Prop size?

[rndn]

My 96 Evinrude 130HP reaches about 4900 with two people on board my 96 Crestliner Phantom 2160. It's a 21 foot open bow aluminum boat and has a dry weight of 1910lbs with a 40 gallon fuel tank, four batteries, and some fishing gear on top of that. I plan to upgrade to a four blade SS prop and can only afford to buy one. The current prop is the original one from the dealer and is 14.25"x21" aluminum three blade cupped prop. The motor is positioned in the bottom hole yet I still get quite a bit of blowout going into turns and high speed. I'm hoping the four blade will help. Question is what size prop will get me to the correct rpm's, whatever that is. I know it's not an exact science, but you guys are a heck of a lot more knowledgeable than I am. Please also let me know what the RPM range is for my motor. Thanks in advance for you knowledge and assistance.

 

[steelespike]

Re: Prop size?

I'm surprised someone with more expierence hasn't jumped in here.I'm not expert but will try to get things rolling.You didn't mention your speed an important number in figureing where you are at now.A 4 blade will probably help your venting but all else being equal you will likely lose some rpm and top end.
Also usually going to stainless will drop your rpm so you have 2 things fighting raising rpm.Just going to a stainless 3 blade may solve venting
and you would need to drop at least to a 19 in pitch just to stay in the 4,900 ball park.You might have to go with a 17 if you want to get your rpm up.Sometimes a dealer will let you try props if you intend to buy.
Lets see if we can see any other reason for your venting.Where is the antivent plate in relation to the bottom of the boat.Has anything been added to the transom or bottom that might disrupt the flow around the motor?Is the prop in good shape?Just a couple of degrees too much trim could cause venting.
Keep in mind I'm just trying to get you started someone should be along with
more inlightened info and this should bump your thread up.

 

[rndn]

Re: Prop size?

Speed is about 40mph. The plate is about even or maybe 1/2 inch above the bottom of the boat. I'm not really interested in top speed of hole shot for that matter. With the 21" prop the boat launches out of the water even with a full load of passengers on board. I really just would like to reduce the blowout and maybe be able to lift the bow out of the water a little more without blowing out while reaching the desired rpm's.

 

[steelespike]

Re: Prop size?

Unfortunately I don't have max rpm info for that motor but a 89 120 & 140 hp is rated at 5,500.You will certainly be safe to shoot for at least that number.
You may want to verify your tach is accurate before making a final decision.
Now keeping in mind I'm not the pro If it was mine I would go with a 3 blade stainless with a 17in pitch.

 

[Ben Paquin]

Re: Prop size?

Speed is about 40mph. The plate is about even or maybe 1/2 inch above the bottom of the boat. I'm not really interested in top speed of hole shot for that matter. With the 21" prop the boat launches out of the water even with a full load of passengers on board. I really just would like to reduce the blowout and maybe be able to lift the bow out of the water a little more without blowing out while reaching the desired rpm's.

Trying a different prop with more cupping should definately reduce blowout. It did on mine. I raised my engine 1" and the factory 21 pitch yamaha had blowout even with the trim completely down. That was when turning and even on a hard acceleration. Changed to a turning point prop 19 pitch and no more blowout even with a fair amount of trim. I can trim much more now and have a small rooster tail at top speed trimmed out.

You might not gain much speed but I would go with no smaller than 19 pitch well cupped prop for what you are looking for.

 

[steelespike]

Re: Prop size?

Hello Ben, I'm glad to see someone else pick up on this thread.
I'm just wondering about only dropping down to a 19.He presently is running
4.900 rpm W a cupped Al.21 pitch;needs to be at 5,500 just to meet minimum specs.As I understand it going to stainless same specs will drop rpm about 100.It would seem to me 17 is closer. If I understand right that would
put him about 54 or 5,500.Still a little low but 15 seems extreme.What do you think.

 

[rndn]

Re: Prop size?

So a 19 or 17 pitch with cupping? I know props are all different, so what's your suggestions? Three blade or four blade? I assume a four blade would help lift the stern and I'm trying to lift the bow instead so I may want to stick with a three blade?

 

[Silvertip]

Re: Prop size?

In your original post you said you experience blow out "in turns" and "at high speeds". If it's blowing out at high speed and the 4900 RPM is not WOT, then that is not a valid number for determining a prop change. You really need to look around for a shop that will let you try a prop that's the same as you currently have so you can get a true WOT RPM run without blowing out. Two props of the same diameter and pitch but from different manufacturers rarely perform the same so simply using a different prop design may solve the proplem. I definitely would not expect a 15P prop to be a solution to your problem. Blow out in hard turns is a relatively common problem on boats with a wide beam that are set up for performance. As you bank into a turn, the engine banks as well which brings the prop closer to the surface and hence it will ventilate so one must throttle back in turns if ventilation is an issue.

 

[rndn]

Re: Prop size?

Silvertip, I mistated my problem. I should have said going into turns at high speed, not going straight at high speed. It really doesn't take much of a turn to have the prop blow out. The only other boat I've owned was an I/O so mayb e I'm expecting to be able to take a moderate corner at full throttle without the prop losing grip. Do anyone know the correct WOT RPM's so I can choose the correct prop?

 

[rolmops]

Re: Prop size?

Hello,RNDN
After I saw your boat,I did some checking.
Ideal WOT for your boat is stated between 4500 and 5500,so with 4900 you are nicely in the middle.
Because of the V shape of your boat,there is a bit of sideways planing at high speed turns.Add to this the width of your boat and the result is an actual lower water level at the back of your boat while you turn at high speeds. The only thing that will prevent venting in turns is trimming the engine as far down as you can,but remember, your outboard weighs only about a third of a comparable I/O, with this type of V you can plane sideways too much and your boat is not as deep in the water as the I/O you were used to.
You mentioned wanting to change to a stainless steel prop.These are about 3 to 4 times as expensive as an aluminum prop,but their performance is not that much better.When you hit a rock with an aluminum prop,your damage is usually limited to the prop.With stainless,it might be a drive shaft.
If you want to be on the high end of your range,a cupped 19 inch pitch should do great.
Good luck,and thanks again for the down riggers.

 

[Silvertip]

Re: Prop size?

It is the "observed" rpm that you need when working with props. Yes -- you try to get at the upper end of the RPM band so 4900 is still 600 RPM shy of the upper end. And yes -- you definitely want to be at the upper end. If you are blowing out in turns, then you need to trim down a little entering the turn. What you are experiencing is not unusual. My engine is mounted four holes up and even though I run trim tabs my prop will ventilate in a turn, although it has to be relatively sharp turn before it happens. Hee's the deal. I ran some numbers through the slip calculator and at 5500 RPM, 2.0 gear ratio (your engine might have a 1.86 gear set), a 19P prop, and 12% slip, your speed would be 40 MPH. Slightly faster if the engine has a 1.86 gear set. Hole shot would be improved considerably. In your case I would be tempted to try a 19P stainless prop with lots of cup. That's about all I can do for you with the current numbers. If you can get some numbers that provide something other than negative slip we can go from there.

 

[steelespike]

Re: Prop size?

I was hoping if we kept this thread active someone with the proper knowledge would be along to help.Rolmops made a good point about stainless
props.If you run in an area you might be hitting stuff you may want to stick with a good aluminum prop.Thanks Silvertip for the real poop.

 

[rndn]

Re: Prop size?

You guys are great, thank you very much for your time and effort you put into this for me. I know you guys must get sick of hearing and answering the same questions over and over, but for us novices we all think our problem is unique and want to make sure the same answer applies. Thank you for letting me know that because my boat is an outboard as compared to my old I/O that the handling I'm experiencing is normal for outboard boats.
Rolmops, thank you for the information you provided while you were at my house yesterday, I really appreciate it and good luck with the downriggers.
Silvertip, thank again for the prop recommendation as I didn't want to spend that much money without being very sure I was getting the right one.

One last question, what prop(s) would you recommend for my boat?

 

[Silvertip]

Re: Prop size?

Walleyhed has a prop study at the very top of this forum. He's tested just about anything that spins so you can study that list and make your choice. I would really suggest you work with a prop shop that would be willing to let you try before you buy. Just be a sport and buy from them when you find the one you like.