90 hp rpm/prop ?

[wjphunter]

I've recently posted a question about prop size.<br />The book I have (seloc brand )show the wot rpm for my 87 90 hp mariner to be 5000-5250.But when I talked to merc.they said 5000 to 5500 rpm .Can anyone confirm this .here are my current specs.<br /><br />1998 fisher f-16 s/c bassboat<br />1987 90 hp mariner w/ fresh powerhead (150psi)<br />boat weight dry hull 975 lbs.<br />motor weight 265<br />current wot rpm 5250 w 13.25 x 19 pitch alum. prop<br />gps speed @ wot 41-42 mph<br /><br />I'am still trying gain speeds with this boat.Does anyone have a boat with close specs. If so what prop size pitch? <br />I'am also planning to switch to stainless steel.But before I spent 300- 500 dollars for a prop I need to verify rpm.The only reason I question what merc said is because the guy that told me this spoke very poor english.<br />I herd of guy getting arond 50 mph with similar setups is this possible. <br />The person that replied to my last post was very helpful and hopefully he will reconize this and respond for I've forgotten who this was<br /> Sorry this post is so long<br /> thanks wjphunter

 

[steam_mill]

Re: 90 hp rpm/prop ?

Wjphunter:<br /><br />I'm running a smilar setup:<br /><br />2004 16' Cutter(1200 lbs.) with a 2003 90 HP Merc.<br /><br />The boat came with a 17" prop last year. Part of the ski pachage. Unbelievable pickup and on plane in no time. My boat speedo registered 47 miles at 5500 RPM. I could easily over rev.<br /><br />I went to a 21" prop. Now at WOT I'm at 5500 (actually with half a tank, good water and only me , I can spin around 5600).<br /><br />I gained about 6 MPH according to speedo. I still ski with this same prop.<br /><br />I think that if you go to 21" though, you will turn about 5000. You really should go to 17" if you want to spin 5500.<br /><br />I choose to run aluminum. I think that if I hit something (Really lucky so far!!!), the aluminum will absorb the damage.<br /><br />Also, WOT RPM has a lot to do with prop design. I stuck with a merc prop like the original one. You may get a 17" with very cupped blade that will reduce WOT RPM buy may enhance performance.<br />Tx.<br /><br />Joe

 

[Clams Canino]

Re: 90 hp rpm/prop ?

Is that 90 a three or a six? For 1987 is outta be a 6, but I jut wanna make sure - mistakes are often made in dating an outboard.<br /><br />-W

 

[wjphunter]

Re: 90 hp rpm/prop ?

It is a 1987 3 cyl numbers confirmed by merc.<br />Serial # 0b167058.My big question is what is rpm range for this motor? Merc said 5-5500.My book as listed above says 4750- 5250. If this is correct I should be able to go up in pitch and achive more top end speed. Now the boat only does 41-42 mph.For that light of a boat that seems week. It will turn the alum. 13 1/4 x 19 5250 @ wot. So depending on who I listen to will determin up or down on pitch. And I don't want to loose speed.<br /> Thanks guys for the feedback<br /> wjphunter

 

[Clams Canino]

Re: 90 hp rpm/prop ?

I'd try same pitch in stainless. <br /><br />And anything over 40 with a 90 isn't "only". Boats don't work like cars, there's a lot of drag.<br /><br />I get 47 with a 115 after 1/2 the tricks in the book... if I ever break 50 it'll be by cheating. <br /><br />-W