Cruising Speed Prop

[TunaFish389]

Boats a 1984 Ebko v165 16.5' with Mericruiser 3.0 and Alpha One Gen 1 drive. Currently have a Solas 14.5" 17p four blade prop. Running 25.5mph at 3600rpm at cruise and 33mph at 4600rpm at wot with usual load. I would like to lower my cruising rpms while keeping my speed.

I am thinking of going to a three blade prop (Solas) but not sure what effect that will have on my prop slip. If the three blade will have more slip then a prop change might be out of the question. If the slip is unchanged then I could cruise 26mph at 3300rpm with a 14.5" 19p three blade which would be idea.

I am guessing I would go down 300rpm on the top end; minus 400rpm going up two pitches and plus 100rpm losing a blade. This will in theory keep me in my max rpm range and might gain a mph on top but not to worried about top speed.

What do you all think/suggest? Thanks.

 

[alldodge]

Rule of thumb, going from a 4 blade to 3 blade need to go up 2 inches in pitch (19P). You will always have slip, it just needs to be managed. There are so many folks that go to 4 and 5 blade props trying to get more speed, or less slip.

Less slip just means more load, and so long as it is within limits, all is good. Where talking blades in water, there will always be slip. I would think that there would be an increase of 200 to 300 rpm at wot, which could help.

A motor running easy at 3500 rpm is better then the same motor running a bit hard at 3100

 

[Texasmark]

Back in the days of carbs and vacuum windshield washers, any time you stepped on the gas the WW quit working until the intake manifold vacuum restored itself. On fuel consumption, if you could unload the engine, thus keeping manifold vacuum up, for a given task mpg increased. On injected engines I have no idea.

For cruising, the more blades in the water and the shallower the pitch, the better control you have of the speed and in doing so, the higher the rpms for that task which would amount to the better mpg.

It's a published fact that just over "planing" speed is your best economy, and the spot or a few mph higher is what most folks prefer as cruising speed....course defining that point depends a lot on the type of hull.....semiVs easily determined; deep Vs with lots of dead rise at the transom.......when are you really on plane.

As the blades decrease and the pitch increases, the ability to keep an even speed at that sweet spot is challenged....few blades with high pitches are made for light fast boats.

My 2c is you are not doing yourself a favor in your aspirations.

 

[porscheguy]

Switching from 4 blades to 3 is not a guarantee of increased prop slip because you’re decreasing your total blade surface area which means a reduction in drag. It’s not uncommon to see a 1-2 mph difference in speed between 3 and 4 blade props of similar style and diameter running at the same rpm with the 3 blade almost always being faster.

I assume you’re going to be using aluminum props? If so, they’re fairly inexpensive. If you want to play it safe you can jump up to a 19” 3 blade and see what happens. There are even guys with boats your size with your engine/drive ratio who are running 21-23” pitch. I can’t remember what prop I used on it, but 25 years ago I had a Chris craft scorpion 168 which was quite similar in size ;and probably everything else) to your boat, with the same engine/drive ratio (assuming you do have 1.98 gearing.). I routinely saw cruise speeds of 28-30 and WOT of 42-44mph.

 

[H20Rat]

. I would like to lower my cruising rpms while keeping my speed.

Why? What are you trying to accomplish? It takes X amount of horsepower (fuel) to move a boat at Y speed. Notice there isn't anything about RPM in that equation? Doesn't really matter what your rpm is, your fuel to move your boat at a certain speed is going to be more or less consistent.

You aren't saving the life of the engine either, there are about 100 other things that will kill a marine engine before a bearing wears out.

 

[TunaFish389]

Thanks for the information to think about. I might try the 19p just to see. I didn't put in that I ran mostly in higher elevation 5000ft+. I think this might limit me. I bought my first prop from iboat and the cover for my boat. Both I am happy with.

I was ask what I wanted to accomplish, well it was quoted. Lower cruising RPM while keeping current speed and staying in my max RPM range. It has to do with what RPMs I would like to run the engine at. I think cruising for hours at 3600 is right to the top of where I would want to be. A 3300 cruising RPM seems to what is usually recommended at 75% of WOT. I am not looking for MPG increase, I can be out all weekend on a tank of gas. Just trying to stay within my max RPM range while lowering my cruising RPM and keep current speed. I thought this was implied in opening question but am proved wrong.

Looking at the Solas 19p prop was a let down on this site. Search comes up with $102.24 but when you enter item page its $156.69. Probably a typo on the $156.69 but that is what is charged. Too bad will have to look elsewhere.

 

[jimmbo]

Your engine has a WOT rpm range of 4200 - 4600. I would have to say you are pretty much propped ideally. Yes you could add an inch of pitch and stay within the RPM range. Myself, I would ditch the 4 blade, put a 3 blade with another inch or two of pitch, that would increase your top end, thus lowering the rpm you need for 25 mph.

 

[QBhoy]

Agree with Jimbo here.
I could honestly only excuse a 4 blade over a 3 blade if there was a specific reason or intention behind it. You don’t need a 4 blade for sure. 9/10 times, a well chosen 3 blade will be so much better suited and always more efficient that a 4 blade.
On the solas prop choice...personally I’d be tempted to get a better prop. A well chosen merc based prop and a 3 blade of 19” would almost certainly get you what you are after. Laser 2 if you can find one. But generally any 3 blade merc prop at 19” will see you right. I’d even go so far as to say a 19” black max ally prop would be a better choice than what you have or the solas you are thinking of. It’s well documented that a well suited black max can out perform most of the cheaper stainless prop of the same pitch.
Good luck.

 

[TunaFish389]

I am going to pull the trigger on a Solas 19p three blade and report back. I am happy with the brand, while the Mercs are better i going to stay with Solas.

It will be after summer when the kids come down. They like to be pulled and the four blade does the job. Thanks.

 

[TunaFish389]

I was able to try the new three blade prop today. My prop slip actually decreased which was a surprise to me. Both props are Solas Amita with one being a 14.5x17p four blade and the other 14.5x19p three blade, they were both bought new and the four blade is a year old with no damage.

The four blade max rpm was 4600 at 33mph and cruising at 3600 at 25.5mph(GPS from fish finder).

The three blade max rpm was 4400 at 36.5mph and cruising at 3600 at 29.5mph. I actually cruised at a lower rpm and still was faster then 25.5mph but for comparison kept the same rpms.

I had the same load, GPS and water conditions for both test. The three blade had less slip which I was worried about more. I only dropped 200rpms at wot but gained 3.5mph. I can hit 25.5 at 3150 with new prop vs 3600 with four blade, which I think is a little low for cruising so now my cruising speed is a little faster.

I put the four blade back since I will be towing kids around but once summers over and it just me and a friend I going to stick with the three blade. Anyways maybe someone can find this helpful.

 

[alldodge]

From my view the 4 blade is giving you more lift and also more slip because the prop isn't getting full bite. The 3 blade has less lift and better bite. More is not always better. I think you would do best with a 3 blade 17 pitch for towing things, and might even do better then the 19 with different trim angle

 

[Scott Danforth]

I had 3 props in my boat with a 3.0. one for cruising with light load (19p), one for general boating (17p), and one for water sports and trolling (15p)

 

[TunaFish389]

I did think about the 17p three blade. Its on the table but for now I am set. I think I will try it out next season. I know it would faster but might put me slightly over on the wot rpms. Thanks for the input.

Question about the 15p for trolling. What is your speed? The 17p runs 3mph at idle which is a little fast for me. Also is trolling with the I/O bad for engine/drive? I know this is kinda off subject but has to props and speed... Thanks.

 

[jimmbo]

Extended operation at idle has its issues with any engine. Engine Oil may not get hot enough to boil off any water that has condensed in the crankcase and oil, At idle the fuel-air ratio has to be on the rich side for the engine to run smooth, and that results in fuel condensing and eventual dilution of the oil. Again the oil needs to hot enough for the the gas to boil off. Spark Plugs might carbon up a bit, then when a High Speed run happens, the deposits melt and the insulator takes on a glazed appearance, and may misfire.
Is it Bad for the engine? With proper oil pressure and proper maintenance, the engine should survive. Neglected, it will die a Horrible Twitching Death

 

[dingbat]

Question about the 15p for trolling. What is your speed? The 17p runs 3mph at idle which is a little fast for me.

I ran a 14 x 17” 4 blade for years. Ran 2.4 - 2.6 Kts. @ 850 rpm.

Switched over to a 15x15 Enertia prop the beginning of next year. Now running @ 950 to achieve the same speeds.

 

[alldodge]

2.4 to 2.6 kts = 2.7 to 3 mph

 

[TunaFish389]

Yeah I think trolling is going to be out. I did notice more bow rise and planing speed with the three blade. I guess that what was meant with the four blade providing lift. Lifting the stern, causing less bow rise-faster plane.

Thinking about it, I think I am going to stick with the four blade for towing. I do tubes with the little ones and don't go that fast. I would hate for forward vision to be an issue with bow rise and have to go faster just to get on plane. I am happy how the four does, for this job.

The 19p does well and will be using lightly loaded but wouldn't tow with it. I bet in lower elevations I could even run a 21p or maybe not. Anyway thanks for all the help.

 

[jimmbo]

What are your WOT rpms with the 15 Enertia?

 

[dingbat]

What are your WOT rpms with the 15 Enertia?

@5600-5650

 

[jimmbo]

deleted.