Who's good with props..?

mnm99

Well-Known Member
Oct 2, 2015
2,445
Long Island
Boat Info
2004 340 SeaRay Sundancer
Engines
Twin 8.1 Merc
Yea I know, It's never good enough...lol. Winter is here and I want to play again.

A few years ago I had my props re-pitched because I was above my WOT range of 4200-4600. I was at 4730 so I ended up going from a 18x21 with a .125 cup to a 18x22 with a .075 cup. After that I was at 4620 for some reason and gained like 3mph. So last year I had them go over the props again and add a .250 cup. My speed right now is 28 to 28.5 mph at 3450rpm and a WOT of 38.5 at 4560-80 rpm. I called ACME props and talked to him for a while. He suggested there 18x23 with a .043 cup. He told me it would gain around 1 mph on the top end at 4450rpm and .75 mph at cruise. Well if you do the math, there is no way i'm spending $3500 for that. So I'm thinking...safe RPM range is 4200-4600. What's stopping me to add another 1 degree of pitch and take out .125 cup or so. I would drop like 150 rpm bringing me to around 4400 rpm. I should gain better mph at cruise. I know optimal rpm is 4600, but if the range is 4200 to 4600 I should be good? right? I'm not sure how my gas mileage will do also after? Right now I'm burning 32GPH right at 28-28.5 mph. Here are some scenarios.

Current =WOT 4580 rpm with 22 pitch prop .15.26 slip at 38.5 mph

Proposed =WOT 4400 rpm with 23 pitch prop .16.73 slip at 38 mph
Proposed =WOT 4400 rpm with 23 pitch prop .15.63 slip at 38.5 mph
Proposed =WOT 4400 rpm with 23 pitch prop .14.54 slip at 39 mph
Proposed =WOT 4400 rpm with 23 pitch prop .13.44 slip at 39.5 mph
Proposed =WOT 4400 rpm with 23 pitch prop .12.35 slip at 38.5 mph


Current =CRUISE 3450 rpm with 22 pitch prop .18.19 slip at 28 mph

Proposed =CRUISE 3450 rpm with 23 pitch prop .21.74 slip at 28 mph
Proposed =CRUISE 3450 rpm with 23 pitch prop .20.35 slip at 28.5 mph
Proposed =CRUISE 3450 rpm with 23 pitch prop .18.95 slip at 29 mph
Proposed =CRUISE 3450 rpm with 23 pitch prop .17.55 slip at 29.5 mph
Proposed =CRUISE 3450 rpm with 23 pitch prop .16.15 slip at 30 mph
 
Yea I know, It's never good enough...lol. Winter is here and I want to play again.

A few years ago I had my props re-pitched because I was above my WOT range of 4200-4600. I was at 4730 so I ended up going from a 18x21 with a .125 cup to a 18x22 with a .075 cup. After that I was at 4620 for some reason and gained like 3mph. So last year I had them go over the props again and add a .250 cup. My speed right now is 28 to 28.5 mph at 3450rpm and a WOT of 38.5 at 4560-80 rpm. I called ACME props and talked to him for a while. He suggested there 18x23 with a .043 cup. He told me it would gain around 1 mph on the top end at 4450rpm and .75 mph at cruise. Well if you do the math, there is no way i'm spending $3500 for that. So I'm thinking...safe RPM range is 4200-4600. What's stopping me to add another 1 degree of pitch and take out .125 cup or so. I would drop like 150 rpm bringing me to around 4400 rpm. I should gain better mph at cruise. I know optimal rpm is 4600, but if the range is 4200 to 4600 I should be good? right? I'm not sure how my gas mileage will do also after? Right now I'm burning 32GPH right at 28-28.5 mph. Here are some scenarios.

Current =WOT 4580 rpm with 22 pitch prop .15.26 slip at 38.5 mph

Proposed =WOT 4400 rpm with 23 pitch prop .16.73 slip at 38 mph
Proposed =WOT 4400 rpm with 23 pitch prop .15.63 slip at 38.5 mph
Proposed =WOT 4400 rpm with 23 pitch prop .14.54 slip at 39 mph
Proposed =WOT 4400 rpm with 23 pitch prop .13.44 slip at 39.5 mph
Proposed =WOT 4400 rpm with 23 pitch prop .12.35 slip at 38.5 mph


Current =CRUISE 3450 rpm with 22 pitch prop .18.19 slip at 28 mph

Proposed =CRUISE 3450 rpm with 23 pitch prop .21.74 slip at 28 mph
Proposed =CRUISE 3450 rpm with 23 pitch prop .20.35 slip at 28.5 mph
Proposed =CRUISE 3450 rpm with 23 pitch prop .18.95 slip at 29 mph
Proposed =CRUISE 3450 rpm with 23 pitch prop .17.55 slip at 29.5 mph
Proposed =CRUISE 3450 rpm with 23 pitch prop .16.15 slip at 30 mph

Just my opinion I wouldn't be too concerned with WOT 4200 -4600 your just going to tear your motor's up.
What does it take to keep you on plane at a cruise speed you can live with. I just hear of so many people having motor problems with todays newer styles I think running hard destroys them. Again my opinion!
 
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When selecting props you NEED to know WOT. That doesnt mean that's what I'm using it for. Your absolutely wrong though. Sorry. Wot determines where your cruise will end up. If you dont keep it in that range that's what kills motors. Too low and your lugging too high and its over revving. I never run at wot unless I'm testing out my prop.
 
My intention was not to insult you the specs are under ideal conditions witch we don't have. Wind and current all play a part here all i'm saying , Is don't get crazy about your top number. As I Said Just my opinion !
 
My intention was not to insult you the specs are under ideal conditions witch we don't have. Wind and current all play a part here all i'm saying , Is don't get crazy about your top number. As I Said Just my opinion !
Sorry! I didn't mean to come off that way. I apologize.. The top number is what you really have to go by though. Anything you do to a prop is determined in the end by the top rpm range (ie 4200-4600). below 4200 and your lugging as i mentioned above your over revving. I know perfect is keeping it at 4600, but wondering if 4400 would really hurt engine and gas mileage.
 
You don’t have to apologize just having a Boat discussion what does SeaRay spec the prop size at?
 
Searay specs it out with 21 and slight pitch and 21.5 slight pitch. I confirmed that we can go higher than spec. All my numbers are with 8-10 people and full of fuel. If I tested with 2 people and 1/2 tanks I'd probably be higher rpm.
 
Just stopped at my prop shop and gave him my spec sheet. Curious to see what he says. I'll post back
 
IMG_4230.JPG
If it was me, I would have accepted the 4680 and called it a day.
 
Prop shop got back to me. He want's to leave the cup and all 1 degree of pitch. Said I would drop 150 rpm. This will get me at 4400 rpm WOT within the range of 4200-4600. My cruise at 3450 should get me to 29 - 29.5 mph. I'll be happy with that! Every time they worked on my props, they were right on with the RPM.
 
Okay, I'm going to wait till the end of next season. I can't justify spending the money right now with 2 vacations coming up..lol I guess I'll have to go 1 more season before I try it out.
 

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