rpm and mph

[big ken]

i have a 96 18 ft sea ray[175 series] with 3.0 mercruiser took it out today and the boat seemed to be flying across the water. only thing i noticed at 35 mph[about 3/4 throttle] rpm was 4500. i thought this was kind of high. what would be normal? or is this ok?. i dont know the prop pitch off hand but i do know the guy i bought it from said it was a power prop. he used to haul his family around and did some boogie boarding . his best friend was over 400 lbs. thanks for any input. ken

 

[krisnowicki]

Re: rpm and mph

you need to find out what prop you have an what gear ratio your outdrive.... Then plug those number in to a prop calc like
http://www.rbbi.com/folders/prop/propcalc.htm
and it will tell you how you are doing in the form of a slip %. The lower this number is the more efficent your engine rpms is being translated in to speed.

 

[Silvertip]

Re: rpm and mph

If the tach reads 4500 rpm at 3/4 throttle the rig is under propped and the seller was correct in that it was a "power prop". If you went full throttle the engine would over reve. The term power prop by the way is meaningless since it may be a power prop for your boat but may be a speed prop for another. You need to do as suggested but it is obvious you need a prop with more pitch (provided the tach is correct)

 

[MikDee]

Re: rpm and mph

Your "power prop" is probably designed for hauling extra weight, or towing heavy tubes, or boogie boarding. It is a prop with less pitch that will spin up too high an rpm at WOT with normal loads. Your goal is to find a prop that will let your engine reach 4500rpm at full throttle, at about 45mph.

On your prop, there are a series of numbers with usually either, 15, 17, 19, & 21,

I'm guessing you probably have a 17" pitch prop, and need 2 sizes bigger like a 21" pitch prop. If you can borrow one to try, it would be the best idea.

 

[hwsiii]

Re: rpm and mph

Ken, I think I found the correct information on your boat and motor I show it to be 18' with a 135 HP Mercruiser engine that turns between 4,400 and 4,800 RPM at WOT, with a 1.94 Gear Ratio. It weighs about 2,000 pounds empty, I added 25 gallons of gas 2 adults and 1 child and various other weights to come to a total of 2,800 pounds fully loaded. Please check this and let me know if it is right, because if it is wrong then the informtaion I am about to give you is wrong.
When I run this through my formulas I find that at 4,500 RPM with a 17" prop you should be running about 34 MPH with 9% Slip ratio if your tach is correct. My opinion is you are probably running a theoretical 17" prop at this time, beause the numbers are almost spot on to what you are getting right now.
When I also calculate which prop you should be running I come up with 19" prop in aluminum, it should run right at maximum RPM of 4,800 and about 40 MPH if you use the same manufacturer and model of prop you are using now, but since you don't have the speed and RPM at wide open throttle I can't be positive. MikDee could be right with the 21" pitch, but I believe that I am correct with with the 19" prop. If you can borrow a 19" or 21" to try you will find out very quickly which one is right.
I hope this helps you with your quest for the right propeller.

H

 

[big ken]

Re: rpm and mph

Ken, I think I found the correct information on your boat and motor I show it to be 18' with a 135 HP Mercruiser engine that turns between 4,400 and 4,800 RPM at WOT, with a 1.94 Gear Ratio. It weighs about 2,000 pounds empty, I added 25 gallons of gas 2 adults and 1 child and various other weights to come to a total of 2,800 pounds fully loaded. Please check this and let me know if it is right, because if it is wrong then the informtaion I am about to give you is wrong.
When I run this through my formulas I find that at 4,500 RPM with a 17" prop you should be running about 34 MPH with 9% Slip ratio if your tach is correct. My opinion is you are probably running a theoretical 17" prop at this time, beause the numbers are almost spot on to what you are getting right now.
When I also calculate which prop you should be running I come up with 19" prop in aluminum, it should run right at maximum RPM of 4,800 and about 40 MPH if you use the same manufacturer and model of prop you are using now, but since you don't have the speed and RPM at wide open throttle I can't be positive. MikDee could be right with the 21" pitch, but I believe that I am correct with with the 19" prop. If you can borrow a 19" or 21" to try you will find out very quickly which one is right.
I hope this helps you with your quest for the right propeller.

H

thanks for the info, i do have the slightly damaged original prop i got with boat. i will go out and check the #s and try to find the # on the stainless prop on boat to compare thanks ken

 

[big ken]

Re: rpm and mph

ok guys here is the # of ss prop on boat now 1 3/4x21 and the # of the damaged alum prop is 48-781-21 im thinking they are the same? should i go smaller? thanks ken

 

[hwsiii]

Re: rpm and mph

Ken, the diameter you stated for the SS prop is not complete. But from the information you did give us it is obvious both props are 21" Pitch. And at 4500 RPM with your motor with the gear ratio I stated as 1.94 you are showing a slip ratio on that prop of 26%, and that is way out of line with what you ought to be showing. Did you check the information that I gave you about the motor and gear ratio, like I asked.

 

[big ken]

Re: rpm and mph

yes all the info you stated is on the money? the alum prop i was given with boat was probably a power prop as well. he just put on a stainless because he chiped the alum one pretty bad. as far as #s on stainless prop that is all i could see, they were on the hub area. what size would you reccemend? [17 or 18?] not looking for speed as much as getting the rpm down and maybe not using so much fuel. wont be doing any towing , sking etc. just taking family of maybe [4] out for pleasure ride. will be in the patapsaco river can be a little rough though . thanks ken

 

[hwsiii]

Re: rpm and mph

Ken, what I would like you to do is put the aluminum prop on the boat and get me the WOT RPM, but don't exceed 5000 RPM and don't run it very long. I just need to see what speed can be attained, if it vibrates a whole lot don't even try that, we will figure it out.There is a problem with something here and it could be for a multitude of different reasons. If you put a smaller prop on the boat it will GAIN RPM and NOT lose them, we need to get the RPM DOWN. There are a few possibilities here:
1. Your tach is way off
2. Your stainless prop has spun the hub and is slipping
3. The gears in your lower unit were changed and the gear ratio is not right
4. Something I haven't thought about

H

 

[big ken]

Re: rpm and mph

i wont be able to use the alum prop. i know it will be totally out of balance. we will have it out next week to get some #s. i did get off the phone with my son just now. he told me we were almost full throttle[ just backed off a little] our speed was 38 and our rpm was 4500 this ss prop just about new and i dont think outdrive has been changed. i bought boat from orginal owner. boat is emaculate and garage keep. as far as tach boat in idle position reads 900 rpm[seems to be working ok] motor dosnt seem to be screaming or anything.

 

[hwsiii]

Re: rpm and mph

Ken, when you get the new numbers let me know. I have a couple of forms if you fill them out when you test the boat, it will give me a much better feel for where the problem is, if you will send your email address I will be glad to send them to you.

H

 

[big ken]

Re: rpm and mph

thanks. we should be taking boat down to botel next weekend weather permitting. will take out and test. i thank you for all of your help. i love this site! i will send you my email adress thanks ken

 

[hwsiii]

Re: rpm and mph

You are very welcome Ken.

H

 

[hwsiii]

Re: rpm and mph

Ken I sent you the forms.

H

 

[MikDee]

Re: rpm and mph

Ken, what I would like you to do is put the aluminum prop on the boat and get me the WOT RPM, but don't exceed 5000 RPM and don't run it very long. I just need to see what speed can be attained, if it vibrates a whole lot don't even try that, we will figure it out.There is a problem with something here and it could be for a multitude of different reasons. If you put a smaller prop on the boat it will GAIN RPM and NOT lose them, we need to get the RPM DOWN. There are a few possibilities here:
1. Your tach is way off
2. Your stainless prop has spun the hub and is slipping
3. The gears in your lower unit were changed and the gear ratio is not right
4. Something I haven't thought about

H

You've done all the groundwork hwsiii, In my opinion, if the hub is not spun on that stainless prop, (and they are pretty hardy!) then your engine coupler could be slipping from running what appears to be previously too much stress, & weight, with a prop that has too much pitch for those conditions.
On that boat, a 21" pitch SS prop is made for a light, to medium load.