SS vs. AL = different performance?

[oldslowandugly]

I had my setup dialed in perfectly. 1985 Evinrude 30 hp with an '80 35 hp carb with a #58D jet on an 18' Lund aluminum skiff with an old 10 X 13 SS OMC prop. It tached out at exactly 5800 rpm with a good holeshot with me alone or with my 10 year old Grandson aboard. With another adult aboard it lost about 150-200 rpm, still good. Then the rubber hub let go and I needed a new prop. I may be selling the boat soon, and didn't want to invest in another OMC SS prop. So I bought a new Solas 10 X 13 aluminum prop thinking it would act the same as the SS. Wrong. Above 4500 rpm it surges up and down about 500 rpm, and it struggles to reach 5500 rpm. Thinking it was the current heat wave (97? -100?) maybe causing trouble I posted a topic about the heat affecting WOT but today I disproved that by temporarily switching to my spare prop, a 10.5 X 11 SS OMC. Even though it was 94? today the motor revved right up to 6200 rpm effortlessly, which is why it is a spare, it over revs. So my question is- why is the Solas prop a dog even though the diameter and pitch are the same. Is aluminum that different performance-wise than SS? Maybe the manufactures have different standards concerning specs? And finally, the original 35 hp jet was a #59D but the #58D gave "crisper" mid-range and top end. Would the richer jet overcome the surging, or is that strictly a prop issue? As for why I am using the '80 35 hp carb over the '85 30 hp carb which has a slightly larger throat and a #63D jet, I prefer the electric/manual choke over the troublesome primer system. If it's the smaller carb that is not working with the new prop, I can always switch back, but it ran great with the SS prop.

 

[realrez]

Re: SS vs. AL = different performance?

take it to a propshop and have it re-hubbed

 

[444]

Re: SS vs. AL = different performance?

Does the aluminum prop sound like it's cavitating when the engine rpm is surging? The aluminum props can flex more under heavy loads and from what I've heard don't cut through the water quite as easy because they're thicker.

 

[oldslowandugly]

Re: SS vs. AL = different performance?

I may re-hub it myself. It isn't cavitating, in fact I think the bite is better than the SS prop which was pretty beat up. It's when I'm up on plane and it should be humming along, it's struggling. I did a similar prop swap on a 48 SPL and the aluminum prop performed perfectly. So I'm confused why this is happening. Now, OMC props go up 2" of pitch at a time, while Solas props can be had with 1" of pitch difference. They offer a 10 X 12 aluminum prop that theoretically should let the motor rev better, but if the aluminum props by definition act "bigger" than SS, maybe the aluminum 10.5 X 11 is what I need. To re-cap, SS 10.5 X 11 over revved. SS 10 X 13 perfect. Aluminum 10 X 13 struggles.

 

[pootnic]

Re: SS vs. AL = different performance?

Not much to add other then from what i've read/heard.
You normally go down in pitch if going from aluminum to SS,so opposite of what your thinking.
I've also read where sometimes props aren't what they said they are,ie:maybe your SS wasn't a 13pitch or the new one isn't a 13 pitch.
In theory I don't think the aluminum should struggle to reach the rpm's if the SS worked before(at same pitch)

 

[oldslowandugly]

Re: SS vs. AL = different performance?

I had also thought maybe the prop was cupped, but I don't see any evidence of this. Assuming the aluminum prop is, or is acting, "larger", I think I will swap carbs and see if the slightly larger '85 30hp carb will allow the motor to push this prop.

 

[fishndirk]

Re: SS vs. AL = different performance?

Changing your carb will not solve your problem. The reason your loosing rpms is because the new Solas Amita was designed with a larger and wider blade area to take advantage of the higher gear ratio in newer 4 strokes over equivalent hp 2 strokes. So even though both props may have an average pitch of 13" the Amita is attempting to push a larger volume of water underneath the boat. I would suggest if your going to stick with aluminum to drop pitch slightly. If you go back to SS check out the Solas Saturn, their good quality and much less expensive than other SS props in their class. Tip of advice though on the Saturn; they are a very fast spinning prop. I have ran both the Solas Saturn and Amita on my boat, currently running the Amita.

 

[oldslowandugly]

Re: SS vs. AL = different performance?

That's the kind of answer I was looking for! Thank you so much! I know my '85 30 hp has a higher ratio gear case than my '80 35 hp. That's why the 10.5 X 11 came on the 35 and the 10 X 13 came on the 30. I had no idea the 4 strokes had an even higher ratio. So it's a different prop for me and the 10 X 13 Solas goes up for sale. Judging from my data, should I drop down to a 10 X 12 or a 10.5 X 11?

 

[fishndirk]

Re: SS vs. AL = different performance?

Yes, lowering your pitch with that same model prop will bring your top end rpms back up. The general rule of thumb is around 200rpms for every 1 inch of pitch difference. It may not regain the same type of performance you had with your old prop though. Every type of prop is designed slightly different and will affect it at some point in its performance curve. Its not just a straight shot up the scale based on diameter and pitch. Newer 4 strokes typically have a gear ratio of just over 2:1. This gives them the mechanical advantage needed because by their design only every other stroke of the piston is a power stroke. A higher gear ratio also means less top end though because the prop speed is farther from the crankshaft speed. 2 strokes, especially older ones can afford a lower gear ratio because on 2 strokes every stroke is a power stroke. They run typically just under 2:1 in the equivalent hp range in terms of gear ratio. This may not sound like much but at 5500rpms it is a considerable difference. Different props designes have come out to accommodate those differences. I think your best bet to match your old performance would be to do some research on different prop designs to find the closest match to your original. Call the manufacturer if you have to and compare the specs. Any monkey can read and tell you that a 10x13 is a 10x13 is a 10x13 is a 10x13...but there's more to it than that and I think your recent experience as well as others I have seen can atest to that.

 

[oldslowandugly]

Re: SS vs. AL = different performance?

Terrific info. As usual, it's too bad I don't have a large selection of props handy to test-and-tune with. Best I can do is do some research and then make an educated guess which prop will work best. I'm already leaning towards the 10.5 X 11 because the Solas Amita design should stop the over revving I saw with the SS version and still allow me a cushion in case I carry another passenger. When I was considering the Solas purchase my gut said go with the 10 X 12 for the same reason, it was right between the two SS's I had and would raise the rpm's a bit, and not over rev. But that's before I learned the Solas was designed differently. Too bad Solas didn't put a disclaimer in the literature like "if used on an older 2-stroke motor, subtract "X" amount of pitch".

 

[oldslowandugly]

Re: SS vs. AL = different performance?

Here's why I picked what I did. The Solas site compares OEM props to theirs and they make no distinction between 2 or 4 stroke motors and they suggest the Amita 3 series for both. Once again marketing isn't on the same page as Engineering. --------------------------------------------------------------------------- Category Series Engine Dimension Part No. Nearest O/E Propeller Dimension Nearest O/E Part No.


HP B D P R B D P R
B AMITA 3

20
25
30
35

3 11 9 R 2211-110-09
3 10.5 11 R 2211-105-11 ------- 3 10 1/2 11 R 175190
3 10.3 12 R 2211-103-12 ------- 3 10 1/2 12 R 176424
3 10.1 13 R 2211-101-13 ------- 3 10 13 R 175191
3 10 14 R 2211-100-14 --------- 3 10 1/4 14 R 176425
3 10.0 15 R 2211-100-15

20, 30HP(4 Stroke)

3 11.0 9 R 4211-110-09
3 10.5 11 R 4211-105-11 ------ 3 10 1/4 11 R 5032107
3 10.3 12 R 4211-103-12 ------ 3 10 1/4 12 R 5032108
3 10.1 13 R 4211-101-13 ------ 3 10 1/4 13 R 5032109
3 10.0 14 R 4211-100-14 ------ 3 10 1/4 14 R 5032110
3 10.0 15 R 4211-100-15

 

[fishndirk]

Re: SS vs. AL = different performance?

If you use the prop finder on their website and look at the Amita 3 and Amita 4 it says "wide blade design usefull for 4 strokes". This is the only indication they give however but he real reason is that 4 strokes have become more popular and more common now days. The 11 or 12 pitch should bring you back up towards the wot rpms you had before but you had also mentioned something about your rpms surging randomly. I don't know for sure without seeing it but that sounds like your prop was ventilating. The lower pitch may not solve that problem because that is more of a motor trim/blade design type thing. You may find that with the Amita you will end up having to mess with the trim and motor height a little to get it to run smoothly again. Unfortunately there is no way to "try these things out" unless you have a good Marine shop nearby. That's why I was suggesting finding something that is as close to your original design as possible, not just in basic specs but also in material and blade design too.

 

[oldslowandugly]

Re: SS vs. AL = different performance?

Fishndirk you seem to know your props. Do you think a 4 blade prop would work better in my situation? And if I tried one what is the rule for pitch difference? 1" ? 2" ? I've been scouring the prop topics and everyone seems to have their own ideas. But it seems if I switched to a 4 blade I should give up 1" at least, maybe 2". As for the surging I changed nothing about the motor, just the prop and both my 10.5 X 11 SS and my 10 X 13 SS did not ventilate at all. Only the Solas surged and you are probably right, as it acted like it would bite then slip. If I was bent on keeping this boat I would just get a new SS, same as before and call it a day. But I am involved with a 15' Whaler that I intended as a replacement and I can't make my mind up until the Whaler is functional. Bad timing to spin a prop hub.

 

[fishndirk]

Re: SS vs. AL = different performance?

I would not suggest moving to a 4 blade because it does not sound like you are pushing heavy loads or pulling a tube or skiers. 4 blade props are intended to push a larger volume of water and increase low to mid range capacity. Its the larger volume design of the Amita that started these problems in the first place so moving to a 4 blade would probably worsen things for you. Unfortunatley a lot of new al props have a similar design to the Amita. There is nothing wrong with the Amita prop though, I love mine and if your going to stick with it I would say your best bet is to drop a bit of pitch and just play around with the trim and motor height to eliminate any possible venting you may have. Fortunately the Amita is on the lower end of the price range and a lot cheaper to do a trial and error on than a $300 ss prop.

 

[oldslowandugly]

Re: SS vs. AL = different performance?

Great advice, thank you! I do like the Amita I have on my 48 SPL, I just need to get this 30 hp dialed in. That will be easier now, thank's to you.

 

[fishndirk]

Re: SS vs. AL = different performance?

I've wasted my fair share of money on props that were not right for my setup. After that I learned as much as I could so it didn't happen again. Plus I'm just a physics nut. A lot of people don't think that minor equipment differences play a factor in smaller size boats but they do. I wish you the best, keep us updated.

 

[oldslowandugly]

Re: SS vs. AL = different performance?

Update-OK, a year has passed, Sandy and all, and we're still here. And is my face red! Guess what I found when I pulled the boat out just before the hurricane hit? A doormat sized section of bottom paint must have let go early in the season, allowing a HUGE colony of big barnacles to accumulate just out of sight up front. No wonder the prop wouldn't perform! And that's probably why the SS prop let go too, it was killing itself to push the crusty bottomed boat. So now with a clean bottom, the Solas prop did just fine, revving right to 5800, and picking up a bit of top end speed to boot. I guess the 4-stroke design actually helped. But thank's to all who helped me out with info, I appreciate the help. I am posting this for others to remind them to check the stupid things first, unlike how I didn't.