Looking for a new propeller

[adruid]

I have a 1984 Marathon Sunfish 2100 fishing boat w/cuddy cabin. Powered by 1996 GM Mercruiser 140/3.0L engine and Alpha I outdrive. Gear Ratio 1.98:1, The boat weighs 3600Lbs. with no passengers, add 400lbs for two. Length overall is 21’4”, beam is 8’, BWL is 6’7”, depth is @52”, FB is 32”, dead rise is 18.4*. Currently using a Michigan 031023 15.25” 15P AL prop that’s starting to get beat. I would like to replace the prop and hope to find the best match for the boat. Thinking about a four blade aluminum and would like some suggestions from people the understand props better than I do. The hull is never happy with more than 30mph running in the Pacific Ocean and fuel economy is always an issue. Looking forward to your good advice.

 

[porscheguy]

RPM @ wide open throttle is a critical piece of info.

I think you need to recheck the weight as a 21? cuddy of any type is going to weigh about 3000 lbs especially with a 700lb lump of cast iron and aluminum in the back. Every marathon 2100 I?ve seen doing a google image search looks like it weighs more than 1200.

I?m not sure what you mean when you say it?s never happy doing 30 or greater on the Pacific Ocean.

Here?s my .02 so far. Most boats in that size range aren?t going to be too happy running at most speeds in the ocean. That engine is woefully inadequate for a boat that size, but google searching does suggest it was a common install.

Based on the info you did provide I entered some data into the prop slip calculator. With a 15 pitch, you should be topping out at 30mph at redline. That?s factoring an extremely optimistic 13% slip on a beat aluminum prop.

If you?re running around at 30 and at redline, your mileage will suck. Even if you go up to a 17 pitch, you won?t gain much speed and it may not reduce your rpms enough to improve mileage. This is the problem with underpowered boats. You?ve got to run the engine at full throttle to have any speed.

Lid say stick with a 3 blade since you don?t need the extra drag from a 4. You could also go a bit smaller in diameter. This will also reduce drag.

Let us know the top speed and WOT rpm.

 

[Sea Rider]

Before venturing into new prop territory, the ideal is to know achieved max wot revs factory stated as usually loaded on calm water cond. Probably current prop is a top performer and only need same new one ? without wot data impossible to know. A good starting point : How loaded do you usually go boating with ?

Boaters thinks that 4 blade props are magical props, which ain't so. 3 blade ones do their homework right if the watercraft is propped right for the load it will move at plane. Let prop manufacturers worry about the diameter they want to put to each prop, you only need to worry about selecting and dialing the correct pitch for your boating requirements.

Happy Boating

 

[ahicks]

About the only possible advantage I see with a 4 blade is the POTENTIAL for it to hold the boat on plane at a lower speed than what you can with a 3 blade. With a boat like this being used the way it is, that might be an advantage worth seeking, as long as you don't have to pay a lot extra to do that.

You will need to know max rpm you're achieving with your existing prop to avoid any bigger gamble on prop size than necessary. .

 

[jimmbo]

First of all, a 21ft boat is asking a lot of a 3 liter. As mentioned, what is the WOT rpm range for your engine? My guess is 4200 - 4600 rpm. What rpm are you currently getting? Going to a 4 blade will kill your top speed. Increasing pitch will reduce both top rpm and holeshot, and may even reduce your speed if the rpms drop too far. Without knowing the RPM infos, no recommendation can be made

 

[QBhoy]

Hi
I?d firstly say that your boat is probably much heavier than that
Secondly and perhaps backing up the first theory, if it is indeed that light and with that engine and gear ratio...a 15? prop would see the rpm sky high.
Just my assumptions....

 

[adruid]

WOT rpm on this engine is indeed 4200-4600 rpm. Previous listed weight was incorrect. Mfg. Est. weight is 3600. I carry 2-4 passengers max. Yep, the engine is not the best possible for this size boat, but she was built as a walkaround fisher, not a runabout. I've installed trim tabs that help her get on plane much quicker than without, but want to make sure my next prop choice will be the best possible match for the engine, boat and fuel efficiency. I have a couple of pitch choices (17 &19) on hand, but am unfamiliar with prop slip calculators and the like (hence my inquiry.) As I'm in the harbor or ocean exclusively, I rarely run WOT. Those few times it's calm enough to open her up I've been up to @ 30 mph on the gps at @ 4500 rpm. Cruising speed is usually between 14-20 mph. She's 33 yrs. old so gauges are not new... Thanks!

 

[ahicks]

There is absolutely no doubt about the results you'll see going with a 17 or 19" prop on this boat. So you get those results permanently embossed in your memory, suggest you try them.

Then come back with any questions you might have.

 

[jimmbo]

you are currently running 4500 rpm, you are not going to gain any speed by switching to a 17. In fact you will probably go slower as the rpm will drop below the 4200 mark. Adding an extra blade will drop the rpm too. Getting rid of any unnecessary weight will be about the only way to increase performance.

Since you have the 17 and 19 on hand, give them a try. As mentioned, it will give you an education about what higher pitch props will do. Just don't run them for very long as they will overload the engine

 

[porscheguy]

Are you certain on the RPM range Everywhere I?ve looked on that engine says 4400-4800? It?s somewhat important because it?s the difference between operating near the top of the range versus being near the bottom.

Due to your engine limits, you?ve got very little wiggle room in selecting a prop.

By the info you provided, you?ve got 7% slip. That?s really good. Too good for an aluminum prop that you?ve described as ?beat.? When you?re running at WOT does the engine feel like it?s screaming on the edge? In my experience, tachs seem to read lower than actual rpm if they?re off.

Either way, I?d recommend and Quicksilver black diamond. It?s the same as a mercury black max. Search online and you can find them brand new for $90 + hub kit. At most you can change 1? in pitch, up or down (I.e. go to a 14 pitch or a16.)

If you?re actually at or near redline, the 16 might offer a slight gain in performance. If you?re at the low end, going down to a 14 may improve performance + reduce fuel burn because the engine isn?t working as hard. With that said, a 15 could be the best option. If your current prop is in really poor shape, you could be losing significant performance and the best solution is a new 15.

To clarify this. If your rpm range is 4200-4600 and your tach is accurate, stick to a 15 if your current prop is really screwed up. Go to a 16 if it?s really not bad.

If the proper range is 4400-4800 and yourvtach is accurate, you?re on the verge of lugging the engine and you should try a 14P.

 

[adruid]

Thanks porscheguy! I think you have pretty well set in my head where a prop change will take me. Seems the current prop was on the boat for most of the right reasons. It's not all that beat up, but I was thinking that since I've improved everything else, the prop may be next. Looks like I'll play with the 17p, but likely stick with the 15p for a new one... Thanks to all for the great input and lessons on props!

 

[Sea Rider]

If achieved 4500 wot revs from theoretical 4600 max wot revs, engine runs just 100 rpm from max wot range. Buy a new same 15 pitch prop. Going for a 17 will lessen too much max wot rpm into the luging limbo zone, bad for the engine. Would you say that combo was lightly, medium loaded or fully loaded when achieved max 4500 wot revs, load is important to know specially if engine is bit underpowered as they say.

Happy Boating

 

[fishrdan]

I wouldn't stray far from the pitch you have now since the engine is towards the top of it's WOT range, which is what you want.

I have no experience with them, but Mercury Spitfire props are supposed to deliver 4-blade performance without sacrificing speed of a 3-blade. I'm running an Alpha-4 prop on my 140HP Mercruiser (predecessor of the Spitfire) and love the prop, fantastic hole-shot and runs smooooth.

A 15P Spitfire has less diameter than your 15P Michigan 3-blade, so you may not have a RPM loss going to the 4-blade Spitfire. Mercury suggests to stay with the same prop pitch, when going from a 3-blade aluminum to the 4-blade Spitfire.

Quicksilver Nemesis is the same thing. The diameter of the 15" Nemisis/Spitfire props seem off to me, for a sterndrive. IE: 17, 19, 21 and 23P props have 14" diameter, but no 15" pitch with 14" diameter, only smaller. Looks like the 15P props may be for outboards................ A quick call to Mercury would confirm.

 

[centerline]

with the "beat up" 15, you are losing speed... get yourself a new 15P and see the difference. if you have any idea that bigger is better, go to a 15.5 diameter with full cup, as it will give you a slight bit less slip.... and more push at WOT.

trying to determine if you need a different size prop by taking data from a dinged or slightly bent prop is not going to be accurate and you will be disappointed with what you buy...
take the data from a good working prop, and then evaluate what you might need to do to make it better.

with the correct prop, you should be able to get to the top of your rpm range or slightly above, as this will give you the optimum performance from the horse power you have available... THEN cut the throttle back for cruise economy..
more pitch isnt always the answer for better milage, as economy does not happen if the engine is laboring at a low rpm, nor does it happen when the gearing is so low that the engine will easily over rev, but when the boat is propped to the specs of the engine, and then you reduce the throttle, then you are correctly within the torque curve of the engine and it maximizes your fuel economy without damage to the engine.

if you find you are just slightly over propped, go to a prop with no cup or a bit smaller diameter... if you need to move 200rpm or more, change the pitch.. 1" of pitch will change the rpm about 200rpm.... and again, dont take your base figures from the damaged prop to compute what you may need for a new one.