Propeller pitch vs. mph

[charvelplayer]

I have a 1992 Ranger 451V with a 1993 Mercury 150HP XRi motor, 6 inch Detwiller jack plate, I have a 23P Tempest plus propeller on it now, 3 large vent plugs installed. I am running 5100-5200 RPM's at WOT at 58.7 mph with two guys in the boat, Jack plate at 4-1/2 inches below pad. Hole shot RPM is 3500, it is not a great hole shot but not terrible, I am wondering if I should go to a 21P Tempest plus or go with another propeller altogether? Maybe a Solas, Powertech, etc. Some on these boards say an Enertia would run really well on my set up. I want to run closer to 5600 RPM's, but do not want to lose top end speed. I would also like a better hole shot. The nose sticks way up in the air on a heavy wake and hole shot. I would like it to stay more level on a hole shot. I do know that my hull has a very light nose and is stern heavy. I hear some running a Trophy plus with a higher pitch, which gives them a much better hole shot and better top end because you can run higher pitch with the smaller diameter. Most of the props today have a lot of rake. I think maybe a prop with less rake would prevent the nose from sticking way in the air? The boat will porpoise if I ease into the throttle from a stop and get on plane little by little, it won't porpoise if I get the hull moving to a heavy wake and hammer the throttle, it seems like the hull has to have some velocity to stop the porpoise. How many MPH do you lose per pitch increase\decrease? How many MPH gained or lost with every 100 RPM? Anyone own and run this hull before that can chime in on what worked for them?

[JR19]

When you say your "jack plate at 4-1/2 inches below the pad" I assume you really mean your prop to pad height. If your prop to pad is 4.5" that is deep for your combo. 3.5" would be a good starting point. Typically people raise the motor height .25" at a time until they find the sweet spot but I think your first adjustment should be up 1" and if more is needed .25" until you find the sweet spot. Anytime you play with motor height it is important you know what your minimum recommended water pressure is for your particular engine at wide open throttle. Getting the motor too high can cause low water pressure. It can also make your rpm's too high because the prop is slipping instead of biting. If that happens you will see a slight speed loss due to slip. Adjusting motor height can be a tedious job finding the sweet spot. You may see performance gains from raising motor height and then you go up .25" or .5" and you lose performance. If this happens simply lower it back down .25" and your done.

Now the reasons I think you're deep..."typically" a deep motor is sluggish on hole shot and has more trouble achieving full rpm on top end. Your numbers are actually very good for your current set up. 150 horsepower with a 23 Tempest is a good combo for your particular rig. 58.7 @ 5200 is very good numbers. However you are correct you need to target 5600 for top end and that should net you 60 or low 60.

Just so we are on the same page here is a picture on how to measure prop to pad. If your truly 4.5" I would go to 3.5" for a starting point and adjust from there if necessary. This should let the engine build rpm's faster on hole-shot and let it spin more free on top end.

[charvelplayer]

Thank you for taking the time to respond. You are a stand up guy JR19. Forgot to mention 1.87:1 ratio. Yes I mean prop to pad height. I leave it a 4.5" since I cannot achieve 5,600 RPM's anyway. I have measured per the diagram you posted shows. I have the jackplate marked so I know where 3.5" is. I have not tried .25" increments though. I have had the motor at 3.5" below pad several times. It did not get close to the 5600 RPM even with that, but I had two other adults with me at that time so I did not think it was a fair test. I got around 5250 RPM @ 52.4 MPH. It was kind of a hot day also. The water pressure is always in specs at WOT at any engine height that I have tried. I am also aware of that issue. I start out on the hole shot with negative trim, and deeper prop because of the porpoising issue. It does not want to porpoise if I do my hole shots like this. I can raise the motor to 3.5" when I am running at WOT without it trying to porpoise at all. The hull has to have velocity to avoid the tendency to porpoise. It does still have the hooks on the rear. If I start out at 3.5" it will want to porpoise until the hull gets moving at a certain velocity. It actually bounces 2-3 times when it brakes over with the prop at 3.5". Even with 3 guys total in the boat. When that bow comes down and flattens out, I am literally at full speed in 2.5 seconds. I had to put the large vent plugs in the prop to get the 3500 RPM's on the hole shot. It was 3,000 RPM's before that with the medium plugs in. My biggest concern is lugging the engine.
Your thoughts..............

[lpugh]

You will get the best speed by using the prop that achieves the RPM where the the motor develops its peak HP, In your case that may be a 21 Tempest, holeshot and acceleration will improve dramatically
Try it with only one large plug, may need to lift just a little at nose over

[JR19]

Your thoughts..............

The only red flag I saw was 4.5" deep but you said your tried it at 3.5" and that did not help. In my opinion you have the correct prop for the boat size and motor horsepower. The only other suggestion I can think of at this point is ensure the butterflies are opening 100% when the throttle linkage is wide open. You are correct that your rpm's are a little low at WOT. It might be a prop issue but I just don't think so at this point based on the information you posted but I have been wrong before. Hopefully you get your issue resolved.

The only other thing I will add is "typically" porpoise occurs when you don't have enough rpm to keep the bow lifted. This can be caused buy too much positive trim or not enough rpm. If the bow falls it will bounce back up and repeat the up and down process until you give it more rpm to keep the bow up or bump the trim down so its the right trim angle for the rpm your're running.

[charvelplayer]

I do thank you for replying and I do understand it is difficult to assess something you have never seen or experienced. I do appreciate the information and time you have given to try to help. The 3.5" helped slightly, but only about 100-150 RPM's. I actually trim under, "negative" on a hole shot. I trim up to level when it gets on plane. I guess I could order a 22P or 21P Tempest and try it. The cup adds 1 pitch according to the Mercury slip calculator I have. That means I actually have a 24P. I do have the option to send it back for exchange if it does not work. I have 30 days. It is the hassle of worrying if it gets lost in shipping, etc. I have received some items that were pretty beat up through shipping. Also had a door hinge I ordered for my pick up truck lost and never found by Fed Ex. I had to get a refund. Rothwell Marine sold me the 23P Tempest brand new a year ago. They don't give refunds, just exchanges, so if I am wrong I own it. The only other possibility is the tach is off, but it does not sound like it is turning 5,600 RPM's at WOT. I was a G.M. technician for 23 years and am good with engines, etc. I may test the tach with a calibrated RPM meter. If it is off it is not by much.

[lpugh]

Use a timing light with digital tach and check it on the water at high rpm, analog tachs are usually off

[JR19]

At 3.5" what is your WOT rpm?...and what was your load like when you ran it at 3.5"

The reason I ask is you said you was 5100-5200 at 4.5" but gain 100-150 at 3.5" (which is about what I expected).....so if your at 5200-5350 and you have good water pressure go up another .25" If you can get it to 5400-5450 in the heat of summer with a normal load you will be around 5500-5550 in the cool weather. If you can get to within 1-200 rpm of what your target is you can send the prop to Mark Croxten and he can work it to get 100 or possibly 200 more rpm out of it.... http://www.marksprops.com/....when it comes to prop repair or modification Mark is simply the best. Do some research here on BBC. It will be hard to find a negative review on his work because he is simply that good.


Also....Did you verify your throttle linkage is opening the butterflies 100%.

[mdtritn21]

Move the motor back up to 3.5" under and test it again with no more than two in the boat. You got 5250 with that much load but moved it back down because it didn't get 5600 so now it only gets 5100-5200 with only two guys in the boat so that alone should tell you your too low prop to pad.

[mdtritn21]

I'd check the old tach too because you have to be turning more than 5200 rpm's to reach 58.7 mph closer to 5400

[JR19]

Any updates...I was looking for some information about prop pitch that was totally un-related to this thread and ran across an interesting link. As soon as I read it I thought about how a lot of the information applies to this thread...

https://www.inland-marine.com/servic...tch-education/

[charvelplayer]

Thank you for adding your expertise mdtrtn21. I may have to test the tach like you and lpugh recommends. It is an analog tach, the engine sounds strong but does not sound like it is turning 5,600 by ear though.

[charvelplayer]

JR19, thank you for the link. I did have to adjust my throttle cable to achieve WOT with the throttle plates, it was not getting there with the hammer down. I also reset the TPS with the temperature sensor disconnected like the Mercury manual says. This was done in early spring before the tests. my WOT RPM's were 5100-5200 at 3.5" below pad at 54-56.1 mph with three guys in the boat. This was in the morning at around 75 degrees. With two guys it was barely 5250 at WOT at 57.8 mph. This was at 4-1/2" below pad. This run was in the morning at around 53 degrees outside. I did call and talk to Mark Croxton about my porpoising issue and the propeller. He said the 4 hulls of that era are 4 blade boats. Every other moderator, including the one in the Ranger set up forum says the 4 hulls like 3 blades. I may go down to a 22P or 21P since the 23P is technically a 24P due to cupping. I may have to test the tach first. It is analog.

[charvelplayer]

mdtrtn21 thank you for adding your expertise. I can run at 3.5" as long as I am at WOT or close to WOT. If I do that on a hole shot, it wants to porpoise until the hull gets some velocity. Putting the Detwiller jack plate on helped the porpoising 60% over the old Quicksilver Paralift jack plate. I think I will run my tests again with only two guys only to eliminate the back and forth with weight.

[charvelplayer]

Just updating everyone that offered advice on my RPM\speed issue. I took the tachometer out and cleaned the terminals behind it. I also noticed the calibration dial on the back was off a little. It had rotated about 1/16th of an inch. I turned it back & forth a little and set it on "alternator" where it should be. I ran at 3.5" below pad once on plane, 5,500 RPMS, 61 MPH into a 5 MPH head wind, with me and my brother in the boat. Looks like the propeller and tachometer is correct.

I want to try a Solas Scorpion. I know they run 2 pitches stiffer. I don't want to remove the trim tab under the anti-cavitation plate due to torque steer. My tab is offset to the back of the tab base. The tab actually starts right behind the center of the bolt and goes back. Does anyone know if I can run one without taking the trim tab off? Can I turn a 15-1/8" diameter prop with a 150 XRi 2-stroke? I was thinking a 21p.