83' 115hp Mercury, prop slip

[lapl0007]

Hi,

I have a 1974 Starcraft ss (Aluminum deep v hull bow rider) loaded weight at about 2400#. On it is a 115hp tower of power, 1983 max rpm at 5800. Gear ratio is 2.0. On it is a knock off 13x19p 3 blade Aluminum prop. WOT it goes to 5600 rpm at just under 40mph.

My goals are to increase a cruising speed fuel efficiency, decrease slip (which is roughly at 20%) and if possible increase max speed. Those goals in that order. I plan on buying an ss prop. I usually run in big deep water. The boat rides fine, no trim issues like a bouncing bow trimmed up or anything.

I'm looking at a basic (no fancy rake or cupping) 3 blade ss 13x19p prop that I think may do this. Is there another recommendation for a 4 blade that I should look at? Any thoughts at all help. Thanks
Mike

 

[Texasmark]

Having diddled with alum and SS, 3 and 4 blade props on 90 and 115 hp 2 stroke later year Mercs, a 4th blade can help you maintain plane just above planing speed but you will suffer otherwise with the extra blade....that's why Karl K. used 2 blade props on his engines until he came out with the 1000, 100 hp in Phantom Black where he went to 3. But my guess is that the 1000 was built as a pusher for larger, heavier boats, not necessarily for speed so he wanted the 3rd blade to help get the stern up on hole shots and cruising......just my guess but I'll stand by it.

I never looked for a SS prop without rake and cupping. If they are out there in the size you specified, the blades will be thinner (said to be performance enhancer) and you may come on up a couple hundred RPMS with a small increase in top end....I personally wouldn't expect your cruising attitude to be affected assuming the designs of both material props are pretty similar . If you don't do what you said you wanted to do, Your RPMs are right where you want them and the amenities you said you are avoiding in a SS prop are designed to make a prop of a given dimension, bite better and will drop your RPMs in addition to probably not helping your slow cruising.

However, rake (especially with power trim and tilt) will assist in getting the transom up and out of the water and assist in bow lift (at speed), thus getting more hull out of the water and allow for more speed with an increase in RPM too....probably/possibly......resulting in better top end. I don't see them changing your hole shot all that much.

However #2: Changing props is a "crap shoot" (like throwing dice) and the only way you know what will really happen is install the prop and go test it.

 

[lapl0007]

So, if you were to try out a 4 blade, with cup and rake, what pitch would you start at? 17?

Would you try to stay same diameter as well or not worry so much?

Would you share some of your trials with me so I might gleam so info?

Thank you for your help

 

[Chris1956]

Do you have power trim? If so, you will want a prop with cupping, so it may be trimmed up to maximize speed.

Actually, I am unsure if you can even get a new prop w/o cupping. What is your beef against it anyway?

 

[lapl0007]

I do have power trim.
I guess I'm not sure why I am against it, I think in my head it adds slip and extra resistance that I don't need/want. Which may not be the case. I'm looking for economy first and speed second.
I was looking at solas props and some have cupping and rake and others not so much.

 

[Chris1956]

A cupped prop holds the water much better, allowing more trim and therefore more speed and efficiency.

Check out a turning point hustler in 3 blade. They have good rake and cup and are high performance for aluminum.

 

[racerone]

Power trim allows you to lift the bow at speed.-----This reduces wetted surface ( drag ) and you gain speed.

 

[lapl0007]

So, 3 blade hustler Alu at same 19 pitch you think Chris1956?

 

[jimmbo]

An 83 115 has a WOT Range of 5000-5500. The Last year an Inline 6 had a 5300-5800 WOT Range was in 1981, on the 140hp
The best way to improve Efficiency is to reduce Weight. Getting rid of unneeded Weight, like Wife, kids, Mother-in-Law, especially the MIL, will help a lot. Mistresses and Girlfriends can remain aboard, as they generally are lighter.
A 4th blade will really kill the Top Speed, as you will have to drop several inches in Pitch to recover from the extra load that blade puts on the Engine.

5600 rpm and only 40mph? Either there is a lot of Slip, or your Tachometer is way off. I had a 1985 115 with a 19" Mercury Al Prop and was getting just under 50mph at 5400 rpm

Cupping, Progressive Pitch, and Rake, will all allow the Prop to perform better at higher Transom Heights, which is beneficial at speeds over 50mph

Regarding the Turning Point Hustler, examine the Prop closely. 7 yrs ago, I open many Boxes and found that one of the Blades was not the same as the the other 2, like it had sagged or been bent. In Short, I can't recommend that brand

 

[Chris1956]

Unlike Jimmbo, I have had very good luck with turning point props. I have 4 or 5 of them.

You will want a 19 or 21P prop, likely the 19P prop would give better all-around performance.

 

[jimmbo]

I have never owned a TP Prop, I was at the Dealer, and took one out of the Box, it looked Odd, so I took a few more out, and several had 1 of 3 misshaped Blades. Maybe they had a bad Batch, but it shows poor QC and/or Inspection prior to packaging

 

[lapl0007]

Ok, so Jimmbo, what size boat did you have the 19p merc prop on? Similar to mine? Getting close to 50 sounds better than close to 40. What prop was that exactly, it might be what I'm looking for.

I had asked about 4 blades but no one seems keen. Does anyone have a 4 blade at all or are they not worth it?

 

[jimmbo]

It was on a Crestliner Crusader 550. Prop was 19" Mercury Prop. The pic was taken in July 85/ I have a pic of the Tach and Speedo giving their Info, I just have to get it into the Computer


A 4 blade will kill the Top End by several mph... I would NEVER put a 4 blade on my Boat

 

[jlh3rd]

if jimmbo is correct, you're 100 rpm above your max now..( which I wouldn't worry over) but I wouldn't want to go over that.
On paper, increasing by a blade, or a 1" increase in diameter or pitch would drop the rpm by around 200 rpm. These are not my facts, but what I've read on here and other sources.
These are my results of various props on the same pontoon boat. All #'s were me by myself. All aluminum props. 3 mercs, one Solas.
The 4 blade 14x14 gave good performance but rpm's were way down when pulling teens on tubes, so I put/needed a 3-16x12 which solved my torque needs but reduced top end. However, if by myself, I was over my rpm limit@WOT.
My point is I found no magical prop. And 2 of the 3blades and the 4 blade gave the same #'s

 

[lapl0007]

Thank you

if jimmbo is correct, you're 100 rpm above your max now..( which I wouldn't worry over) but I wouldn't want to go over that.
On paper, increasing by a blade, or a 1" increase in diameter or pitch would drop the rpm by around 200 rpm. These are not my facts, but what I've read on here and other sources.
These are my results of various props on the same pontoon boat. All #'s were me by myself. All aluminum props. 3 mercs, one Solas.
The 4 blade 14x14 gave good performance but rpm's were way down when pulling teens on tubes, so I put/needed a 3-16x12 which solved my torque needs but reduced top end. However, if by myself, I was over my rpm limit@WOT.
My point is I found no magical prop. And 2 of the 3blades and the 4 blade gave the same #'s

Thank you,
That probably was the most helpful. I forget that sometimes, there is no best answer in life, just the one you choose. Trial and error and tracking results. Thank you.

I am most interested in your gph calcs. What economizer did you use and is it easy to use. With my first goal of being fuel efficient at cruising speed I never thought of capturing that data by anything other than rpm and speed.

 

[racerone]

Running a 2 stroke at about 3600 to 4000 RPM is best for fuel economy.

 

[jlh3rd]

Thank you

Thank you,
That probably was the most helpful. I forget that sometimes, there is no best answer in life, just the one you choose. Trial and error and tracking results. Thank you.

I am most interested in your gph calcs. What economizer did you use and is it easy to use. With my first goal of being fuel efficient at cruising speed I never thought of capturing that data by anything other than rpm and speed.

I use the smartcraft system built into my 2021 merc motor.
I installed the smartcraft tach to retrieve the data. I also installed a stand alone GPS speedometer. So the tach utilizes and displays fuel used, and gallons per hour at any rpm.
Pontoons are a displacement hull, not a planing hull, so you might see different types of results.

 

[Texasmark]

So, if you were to try out a 4 blade, with cup and rake, what pitch would you start at? 17?

Would you try to stay same diameter as well or not worry so much?

Would you share some of your trials with me so I might gleam so info?

Thank you for your help

As a rule of thumb, it seems that for a given size....lets use 13" +/- for small gearcased higher Hp engines, the diameter will drop fractions of an inch as the pitch is increased....part of the design formula for props. Has little to do with performance like pitch changes.

First of all the 4th blade would be at least an inch of less pitch to account for the losses caused by the 4th blade having to share water with the other 3. That fact has been proven many times on here and designers are well aware of that fact.
Second, Rake should make your current situation better as it will get more of the boat out of the water, reducing drag that reduces speed and allows RPMs to increase with the speed increase.
Third, some say cupping is worth 1" of pitch and I believe that....haven't put a pencil to it but it makes sense since you are grabbing more water which is load on the engine.
So, with that spagetti salad, what's the answer?????? I dunno!!!!

It may be that everything works to your advantage and you can gain with the same pitch in a SS 4 blade with cupping and rake. I could buy in on that but the only way you know is to install the prop and go test it. One common denominator is proper use of your trim position. Standard procedure is to have the engine tucked in (toward the transom) to some degree and as the bow comes down, getting on plane, then push the trim out. In doing that with Rake, you can see/feel the bow lift (whole boat actually) unlike the lift you get getting on plane where the bow alone is up in the air, and watching the location of the surface of the water vs boat position "on" the water you realize that the hull has moved out/up somewhat.

Watching your wake origin too....before you start trimming out the wake will start forward of mid ship...usually. As you have finished getting up on plane and start trimming out you will notice the wake starting farther and farther back toward the transom. If you are doing everything right and have the right equipment, the onset (that you can see from your seat) will be back at the transom corners and your speed will be over 35.

Once you are up to speed and trimming out, you can continue to trim out until your speed starts falling off and your RPMs start increasing signaling that the blades of the prop are no longer totally submerged. Some trim systems and angle of the transom are such that you can't trim up far enough to get the blades out of the water at speed....some you can.

My comments here aren't what to expect if you have your boat loaded down with folks and gear. It's with you and maybe a friend and that's about it. HTH.