My turn to ask for prop help

[StevensMan]

I guess everyone asked this question and now it is my turn: what is correct prop for my boat?
Technical details: 2355 1995 Bayliner, Alpha One with 1:1.65 gear ratio. Those are gears for this boat with 5.0 motor, previous owner put 5.7 in it but left the same outdrive. Boat came with 15 1/4 X 15 aluminum prop.
With this prop WOT was really high (I guess we took it to 5500 RPM - tachometer was not working very good at this time). Max speed on lake was about 37 MPH (3 people) Easy on the plane.
So I went to Mukilteo prop shop and got 16X16 prop for demo. This one had some cupping (it is hard to say how much) and was working much better for my boat, max RPM was 5100, max speed 39 MPH (Puget sound, 2 people and some gear). Easily gets on the plane, cruising RPM ~3400-3500, if I go lower it does not plane well. So I liked this prop and only complain was that at 3500 RPM cruising it eats more gas than in ideal situation.
Got back to the shop, they gave me 15 1/2 x 18. I had boat on the lake Washington with 5 people on board. Really had trouble getting on the plane, max speed only 31 MPH, RPM too high - 5200. cruising RPM about 4000. Got rid of 2 people - RPM 5200, max speed 36 MPH. I was completely lost, I expected that 15X18 prop will drop my RPM but instead it again gave my too much. Now I'm thinking about it and maybe my tachometer did not show it right - I'll need to clean electric contacts there again and retest on Puget Sound.
I'll go back to the prop shop for the advice but anyhow what do you think? It looks like my prop will rotate slower then normal prop on 5.7/Alpha One - normal gear is 1:1.5, mine is 1:1.65. I believe I need biggest possible diameter and not that much pitch. Unfortunately I can not go bigger then 16 on Alpha One. I do not want to change gears on outdrive(any idea how much those parts may cost, BTW?) Next I want to re-test 15X18 and try 4 blades 15X16 but guy at the shop told me that it is probably not what I want. If it does not work I'll ask the shop to add a little bit more cupping on 16X16 and be happy with that. It is stil spins high I'll try to play with gas mixture - leaner gas may drop RPM a little and save me some money.
Please share your thoughts.

Anton
P.S. I know it is not good for the engine to spin at above 4800 RPM - I'm doing it for short periods of time trying to find right prop size.

[SomeSailor]

Anton;

Here are the original ratios:

V8 heavy duty gear set (1.50 / 1.47 Ratio) 20/22 uppers
6 Cyl. inline gear set (1.65 or 1.62 Ratio) 24/24 uppers
V6 gear set (1.84 or 1.81 Ratio) 17/19 uppers
4 Cyl. inline gear set (1.98 or 1.94 Ratio)2 0/24

If your drive has a 1.65:1 ratio, then it was geared for an inline 6 originally... not the 305 we talked about earlier (1.47:1)

At 1.65:1 you are turning the prop much slower than with an appropriately geared outdrive. Going down in diameter will increase the engine RPM as the prop bites less water... Too much pitch and it will just cavitate.

The stock prop with the 5.7L was a 15.25x15 and that should yield WOT in the neighborhood of 4200-4800 RPMs. The 16x16 "big ear" prop would drop the RPMs down to around 4400-4600 at WOT. You might want to look at changing gear sets rather than a prop. You're a lot further off than a prop will correct I suspect.

The gears are around $250 a set.

[3788sam]

Anton - 5500rpm WOT way to much spin -
SS pretty much hit the nail on the head!
And if the Prop shop guys a scratching their heads, gears may be the way to go.
Some of the highly cupped props can cost as much as new set of gears.
They usually have to be made ofstainless steel so they hold their shape, Alum. will flex and lose the shape and defeat the purpose of cupping the prop.

[StevensMan]

Yes, you may be right, with 5.7 and 1:1.65 gears I may not get an ideal result no matter what I do with the prop. I may later think about changing gears but do not want to do it just yet.
I still want to try 4 blades - it may be OK for my application.
Question here: I heard 16x16 "big ear" quite a bit. Does it really have bigger blades compare to other props of similar size. In other words, is there 16x16 "normal ear" out there?
I talked to the shop and I plan to take all 15x18, 16x16 and 15x16 4 blades out and compare them in the same conditions side by side. Will see how it goes and go from there.

Thanks,
Anton

[SomeSailor]

My understanding is they're all "big ears" when you're talking about the 16"x16". The guy at the prop shop said that was the largest prop I could get for an Alpha. There's probably less than 1/4" of clearance under the cav plate when it's installed. :)

Another problem you'll see with fixing that big of a gap in gearing with prop alone is similar to what happens when you change the tire size on a car drastically. You shift much of your mid-range performance one way or another. You want the tallest primary gear set that will give you the widest band of optimum performance for your engine. Obviously with a 350, this band of power is less critical, and hence not as easily seen. If you were to reverse your situation and say, go from a 5.7L to a 4.3 V-6, this problem would be much more exagerated.

I would think first of the gear sets. You can get them online for around $250, and I'm sure you'll make up for it in fuel economy and/or wear on the motor in the long term. It's like riding your 5 speed motorcycle in only the first 4 gears all the time. It'll seem like no big deal... but you are paying for it a little at a time.

Like the Aamco transmission guys say... "Pay me now... or Pay me Later" :)

[StevensMan]

Mike, I hear you and I'll look into changing gears. I do not want to start doing this in the middle of the season so I'll wait a bit for better time.
I thought that in general bigger boats spins their props slower and go slower when smaller ones go faster and spin their props faster. Looking at gear ratio is it not true: boats with 3.0L motor generally have shorter gears and their props spin slower....
I agree that if I can not get my motor to 4800 WOT then it is like using 4 gears when I have 5 available. At the same time I think if I can get to this number (let's say I can find 4 blade prop that does exactly this) it is probably the same as changing gears in the outdrive. There is no additional load on the motor - it does not know about gears, it only can spin to 4800 at WOT. There are no extra load on prop or outdrive - in fact 1:1.65 gears will have less load then 1:1.50 gears. Prop is spining slower, so I think if I can get this magic prop I'll be fine. The only consern is that slower spinning prop may not work optimal for it's design - but smaller boat have even slower props and they do fine...
Another question I still did not find an answer for: my gears are shorter then "standard", they are only ~91% of standard (compare to 1:1.50). 16x16 prop is ~112% of 15.25X15 ( (16*16) / (15.25*15) *100%). So 1:1.65 gear with 16x16 still should be slighly "bigger" then 1:1.50 with 15.25X15 so my motor should spin slower then 4800. It is not the case, maybe I have soem secretly developed 350HP engine? Anyone has good answer? I suspect my method of prop calculation is not correct, anyone knows another better one? I think if I start calculating blade area and pitch instead of diameter and pitch I'll get even more difference.

And my motorcycle has 6 gears, not 5 :D

Anton

[SomeSailor]

Looking at gear ratio is it not true: boats with 3.0L motor generally have shorter gears and their props spin slower....

That's because they have to spin them so much faster to get them in their power band. Remember your boat is only designed to plane at between 18-20 and 40 MPH, any power band below that, is wasted and the boat will fall off plane... even though it feels like it's running well and coming out of the hole with authority.


At the same time I think if I can get to this number (let's say I can find 4 blade prop that does exactly this) it is probably the same as changing gears in the outdrive. There is no additional load on the motor - it does not know about gears, it only can spin to 4800 at WOT.


I disagree. You MAY be able to load the boat down with a super large, highly pitched prop... but this engineering will cost you somewhere else. Either in the ability to come on plane, stay on plane at low speeds or top end. Even worse is IF you do succeed, you will be working your motor VERY hard at cruise RPM. You'll shorten the life of a motor dramatically like that. (no such thing as a free lunch remember?)


... so I think if I can get this magic prop I'll be fine.

And if you find this "magic" prop, you should quit your Microsoft job and join Mercruiser, I'm sure they'd like to pay to see your math :)


It is not the case, maybe I have soem secretly developed 350HP engine? Anyone has good answer? I suspect my method of prop calculation is not correct, anyone knows another better one? I think if I start calculating blade area and pitch instead of diameter and pitch I'll get even more difference.

I doubt that your 2bbl carbed, non-Vortec 5.7L is making 220 HP, much less 350. :)

Prop calculations are very difficult, almost a black art. :) I wrote a java app a few years back where I tried to get my boats numbers to match the math of the prop. It was very difficult and results became more and more unpredictable. Here are some variables you'll need. Google these terms and you'll see why high performance screw developers get paid so very well:

Prop Diameter, True Pitch, Effective Pitch, Blade Pitch Aspect, Dish Area, Cavitation versus Aeration, Dish drag, Blade Friction, Blade Slip, Blade Cross-sectional drag, etc.


And my motorcycle has 6 gears, not 5 :D Anton

Get a "magic" tire ... you'll only need 4 :D

[3788sam]

5.7 2bbl =250 hp

[StevensMan]

your 2bbl carbed, non-Vortec 5.7L
I have 4bbl carb. I guess it is rated 260HP.

I did some searches and there is a couple things everyone seems to be agreeing on: finding the right prop is somewhat of a black magic - tests work better then any formulas. And there is no ideal prop for everything - each prop is compromize between different factors.

I hope I have time to play with different props over the weekend. Next thing will be playing with gears - too bad that they are a little bit more difficult to swap.

I'm trying to get close to those numbers: WOT 4600-4800 with max speed 40MPH, crusing at 3300 ~25MPH. Do you believe those are correct numbers for 2355? Should it be able to go faster or cruse at lower RPM?
So far I'm able to get WOT 5100 at 39MPH, crusing 3500 at 25MPH

Thanks,
Anton

[SomeSailor]

I'm trying to get close to those numbers: WOT 4600-4800 with max speed 40MPH, crusing at 3300 ~25MPH. Do you believe those are correct numbers for 2355? Should it be able to go faster or cruse at lower RPM?
So far I'm able to get WOT 5100 at 39MPH, crusing 3500 at 25MPH

My "best cruise" was 3300 RPM and 30MPH on GPS. I could slow down to 3000-3100 and make 25MPH, but it felt better at 3300. Just quieter all around and better speed for distance. I had a heavy dink on the back and a 2BBL, so your numbers should be a little better.

Don't think an upper gearset could be that hard to change. And at $250 for parts, not quite twice the price of a new prop. Never had an upper apart myself, so I don't know about special tools required. Did you change your impeller and/or check that fluid yet?

[StevensMan]

Yes, your numbers are better, for 30MPH I had to go to about 3700 RPM. I think my boat will go a little slower then yours because of bottom paint - it is not as smooth as gelcoat. I cruised at 3500RPM / 25MPH to San Juans, I could go 3300 at 21-22MPH but it did not feel right at all. I could not go slower on plane - slightly below 3300 and I'm off the plane. I think I got 2.2 - 2.5 mpg and believe it should be a little better.
I checked outdrive fluid as well as all other fluids - all is good, no water in oil and no oil leak anymore. I bought impeller kit but did not install it yet: my temperature is always 170-175 so I do not want to change impeller until I see first signs of trouble or maybe I'll do that when I change gears.

Anton

[SomeSailor]

Yes, your numbers are better, for 30MPH I had to go to about 3700 RPM. I think my boat will go a little slower then yours because of bottom paint - it is not as smooth as gelcoat.

Nice rationale... but that black stuff on the bottom of mine was bottom paint :)


I could not go slower on plane - slightly below 3300 and I'm off the plane.

Again... 3300 RPM was 30MPH... I would stay on plane until just under 20 MPH. With tabs I could stay up at about 18MPH. No less though.


I think I got 2.2 - 2.5 mpg and believe it should be a little better.

Try to think in terms of gallons per hour instead. You're boat will run from 10 or so GPH (on plane), upwards of 22 GPH (WOT). I always like to use this for thinking of fuel over distance. You can EASILY double your fuel consumption... but your speed is effectively limited between 20 and 40 MPH. It isn't linear though... you use exponentially greater quantities of fuel above 28-30 or so.


I bought impeller kit but did not install it yet: my temperature is always 170-175 so I do not want to change impeller until I see first signs of trouble or maybe I'll do that when I change gears.

Two different sections of the outdrive. Changing the impellor is a lower unit maintenance item. You don't even have to pull the drive. I would at least change the impellor, especially since you haven't changed the drive oil. Any leak and you'll ruin an expensive piece for the sake of not wanting to use a $50 impellor. :) Those should be changed every few years, and as far as you know, yours has never been changed?

[StevensMan]

Well, 3500 at 30MPH - it is nice goal and I'll try to tune my boat to reach those numbers. I ordered new gears, it looks like changing them is not a big deal except that there is special tool for shimming required. Any idea where I can rent or borrow this thing?
Now I need fuel flow meter on my boat to be able accurately read GPH data. navman 2100 seems to be good and not expensive around $160. Did not order it yet because if I get Navman fishfinder with flow meter it will give me much better data or I can go with chart plotter or GPS - you know that options are endless and I already have fishfinder so I need to think it over first. Probably just will get simple 2100 flow meter.
I also opened outdrive top cover to make sure I really have 1:1.65 gears and yes, outside decal matches what is inside.
I looked at different test results at www.boattest.com - 30MPH at 3500 is very good result for 2355 boat - it is better then other bayliners they tested. Unfortunately for bayliner express cruisers they tested only boats with 5.0 motor. It also looks like other brands performs better in the same category :roll: maybe better engines or outdrives?
I did not change the impeller because I just lazy. You know, drain oil, unbolt lower unit, all this trouble 8) I'll do it when I get new gears, I'll have to train oil anyway.
Mike, what was the fastest speed and fuel economy numbers you could get on your boat?

Thanks,
Anton

[SomeSailor]

Again... it was 3300 for 30MPH (vice 3500). This was also with the 16x16 installed. I lost some top-end with that prop, and the motor was working harder at lower RPMs.

Remember your best economy is probably somewhere in the 75% RPM range. A Flowscan is the only way to know for sure though. It'll be nice to see what numbers you come up with. I hope you get the flowscan before you install the gears. Those would be great comparison numbers.

[StevensMan]

Tried to test 4 bladed prop this Saturday, not much of a success - I tried to change props while on the water and managed to misplace or drop one of the washers. I really have no idea how and when it happened but as a result I did not test much. Just confirmed one more time that 15.5X18 does not work well for my boat.
New gears and flow meter are coming, will install and retest. 30MPH at 3300RPM - hope I can get close to those numbers.