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Old 08-15-2020, 11:49 PM   #1
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Looking for gasser opinions - stainless steel prop

I am about to finally launch my boat after an almost 2 year over haul. Included is the new Mercruiser sterndrive that is not a marinized auto or truck engine, it comes with a Bravo two leg.

One of the chaps working on my boat I've come to respect in terms of his experience and technical knowledge. I won't bore you with why but no one including the engine installer or Mercury or prop shops want to make a prop recommendation. The closest I have gotten in terms of recommendation is the "prop selector" software Mercury and other use. Input boat data and out pops a prop recommendation..

But I know the prop may or may not be the best match, so my intention was to use the new prop I have just purchased to be my back up prop. Its a Mercury aluminum jobby, I don't want to go to stainless steel until I know the next prop is hitting the sweet spot.

So I was talking to the guy I respect about this and he said if it were him he'd stick with the aluminum prop, even if I have to change prop diameter/pitch. We have lots of logs in our waters and debris here in coastal BC waters, the joys of logging. About 20 % of American homes are built with BC lumber.

My guy says if I hit a log with the stainless steel prop its rigidity will transfer shock along into the leg and create more damage than the softer aluminum prop with more give. This sounds logical to me but I'd love to hear others opinions. Sea Trials is in 9 days.
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Old 08-16-2020, 01:55 AM   #2
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If you arenít looking for the ultimate performance then there isnít a great reason to go stainless. The aluminum prop will bend easier so it might save the prop shaft, or maybe not depending on how hard you hit something. It will certainly be cheaper to buy. I have not kept up with stern drive props lately so I donít know what there is out there now. Good luck, it may take a couple of tries to get it right.
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Old 08-16-2020, 07:31 AM   #3
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If you have a big risk of hitting logs, then the Al prop makes sense.
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Old 08-16-2020, 10:00 AM   #4
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Any place that has a high risk of logs, rocks, shallow water is a good place for aluminum props. Jet drives are popular as well. I suggest you consider a dock water flushing attachment to extend the life of the exhaust risers. Just a few hoses and a plastic cap you unscrew an attach your dock hose to. Unless the water you idle home that last 10 minutes is fresh water. I know there's a lot of that in the PNW.
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Old 08-16-2020, 10:07 AM   #5
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Most prop shops will let you try a prop for best fit as long as you buy from them
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Old 08-16-2020, 12:09 PM   #6
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If you arenít looking for the ultimate performance then there isnít a great reason to go stainless. The aluminum prop will bend easier so it might save the prop shaft, or maybe not depending on how hard you hit something. It will certainly be cheaper to buy. I have not kept up with stern drive props lately so I donít know what there is out there now. Good luck, it may take a couple of tries to get it right.
Yes, this.

And aluminum props are fairly easily repaired, in the likely event you ding yours up. I had to visit a prop shop every 2-3 years in the time we were cruising BC with a sterndrive. Got pretty good at changing propsets out in the wilds.

If you're going to be traveling on plane, before you start testing props make sure to load the boat with weight comparable to what you'll be cruising with.
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Old 08-16-2020, 12:24 PM   #7
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Most prop shops will let you try a prop for best fit as long as you buy from them

That's the theory isn't it. But not so much here on the Island. My engine install guy, definitely not. I called a place in Victoria and the guy I talked to said he'd allow me to try one out and as long as it wasn't dinged he'd be good to go as long as I bought from them. Then when I called back, that guy had left that retailer and the next guy I talked to definitely didn't want to honour the first guys promise.

The prop selector has suggested a RH 18.75 D 17 P. The boat is either 28 feet or 28 1/2 feet and using a very vague guestimation (older boat with wood/glass hull and wood upper and lots of new heavier goodies added) the weight was pegged at 14,656 pounds, WOT 5200 (350 HP) and calculated boat speed of 34.8 knots.
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Old 08-16-2020, 12:26 PM   #8
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I'd give the calculated prop a try. Once you run the boat you'll be able to see if it hits proper rpm at WOT and determine if it would benefit from more blade area or not. With some help from a prop guy, you should be able to get it dialed in pretty well from that point.
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Old 08-16-2020, 12:42 PM   #9
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And an aluminum prop Is much easier on the shaft and gears of the lower unit.
Don’t think I’ll ever use an steel prop.

Somebody said or implied the SS prop is better for performance. Don’t see how.
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Old 08-16-2020, 01:03 PM   #10
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Every source I've come across says SS will add one or two knots to performance. The blades are thinner, more rigid with less flex. When going slower (my average cruising speed of 7 knots) I assumed that would mean the engine was turning at a slightly slower RPM than with an aluminum prop.
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Old 08-16-2020, 02:41 PM   #11
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SS is definitely more efficient, for the reasons you stated, for a boat on plane. That's been our experience switching from aluminum to SS.

Can't swear to it, but I'd be surprised if there were enough difference to notice at 7 knots, with the prop turning at much lower RPM and pushing out much less power.

Our diesel powered 26-footer had aluminum duoprops. Seemed like the right solution in woody BC waters, in part because we traveled mostly at 6.5 knots after our first few years with that boat.
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Old 08-16-2020, 02:44 PM   #12
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I've trashed a stern drive using an Al prop after striking something. I've run SS props on BIIs and BIIIs with no strikes thus no issues. Bottom line, keep a really sharp eye out.
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Old 08-16-2020, 09:32 PM   #13
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At your boat weight, you need to consider a 4 blade versus a three blade prop. Having owned two C-Dory's, (22 and 25), I was generally very heavy leaving the dock for a week trip and the 4 blade (stainless in my case) did much better in fuel economy. The other thing to pay attention to is trim. Whatever prop you select, your engine should get to wide open throttle with the boat trimmed to highest rpm using lower unit trim and/or trim tabs if you have them, and the throttle up against the stops with the typical load on the boat.

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Old 08-16-2020, 09:45 PM   #14
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Originally Posted by rsn48 View Post
Every source I've come across says SS will add one or two knots to performance. The blades are thinner, more rigid with less flex. When going slower (my average cruising speed of 7 knots) I assumed that would mean the engine was turning at a slightly slower RPM than with an aluminum prop.
Do you ever intend to operate at planing speeds? That's a key question. If not then let's reset the discussion. Is this a < 10 knot boat, or a 15+ knot boat, or both?

I'm guessing since you're hanging here you might not be interested in going fast.
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Old 08-16-2020, 11:29 PM   #15
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I'm guessing since you're hanging here you might not be interested in going fast

I'm a go fast for the first half hour to cross the Strait of Georgia quickly going roughly 20 knots, then slow down to enjoy the view. The wind can blow up quickly along the Strait so I liked to get it out of the way quickly, same with returning, fast the last half hour. But generally futz around at 7 - 8 knots.

Last year on a friends Back Cove boat, we left Vancouver and for the first half hour it was WOT (23 knots), then cruised at 14 knots, we jumping over to Nanaimo and the forecast was for the wind to come up to 25 + knots in the early afternoon and we didn't want to be out there. When cruising at 14 knots we were headed into a 20 knot wind.

I like speed but for short periods of time, usually going much slower.
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Old 08-17-2020, 10:53 PM   #16
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I'm guessing since you're hanging here you might not be interested in going fast

I'm a go fast for the first half hour to cross the Strait of Georgia quickly going roughly 20 knots, then slow down to enjoy the view. The wind can blow up quickly along the Strait so I liked to get it out of the way quickly, same with returning, fast the last half hour. But generally futz around at 7 - 8 knots
Gotcha, thanks. In PNW the speed is really useful. I saw a reference to top speed of 35 knots from prop calc. Hard to put faith in that recommendation when you're mostly slow. But I'm out of my depth on this.
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Old 08-18-2020, 12:33 AM   #17
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It is nice that two or three easily reparable aluminum props may me bought for the price of one stainless steel prop. With the affordability of having multiple props in aluminum, each could be pitched for different speed missions, regardless of pitch. And there will always be a backup that could be used if needed. If a replacement prop is needed with an aluminum prop, due to a damaging strike, at least the drive train should be without damage. With the stainless steel prop strike scenario, an extra prop to get home may not be useful if the drive train is damaged. Getting home is always a safety consideration over better speed and economy.

Also, consider a single engine vs. dual engines situation. Single engine ops demand more ability to get home where two aluminum props may help protect the only drive train available. A second aluminum backup prop could make the difference to get home.

A twin engine operation will have better odds with stainless props due to having a second drive train already in place.
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Old 08-18-2020, 01:00 PM   #18
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Also, consider a single engine vs. dual engines situation. Single engine ops demand more ability to get home where two aluminum props may help protect the only drive train available. A second aluminum backup prop could make the difference to get home.

The odds of me getting the right prop the first time are really low, but I hope I'm wrong about that. It is very possible I have got the boat weight as too high by about a 1000 pounds. The guy I bought the boat from was not exactly "Mr. Reliable" when it came to info about the boat - and he was a nuclear physicist so I was inclined to believe everything he told me, what an idiot I was.

So he told me the boat didn't leak. But when I turned over one of the V birth cushions clearly water had stained and bleached a large area. I wasn't that concerned as I guessed it was either a window or stanchion leak - it was both. When I asked about the two fuel tanks, were they attached so that they refilled together and fuel would flow from either without having to turn a tank on and off manually, he said they were both attached and I wouldn't need to fiddle with anything. When the boat was gutted, the engine and leg removed, fuel tanks removed and replaced with new ones, it was noted the two tanks weren't attached to each other and I would have had to throw switches to feed the engine.

Why am I telling you all this? Well in the area my boat is moored (Comox BC) there are no cranes with straps that I can use to weigh my boat. And there is no truck weigh station in this area, the closest is an hour drive away. And Mr. Nuclear Physicist told me the boat weight 5.5 ton. So where did he get this figure? How reliable is it? All a great mystery as he has moved out of the area so I can't discuss the boat weight with him.

And I have added about 1200 pounds to the boat, bow thruster, two additional batteries, a freezer (Norcold 3.5 cubic feet jobbie), and dinghy with motor and davits along with some drawers in spaces that were just hollow nothing. I believe my weight guestimate is fairly accurate, it is possible it is high.

All this to say, I hope I only need to buy one more prop after the current prop, the current one being the back up. But as for extra propulsion if I have motor/prop problems, the dinghy side tied to the boat will get me home.
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Old 08-18-2020, 01:09 PM   #19
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If your first try at a prop is at least close, it should be possible to get the second try to be plenty good enough to be happy with it. It may not be perfect, but as long as it loads the engine acceptably and the boat performs well, it's good. Finding that last little bit of gain might take a lot more work and experimenting.
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Old 08-19-2020, 12:29 PM   #20
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The prop selector has suggested a RH 18.75 D 17 P. The boat is either 28 feet or 28 1/2 feet and using a very vague guestimation (older boat with wood/glass hull and wood upper and lots of new heavier goodies added) the weight was pegged at 14,656 pounds, WOT 5200 (350 HP) and calculated boat speed of 34.8 knots.

I am no expert but I would use al with a spare backup...
But I question one 350hp engine will get a 14,500lb boat to 34.8knots or 40mph at 5200 rpm?
I must be doing something wrong with my boat, twin 230hp small blocks 33' gets me to 25mph at 15,000lb. Granted I am slightly over propped and only turn 4000 wot.
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