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#353751 - 10/22/07 05:27 AM Re: Gearing vs. prop pitch [Re: D-Rod]  
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seabuddy Offline
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I disagree that a 2.20 gear set with 30" pitch props will have the same or lesser slip in real life on a boat as 1.81 gears with 24" props at sea level, if the boat builder calls for different prop pitches and gearing at sea level. The 30s will have more, and will not work out as well.

Maybe its just a difference of opinon.

So...

Mercury goes to the trouble of making, stocking, and offering three different drive ratios for this set up. Boat builders inventory and select from these. Both do this at no charge/no income to cover their costs. So, both engine make and boat builder businesses have real costs without offsetting income.

Why would a business (actually hundreds of businesses at the boat builders) do such a thing?

Of course, the truth is in the pudding. Take the boat to sea level and throw a set of new 30s on it and see if it goes. What, 55 mph? 57 mph? Light, under ideal conditions, and with a good motor condition a boat buyer is looking for at least 57 mph with a 425 Hp in this boat (over 59 - 62 without the Capt Call on the boat's factory speedo.) Heavy, with full gas and six people the speeds would be expected to fall off from that, partly from being able to achieve the same rpms and also from increased slip (I used a higher slip for my numbers as a reaction to the reported load at the lake.)

Used 30s and one might never know if its the "used" or if its the "30s/with the gearing".

One can pay for this type of thing, once or several times to get it right or get it wrong and just accept whatever performance that the set-up gives. Its your boat, not mine. Its your money, too.

What is a gear change cost by the seller, 2-3%-4% of the total-out-the door price of the rig?

I did not ask before so I'll ask now. We are using hard formulas for GPS speeds (two way average), and not a one time, one way speedo reading? Or was the speedo the only speed indicator read to get the 50 mph?

BTW, Most 2001 Cobalts had a Cobalt engine warranty for five years from first purchase date.

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#353778 - 10/22/07 09:05 AM Re: Gearing vs. prop pitch [Re: etyppo]  
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BillyB Offline
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E, just try the other set of props like we "talked" about. All this math is hurting my head. Try the props and if they don't work, try another set. Changing the gears will be a lot more of a hassle than finding props, trust me. And it will cost more too.
Why disassemble a perfectly good outdrive if you don't have to?


I'm just happy to be here!



#353878 - 10/22/07 12:10 PM Re: Gearing vs. prop pitch [Re: BillyB]  
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etyppo Offline
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 Originally Posted By: BillyB
E, just try the other set of props like we "talked" about. All this math is hurting my head. Try the props and if they don't work, try another set. Changing the gears will be a lot more of a hassle than finding props, trust me. And it will cost more too.
Why disassemble a perfectly good outdrive if you don't have to?


Will do. I'm not crazy about a gearing change for cost and potential for problems reasons. My numbers are based on the boat's tach/speedo, so they're not accurate to 2 decimal places. \:\) I had my GPS with me, but forgot to take it on the boat unfortunately. I just wondered if gearing/prop pitch were equivalent ways of achieving the same result. It sounds like there might be a little performance loss with a bigger pitch prop and larger (numerically) gear ratio, but I can live with that. Pending admiral approval of the purchase, I'll try 28" props and see what happens at lower altitude.

Thanks for all the help.


2006 Cobalt 263 Mercruiser 8.1
#353968 - 10/22/07 03:30 PM Re: Gearing vs. prop pitch [Re: etyppo]  
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seabuddy Offline
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Why change props at all? Just don't push the throttle down past 5,000 - 5,100 rpms.

You already know it will go 50 mph on its speedo at the lake. And it seems you are not interested in a 60 mph type boat set-up when you bring it down to sea level.

BTW, you will love the hole shot with the gearing and prop set you have when you take the rig to sea level.

You are buying a 7 year old used boat and seem to be on a tight budget. Before spending any money you might see how she runs for you as is... but at sea level.




#353972 - 10/22/07 03:34 PM Re: Gearing vs. prop pitch [Re: seabuddy]  
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D-Rod Offline
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Seabuddy...I think he said the dealer said they could trade the props out.....

Last edited by D-Rod; 10/22/07 03:34 PM.

-YOLO
#353973 - 10/22/07 03:35 PM Re: Gearing vs. prop pitch [Re: D-Rod]  
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seabuddy Offline
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Good. What would be the gain?

#353974 - 10/22/07 03:38 PM Re: Gearing vs. prop pitch [Re: seabuddy]  
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D-Rod Offline
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Not having to worry about catastrophic engine damage done by accidental over-revving? Slight increase in top-end. Better fuel economy?

A 263 with a 425hp 8.1 is not under-powered. Holeshot shouldn't be a major problem...

Last edited by D-Rod; 10/22/07 03:38 PM.

-YOLO
#353984 - 10/22/07 04:18 PM Re: Gearing vs. prop pitch [Re: seabuddy]  
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BillyB Offline
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Seabuddy, my dad installed a Mercruiser 5.7LX for a guy in place of the 3.7LX that was there. The motor would way over-rev with his old prop. But he thought like you, "I won't push it that far", and didn't buy a new prop. Well three months later my dad was replacing a bent valve for the guy. Doesn't hardly seem worth the risk to me, especially since etyppo said his dealer will swap out the 26s with 28s. Now if the 28s aren't enough, he's in no better situation, but he might know for sure if 30s will work then.

Last edited by BillyB; 10/22/07 04:19 PM.

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#353998 - 10/22/07 05:02 PM Re: Gearing vs. prop pitch [Re: BillyB]  
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seabuddy Offline
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You are both right.

But for a single driver, with a used 7 year old rig going one way or another may not matter. There is no warrantee rules to be concerned about and there is a rev limiter if the engine does go too high of rpm.

I guess it bothers me that with a dealer sale, you are looking at a bigger fee to buy the boat than the gear change will cost and the gear change opens up and allows a good look at some expensive parts while the deal can still be backed out of if they inspect poorly.


Couple of thoughts

1) The boat is moving to Sea Level for the rest of its life, most likely.

2) That 425 Hp and drive package has a replacment full retail value of just over $30,000 today, installed. Inspection is good insurance.

3) Its is the absolute most expensive parts to replace if they are mis-used in this rig.

4) 8.1 425 Hp engines/drives are not like small block Chevies that one can torture again and again and they come back for more.

I tend to think that such possible exposures impact and would shade what would be the best course of action when someone is asking for advice. Its like the full load versas light load rpms, speed, slip, and propping. Just more fine tuning the answer to the situation.

Heck, going a route that has been tested by the boat builders and engine makers and rejected even when it costs them money and holds no profit is something I might do with lesser costs on the table. I am 58 years old. Less than a month away from turning 59. Maybe I need to get a crazy boat to re-live some of the the stuff I did when I was younger. I once put a engine and boat rig together with my Uncle that was good for only two weeks before the boat had to be junked. The engine lasted 4 months longer. But, boy, what fun those two weeks were that that engine on that boat were. Now I seem to be less of a risk taker. As Kelly says, "oh well".

#354008 - 10/22/07 05:28 PM Re: Gearing vs. prop pitch [Re: seabuddy]  
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etyppo Offline
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Seabuddy,

Good points. The issue for me isn't that I couldn't afford a $2,000 (or whatever it would cost) gearing change, just that for me, the additional cost of a gearing switch wouldn't be worth it if a prop switch would have largely the same impact on performance since the prop switch would be free. I guess I could inquire about a gearing change done as a condition of the sale. If boat sales are as bad as they sound on these forums, they may be pretty motivated to make a sale. Do you have any idea what a gearing change would cost? Also, is it something that has a high potential for noise or other issues?

The holeshot at Tahoe was very good for a boat of that size and weight, so I'm guessing it will still be fine with a somewhat bigger prop at sea level. I could live with it being underpropped, but if I can fix it for free, I might as well do it. My current boat was underpropped (for watersports use) when I bought it, and I feel more comfortable after installing the correct prop not having to worry about overrevving it when I open it up. I'm not sure about the 496, but as I discovered with the original prop, the 5.0L Volvo in my current boat will allow 200-300 more rpm than the max recommended rpms before the rev limiter kicks in. If you're saying the 496 is more fragile than a SBC, I'm not sure I want to be able to run those extra rpm accidently. Does the 425 hp 496 have any special "issues", or is it just more sensitive to abuse?

I'm definitely getting surveys done on the boat and engine before purchase on a boat that expensive. Thanks for the input. It is very useful.


2006 Cobalt 263 Mercruiser 8.1
#354035 - 10/22/07 06:37 PM Re: Gearing vs. prop pitch [Re: etyppo]  
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D-Rod Offline
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Etyppo:

Seabuddy has a very good point. You do *NOT* want to over-rev/abuse the big block like you did your 5.0.

At 5100, you have a cylinder speed of 3714.5. That number is getting high.

If you revved your 5.0 Volvo to 6000 rpms, you would have a cylinder speed of 3480. At 5500rpms, it is only 3144.2. Now, the big-block has more rotational mass it is accelerating with every stroke to higher speeds. Each engine is designed to handle certain stresses, but I think you catch the drift?

Also...please don't discount Seabuddy's comments. He's a very well respected man across the entire boating industry, not just on BABC.

More Later...



-YOLO
#354057 - 10/22/07 07:51 PM Re: Gearing vs. prop pitch [Re: D-Rod]  
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etyppo Offline
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D-Rod,

I've only overreved the motor on my current boat once briefly when I first got it. I didn't realize it was underpropped and it hit 5200 or 5300 rpms when I opened it up (it was a used boat with 140 hours). I looked up the maximum rpm (5000) and haven't exceeded it since.

Also, I lurked for a long time before having a need to post and realize Seabuddy is a great source of information. I'm not disagreeing with him, just clarifying why I was leaning toward a prop switch rather than a gearing change. I understand his concerns, and hadn't thought about the benefits of getting a look at the outdrive internals if gearing was changed. That makes the gearing alternative more appealing that it orignally sounded.


2006 Cobalt 263 Mercruiser 8.1
#354066 - 10/22/07 08:18 PM Re: Gearing vs. prop pitch [Re: etyppo]  
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D-Rod Offline
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Yep...I agree, his response to the gearing was a good one.

BTW: Merc/Volvo now rate the 5.0 to 5200 rpms max. I don't think a whole lot (if anything) changed. I *doubt* 5200 rpms would cause your 5.0 to blow.


-YOLO
#354068 - 10/22/07 08:23 PM Re: Gearing vs. prop pitch [Re: etyppo]  
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seabuddy Offline
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Get a "Master Tech" Merc tech to survey the engine and drive. That is a real title within Merc training levels for their Techs. And that is a separate survey from a boat survey. Its prudent here due the value of these two parts when you look at the entire rig value.

Also, some of the Cobalt made parts should still be under Cobalt warrantee if the ownership records were indicated back to Cobalt. Some of these warrantee aspects may go out to ten years for the original owner, check with Cobalt via shooting a HIN number back to them to know what is still covering your boat and how you transfer it.

Small block Chevies are in a different book from almost any engine in my book.

Prior to the 8.1 GM sold a 454 to Merc and Volvo in two Hp ratings. Roughly 330 Hp and 365 / 385 Hp. With this engine, everyone bought the top power for the upgraded internal parts. Crank, pistons, rods, even head castings, etc were different between the 330 and the 365/385 ratings. Ever hear of "retangular heads"? Then they had a 502 engine. When GM went to the 8.1, they kept the names hanging around to trade on in the magazines, but the internal parts between a 375 and a 425 Hp are the computer chip and the cam, as far as I remember, but I am checking if there is something I am forgeting and will post back when I know more. Give me some time.

Without the upgraded internal parts inside as standard from GM in any form, MercuryRacing stuck with a version of the 502 block for their higher Hp engines. Entirely different egnine series and internal parts. Aftermarket engine builders are quick when pressed to really re-do the internals of any 8.1 once they are asked for more Hp AND long life. I feel fairly comfortable that there is nothing upgraded inside a 425 Hp 8.1 that would make one feel that it will take more high rpm running than the 375 Hp except the computer chip.

As D-Rod says, there are piston speed issues, too.

Two different thoughts, but they say the same thing.

Keep the rpms down, if you want long life. Keep the rpms up (to 5,150 rpms max) if you want max top speed if you can prop to that. The 425 Hp cam dictates that. 375 Hp cams seems to work at up to 4,700 rpms and often lower than that rpm. Its not as "peaky" a cam, I guess.

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