Problem with Prop/pitch

[DagoExpress]

Hello,
I am having a problem with figuring out the proper pitch and prop size for my boat to get it on plane quicker. I purchased a 1984 23 ft. 5000 Century Sun Express Cabin Cruiser in Feb. of this year. It has a Mercruiser 228 305 w/ 4bbl carb (non vortec) with a Alpha one outdrive. The alum. prop that was on the boat when I purchased it had a piece of one blade missing (prop says Mich. smc 376 cup) I was told that this transfers to a 15 3/4 X 17 which I cant find, correct or not? I do not know if this is even the original prop for the boat. The owner gave me a new prop in the box which is a Turning Point Hustler 14 1/4 X 17 cupped. He said it was the correct replacement (NO WAY). I took boat out with new prop on and the bow rides high and takes a very long time to plane out even with trim tabs down at 6000 RPMs with 1/2 tank of gas (35 gal./74 max.). A friend gave me a prop to replace the new one which is a 15 1/4 X 15 pitch (no cup and similar to old one elephant ear style) and it is the same problem. It still only planes out at high RPMs. IM NOT LOOKING FOR SPEED but top speed is between 32 & 34 MPH @ 6000 RPMs. and planed out. I know i shouldnt run it that hard but in order to plane out I do. When cleaning the boat in Feb. I found 5 sand bags up in the bow where the anchor rope stores. Was he compensating for improper pitch? Im not looking for speed, I just want it to plane properly. I dont store water in the holding tank (plastic) nor do I have any unnecessary weight on board (unless you include the wife) only overnight cloths and a cooler. It does not have ANY water inside. Could you PLEASE help me figure this problem out because I dont need a garage full of unwanted props! Thank You Much, DagoExpress.

 

[hwsiii]

Re: Problem with Prop/pitch

Evidently I am very lost, you are talking about a 305 IO in one statement and then you talk about 6,000 RPM in the next. Please tell us exactly what HP engine, the recommended WOT RPM and the gear ratio for your outdrive is.
6,000 RPM and an Inboard/Outboard don't match up. Please don't use your boat again until we get this straightened out. Your tahoneter certainly can't be working right.

H

 

[hwsiii]

Re: Problem with Prop/pitch

Error

 

[MikDee]

Re: Problem with Prop/pitch

I agree with hwsiii, but you also need trim tabs if you're having problems planing out, probably Bennet adjustable ones to keep that cabin cruiser level also.

 

[steelespike]

Re: Problem with Prop/pitch

Are your tabs working correctly?is it trimming in all the way?
Your tach must bad or set wrong?Are you sure the motor is on all cylinders?
Is the throttle opening all the way.Have you checked the compression?
Unless that boat has a dud hull design it has to get up reasonably well.
Sand bags in the bow may be a clue.Either the po was to cheap to check out the motor or they couldn't solve the problem.If he was doing his own work he wouldn't be the first to miss an 8 cylinder running on 7.
If a 15 won't get the job done a 13 borders on the rediculess.
The Hustler is an excellent prop usually providing an over all improvement in performance.

 

[MikDee]

Re: Problem with Prop/pitch

I bet this is a heavy boat with an older non-vortec 198hp 2bbl, or 228hp 4bbl, 305motor, not much power to weight ratio there. You probably need at least a 15" diameter prop, the Hustler is probably great, for a lighter boat, and with more power, but a mismatch here.

 

[DagoExpress]

Re: Problem with Prop/pitch

Evidently I am very lost, you are talking about a 305 IO in one statement and then you talk about 6,000 RPM in the next. Please tell us exactly what HP engine, the recommended WOT RPM and the gear ratio for your outdrive is.
6,000 RPM and an Inboard/Outboard don't match up. Please don't use your boat again until we get this straightened out. Your tahoneter certainly can't be working right.

H

Hello, Thank You for helping out. The engine is a 1984 305 stern drive Mercruiser 228 4 bbl. The engine runs VERY smooth with NO hesitations or hiccups. Im not sure what the WOT RPM is for a 22.67 ft. boat and not shure where to find the info for the gear ratio. I do know that the tach winds up to 5500+ in order to plane out with these 2 props I have tried. The tach works smoothly at all speeds and reads upto 6200 rpms. Even with the trim tabs in working order I feel it is taking too much time. (maybe its just me?) I just replaced the 2 Bennett cylinders and the pump is in good condition. I checked and rechecked to make sure they were working properly. The hull is smooth. I think that with all the weight (engine, outdrive & 74 gal fuel tank) at the stern is most of the problem. Not sure. I just dont want to have sandbags for counterweight. Im asuming that the original prop was incorrect and that was the reason for the sandbags. I like things to work the way they should WITHOUT a bunch of nonsense to correct/hide the problem. Thank you again. I hope this helps out some.

 

[DagoExpress]

Re: Problem with Prop/pitch

I bet this is a heavy boat with an older non-vortec 198hp 2bbl, or 228hp 4bbl, 305motor, not much power to weight ratio there. You probably need at least a 15" diameter prop, the Hustler is probably great, for a lighter boat, and with more power, but a mismatch here.

Hello, It is a 1984 non vortec 228 4bbl. and the boat weighs between 3000 & 3200 lbs. The old broken prop (Mich. smc 376 cup) equals out to be a 15 3/4 x 17 so im told by the local marina BUT I dont even know if this is the right prop. I am the 3rd owner so who really knows. Im NOT looking for speed but only to take out the wife & kids (200 lbs) and be able to fight the upstream current on occasion without winding up to the point of no return. Thank you for helping me out.

 

[DagoExpress]

Re: Problem with Prop/pitch

Hello, It is a 1984 non vortec 228 4bbl. and the boat weighs between 3000 & 3200 lbs. The old broken prop (Mich. smc 376 cup) equals out to be a 15 3/4 x 17 so im told by the local marina BUT I dont even know if this is the right prop. I am the 3rd owner so who really knows. Im NOT looking for speed but only to take out the wife & kids (200 lbs) and be able to fight the upstream current on occasion without winding up to the point of no return. Thank you for helping me out.

The old prop is the elephant ear style (mich smc 376 cup) and the NO WAY replacement Hustler (14 /14 X 17) has small blades about the size of a cupped hand. I know that this does make a difference an therefor put it back in the box for an emergency spare.

 

[QC]

Re: Problem with Prop/pitch

Comment/questions . . .

1) Is this her? That boat is easily 5,000 lbs plus. New and dry IMHO. Probably pushes 6000 in use.

2) You need to verify the tach

3) Although I am skeptical, if the tach is right, the prop(s) is ventilating really BAD. You mentioned 5500 to plane, what happens when she does plane off? If RPM comes down and is under 4800 at WOT underway, then I would almost guarantee ventilation . . . Does seem like an excessive case though. Typically RPM goes up once over and on plane. If it drops then the clues are there

Ventilation is the prop sucking air off of the surface. It can be caused by a variety of things. I see no mention of drive trim in any of your posts. Steelespike touched on it. Should be all of the way down (in) for planing.

4) Yes, the sandbags are a hint, but we can get this right with some time and lots of good info.

 

[MikDee]

Re: Problem with Prop/pitch

Yep, that's about what I was picturing QC

Dago, by the way, I had that same engine in my old 79'-20' SeaRay bowrider, (a much lighter boat) so I know exactly what it is, and what it can do, it's not even prop hp But, I got 50mph out of it

 

[hwsiii]

Re: Problem with Prop/pitch

Dago, That motor is NOT turning 6,000 RPM unless it is WAY over 228 HP. So you need to borrow a tach and see how many RPM it is actually turning. I bet you will find out that the tach is set in the wrong position with the switches in the back. It maxes out at between 4,600 and 4,800 RPM.
WE need the GEAR RATIO as well to see how much prop slip you are getting at this time, go by a mercruiser dealer and find out what gear ratio and RPM you should be turning and get the model and serial number of both the motor and outdrive before you go by there so you can verify the information. It appears you might have a boat with about a 20 degree deadrise in the hull and you only have an allup weight of about 4,500 pounds, if that is correct the deadrise is the problem NOT the weight, AND all the weight in the stern not distributed evenly. You should probably be running a prop with Zero rake and trailing edge cup for more STERN lift, not high rake with bow lift, as that just digs the stern deeper in the water especially with the stern weight you already have.

Let's get the REAL RPM and GEAR RATIO before we go any further.

In my opinion you have 228 HP motor that should be running at about 4,800 RPM and probably a 1.65 gear ratio and probably should be running closer to 40 MPH instead of 33.


Boat and Motor

H

 

[DagoExpress]

Re: Problem with Prop/pitch

Hello to all,
You guys are great! With all the info we should come up with something. First off, that is the boat I have. I Pulled tach and was unaware of settings on the back. It was set between 3 & 4. What would be the proper setting? Lifted hatch and located tag with all info. INFO; Mercruiser w/Thunderbolt ign., Motor serial # 6759611, Model # MCM 228, MAX WOT 4200 - 4600 rpms., Outdrive; Alpha One, Serial # 6747249. I called Marina for help on the gear ratio. The stamp was worn off so he gave me 5 variables. I have to remove outdrive cover and count the teeth to figure out what gear ratio I have. The Variables are; 1.98 20-24, 1.65 24-24, 1.84 17-19, 1.50 20-22, 1.32 16-20. When I do reach plane around 30 mph @ 6000 rpm I trim accordingly and she rides great with trim tabs up. When I back off throttle and trim accordingly stern seems to sink and bow comes up & engine seems to work harder even with the help of the trim tabs down and I have to start all over again. it is difficult to find a happy median. The prop that is on and used this past weekend is the 14 1/4 X 15 My first boat was a 1979 Imperial trihull w/ 90 hp Merc outboard on it so maybe im expecting too much response from such a big cruiser but none the less the prop is still the biggest issue since I dont know which one would be correct to start troubleshooting with. Thank all of you again. I will figure out gear ratio ASAP.

 

[hwsiii]

Re: Problem with Prop/pitch

Get the correct gear ratio and RPM you are turning and we can fix your problem, but I can tell you right now thw prop you are turning does NOT have enough blade surface area for the blade geometry it has.

H

 

[DagoExpress]

Re: Problem with Prop/pitch

Hello Again,
I took the cap off of the outdrive and counted the gears.The lower gear had 20 teeth and the top gear had 22 teeth. I counted them twice even though they were engraved. Given the info from the marina the 20-22 = 1.50 Gear ratio. Max WOT is 4200-4600. I honestly dont know how to get the RPMs I am turning or even what it should be or how to aquire this number. I dont know all the terminology. Is WOT the #s I am looking for? All of you have been GREAT with the help you have given me over the last few hours. What would I need to do next to find the right pitch/prop/cup/rake? Thank You Again, Getting closer! DegoExpress.

 

[hwsiii]

Re: Problem with Prop/pitch

That is great now we know that a 17" pitch prop is the right prop for your boat. Considering the length of your boat, the weight and the gear ratio of the outdrive a 17" Pitch is the correct one. NOW we need to know exactly how many RPM we are turning so we can decide what diameter and what geometry of prop blades we need. You can hook a water hose up to the outdrive with a set of ear muffs and leave the switch on the back of the tach like it is. See what the RPM are at idle, should be around 800 but it won't be. Then shut everything down and move the switch to 4, hopefully that will be the key and show around 800 RPM at idle.
If this gives us the correct RPM you need to take the boat out and run it and give us the RPM and speed in two different directions from 3,000 RPM up to Maximum RPM in 250 RPM increments. Then back off the throttle and tell us at what RPM the boat falls off of plane with the trim tabs down. It is imperative that you give us REAL numbers for RPM and MPH, so take the time to get them right please.
Then we will be able to discern what diameter and blade geometry we need for the best prop for your boat and motor combination.
At this time, in my opinion you will probably want to get a 16" Progressive Pitch prop with about 1" of cup on the trailing edge for maximum stern lift as that is probably why the boat is falling off plane, too much stern weight. With the 1" of theoretical cup on the trailing edge the prop will react speedwise like a 17" prop. Between the motor, outdrive and gas you probably have over 1,300 pounds or more sitting in the stern area which is causing it to fall off plane when the boat slows down, even with the trim tabs.
In one statement you say you used the 14 1/4 x 15" prop, but in another statement you say the prop is a 15 1/4 x 15", I need to know which it is and also the make and model or serial number . Also please check the degrees of deadrise in the boat and tell us what that is please.

H

 

[hwsiii]

Re: Problem with Prop/pitch

Dago, after checking my database of props I find that the prop you had was a Michigan Replacement Prop with a 3" Regressive Pitch prop with 3 degrees of rake built into it. According to Michigan wheels online catalog they quit making that prop anymore.
By regressive pitch I mean that the prop had a leading edge that is an 18" Pitch then the middle is a 17" pitch and the trailing edge is actually a 16" Pitch. It is the exact opposite of all of the progressive pitch props that are so popular now. It was designed for a better hole shot and with the regressive pitch as the RPM increase there is more power available to hold the boat up. The higher pitch on the leading edge enhances the hole shot by inducing drag, and after the boat gets on plane the lower pitch on the trailing edge helps keep the boat up on top of the water.
I would recommend having the prop rebuilt, but I seriously doubt there are many companies that have the correct pitch block to repair a Regressive pitch prop, and if they don't then you would have paid for something that will not work correctly as it was designed and you will have spent your money for nothing.
I am searching for an alternative that has about the same blade geometry and dimensions, but I am also looking for a newer alternative to this design as well.

H

 

[QC]

Re: Problem with Prop/pitch

Yuck ^^^^

 

[hwsiii]

Re: Problem with Prop/pitch

QC, may I ask why you posted Yuck?

H

 

[QC]

Re: Problem with Prop/pitch

Prop design . . . Sorry, definitely not you