Prop change for 90HP Yamaha?

[mrmamiller]

I'm not sure if I should be posting in the Prop section or the Yamaha section, but here goes. I need a bit of help determining whether my issue is the prop or something else. My goal is to get on plane quickly and top speed is a much lesser issue. I boat in skinny water and want to be on plane quickly. (thought of Smart Tabs, but a transducer would need to be relocated, not ready for that yet).

The boat is a Century 1700CC, weight is 1540# without motor. The motor is a 2003 Yamaha 90HP 2-stroke (90TLRB), weight is 262#. Passengers and gear weigh 360#. The prop is an original Yamaha SS prop, 13" x 19". No visual problems with prop (no doings or anything bent). Stamp shows "13 x 19 - K".

Originally I was wanting a faster time to plane, but now I realize that something else may need changing. I didn't measure the time from dead stop to plane, maybe next time. On smooth water, WOT is 4400 RPM (without trimming up to ventilation point) and the speed at WOT is 33MPH, as measured by GPS.

90HP is the max rated motor for this boat, as per the mfgr card.

I am attaching pictures of the motor position relative to the transom. Notice that the transom is "under-cut", which leaves me puzzled as to exactly where the ventilation plate should line up. In the pics I placed a piece of wood on top of the anti-cavitation (ventilation) plate so you can see where it lines up.

There are 4 mounting holes on the motor and I am in the 2nd to the top. Raising the motor further would raise it 1 1/2 inches.

I need a bit of advice and any suggestions would be most welcome. Thank you.

 

[steelespike]

Re: Prop change for 90HP Yamaha?

Does your weight include fuel?
It does appear you have some sort of issue.
While it's possible the 19" is a little too much prop I think the rpm should be over 5000 and speed in the 40 mph range.
Setup looks good to me.
Hopefully its something simple. Check that the throttle opens all the way.Motor in excellent tune.Good compression.
Is the gps set for mph?

 

[mrmamiller]

Re: Prop change for 90HP Yamaha?

Spike, weight included fuel, extra battery, 2 passengers, etc. BUT, my math was wrong, it should have been 460#. GPS showed 28.5 knots and I did the conversion to MPH. Compression is 130 on all 3 cyls +/- 1 PSI. Tune seems OK. I think I'll check the throttle stop next trip out. Thanks, Mark

 

[jestor68]

Re: Prop change for 90HP Yamaha?

The Mercury prop selector(has a Yamaha program) says you need a 17 pitch prop.

Measure how far below the keel your prop shaft center line is. 3.5 inches is a good starting point.

 

[limitout]

Re: Prop change for 90HP Yamaha?

you need to confirm the min-max rpm range for your motor to tell you where you need to be but as others have said you should be in the 5000 range numbers so either you have an issue with the motor or you are way over propped and a motor that is over propped will play hell getting on a plane quickly.

I saw a post on here somewhere about your motor saying it should be 5500rpm but I wouldn't trust that as the correct number to shoot for even though it sounds about right but you need to confirm what it really is

 

[SkiDad]

Re: Prop change for 90HP Yamaha?

if that engine has a 2.27 ratio then your numbers don't make too much sense unless that 19 is really lugging. I would be temped to drop down to a 15 with that much weight if you are sure the rpm is correct. You want to get close to 5500 with that engine for it to run efficiently. Your engine height looks great

 

[steelespike]

Re: Prop change for 90HP Yamaha?

My info indicates a 2.00 ratio for the 2 stroke 90.
Near as I can tell typical prop is 17 or 19 and while the 19 may be too much prop the rpm should be at least in the low 5000.with the 19
and speeds in the area of 40.
He needs to be sure the motor is right.

 

[mpsyamaha]

Re: Prop change for 90HP Yamaha?

I agree, I had a similar setup before, should be turning more rpm/going faster, even with the 19p prop. Weight seems high for that boat, is this an actual weight from a scale? Regardless, I think a 17p or maybe a 15p prop would be in order, particularly because your more concerned about hole shot. And yes, the gear ratio is 2.00 on that 90hp.

 

[steelespike]

Re: Prop change for 90HP Yamaha?

I did a little research;NADA says a 2003 1700 CC is 1158 dry weight. 262 for the motor 460 for passengers and gear= 1880
if i'm adding right.

 

[Texasmark]

Re: Prop change for 90HP Yamaha?

Agree setup looks good to me too. Doesn't look like the Yammie prop is cupped. But he didn't gripe about blowout so I guess it's no big deal.

Also agree on getting rpms up at the top or so of operating range at WOT. Never saw a 2 cycle that liked to lug.

Mark

 

[mrmamiller]

Re: Prop change for 90HP Yamaha?

Spike, I've seen widely varying specs on the 2003 Century 1700CC hull. But, I have the original factory color brochure in hard copy and it says "Wt. w/o Engine 1540 lbs (698.54 kgs). All of the pics and equipment spec'd match up with the boat, so I have to trust the spec. And it trailers like the bare hull weighs 1540.

Mark, blowout; I find that if I trim out beyond "3" on the Yamaha digital gauges, I will ventilate. And yes, I have a cupped prop on my 110 Johnson at my northern base and to me, this Yammi prop looks different regarding cupping.

Took another ride 2 days ago, just me on board, but more 10 gal more gas in tank. Seas were a bit choppier so more air under the hull. I got around 34 MPH (per GPS) and around 4500RPM on the Yammi gauge, throttle burried. This is with the motor trimmed out as far as I can without blowing air. (I still have yet to look at the throttle mechanical stop).

I'm starting to consider a 4 bladed Solas, but not sure if dropping to 15 pitch is far enough. I know that I need to do more testing. Thoughts?

Mark (the other one)

 

[mrmamiller]

Re: Prop change for 90HP Yamaha?

Finally back in FL, put on a Solas Amita 4 blade 13 x 15. (3 blade 13 x 19 SS is in the shed). All numbers are much better, WOT is 5200, hole shot is better and boat is much more responsive to burying the throttle. I was convinced that 15 pitch was right after looking on Yamaha's site where there was a test scenario with the same motor and a similarly equipped boat (weight, length and design) with a 3 blade; the prop was 17 pitch, not the 19 pitch that I had. Thanks for all your comments and suggestions. I'm back in the game. Mark

 

[SkiDad]

Re: Prop change for 90HP Yamaha?

glad to hear it ! Did you loose and speed or stay about the same ?

 

[mrmamiller]

Re: Prop change for 90HP Yamaha?

I don't think I lost more than 1 MPH. Changing water conditions make it hard to measure these things closely. I really appreciate the responsiveness to throttle changes.

 

[Texasmark]

Re: Prop change for 90HP Yamaha?

I really appreciate the responsiveness to throttle changes.

That says you're not lugging your engine which the 5200 rpm supports. 2 strokes have to breathe. Lug them and they die. Lug your 2 stroke chain saw and see what happens. Same thing with OBs.

Mark