Seems a little slow for a 90

[Indyglenn]

My prop is a 13x15 alum. my gps is 35mph my rpms are at 5500.
For a alum 16.5 ft boat with a 90hp yamaha I thought I would see speed in the low 40's am I just wishful thinking?
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Iboats Boat and Motor Info
1. Year, make and model of boat
2009 G3 V 162C
2. Length, width and base weight of boat, look for boat decal on back of boat
16'5 7'6 1200 pounds
2a.What is the recommended HP for your boat
90hp
3. Number of people and gallons of gas normally on boat
2 adults, 16 gal
4. What do you use the boat for
fishing
5. Is it a Deep Vee and if so how many degrees of deadrise
Deep Vee
6. Year, make and model of motor
2008 Yamaha 90TLR
7. HP and gear ratio of motor IMPORTANT
90 HP, 2.0 Ratio
If you don?t know the ratio, you need to pull the plugs out and put a piece of tape across the prop and the lower unit and then cut it between the prop and the housing and turn the motor until the tape lines up with each other, it is easier if two people do this, so one can watch the prop while the other counts the revolutions of the motor
8. Manufacturer?s recommended Wide Open Throttle (WOT) range
4500-5500
9. Anti-ventilation Plate height above keel of boat if it is an outboard in inches
Not sure
10. Is it a bass boat or does it have a pad bottom

11. Does it have a hydrafoil, dolefin or trim tabs
Trim tab
12. Make, model, diameter, pitch and whether SS or aluminum prop
Aluminum Prop, Make: Yamaha 13x15
13. WOT RPM and speed from your current prop and how much gas and how many people were in the boat for the test data and is the speed by GPS. If you do not have a tach you can buy a Tiny Tach for $ 50
WOT: 5500 35gps
14. Are you at sea level or a higher elevation, give us the elevation in feet
Indiana
15. Has your motor been tuned up lately and have you checked that the carburetor butterfly is opening all the way, checked compression, and looked at the plugs and checked spark, all of the foregoing could be the reason your prop is not attaining full RPM Motor is new
16. How long has this prop been on the boat and why, at this time, do you think it is the wrong prop
Just bought the boat and it is dealer prop
17. Does the prop show any damage that you can see
No...
18. What problems are you trying to cure or what are you looking for the boat to do that it is not doing the way you think it should or to your expectations

 

[mpsyamaha]

Re: Seems a little slow for a 90

if the anti-ventilation plate is even with or below the bottom of the boat that will slow you down for sure. if the motor is low move it up to where the plate is 1" above the bottom of the boat.

the speed does seem low for a 90 horse. i had a 16ft fiberglass boat that was probably heavier than yours with a 70hp yam on it, it turned 5800 with the factory alum 13.25x17p prop hitting 38mph on gps. this was with only me and a very light load. before raising it up on the transom it would only do 35 on gps.

 

[andrewsailor]

Re: Seems a little slow for a 90

I have an Alumacraft 165cs with a 75hp four stoke,13x19 prop and at 5600 rpm im doing 38 mph with a GPS,my boat is a little lighter though,I think you may need to change props...

 

[Tig]

Re: Seems a little slow for a 90

35 sounds about right for a 13x15 prop at 5500
You need more prop.
Right now your prop slip is 10%
To get 40 mph (assuming the same slip %) you need a 17 pitch prop.
How's your hole shot? It will suffer somewhat from this change. On a fishing boat it's no big deal. On a ski boat, well, that's your call.
I'm thinking it should still be pretty good.

 

[hwsiii]

Re: Seems a little slow for a 90

Indyglenn, I ran some numbers on your boat and I show you are just a little low on your speed, but you are not going to get speeds in the 40's. 38 or 39 is about the max you will get. I also show that your prop slip is only 10% and that is very good. In my opinion you might gain a few MPH by raising the engine a little higher on the transom and going to a stainless prop.

Exactly what make, model, prop material and diameter and pitch prop did you have on it before when you were doing 38 at 6,100 RPM. Your RPM were too high, but were getting better speed.

You might try reading this for a better understanding of changing props for better performance, and for the major differences between aluminum props and stainless steel.

http://forums.iboats.com/showthread.php?t=369057



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[Dhadley]

Re: Seems a little slow for a 90

Actually the slip is almost 13%. I think we have room for improvement. Just a couple of things to keep in mind - you have a 3 cylinder motor, a deep V has a fair amount of drag and you have an aluminum prop. If the motor is mounted low that'll be more drag.

Let's finish getting the baseline info. Check out the AV plate height and let us know. When it's parallel to the keel it should not be below the keel. Even with an aluminum prop you should be able to go up some.

 

[hwsiii]

Re: Seems a little slow for a 90

No Dale, the slip is 10% he should be doing 39 and he is only doing 35, you must have misread the numbers. I NEVER make an ERROR, UNLESS I am breathing. LOL


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[Dhadley]

Re: Seems a little slow for a 90

I ran it again and came up with 12%, missed it the first time. There's still room for improvement though. We just need to keep hull design and motor size in mind.

 

[hwsiii]

Re: Seems a little slow for a 90

Dale, in an earlier post he had a 13" pith prop and he said he was getting 38 MPH at 6,100 RPM.


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[Indyglenn]

Re: Seems a little slow for a 90

Thanks to everyone for helping me out. I measured from the floor to the antivenalition plate and then from the floor to the very bottom of the boat and came up with the same measurement. I don't know if this is the correct way to measure? I did start out with a 13x13 prop and was running 6100 rpms and 38 with the boat spedo, not gps. So I went with the 13x15 which brought down the rpms to 5500. My hole shot seems to be very good, no problem getting up. When I am running I seem to get alot of spray on both sides of the boat until I am all the way trimmed up. This is my first deep V so I don't know if this is the norm for this design. At WOT trimmed all the way up it seems to easily loose grip in the water when I turn or hit waves, not that I turn very hard or anything. Thanks for the help

 

[Dhadley]

Re: Seems a little slow for a 90

The measurement should be taken from the outside with the AV plate parallel to the keel. Then see where the plate is inrelation to the keel.

If it's spraying water straight up the transom then the motor is too low. The spray is caused by the AV plate being below the bottom. Raise the motor until the plate is 1/2 to 1" above the bottom.

 

[hwsiii]

Re: Seems a little slow for a 90

Indy, I am sure Dhadley will take good care of you, he is superb at this.

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[Tig]

Re: Seems a little slow for a 90

You guys do get your homework done. RPMs have dropped far enough. More prop is a bad idea.
Maybe once there are new RPM numbers with the height sorted out it can be reviewed again.
As for slip, even by hand with a calculator I only get 10.4%
(39.0652 Theoretical - 35 actual)/39.0652 Theoretical

Keep posting the numbers Indy. I have the same task ahead of me this spring. Sort out the height (raise), then pick an SS prop.

 

[hwsiii]

Re: Seems a little slow for a 90

Glenn, here is a picture of a motor mounted at the right height for the anti ventilation plate.

Right Anti Ventilation Height

If you will notice you can see the anti ventilation plate cearly, that is right where you want it.

Induced Drag too Low

The motor should actually be lower than it looks for the extra drag from being too low.


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