How fast should a 18 classic go with 400 horse?

[donzi18 classic]

What speed should i be seeing in my 18 classic with a 383 400 horse any help would be great.im running a bravo1 24 pitch four blade at like 5200 and 70.2 on gps.You can feel the boat serge at top speed like the prop wants to blow out.Ive tryed a 21 four blade and runs at 6000 rpm at the same speed and also a 22 bravo1 at 5600-5800 at the same speed im stuck at that speed and i dont have a nose cone but i know i need one.I have also tryed three blades and they blow out on top end

[donzi18 classic]

and it has a alpha gen 2 on it

[RedDog]

I'd say 70 is about the right number. But I would try a 3-blade in 25 inch - either a Turbo I or Mirage Plus

[The Hedgehog]

I'd say 70 is about the right number. But I would try a 3-blade in 25 inch - either a Turbo I or Mirage Plus

+1. I agree on the speed and prop recommendation. That is probably a good "any day" number for that prop.

You should be able to pick up a few mph by trying one of those 3 blades mentioned above.

I have a 383 with 350 hp and a Bravo drive (requires more hp to turn). I picked up a few mph and better handling by using a Turbo vs the Bravo I.

I get 68-69 on any day and just north of 70 on the good day (low humidity, cool weather and slight chop).

I dunno about the nose cone on that combo. Typically they don't do anything and may be hurtful below 80 or so. I have heard of slower combos benefitting but it is hit and miss.

[MOP]

Try a few props from my experience with my 383 (in a 22) the engine likes to be loaded, mine runs faster at 5000 then it does at 5600. I went through a few props before I went to a Turbo Ultima, I have a new Turbo 2+2 also but the Ultima is so much better in many ways.

[donzi18 classic]

Thanks for your input i tryed some 3 blades and without a nose cone i can not run them tey blow out at top speedi will have to get a nose cone on and then try them again i think we tryed 5 three blades and 3 four blades and the four blade 24 pitch ran the best

[RedDog]

Thanks for your input i tryed some 3 blades and without a nose cone i can not run them tey blow out at top speedi will have to get a nose cone on and then try them again i think we tryed 5 three blades and 3 four blades and the four blade 24 pitch ran the best

Maybe you tried the wrong 3 blade props. No reason the right prop should blow out. In fact blow out should be more of a problem at lower speeds than at high speeds.

[BlownCrewCab]

What year is your Boat? I assume you have the rounded Keel? On mine (with the V Shaped keel) The 24" 4 blade made it chine walk whereas the 23" 3 Blade Mirage ran solid and was 4-5mph faster. I had a 408ci with a bravo drive, Rev Limiter set at 5500. with the 3 blade I'd get 77mph at 5500, the 4 blade went 72-73 at about 5200, it never made the rev limit.

[miike]

keep in mind your drive will take less stress with your engine WOT at 6000 rpm

[MOP]

Drives do not like high RPM, high 5's and better will cut its life span.

[Just Say N20]

In fact blow out should be more of a problem at lower speeds than at high speeds.

Ventilation is more of a problem at lower speeds, at least when going straight, when the prop spins, partially biting in the water, until "clean" water (not a lot of air around the blades) allows it to get a grip.

Blow out happens when the lower unit is "punching" a hole in the water, and that hole gradually gets bigger and bigger as the speed increases, until the hole is so large that there isn't enough water getting to the prop blades for them to maintain bite in the water. At this point the prop loses bite, and begins to spin freely, and thrust is lost, as is any bow lift, so the nose falls and you can lose control of the boat.

[donzi18 classic]

My boat is a 97 and yes it does have the rounded keel who makes a nose cone for the alpha drives?

[RedDog]

I still don't think the addition of a nose cone is the answer. Too many others on this board are running Alphas, no nose cone, and no blow-out.

You may have locked on to a solution that doesn't address the problem

[Lenny]

...THIS IS OF NO HELP, ... but ... add an Aplha SS onto a Gen 1 gimbal and with your 400 ponies and a 25 MERC Racing cleaver, expect about 77-78. :D

How do I know this ??? :outtahere:

[A2VeeDub]

I ran a 23" 3 blade mirage on my 18 with an alpha. Never blew out at all. Topped out at 70mph. Also ran a 23" cleaver and it would blow out mildly at very high trim at mid speed (50ish). Never blew out at top speeds. Boat handles much better with the cleaver. Turns harder and feels like it has brakes when you let off the throttle.

[Rootsy]

If you are having trouble with props ventilating or blowing out on this boat then you have another issue you need to solve.

Bottom paint?

Algae growth?

Damaged gearcase?

Algae on gearcase?

Has the engine been raised, in turn relocating the gimble and raising the propshaft height relative to the keel?

Do you have transducers or anything on the bottom of the boat?

when is it blowing out the prop? attempting to get on plane? In hard turns?

When getting on plane do you have the drive all of the way "in" or is it trimmed "out"? Are you sure the trim limit is set to allow the drive to go all of the way "in"?

Does the drive have anything aftermarket, etc attached to it?

Just a few items to look at...

[donzi18 classic]

The bottom is clean and the drive has no damage.the 3 blades blew out at about 50ish straight line.I tried a 23 cleaver that was the worst one 25 mirage but still blew that out i am running in fresh water.The motor is in its original location it is a gen 2 drive if that makes any diff i was just thinking cone and then i would try the 3 blades again.I appreciate all the help on here. I wish i could afford a ss alpha but not rite now i dumped all my money into the motor to get back on the water.maybe that is all she will do with this setup but i need to find out why i cant get a 3 blade to bite.

[mrfixxall]

The bottom is clean and the drive has no damage.the 3 blades blew out at about 50ish straight line.I tried a 23 cleaver that was the worst one 25 mirage but still blew that out i am running in fresh water.The motor is in its original location it is a gen 2 drive if that makes any diff i was just thinking cone and then i would try the 3 blades again.I appreciate all the help on here. I wish i could afford a ss alpha but not rite now i dumped all my money into the motor to get back on the water.maybe that is all she will do with this setup but i need to find out why i cant get a 3 blade to bite.

What is your drive ratio? if its higher then a 1.65 that might be ypur problem..How much timing is in the engine?

[Rootsy]

If you are on plane and running at speed and then she blows out I would venture to say you have an excessive trim situation.

A nosecone isn't going to do anything for you below 85ish cept slow you down due to added surface area on the gearcase... The bubble is nowhere near the prop at the speeds you are speaking of.

Checked the coupler yet?

[donzi18 classic]

My drive ratio is 1.50 i think and my timing is 35 degrees.I read a post i think on a different site that the alpha above 65 you need a nose cone cuz after 65 it causes blowout.Have anyone of you read this same thing?

[Carl C]

My drive ratio is 1.50 i think and my timing is 35 degrees.I read a post i think on a different site that the alpha above 65 you need a nose cone cuz after 65 it causes blowout.Have anyone of you read this same thing?

No, I've always read and heard just what Rootsy said: A nose cone will most likely slow you down below 80ish. Something else is going on.

[miike]

Drives do not like high RPM, high 5's and better will cut its life span.


I see what you are saying because 6000 rpm is an extreme, but my point is that the drive is less stressed moving the boat at the same speed with higher crankshaft speeds. So under normal circumstances for example, its better on the drive to operate the boat at 4800 rpm than 4200. If you can only turn 4200, you are overpropped and putting too much stress on the drive and engine. The OP used 6000 in his example so thats what I used as well. You might not be able to turn it 6000 rpm forever, but by the same token, its not going swing a lot pitch forever at 5000. Thats a function of how much power you are feeding it and if its rated for that much power at THAT rpm.

[CaribouLou]

I see what you are saying because 6000 rpm is an extreme, but my point is that the drive is less stressed moving the boat at the same speed with higher crankshaft speeds. So under normal circumstances for example, its better on the drive to operate the boat at 4800 rpm than 4200. If you can only turn 4200, you are overpropped and putting too much stress on the drive and engine. The OP used 6000 in his example so thats what I used as well. You might not be able to turn it 6000 rpm forever, but by the same token, its not going swing a lot pitch forever at 5000. Thats a function of how much power you are feeding it and if its rated for that much power at THAT rpm.


Well put, now THAT makes sense.

[mrfixxall]

My drive ratio is 1.50 i think and my timing is 35 degrees.I read a post i think on a different site that the alpha above 65 you need a nose cone cuz after 65 it causes blowout.Have anyone of you read this same thing?


tmdog ran over 72 with his alpha 1 and didnt have any blow out...like rootsy stated you coupler may be slipping at that speed..do you ever rubber burnnig in the engine bay? try settin your timing at 32 deg.your motor might like it..

[donzi18 classic]

How do you check the coupler to see if that is slipping? And why would it be slipping if thats the case?

[Rootsy]

How do you check the coupler to see if that is slipping? And why would it be slipping if thats the case?

In your case the coupler is just rubber bonded inside of a steel shell with a steel splined hub bonded inside of the rubber... They do fail... Generally you'll notice it at speed or upon attempting to plane... Kinda feels like a clutch slipping in a car... Should be able to smell burnt rubber in the bilge. If you look at it through the gimble bearing with a flash light you may be able to see boiled rubber at the hub / rubber or rubber to outer housing interfaces...

[donzi18 classic]

Ok just ran 3 differant props.The boat still surges but no blow out it might be carburation. checked plugs and they are on the lite side for color. running a holley 750 modified carb i have 74 primary and 77 secondary jets.My tach bounces around i need a new tach but the rpms are close though

Marage plus 25 pitch ran 70 on gps 4800-4900
mach 4 blade 23 pitch ran 68 on gps 5400-5600
rev 4 4 blade 23 pitch ran 71 on gps 5000-5200

Checked coupler and its not slipping from what i can tell and does not smell like rubber in bildge.

Is there hook in the bottom of these hulls?Does that need to get taken out?

[BUIZILLA]

it might be carburation. checked plugs and they are on the lite side for color. running a holley 750 modified carb i have 74 primary and 77 secondary jets. if you have a 6.5 PV in the front and 5.5 PV in the back the jetting is pretty close.. check your timing for 12* at idle in gear :angel:

[MOP]

This may sound a bit dumb, but many years back we chased our tail on a similar condition, simple as it sounds it turned out to be someone had put an extra washer under the fill plug. The head protruded out a little over 1/16th but would blow the prop at speed! Not to say you have the same thing but it does not take much, a nose cone I doubt would do anything outside possibly cause another issue. Do a lot of research befroe slapping one of those on!

[BlownCrewCab]

Seems that the Rev 4- 4 Blade is your best prop in speed/slip/rpm #s Hows it handle????

[donzi18 classic]

The rev 4 handles really well all the 4 blades work good.It has some chine walk but its driveable and with the three blade it chine walked but it felt looser in the back of the boat.im going back out today and im puting that rev 4 back on and taking the flame arrester off to see what i gain.I have a stock merc arrester on and i know that it restricts it i pulled it off before and noticed a big differance.

[Rootsy]

Never noticed a difference between a 2 inch tall x 6ish inch Zenith on and off.. and that was in excess of 400 ponies...

Check your battery cable connections also...