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Re: Propeller selection info and speeds
Posted: Mon Jun 11, 2018 11:46 am
by cmattj
liquid wrote: Mon Jun 11, 2018 8:50 am
Hmmm. You think it'll slow me down? I already can't get to top RPM. This is what Cobalt recommends for my boat. Should get 52 at 4500 according to them.
With 12-14 % slip which is common for 3 blade props it is more like 48 mph with a 21p and your 1.6 gear ratio.
Not being able to already come close to "touching the rev limiter" so to speak may be a concern. You may be a tad down on power as I know you trim up . and your elevation of 600 ft is only a small power robber.
The same prop pitch (17) just going up in diameter 14.25-14.75 will kill 100 rpm just in drag. Optimally you almost want a single 3 blade system to slip a little when trimmed fully to be the most efficient and have the least drag .
I'm still with you on increasing pitch but adding cruise speed is the only gain you may get. It is going to strain and become problematic to hold it wide open as it will lug and not pull through the rpm range .
I grabbed the sheet from Cobalt as I see what your referencing as far as set up which would almost lead to believe as I said you may be down on power.
Try another 14.25 or 14.5 x 19 cheap aluminum first as I worry about the $$$ and being disappointed. The we can look at Possible engine power issues.
Re: Propeller selection info and speeds
Posted: Mon Jun 11, 2018 2:24 pm
by liquid
cmattj wrote: Mon Jun 11, 2018 11:46 am
liquid wrote: Mon Jun 11, 2018 8:50 am
Hmmm. You think it'll slow me down? I already can't get to top RPM. This is what Cobalt recommends for my boat. Should get 52 at 4500 according to them.
With 12-14 % slip which is common for 3 blade props it is more like 48 mph with a 21p and your 1.6 gear ratio.
Not being able to already come close to "touching the rev limiter" so to speak may be a concern. You may be a tad down on power as I know you trim up . and your elevation of 600 ft is only a small power robber.
The same prop pitch (17) just going up in diameter 14.25-14.75 will kill 100 rpm just in drag. Optimally you almost want a single 3 blade system to slip a little when trimmed fully to be the most efficient and have the least drag .
I'm still with you on increasing pitch but adding cruise speed is the only gain you may get. It is going to strain and become problematic to hold it wide open as it will lug and not pull through the rpm range .
I grabbed the sheet from Cobalt as I see what your referencing as far as set up which would almost lead to believe as I said you may be down on power.
Try another 14.25 or 14.5 x 19 cheap aluminum first as I worry about the $$$ and being disappointed. The we can look at Possible engine power issues.
I appreciate your help here. Sounds like you know what you're talking about. You think I might have a power issue??? That had never occurred to me before. I've never felt like it lacked power as she gets up and goes quickly. I also would never hang around at WOT for long - in fact the only time i've done it is to test.
Re: Propeller selection info and speeds
Posted: Mon Jun 11, 2018 2:48 pm
by cmattj
I've done my share of prop testing and it is expensive and inconvenient. Most Boat Mfg have done there homework thankfully and give the boat all around performance and drivability with what they feel the weights and conditions we will see.
Like you I don't keep it WOT for very long. But testing for props does require what may seem like an eternity.
Run her up 3500. Trim to the verge of porpoise or slight slip. Then slowly bring her to full. And really only 30-45 seconds she optimumly should reach max rpm or slight surge from the limiter.
The faster cruise speed is easy to get with more pitch but it comes at a price too because now your further down on the throttle to maintain it and that's when things get worn out quickly.
Let's baby step a 19p ugly black aluminum one
Re: Propeller selection info and speeds
Posted: Mon Jun 11, 2018 8:24 pm
by Big Block Power
Sorry liquid ive been busy running around the south lately. 17 p rev 4 and be done with this. Lol
Yea the prop you picked will never work. Sorry. Too large too much pitch. I can't remember what your running now for a prop and pitch and rpm. What is your max rpm on your engine?
Re: Propeller selection info and speeds
Posted: Tue Jun 12, 2018 3:32 pm
by liquid
cmattj wrote: Mon Jun 11, 2018 2:48 pm
I've done my share of prop testing and it is expensive and inconvenient. Most Boat Mfg have done there homework thankfully and give the boat all around performance and drivability with what they feel the weights and conditions we will see.
Like you I don't keep it WOT for very long. But testing for props does require what may seem like an eternity.
Run her up 3500. Trim to the verge of porpoise or slight slip. Then slowly bring her to full. And really only 30-45 seconds she optimumly should reach max rpm or slight surge from the limiter.
The faster cruise speed is easy to get with more pitch but it comes at a price too because now your further down on the throttle to maintain it and that's when things get worn out quickly.
Let's baby step a 19p ugly black aluminum one
Will test on GPS this weekend. Thanks for your help with this.
Re: Propeller selection info and speeds
Posted: Tue Jun 12, 2018 3:39 pm
by liquid
Big Block Power wrote: Mon Jun 11, 2018 8:24 pm
Sorry liquid ive been busy running around the south lately. 17 p rev 4 and be done with this. Lol
Yea the prop you picked will never work. Sorry. Too large too much pitch. I can't remember what your running now for a prop and pitch and rpm. What is your max rpm on your engine?
14.25 x 21 - will check what RPM I'm getting this weekend as well. I was hovering around the low end before - 4200 ish.
If memory serves I'm 4200 - 4800.
Cobalt recommends that 3 blade SS for my setup - 3860709. Should hit 52 at 4500 rpm. I tend to agree with you both and that the 19 would make more sense.
But, let's start again! from the beginning! CMatt - FYI I'm an awful pain in the ass.
Re: Propeller selection info and speeds
Posted: Tue Jun 12, 2018 7:41 pm
by Big Block Power
What does mercury recommend. Lol.
Your killing me man
Re: Propeller selection info and speeds
Posted: Wed Jun 13, 2018 2:44 pm
by cmattj
liquid wrote: Tue Jun 12, 2018 3:39 pm
But, let's start again! from the beginning! CMatt - FYI I'm an awful pain in the ass.
Ha!! Your actually a fun pain in ass.. or I would have lost interest many posts ago
. No worries pal we'll get you figured out.. pulling skiers will be out of the question but after you do some water recon and get some data we will find the best combo.
Re: Propeller selection info and speeds
Posted: Wed Jun 13, 2018 10:24 pm
by Big Block Power
I'm 2 or 3 years into this hunt. Thanks for helping Matt.
Re: Propeller selection info and speeds
Posted: Wed Jun 13, 2018 11:00 pm
by cmattj
Maybe he'll fly us out for a Test session. I speak NY
Throttle against the dash.. trimmed to the moon using the " trailer trim button" ! We'll make her right
Re: Propeller selection info and speeds
Posted: Thu Jun 14, 2018 6:06 am
by Big Block Power
cmattj wrote: Wed Jun 13, 2018 11:00 pm
Maybe he'll fly us out for a Test session. I speak NY
Throttle against the dash.. trimmed to the moon using the " trailer trim button" ! We'll make her right
Now we are talking. Maybe pull some snakes behind her.
Re: Propeller selection info and speeds
Posted: Thu Jun 14, 2018 8:37 am
by liquid
Re: Propeller selection info and speeds
Posted: Mon Jun 25, 2018 9:40 pm
by Midlifecrisis
New owner here with a question (and I apologize in advance if this has been answered elsewhere - my search didn't come up with anything)…
I have a new (to me) 2006 220 with 5.7 GXi (320 HP) Volvo engine/drive. I'm pretty sure the props are F9s. I'm using it at about 5,000 feet (Lake Arrowhead, CA), so performance really suffers compared to my test drive at sea level. WOT comes in at about 4400 rpm. I'd really like to improve hole shot, since we do a lot of skiing. If that takes some off top end, so be it. Is changing out to F8s the answer? And, for a good backyard mechanic, how difficult is it to swap the props?
Thanks for your help...
Re: Propeller selection info and speeds
Posted: Mon Jun 25, 2018 9:52 pm
by Big Block Power
I'm really surprised it had f9's on it. You might have to go to f7. It's about 200 rpms for every pitch. You want to be about 5200 rpms
Bfuns 5.7 has f6 on it. I think 1.92 ratio.
Props are easy to change if you have the special sockets. Retorque them back on though only 55ft lbs.
Re: Propeller selection info and speeds
Posted: Tue Jun 26, 2018 1:21 pm
by Midlifecrisis
Thanks for the help. I’ll take another close look at the writing on the prop this weekend. Am I on the right track: shallower pitch will improve the hole shot?
