new at this

[Skinny]

I have a 4.3 Merc and I was wondering what prop I should use. I think it has the original one on it. The boat is mainly used for tubing and touring.

Thanks

 

[Silvertip]

Re: new at this

We are very happy that you have a 4.3 Merc -- however when it comes to props you need to provide us with a little more to go on. Like:
1) What boat make model, length and weight if possible
2) What prop (manufacturer, stainless or aluminum, diameter and pitch) does it have now
3) what is the current observed wide open throttle operation with an average load
4) what is the current observed wide open throttle speed with an averaage load.
5) do you tow water toys, ski or wakeboard?
6) Does the boat currently have trouble getting on plane?
7) Any performance issues you want to solve. Sorry -- Doubling your fuel economy is not possible.

You see -- saying you have a 4.3 Merc is like me saying I have boat -- what prop do I need. The prop is the transmission for your boat and it has only one gear (as in high gear for your car). It needs to get the boat out of the hole, cruise comfortably and keep the engine in its recommended wide open throttle rpm band. Give us some numbers to work with and we can help.

 

[Skinny]

Re: new at this

Sorry, like I say I am new at this.

Starcraft 1810 SS 18'
Prop manufacturer

Planes well, just not sure if it will be good for tubing.
49-mph with 3 adults.
just looking for a prop that will tow larger adults.

thanks

 

[Silvertip]

Re: new at this

Still need to know the current wide open throttle rpm and the diameter and pitch of the current prop. Can't do much without that. For all you know you already have the best prop. Make some wide open throttle runs with an average load and note the engine rpm and speed. Speed really isn't that important. The prop must be selected so at wide open throttle the engine is running in its recommended rpm band. If it over revs, you add pitch. If it cant reach its recommended rpm band you decrease pitch. Look at the prop and you will find some numbers like 15 x 23 (15 inches in diameter x 23 inches pitch), or there may be a string of numbers that represent a manufacturers part number. Those numbers will be on the barrel of the prop or behind the prop nut on the hub of the prop.

 

[Skinny]

Re: new at this

thanks for all the info. I guess I will have to wait for spring. The boat is instorage and I did not get that info off the prop. As for the RMP at wide open is 5200.

Thanks again

 

[Silvertip]

Re: new at this

At 5200 you are a bit under propped meaning the engine is reving too high. I believe the upper end of the range for that motor is 4800. With a heavier load and towing however, that would be a good prop. you just need to watch the tach and limit top end to about 4800 - 5000. You may even look at two props -- one for cruising (more pitch) and the one you have now for towing water toys.

 

[Skinny]

Re: new at this

Thanks for all the help. Again I am not sure what pitch or size is on the boat. I may have to go to the storage and get the numbers off the prop. I have never changed a prop before are they hard to do? We only ran the boat for a short time at that rmp. That will not hurt the motor? What is a good manufacter of props?

thanks

 

[Silvertip]

Re: new at this

When a manufacturer recommends an rpm band for an engine (typically 4600 - 4800 for I/Os or 5000 - 6000 for outboards) they don't want you to exceed that upper end. A short burst a couple hundred above the top end won't typically hurt anything and two strokes can handle that better than a four stroke. If you intend to do any work on your boat yourself you are advised to buy a service manual specific to your engine/outdrive. Attempting repairs without knowing how to properly perform them can result in a great deal of damage. That cost makes the cost of the manual pale in comparison. Changing the prop involves removing a cotter pin, removing the nut and pulling the prop. There are some variations depending on outdrives. Do not buy any prop until you have run the boat long enough to know what you need and have some good baseline load and rpm numbers. You can buy props from virtually every manufacturer right here on iBoats. Click on the Marine Store button at the top left corner of this panel.