost less than most.

So, do I drop an inch of pitch and keep my WOT around 4800 rpm, or do I keep the same (19) pitch and still be able to max out at around the middle of the recommended rpm range (I estimate 4600 rpm)?

Which do you think would give me the result I'm looking for (lower planing speed) in the mid-range??

 

[JoLin]

Putting it all on paper (so to speak) might have clarified something for me. Anyone know where in the rpm range these motors develop peak torque? I can't seem to find it. I think I'd want to keep the engine in the high torque rpm range if possible, and that should shed some light on the right prop pitch for me.

 

[Starcraft5834]

"On a previous boat (same goal- lower planing speed) I got good results from a Solas Titan HR 4-blade stainless and I'm leaning in that direction again. They're good props and cost less than most."

when I was reading this....putting (2) 4 blades on was my first thought........Think you answered your own ?.. (2) 4 blades shoving you along would be my fix....for what you want and do, that's the ticket....

 

[JoLin]

"On a previous boat (same goal- lower planing speed) I got good results from a Solas Titan HR 4-blade stainless and I'm leaning in that direction again. They're good props and cost less than most."

when I was reading this....putting (2) 4 blades on was my first thought........Think you answered your own ?.. (2) 4 blades shoving you along would be my fix....for what you want and do, that's the ticket....

Yeah, I already decided on 4-blade props. Question was about pitch, given my goal and current performance on the 3-blade 19's. Thanks.

 

[steelespike]

If your going 3 blade to 4 blade both ss props drop 1" of pitch.The 4 blade and the lower pitch should struggle less trying to stay on plane

 

[Fed]

Have you already played with your trim tabs and motor trim settings? (Maxed out down)

 

[QC]

Anyone know where in the rpm range these motors develop peak torque?

How would you use that information? Despite all of the hype surrounding torque it is not relevant in a boat and precisely why you can't find it.

I second the comment on trim tabs. If you are not using them try her with them buried down for a reference point. And if that doesn't work I'd increase tab size if you already have them.

 

[JoLin]

Guys, trim tabs and I/O trim are already in the mix. I maintain a (approximately) 20-mph planing speed by having the tabs down and drives trimmed in. I hit 39 mph yesterday by having the tabs up and drives trimmed out. I know how to use them.

I know that the rule of thumb is to drop an inch of pitch when going from 3 blades to 4 with the same prop material. I'm asking if anyone has any better insight to achieving my goal than I have. If nobody does, that's what I plan to do.

QC, I bow to your expertise. As to how I planned to use that torque info, I wasn't sure myself. Just thought it might be something to factor in. I have now factored it out

 

[Sea Rider]

Max torque is achieved near max wot range. If can't dial a proper prop to suit you needs, how about telling you boating mates to speed up to catch with you if plan going in caravan LOL!!

Happy Boating

 

[JoLin]

how about telling you boating mates to speed up to catch with you if plan going in caravan LOL!!

Happy Boating

Yeah, that didn't work out so well. Big boats, big block motors, semi-planing hulls. Those guys live in a different world whre performance is concerned. Now, if I were to move on to the 30' Luhrs Alura I always wanted.... :joyous:

 

[Watermann]

Very different reason for wanting to change props from most of the reads here! I guess we all use our boats differently. Be sure to stop back by and let us know how the prop change works out.

39 MPH with that size boat is pretty darn fast, I'm thinking you can stand to loose a mile or two an hour with the prop change. :happy:

 

[JoLin]

Yeah, Watermann- I always seem to be the 'odd man out' :facepalm:

Whatever I do will be a winter project and won't be tested until next Spring, but I'll post back after I've made my 'improvement(s)'. I'm investigating two changes and might go with both if I can scrape up the money.

QC's suggestion of bigger tabs has a lot of merit and there's no downsde. Can't afford to replace the whole system (InstaTrim), but may be able to change out the tab plates/hinges. I'd have to retain the current span (believe it's 12 inches) to keep the actuators in the same place, but could probably increase cord. I'll measure them when the boat comes out of the water and take it from there. If I do that, I'll also want to replace the old hydraulic hoses.... in other words, another sore back, stand on my head, skinned knuckles project.

If I replace the props, I'll likely just drop an inch of pitch (to 18) and go with the Solas 4-blades. They have part numbers for both the Titan and Rubex in a counter-rotating pair. I want to give them a call sometime- been curious to find out if there's any difference between the two models aside from the hubs. The descriptions are identical.

 

[steelespike]

My experience has been that the Rubex and Amita are the same but for the removable hub in the Rubex.
I think the Titan is a different animal entirely.

 

[45Auto]

Max torque is achieved near max wot range.

I'd be very surprised if max torque is anywhere near WOT RPM. Probably 25% or so below it. But as QC said, it's really pretty irrelevant.

HP/Torque chart for a new marine V6 is here (285HP at 5300 RPM, max torque 305 lb-ft at 3900 RPM), you can probably find some older ones in the archives if you're that interested in it.

http://www.gmpowertrain.com/2013_pdf...ert_020315.pdf