Which pitch 4 blade should I get?

[watered down]

Ok, got the boat out today and did some quick WOT runs. I currenly have a 14 1/2 x 19 aluminum prop. Top speed on speedo was 56mph, at 5200rpms. the recommended WOT range for my volvo 5.0L GXI is 4600-5000 rpm. Thus, I'm running about 200 rpms too high. <br /><br />I would like to go to a 4 blade prop. However, I'm only finding 18 or 20 pitch...no 19. Should I drop the pitch down to a 18 on the 4 blade? I would like to remain on plane at a slower speeds, and like more out of the hole<br /><br />So....what do you think?????

 

[TilliamWe]

Re: Which pitch 4 blade should I get?

First, get trailer boats magazine's July 2005 issue. <br />Were those runs with your normal load aboard? If you normally run full of gas, people, and coolers, test it that way first.<br />If your test was done the way you use the boat, you'll want the 20 inch. Rules of thumb:<br />1" increase in pitch will lower WOT RPM by 100-200. Add a blade decreases WOT RPM 100-200. Aluminum to Stainless Steel decreases WOT RPM by about 100.<br />You still will not know excatly, until you try out a specific prop. And for some reason, most 4 blades seem to be even numbered pitch. But there are odds out there.

 

[gatorred]

Re: Which pitch 4 blade should I get?

you go up in pitch to lower top rpm try 21

 

[stevens]

Re: Which pitch 4 blade should I get?

TilliamWe is right. In a four-blade, you should probably try a 20" first.<br /><br />They usually say to drop an inch of pitch when going from 3 to 4 blades, hence the practice of three-blades in odd and four-blades in even numbered pitches.<br /><br />There's a sale on 4-blade Stilettos at this link.

 

[walleyehed]

Re: Which pitch 4 blade should I get?

Let's take into account this appears to be the WOT with just the operator...200RPM over max is actually perfect if you're by yourself.<br />Remember, you want to be at or near the top of the RPM range WITH a LOAD.<br />I'd expect a serious drop in RPM the first time you put 3 or 4 people in the boat, if you go Up in pitch...I'd be looking the other way .

 

[stevens]

Re: Which pitch 4 blade should I get?

Walleyehed - good point! So he should go back out and check the RPM with an average load on board (i.e. people, fuel, gear) before deciding anything. WateredDown - listen to Walleyehed here, he knows.

 

[TilliamWe]

Re: Which pitch 4 blade should I get?

Or he could listen to me, because his whole question was answered in the first reply, mine!<br />Of course it was the same answer I gave you Stevens! Plus, he's on another site, and someone copied the trailerboats article & sent it to him. So he's all set.<br />How's your experimenting going Stevens?

 

[stevens]

Re: Which pitch 4 blade should I get?

So you were both right on that one. The guy is getting some good advise here!! <br /><br />What other board is he on? - you mean, there are others? And, somebody sent him a copy of the article even? Wow.<br /><br />My experimenting is done with for the year, the boat is on land, and I am having abstinence pains. I have one Vengeance propeller waiting to be tried out and believe it will work out OK.<br /><br />Am eagerly waiting to hear about Walleyehed's most recent testing!

 

[walleyehed]

Re: Which pitch 4 blade should I get?

His question was not answered in the first reply....you aren't taking into account the 200 per inch rule does not even come close when going from 3 to 4 blades, and then comes the blade design, and as I said, I would take the 18 over the 20 and 4 blades ARE available in odd numbers too.

 

[LubeDude]

Re: Which pitch 4 blade should I get?

Gee Walleyehed, I guess Tilliamwe dosent know who he is talking to. <br /><br />I guess the fact that you design props doesnt mean anything. <br /><br />But you are being pleasant though. <br /><br />He seems pretty antament about being right, maybe you should just let him have his day in the spotlight!

 

[walleyehed]

Re: Which pitch 4 blade should I get?

Good point, LD...I'll lay off.<br />Some days it's just like being at the shop...<br />I forget where I am...<br />I'm outa here............................<br /><br />EDIT; After looking at this again, I owe you an apology, TilliamWe...Not a good excuse, but I've been working with 3 manufacturers on testing some prototype props and I've had to run several other brands for comparison, and due to one of the manufacturers throwing in another line of props, I basically had to start over(with 51 props) to get accurate data during the same atmospheric conditions. I guess I've gotten "too" particular about a standard set-up, as I now consider success in tenths of a mile per hour.<br />I've gotten a bit testy, and that was out of line, I'm sorry.<br />Kenny

 

[TilliamWe]

Re: Which pitch 4 blade should I get?

First, the original poster also posts at BoatingABC.<br />Second, just as you lubedude want credit for all oil questions, yes I wanted credit for answering his question with the best information available. And without being a prop designer tester, the answer I gave covered all the basics, which is all the poster is gonna learn this time around. He doesn't need to design props like Walleye to get the right one, he needs to test in "real world" conditions, and experiment a little on what's offered.<br />Walleye, I respect you obvious techinal and theoretical knowledge. In this case, the original question left us some blanks. You assumed one thing (that he tested alone) and based your sugesstion on that. Not a bad assumption, either. What I did was ask him if that's what he did, and if that was the way the boat was to be used. I tried to get the poster to provide us with Exact info, so that if you or anyone else had the same Exact setup, they could share their Exact experiences with him. As it is, we are left with generalities, and I gave a "general" answer. And a good one too. <br />Third, you owe me no apology. I understand your perspective completely. You have even more knowledge of specifics that you want to impart. So all we have is a different starting point. You wanted to give specifics, I offered generalities.<br />In my opnion, my generalities will work, until the guy needs specifics.

 

[walleyehed]

Re: Which pitch 4 blade should I get?

OK, all aside..what does the article in trailor boats have to do with his boat???

 

[TilliamWe]

Re: Which pitch 4 blade should I get?

it gives good general info. It lists props by manufacturers. It also provides the diameter/pitch, number of blades, material, "type of application, and list price for the props. Of course the prop listings are also on their website.

 

[watered down]

Re: Which pitch 4 blade should I get?

Ok, thanks for all the info. I am over 20hrs on the boat now so I feel comfortable opening her up more. My previous run at WOT was only with two adults and about 3/4 tank of gas.<br /><br />Had the normal load on her this past weekend...2 adults, 2 kids, gear and almost full tank of gas. WOT was exactly at 5000 rpm, but the speed of course was less. Again, top speed isn't as important as hole shot for water sports, slower plane speed and handling.<br /><br />I plan on staying with an aluminum due to where I boat and cost. <br /><br />So, would going to the 4-blade aluminum 14x20 decrease my top rpms to 4800 or is there more of a difference when you go from 3 to 4-blade?<br /><br />My problem is that there are no prop shops around here that I can find so I'm going to purchase online.

 

[walleyehed]

Re: Which pitch 4 blade should I get?

As I said in my previous posts, I would be looking at lower pitch in 4-bld...the 18.<br />The 20 will drop your RPM (likely) to around 4700RPM...wrong side of the range. For best engine life, best fuel economy and best performance for what you want to do, the 18P 4-blade would be very, very close in my opinion.<br />See what others will recommend as well...there may be some other factors you may want to consider...

 

[watered down]

Re: Which pitch 4 blade should I get?

Thanks walleyehed, even after reading the article I'm still know very little about this. So you don't think I will run over recommended rpms with the 18? <br /><br />Do you know of anyone who makes a 19 pitch 4 blade? I assume that would be the best since I'm right at the max rpms with the normal load.

 

[walleyehed]

Re: Which pitch 4 blade should I get?

When you go from a 3 blade to a 4 blade in the same pitch, you can expect anywhere from 150-300RPM drop depending on the prop style..each manufacture has it's own features, so in most cases, it will vary..there is no formula to tell you which manufacturer prop you need.<br />You'll find a 19 in SS by a few manufacturers, but that will drop it another 100-150+ a drop of 100-300 going to a 4 blade. So...if you go with a 19 in SS, you will be around 4600-4700, and if you would happen to find an aluminum 4 blade, you'll be looking at 4750-4850..still too low.<br />You need to understand 5000 is perfect for what you are doing. If you are by yourself, don't run it much over 5100-5200, or, if it makes you feel better, keep it at 5000 when you're alone.<br />The 18 4 blade has no reference to an 18 3 blade, as the pitch numbers are more theory than actual.

 

[watered down]

Re: Which pitch 4 blade should I get?

Thanks, I think I'm getting it now. <br /><br />When looking at Caravelles, the dealer showed me a new 4 blade, thru-hub exhaust aluminum prop he just got in stock. My 3 blade is not thru-hub. If I went to the dealer and got that prop, would the thru-hub exhaust change your figures or thinking on pitch?

 

[walleyehed]

Re: Which pitch 4 blade should I get?

Is this a Volvo SX drive??? It isn't, is it...