do I need a different prop?

[pheasanthunter]

I just bought my first boat in January. I took it out for the first time last weekend and it seems very sluggish out of the hole. Plan to use it as a fishing and skiing boat. I have a 2008 Tahoe 216 deck boat W/ 270 h.p. 5.0 litre stern drive. My prop is stamped w/ 48 16318 21p. I'm guessing this means 21 pitch??? If I remember correctly it would top out at about 40 mph at 4000 rpm. I live in Denver at 5280' form sea level. What would I do to determine if I have the right prop and how do I get a better hole shot.

 

[jopes]

Re: do I need a different prop?

your vavle cover should have your engines operating rpms on it. My guess is your a little low in the rpms, and need to go down in pitch to increase rpm's.

http://boatpropellers.iboats.com/propeller-tutorials/rpm/?**********=586612089&*******=367508071

 

[pheasanthunter]

Re: do I need a different prop?

Thanks for the help. So if WOT rpm is 4600-5000 should I go to a 17??

 

[pvmtsux]

Re: do I need a different prop?

17p seems a little small. I have a 20' regal deck boat (heavy) and run a 19p works pretty well for overall performace and pulling inflatables and skiers. I'm right about 4800 RPMs WOT.

 

[metriccrescentwrench]

Re: do I need a different prop?

. 17p seems a little small.......I'm right about 4800 RPMs WOT.

You've also got to factor in altitude. At over 5000 ft that 270 HP engine is putting out a lot less than 270 HP.

 

[1980Coronado]

Re: do I need a different prop?

You've also got to factor in altitude. At over 5000 ft that 270 HP engine is putting out a lot less than 270 HP.

Agree 100%...but wouldn't the altitude already be factored in since he's reported his actual max WOT rpm at that altitude? He's at 4000 but needs to get closer to 4800 rpm at altitude. Wouldn't you be able to use the 2" in pitch is 300-400 rpm rule of thumb and go to a 17" pitch and gain roughly 600-800 rpm...that is as long as the diameter, material, type, or mfg of prop doesn't change? Or am I not thinking this through?

A 4" pitch is a big change I know...but using the rule of thumb maybe it's not so crazy considering he needs to pick up 800 rpm to get into the middle of his WOT range.

 

[75TowerOfPower]

Re: do I need a different prop?

What's the ratio on your outdrive? 1.50 or 2.62? This will help determine if the motor is sluggish or too much prop. morethan likely a 17 with a 4th blade would be great for you since it will throw you on to plane and still push a full boat of people around.

you guys do water sports any? tubing, skiing, or wakeboarding?

So you have the 5.0 EFI Merc?

 

[steelespike]

Re: do I need a different prop?

Tried to find the boat in NADA for weight;only found pontoons.?Is it a deck boat( with a hull)
or a pontoon?
Usually you would shoot for the max rated rpm 5,000 that would require about a 5" reduction in pitch. 16" seems extreme to me.Unless it is a pontoon.While a 21 might not be ideal for hole shot I think it should create more rpm and speed.Before a prop change be sure the motor is in excellent tune,the throttle opens all the way and the bottom is clean and smooth.Do you trim up for best speed?

 

[1980Coronado]

Re: do I need a different prop?

Tried to find the boat in NADA for weight;only found pontoons.?Is it a deck boat( with a hull)
or a pontoon?
Usually you would shoot for the max rated rpm 5,000 that would require about a 5" reduction in pitch. 16" seems extreme to me.Unless it is a pontoon.While a 21 might not be ideal for hole shot I think it should create more rpm and speed.Before a prop change be sure the motor is in excellent tune,the throttle opens all the way and the bottom is clean and smooth.Do you trim up for best speed?

Google the 2008 Tahoe 216....It is a 21' deck boat...not a toon...don't know why NADA doesn't list it...Tahoe Pontoons is all I saw too. However, I notice that these boats came standard with a 4.3L 190hp....I couldn't find the 5.0L 270 hp option....Hmmmm...I wonder?

 

[pheasanthunter]

Re: do I need a different prop?

O.K. my outdrive ratio is 1:65. The motor is tuned up and runs great. The boat will be used alot for tubing and water skiing. I also just put 3 batteries and 3 bank charger in the back next to the motor as close to the center as possible to run a 36 volt terrova for fishing. the boat weighs 3400 lbs stock. I do deep water starts on a slalom ski and I need a decent hole shot. The 5.0 was an option in 2008. It's not available any more. The current prop is a Vengeance 3 blade stainless.

 

[1980Coronado]

Re: do I need a different prop?

I think this is something you're going to have to experiment with. It would not be unreasonable for you to have to reduce the pitch of a prop by 3" or 4" at that altitude to regain the rpm lost due to the reduction in Hp at altitude. You could also try a reduction in diameter in combination with a reduction in pitch, both will increase rpm, but you wouldn't reduce the pitch by 4" as well as reduce the diameter by 1". If you do decide to reduce the diameter, you wouldn't need to reduce the pitch as much.

The bottom line here is that obviously the boat is over propped for the conditions. The rules of thumb say you need to reduce pitch by 4" to gain 800 rpm +/-....it's not an exact science. If it were me, I'd probably drop down to a 17" pitch in the same prop diameter (if available) and test it. It will definitely, increase your rpm and get you up on plane more quickly.....you just need to test it to see where the max WOT rpm increases to. IMO, if you get close to the 5000 without going over, call it good.

 

[pheasanthunter]

Re: do I need a different prop?

So i'm guessing is if I find that 17" works for Colorado, when we go to Minnesota in the summer I am going to want to put the old prop back on. This boat was originally from California which kind of explains the prop that is on it now. Also, would a four blade prop be better. What is the Advantage?

 

[1980Coronado]

Re: do I need a different prop?

So i'm guessing is if I find that 17" works for Colorado, when we go to Minnesota in the summer I am going to want to put the old prop back on. This boat was originally from California which kind of explains the prop that is on it now. Also, would a four blade prop be better. What is the Advantage?

Yes....you would need to reprop when you come back closer to sea level. I'd keep that prop if that's what you intend to do. Don't get rid of it until you try it at a lower altitude and determine it's performance.

Well....that's one of those great debates .....I'm looking into this 3 vs 4 blade for my inboard right now....I have to change the prop because the PO had the pitch reduced and the diameter reduced...I believe he hit something, bent the shaft, damaged the prop and had to rework it....then sold the boat with a bent shaft through dealer. I fixed the shaft, but the prop allows my 454 to over rev at WOT......So anyway back to your question.....I discussed this very issue with Eric Johnson at OJ propeller....(OEM of inboard props)....He said there would be no difference in rpm between the 3 and the 4 blade, but the 4 blade would run smoother and provide a better hole shot.....now...being an engineer, I said that's not possible because you increased the blade area by 25%....rpm has to be affected with the increased load. He then told me they reduce the diameter of the 4 blade to compensate, and they cup them differently. So...I'm assuming that the I/O prop OEM's are doing similar things. The theory is that a 4-blade will improve hole-shot and generally perform better on the low end w/o affecting WOT rpm....I'm still skeptical about it not affecting WOT, but may try both on my boat to prove it to myself. Many people use the 4 blade for wakeboard boats because they "perform better" under load in the low end. I've never run a 4 blade on my old 16' 1968 Starcraft 140 hp Merc. My guess is, you're not going to notice a huge difference between the 3 blade or 4 blade if it's properly sized. Some guys will swear there's magic in the 4-blade...again I'm skeptical until I see it myself.

 

[pheasanthunter]

Re: do I need a different prop?

Thanks alot for all your help.

 

[1980Coronado]

Re: do I need a different prop?

Thanks alot for all your help.

No problem....post back with what you tried and how it worked!

 

[Tail_Gunner]

Re: do I need a different prop?

Lets look at like this..

1.65 gearing... 21p prop...4900rpm....theorectical speed 60 0% slip....12%...54........21' deck boat with a 270hp motor thats pretty strong. Now your actual numbers shows 17% slip....Thats not good either. Are those speeds gps speeds or boat speedo....Two have your tach checked all the way to 4000...spend a very short time at 4000


Or Heres a WAG 18p 4blade SS Solas that should net you 45mph and a nice hole shot

 

[pheasanthunter]

Re: do I need a different prop?

This is all very helpful.. So here is my understanding... A four blade should give me a better hole shot for towing a skier. Sounds like I should try a 17 or 18 pitch...A four blade stainless prop is almost $400 so if I don't get the right one on the first try I will have to buy another so now I'm into it for $800... So should I try a 3 blade for half the money?? then if I have to go up or down a size I'm out half the money. I guess i'm trying to avoid just buying props to find the one I need, or will boat shops take them back if you try it and it's the wrong one. My wife will not understand if I have three or four props hanging on the shop wall because they were the wrong pitch..after buying a boat and several grand worth of extras just to use it...

 

[1980Coronado]

Re: do I need a different prop?

This is all very helpful.. So here is my understanding... A four blade should give me a better hole shot for towing a skier. Sounds like I should try a 17 or 18 pitch...A four blade stainless prop is almost $400 so if I don't get the right one on the first try I will have to buy another so now I'm into it for $800... So should I try a 3 blade for half the money?? then if I have to go up or down a size I'm out half the money. I guess i'm trying to avoid just buying props to find the one I need, or will boat shops take them back if you try it and it's the wrong one. My wife will not understand if I have three or four props hanging on the shop wall because they were the wrong pitch..after buying a boat and several grand worth of extras just to use it...

I've had the same issue.....I don't know what iboats prop guarantee is.....some have a satisfaction guarantee were you have 30 days to try and send back if you don't like it.....some, if it's installed on a boat it's yours......so you just have to check the policy where you buy....ask the question. Do you have a local marina/shop that might let you try a couple?

 

[Tail_Gunner]

Re: do I need a different prop?

This is all very helpful.. So here is my understanding... A four blade should give me a better hole shot for towing a skier. Sounds like I should try a 17 or 18 pitch...A four blade stainless prop is almost $400 so if I don't get the right one on the first try I will have to buy another so now I'm into it for $800... So should I try a 3 blade for half the money?? then if I have to go up or down a size I'm out half the money. I guess i'm trying to avoid just buying props to find the one I need, or will boat shops take them back if you try it and it's the wrong one. My wife will not understand if I have three or four props hanging on the shop wall because they were the wrong pitch..after buying a boat and several grand worth of extras just to use it...

Go with a 18p alum.... Again Solas mke a great prop in alum...about a 100 andyouhave a good back up it works

 

[pheasanthunter]

Re: do I need a different prop?

O.K. gentlemen, I bought a used 17p four blade stainless at a local prop repair shop. I used the boat this weekend and it seems to exccelerate better out of the hole but I am still only getting 4200 rpm wot. That is with my wife and two teenage daughters and my self in the boat I'd say around 600 lbs. and have to trim the outdrive up a bit. When it is fully down, 4000 is still all I can get. Also added after first test was a Minn Kota trolling motor on bow, and three batteries and a charger in back. Guessing about 200 more pounds. Do you have any thoughts on these results.