Need more prop!

[Strecker25]

Hey guys, tried going from 3 to 4 blade this year and bouncing off the limiter at WOT so it seems like we need more prop. OEM on our boat is 14x19 SS Vengeance. They were a little dinged but performed OK, just had a high minimum cruise/planing speed.

1) Performance issue you are trying to correct.
Better low planing speed, reduced RPM at cruise.
2) Current prop manufacturer, model, aluminum or stainless as a minimum.
Went from OEM 14x19 SS 3 blade Vengeance, currenrly have 14.5x18 Quicksilver Diamond 4
3) Current prop diameter and pitch (required).
14x18 4 blade Aluminum
4) Wide open throttle RPM and speed with an average load (very helpful)
5000, very slightly touching rev limiter
5) Engine/drive make, model, year, and HP
Twin 1998 5.7EFI, Alpha I gen II drives (standard/counter)
6) Boat make model, year, length and weight
1998 Searay 290 Sundancer, 31' overall including platform. 10,500 dry, 130 gallons fuel, 40 gallons water, gear, 2 people.


With the change to the diamond 4's we see a nice low planing speed and decent hole shot with no tab needed to get on plane. I never run WOT or anything much faster than 3400 or so. Tested speeds today on a slight tailwind, full fuel, full water, 2 people and gear. Another owner recommended 15.5x17 3 blades instead of 4. I really like the extra bite at the dock with the 4 blades but it seems to need more prop based on today's test.

1200RPM - 7mph
1600RPM - 8mph
2000RPM - 9mph
3000RPM - 24mph (planing, drives fully tucked, no tab)
5000+RPM - 40mph (rev limiter, drives tucked)

Sticking with 4 blade - do I go up 1-2 inches in pitch and keep the size? Or try the big 3 blade?

let me know what other info you need. thanks!

 

[Sea Rider]

Whichever prop you test must be full hammer down preferably on calm no wind water cond as stated loaded. Ideal is to dial a prop that won't kick the rev limiter in and make OB to run at max wot rpm range factory stated.

Happy Boating

 

[jimmbo]

Your max rpm range is 4400 - 4800? Try the 4 blade in a 19 inch. Myself, I would be trying 20 inch Merc Enertias, but I like 3 blades and really like the Enertia

 

[Strecker25]

Yep, 4400-4800. I'm not opposed to the 3 blades if it allows us to cruise at a low RPM like we're seeing with these 18's. With the vengeance we were around 3600 and 28mph cruise which I felt was excessive, and getting 35%ish slip.

I've read that 1" pitch =200RPM, but what does the diameter equate to? I can't find 14.5x19 or 14.5x20 but they get close. Solas has 14.25x19x4, and Merc has 14x20x4

this is a very stern heavy boat (genset, 3 group 27's, water and waste all in the rear) so the more lift the better.

 

[jimmbo]

Simplistically, more dia should lower rpm, but blade shape and area also play into it. I never worry about dia, I let the prop engineers do that. In most propeller lines, but not all, as pitch increases, dia decreases. Might be to reduce drag as speed increases.

How are you calculating your slip? I can't do any calculations as I don't know the gear ratio your drives have

 

[Strecker25]

just the slip calc on the merc site.

drives are 1.47 Alpha gen2's. Speeds are GPS measured and tachs have been checked for accuracy with a gun while underway.

That makes sense about the diameter I assume until you get into the big elephant ear 3 blades. I was able to find the current diamond4 at walmart so I can return them easily but unfortunately they don't carry the 19's or 20's so I'll have to get it right this time around. I'd like SS over the aluminum but just can't swing it right now.

 

[jimmbo]

What happens when you trim the drives up for max performance?

 

[Strecker25]

Barely touched trim since it was already so high in the rev range but just one shot of uptrim caused them to over rev. I am an Amazon prime member and they had the 14x20s so I just ordered them. Prime will take them back no problem, so that will be a good test. Worst case I'll return and seek out some 19s but if we drop WOT to 4600 with the 20s I'll be content with that.

 

[jimmbo]

Too much inward trim increases drag and might be a contributing factor for your high slippage. Proper propeller choice requires the boat trimmed for best performance as that allows the engine to attain highest rpm. Once you get your 20s on and trim out, not so far as to blow out, you may find you still need more pitch

 

[Strecker25]

Ah I gotcha. Thanks, I wasn't aware of that. Just out of curiosity, is there a risk for any driveline/engine damage for being underpropped if we never run WOT or high end of the range? I'm sure it's inefficient compared to the "right" prop but I was really happy with the 24/3k cruise speed. If the 20s get even better than that I'll be very happy.

 

[Scott Danforth]

I prefer to be under-prop'd vs over-prop'd as I can control RPM with my right hand on the throttle if need be and if I add a bunch of people or gear, the motor still hits it upper RPM.

boats by nature are inefficient.

 

[Strecker25]

I prefer to be under-prop'd vs over-prop'd as I can control RPM with my right hand on the throttle if need be and if I add a bunch of people or gear, the motor still hits it upper RPM.

boats by nature are inefficient.

That's what I was thinking. No one buys these things to save money on gas, so I was thinking slightly underpropped should be ok as long as I can get still get the revs up if needed.

 

[jimmbo]

Under-propping and limiting the throttle sounds like a great idea, but... An engine designed around an expected WOT Rpm range has its fuel delivery curve, and to a lesser extent, Ignition advance curve based on that. An under-propped engine will be reaching its upper range at less than full throttle opening. This could result in higher intake manifold vacuum, which might be get high enough to alter the air-fuel mixture by closing the power enrichment system on a carb, or the MAP sensor sensing the higher than nominal vacuum

 

[tednv]

i think when picking prop pitch/type/size, there is really only one simple tradeoff... speed vs torque

I have a bunch of different pitch and size props on the shelf. Some work great when skiing or pulling a bunch of weight. Those are also the ones that have a lot more grip in bad weather. The ones that are closer to factory pitch will spin up a little faster and allow me to go faster under ideal weather conditions, but do tend to slip a little in bad weather and won't go up on plane as fast. Most of the time I leave the aluminum factory pitch/size prop since it's good enough. If I'm going to have a bunch of people on the boat or pulling skis, i'll switch to the heavier stainless lower pitch prop that doesn't spin up as fast but does get a lot more grip and torque. With this prop, going up on plane is almost effortless, but once I'm there, the boat will not go as fast as it could and there is more vibration at higher RPMs. I haven't tried a four blade prop, but I would expect it to drop your RPMs some more but provide more grip since it'll be grabbing even more water in each rotation. It also depends on the size of the boat and what you're looking to improve. If your boat goes up on a plane easy and you're mostly out in good weather, go with factory size/pitch for speed and it'll be better on gas too. You can try a heavier stainless prop with a slightly lower pitch to get more torque for pulling heavy loads or bad weather

if you've ever played around with RC airplanes/quadcopters/helicopters, it's the same concept. The more air (or water in this case) a propeller tries to grab in one revolution, the slower it will spin. This will give you better low-end torque, but you will use more energy and it will never go as fast as one that is allowed to spin up faster by grabbing less air. When you have one that doesn't grab enough air, you will simply spin your motor too fast without actually allowing it to do any work

I know it's just an over-simplification of it, but that's what I think of it after trying a bunch of different ones

 

[walleyehed]

Here's the bottom line based on what you say you want... #1, you want to stay on/get on plane easier. 4-Bld will be about the only answer as far as staying on plane at a slightly slower speed-you'll never get that with 3-Bld.
I might add also, that's a LOT of boat for alum props. You would benefit greatly from SS designs, some better than others but the rewards would be worth the money spent. Merc has some real good designs as does solas for exactly what you are wanting.

 

[Strecker25]

Thanks everyone. To walleye, completely agree. My factory SS 3 blades are in what I'll serviceable condition but could use some help. The goal with trying the 4 blade aluminum was to get into the 4 blade world for quite cheap and see how the boat responds. I may run them for just this season and scoop up the proper SS 4 blades for next year.

Since I'm trying out the aluminum currently, is it fair to assume once I find the proper aluminum 4 blade pitch that the SS equivalent will be an inch less to spin up to the same WOT revs? In other words, if 20" aluminum is in the correct range should we look for 19" SS? Or is that just too simplified?

I know prop design is going to be different for each line but seeing as no one I've found will return SS props after they've been used I'd hate to drop $1000+ on two incorrect props.

 

[jimmbo]

Comparing AL to SS is a crapshoot at best, unless the SS has the same blade shape and thickness, same cup and same rake. Except for heavy duty work props, all SS props deviate from the AL props in blade shape, rake, and cup, some more, some less.