Decrease in top speed

svkb

Seaman Apprentice
Joined
Jun 4, 2012
Messages
36
Hi all,

I've been having issues with obtaining the correct WOT with the original prop that was on my boat. The original was a 15.25 x 15 prop. With that prop I was getting 3900 rpm WOT and 34 mph both rpm and speed have been confirmed. RPM confirmed by timing light and speed by GPS. Yesterday I went to a 16 x 12 prop, I was expecting to lose a bit of top speed but not as much as I did. With the new prop I was getting 4300 rpm and a top speed of 25 mph. (This was loaded with 2 adults, and about 100 lbs of children and associated gear). With the original prop it didn't matter how much weight was in the boat it could be just me or the whole family and it would still do 3900 rpm and 34 mph.

Other info:

Engine: 3.0 litre merc (Compression is good with 160 - 165 on each cylinder)
Gears: 1.62:1 (somebody switched gears at one time)
Boat: 1991 Sea Ray 170 bow rider

An online speed calculator shows that my slippage with the original prop was .5% which is unrealistic, with the new prop it shows 17%.

The boat gets up on plane pretty fast and responds ok to trim.

My only explanation is that the original prop can not possibly be a 15 pitch (It has been repaired at least once by me, not sure about previous owner). However if it was more than a 15 pitch I would have expected a larger jump in rpm by dropping to a 12 pitch. A 400 - 500 rpm increase is about right for a 3 inch drop and increase in diameter.

Any ideas? Something just doesn't add up to me.

PS: I know the gears are wrong for the boat and I'll likely never get it to work the way it was designed and I've come to terms with that. (it is our first boat and we'll likely upgrade in a few years)
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
Re: Decrease in top speed

I think before going any farther i would confirm the tach is accurate.Using 10% slip your rpm becomes
about 4300.
Also is the speed by gps?Its obvious the 15" prop is about the right number.
After we know the tach is accurate I think you need to shoot for an rpm in the bottom end of your range With some luck you may allready be the there.It seems that a boat with fairly quick hole shot should be closer to its rpm range.
 

svkb

Seaman Apprentice
Joined
Jun 4, 2012
Messages
36
Re: Decrease in top speed

I should have mentioned in the initial post that the rpm was confirmed using a timing light. My only thought was perhaps my timing light was not reading right. The rpm withe the original prop was 3950 confirmed with a timing light. I didn't use a timing light with the new prop but the tach is pretty close to the timing light. Speed was confirmed by GPS.

I went back and edited my first post to include that info.
 

svkb

Seaman Apprentice
Joined
Jun 4, 2012
Messages
36
Re: Decrease in top speed

The only unknown really is if my timing light is reading correctly. I clamped onto the #1 plug wire. I have no reason to doubt its accuracy.
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
Re: Decrease in top speed

So we seem to have a mystery prop that under the circumstances produces some decent numbers.
We need to confirm its size and if any modifications And work from there.
If the prop is doctored maybe remove a little cup or pitch Just to get close to the bottom.
I have seen some impressive results removing some cup from a prop to increase rpm and speed.
 

jestor68

Commander
Joined
Jun 12, 2012
Messages
2,308
Re: Decrease in top speed

I have to ask; how are reading engine rpm with a "timing light"?
 

svkb

Seaman Apprentice
Joined
Jun 4, 2012
Messages
36
Re: Decrease in top speed

My timing light has an rpm function. Many of them do. Every time #1 cylinder fires is one revolution, so it just counts the number of "fires"
 

svkb

Seaman Apprentice
Joined
Jun 4, 2012
Messages
36
Re: Decrease in top speed

I'm at the point where I'll probably keep the two props that I have (one for pulling and one for cruising). I'm pretty sure I'm not in the right RPM range with the one prop, but we have no intentions of keeping this boat for more than 3 or maybe 4 years until we upgrade.
 

Scott Danforth

Grumpy Vintage Moderator still playing with boats
Staff member
Joined
Jul 23, 2011
Messages
51,205
Re: Decrease in top speed

the 1.62:1 gears are usually behind the 4.3 and 5.0. that being stated and you acknowledging it we will move on

my first thought is drop down in diameter to about a 14 - 14.5 diameter at 15p. best bet to find a prop to match your boat, drive gears, and motor would be to try a few dozen. many marinas have demo props.
 

svkb

Seaman Apprentice
Joined
Jun 4, 2012
Messages
36
Re: Decrease in top speed

the 1.62:1 gears are usually behind the 4.3 and 5.0. that being stated and you acknowledging it we will move on

my first thought is drop down in diameter to about a 14 - 14.5 diameter at 15p. best bet to find a prop to match your boat, drive gears, and motor would be to try a few dozen. many marinas have demo props.

I would love to test a prop before buying however, we don't have marinas in my area and I have yet to find a shop that will let you "test" a prop before purchase. Now that I've bought 2 new props (the first one was an error on my part and I won't even discuss it because it was a stupid mistake and the second I purchased here from Iboats) and have not had the results I'm looking for, I'm hesitant to throw any more money at a problem that I'm not even sure that I can solve given the gear ratio problem.


Given the information that I have supplied already, is it normal for a boat to see a decrese of 10 mph when the rpm jumped about 400 - 500 higher? I mean the increase in rpm indicates that I dropped 3 inches of pitch so it would seem that my original prop was indeed a 15. I find it strange that I would see that much of a drop in speed.

I was expecting even less slippage given that it is a larger diameter prop and just looking at the two, you can see a significant increase in blade area on the new prop compared to the old.

I'm confused!!!
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
Re: Decrease in top speed

If your test numbers are correct; you have one prop with less than 1% slip and another with 17%.
I think it is very obvious that a 12" prop with 17% slip would be slower than a 15 with 1%.
The vast disparity of performance is either the props or some variable in the test perameters.
 

svkb

Seaman Apprentice
Joined
Jun 4, 2012
Messages
36
Re: Decrease in top speed

If your test numbers are correct; you have one prop with less than 1% slip and another with 17%.
I think it is very obvious that a 12" prop with 17% slip would be slower than a 15 with 1%.
The vast disparity of performance is either the props or some variable in the test perameters.

Yes, I suppose when you look at it from an efficiency standpoint it does make perfect sense. I guess where I'm getting confused is why would a prop with (what I consider) a noticeably larger blade area have more slip than one with less?

I'm pretty confident in my test numbers, the speed confirmed via my phone GPS and the rpm by my timing light clamped to #1 plug wire.
 
Top