1986 v4 120hp basic prop question

[bbach]

First a dumb question. Are props interchangeable between manufactures (are all shafts the same) or do I need to look for a OMC specific prop? Any thing specific in a prop (like the number of splines, shaft diameter, shaft length) I need to look for this motor?<br /><br />Now, more specifically, the boat is a 18' center console - Boston Whaler 18 Outrage. Seems slow to plane. Current prop is an aluminum prop stamped 14x17. WOT RPM is 5300. I think the 14" diameter is a little big for this motor. If I go to something like a 13-3/4 x 15, would that get me up around 5700 and better hole shot? <br /><br />What if I went with a stainless steel that size?<br /><br />One other thought is to keep the current prop and add a hydrofoil of some sort. I may do this anyway. Should I do that before I start messing with the prop?<br /><br />I'd also be happy for any advice on specific props to look at.<br /><br />Thanks. -- Bud

 

[reeldutch]

Re: 1986 v4 120hp basic prop question

adding hydrofoil or dualfins or wathever might get your boat to plain easyer but your WOT will suffer a little caused by drag.<br />so running 5300 at wot and adding fins will make it worse. you want to gain rpms!!! not loose.<br />make shure the motor is set at the propper hight.<br /><br />reeldutch

 

[JB]

Re: 1986 v4 120hp basic prop question

http://www.iboats.com/cgi-bin/ubb/ultimatebb.cgi?ubb=get_topic;f=36;t=000030 <br /><br />Good luck.

 

[bbach]

Re: 1986 v4 120hp basic prop question

Reeldutch, I agree with what you are saying. That seems to imply that if I want to add a hydrofoil, I should do that first to see what the impact is. Agree?<br /><br />JB, I should have mentioned that I had looked at that FAQ entry. It really doesn't answer my questions though. From what I've read though, it seems that props(hubs) are manufacture specific. Still not sure how many splines I need to look for this particular motor (the boat is in NY and I am in IL).<br /><br />I still haven't seen anything that indicates approximate rpm gain when going down in diameter or the impact on rpm in going from aluminum to stainless. Thanks. -- Bud

 

[bbach]

Re: 1986 v4 120hp basic prop question

JB, Just re-read the faq to see if I missed anything and saw this:<br /><br />'Smaller diameter props in a given pitch tend to be "slightly" faster for the same reason..LESS drag from less blade area. The smaller diameter props are usually better suited to "Lighter" boats.'<br /><br />I'm guessing that going down in diameter 1/4" will have a 25-50 rpm increase. Does that sound reasonable?<br /><br />How about going from aluminum to stainless? About the same or would there really be no impact to rpm only to speed?<br /><br />Thanks! Oh and BTW, the FAQ was really valuable too!<br /><br />-- Bud

 

[Dhadley]

Re: 1986 v4 120hp basic prop question

A properly installed fin (what you call a hydrofoil) will actually increase top speed, not lower it.<br /><br />A larger diameter prop will, in most cases, increase top speed. Mainly because a larger diameter prop allows for a higher X dimension which equals less drag.<br /><br />A ss prop is going to give you better all around performance compared to an aluminum prop of the same basic design because it has less flex -- for starters.

 

[walleyehed]

Re: 1986 v4 120hp basic prop question

I should have been a bit more clear on the diameter issue on props...smaller diameter using "X" as a mid-point will allow greater RPM (Engine speed, not boat speed), hence the application being best on light-weight boats. Yet when tuning for performance, larger diameter allows higher engine heights for less drag, (More boat speed and engine RPM)<br />Sometimes it's confusing as to what works best on what, and some props react differently than others, SS, Alum, etc...<br />SS is thinner, has less flex, but usually has a bit more aggresive cup adding up to higher efficiency. It's not a trade-off, it's more like a step up.

 

[bbach]

Re: 1986 v4 120hp basic prop question

Walleyehed, Thanks for the additional info.<br /><br />Just a quick update. I was wrong on the current prop size. It is a 14x19. I picked up a 13 7/8 x 17 SST prop on eBay that I plan to try out within the next few weeks. <br /><br />I also believe the engine may be mounted to low and should be raised up one hole. The ventalation/cavitation (?) plate is currently 1 1/2" below the keel. I think I want to be above that correct?<br /><br />Thanks. -- Bud

 

[moderator1]

Re: 1986 v4 120hp basic prop question

1-1/2" would be 2 holes to bring it up to even with bottom of the boat.<br />Holes are 3/4" center to center.<br />Walleyehed may come along and say different though...

 

[ob15]

Re: 1986 v4 120hp basic prop question

The motor is too low. I would raise it up 2 holes. You might even be able to go higher. It's one of those things you just have to try as each boat if different. You may not need another prop after you raise the motor. That's why the experts always suggest getting the motor set up correctly prior to changing props.

 

[Dhadley]

Re: 1986 v4 120hp basic prop question

Just a few things to keep in mind --<br /><br />The larger the diameter, the higher you can mount the motor (X dimension).<br />A ss prop will, generally, have more "bite" than an aluminum prop. Therefore allowing a greater X dimension.<br />A fin is a tool that allows a greater X dimension.<br />5300 is lugging that motor and its killing it.

 

[bbach]

Re: 1986 v4 120hp basic prop question

OK. Plan is to raise the engine and install the 13 7/8 x 17 stainless prop and see what impact is. I'll report back in a few weeks. Thanks again. <br /><br />-- Bud