30 mph top speed?

[craze1cars]

Re: 30 mph top speed?

1.98 is a very common Mercruiser Alpha drive ratio.

Inexpensive shop tachs are invaluable....and have other features:
http://www.amazon.com/Actron-CP7605.../ref=cm_cr_pr_product_top/179-1834556-7823119

I have this particular product...digital, and works fantastic as a tach and a general multimeter:
http://www.amazon.com/Actron-CP7677..._1?s=automotive&ie=UTF8&qid=1314554100&sr=1-1

State of carb tune and having proper ignition timing is EVERYTHING in getting peak performance out of an engine. Do you have a timing light? Or can you rent one and set/adjust timing and confirm the advance is working according to Mercruiser specs? Years of corrosion inside a marine distributor can wreak havoc on ignition advance weight function...and if your ignition curve is retarded even a few degrees at any point in the RPM range, your HP will suffer substantailly. Soooo many older boats have found substantial extra hp by setting and/or repairing faulty ignition timing systems...

If your current setup is capable of running 5,000 and you just don't like running that fast (and rightly so), then you are most certainly underpropped, and this can lead to slow speeds. I agree with Maclin that you will need to go up in pitch. Most likely this will lower your peak RPM to acceptable level, and will increase your top speed at the same time. You want to target 4,800 as a max rpm for your motor, and a 19 pitch prop is certainly the next logical step to try to get you there.

What is the condition of your current prop? If it's a bit chewed up or has been home-filed of some nicks & dings, this may be the cause of your extra slip. And extra slip causes high RPMS without a gain in speed. So if your current 17 pitch prop is a bit chewed up, it's also entirely possible that a newer/less slippery/more efficient 17 pitch will bite the water better, bring your rpms down, and your speed up. I will also assume your current 17 pitch prop is 3 blade. Keep in mind if you get a replacement 17 pitch with a 4th blade, you may very likely drop 200 rpms or better...in the same pitch...and this might put you right where you want to be at 4,800-ish. And 4 blade props improve holeshot generally...but they do not usually improve top speed.

 

[roscoe]

Re: 30 mph top speed?

I only ran it up to 5000 to see if would make a difference, 4500 is most i run it. But ya i guess my very best is 32 at 5000.

The point is, when you go WOT, you should not be able to get 5000 rpm.

 

[wyoboat]

Re: 30 mph top speed?

Thanks, you guys are a big help. Seriously. I did forget to mention that i did time it, it is set at 8degrees before tdc. It doesn't appear to have any vacuum advance in it. do boats have a different set up than cars? it is a thunderbolt ignition first generation i assume from reading other threads on here. the prop looks fine no nicks or scrapes or anything it looks good. I've been reading a little on hear about guys using props with more cupping. wonder it a more aggressive 17p prop might work good? thanks again i've been learning alot on here just reading :]

 

[Maclin]

Re: 30 mph top speed?

If 30mph is ok with you then just keep what you have. If you want 35+ then the pitch need to change.

Thunderbolts have a static timing procedure, then once you put it back in run mode the ignition computer takes care of the total advance at the different rpm's. No vacuum advance on boat engines.

 

[craze1cars]

Re: 30 mph top speed?

That 8 degrees you set it at is a red flag to me. It is VERY important you follow correct procedure to set timing on these boats. Do you have a service manual? Many times you CANNOT just hook up a timing light, let it idle, and get an accurate reading. Sometimes you need to bypass/unplug/jump electrical terminals on some connections first, etc.

I found these specs in another thread, can't vouch for accuracy but I believe them to be correct:
8 initial
14 module advance
22 total advance at 2500rpm

So if those specs are accurate, and you found it to be set at 8, is it possible that 8 degrees advanced was with the module advancing it? Where it's supposed to be 14? If yes, that means you might actually be set at 6 degrees retarded. And THAT, my friend, could EASILY cost you many many HP....I find it quite possible that you, or someone who owned the boat before you, set this boat's timing to 8 degrees without using the proper procedure and had no idea that dialed out a bunch of horsepower. This error is very VERY common for shadetree mechanics who treat these boat engines like they're tuning a classic car...without following proper procedure.

Let me make it clear that I DO NOT KNOW THE CORRECT PROCEDURE FOR TIMING YOUR BOAT. So please don't blindly advance your timing 6 degrees because I implied you might need to. I could be wrong, and if I am detonation will destroy your engine! But please do make sure that YOU know the proper procedure for setting timing...

Yes a more aggressive 17 with extra cupping might dial down your RPMS a couple hundred while reducing your slip. On paper an aggressively cupped 17 will act like an 18 pitch prop. It also might not bring it down enough depending on what you're starting with for a prop. The only way to know for sure is to try it...

And if you actually are maybe 6 degrees retarded, you need to confirm and set this FIRST. An extra 6 degrees of advance might push your motor into a whole new HP range which may result in it needing a 19 or 21 pitch prop real easy....

 

[wyoboat]

Re: 30 mph top speed?

I didn't touch the timing, I'm going to research this and see what i can find out, has anyone on here timed a 4.3 with the thunderbolt? I agree timing is the cheapest power out there. thanks for keeping me thinking carze1cars

 

[craze1cars]

Re: 30 mph top speed?

For timing procedure, you will get many responses from knowledgeable people if you start a new topic over in the "Mercruiser I/O Repair" subforum of Iboats. You won't get many here on the Prop forum that will know your answer.

For best response, when you ask about timing procedure, provide the exact year/make/model of your engine, and even the serial number if you have easy access to it....they love that over there, and with that info they can get you very accurate answers...and often free links to sections of the Mercruiser service manual, etc.

In fact, go to this link and look at #7. Pretty sure it's a free download of the Mercruiser service manual for your V6:
http://forums.iboats.com/showthread.php?t=457054&p=3084839#post3084839

If you can't find your procedure in there, you'll find a lotta helpful folk over there in the Mercruiser forum...

 

[craze1cars]

Re: 30 mph top speed?

Wyoboat, I think this guy had similar problems to what you may have...and recently reported that he solved them WITHOUT buying a prop. I'm just hoping it will help you think a bit more "outside the prop" to find your problems:
http://forums.iboats.com/showthread.php?t=505819

 

[Maclin]

Re: 30 mph top speed?

Craze, the other boat had low rpm issues due to needing a tuneup, it had been doing ok before and the performance had deteriorated.

Wyoboat's engine is running great as far as rpm's, but will not be able to get more mph, which I think is what he wants, without prop or gearing change.

A prop change may show that Wyo's engine needs a tuneup but right now it can zing right on past the "redline". The limiting factor in mph is the physics of the ratio and prop combo he has.

 

[craze1cars]

Re: 30 mph top speed?

You may very well be correct...no objection here.

I guess I'm just sayin' leave no stone unturned, and I am especially questioning whether he has correct ignition timing more than anything else. I always believe it is critical to assess the health of the complete drivetrain FIRST. THEN re-prop if needed.

Of course in Wyoboat's case here, he needs a bigger prop to load the engine properly first...and without a big enough prop he can't really assess anything else. So I absolutely can see where your comments are coming from. Thanks for helping clarify.

 

[wyoboat]

Re: 30 mph top speed?

the boat does run really well, last weekend i had 6 BIG adults probably 1100 pounds and gear and still pulled the tube and skiers just fine no power problems, got on plane fast. I do appreciate craze1cars point of view I'm the same way with my vehicles, I grease this boat and trailer every time i go and check everything, there is only a couple things i havn't touched and i plan on doing that before the weekend. I did look into timing it and is right just need to see if the advance is working but i think it is. I'm going to tune up the carb and distributer. But my gut tells me this wont get me any more speed as it runs good now.

So all and all i would like to try a new prop what are your guys suggestions? thinking of trying a ss in a 19pich what do you guys think?

 

[Maclin]

Re: 30 mph top speed?

A 19 would be my first step. The SS will not flex and should reduce slippage. By the numbers you should still have a good hole shot and pickup 2-4 mph on the top end.

 

[craze1cars]

Re: 30 mph top speed?

Whatever you try, I'd try to make sure it's exchangeable. Keep in mind changing from alum to SS is worth an inch or so in pitch alone due to less slip. If the SS prop you pick has more cupping/rake it's kinda like another inch of pitch.

So a 19 in a SS might bring your rpms own more than you realize...hopefully not too far. But I tend to agree it's a good place to start to see where you're really at. Experimenting with non-returnable SS props via mail order can be a very expensive proposition.

 

[Maclin]

Re: 30 mph top speed?

Ok Wyo, now you know I don't get out much and I boat "vicariously" thru these posts, so be sure to come back with your story. and pics...!

 

[wyoboat]

Re: 30 mph top speed?

I will be sure and tell you guys what i find out, this is my first boat but i'm defiantly hooked really like this site i find my self reading a lot on hear. thanks again for all the help.

 

[jimmbo]

Re: 30 mph top speed?

3800 ft above sea level will drop you HP more than 10%. You have two, maybe three things hampering you speed, all due to elevation. Less air density equal less power and rich carburation compounds that. Now the gearing, often when a boat is sold for high altitude use a lower gear ratio is utilized. This allows the engine to attain its rated rpm range without having to utilize much lower than usual prop pitches. Some people will say you could use the std gearing with lower pitches and it works out the same. Well there is minor detail often forgotten... In a given prop line, as the pitch gets greater, the diameter gets smaller, overall, propeller efficiency rising as the pitch increases.

 

[wyoboat]

Re: 30 mph top speed?

food for thought