Alpha one gen 2 1 mounted too high.

[LassePelle]

Hi. I have a mercruiser 4.3 With a alpha one gen 2 outdrive. The engine has New filters and ignition is correct. My transom was totally rotten so it was rebuildt, but now the anti ventilation/cavitation plate is 1 inch over the bottom of the boat. When i was on the water this summer i could only Reach 4100rpm at wot and 37 mph. The prop is a standard merc 14,25 x 21 and when i try to trim the drive out the prop ventilates. Alot of rpm and less speed. When trimmed in it looses rpm and Gaines speed.

The boat is a 21 feet cabin @ 3700lbs. Do i have to lower the transom, Mount a spacer or can a ss prop help me?

 

[Don S]

Re: Alpha one gen 2 1 mounted too high.

I doubt being too high has any thing to do with it for the simple fact you also have low WOT rpm. Being mounted too high cause the prop to come out of the water which would increase the rpm.

Might recheck your measurement, you can't measure it when all the way down, you have to trim it up so the cavitation plate is parallel with the bottom. Then measure.

 

[LassePelle]

Re: Alpha one gen 2 1 mounted too high.

Hi. The measurement is With the plate parallel With the bottom. The measurement is 24,6 mm to be exact. The outdrive has 1,62 ratio.

 

[steelespike]

Re: Alpha one gen 2 1 mounted too high.

!" doesn't seem excessive.Is the prop in new condition?
You might try an aggressive prop like a Turning Point Hustler.
I assume some of your lack of speed is not being able to trim up far enough.
Is there anything on the transom or the bottom that may be disturbing water near the prop?
Some time ago I read a post here that said smart tabs mounted fairly close to the to the leg
incidentally helped with ventilation problems. 4 blade props are said to resist venting as well.

 

[LassePelle]

Re: Alpha one gen 2 1 mounted too high.

The prop is not looking too good, but there is nothing on the bottom that gives trouble.

 

[Bondo]

Re: Alpha one gen 2 1 mounted too high.

The prop is not looking too good, but there is nothing on the bottom that gives trouble. View attachment 178722

Ayuh,.... That prop don't look to bad,....

Has anybody crawled under the hull, 'n put a 6' straight edge up against the Keel Line,..??

It sounds like yer hull has a 'ell of a Hook in it...

It would make sense if the transom was rotten, the stringers are probably also failing...

 

[Tail_Gunner]

Re: Alpha one gen 2 1 mounted too high.

Something like that..The cav plate is 1.5" above the bottom... keel...hull..or transmon

what is the gearing on the drive....

When trimmed in it looses rpm and Gaines speed. that's good new's

What else have you fixed or tuned up

Where did you get the prop...is it a new addition to the boat???

 

[LassePelle]

Re: Alpha one gen 2 1 mounted too high.

Here is a pic when the transom was finished. I dont have stringers like many other boats but there is steel bars glassed in (the round things) bechause the boat is pretty tall. The transom was rebuildt since some of the last owner put a omc cobra in and just filled in the hole after vp 280 drive. I am pretty sure the hull is in good condition, but the boat is from -68 so who knows

 

[Howard Sterndrive]

Re: Alpha one gen 2 1 mounted too high.

i could only Reach 4100rpm at wot and 37 mph. The prop is a standard merc 14,25 x 21

The boat is a 21 feet cabin @ 3700lbs. Do i have to lower the transom, Mount a spacer or can a ss prop help me?

try a 19P prop or even a 20P prop
your speed seems fine at 37 mph, you just need to lower prop pitch to get the rpm's up a bit for the engine's sake.

 

[jestor68]

Re: Alpha one gen 2 1 mounted too high.

If your AV plate is about one inch above the bottom, that is fine.

Your problem is that you are using the wrong prop. :grumpy:

Before making my recommendation, I ran your numbers through the Mercury prop selector. With a 4.3L and a calculated weight of 4371 lbs(assumes a dry weight of 3700), it comes up with a 15.5 X 18 pitch Black Diamond, and calculates that it will do 4600 rpm at 41 mph.

This confirms what was going to be my suggestion for a prop. A Solas aluminum 15.5 X 17 will turn 4800 rpm at about the same speed(40 mph). If you wanted to spring for a SS prop, the 15.5 X 17 Solas Rubex would be my choice.

BTW; your WOT rpm range is 4400-4800 rpm.

What you need on a heavy boat ( over 3000 lbs) is a larger diameter prop with less pitch.

If I added 200 lbs to the weight, the Mercury prop selector went to a 16" diameter, instead of 15-15.5.

 

[LassePelle]

Re: Alpha one gen 2 1 mounted too high.

If your AV plate is about one inch above the bottom, that is fine.

Your problem is that you are using the wrong prop. :grumpy:

Before making my recommendation, I ran your numbers through the Mercury prop selector. With a 4.3L and a calculated weight of 4371 lbs(assumes a dry weight of 3700), it comes up with a 15.5 X 18 pitch, and calculates that it will do 4600 rpm at 41 mph.

This confirms what was going to be my suggestion for a prop. A Solas aluminum 15.5 X 17 will turn 4800 rpm at about the same speed(40 mph). If you wanted to spring for a SS prop, the 15.5 X 17 Solas Rubex would be my choice.

BTW; your WOT rpm range is 4400-4800 rpm.

What you need on a heavy boat ( over 3000 lbs) is a larger diameter prop with less pitch.

If I added 200 lbs to the weight, the Mercury prop selector went to a 16" diameter, instead of 15-15.5.

Thank you for the advice Guess i will try a New prop and og from there. Unfortunately we have Winter here in norway, so i cant test before april/may

 

[jestor68]

Re: Alpha one gen 2 1 mounted too high.

When I put your numbers through the selector another way(best way) based upon the current performance, I get these results:

Using a 1.62 gear ratio: 21 pitch at 4100 rpm=theoretical speed of 50 mph. actual 37 mph=26% slip. :grumpy:

Using a 1.62 gear ratio: 18 pitch at 4600 rpm=theoretical speed of 48 mph. projected 42 mph=13% slip. :joyous:

Either way, the selector comes up with Black Max (black diamond) in 15.5 X 18.

As mentioned, the Solas Rubex aluminum equivalent of 15.5 X 17 would run 200 rpm faster at the same speed. So either prop would satisfy your need .

The Solas SS Rubex 15.5 X 17 will operate at about the same(4600) rpm as the aluminum 18 but provide 1-2 mph more speed, further reducing the slip to a range of 7-8%. :faint2:

 

[LassePelle]

Re: Alpha one gen 2 1 mounted too high.

When I put your numbers through the selector another way(best way) based upon the current performance, I get these results:

Using a 1.62 gear ratio: 21 pitch at 4100 rpm=theoretical speed of 50 mph. actual 37 mph=26% slip. :grumpy:

Using a 1.62 gear ratio: 18 pitch at 4600 rpm=theoretical speed of 48 mph. projected 42 mph=13% slip. :joyous:

Either way, the selector comes up with Black Max (black diamond) in 15.5 X 18.

As mentioned, the Solas Rubex aluminum equivalent of 15.5 X 17 would run 200 rpm faster at the same speed. So either prop would satisfy your need .

The Solas SS Rubex 15.5 X 17 will operate at about the same(4600) rpm as the aluminum 18 but provide 1-2 mph more speed, further reducing the slip to a range of 7-8%. :faint2:

Thank you I tried to search the internet for advice in props, but it is a jungle hehe. But the part With bigger diameter sounds reasonable as it should push more water.

 

[LassePelle]

Re: Alpha one gen 2 1 mounted too high.

Btw i Guess this is for 3 blade props? Any idea about what 4 blade i could try? I am most interrested in mid range cruising and low speed handling

 

[steelespike]

Re: Alpha one gen 2 1 mounted too high.

Typically you would go with one inch less pitch with a 4 blade.Don't know if the prop selector would come up with that.
Usually the 4 blade would indeed improve low to midrange performance with added resistance to venting, better responce at the dock
and less vibration.

 

[jestor68]

Re: Alpha one gen 2 1 mounted too high.

Btw i Guess this is for 3 blade props? Any idea about what 4 blade i could try? I am most interrested in mid range cruising and low speed handling

The Mercury aluminum Alpha 4 (14.5 X 18 )was on the list also. Due to the inch reduction in diameter, expect about the same WOT rpm as the 15.5 X 18 3 blade(4600 rpm).