Mercury 75hp on aluminum 16’ Starcraft

[Moserkr]

Hey everyone,

After rebuilding my boat, I recently put a new motor on her. I started with a standard propeller on the recommendation from the shop where it was installed. Id say they guessed pretty well. I will list propeller stats below. My goal is more stern lift, better handling, and mid range cruise - Id like to move to a 4-blade prop. Id like to stay between 5000-5500 rpm. Boat is used for cruising, towing a tube, and trolling for fish. We usually boat at high elevation so 4-6000’ is normal. The current prop can be used at sea level as it hits my max WOT operating range.

Current Propeller and Data
Mercury Black Max 3-blade Aluminum prop
15 pitch
13.75” diameter
WOT operating range 4500-5500 rpm
Heavy load = 5 ppl + gear
Average load = 3 ppl + gear

WOT 5200 rpm average load @ 5000’ elevation, trim full down, 32 mph

WOT 5100 rpm heavy load @ 5000’ elevation, trim up slightly, 31 mph

WOT 5000 rpm heavy load @ 5000’ elevation, trim full down, 30 mph

WOT 5500 rpm average load @ sea level, trim full down, 33 mph

Motor currently sits on the 2nd hole so my cav plate is slightly (1”) above the keel.
I think thats all the data that should be listed. Id like to stick with an aluminum prop for now, but once I get a 4-blade AL dialed in, then switch over to SS.

So my thought is I could possibly move down a pitch or 2 to a 13 or 14 pitch. Seems like those are geared more towards pontoons though. Another thought is stick with the 15 pitch but in 4-blade, then eventually add a manual jack plate with setback. Would like to stay in the mercury family of propellers so all things stay equal in general. Thanks in advance for any thoughts and suggestions.

 

[airshot]

Probably a four stroke ??? Stern heavy it sounds like....I have a 16' Sylvan alum. with a 40 hp Merc classic two stroke abd I get 30 plus mph ( gps) trimmed out a bit. You may have ro move some weight forward to counter that heavy four stroke.. Just an educated guess here.

 

[Moserkr]

Yes, brand new 2023 4-stroke. 356 lb motor but I already balanced the boat out a bit by putting the 18 gallon fuel tank forward. Rides nice but yes, a little stern heavy depending where people sit. With my rear bench seat Im expecting even more weight in the stern. So its mainly extra people weight and pulling a tube that will require more stern lift. With just my Admiral and young kids in the boat, she has great balance.

 

[jimmbo]

A 4 blade will likely drop you about 4 - 5 mph. And and will likely require 3 to 4 drop in Pitch to restore the Rpms.

Your 18 gals of Ballast is a variable Ballast, as the day goes by, the boat's trim will be different resulting in changes to the Boats Trim.

Switching from a AL to a SS, is often a lot more than just getting a SS of the same Pitch. Factors like Blade Contour, Cup, Rake, become factors when picking a different Prop, AL or SS.

 

[airshot]

There are special props to help lift a heavy stern, you will need a prop shop/ specialist to explain and guide you. Wow...350 lbs...my 4 cylinder two stroke is 195 lbs...big difference !!....afraid your gonna need more weight up front !! My boat itself weighs just under 1000 lbs, how much does yours weigh ??

 

[Moserkr]

Im not concerned with top speed but keeping my rpms within the 5000-5500 range is important to me. A 3-4 drop in pitch is a lot more than I was expecting moving to a 4-blade.

I hardly use the trim at all, only when people are in the bow of the boat and I need to lift the front end a tiny bit for top speed or handling. Otherwise she wants to drive hard nose down once on plane and it feels good. Getting on plane is decent but I like when its faster.

I was hoping to go from the mercury 4-blade AL directly to a mercury 4-blade SS in the spitfire family. From what Ive read they are exactly the same except for AL or SS. Was hoping to pick up some of the speed lost with 4 blades by switching to SS in the future too.

@airshot The max weight capacity of my boat is 1500 lbs with motor gear people etc. Yes the stern is heavy but I am not having an issue getting on plane with a heavy load of 5 people plus gear. She still rips and throws you back. That motor is plenty powerful to do it, I just want to maximize it. Shave a second off my hole shot. The balance works well as it sits. My buddy who weighs 200 lbs, plus our 2 young kids were in the bow and we almost swamped going into some big wakes. I dont want or need more weight up front permanently, a 4-blade prop is the only solution needed. There were plenty of old 16’ aluminum starcrafts with the 85 hp Johnson which weighs 300 lbs, and even more with the 70 hp johnson plus a kicker, also weighing in at 300+. My previous 3 cylinder 2-stroke johnson was 220 lbs. Theres no way your 4 cylinder 2 stroke is 195 lbs. - prove me wrong.

Goal is to figure out which pitch 4-blade prop to buy without buying 3.

 

[jimmbo]

Having the Bow Plowing when on Plane is not good from a Safety Standpoint. It can create a Condition where the Bow becomes a pivot Point and the Stern will Violently snap around when the Motor is steered

 

[Moserkr]

@jimmbo Maybe I wasnt clear in what I said. The bow pushes down enough to feel like it just wants to go faster. Trim can lift the bow by barely trimming up. In no way does the motor want to come out of the water unless Im screwing around on sharp turns going too fast, and even then its hard to make that happen. The boat feels great when its running and driving. This is not the issue. Im more interested in what you said about the changes in prop pitch and its relation to rpms. I have an idea how it will affect performance. Trying to figure out a pitch to choose.

 

[jimmbo]

Gaining speed by swapping to a SS is maybe, maybe not. Some SS props are literally SS copies of AL props, sometimes not. If there are no Differences in Blade Shape, Blade Thickness, Blade Area, Rake, Cup, Dia, and Pitch, any speed difference will be minimal, in fact the Heavier SS might be slower. SS being stronger than AL, can be utilized to make the Blades Thinner, and Thinner Blades are slightly more Efficient, but the SS is still heavier. Changes to the Blade Area, Shape, Rake and Cup, can and will affect how a Prop Performs. Once you change Prop Lines/Models you are kind of back to square One. I have tested many different Props and even when several all claimed the same Dia and Pitch, they gave very different Results. Some even changed how high the entire Boat rode out of the Water. My Avatar shows how a prop can lift and hold the Entire Boat out of the water.

 

[Moserkr]

Thanks @jimmbo Ive read all about the cup rake shape thickness dia etc and everything seems to be a big IF hahaha. Thats a cool avatar pic of the boat flying out of the water.

In a nutshell, basically I just need to try a prop and see if it works cause theres no way to accurately guess which will do what, even if they claim to be the same. Didnt even consider the weight of the SS vs AL, forgot that factor cause there are so many. Sounds like going to SS is just starting over and if Im not for sure gaining speed from it, then I wouldnt do it.

I need to find a place that takes returns. I got away with it once through bass pro but they dont have the props in stock Im looking for.

I own mercury, solas, and turning point props. The turning point and black max look exactly the same and have the same specs for my new motor. The solas only fits my old johnson but is a 4-blade. Nothing bad to say about solas though.

What other companies should i consider then? Im not 100% glued to merc props. Im willing to buy a few if need be and Im always looking for used to try to save a buck.

 

[jimmbo]

Here is the whole pic, and the one just prior to it

 

[jimmbo]

In 1985 I bought my first SS prop. It was a Mercury Prop, it and the AL it replaced were both 19in. Pitch, the SS had a 1/4 inch more Diameter. Top Speed was Identical, well within 1/2 mph. However the SS did have better Bite on the water, and didn't ventilate as much on turns, and allowed more Positive Trim.
In 1999, I tried several more SS Props, all of them more Higher Performance, and they were within 2-3 mph, but they allowed me to raise the Engine up higher on the Transom and that yielded another 2 mph with very little Blowout. I ended up running an OMC Raker. That Prop has a lot of Rake, and does very well running only partially in the water
The pics above were in 2012, and I was running the OMC RAKER on the Volvo I/O, after rehubbing it for the Volvo. It still did very good buried. It has been replaced by a Mercury Enertia, which is a great Prop for Top End and Holeshot.
I am lucky, I can still walk into the Dealership, and borrow a Prop for testing. I have only dinged one up, and it was repaired at my expense.

 

[airshot]

Im not concerned with top speed but keeping my rpms within the 5000-5500 range is important to me. A 3-4 drop in pitch is a lot more than I was expecting moving to a 4-blade.

I hardly use the trim at all, only when people are in the bow of the boat and I need to lift the front end a tiny bit for top speed or handling. Otherwise she wants to drive hard nose down once on plane and it feels good. Getting on plane is decent but I like when its faster.

I was hoping to go from the mercury 4-blade AL directly to a mercury 4-blade SS in the spitfire family. From what Ive read they are exactly the same except for AL or SS. Was hoping to pick up some of the speed lost with 4 blades by switching to SS in the future too.

@airshot The max weight capacity of my boat is 1500 lbs with motor gear people etc. Yes the stern is heavy but I am not having an issue getting on plane with a heavy load of 5 people plus gear. She still rips and throws you back. That motor is plenty powerful to do it, I just want to maximize it. Shave a second off my hole shot. The balance works well as it sits. My buddy who weighs 200 lbs, plus our 2 young kids were in the bow and we almost swamped going into some big wakes. I dont want or need more weight up front permanently, a 4-blade prop is the only solution needed. There were plenty of old 16’ aluminum starcrafts with the 85 hp Johnson which weighs 300 lbs, and even more with the 70 hp johnson plus a kicker, also weighing in at 300+. My previous 3 cylinder 2-stroke johnson was 220 lbs. Theres no way your 4 cylinder 2 stroke is 195 lbs. - prove me wrong.

Goal is to figure out which pitch 4-blade prop to buy without buying 3. View attachment 384362View attachment 384363

Just look up the specs on a 1992 40 hp Mercury classic two stroke, 4 cylinder...dry weight is 192 lbs according to factory service manual !! I had an 85 hp omc V4 and it weighed 225 lbs according to factory specs in the manual. These new four strokes are heavy and hanging over the stern like they do is the worst case. Not bad mouthing you in any way, just trying to get you to understand how that extra weight affects a 15' aluminum boat. That 350 lbs is taxing the stern capacity of that boat, and getting the proper balance is gonna take some work. Often passengers need to sit where the weight is needed not where they want. Been an aluminum boat owner for over 60 years, had to figure proper balance on everyone of them. Just understand that when going to a 4 blade prop, you normally drop 2" in pitch to make up for the extra blade ! There are pros and cons between 3&4 blade props, while changing props often helps, weight shift can offer bigger advantages.

 

[airshot]

In 1985 I bought my first SS prop. It was a Mercury Prop, it and the AL it replaced were both 19in. Pitch, the SS had a 1/4 inch more Diameter. Top Speed was Identical, well within 1/2 mph. However the SS did have better Bite on the water, and didn't ventilate as much on turns, and allowed more Positive Trim.
In 1999, I tried several more SS Props, all of them more Higher Performance, and they were within 2-3 mph, but they allowed me to raise the Engine up higher on the Transom and that yielded another 2 mph with very little Blowout. I ended up running an OMC Raker. That Prop has a lot of Rake, and does very well running only partially in the water
The pics above were in 2012, and I was running the OMC RAKER on the Volvo I/O, after rehubbing it for the Volvo. It still did very good buried. It has been replaced by a Mercury Enertia, which is a great Prop for Top End and Holeshot.
I am lucky, I can still walk into the Dealership, and borrow a Prop for testing. I have only dinged one up, and it was repaired at my expense.

Yes...SS has thinner blades giving more bite on the water and in turn reacts to trim adjustment better. When you have a stern heavy boat, there are things that can be done to props to increase stern lift and that is rake angle. He needs to meet with a prop expert and have a prop matched to his particular situation. But, no matter what prop...weight distributuon is gonna be crirical in that short of a boat..

 

[jimmbo]

Increased Rake, holds the Bow higher, at lower Trim settings. Props with less Rake are better for raising the Stern

 

[Moserkr]

Dang @airshot I would not expect a 4 cylinder 2-stroke to weigh so little. Those must be small cylinders!! Considering I was 7 when that motor was built, its no surprise I havent seen one. Wasnt calling you a liar, I was just in disbelief.

So weight distribution…. All there is to move around is my small starter battery. After having 250 lbs of friend and kids up at the very front of the bow, I would not want any more weight up there. The fuel tank sits between and slightly forward of the consoles, low on the hull. I usually keep it full and its never been less than half full. The boat actually sits flatter in the water with a 356lb motor on the back VS the old 220lb motor plus 2 batteries plus 12 gallons of fuel. I used to have water splashing into my splashwell through the drains while idling and dont anymore. The only weight distribution I seem to need is port to starboard when theres people on board which is to be expected. If I move permanent weight forward I would be concerned about having too much weight up there with people sitting in the bow.

Not sure how much rake my prop has now but it sounds like I want minimum rake. I barely use my trim at all and its usually in the full down position. I can drop 2” of pitch and try a 13p 4-blade prop. If that doesnt work then Ill be looking at an 11p 4-blade prop which seems low for my boat - those are definitely pontoon type props at that point.

I could do the manual jack plate first bring the motor up as well as set it back a little. That could bring my rpms up and theoretically improve hole shot. Doubt it will lift the stern anymore and if anything, leverage more weight to the rear. I understand that will stress the transom a little more. Im pretty confident in my work when I rebuilt the transom though. Its built far beyond what originally came with the boat, and the structure is tied into the hull in places it was not before. A guy in the starcraft forum was getting 39mph with a manual jack plate, 5” setback, 75hp merc 2-stroke. I know I should be able to squeeze more hole shot and top speed out of my motor and boat.

 

[airshot]

That two stroke is probably a little faster than a new four stroke...On a 15' boat, there is only so much room, I know, getting your passengers setting where they do the most good can be a problem. Especially when they are heavier people, been there, done that, even had to make some get out that wouldn't listen !! As I said, you need to do some experimenting with where folks sit, it can make a big difference on a light alum boat. My guess is tou should get at least 35-36 mph with that heavy four stroke, once you find that magic setup...good luck

 

[flyingscott]

I have that boat. the weight is fine as the boat is rated for either a 85/90 hp motor. Those motors were around the 305 #s so I would look at your prop design. Is that the big foot or whatever they call it now.

 

[Moserkr]

@airshot I have no problem telling people where to sit and what to do. Their other option is swimming. 35mph would be fast enough for me. Im happy anywhere from 30-35.

@flyingscott It is not the big foot. The prop for my motor is for the standard gear case. 4.25” hub exhaust, 2.07 gear ratio.

For prop design, so far it looks like I want less rake. A little cupping is nice. Found a reputable place today that accepts returns and will probably talk shop with me. That would be huge and let me test a few different props. I may even skip right to a SS if stuff is returnable.