340 props?

mnm99

mnm99

Well-Known Member
Oct 2, 2015
2,447
Long Island
Boat Info
2004 340 SeaRay Sundancer
Engines
Twin 8.1 Merc
340 props? ( UPDATED 5-14-2017 )

I'm going to take my props off this winter and have them scanned. I have a slight vibration and I know when I bought the boat in April it had a dent in one that they bent back for the seatrial. So before next season I just want everything right. I have 18x21 and 18x21.5's ..Here's my question...I did a few tests this year. During the Seatrial I was able to hit 4800 rpm WOT. This was with 1/2 tank of gas , 2 people on board with NOTHING else. No water tank , beer , tools ect.. At the end of this season with 4 people and loaded normal I can hit 4630 WOT, , no new bottom paint since 5 years ago, Full tank of gas, fresh water and loaded with beer. Even with the 4630 rpm do you think I can add another degree of pitch? Of course the prop guy will advise me, but I'm thinking my RPM will drop 200rpm to 4400 and still be good. I'd like to gain a couple MPH if possible. At 3440 I'm at 26mph, 3500 I'm at 27mph. The range is 4200-4600.
Thanks
 
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I'm no prop expert but, have been instructed and believe that as long as your able to reach the W.O.T. as per the specs on the engine information sticker...your good.

I added some cup to mine with the same thinking as you and have been happy with the results.
 
I'm getting similar numbers. I lost about 200 rpm since launch do growth. I'm planning to pull my props this winter as well and have Bossler Sweezey check them. I suspect the will recommend adding cupping to improve cruise speed and mpg. So far I'm getting .7 to .8 mpg at 3400 rpm doing around 25 mph. Seems mpg doesn't change much from 3300 to 3800 rpm loaded or empty.
 
saopm said:
I'm getting similar numbers. I lost about 200 rpm since launch do growth. I'm planning to pull my props this winter as well and have Bossler Sweezey check them. I suspect the will recommend adding cupping to improve cruise speed and mpg. So far I'm getting .7 to .8 mpg at 3400 rpm doing around 25 mph. Seems mpg doesn't change much from 3300 to 3800 rpm loaded or empty.
Click to expand...

Yea I'm bringing them to Bossler also. Look at your GPH on your smartcraft. I'm at 30gph at 3450=26mph and 32gph at 3550=27mph and keeps going up from there. I don't know how your's can stay the same.
 
As you can see from the attached service bulletin, Mercruiser recommends propping as close as possible to 4600 (in your case) as loaded with half fuel, full water and 4 people. Moreover, that 1 inch of pitch gives about 150rpm change. I've highlighted the relevant parts in yellow.

FWIW Veem recommends carrying out seal trials in at least 10 metres of water.

Propping_Page_3.jpgPropping_Page_4.jpg

I'm looking at props too

Top Speed (05.06..2016).jpg
 
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The bulletin above is about just how I tested. and came up with 4630 WOT. This was also at the end of the season. When I bought the boat in April the Marina wanted $2500 to bottom paint the boat. The paint looked OK since it was in Fresh water, but it was also 5 years old. I told him to hold off. I had it in Salt water this year , so There is some growth I'm sure. From the looks of my trim tabs I'm sure there is. I have a slight vibration also. When I went to seatrial the prop had a nice ding in it. The marina took it off and just hammered it as straight as they could get it. This is how it's been all season, so I could imagine if it was balanced correct I could get a little more out of it also.
 
Once you swap to the cast iron manifolds from the ali ones you will probably lose a few knots :lol:

Given your wot is close to 4600 I wouldn't touch the pitch.
 
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Wow massive difference between speeds of 340's of JJ Chones and Scotts boats at the same rpm's. If that is legit - props to you (pardon the pun) :lol:.

What props an specs?
 
Ok ok. I confess. They're not 5 blade props. Must have been a gps glitch or something. I looked down and saw it and yes, had to take a picture. It went back to normal after a while.
One can wish.
 
jj chones said:
ok ok. I confess. They're not 5 blade props. Must have been a gps glitch or something. I looked down and saw it and yes, had to take a picture. It went back to normal after a while.
One can wish.
Click to expand...

lol........
 
JJ Chones said:
Ok ok. I confess. They're not 5 blade props. Must have been a gps glitch or something. I looked down and saw it and yes, had to take a picture. It went back to normal after a while.
One can wish.
Click to expand...

If it were legit, I was thinking you were in one hell of a current. :wow:

I know from my recent discussions with Veem 5 blade props wouldn't have provided those gains. In fact the naval architect at Veem recommended 4 blades over the 5's for the best all round performance for my application
 
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sfergson727 said:
With no offense, I don't think that is legit, unless he is at WOT, which would mean he's wayyyyyy over-propped. Probably a GPS glitch, but hey, if I saw that on my display, I'd snap a shot too.
Click to expand...

I was attempting to tease out the story behind the post. :smt001
 
I got some results...Very pleased so far....I can't really give a real accurate result since there was a pretty bad chop and I was heading into 15mph winds, but even then I think you will be impressed! So a recap of the first post.

"At the end of this season with 4 people and loaded normal I can hit 4630 WOT, , no new bottom paint since 5 years ago, Full tank of gas, fresh water and loaded with beer. Even with the 4630 rpm do you think I can add another degree of pitch? Of course the prop guy will advise me, but I'm thinking my RPM will drop 200 rpm to 4400 and still be good. I'd like to gain a couple MPH if possible. The range is 4200-4600.
3440 I'm at 26 mph,
3500 I'm at 27 mph. "

New results....

I can stay on plane at 18 mph was 22 before
3400 I'm at 28 mph = 2 plus gain
3500 I'm at 30 mph = 3 gain

Went from 21 pitch to 22 with med cup.
21.5 to 22.5 with med cup.

I gained 3 mph and I was still able to hit 4650 rpm at 39 mph. It was even a bit more than last year and gained speed., but really wasn't a test because it was so choppy and I pulled back quick.. The bottom paint was pretty good last year. The changes I did was add an additional pitch and balance. Boat is smooth a silk too. These results were with the same load as last year. Full tanks of gas, full water tank, loaded with beer, bedding , 2 people exc...
 
juggernaut1 said:
Nice improvement given chop and wind condition.
Click to expand...

I can't wait to see what a smooth day brings. If I can still hit my max 4800 rpm I'm wondering if I can even go one more pitch or more cup next year. I'll post back when it's nice out.
 
I hit a new top speed wit hmin 41.8mph ... stock 8.1s, and 18X22 3blade props.

I am told the newwer body 4blade hlep my hull a good amount.
 
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