Prop Question

My WOT is at about 3800 RPM, speed is 22 to 25 MPH
Black Diamond Prop 15 x 17 with plastic center hub (new prop)
Boat dry weight is 5500 LBS with 100 gal tank of gas, 25 fresh water, 9 gal waste, and lots of beer and ice, and usually only a couple of people on it.
Bottom paint is smooth and active, have trim tabs
Mercruiser 5.7 GMC looks like 260 HP 1988 (1989 carver 2557 boat) 25 foot long but about 30' OAL

Question is should I be able to achieve my suggested WOT 4200 - 4800 with this prop? I would hate to spend my money changing props without the research first.

Thanks

Re: Prop Question

What prop was on it before the replacement and what were the WOT throttle rpm with that prop? When RPM is that far off one needs to question the following:
1) Is the tachometer correct?
2) is the throttle opening fully?
3) If this is a carbureted engine is the choke opening fully when warm.
4) is the engine properly tuned?
5) Is the engine making full power?
6) Does the outdrive have the correct gear ratio
7) Is boat full of water soaked foam flotation?
8) Are you certain about the engine size and HP. ("Looks like is not a good answer")

Note that most of these checks cost you nothing but they must be answered objectively -- not with an "I think so". To pick up 1000 RPM you would need to drop pitch 5-6 inches which doesn't seem practical. You should not buy a prop in an attempt to fix a problem that is not prop related. An example is to decrease pitch just to get RPM up when the real problem is the engine is tired and needs a rebuild, or a tune-up.

Re: Prop Question

It makes you wonder that i will say. But as in Tony's thread his grady should turn a 15 by the numbers and thats with a fresh 350 mag. Here we have a older 7.5 and larger hull already 600 rpm under the rating and it fall's off plane quickly.

Remeber propping to max wot in reality increase's the boats lower and midrange response aka a 15" prop turning 4200 is going to place a far heavier load on the motor leg combo than a 13 turning 4800.

Id check the tach closely and if it is right...then your 13p is correct only tabs will help you from there. As to prop's there is little avaibilty 13 aka a large blade design..But i-boats does have a 13 stilletto and it would greatly improve things if your tach is right and motor is in tune
Stickly a opinion your milage may vary.

http://forums.iboats.com/showthread.php?t=309526


There seems to be some commonality going on here 12-15 pitch on these larger boat's..But as Spike has said make sure the motor is in order and the tach is on.

Re: Prop Question

Tail_Gunner said:

It makes you wonder that i will say. But as in Tony's thread his grady should turn a 15 by the numbers and thats with a fresh 350 mag. Here we have a older 7.5 and larger hull already 600 rpm under the rating and it fall's off plane quickly.

Remeber propping to max wot in reality increase's the boats lower and midrange response aka a 15" prop turning 4200 is going to place a far heavier load on the motor leg combo than a 13 turning 4800.

Id check the tach closely and if it is right...then your 13p is correct only tabs will help you from there. As to prop's there is little avaibilty 13 aka a large blade design..But i-boats does have a 13 stilletto and it would greatly improve things if your tach is right and motor is in tune
Stickly a opinion your milage may vary.

http://forums.iboats.com/showthread.php?t=309526


There seems to be some commonality going on here 12-15 pitch on these larger boat's..But as Spike has said make sure the motor is in order and the tach is on.

Click to expand...

Make sure all else is good with your motor mechanically first. Next, that's a big, heavy boat for a single 5.7 V8, from my experience, a 17" prop is too much pitch. In order to get in the right rpm range, I would suggest a 13" pitch prop, and stay with the same, or a bit bigger diameter (I don't think a Stilletto comes in a big enough diameter at that pitch?) Or, you will find it difficult to stay on plane, & fall off plane in turns (been there, done that). You may find freeing up the load on the motor with a 13" pitch prop, will put it in it's sweet spot, to reach it's full rated rpm.

Re: Prop Question

Thanks for the replies,

The engine has 990 hours on it, Rated at 260 HP and I did a compression test before I purchased it (was sitting for months) the results were as follows:

#1 125 #2 145
#3 140 #4 145
#5 125 #6 145
#7 150 #8 145

I've ran it about 6 hours since then after replacing the plugs to AC Delco, new cap and rotor and checked and moved the timing to about 12 BTC (when I set the timing I just put her to 1000 RPM, I did not mess with the thunderbolt ignition system, because I'm not sure if I had to) but should be close enough to get the RPM needed. I havent reved it up that high whithout a load. I need to look around at more tests to do to the engine to make sure its not tired and getting full power... (any pointers would be appreciated). I have had it at a higher RPM (4200 max) with a chewed up prop but it was just churning the water. Silvertips #7 scares me, how do I check for water filled floatation? When I surveyed it the stringers were ok but I want to check this out. 15 inch diameter looks to be the biggest I can go on the prop

It takes a minute to plane then runs pretty good just doesnt get much over 3800 RPM, havent checked when it comes out of plane but probably around 2500

You know if my setup is that much junk I would go with a whole new one, new engine and outdrive maybe counter prop? Wonder what that would cost me and what the best setup for a beast like this is?

Anyway 13" pitch is what I'm hearing if my setup is holding up, so it would take me longer to get on plane right? but then it would turn faster so I would gain top end and my fuel economy should be better achieving my rated WOT?

Re: Prop Question

Your going to have to jump around a bit in this forum to get good answer's, As to the motor's There is the I/O forums and wet foam will get you a loud response in restoration....But id go post in both forum's and see what happens..You did have it surveyed a huge plus in your favor..there should not be any wet foam if done correctly..

Beware of the restro forum they will have there drills out hunting for wet foam with a vengance..

Re: Prop Question

houstoncajun said:

Thanks for the replies,
Anyway 13" pitch is what I'm hearing if my setup is holding up, so it would take me longer to get on plane right? but then it would turn faster so I would gain top end and my fuel economy should be better achieving my rated WOT?

Click to expand...

Well, cyl 1, & cyl 5, look a bit weak, but not bad considering the age, load, & total hours, and actually having less pitch like a 13", would get you up on plane quicker, like being in a lower gear, and will let your engine rev out more,,, towards that sweet spot range on your motor.

Re: Prop Question

Cajun, I found your motor to have a 1.32 gear ratio with a WOT RPM range of 4,200 to 4,600. I used your allup weight of 7,100 pounds, and I took your 3,800 RPM at 25 MPH as a basis for your boat and figured the numbers from there. At this time your Prop Slip is 46% using the 3,800 RPM with a 1.32 gear ratio and your 25 MPH speed you quoted. Please check my gear ratio and WOT RPM I am using, because if these are off the all my calculations are off. My Prop Picker software shows you need a 14" Pitch prop, if you use a 13" prop you will be turning about 5,100 RPM and that is way too fast. But if you use a 14" prop you will be turning about 4,700 RPM, about 100RPM more than recommended, and I would recommend you buy a 4 blade aluminum prop with Zero rake to pick the stern up better and that has a lot more surface area and it will lower the RPM down to within the recommended RPM range. With this prop you should be able to do about the speed you are attaining now maybe a little better, but your climb to plane will be MUCH quicker and you should not fall off plane anymore. If you do not have trim tabs I would also recommend Nauticus Smart Tabs, they are only about $ 125 and a very good cheap investment for your problems and it could gain you a mile or two an hour.

Here are my numbers for the prop change to a 14" prop and a 13" prop.

14" Prop



13" Prop



H

Re: Prop Question

hwsiii said:

Cajun, I found your motor to have a 1.32 gear ratio with a WOT RPM range of 4,200 to 4,600. I used your allup weight of 7,100 pounds, and I took your 3,800 RPM at 25 MPH as a basis for your boat and figured the numbers from there. At this time your Prop Slip is 46% using the 3,800 RPM with a 1.32 gear ratio and your 25 MPH speed you quoted. Please check my gear ratio and WOT RPM I am using, because if these are off the all my calculations are off. My Prop Picker software shows you need a 14" Pitch prop, if you use a 13" prop you will be turning about 5,100 RPM and that is way too fast. But if you use a 14" prop you will be turning about 4,700 RPM, about 100RPM more than recommended, and I would recommend you buy a 4 blade aluminum prop with Zero rake to pick the stern up better and that has a lot more surface area and it will lower the RPM down to within the recommended RPM range. With this prop you should be able to do about the speed you are attaining now maybe a little better, but your climb to plane will be MUCH quicker and you should not fall off plane anymore. If you do not have trim tabs I would also recommend Nauticus Smart Tabs, they are only about $ 125 and a very good cheap investment for your problems and it could gain you a mile or two an hour.

Here are my numbers for the prop change to a 14" prop and a 13" prop.

14" Prop

13" Prop

H

Click to expand...

Isn't that Alpha one drive usually a 1.47, or 1.5 gear ratio?
Besides it is struggling to spin that 3 blade 17" to 3800rpm, theoretically a 15" pitch prop will get it to 4200rpm, and a 13" pitch should get it to the target 4600rpm. If his engine is strong enough, another blade (a 4blade prop) is just gonna add more drag to the mix, but that's my opinion, for whatever it's worth.

Re: Prop Question

Mik, you could be 100% right, but my data shows a 1.32 gear ratio until 1991 when i show it changed to a 1.65 gear ratio. I asked cajun to check my WOT and gear ratio, because if they are wrong then everything I told him is wrong and worthless, and you could be 100% right. But I have found my calculations to be right in almost all cases that I have run them on, so I believe in them. The only formula I know for almost correct RPM change between prop pitches is 3,800 RPM times * (17" prop/15" prop)= 4,307 and then I use a proprietary calculation after that to get to what I consider the correct answer. That is the normally accepted mathematical calculation for prop pitch changes RPM * (old prop pitch/new prop pitch)= New RPM, but this does not take into account all the variables in my opinion. I am not saying you are wrong Mik. I could be totally wrong and you right, but I do believe that I am right. You could also be very right about the 4 blades as well, but I believe he needs the extra surface area and zero rake to help keep the boat on plane at slower speeds. I am sure you are well versed in motors and props Mik, because of posts I have seen you make and the great advice you gave people. But I believe in my program and the results it returns, unless I have the wrong data entered. There is always room for logical disagreement between two people who have good intentions, and I am sure we both fit that description.

H

Re: Prop Question

Have at it hwsiii with your formula , Basically, the only thing I'm questioning, is the gear ratio? 1.32 I thought was a Bravo drive ratio, not an Alpha? and the 1.65 ratio Alpha as far as I know, was only used on the Chevrolet 4.3V6, and maybe even the 250cu inch Chevy inline 6. Most of the V8 Type one, or Alpha drives I know of are either 1.5, or 1.47 ratio's.

I have my own simple rule of thumb formula for figuring top speed, that works quite well for me, I got it out of an old outboard manual about 30 years ago

Re: Prop Question

Personally he needs to redo the timing the right way. he set it at 1000 rpm and set it at 12 degrees btdc. needs to be set at 8 degrees at idle speed of 650-700. Dont have to do anything with the thunderbolt 4 igniton to set timing. BUT he probably does need to go to a 13 pitch prop. Had to throw my 2 cents in.

Re: Prop Question

Mik, I am sure you are right about the gear ratio, but that is the information I have. I must have transposed the numbers.

H

Re: Prop Question

Thanks for the great info guys,
Guess I'll have to take a drive out to the boat after work one evening, (I have the original paperwork in there) so i'm not sure of the ratio myself. Is there a stamp or something I can find to tell for sure? I know its an alpha 1 and its a 1989 carver 2557 and says it was made in 1988 so the engine and drive would be 88?
I can retime it with no problem.
It also does have trim tabs.
I have the quicksilver torque 3 hub system and will see if I can get the prop you are recomending, I just wish I could try before I buy kinda thing

Re: Prop Question

Ok here is the info from my owners manual:

WOT 4200-4600
Idle 650-700 @ 8 BTDC
1.50:1 overall drive gear ratio

The original prop Carver put on it was a 15 3/4" x 16, but there is a statement in there " 25 to 35% prop slip is common for a cruiser type boat operating at crusing speed"

"variations in load, operating conditions, enviornment, the individual engine and hull performance may necessitate the purchase and use of another prop"

Hwsii, can you rerun your software for me?

Is there anyother way to find out if my engine is making the proper power without a dyno test? I dont want to change the prop if I have a engine problem instead

Re: Prop Question

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Theres a very good design spefic prop again in ss
http://boatpropellers.iboats.com/Mer...****=759923513

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Your old prop is cavitating badly and looks a little lean..At this point because of the lack of good data or info..(worn prop) id suggest you go with the 15 alum and if it performs well with the boat you can alway's use it as a spare and go to the SS which will give you lower planing and higher speeds ..but they will be not be that large of a gain



__________________
Stickly a opinion your milage may vary.

Re: Prop Question

Personally I don't recommend a 4 blade 15" pitch prop, if you can't get the rpm up to the recommended range with a 3 blade 17" pitch. The drag of the surface area of an extra blade, would put you right back where you started.
IMO a 4 blade prop is for middleweight offshore, high revving outboard boats, I really doubt your motor has the power to spin it on such a heavy boat.

I was tempting fate putting a 17" pitch 3 blade prop on my old heavy 24' SeaRay cuddy with the 260hp/350 Merc. Only because it had a slick bottom, thru-hull exhaust, never had more the 2 people, was lightly loaded (especially on fuel ) and was rack stored (not waterlogged), even then I had to trim it up as far as I could at WOT to hit 4600rpm, & 40mph.
Or else it would have been a 15" pitch 3 blade!

And you have a boat with a similar engine that is probably 1000lbs heavier,

I still recommend a 13" pitch 3 blade aluminum prop.

Hey TG, what does "Stickly a opinion" mean? Is that like "Strictly my opinion" ?

Re: Prop Question

Hey TG, what does "Stickly a opinion" mean? Is that like "Strictly my opinion" ?
__________________


Indeed and your milage may vary...

He's really plowing with that bow..his timing was a bit off and if you go to the I/O forum you notice the same posting....All in all it sounds like a long over due all around type of tune up. That prop cant have any bite left, by the time hes done that 15p 4bl should pop it right out...notice when it trims out that old blade cannot bite at all. I believe we can agree the drag of the 4th blade comes into effect only above 35 mph...it will only aid in push and planing which should accour somewhere arond 3000-3500 rpm

Id bet with some seafoam and a good tune thing's will be good..

But that is strickly a opinion and your milage may vary..

Re: Prop Question

Cajun, with the new numbers you will be doing right at 4,600 RPM with the
14" Aluminum that ought to have zero rake to pick the stern up more and that is my recommendation. The 13" would be turning 4,900 RPM which is too high. But lets go with Mik's recommendation on the three blade because of his past numbers with his Sea Ray.





H

Re: Prop Question

Tail_Gunner said:

Hey TG, what does "Stickly a opinion" mean? Is that like "Strictly my opinion" ?
__________________


Indeed and your milage may vary...

He's really plowing with that bow..his timing was a bit off and if you go to the I/O forum you notice the same posting....All in all it sounds like a long over due all around type of tune up. That prop cant have any bite left, by the time hes done that 15p 4bl should pop it right out...notice when it trims out that old blade cannot bite at all. I believe we can agree the drag of the 4th blade comes into effect only above 35 mph...it will only aid in push and planing which should accour somewhere arond 3000-3500 rpm

Id bet with some seafoam and a good tune thing's will be good..

But that is strickly a opinion and your milage may vary..

Click to expand...

Thanks for the tip TG, I see (said the blind man ) There's a WHOLE NOTHER thread going on in the I/O forum

With a few more points of interest, & pictures