How much of a pitch change do I need?

[Elkins45]

Edit: I just realized I have asked a similar question before and there were questions about the accuracy of my tach. I THINK it's accurate, but how does one verify this? Is there an electronic gizmo I can buy that will count sparks or touch that I can test it with?

My boat is a 16ft. G3 V167C, deep V aluminum. The outboard is a 50hp Yamaha 2 stroke. I don't know the pitch of the prop that's on it (I will need to remove it to find the markings) but I think it's probably pitched too steeply for my boat. According to the tach it only makes 4400rpm at WOT and GPS confirms a speed of 27mph at that RPM. It will plane quickly at 3400 and will slowly climb onto plane at somewhere around 3000. I'm curious how much less pitch I would need to be able to get the engine up to 5000 at WOT?

I'm going to update this post with the actual pitch, but in the meantime just some rule-of-thumb suggestions would be helpful. In case it matters it's a three-blade stainless prop and I would want to replace it with another stainless prop.

 

[HighDesertMan]

I have the same boat, but with a 60 hp 4-stroke. IIRC, the max rpm range on your motor is 5-6,000 with the HP rating of 50 at 5500 RPM. If you're only turning 4400 you have a long way to step down. Definitely pull the prop to see what it is (should be stamped around the hub, it's very easy to see). The rule of thumb I've gone by is that going down 1" of pitch will give you 100-150 RPM increase, but there are guys on here with a LOT more knowledge on props/pitch.

 

[Sea Rider]

When achieved 4400 wot rpm, how loaded was the combo, state number of passengers on board. When playing with pitches will find that the wot rpm varies accordingly to the current boating load. Less wot rpm when heavily loaded, more wot rpm when lightly loaded when going one pitch down.

Should prop right that motor to rev at least towards the middle of its wot rpm range factory stated as currently loaded.

Happy Boating

 

[Elkins45]

When achieved 4400 wot rpm, how loaded was the combo, state number of passengers on board. When playing with pitches will find that the wot rpm varies accordingly to the current boating load. Less wot rpm when heavily loaded, more wot rpm when lightly loaded when going one pitch down.

Should prop right that motor to rev at least towards the middle of its wot rpm range factory stated as currently loaded.

Happy Boating

Just me and my gear, which is how it's always loaded. I almost never have anyone else in the boat with me.

 

[WesNewell]

How about engine mounting height? Mounted too low can reduce both speed and rpm by as much as 20%

 

[WesNewell]

From prop selector.

• Boat type: Aluminum - Console
• Usage: Overall good performance/0 ft
• Engine Manufacturer:
  Yamaha
• Engine Type: Outboard
• Engine Year: 2003
• Number of Engines: One
• Engine Family: 3 Cyl 2-Stroke
• Engine: 50
• Drive / Gear Case: Yamaha 25-50 hp 2S, 40-60 hp 4S 1.85 Ratio

[h=3]Details[/h]

• Calculated Pitch: 16.10
• Calculated Weight with engine(s): 1,382.40 lbs
• Calculated Engine RPM: 5,000
• Calculated Boat Speed: 34.79 mph

 

[jimmbo]

Before you go changing props.
How long have you owned this boat and motor?
What condition is the Engine in, as in compression test numbers?
Is it firing on all cylinders?
When throttle control is at full, are the carbs opening fully?
Is the spark advancing?

 

[Elkins45]

Before you go changing props.
How long have you owned this boat and motor?
What condition is the Engine in, as in compression test numbers?
Is it firing on all cylinders?
When throttle control is at full, are the carbs opening fully?
Is the spark advancing?

I’ve had it three years now and it’s been like this since I bought it. Haven’t tested the compression but it’s running beautifully at all speeds. Nothing in the behavior of the engine leads me to believe it has any sort of issues. I will check the throttles.

 

[jimmbo]

So it has never ran over 4400 rpm?
While you are checking the carb opening, take the prop off and get the Dia and Pitch info

 

[Elkins45]

How about engine mounting height? Mounted too low can reduce both speed and rpm by as much as 20%

Seems normal to my admittedly untrained eye.

 

[WesNewell]

Looks to be too low to me from that photo. Here's mine, which is as high as it can go.

reccomended heights

 

[Elkins45]

The prop is a 10 1/4-15-G. I know 10 1/4 is the diameter and 15 is the pitch. I don’t know what G means. That doesn’t seem like a crazily over pitched prop to me.

The motor is mounted in the top hole, so as low as possible. It seems like the lowest-cost option would be to try lifting the motor up at least one hole and see if that makes a difference. I’ve read a couple of posts that say you can do that by using the trailer jack but I have a tractor with a loader if that would be a better option.

 

[Sea Rider]

Some issues :

To verify if a tach reads correct numbers will need another one, a cheap option would be to buy a Hardline model HR-8062-2 good for up to 8 cylinder motors.

WN do you happen to have the exact same boat and motor, if not to each their own motor installation. BTW those pictures you posted correspond to oldie Merc ones in which the distance from lower to upper plate is only 2.5 inches. A newer Yam 50 HP motor has 5.0 inches between plates so those parameters doesn't apply at all. What works best for oldie motors doesn't work well for newer motors.....

Those idiotic lower leg/transom parameters doesn't say a thing as won't know when combo is on plane at which lower leg height will water flow skim by at speed. If over small plate, an excessive back water splash will be produced along increasing exponentialy the lower leg water drag while decreasing top end speed.

If water flow skims around AV plate a prop aeration at close tight turns and at choopy water cond will occur, only good to run at straight water courses on flat glass water cond. Best lower leg/transom height match for Yams is when water flow skims at speed right under the small upper plate, motor must run trimmed to 90 deg while the combo is on plane and deck load evenly distributed.

The motor is revving 900 wot rpm less than its middle wot rpm range of 5500. If the also idiotic Rule of Thumb 2 applies that states + 200 wot revs per one pitch down will need to find a prop 4.5 less numbers in pitch from current stock one for motor to rev around middle wot rpm range. 2 less prop numbers will do the trick.

Factory delivered props with any brand motor are medium pitched ones, as the motor manufacturer won't know which type of boat such motor will be powering it. All motors needs a prop maximization soon after the break end period ends or when buying second hand motors as well.

Happy Boating

 

[WesNewell]

I've got a 2017 Lowe stinger 175C. 17' 5" 995 dry weight with 60hp 4s merc 20" transom. Original mount had the av plate even with the bottom of the stringer going down the middle of the hull., which is actually about 1" below the actual bottom of the boat. I raised mine 1.5" (2 holes which put it about .5" above the actual bottom. That was the highest it would go since it originally was mounted in the center holes. I went from 35mph to 38.5mph and a smoother ride without any porpoising and gained ~200rpm. If he's got a 20" transom and it's mounted as low as it will go, I guarantee his engine is mounted too low by 2 or more holes. Mounted right he should get a top speed close to 35mph, which is what I get with a 700# people/gearload and full 19gal of gas

 

[WesNewell]

As for raising the engine, I've heard of people doing it by using the trailer jack, but I don't have any personal experience with that. As long as the skag is strong enough or you have another way of stabilizing the lower unit, it should be fine, but it's more than a one person job that way.

 

[Elkins45]

As for raising the engine, I've heard of people doing it by using the trailer jack, but I don't have any personal experience with that. As long as the skag is strong enough or you have another way of stabilizing the lower unit, it should be fine, but it's more than a one person job that way.

Since it has sliders for the lower bolts it won’t ever be completely free of the transom, and being a 2-stroke 50 it really isn’t all that heavy. But it would probably be the smart thing for me to use the tractor loader. It needs to be started anyway.

Any tips on sealing up the bolts so I don’t rot my transom?

 

[Sea Rider]

Can you post a nice pic of the motor sitting at back transom to have a look at your current motor height installation..

Happy Boating

 

[Elkins45]

Can you post a nice pic of the motor sitting at back transom to have a look at your current motor height installation..

Happy Boating

I’m not sure what you’re asking for the photo to show.

 

[Sea Rider]

I’m not sure what you’re asking for the photo to show.

The type of transom the boat has, the sliders, how the motor sits on transom...if possible, if not never mind...

Happy Boating

 

[Elkins45]

Do these help?