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thedeadsea

Stainless Prop: 3 vs. 4 blade

18 posts in this topic

I'm going to drop the pitch of my prop and am looking around for a new prop. Currently have a 3 blade but wondering if you pick up any additional, noticeable benefits by going to a 4 blade prop? The motor is a 150 hp Yamaha 2-stroke pushing a 19' Crestliner.

To achieve an additional 300 RPM at WOT, should I drop one or two inches in pitch?

Any brands that are better or worse? I have a Yamaha prop on currently.

Thanks,

Steve

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I presume, since you're trying to get an additional 300 rpm at WOT that you know where your max rpm is...

Generally you'll get more hole shot with a 4 blade... are you running a stainless now, or considering it?

I'd also touch base with Jay Soderbloom at Soderblooms and see what he has for recommendations.

Most often a three blade is enough to do it, and if your RPM numbers are good dropping 1" pitch of prop will put you 100 rpm short and 2" pitch would put your 100 rpm too much... so you may have to experiment.

marine_man

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Thanks Marine Man!

I already have the original stainless prop on that came with the boat but it has seen some use and I figured I would get a new prop that had a little lower pitch and keep this one for a spare.

I checked some sources to determine what max rpm should be vs what I was getting. Probably drop one size as I would rather rev a little low than high.

I will have to get in touch with Jay, it works out nice that he is located so close.

Steve

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What brand motor and hp do you have? Also, what year?

I can get you the rpm specs.

marine_man

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I don't know about your motor, but for my 175 Opti I was running a 21 pitch Tempest (3 blade) for a while and replaced it with a 19 pitch Rev 4 (4 blade). I picked up some rpms over the 21 pitch Tempest but not as many rpms as I got when I tried a 19 pitch Tempest. But for my boat the Rev 4 was clearly the best for overall performance - hole shot, top speed, and especially handing a heavy load in the boat.

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92' Yamaha 150 Pro-V.

Also do you know what the max RPM is for 2000' 4.3 Chevy on a Mercruiser Alpha One?

I'm heading out for a fishing trip later today so won't be on here for a few days.

Steve

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5500 RPM at top end on the Yamaha.

4800 RPM should be correct for the Mercruiser... did you check on the carb cover to see if it's stamped there?

marine_man

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Thanks.

Best I can get with the Yamaha is 5200 and an empty boat, less if I have other adults.

Running 4900 on the Mercruiser (with just me) so that's spot on. I didn't check the cover.

Steve

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Also, on the walleye boats in the early to late 90's I think the dealers would often mount them too low. On our tp175 with 150efi, we raised the motor to the middle hole, got a good SS prop after much testing, and it made a big difference.

Each SS prop model and pitch makes a big difference in how the boat handles and how the motor runs, and you just need to try out a bunch before deciding what is best for your use.

Good luck.

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I think it is a 21' but can't remember. Will have to check later.

Will raising the motor up one "hole" make a difference in speed and handling? No complaints in that department but certainly something to consider if things can be made even better.

Steve

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It depends... which hole is it on now? I think most people call it the "first" hole when it is all the low as it can go. If it is on first hole, I would say you can go one hole up for sure, maybe two. It usually does help to have it mounted as high as it can be, but still having water pressure and not blowing out in corners. Also, the better prop you have generally the higher you can mount it, as long as it is still getting "bite".

Less lower unit in the water means less drag. Also able to get more bow out of the water, usually, againt, less drag.

This alone may get you more rpms out of your current prop. Dealers charge about $50-$75 to raise a motor. A 150 is pretty heavy... Last motor we did ourselves was a 90hp, and after that one, we said all bigger motors go to a dealer to get rasied smile

Good luck.

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I'll have to check that as well when I get home. I would use an engine crane if I did. I have installed 4 strokes up to 200 hp when I worked for a marine dealer in college so it would be an easy project.

The dealer we bought it from was more of a "fishing boat" place than a performance boat place. I have found that while many marine dealers can fix things in their sleep, they really don't pay attention to the details that can greatly change performance. That's why I like to ask performance based questions on here to get a better range of opinions.

This is the standard Yamaha SS prop that came with the boat. New we could get 54 mph out of the boat (never paid attention to the tach), now it gets 50-51 (both on GPS and speedo) but isn't revving. Although not bent, the prop does have some minor dings, so if I raise the motor and get a 4 blade of the same pitch, I may be just fine as well. Probably will try raising the motor first. I'll post up later what I find about the location of the holes.

Thanks,

Steve

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Steve,

Here are a couple of things to consider. When your rig was new, you could get about 54 mph on the speedometer at 5500 rpm, right at the motor's sweet spot. Now you only get about 50 mph and RPM has dropped by 300. Ask yourself what has changed. I bet it is the "dings' on the prop that you mentioned. Damage to the prop can greatly affect both speed and rpm. Another thing to watch is that if you switch to a 4 blade prop, you will probably have to drop a little in pitch for the motor to turn it.

Second, try taking a couple of measurements to see where the motor is at. With the boat doing upwards of 50 mph, you are getting to a set of parameters most "fishing boat" setup guys don't understand too well. As mentioned above, you want to reduce drag, and the first step is to raise the motor if that makes sense. Try having a helper put a straightedge on the lowermost part of the hull, and measure the distance up to the cavitation plate and down to the propshaft. Post those dimensions and we can see if raising the motor makes sense.

One caution when raising the motor, if you do not have a water pressure gauge, be sure to watch the temp carefully while testing. If you get the motor too high, the pickup can pull air and you lose water pressure.

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The prop is a 21 pitch. It is marked 21-M. Does the M have any significance?

The motor is all the way down on the transom and bolted through the top of 4 holes(so that the saddle of the transom bracket is touching the transom) and I would say the cavitation plate is a good 3" above the bottom of the boat. I didn't have a ruler or straight edge to get the exact measurement to the propshaft, but can get that if you need it.

Steve

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Steve,

If your cavitation plate is 3" above the bottom your propshaft would be about 5 1/2" below, assuming the gearcase is about the same size as my Merc 200 (probably so, since you can swap props between the two motors). With the motor at that height, your water pickups are plenty deep and you certainly could try raising it one hole. Here is what to watch for: Check your steering torque before and after raising; it should be a little less with the motor higher (good thing). At WOT you should see your RPM's come up a bit as well. With the motor trimmed neutral do a hole shot to plane. Does it ventilate or "blow out"? Try it again with the motor trimmed all the way in- is it better? If your hole shot still works without much ventilation, try a couple of hard turns, both left and right at about 45 mph. Does it ventilate anywhere into or out of the turn? If you notice significant blow out in either hole shot or the turns your motor is too high for your prop. If you have ventilation problems you can go two ways here, either lower the motor back down, or look at a prop designed to run higher (either way you need to get that prop repaired!). Typically they will have more cup in the trailing edge to hold the bite on the water. I went through this process when I got my boat. It had the motor set at 5 1/2 " below the pad and at WOT it did mid 50's with awful steering torque. Through raising the motor and tuning the PVS holes in my prop I now have it running in the mid 60's with very mild steering torque. All with no prop change. My boat is very different than yours but the ideas should transfer to your application.

I run a Merc Tempest Plus 23" 3 blade on my Coral at 3 1/4" below the pad. It has tuneable PVS ventilation holes to allow it to spin up on the hole shot and the cupped blades hold very well at speed. A 21" pitch Tempest Plus would fit on your Yamaha although you may have to change the hub. Here is my boat:

Image008.jpg

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I'll try raising it one hole in a few weeks when I get it back to the cabin. The steering torque right now is pretty extreme when the motor is trimmed all of the way down, so if this could be improved that would be great.

Thanks much,

Steve

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