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