Prop question....120 v4

[Mickyfinn]

Hi,

Once again looking for sage advice. I spun the hub on my prop and I decided to replace it with new. The problem is, I can't figure the size from the numbers stamped into it, "176884". I've just spent the last few hours digging through the faq's and forum for prop advice and think I have a decent handle on where to go but still have some questions. The Michigan Wheel prop-finder for my 1990 120hp v4 spits out three different props. 13 1/4x17, 13 3/8x17 and 13 3/4x15. The best I can figure my current one is to be 13 3/8 and I'm guessing 17 because of the number stamped on it. If I measure out the 13 3/4 from the shaft if leaves me with @1/4" clearance to the ventilation plate. Is this too close? My boat is a rebuilt 1978 Wellcraft 20' CC that's pretty heavy. The current prop moves it along pretty well, but the WOT is only @ 5200-5300. Will the extra size negate any benefit from the smaller pitch?

I'm open to any advice on brands/materials/size etc. The boat also seems to be guzzling fuel (not that I expect it to be miserly...) I'd gladly sacrifice some speed for better fuel comsumption, if a different prop can help that. One thing I did realize through my digging today, was that my motor is sitting too low. The ventilation plate is @ 1 1/4" below the keel. I'm gonna raise that tomorrow.

Thanks for any help!

Evan

 

[JB]

Re: Prop question....120 v4

Did you try digging in the Prop Forum?

Let's try this there.

 

[Mickyfinn]

Re: Prop question....120 v4

Oops...didn't know there was a prop forum. I have this one bookmarked directly. I'll check that out and follow up there if needed.

Sorry!

 

[Texasmark]

Re: Prop question....120 v4

The prop/hub should be symmetric with no wobble in it. So, there is nothing wrong with 1/4 inch spacing. I happen to be running that right now with 1/8 spacing in front of my trim tab which I had to notch to accept the current prop.

You have a problem in that you can't raise your engine before you change props since you spun your hub. Also I don't know if you are in alum wishing to stay or in alum and looking at SS. The latter will/should have a big impact on which is the right replacement. The only way you ever really know which is the right answer is to run it. There are just too many variables. You can only predict....sometimes they are true, and other times the converse is true.

You'd expect raising (that much) to have a profound affect on performance. With that said, the 17P might work just fine. However, the engine would run easier with the 15 and that translates to lower speed and higher MPG......maybe, as long as you didn't loose a lot of mph in the process. You may have to watch your WOT rpm's with that prop.

Having had a 125 John (powerhead rated) on an 18' tri-hull (heavy boat) a 17P SS gave me a good combination of speed (39) and power to pull waterskiers. Slight diameter changes has little to do with performance as compared to inches of pitch change. Larger diameters (and shallower pitches) help with heavy loads.

Good Luck,

Mark

 

[Mickyfinn]

Re: Prop question....120 v4

Hi Mark,

Yeah, I kinda figured this wouldn't be easy. I wish I had noticed the engine height before I lost the prop. I'm tempted to put the same prop back on, but I don't really know for sure that it's a 17. I mostly use the boat inshore and running along the coast fishing. so speed is not the most important factor. Fuel economy is though, this motor sucks fuel like a pig. I'd like to think that I'll gain some by raising the motor, but it's hard to say what will happen with a prop change for sure. If I'm not mistaken, the 15 should give me better slow speed handling, which with the heavy deep V, I haven't thought has been too good so far. The WOT doesn't concern me too much given what I've read on the evin/john forum about top speed. It sounds like my motor "likes" to be run towards the high end of things, and I have 600-700 rpms to work with (assuming raising the motor doesnt' affect that too much....gets complicated quickly, doesn't it??).

As for alum vs SS, I'm thinking alum, for cost reasons if nothing else. I also like the more forgiving nature of the alum given where I'm running. I've been looking at a Mich. Wheel vortex as a choice. Any thoughts on that?


Thanks for the input!!

Evan

 

[sabastianunf]

Re: Prop question....120 v4

Lots of people say that 4 blade can slightly improve gas milage. The vortex is a good prop but the blades are heavily cupped for a better hole shot. The smallest they make is a 16" pitch in the 4 blade. In my opinion I think that prop would decrease your rpm's by @ 200 that's what it did to me. I was running a 14.5x17p 3 blade at 42-4300 rpm with the 4 blade vortex it knocked my RPM's down to 4000. I have a mercruiser stern drive that's why mine runs so fewer rpm's that yours. However, everyone will tell you that props have different results with different boat/motor combinations. If you want a 15x16 vortex you can buy one for 1/2 the price of a new one on ebay right now - see my thread 4 blade 4 sale.

 

[Texasmark]

Re: Prop question....120 v4

I know Hustler for one, has a replaceable hub. I may have bought mine from I boats. That means that you could buy the alum prop (shell) for around $65 (hub goes for $35).....total $100 if you bought the whole prop initially, and if you got the wrong pitch you only to have to buy the different prop shell.

They are alum and have a lot of things SS have, including ports (holes in the side) which help the hole shot.

Might give that a try and keep the first one you buy for a spare....if you don't get it right the first time.

The 125 I had was a crossflow design. Loved it's gas. Also, heavy boats cause the engine to work harder and that translates to more gas.

Mark

 

[Mickyfinn]

Re: Prop question....120 v4

I ended up finding a local shop that will let me exchange a new prop for another if it isn't right. Of course they are a bit more money but I certainly don't mind giving them the business for that convienence. They unfortunately couldn't identify what I had to start with, so I opted to buy the 13 3/4 x 15 AL to give it a try. With the motor raised and a new (in theory) prop, it should be quite a change with the boat. I'll run her out and decide from there where to go. Unfortunately, it'll be a good week or so before I get the chance. I'll report back with the findings.

Thanks for all the help!
Evan

 

[Texasmark]

Re: Prop question....120 v4

That's the right answer. Yes it costs more but you are getting more of their time and you can get it right the first time.

Just be sure and run the prop a minimum of time and don't ding it up. Might just take both with you and you can come home a happy camper and their undesirable prop will be untarnished.

Good Luck and let us know.

Mark

 

[Mickyfinn]

Re: Prop question....120 v4

Conclusion:

Raised the motor one hole, it's still a hair below the hull but raising it another might be an issue (my inexperience with the transom re-build and re-drilling the motor holes makes raising it anymore a sizable hassle...). I installed the Johnson 15 1/2"x 15 prop and took it out for a spin. RPM's increased to about 5600, top speed dropped @ 4 kts and it appears as if my fuel economy has improved quite a bit. I've run it a few times and I'm very happy with it. I think the weight of my 20' 1978 Wellcraft CC needed the power of the lower pitch. I was tempted to try out the 17 pitch but it's a drive up to the marina and this one is working well.

Thanks again for all the input!

Evan