Wheel diameter and pitch Not getting top RPM

Fvdestiny

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without getting too crazy ,I have a 38 Northern Bay with a NEF 450 Iveco which is a 3000 rpm engine . I am getting around 2700 rpms . I know I am putting more load than normal by not operating at the most efficient Rpm. I cruise now at between 17-18 knots at 2350 burning 15-16 gph. how best can I achieve the peak performance and efficiency ,but also reducing the load and not loose my cruise speed.
Warning Go easy on the technical stuff . i have a 4 blade nibral 28 *28 ..Will less diameter maybe solve some of the issue?
thanks in advance for help..
 

steveinak

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I'm not a expert wheel guy but it think i'd start by taking out a couple of inches of pitch out of the wheel that i think will get ya close on the WOT rpms's.
 

tunafishhkg

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Agree with steve and Cpt Larry should come in with his knowlegeable opinion on how much. You need to get over max rated RPM FULLY LOADED by at least 100RPM unless electronic control (Peter from FPT should chime in)does not allow over 100RPM max. Your speed at cruise and top end will be what it is, perhaps slower then now, but your engine will love you and last!!
 

F/V First Team

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Focus on your pitch. Using the %load feature on your engine panel write down what you have throughout the throttle response range. For example at idle the %load is such and such, at 1000 it's this, 1500 it's that, etc etc. What were the conditions when the boat was launched and you were able to hit maximum rpm, if that ever happened. Sometimes boats are overloaded from day one. I know of a boat that had 54% load at IDLE. Needless to say it took a long time for that engine to get up to full rpm (it felt as though it had like a 7 minute turbo lag).

If this is a recent development, check the easy stuff first.

How's the bottom? Is it scaled up or covered with slime? This can peel speed, and therefore efficiency, away right from idle all the way to WOT.

Did you just load up the boat with a whole bunch of stuff, primarily in the bow. It takes a tremendous amount of energy to lift a bow out of the water, and now it has more stuff in it. Think of it as a see-saw at the park when you were a kid. On your end you could lift a similarly shaped kid. Toss another on that end, or swap them out with a larger kid and suddenly you didn't have enough lead in your ass to get the job done. It's all about isometrics, only in a boat your fulcrum is moving as the boat is doing its thing on top of the water column. If you watch closely, these boats aren't so much riding on top of the water, as they are pushing a controlled amount of water that they are on top of.

Maybe your angle of attack is off a bit, no different than an airplane. Big difference between a boat and an airplane (no, not the stewardesses) is that an airplane has flaps built in so it can change it's angle of attack. The downside is they have to adjust these flaps constantly, so with a boat you're trying to get the best angle for the entire range. Try filling up a barrel or two full of water in the aft corners of your deck, lift that bow up a little bit statically so that you don't have to do it mechanically with the engine. Don't worry about the transom being a little further in the water, we're trying to get that bow up so you're higher in the water column while at the same time reducing wetted surface.

If your bottom is grade A, your bow isn't full of stuff and its up in the air right from the start then yeah, your pitch is probably in need of adjustment. It is far easier to wiggle around with the pitch than it is to start shaving off diameter. If you have access to friends who have different propellers give those a whirl before you commit to something else.
 

Duke

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Nighthawk

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Has the boat ever reached 3000 rpm? Duke

Key question to solving this.

You don't want to go and pull pitch out of your wheel until other possibilities are investigated. If the boat has never reached correct rpms that is a different matter.
 

FPTMarineDiesel

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Agree with steve and Cpt Larry should come in with his knowlegeable opinion on how much. You need to get over max rated RPM FULLY LOADED by at least 100RPM unless electronic control (Peter from FPT should chime in)does not allow over 100RPM max. Your speed at cruise and top end will be what it is, perhaps slower then now, but your engine will love you and last!!
Hello, I have spoken with Mike yesterday due to a derate caused by fuel restriction, we discussed how I would proceed by phone, it is up to him to try my suggestion now. You are correct that he should achieve rated speed for longevity and fuel efficiency. Right now he is overloaded in all speed ranges therefore is using more fuel then he should. Theoretically every inch of pitch is an inch in forward motion, I suggest trying a wheel with a couple of inches less diamter and keep as much pitch as possible. Obviously there is some efficiency that will be lost when heavily loaded and the LWL changes but his speed may not change that much from removing the diameter as the engine will now turn the wheel faster due to the loss in over load. This in turn should theoretically allow for more speed as the pitch shouldn't change and there will be more revolutions to grab more water and theoretically speaking this should allow for more speed and higher efficiency if everything is close to perfect. I suggest trying someone elses wheel first if there is one that can correct in this manner. I am not arguing here with anyone, but am going based off of my own experience on this. This boat is a heavy boat.
 

Tuna Pursuit

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I beleive the procedure is to:

#1 Make sure your tachs are accurate, if not use a photo tach.

#2 Make sure the engine is reving up to rated RPM under no load(at the dock)

#3 Record rpms at idle and WOT in 4 direction while fully loaded, clean bottom

From there can't the prop shop figure the wheel?
 

MDI45

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without getting too crazy ,I have a 38 Northern Bay with a NEF 450 Iveco which is a 3000 rpm engine . I am getting around 2700 rpms . I know I am putting more load than normal by not operating at the most efficient Rpm. I cruise now at between 17-18 knots at 2350 burning 15-16 gph. how best can I achieve the peak performance and efficiency ,but also reducing the load and not loose my cruise speed.
Warning Go easy on the technical stuff . i have a 4 blade nibral 28 *28 ..Will less diameter maybe solve some of the issue?
thanks in advance for help..
You left out one thing....what gear reduction do you have........2 or 2 1/2 to 1....also what rpm off the top does the motor manufacturer recommend....200 or 400?
 
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captainlarry84

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All good information on this post and a lot to consider before re-pitching a propeller:

Photo Taching is a must. You need to make sure that the boat tach is correct

Great question did the boat ever turn that 28 X 28. If so may it is slightly bent and one blade is holding her back from full RPMs.

I would change all fuel filters just to be sure

Run the boat with the hatch up make sure you are pulling enough air

As for gear, on a re-pitch these are the numbers
1 of pitch = 200 RPMs on a 1:5, 150 RPMs on a 1:77, 100 RPMs on a 2:0
75 RPMs on a 2.5:1.
 

captainlarry84

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Something is very wrong. 300 RPMs short would be 4s of pitch out. Making the wheel a 28X24. Back to the million dollar questiondid you ever turn the 28 x 28 to 3000 RPMs. That is a lot of wheel for a 450 HP motor in a 38 N/B even with a 2.43 gear.
 

FPTMarineDiesel

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Something is very wrong. 300 RPMs short would be 4s of pitch out. Making the wheel a 28X24. Back to the million dollar questiondid you ever turn the 28 x 28 to 3000 RPMs. That is a lot of wheel for a 450 HP motor in a 38 N/B even with a 2.43 gear.
No, we have on record that the engine never made all of the turns, with the prop that is on there. Mike knows this and is trying to solve that.
 

FPTMarineDiesel

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All good information on this post and a lot to consider before re-pitching a propeller:

Photo Taching is a must. You need to make sure that the boat tach is correct

Great question did the boat ever turn that 28 X 28. If so may it is slightly bent and one blade is holding her back from full RPMs.

I would change all fuel filters just to be sure

Run the boat with the hatch up make sure you are pulling enough air

As for gear, on a re-pitch these are the numbers
1 of pitch = 200 RPMs on a 1:5, 150 RPMs on a 1:77, 100 RPMs on a 2:0
75 RPMs on a 2.5:1.
The boat tach is based off of crankshaft speed and we are aware that the engine never made all of its turns, it wasn't because the throttle did not make full stroke or the engine did not achieve full high idle governed speed. If the crankshaft signal was incorrect there would be an error message displayed on the dash and a plausibility error active with camshaft sensor and a hard to start issue occurring which is not the case.

This is due to a prop overload and likely an exact unkown weight/displacement of this boat as well as a LWL that may have not been accurate when sizing the prop originally. Normally we see a 26x26 or a 25x27 wheel on a boat of this size with this power, gear ratio at this displacement.

I have advised Mike to try and get the exact LWL and displacement when he hauls out, hopefully the shipyard has scales on the travel lift.

Peter
 

Fvdestiny

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Peter thank you again for your help. From the original notes the wheel was under turning 4% since the launch. We will get this figured out. Hauling tomorrow but don't have scale to get weight. Looking for one
 

tunafishhkg

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I think at this point there is no need to guess anything since you have or should make sure to get all the real time info while running in the water fully loaded, tach verified, with clean filters. This is the best info for prop guys to advise on correct prop now. The weight,Trans, and waterline stuff is for a general starting point for prop size before first launch, especially weight that will always change both in added equipment and water absorption after first launch. One of the main reasons why you want to be 100RPM over rated RPM.
 
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