How fast should I be going??

[demmer]

I just recently purchased my 1st boat. It is a 1989 Wellcraft, 17'(1100 lbs)It has a merc 140 L6 pushing it. My speedo says I'm going 28 mph, 5700 rpms. To me that seems slow for the rpms. My friend has a 16' ski boat with a 90hp merc and reaches 47mph. He mentioned a new prop. Will a different prop help with speed?

 

[groovy.dude]

Re: How fast should I be going??

Definitly. My buddy has a 175 and bought 2 different props when he purchased his boat. Although I am not sure of the exact top speed differences, the amount of time it took to pull me out of the water on 1 ski was significant(I am heavy) With less top end there was a lot of pull on the bottom end. I would suggest trying a different prop. The manufactures will be able to tell me what they are rated for regarding speed etc...<br /><br />Another indicator would be how fast you hit top speed.<br /><br />28 MPH seems a little slow, even for a pulling prop(Could be your speedo), but this would be easy to check out if you pull along side your friends boat.

 

[Parkstribe]

Re: How fast should I be going??

Simply stated, the maximum speed you should go is whatever the motor and prop combo will carry you at, while still staying within the WOT (Wide Open Throttle) spec that your motor is rated for. With that said, here is the proper way to determine what pitch prop you should have:<br />1) Look in the owner's manual of your specific motor and determine what the WOT RPM rating is for your motor (common ranges are 4500-5000 RPM or 5000-5500 RPM). If you don't have a manual, call a dealer and ask them if they can tell you the WOT rating for your Make & Model of motor. The WOT rating of your motor means that the motor is designed to run at this RPM when you have a normal-sized load in your boat with normal operating trim (you define "normal"). <br />2) With a normal sized load on your boat, and with the motor in the normal trim position, run your boat at Wide Open Throttle with the current prop and see what RPM's your boat is running at. <br />3) If your boat is operating within the stated RPM range, you have the correct prop on your boat. If your boat's RPM's are higher than the stated RPM range, then the pitch of your prop is too low. If your boat's RPM's are lower than the stated RPM range, then the pitch on your prop is too high (this is the most damaging scenario for your motor). <br />4) For most boat/motor combinations, 1" of prop pitch will equate to approx 500 RPM's. In other words, if you are running 1000 RPM's too low with your current prop, try a prop that is 2" lower in pitch. Visa Versa if you are running 1000 RPM's too high. <br /><br />FYI, it is acceptable to run a prop that is a little too low in pitch if you are trying to get extra "hole shot" power for pulling water skiers out of the water. However, because running a "too low" pitch prop means that the motor will run above the spec'd WOT RPM max value range, you should not make it a point to run the motor wide open and put it above the stated max RPM's. Conversely, if you are looking to get more "top end speed", you can run a prop that is a little too high in pitch; however, that puts excessive strain on the motor over the long haul and can cause serious damage to your motor. Bottom line is that if you want your motor to live a long and prosperous life, you should run it within the mfr's WOT specs. <br /><br />Hope this helps ... Fred.

 

[demmer]

Re: How fast should I be going??

Thanks a bunch guys for the info. I'll check the manual again and see what the recommended rpm range is. I'm not looking for pull power, looking for SPEED to make tubbing more intense. Thanks again. I think my friend said he is running a 19 pitch on his motor, 90hp merc.

 

[KCLOST]

Re: How fast should I be going??

Parkstribe is correct...<br /><br />However, 1" of pitch change in the prop normally results in a change of 200rpms not 500... Higher pitch = lower rpms and visa versa as he mentioned...

 

[Chris1956]

Re: How fast should I be going??

What prop are you running? I would think that a 19" pitch prop may be a good starting point for you. It is likely that your friends prop will fit your motor. Do you think he will lend it to you?

 

[wilde1j]

Re: How fast should I be going??

You might try checking the speed with a handheld GPS. Boat speedometers are notoriously inaccurate.

 

[demmer]

Re: How fast should I be going??

I checked the manual and it says the recommended rpm range is 5300-5800. I also noticed the engine is sucking the gas like crazy. Would a different prop pitch help with gas? I was told this motor will use 2-4 gallons an hour. Does this seem accurate?

 

[Dhadley]

Re: How fast should I be going??

When set up correctly a 140 hp gas motor will burn about 14 gallons an hour at wot.

 

[Parkstribe]

Re: How fast should I be going??

If you have a "normal" load with "normal" trim on your boat at 5700 RPM's, then the 17 pitch prop is the correct prop for your boat (since your manual says 5300-5800). As stated before, if you are looking for a little more top end speed, you could go up to the 19 pitch prop but you will be operating the motor outside of the spec'd range and it could cause you wearout problems over time (it won't blow up, it'll just tax the motor more than intended ... and use more gas in the process).

 

[mjbrueck]

Re: How fast should I be going??

From what you've said, if your numbers are accurate (28mph @ 5700 rpms), your motor is not making power. With 140hp on the back of an 1100 pound boat, I'd expect your speed to be in the upper 40's. If the motor's turning 5700 rpms, it should be close to making it's maximum power. Re-propping isn't going to significantly change your speed, you have more than just a prop problem.<br /><br />Since you said it's sucking gas, have you checked to see if the carbs are leaking. You could have one or more stuck needle valves or bad floats.

 

[demmer]

Re: How fast should I be going??

Checked carbs yesterday. The top one is dripping fuel. Not sure from where. I'm sure this has an effect on the speed and performance of the motor. I called my local marine service and he quoted me 70$ to rebuild it(per carb) and another 70$ to do a tune up on it. I've been reading and studying the manual since I got the boat and motor. I'm going to do this myself(rebuild the carb), any good advice for a rookie boat/motor owner? What's the average time to rebuild a carb?

 

[mjbrueck]

Re: How fast should I be going??

The carbs should only take you less than an hour each (even for your first time). The important thing though, is to not rush the soaking. Let them sit overnight in carb cleaner. Since it's your first time, you may want to clean half the carbs at a time. Just in case you wind up with extra parts, you can have another one to take apart to figure out where they go.

 

[demmer]

Re: How fast should I be going??

Can I get away with rebuilding just the one that is leaking or do I have to rebuild all 3??

 

[CFronzek]

Re: How fast should I be going??

One thing that hasn't been mentioned is the condition of this motor. It's been around for about 15 years. If it's newly purchased the treatment by previous owners is unknown. One thing is sure. It's probably not making peak horsepower any more. It may be just out of tune or it may past it's prime because of hard use.<br />Time to put a compression tester on it. It's cheap to do and may help avoid chasing your tail looking for HP that won't materialize because of age/wear/abuse.<br />Let us know what you find.<br />Charlie

 

[jamietaschetti]

Re: How fast should I be going??

i have a 88 merc 150 which i just purchased and the speedo says 55 at norm trim and 65 trimmed out. Im thinking theres o was that boat does 65 its a 17'6" ebbtide

 

[emckelvy]

Re: How fast should I be going??

It's just about physically impossible to be going 5700 rpm and only 28 mph. And the difference between under-30-mph and over-45-mph is staggering and should be quite noticable.<br /><br />Take a look at the side of the prop (the body) and there should be a series of numbers if it's a Merc prop. This series of numbers will end with a -##<br /><br />The ## being the pitch of the prop. I'd expect to see something like "-19" or "-17". Of course, maybe you do have a 13" pitch prop and are actually going that slow and 5700 rpm! But I doubt it. Probably the speedo is fouled and not registering correctly.<br /><br />Otherwise the tach is terribly inaccurate, or a combo of both. <br /><br />Maybe you and your friend should get out in the water side-by-side and compare notes. That should give you a better idea of what's going on.<br /><br />I had a Seaswirl Spyder 17'-4" ski boat hull which would do around 45 with a 1350-I-6 Merc propped for skiing. The hull weight was around 1200 lb bare. You should be running somewhere in the mid-40's if everything is set up right.<br /><br />HTH...........ed

 

[cwilt]

Re: How fast should I be going??

just another compairson i have a 17ft boat with a 90hp 1200lbs with a 23 pitch prop and i get 50.2 mph not knots ...GPS

 

[jsr21]

Re: How fast should I be going??

You may have a prop that is slipping..like a clutch in a car. One way to tell is to use a punch and mark the inner hub and the outer part of the prop hub. The hub may be spinning inside the prop....hope this makes sense.