a more efficient prop? or just be happy?

[cleve]

boat-16' alum. grumman(semi-v).approx.600lbs.max hp 50
motor-yamaha 2s 60(2004).228lbs. 2.33 gear.
prop-aluminum oem 13x19
wot-55-5600 rpms, 34-36mph(gps)
av plate is @ 1" above keel(can't go any higher or it vents)

The boat performs ok, but i would like a few more mph's. i was curious about a ss prop i saw on powertech's website. it's designed for low cubic inches, big gear (bigfoot) engines,which is what i have. they have smaller diameters,12.75" i think. i tried a stiletto 21 pitch and mph was 39 but rpms fell to 4500 which is unacceptable but that shows how aggressive that prop was. i figured my prop slip to be around 15%(if i figured right?). all of these numbers are with 2 people,2 bateries,and an otherwise avg. load in glass or light chop,no current conditions. Questions: i'm i getting about all i can get or is there a better prop for me that will at least not decrease rpms and gain a couple mph.

 

[steelespike]

Re: a more efficient prop? or just be happy?

I'm not a pro but it seems to me that something like the stiletto, because you have test specs from the 21, in a 19 would be better than the 21.As you said an agressive prop;doesn't make sense to increase pitch when going agressive.
If the 19 is still too much I have seen props respond really well after removing some of the cup.

 

[cleve]

Re: a more efficient prop? or just be happy?

i was thinking at the time adding 2" of pitch would still give me 5200 or so rpms. i did not take into account the cupping. i'm thinking now i should have went with a 17. the yamaha aluminum i have now looks more like a semi cleaver and has no cup at all. i've seen it referred to as a stern lifter.

 

[hwsiii]

Re: a more efficient prop? or just be happy?

cleve, You might try reading this for a better understanding of changing props for better performance, and for the major differences between aluminum props and stainless steel.

http://forums.iboats.com/showthread.php?t=369057

You have a very lightweight boat, BUT you have a big foot lower unit on the motor, and this tends to create a lottt of drag on your speed. An easy way to cure this, to some degree, is to raise the motor UP on the transom, and with the right prop you can move the motor up and gain some RPM and speed, the prop you have now is very limited in design elements and is showing 18% prop slip.

After you read the above mentioned article if you have any more questions I will be glad to answer them.



H

 

[cleve]

Re: a more efficient prop? or just be happy?

very informative post,hwsiii. i'm reaching my rpm's but i'm not efficient. i can't raise motor anymore with the prop i have and i havn't looked to see where my av plate is while on plane. the guy i bought the motor from had it raised to the 4th hole on a flatbottom carolina skiff and he was turning 6000 rpms and 37 mph with a 17 pitch yamaha aluminum and it did not vent. i'm assuming because of the boat's design. if i lowered prop slip by getting a more efficient prop and try to get 5500-6000 rpm's, would i see more speed? i know that there are no 'magic' props out there that will give me 5 mph, but in your opinion, what are a couple of props that you think would help. i did raise motor 1 more hole with the stiletto with no venting. i'm also thinking a 17 or maybe even a 15 if it's an 'agressive' prop, i guess i don't know much about the different blade geometries to choose from. thanks for the reply, you always give good advice. i've owned several boats over the years and never really understood the prop. i always thought as long as you were turning max rpms you were at your best performance.

 

[hwsiii]

Re: a more efficient prop? or just be happy?

Cleve, in all props the higher the pitch is in any particular series or model of prop the higher the top end efficiency is for that particular series. and when we lower the pitch by 4" it goes down a good bit, but by being able to turn the lower pitch prop at much higher RPM we gain almost all of that back.

Spike has the right theory, as always, even though he says he is not a pro, he has great gut instincts, whereas I have to do it mathematically.

Prop Slip

The key to this will be being able to lower the pitch on the Stiletto and being able to raise the motor high enough on the transom to lower the drag on the big foot drive, that drive unit is actually designed to be used on large heavy boats with large diameter props and NOT light fast boats, that is why the gear ratio is so high, but it certainly creates excessive drag in the water on faster boats.

I show the maximum manufacturers recommended RPM should be, 5,500 RPM, please verify this.

If you will buy the Stiletto prop in a 17" pitch and we can raise the motor up TWO notches on the transom I believe there is a possibility to get to 40 MPH. In my prop slip analysis I only show 38.5, because if we can only get to raise the motor one notch then that is about how fast you can go. That is not an inexpensive prop to gain just a few miles an hour, but that is your decision, and these are my opinions.

I don't know where you got the first Stiletto from, but if you can't get the Stiletto in a 17" pitch with the right to exchange it for a small fee, let me know and I will put you in touch with someone who will, and they will take care of you until you get the right prop.



H

 

[cleve]

Re: a more efficient prop? or just be happy?

yes the max rpm is 5500. i bought the stiletto advantage here from iboats last year when it was on sale for @ $220 and free shipping. i later sold it and got my money back. i will look for a 17 and try to find another deal like that one, though probably not that good.another question. if the 21 turned 4500 rpm's and for every inch down it gains @ 100 more wouldn't that put me @5000 or will raising motor 2 more notches(if i can) get me closer to 5500? would i be okay turning 5000-5100 at wot or should i be a little closer to 5500?

 

[cleve]

Re: a more efficient prop? or just be happy?

disregard those last 2 questions, it's @200 rpms per inch which should put me around 5300 -5400.

 

[hwsiii]

Re: a more efficient prop? or just be happy?

cleve, you are learning.



H

 

[hwsiii]

Re: a more efficient prop? or just be happy?

Please let me know the new numbers when you replace the prop, for my database.



H

 

[cleve]

Re: a more efficient prop? or just be happy?

hwsiii,i checked ebay and craigslist and found nothing. the best deal i found online was at boatownersworld.com. stiletto advantage I 17 is $239,free shipping and a turbo I 17 is $259 free shipping and you can exchange turbos for $40 plus freight,but not the stiletto. the stiletto on iboats is $300,free shipping. the marine dealer where i live carries yamaha and powertech and he will let me exchange props under the impression that i end up buying one from him. a yamaha is out of the question because they are very expensive, $400 and up. should i try a powertech or go with the stiletto or turbo?

 

[cleve]

Re: a more efficient prop? or just be happy?

scratch the powertech. i went by there today to buy another bilge pump and look at the props. they only carry aluminum yamaha and powertech stainless which are $399 and he said they were designed similar to stiletto,turbo and rapture.

 

[hwsiii]

Re: a more efficient prop? or just be happy?

cleve, they are both great props, but I believe the Turbo1 is better for your particular circumstances and uses. And I am sure you will be able to raise it two notches on the transom, so I would go with it. There are probably better props for your boat, but like you said, they are very pricey for the little bit of gains that you will receive.



H

 

[cleve]

Re: a more efficient prop? or just be happy?

i'm probably gonna order the turbo I 17. if it's not right at least i can exchange it. thanks for your help and i will post my results, but it may be a few weeks.

 

[hwsiii]

Re: a more efficient prop? or just be happy?

Thank you very much cleve.



H