Which prop to get?

[kmagnuss]

Have a 2007 Mercury 90 on a 17' Action Craft flats boat... no idea on weight of the boat, sorry. Any ideas on a pitch I should try first? Don't really care about top end speed.... more concerned with getting up on plane quick.

Thanks.

-Mag

***UPDATED BELOW AT POST #16 ***

 

[jestor68]

Re: Which prop to get?

You need to run the boat with the prop that's on it and tell us what it does in terms Of WOT rpm and speed, as well as what motor you have. Mercury has 3 different 90's in 2007; all with different rpm ranges.

Tell us the size/make of the current prop so we have a starting point. And we'll go from there.

 

[kmagnuss]

Re: Which prop to get?

That's the tricky part... current motor is a 95' Evinrude 115 and has a 13 7/8” x 17 prop 15 spline on it...and I've never had it in the water... motor is blown. Not gonna rebuild it. So I'm buying this Merc and it doesn't come with a prop. So... any ideas on a good starting point? I'm thinking something around a 15-16 to start... props aren't THAT expensive that I'll hate myself for buying a second one... plus it's good to have a spare I suppose.

 

[kmagnuss]

Re: Which prop to get?

Thinking I might try this one first: http://www.**********/parts/detail/props/R-1411-135-15.html

 

[jestor68]

Re: Which prop to get?

The Mercury prop selector recommends a 13-3/4 X 19" Trophy Plus(4 blade) on a 90-115 Optimax pushing a 17 ft flats boat.

The Mercury part number is 825932A46.

 

[kmagnuss]

Re: Which prop to get?

Hmmm... I was just on that site and didn't get that far into it I guess.

Thanks!

 

[kmagnuss]

Re: Which prop to get?

Weird... when I do it, it says I'll need a 22-23 pitch prop. But I don't have a optimax either... it's a classic I guess... carb'd 2 stroke.

 

[jestor68]

Re: Which prop to get?

Another likely candidate would be the Quicksilver Light speed (3 blade)13.5 X 20 pitch. It would out run the Trophy Plus; for a lot less money.

 

[kmagnuss]

Re: Which prop to get?

Are solas props decent? They're WAY cheaper than the Trophy Plus.

 

[jestor68]

Re: Which prop to get?

Weird... when I do it, it says I'll need a 22-23 pitch prop. But I don't have a optimax either... it's a classic I guess... carb'd 2 stroke.

Did you select "acceleration" on the type performance you wanted? I first got a 21" then click on the prop picture and read the notes. It'll say use a 19" on a 90 hp.

 

[jestor68]

Re: Which prop to get?

Are solas props decent? They're WAY cheaper than the Trophy Plus.

Yes. The Solas HR Titan series are excellent performance props.

 

[kmagnuss]

Re: Which prop to get?

I tried acceleration as well as top speed and they came back with pretty much the same results. Lol.
When I do it there's no option for a 90-115... only 90.

 

[jestor68]

Re: Which prop to get?

Ran it again using the following weights. Boat/motor=1400. people/gear=500. Gas=30gal

Total weight=2071 lbs . Came up 19" pitch right down the line, except for a 20" Laser II if I checked for top speed.

When using the selector, you need some weights to get a more accurate answer.

The 20" Light Speed prop mentioned earlier is the same design as a Laser prop; only not as expensive.

The classic 90's rpm range is 5000-5500 rpm.

www.actioncraft.com

 

[batman99]

Re: Which prop to get?

.

If one doesn't worry about WOT / top speed, do focus on 4-blade props. 4-blade has better hole shot grip and gets on plane faster. WOT might drop a few mph but if 1/2 to 2/3 throttle range is your normal crusing speed, the 4-blade is a good choice. When going with 4-blade, remember to go down 1-2 pitches as well. For example (as a general startement), a 17" 4-blade = 19" 3-blade (using same prop model). The 4-blade is for more hole-shot traction and the 3-blade is for faster crusing speed (with slightly better mpgs).

For shallow or unknown water depth, I'd recommend using only Aluminim props. If/when they hit something, they chip and split. SS is much, much stronger. When SS props hit something, they are too strong. And, it would rip out the bottom end (very expensive) instead of breaking a few prop fins. For deep water, go with SS props. For shallow or unknown water, go with aluminum blades.

Also... I'd recommend 2 x props for each boat. And, apply the 80%/20% rule as well. If 80% is tubing / low ground speed crusing, then have a 4-blade as your main prop. If little tubing/skiing and mostly fast ground trips, then go with a 3-blade. And, use the 4-blade as backup. Backup prop is stored "in the boat". Along with a prop wrench and a prop wedge board. If one destroys the main prop on the other side of the large lake or river, one can install their backup prop (to get home). Storing your backup prop and wrenches on the shop's wall isn't a good idea. Especially if you like doing long distance cruising in unknown depth waters. And if one does a long distance cruise that day, simply remove the 4-blade (used for kids tubing / sking) and install the 3-blade (which gets slightly better mpgs as well). Works for me....

If wanting to get on plane even faster, do investigate the SX Smart tabs "install it and leave it" product as well. re: Smart Tabs SX Trim Tabs I have SX smart tabs on my 17" boat and love them. Makes the short boat handle like a longer 20-22 ft boat.

Hope this helps..

 

[batman99]

Re: Which prop to get?

Fogot to mention...

When testing props against WOT or "for what feels right to you", remember to test using "average" loaded boat as well. Some folks test prop sizing using a single driver. Yes. After 10 test runs with 6 store visits, they finally found the right size prop. Then, they "load up" the wife, kids, dog and that previously perfectly matched prop now feels sluggish. Too sluggish. If one does "most often" boating with the family, find the correct prop pitch using single driver, then go lower pitch for your "heavier" loaded boat. For example, felt great on 20" pitch. But with "most often" loaded boat, the 19" prop feels much better. Or, felt great with 19" but loaded boat feels much better using 17" pitch.

Also... Not all prop brands of same pitch number feel the same either. Even though they might be 19" pitch size on the outside, brand A might feel like a 20" pitch. And, brand B might feel like a 18" pitch. I use Solars aluminum props on my boat. Was told their 19" "feels like" a 19.5" (when compared against other brands of 19" props). But their 17" 4-blade is more aligned with other 17" 4-blade brands. Just like human body clothes sizes, not all xx size is exactly the same (when compared to other makers).

I use Solars aluminium props because they are low cost (re: get 2 for the price of 1 x SS prop) and for general family boating tasks, Solars is good value for the product (especially for my needs). Might be much better props on the market. But, why over spend if one doesn't need to?

Hope these sizing influence helps as well...

 

[kmagnuss]

Re: Which prop to get?

Thanks for the help guys... gonna revive this thread now...

Ended up getting a Solas 4 blade 19"... worked great but RPM's were up above 6200 at WOT so I bought a 21" 4 blade Solas...
The 21" 4 blade kept WOT RPM's around 6000... still not low enough... although this prop 'feels' right at hole shot and cruising speeds.
The only other aluminum prop I could find with a higher pitch is a Mercury Black Max 23" 3 blade... so I gave it a try. Cruises better than the 4 blade, hole shot is worse, rpm's still up at 6,000.

Well I have a really hard time believing that I need to bump up to a 25" prop on a 90hp motor... so now I'm looking at other options like maybe my motor is mounted too high. I have it one hole up (I could only drop it one hole and it would be as low as it gets) and haven't visually looked at the cavitation plate while running... gonna do that this week. I will say though, that when I take off I have it trimmed all the way down... I can only blip the trim up a couple times and I have blow out already.

-Cavitation plate is 2" above the bottom of the hull, but 2" below the top of the pocket tunnel. (when motor is trimmed to be perfectly level with hull)

-Bad tach is always a possibility, but man when that motor gets up to 6,000 rpms it really sounds like it's screaming. Tach is set properly with dial at "5".

-Did a prop slip calculator and it says I have a slip of 37% !!!!!! 6,000 rpm, 23" pitch, 35 mph, gear ratio 2.33. This is the reason I think it may be a height issue at this point.

-Also added a set of those Nauticus Smart Tabs... HUGE difference. Love em. Took em off to mess with the prop issue and nothing changed except it took forever to get on plane and rode like crap... put them right back on.

Thanks in advance.

-Mag

 

[steelespike]

Re: Which prop to get?

37% slip I would be very suspicious of the tach. If we use a more reasonable slip of 14% rpm would be about 4400.
Poor hole shot and 6,000 really don't go together on a 17 ft boat,21" prop.
What model Action craft?Weights could be as little as 650 to over 900 dry weight.

 

[kmagnuss]

Re: Which prop to get?

What model Action craft?Weights could be as little as 650 to over 900 dry weight.

It's a 89' 17' Bayrunner... weighs about 1200 with motor. 1350 with fuel. 1600 with gear and me.

 

[craze1cars]

Re: Which prop to get?

Your experiments are relatively cheap and easy. Much cheaper than guessing on another prop. So frankly the only way to get your answers is to experiment and answer your own questions. Anything here is just educated speculation.

If you suspect motor is too high, drop it a couple holes and test drive.

If you suspect your tach might be reading too high, get an inexpensive optical digital tach off Ebay, and with that you can at least verify it at lower RPMS while running on muffs in the drive-way.

Is your speed source GPS or boat speedo? Maybe you're going faster than you think you are...

Basically the ONLY way you can have 37% slip on a boat that's on plane is to have a huge amount of ventillation or a drastically inaccurate tach. You just need to figure out which of these two is the case. If your tach is an original on a 1989 model, it is probably wrong. In my experience, tachs rarely last that long on boats. And usually when they fail they start to read high. I agree fully with Steelspike that your tach is suspect.

 

[kmagnuss]

Re: Which prop to get?

Ordered a new tach. Will see if that changes things. Going to take a look at the cav plate while on plane this week. Thanks again.