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Old 08-19-2020, 12:37 PM   #21
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Originally Posted by RonR View Post
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.
It depends on hull design somewhat too. Prop selection does matter, as you pointed out. And a sterndrive is somewhat more efficient in theory than inboards. That said, my pair of 340hp big blocks will push my 26,000 lb 38 footer to about 27 kts on a good day.
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Old 08-19-2020, 12:50 PM   #22
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Earlier in the thread, it was pointed out that an aluminum prop is a good choice when hitting hard submerged items are possible. True. Particularly on outboard engines because the whole power unit will kick up with the full range of tilt that outboards have. With outdrives, I don't think that they kick up like outboards do. What this means is regardless of what prop type you have, if that lower unit/outdrive hits something hard, you're likely to have damage. Nothing you can do about it. But just realize that an aluminum prop won't be a gear case saver like it is on outboards.
I do think that aluminum props are a cheap way to find the right diameter and pitch. Once you do, then you can see how well that final prop holds up after a few months and decide if upgrading to SS is right for you. At least with aluminum, you can take a hammer and wood block to straighten the blade edges from minor hits. Hope this helps.
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Old 08-24-2020, 01:06 PM   #23
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Anyone have any experience or hearsay about the Sharrow props?
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Old 08-24-2020, 01:44 PM   #24
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But I question one 350hp engine will get a 14,500lb boat to 34.8knots or 40mph at 5200 rpm

I totally agree with this sentiment. I phoned Merc in Wisconsin when I was trying to figure out if I could go with a Bravo 3 instead of a Bravo 2, the point of speed came up. The guy I was talking to thought the boat would hit 30 +, actually the high 30's and I didn't believe him. The Bravo 3 was definitely not recommended and a number of sources recommended the Bravo 2, which is what I ultimately did.

Adding a second motor doesn't buy the mph performance one would assume, I think it is because of the extra weight but I know two friends when they have one of their diesels on, the boat will go about 6 knots slower than when the second diesel is added.

I also know that older gas motors were measured at the engine, hence my old 270 hp motor, severely over propped by the previous owner could go 12 knots WOT. But several motor retailer guys told me my 270 hp was really a 240 hp measured at the prop. So I have added 45% more hp with Merc claiming it has more torque (their newest non-automotive engine).

Anyways, tomorrow - Tuesday, 24 August - is the big day and I will know how fast it goes at 3900 RPM (not suppose to go to 5200 until hour ten of engine break in).
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Old 08-24-2020, 02:21 PM   #25
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Stainless vs Al

I had a 30' with Mercruiser MPI 7.4l Bravo 2. It came with an Al 3-blade prop. It performed pretty well but seemed slightly sluggish getting out of the hole. It also had a 4-blad stainless spare prop which I put on when I ate a log (or it ate me) with the Al prop.. The SS 4-blade made a noticeable difference in climb out performance, and maybe a little on the top end. I think this would also transfer into improved fuel economy at slower speeds. I did install a Garmin fuel flow meter interfaced to Garmin chartplotter which then told me flow rate, Mpg, fuel in tank,etc. but this was after going from Al to SS.

But, I also heard the story about Al transferring less shock to the drive train than SS, and that makes sense to me. When I sold the boat, it still had the SS in place.
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