ELPTO 90hp 17.5ft alum boat--what Prop??

[mystro]

Please suggest which prop your using with this fairly common engine/boat combo..dealers seem clueless unless they have the old prop..I asked them what happens if someone comes in with a missing/stolen prop..duh, they could only suggest going back to the manufacturer etc etc.

 

[Chris1956]

Re: ELPTO 90hp 17.5ft alum boat--what Prop??

The correct prop will allow the boat to get to the recommended operating RPM, under normal load. For that boat I would predict 15, 17 or 19 I pitch would be correct. I would see if you can get a loaner prop of 17" pitch and go from there.

 

[mystro]

Re: ELPTO 90hp 17.5ft alum boat--what Prop??

The current prop has such heavy coatings on it but it looks like the last number is 17 so I'll work from there...thanks

I noticed this messages was moved to Boating and Outdoor Activities..how weird the forums are here..they have Tohatsu/Nissan engines aligned with Honda--with Honda and not Mercury who they actually make small engines for-- so now I ask a question on props for mercury and I get a BBQ or whatever forum..who makes these moves and decisions?

I really wonder if the forum manager realizes how important it is to properly categorize or sub categorize things-

Really..it's the "Outdoor Activities" that throws most people..odd placement name for what really is just a motor related question.

 

[Silvertip]

Re: ELPTO 90hp 17.5ft alum boat--what Prop??

75 and 90 HP Mercs use the same lower unit (2.33:1 gears). You definitely don't need a 15P prop. your engine will be very happy with a 13-1/4 x 19P. If the boat happens to be very light, you may be able to twist a 20 SS.

 

[steelespike]

Re: ELPTO 90hp 17.5ft alum boat--what Prop??

Silvertip is probably on the nose; but you should check the max rpm and not exceed the recommended range.

 

[Silvertip]

Re: ELPTO 90hp 17.5ft alum boat--what Prop??

75 HP is 5250 and the 90 is 5500. The 19P on my 75 runs exactly at 5250 and on the 90 would very likely run 5500 but as I said, it would depend on boat weight, hull design, and setup.

 

[CaptAhab]

Re: ELPTO 90hp 17.5ft alum boat--what Prop??

The make and model boat would be helpful, I have an 18 foot aluminum boat with a classic 90 hp spinning a 21 pitch aluminum prop, I am waiting for the delivery of a 13.5x22 pitch laser II which I think will do nicely on my boat, but it's light...

 

[mystro]

Re: ELPTO 90hp 17.5ft alum boat--what Prop??

Thanks all. It's a 17.5 Alumacraft Trophy duel console walk-thru--it has the extra large tank (37gal) and I think I see 17 as the last two numbers on the prop. With just me onboard and a full tank it seems to slightly over tach past what I heard for this engine was 5500--if anyone has what this engine should run please advise. Maybe a 19 would be more appropriate? Also, is the manufact prop size on the prop shaft in the 2001 engine?

 

[Irv964]

Re: ELPTO 90hp 17.5ft alum boat--what Prop??

Please suggest which prop your using with this fairly common engine/boat combo..dealers seem clueless unless they have the old prop..I asked them what happens if someone comes in with a missing/stolen prop..duh, they could only suggest going back to the manufacturer etc etc.

This link may be of help, I have a similar boat/motor combo and my gear case is also a 2:33 ratio.
I read lots of info and asked lots of questions(many pages back were read, searches done etc before I chose this prop) Good Luck...............Irv964
http://forums.iboats.com/showthread.php?t=269778

 

[mystro]

Re: ELPTO 90hp 17.5ft alum boat--what Prop??

Thanks for the thread link, though mostly dealing with the 4 stroke it had some good helpful info. I purchased for this 90 2 stroke knowing it was torquey especially at low end, I tested a 19p to see if I can settle down the max rpm from the orig 17p which I damaged but keeping so I'll have a backup regardless--- but all of this may be to no avail..I tested the 19p and my tach showed 6100 wot (600 over max)..replaced it with a 21 and the tach showed 5800..only three hundred diff. From what I understood you should see a good 500 rpm diff at each step 17-19-21p so the tach may actually not be correct. I'll try and have a friend put a shop dwell tach on and go out and test the 19 and 17 again.

I want to have which ever pitch hits the 5500 but no higher mark. I don't currently have the 17p but isnt the smalled pitch usually higher RPM..just making sure?

 

[CaptAhab]

Re: ELPTO 90hp 17.5ft alum boat--what Prop??

Yep lower pitch higher RPM.

 

[Silvertip]

Re: ELPTO 90hp 17.5ft alum boat--what Prop??

The 200 rpm drop per inch of pitch holds true only if the props are all from the same manufacturer and are of the same identical design. If you compare two 19P props from two different manufacturers they very likely will not perform the same. I would also verify that the tach is accurate.

 

[comfun1]

Re: ELPTO 90hp 17.5ft alum boat--what Prop??

How do you verify that a tach is accurate?

 

[Silvertip]

Re: ELPTO 90hp 17.5ft alum boat--what Prop??

With a shop tach. If the engine is idling at it's recommended idle speed you can assume the tach is reasonably accurate. If you don't know what the idle RPM is, you will need to check the service manual.

 

[CaptAhab]

Re: ELPTO 90hp 17.5ft alum boat--what Prop??

Was the 21 pitch a stainless? Probably not, a SS 21 would bring em rpm's down to be sure..
Just by visually comparing my old 21 alum. with the new SS 22 the difference is easy to see, the blade area especially was much larger on the SS prop as well as cupping and higher rake..
I don't know for sure if a 21SS would work well for you but I would bet a SS 20
would be pretty close to perfect as long as you don't mind a little less of a hole-shot for the benefit of higher top speed.It all depends on what you want.
Good luck anyways I hope you got it sorted out.