want more top speed

[rknrollo]

I just bought a 94 chris craft 217 concept bowrider about a month ago, I have never owned a boat before, it has a 5.8 cobra efi, it was mainly used for a ski boat, the holeshot is great and gets on plane real fast but 3500 rpm is about 25-28 mph and 4000 rpm is about 30-35 mph, 4800 is about 40 mph I did a short few second blast up around 5500-6000 and it hit almost 50 but I dont want to over rev the engine. My wife broke the prop so I replaced it with the exact prop michigan wheel 161002 15x17, I would like to get a bit more top speed at a lower rpm what prop size would help? I want to stick with an alum prop as we run the Colo river in AZ and I would rather break the prop than something bigger. Also is there any prop design that will help low speed steering, when going about 5 mph I'm constantly steering left and right to keep the boat straight. My outdrive is the omc/volvo sx model.

 

[rallyart]

Re: want more top speed

Slow speed is just going to wander. You could use a much higher pitch prop. A 20" pitch is about 20% change so your 4800 would give you about 48 MPH.
It's not exactly linear like that, but your hull and engine have hit that speed so it should be no problem again. Your hole shot will be noticeably slower.
I can't guess as to diameter as I'm not familiar with your outdrive.

 

[Tail_Gunner]

Re: want more top speed

Your tach is bonkers...Efi will not let a motor turn more that 5000. On the back of the tach there should be setting 4-6-8 make sure there set correctly and if they are....Use another tach to verify what you are posting

 

[Bifflefan]

Re: want more top speed

Is that speed on a GPS or the speedo? GPS is best.
You didnt say anything about skiing or pulling a tube so, id keep the one you have as a spare or ski prop and get a 21p (dont worry about the diameter) for everyday use. You should have plenty of power with that motor and could hit close to 60 MPH. If not a 23 will.
A 4 blade will help some with the low speed steering and so will smart tabs, they will also help get you on plane faster with the longer prop.

 

[steelespike]

Re: want more top speed

Low speed wander is a matter of getting used to low speed handling.
You need to be aware of the boats movement; correct the wander early and avoid over correction it takes careful attention and practice.Trim tabs are said to reduce wander.Either active tabs or passive tabs like the smart tab.

 

[Tail_Gunner]

Re: want more top speed

Low speed wander is a matter of getting used to low speed handling.
You need to be aware of the boats movement; correct the wander early and avoid over correction it takes careful attention and practice.Trim tabs are said to reduce wander.Either active tabs or passive tabs like the smart tab.

This thing can be called cycling...Notice one thing...its like the miltary...left right left right left right. Its a pattern and i believe it has to do with the a deep v hull digging in.......heres somewhat of explantion

http://www.wavetrain.net/boats-a-gear/74-crunching-numbers-why-longer-is-faster

 

[scipper77]

Re: want more top speed

power steering while not the cause is a major factor in wandering. With the PS you can't feel any feedback through the wheel. You must over-correct to stop the boat from wandering in one direction. It will feel like the steering is pointed straight ahead when you are actually still a little over corrected causing you to wander in the other direction.

If you keep this in mind you can stop yourself from leaving the out-drive over-corrected and greatly reduce the severity of the wander.

I hope this explanation makes sense.

 

[Robbabob]

Re: want more top speed

Check the tach as suggested. If it checks out

The previous owner had the prop set for what he did the most, skiing. By the numbers, you would need to go to a 15/21 or even a 15/22 to get your rpm back into the safe zone during WOT.

You will lose some serious hole shot, but you'll have a good all around set up.

 

[BLUEDOT]

Re: want more top speed

I have some experiance to share. I have a 20ft. bowrider with a 5.7 L. MPI Mer-cruiser, Bravo outdrive. The boat came with a 23' prop ss. I love the performance from like 25mph to 55 mph, and listing to the motor tells me it's not to close to red line, { my tach gets stuck at 3900 rpm }. Everybody told me the 23" is to tall, and you do better with a 19". I got one and tried it out . Yeah, it gets up on plane faster, and will be good when I start pulling the kids on tubes, but I can over rev the engine now.
Long story short, I'll be getting a 21" prop. and probadly be happy. I want to be able to jump on it , and not have to worry about over reving.

I hope this is helpful, Just chill....

 

[rknrollo]

Re: want more top speed

I went into Havasu today as it was blown out on the water, I found a 19 pitch prop I couldnt pass up for 80.00 brand new in the box (volvo penta), so I figured I'd give it a try, it will make a good spare anyways.

I did check the tach with another one I had in my garage and the tach on the boat is right on the money, for some reason this omc cobra 5.8 will rev up to 5500 and smack almost 6000 if I push it, I would think it would have a rev limiter on it, obviously I only did it for a couple seconds, from what I read max rpm is like 4400.

 

[Bondo]

Re: want more top speed

I would think it would have a rev limiter on it,

Ayuh,.... There Is,... The Prop,.... Your's is much too Small....

 

[fossill]

Re: want more top speed

My boat wandered real bad at slow speeds too, even with a 4 blade prop. I went to a 5 blade Mercruiser High5 prop (stumbled on a deal I couldn't pass it up) and found it made a huge difference with the wandering. Unfortunately it only comes in stainless steel if you're stuck on aluminum. I also replaced the flat anode and put in the fin anode on the outdrive. Not sure if that also helped or not.

 

[jason32038]

Re: want more top speed

try a 15x18 4 blade. Should lower the max rpm 400-500 and the 4 blade will give you smoother acceleration and less vibration.

 

[barbosam]

Re: want more top speed

You need to find out for sure what the maximum safe rpm is for you engine, then you can find the prop that will let you just reach that rpm when the boat is lightly loaded. It sounds like it may be a prop with 21-22" pitch or maybe a larger diameter, but you can't make a final decision on your prop without first knowing for sure what your maximum rpm should be. Is this engine modified in any way? if not then it should be easy to look up the max recommended rpm.

side note: for skiing and tubing you should be able to reach or come very close to max recommended rpm with a heavy load.

 

[rknrollo]

Re: want more top speed

put the 14 1/2 x 19 prop on today, holeshot feels about the same but now at 3500 rpm I'm cruzing about 32 mph so it definetly worked good.

Just for the heck of it I got it up to 52 mph for a few seconds but turning about 5800 so I am sure I can use a 20-21 no prob.

 

[rallyart]

Re: want more top speed

Your rated maximum engine speed is probably around 4800 RPM. When you over rev the engine you run the risk of stretching a rod bolt which could lead to spun bearings, lost oil pressure, or a broken connecting rod. It could also float the valves which could lead to the piston hitting the valves, broken valve springs, or broken valves. You could also break the crank. When something does fail you will be able to look through the new inspection hole in the side of the block and see what went wrong.
Now that you've been warned, you could get away with over revving an engine that has been well built by quite a bit but the older the engine is the less you can rev it, and the longer it has run at high revs the less life the engine has.
I spent two years seriously over revving a Merc 470 until we found out what the operating range was supposed to be and that engine is still running 30 years later.
Just try to keep the engine slower. Your life will be easier.

 

[rknrollo]

Re: want more top speed

Your rated maximum engine speed is probably around 4800 RPM. When you over rev the engine you run the risk of stretching a rod bolt which could lead to spun bearings, lost oil pressure, or a broken connecting rod. It could also float the valves which could lead to the piston hitting the valves, broken valve springs, or broken valves. You could also break the crank. When something does fail you will be able to look through the new inspection hole in the side of the block and see what went wrong.
Now that you've been warned, you could get away with over revving an engine that has been well built by quite a bit but the older the engine is the less you can rev it, and the longer it has run at high revs the less life the engine has.
I spent two years seriously over revving a Merc 470 until we found out what the operating range was supposed to be and that engine is still running 30 years later.
Just try to keep the engine slower. Your life will be easier.

I dont do it regularly, I have lots of experience with the 351w ford (in cars and trucks) and I have built many, my last 351w used stock rods/crank and I ran it 6500-7000 on a regular basis on the drag strip, 351w's are very durable, I beat the crap out of a 7000lb work van with a 351w floored and reved it to 5000 constantly and it was still running great past 100k
and like I said I dont do it regularly or for more than a couple seconds and frankly with the cam thats probably in this engine it wont make any power past 4500, thats my objective of getting the right prop as I want to keep the rpms down at higher speeds so the engine will last longer.

 

[rallyart]

Re: want more top speed

No problem rknrollo.
If you've got experience with the engine you should be fine. Some people don't which was the cautionary reason for my post. It sounds like you could go up 2" in pitch from what you describe. Occasionally you will find a prop that is not actually the pitch it has stamped on it, even from new. If you find too big a change or not enough that might be something to look at while you continue to work it out.

 

[Sparkinator]

Re: want more top speed

Up in pitch will definitely help your top end. I run a 21 pitch VP stainless on the same SX cobra drive. It is pushed by a 302 carbed and with that setup, it will run 4600 rpms and 50 mph. It has to be super calm water and trimmed up to reach 4600 rpm and that is the only time I can max out my rpms with that big of a prop.

You could probably run a 23 on that 351, but I don't know if you would like the sacrifice you would make on the holeshot though. A good prop shop should let you try a few to figure out which one is the most versatile for your application. Until you try a few, it's just a guessing game.

Good luck.

 

[Sparkinator]

Re: want more top speed

Here is a LINK to an engine and drive identical to yours, in a boat that specs out similar. It has recommended props and expected MPH and RPMs.

It's a 95 Four Winns Horizon 220. 21' 6" overall length.

Hope it helps.