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Old 06-18-2014, 03:39 PM   #1
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Exclamation Need prop help

I am doing my very best to try to figure out what props my boat originally came with and the best pitch and size for the boat, info: 1989 35 mediterranean, gas powered 454 crusaders. Current props 21x24 4 blade. 1.75 shaft. The boat usually has a heavy load on it but even with just my self, no water, and 1/2 tank of fuel it seems very sluggish. The engines and transmissions are great and are operating to spec. I know they originally came with 3 blades. My wot rpm is about 4200, from what I've read this is pretty close to normal for my set up. But 21x24 seem a little over kill and expensive for a new set. Just want to make sure if I'm gonna spend the money it's on somthing that's proven on the water and not from a prop calculator. Thanks for the help
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Old 06-19-2014, 09:33 PM   #2
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I see you haven't received any response from your well stated post.

I suggest getting as many specifics as you can. Start w the engine. A Crusader should be easy to find numbers on. What you need is the rpm your engine develops it's maximum power. Then a more difficult task of "tuning" your props so that at full throttle your engines attain the maximum power rpm.

The props can be bent slightly so they will assume a different pitch of 1 or 2". If your props have too much blade area preventing the engine from reaching the correct rpm the blades can be modified by grinding of some of the ends or edges ... or both. Since you have 3 blade props that's not very likely but I'm not familiar w your boat. There's a possibility (since you have 3blade props) that you have too much power for the blade area of your props. Four blade props would then emerge as a possible option.

Of course you can change the load on your engine by changing the gear ratio of your transmission and hence the speed of your props. Not an option you will be likely to do as the cost would be high.

Calling Michigan is a good option. They can be very helpful. Talking to a prop repair man probably would be helpful but they can be very biased and opinionated.
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Old 06-20-2014, 09:37 AM   #3
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I would not worry about the original props. What you want is to prop for the current weight and other conditions of the boat today. I don't know what type boat you have but a 35' most anything is not going to be a racehorse.

First you have to make sure the engines are producing rated power but if the WOT on both is the same my first guess would be they are OK or equally tired. Ethanol gas reduces power by as much as 10%. Just to be sure verify engine timing at 3000 RPM.

Next you need to make sure the bottom is clean. It doesn't take much growth to mess up prop efficiency or add bottom drag. Next make sure the props are actually pitched as you think they are supposed to be. Four blade props do provide added efficiency over three blades so that is probably why they were replaced. Some people purposely over prop thinking, incorrectly , that will add efficiency and reduce fuel consumption.

I don't know the specs of your engines off hand but 4200 may be a bit low. Check the engine specs.

Weight is a big factor influencing speed. Have you weighed the boat and compared that to the makers weight numbers? We all add a lot of weight to our boats that we don't even think about.

How is it sluggish? Does if go slower that it did in the past? Over propping would reduce initial acceleration.

Are you sure the tachometers and speed measurement are accurate?? A phototach is a good way to check.

A prop scan shop will computer scan your props for free and tell you if they are set properly. If you give them the current performance info they can give a good estimate of what any changes will do.

If this is a new to you boat my guess is that someone changed props and perhaps other stuff to increase gas mileage, It probably didn't work.
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Old 06-20-2014, 10:50 AM   #4
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Quote:
Originally Posted by bayview View Post
Weight is a big factor influencing speed. Have you weighed the boat and compared that to the makers weight numbers? We all add a lot of weight to our boats that we don't even think about.
That is something I think most people seldom take into account. Full time live aboard folks and distance cruisers naturally will have more stuff packed away, that's just a fact. What baffles me is the amount of pure crap some folks have aboard for weekend use. Talking to folks at the marina and reading posts on this and other forums simply amazes me.

Fair enough if your weekend trips are in Alaska(or some other equally remote location) but most folks are never more than 10 miles from a marina and have more crap stuffed away than they could ever hope to find, let alone use. I know a weekend warrior with 6 anchors and enough parts to rebuild both his engines and generator and enough tools to construct a dock to tie up his 35 foot boat. There's a fine line between being prepared and warehousing. Nothing wrong with either but if you use your boat as a warehouse don't be surprised if the top end ain't what it could be.

I recommend seeing if your boat needs to go on a diet before digging too deep into the problem Chris. An extra thousand pounds of stuff(not at all hard to accumulate) wont hurt the performance of a 40' long 7-9 knot boat much but it will really be felt in a 35' planing hull.
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Old 06-20-2014, 04:15 PM   #5
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Years ago I had a 34' Searay with twin 454's in it. Top loaded
RPM was 4600. The props on that boat were 18 x 18 if I recall.
I'm thinking that your way over propped. You should be able to "borrow" a set of used ones from Johney's in Detroit or any other prop repair shop.
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Old 06-20-2014, 10:29 PM   #6
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Propping

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Old 06-20-2014, 11:23 PM   #7
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Witchcraft????

I think just the opposite.
Pure science and engineering. Cut and dry stuff. Proper dia/pitch ratio, good blade area, spot on wot rpm and good prop tip clearance.
That's just about it.
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